Organogermanium Compounds: Theory: Experiment, and Applications, 2 Volumes [2] 9781394177561, 9781394177585

Citation preview

Organogermanium Compounds

Organogermanium Compounds Theory, Experiment, and Applications

Edited by Vladimir Ya. Lee

Volume 1

This edition first published 2023 © 2023 John Wiley & Sons, Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions. The right of Vladimir Ya. Lee to be identified as the author of editorial material in this work has been asserted in accordance with law. Registered Office John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com. Wiley also publishes its books in a variety of electronic formats and by print-on-demand. Some content that appears in standard print versions of this book may not be available in other formats. Trademarks: Wiley and the Wiley logo are trademarks or registered trademarks of John Wiley & Sons, Inc. and/or its affiliates in the United States and other countries and may not be used without written permission. All other trademarks are the property of their respective owners. John Wiley & Sons, Inc. is not associated with any product or vendor mentioned in this book. Limit of Liability/Disclaimer of Warranty In view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of experimental reagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. While the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. A catalogue record for this book is available from the Library of Congress Hardback ISBN: 9781119613435; Set ISBN: 9781394177561 (Volume 1); ePub ISBN: 9781119613527; ePDF ISBN: 9781119613473; oBook ISBN: 9781119613466 Cover Images: © ALFRED PASIEKA/Getty Images; © Intothelight Photography/Shutterstock Cover design by Wiley Set in 9.5/12.5pt STIXTwoText by Integra Software Services Pvt. Ltd, Pondicherry, India

v

Full Table of Contents



Volume 1:

Preface  ix List of Contributors  xiii 1 Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds  1 Miriam Karni and Yitzhak Apeloig 2 Organogermanium Compounds of the Main Group Elements  103 Kirill V. Zaitsev 3 Transition Metal Complexes of Germanium  195 Kohtaro Osakada 4 Germanium Cages and Clusters  225 Tanja Kunz and Andreas Schnepf 5 Arylgermanium Hydrides, ArnGeH4-n (n = 1–3) - Synthesis, Characterization, Reactivity  277 Ana Torvisco and Frank Uhlig 6 Germylium Ions and Germylium Ion-like Species  299 Thomas Müller 7 Germanium-Containing Radicals  339 Alexander Hinz and Frank Breher 8 Germanium-Centered Anions  361 Christoph Marschner 9 Germylenes 387 Norio Nakata 10 Multiple Bonds to Germanium  435 Vladimir Ya. Lee

vi

Full Table of Contents

Volume 2: Preface  vii List of Contributors  xi 11 Germaaromatic Compounds  477 Yoshiyuki Mizuhata and Norihiro Tokitoh 12 Germanium-centered Ion Radicals  507 Mikhail P. Egorov, Viatcheslav V. Jouikov, Elena N. Nikolaevskaya, and Mikhail A. Syroeshkin 13 Donor-acceptor Stabilization of Species with Low-coordinate Germanium  561 Sakya S. Sen and Herbert W. Roesky 14 Synthesis of the Penta- and Hexacoordinate Germanium(IV) Complexes  597 Naokazu Kano 15 Dynamic Stereochemistry of Penta- and Hexacoordinate Germanium(IV) Complexes  629 Vadim V. Negrebetsky and Alexander A. Korlyukov 16 X-ray Crystallography of Organogermanium Compounds  667 Catherine Hemmert and Heinz Gornitzka 17 Organogermanium Photochemistry  745 William J. Leigh 18 Oligo- and Polygermanes  787 Charles S. Weinert 19 Bioorganic and Medicinal Organogermanium Chemistry  839 Takashi Nakamura, Yasuhiro Shimada, and Katsuyuki Sato Index  867

vii

Contents

Preface  ix List of Contributors  xiii 1 Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds  1 Miriam Karni and Yitzhak Apeloig 2 Organogermanium Compounds of the Main Group Elements  103 Kirill V. Zaitsev 3 Transition Metal Complexes of Germanium  195 Kohtaro Osakada 4 Germanium Cages and Clusters  225 Tanja Kunz and Andreas Schnepf 5 Arylgermanium Hydrides, ArnGeH4-n (n = 1–3) - Synthesis, Characterization, Reactivity  277 Ana Torvisco and Frank Uhlig 6 Germylium Ions and Germylium Ion-like Species  299 Thomas Müller 7 Germanium-Containing Radicals  339 Alexander Hinz and Frank Breher 8 Germanium-Centered Anions  361 Christoph Marschner 9 Germylenes 387 Norio Nakata 10 Multiple Bonds to Germanium  435 Vladimir Ya. Lee

ix

Preface Germanium is one of the few chemical elements in the Periodic Table, for which the theoretical prediction of its very existence has preceded its actual experimental discovery. This prediction was made by the Russian chemist Dmitri Mendeleev based on the general trends of valence and atomic weights within his Periodic Table of the chemical elements (1869) [D. Mendelejeff “Ueber die Beziehungen der Eigenschaften zu den Atomgewichten der Elemente”, Z. Chem. 1869, 12, 405–406]. In an updated version of this Periodic Table (1871-1872) [D. Mendelejeff “Die Periodische Gesetzmässigkeit der Chemischen Elemente”, Ann. Chem. Pharm. 1872, Suppl. 8, 133–229; D. Mendelejeff “Zur Frage über das System der Elemente”, Ber. Dtsch. Chem. Ges. 1871, 4, 348–352], Mendeleev proposed that there was a missing element in the carbon family with the atomic weight 72 that should be placed in the fourth row, just below silicon and just above tin within the carbon group. He named this non-existing (at that time) element as “eka-silicium”. Following this seminal Mendeleev prediction, German chemist Clemens Winkler finally succeeded in 1886 in the isolation of “eka-silicium” from the mineral argyrodite (Ag8GeS6) and named this new element as germanium (Ge) [C. Winkler “Germanium, Ge, Ein Neues, Nichtmetallisches Element”, Ber. Dtsch. Chem. Ges. 1886, 19, 210–211; C. Winkler “Mittheilungen über das Germanium”, J. Prakt. Chem. 1886, 34, 177–229]. Winkler also pioneered the preparation of the first organic derivative of germanium, namely, tetraethylgermane Et4Ge, in 1887 [C. Winkler “Mittheilungen über das Germanium”, J. Prakt. Chem. 1887, 36, 177–209]. Since then and up the present date, the chemistry of organogermanium compounds (that is, compounds featuring Ge–C bonds) has experienced an explosive growth, especially after the recognition of the key role of metallic germanium in semiconductor electronics in the midtwentieth century, followed by the extensive use of germanium and its organic derivatives in optical fibers, polymerization catalysts, microchip manufacturing, and biomedical applications. Given the undoubted importance of organogermanium compounds, it comes as no surprise that the field of organogermanium chemistry is continuously growing, thus requiring regular reviewing and updates on its latest advances. Among the most important previously published books on organogermanium chemistry, one should first of all mention excellent monograph by Satgé and coworkers [J. Satgé, M. Lesbre, P. Mazerolles, “The Organic Compounds of Germanium, Wiley, 1971] and two comprehensive volumes of the Patai’s series of books [(a) The Chemistry of Organic Germanium, Tin, and Lead Compounds (Eds. S. Patai, Z. Rappoport), Wiley, 1995; (b) The Chemistry of Organic Germanium, Tin, and Lead Compounds, Volume 2 (Ed. Z. Rappoport), Volume 2, Parts 1–2, Wiley, 2002]. Patai’s latest book was published 20 years ago, and since then, critical progress has been made in organogermanium chemistry with the majority of milestone developments achieved since 2000. That is why we have attempted in this book to survey, analyze and summarize the current state of affairs in the field of organogermanium chemistry, focusing on the latest (published mostly after 2000) groundbreaking advances with comprehensive and up-to-date literature coverage up to the end of 2021. Our “Organogermanium Compounds” book is organized into the three major parts, Theory, Experiment, and Applications, made up of a total of 19 chapters, each written by leading experts in their respective fields comprehensively covering the relevant literature, first published within the last two decades. 1) The first part, Theory (one chapter) includes a contribution “Computational Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds” from Miri Karni and Yitzhak Apeloig (Technion – Israel Institute of Technology, Haifa, Israel). In their chapter, the authors present state-of-the-art computational approaches to the flourishing field of doubly bonded organogermanium compounds and thoroughly discuss their structural and bonding aspects depending on the nature of the double bond and its substituents.

x

Preface

2) The second part, Experiment (16 chapters), deals with the most fundamental experimental advances that were achieved in synthetic and physico-chemical studies, and is accordingly divided into two sections, Synthesis of Organogermanium Compounds and Physico-Chemical Studies of Organogermanium Compounds. The first section, Synthesis of Organogermanium Compounds, is further subdivided into three subsections (in accord with the coordination number of the central germanium): 1) Organogermanium Compounds of Tetracoordinate Germanium, 2) Organogermanium Compounds of Low-Coordinate Germanium, 3) Organogermanium Compounds of Hypercoordinate Germanium. In the first subsection, Organogermanium Compounds of Tetracoordinate Germanium, there are four contributions. Kirill V. Zaitsev (Moscow State University, Moscow, Russia) in his chapter “Organogermanium Compounds of the Main Group Elements” provides a detailed overview of synthetic and structural aspects of organogermanium compounds containing Ge–E bonds (where E is an element of groups 13–17). The following chapter “Transition Metal Complexes of Germanium” by Kohtaro Osakada (Tokyo Institute of Technology, Tokyo, Japan) comprehensively covers chemistry of transition metal complexes featuring metal–germanium single bonds. In their chapter “Germanium Cages and Clusters”, Tanja Kunz and Andreas Schnepf (University of Tübingen, Tübingen, Germany) discuss syntheses, structural characterization, and reactivity of germanium clusters of the polyhedral-, metalloid-, and Zintl-type. In the final chapter of this subsection, “Arylgermanium Hydrides RnGeH4–n (n = 1–3) – Synthesis, Characterization, Reactivity”, Ana Torvisco and Frank Uhlig (Graz University of Technology, Graz, Austria) describe a variety of aryl-substituted organogermanium hydrides focusing on their synthesis, structural aspects (including 73Ge NMR properties), and selected applications. The most abundant second subsection (eight chapters), Organogermanium Compounds of Low-Coordinate Germanium, focuses on the emerging field concerned with the stable germanium analogues of pivotal organic reactive intermediates, such as germanium-centered cations, free radicals, anions, ion-radicals, germylenes, multiply-bonded organogermanium compounds, germaaromatics, and donor-acceptor complexes of low-coordinate germanium. In the first chapter in this subsection, “Germylium Ions and Germylium Ion-Like Species”, Thomas Müller (University of Oldenburg, Oldenburg, Germany) summarizes the chemistry of germylium ions [R3Ge]+ and ion-like species [R3Ge(Do)]+ (Do = electron donor) with particular emphasis on their synthesis, characterization, and specific reactivity. The chapter “Germanium-Containing Radicals” by Alexander Hinz and Frank Breher (Karlsruhe Institute of Technology, Karlsruhe, Germany) discusses general methods for generation and isolation of Ge-centered radicals (including those with redox-non-innocent ligands), polyradicals, as well as their reactivity and synthetic applications. The following chapter “Germanium-Centered Anions”, written by Christoph Marschner (Graz University of Technology, Graz, Austria), focuses on the general methods for the preparation and synthetic utilization of a variety of germyl anions, as well as germyl dianions, and compounds with a negative charge on the sp2 Ge atom. Recent advances in the field of the stable germylenes (including N-heterocyclic germylenes) and their transition metal complexes, with focus on their preparation, characterization and specific reactivity, are presented in the chapter “Germylenes” by Norio Nakata (Saitama University, Saitama, Japan). The story of the low-coordinate organogermanium compounds is further continued by Vladimir Ya. Lee (University of Tsukuba, Tsukuba, Japan) in his chapter “Multiple Bonds to Germanium,” which summarizes the latest developments in the field of doubly and triply bonded organogermanium derivatives containing both homonuclear and heteronuclear multiple bonds. Another challenging topic of contemporary organogermanium chemistry, namely, germaaromatic compounds (both neutral and charged) including non-classical aromatic systems, is overviewed by Yoshiyuki Mizuhata and Norihiro Tokitoh (Kyoto University, Kyoto, Japan) in their chapter “Germaaromatic Compounds”. The following chapter “Germanium-Centered Ion Radicals”, written by Mikhail P. Egorov, Viatcheslav V. Jouikov, Elena N. Nikolaevskaya, and Mikhail A. Syroeshkin (Institute of Organic Chemistry, Moscow, Russia and University of Rennes, Rennes, France), covers recent progress in the field of anion-radicals and cation-radicals discussing general methods for their generation and identification, both experimental and computational. The Organogermanium Compounds of Low-Coordinate Germanium subsection is completed by a contribution “Donor-Acceptor Stabilization of Species with Low-Coordinate Germanium” by Sakuya S. Sen and Herbert W. Roesky (National Chemical Laboratory, Pune, India and University of Göttingen, Göttingen, Germany), which highlights the progress in the field of compounds with low-coordinate germanium stabilized by donor-acceptor interactions. The Synthesis of Organogermanium Compounds section ends with the subsection Organogermanium Compounds of Hypercoordinate Germanium containing two contributions. In the first one, “Synthesis of the Penta- and Hexacoordinate Germanium(IV) Complexes”, Naokazu Kano (Gakushuin University, Tokyo, Japan) classifies the title compounds bearing bi-, tri-, and tetradentate ligands (as well as carbene ligands), and discusses their synthesis and reactivity. The second contribution “Dynamic Stereochemistry of Penta- and Hexacoordinate Germanium(IV) Complexes”, written by Vadim V. Negrebetsky and Alexander A. Korlyukov (Russian National Research Medical University, Moscow, Russia and Institute of Organoelement Compounds, Moscow, Russia), summarizes and analyzes the data on stereochemical processes involving organic complexes of hypercoordinate germanium.

Preface

The Experiment part is completed by the Physico-Chemical Studies of Organogermanium Compounds section which comprises two contributions each dealing with state-of-the-art instrumental techniques for assessing the structures of organogermanium compounds. In the first one, “X-Ray Crystallography of Organogermanium Compounds”, Catherine Hemmert and Heinz Gornitzka (University of Toulouse, Toulouse, France) comprehensively overview the vast literature on the structurally characterized organogermanium compounds discussing their particular X-ray crystallographic features. The following chapter “Organogermanium Photochemistry”, by William J. Leigh (McMaster University, Hamilton, Canada) deals with developments in the photochemistry of organogermanium compounds, particularly emphasizing systems involving low-coordinate organogermanium compounds as either photoproducts or photoreactants. 3) The last, third, part of the book, Applications (two chapters), reports on the synthetic approaches towards materials based on organogermanium compounds and their practical use. In the first contribution, “Oligo- and Polygermanes”, Charles S. Weinert (Oklahoma State University, Stillwater, USA) reviewed and updated the recent advances in the field of the oligo- and polygermanes, discussing general synthetic methods for their preparation and also their peculiar physical properties (luminescence, electrochemistry). In the very final chapter of the Applications part (and the whole book) “Bioorganic and Medicinal Organogermanium Chemistry”, Takashi Nakamura, Yasuhiro Shimada, and Katsuyuki Sato (Asai Germanium Research Institute, Hakodate, Japan) give an account on the production and use of bioactive organogermanium compounds (such as Ge-132) in medicinal chemistry. As an editor of the volume, I highly appreciate the works of all above-mentioned authors whose excellent contributions will hopefully turn this book into a guide to contemporary organogermanium chemistry and a useful reference source, to further inspire those already working in the field and to attract newcomers to join the fascinating world of organogermanium compounds. Although the book is primarily and foremost addressed to specialists in the field of organogermanium and organometallic chemistry, it could also be of interest to both Main Group and transition metal chemistry experts, as well as to computational and material science chemists, and to the rest of the chemical community, including undergraduate, graduate, and post-graduate students of the advanced levels. Vladimir Ya. Lee (Editor) University of Tsukuba, Tsukuba, Japan April 2022

xi

xiii

List of Contributors Yitzhak Apeloig Schulich Faculty of Chemistry Technion-Israel Institute of Technology Haifa, Israel

Miriam Karni Schulich Faculty of Chemistry Technion-Israel Institute of Technology Haifa, Israel

Frank Breher Institute of Inorganic Chemistry Karlsruhe Institute of Technology (KIT) Karlsruhe, Germany

Tanja Kunz Institut für Anorganische Chemie Universität Tübingen Tübingen, Germany

Mikhail P. Egorov N. D. Zelinsky Institute of Organic Chemistry Moscow, Russia

Alexander A. Korlyukov Pirogov Russian National Research Medical University Moscow, Russia

Heinz Gornitzka Laboratoire de Chimie de Coordination du CNRS Université Toulouse Toulouse, France Catherine Hemmert Laboratoire de Chimie de Coordination du CNRS Université Toulouse Toulouse, France Alexander Hinz Institute of Inorganic Chemistry Karlsruhe Institute of Technology (KIT) Karlsruhe, Germany

Vladimir Ya. Lee Department of Chemistry University of Tsukuba Tsukuba, Japan William J. Leigh Department of Chemistry & Chemical Biology McMaster University Hamilton, Ontario, Canada Christoph Marschner Institut für Anorganische Chemie Technische Universität Graz Graz, Austria

Viatcheslav V. Jouikov N. D. Zelinsky Institute of Organic Chemistry Moscow, Russia

Yoshiyuki Mizuhata Institute for Chemical Research Kyoto University Kyoto, Japan 

Naokazu Kano Department of Chemistry Gakushuin University Tokyo, Japan

Thomas Müller Institute of Chemistry Carl von Ossietzky University Oldenburg Oldenburg, Germany

xiv

  List of Contributors

Takashi Nakamura Asai Germanium Research Institute Co., Ltd. Hokkaido, Japan Norio Nakata  Department of Chemistry Saitama University Saitama, Japan Vadim V. Negrebetsky Pirogov Russian National Research Medical University Moscow, Russia

Sakya S. Sen Institute of Inorganic Chemistry Georg-August-University Göttingen Göttingen, Germany Yasuhiro Shimada Asai Germanium Research Institute Co., Ltd. Hokkaido, Japan Mikhail A. Syroeshkina N. D. Zelinsky Institute of Organic Chemistry Moscow, Russia

Elena N. Nikolaevskaya N. D. Zelinsky Institute of Organic Chemistry Moscow, Russia

Norihiro Tokitoh Institute for Chemical Research Kyoto University Kyoto, Japan 

Kohtaro Osakada   Chemical Resources Laboratory Tokyo Institute of Technology Yokohama, Japan

Ana Torvisco Institute for Inorganic Chemistry Graz University of Technology Graz, Austria

Herbert W. Roesky Institute of Inorganic Chemistry Georg-August-University Göttingen Göttingen, Germany

Frank Uhlig Institute for Inorganic Chemistry Graz University of Technology Graz, Austria

Katsuyuki Sato Asai Germanium Research Institute Co., Ltd. Hokkaido, Japan

Charles S. Weinert  Department of Chemistry Oklahoma State University Stillwater, Oklahoma

Andreas Schnepf Institut für Anorganische Chemie Universität Tübingen Tübingen, Germany

Kirill V. Zaitsev Department of Chemistry Moscow State University Moscow, Russia

1

1 Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds Miriam Karni and Yitzhak Apeloig Schulich Faculty of Chemistry, Technion - Israel Institute of Technology, Haifa, Israel

In memory of Professor Robert (Bob) West, a pioneer in main group chemistry and group 14 compounds in particular,  and a unique human being.

List of Abbreviations 1-Ad AIM Ar B3LYP Bbt BDE CCSD(T) CDA CGMT CV Dip (or Dipp) DMAP Dmp DSSE EDA Eind EMind EPR Etind HOMO LDA/NL LDF LUMO Mes Mes* NBO NHC NICS NMR NPA NRT PES

1- adamantyl atoms in molecules aryl Becke, 3-parameter, Lee–Yang–Parr functional 2,6-[CH(SiMe3)2]2-4-[C(SiMe3)3]-C6H2 bond dissociation energy coupled cluster, singles, doubles (triples) charge delocalization analysis Carter-Goddard-Malrieu-Trinquier model cyclic voltammetry 2,6-iPr2-C6H3 4-Dimethylaminopyridine 2,6-dimethylphenyl divalent state stabilization energy energy decomposition analysis 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl 1,1,7,7-tetraethyl-3,3,5,5-tetramethyl-s-hydrindacen-4-yl electron paramagnetic resonance

Et

Et

Et Et

highest occupied molecular orbital local density approximation with non-local corrections London Dispersion Forces lowest unoccupied molecular orbital 2,4,6-Me3-C6H2 2,4,6-tBu3-C6H2 natural bond orbital N-Heterocyclic carbene nucleus independent chemical shift nuclear magnetic resonance natural population analysis natural resonance theory potential energy surface

Organogermanium Compounds: Theory, Experiment, and Applications, Volume 1, First Edition. Edited by Vladimir Ya. Lee. © 2023 John Wiley & Sons, Inc. Published 2023 by John Wiley & Sons, Inc.

2

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

RRKM SDD SOJT Tbb Tip VB WBI Xylyl

Rice-Ramsberger-Kassel-Marcus theory Sttutgart-Dresden Double zeta Electron Core Potential (ECP) second-order Jahn Teller 2,6-[CH(SiMe3)2]2-4-tBu-C6H2 2,4,6-triisopropylphenyl valence bond Wiberg bond index (CH3)2C6H3

1.1 Introduction Unsaturated hydrocarbons having C=C double bonds; heteroelement- substituted doubly bonded carbon compounds, e.g., ketones; alkynes with C≡C triple bonds; hydrocarbons with an extended number of double bonds, such as dienes and allenes; all are of fundamental importance to chemistry and the chemistry of life. In contrast stable heavier group 14 analogues, with Si, Ge, Sn and Pb atoms, were believed to be nonexistent until several decades ago (see below the historical overview). The last four decades have witnessed an enormous progress in the synthesis and in the theoretical understanding of the properties of these groups of compounds. The experimental progress was accompanied, and in many cases even preceded, by theoretical and computational studies. These advances were enabled by two important developments: (a) the substantial advances in quantum mechanical computational methodology [1–10] that now enables reliable, accurate and efficient calculations of the properties of relatively large molecules possessing heavy elements. These developments make quantum chemical calculations a reliable tool for the quantitative analysis of chemical phenomena and a reliable guide to experiment; and (b) the dramatic developments in computer technology that have accelerated calculations dramatically [11, 12]. The synthesis, molecular structure determination, physical and chemical properties, and the development of bonding models of multiply bonded compounds of heavier group 14 elements, Si, Ge, Sn and Pb, have been reviewed extensively in the past three decades [13–26]. The numerous experimental and theoretical studies carried out in this field have revealed fundamental differences in the structures, physical properties and chemical behavior between carbon multiply bonded compounds and analogous group 14 heavier congener compounds. The heavier group 14 compounds were considered “unusual”. In contrast, Kutzelnigg suggested that the heavier main group element compounds have “regular” behavior, while carbon compounds are the exception [27]. This revolutionary view is now widely accepted by main group researchers (not only for group 14 elements). The trends in the physical properties of group 14 elements, E, are largely responsible for the unprecedented structures and chemical behavior of the heavier group 14 compounds compared to those of the analogous carbon compounds. Selected properties of group 14 elements are given in Table 1.1. A comparison of the important physical properties of group 14 elements was also provided by Basch and Hoz [28]. The effects of the trends in the properties of the E atoms shown in Table 1.1, on the structure and nature of bonding of group 14 multiply bonded compounds, are discussed in section 1.4.1. The unusual structures and bonding motifs of multiply bonded heavier group 14 molecules attracted the attention of many theoreticians, who studied their molecular structures, nature of bonding, kinetic and thermodynamic stability, potential energy surfaces for a variety of their reactions, etc. Many of these theoretical studies were carried out before stable compounds of these classes were synthesized, and were therefore truly predictive, and in some cases directed experiment. Two comprehensive reviews on the theoretical studies of the chemistry of organic Si, Ge, Sn and Pb compounds were published: one by Apeloig, Karni and Schleyer in 2001 [44] and one in 2002 by Frenking and coworkers [45]. In this chapter we review the theoretical studies of doubly bonded organogermanium compounds, mainly those published in the years 2000–2020 but occasionally we added important studies published in 2021. We have highlighted the insights that these theoretical studies provide on molecular structures, nature of bonding, stability vs. isomeric forms, and the physical properties, of these intriguing compounds. We do not include in this review their reactions and reaction mechanisms. Where available, we relate and compare the computational results to reported experimental studies. In our view, it is important to examine organogermanium compounds in comparison to their closest neighbors in the Periodic Table, Si and Sn. Thus, when available, we discuss comparisons with analogous organosilicon compounds (the element most similar to germanium) and other group 14 elements, mainly Sn. For pre-year 2000 theoretical studies, the readers are referred to the above-mentioned reviews [44, 45] and to specific papers cited in this chapter.

1.2  Computational Methods

Table 1.1  Selected physical properties of group 14 elements. E

Electronegativity Allred-Rochow Pauling Allen

C

Si

Ge

Sn

Pb

References

2.50

1.74

2.02

1.72

1.55

[29,30]

2.55

1.90

2.01

1.96

2.33

[31]

2.54

1.92

1.99

1.82

–

[32]

Atomic and ionic radii (pm) Neutral 2+ 4+

77 16

118 40–42

121 73 53

140 93 69–71

175a 118–120 78–84

[33–36]

Valence orbital energy (eV) s p Energy difference

−19.39 −11.07 8.32

−14.84 −7.57 7.27

−15.52 −7.29 8.23

−13.88 −6.71 7.17

−15.41 −6.48 8.93

[37]

Δr = (rp − rs) (pm)b

−0.2

20.3

24.9

28.5

35.8

[37]

Ionization energy (eV) nsc npd

16.60 11.26

13.64 8.15

14.43 7.90

13.49 7.39

16.04 7.53

[38–40]

Electron affinity (eV)

1.26

1.76

1.81

1.68

1.91

[41]

Hybridization of E in EH4e

sp3.17

sp2.08

sp2.05

sp1.79

sp1.75

[42]

sp2 p

sp1.79 sp18.0

sp2.07 sp7.40

sp2.22 sp5.78

sp2.52 sp3.82

[43]

Hybridization of E in H2E=EH2 (C2h) σ(E-E) “π(E=E)”

f

a

Metallic distance; Difference of orbital radii between the valence p and s orbitals; c For the transition: ns2np2(3P) → ns1np2(4P); d For the transition: ns2np2(3P) → ns2np1(2P); e Calculated using NBO analysis; f Calculated using NBO analysis at B3LYP/6-31++G(d,p). b

The theoretical literature of organogermanium compounds is vast. When we collected the literature for this review, we were amazed how vast is this field, and because of space limitations we had to limit the scope of this review, which even so includes more than 540 references! In this chapter, we concentrate on doubly bonded organogermanium compounds, which in their bonding and other fundamental properties are significantly different from analogous carbon compounds. We discuss homonuclear and heteronuclear organogermanium compounds, R2Ge=ER2, E=C, Si, Ge, and extended doubly bonded organogermanium compounds such as germadienes, germaallenes, and germaaromatic derivatives, i.e., germabenezene analogues. For lack of space, we do not discuss doubly bonded compounds between group 14 elements and groups 13, 15 and 16 elements or triply bonded compounds. For completeness, we present a brief historic overview of the field, since the isolation of the first stable doubly bonded heavier group 14 element compounds in 1976.

1.2  Computational Methods For the benefit of readers unfamiliar with the theoretical methods and terms, we bring a brief overview of the theoretical methods that are currently frequently used in theoretical calculations of organogermanium compounds. Details of the specific computational methods or of the basis sets mentioned in this review can be found in the cited papers. The rapid development of computational packages for electronic structure calculations in the last three decades has been recently reviewed [10] (for a comprehensive list of available software packages see Ref. [46]). These computational software packages apply state-of-the-art theoretical methods for electronic structure modelling of molecules in the gas phase,

3

4

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

solution, and the solid-state. Currently, these software packages apply mainly two theoretical approaches, ab initio wavefunction based molecular orbital (MO) methods [1–5], and electron density based Density Functional Theory (DFT) methods [6–8]. These software packages enable highly reliable calculations of molecular structures, potential energy surfaces, electron density and population analysis, vibrational frequencies, UV-Vis, NMR, EPR, IR and Raman spectroscopic properties, electronic circular dichroism (ECD), excitation energies, and other properties [1–9, 47]. Below we briefly summarize the theoretical approaches most commonly used for the calculation of the above-mentioned molecular properties. A detailed description of the field is given in the monographs cited above. Specific definitions and description of the computational methods used in the studies reviewed in this chapter are provided in the cited papers and references therein.

1.2.1  Ab initio Methods Ab initio MO methods are based on Hartree-Fock (HF) self-consistent field (SCF) calculations, in which the total multielectron wave function Ψ is composed from single electron spin orbital wave functions ϕl (MO) via a single anti-symmetrized determinant (the Slater determinant). The single electron wave functions are expressed by a sum of basis functions (basis sets). A very large choice of basis sets exists; they differ in size and in their adaptation for a specific calculated property [48, 49]. In addition to choosing the appropriate computational method, a correct choice of a basis set determines the computational accuracy and efficiency. However, even with the largest basis set and reaching the HF limit, the HF method recovers only 95–99% of the total energy. The missing fraction is relatively small in absolute terms, but most chemistry happens in this small remaining fraction. The missing fraction is corrected by including electron correlation energy. The most frequently used methods for including electron correlation are by perturbation theory, i.e., Møller-Plesset [50] in the nth order, (MPn) [51], configuration interaction (CI) [52], and coupled-cluster (CC) [53] methods [1–5, 46]. Today, coupled-cluster methods provide the most accurate results among the practical ab initio electronic structure theories and often serve as a benchmark for evaluating, in the absence of experimental results, the reliability of new computational methods, e.g., density functionals. Ab initio methods can be improved systematically by increasing the size of the basis sets and the amount of correlation energy that is included. It is assumed that with a complete basis set and full CI the exact solution to the Schrödinger equation can be obtained. However, the major drawback of these methods is the large computer resources required, and they are therefore applicable only to moderately-sized molecules.

1.2.2  Density Functional Theory ( DFT) Methods DFT methods [6–8], which include electron correlation indirectly, have become the most popular electronic structure methods in computational chemistry over the last 30 years. Recent advances in the development of density functionals (DF) allow one to reliably describe numerous properties of large systems at an affordable computational expense. A major difficuly in developping reliable DFs is that in contrast to the rigor of ab intio methods, DFT methods do not allow systematic improvement of the theoretical method. The strategy for improving DFs is by adding new ingredients and parametrizing them by fitting to a set of properties, e.g., atomization energy, thermochemistry, barrier heights, etc. For example, addition of a small percentage of exact HF exchange (i.e., hybrid DFs) improves significantly the performance of local DFs as shown in Figure 1.1. For example, the performance of the PBE (Perdew–Burke-Ernzerhof) functional is improved by a factor of two in PBE0, which mixes the PBE exchange energy with 25% exact Hartree–Fock exchange energy [8, 54]. Figure 1.2 shows the importance of including London dispersion corrections (D2 [55] and D3(BJ) [56]) that capture noncovalent interactions more effectively. Based on an assessment of 200 DFs tested on 84 data sets including thermodynamic data, isomerization energies, barrier heights, etc., it was concluded that addition of dispersion corrections should be applied with caution [8]. Recommendations for effective use of DFs that are suited for specific applications are given in Refs. [8] and [57], and the reader is advised to consult these references before applying a specific method to his own research or when evaluating the research of others. As can be seen in Figures 1.1 and 1.2, calculated errors can be very large, e.g., with BLYP, B3LYP, and BP86. The common notations of the theoretical methods used in the literature are the following: each computational level (ab initio and DFT) is defined by the type of method and the basis sets which are used. By convention, the computational level is designated as follows: [method 1/basis set 1]//[method 2/basis set 2]. Method 2 and basis set 2 are those used for geometry optimizations while method 1 and basis set 1 are those used for obtaining the final energy or other properties that are calculated.

1.2  Computational Methods 30

NCD

ID

BH

RMSD (kcal/mol)

25 20 15 10 5 0 BLYP

B3LYP

BP86

B3P86

PBE

PBE0

revPBE

revPBE0

TPSS

TPSSh

revTPSS revTPSSh

Figure 1.1  Stacked root-mean-square deviations (RMSD, in kcal/mol) for a set of six popular local/hybrid pairs (e.g., BLYP/B3LYP, BP86/B3P86, etc.) derived from 155 isomerization energies (ID), 206 barrier heights (BH), and 91 noncovalent (NCD) interactions. Reproduced with permission from Ref. [8]. Copyright (2017) Taylor and Francis, Ltd. 6

NCED

IE

-D3(BJ)

TPSS

RMSD (kcal/mol)

5 4 3 2 1 0 BLYP

-D2

-D3(BJ)

B3LYP

-D2

-D2

-D3(BJ)

TPSSh

-D2

-D3(BJ)

Figure 1.2  Stacked root-mean-square deviations (RMSD, in kcal/mol) for a set of four DFs and their dispersion corrected (D2 and D3(BJ)) counterparts. Based on 1744 non-covalent dimer interactions (NCED) and on 755 isomerization energies (IE). Reproduced with permission from Ref. [8]. Copyright (2017) Taylor and Francis, Ltd.

1.2.3  Methods for Analysis of Electronic Structure Important information and deeper insight into the chemical and physical properties of the studied molecules can be gained by analyzing their wave function, the electron distribution, hybridization at various atoms, the nature of the chemical bond, etc. Several analysis methods are mentioned throughout this chapter. The most popular is the Natural Bond Orbital method (NBO) [58–63] that calculates the Natural Population Analysis charges (NPA). The NPA charges are believed to be more accurate [64, 65] than the Mulliken atomic charges [66, 67], which are still often used. Several other methods for calculating atomic charges are available and a comparison of their accuracy for specific purposes was discussed, e.g., in References [64] and [65]. NBO analysis, based on localized orbitals, provides a Lewis structure of the molecule, bond orders, hybridization at the various atoms, and suggests possible resonance structures that contribute to the molecular electronic structure and their relative weights (NRT)[68], thus providing important information for analyzing the structures and bonding of the studied molecules. Other frequently used computational procedures are: (a) Atoms in Molecules (AIM), developed by Bader [69], which is based on a topological analysis of the electron density in atomic basins, and calculates also atomic AIM charges; and (b) Energy Decomposition Analysis and Natural Orbitals for Chemical Valence (EDA-NOCV), which proved to be a powerful tool for bonding analysis. For a detailed description of the above-mentioned methods and their performance, the reader is referred to the recent review by Frenking, Schwerdtfeger, and coworkers [70].

1.2.4  How Important are Relativistic Effects for Ge? As the nuclei become heavier the strong attraction of the electrons by the heavier nuclear charge causes the electrons to move faster and behave relativistically [71, 72], i.e., their relative mass increases and the effective Bohr average radius for

5

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

(a)

(b) –12.0

–14.0

relativistic energy

–16.0

–18.0

–20.0

Bond length relative to NR (a.u)

nonrelativistic energy

s orbital energy (eV)

6

0.02 0 –0.02 –0.04 –0.06 –0.08 CH4

C

Si

Ge

Sn

SiH4

GeH4

SnH4

PbH4

Pb

Figure 1.3  (a) Stabilization of the ns valence orbital due to relativistic effect. Reprinted from Ref. [73] by permission of Gordon and Breach science publishers, 1999; (b) The contraction of the E–H bond length of EH4 (E=C, Si, Ge, Sn and Pb) calculated using FockDirac (rectangle) and Fock-Dirac-Breit (*) relativistic approaches relative to non-relativistic calculations. Reprinted with permission from Ref. [74]. Copyright (1992) Springer Verlag.

the inner-electrons contracts. It was found that the relativistic contraction of the 1s orbital of Ge, Sn, and Pb, is 3, 8, and 20%, respectively [71]. Due to the orthogonality of the higher ns orbitals to the 1s orbital, the relativistic effect on higher ns orbitals is similar [71]. As shown in Figure 1.3a, the relativistic effect stabilizes the ns orbitals of Ge–Pb atoms. However, the effect for Ge is relatively small [73]. The relativistic contraction of the E–H bond in EH4 is 0.01% for CH4, 0.05% for SiH4, 0.37% for GeH4 1.16% for SnH4, and 3.9% for PbH4, leading to nearly identical E–H bond lengths in SnH4 (1.734 Å) and PbH4 (1.737 Å). These data show relatively small relativistic effects for Ge, and consequently most calculations of Ge compounds do not include relativistic effects.

1.3  Historical Overview Until the early 1970s, compounds with heavier group 14 elements, E, having double and triple bonds, were unknown (i.e., homonuclear or heteronuclear compounds possessing E=E’ double bonds and E≡E’ triple bonds (E, E’=Si, Ge, Sn, and Pb; E’=group 13–16 elements). Many experimental failures [75–78] to synthesize such compounds and some theoretical studies [79, 80] led to formulation of the classical “double bond rule” stating that “elements having a principal quantum number greater than 2 should not be able to form (p–p) π bonds with themselves or with other elements” [81]. Therefore, this group of compounds were believed not to exist. However, the pursuit for such compounds continued, leading in the late 1970s to early 1980s to detection of transient intermediates having E=E’ bonds of heavier group 14 elements [22, 75–78, 82, 83]. The first important breakthrough occurred in 1976, interestingly in germanium and tin chemistry, with the synthesis, isolation, and characterization of alkyl substituted digermene and distannene, [(Me3Si)2CH]2E=E[CH(SiMe3)2]2, (E=Ge (1) and E=Sn, (2)) by Lappert and coworkers [84–87]. The X-ray structure of digermene 1 shows a Ge–Ge bond length of 2.347 Å [86, 87], significantly shorter than the single Ge–Ge bond length of 2.463 Å in (GePh2)6 [87]. The Ge atoms in 1 are pyramidal featuring a trans-bent structure with sum of angles around Ge, Σ∠(Ge) = 348.5°, and a trans-bent angle of 32°. The X-ray structure of 2 [84, 85] revealed a Sn–Sn bond distance of 2.764 Å, similar to typical Sn–Sn single bond distances of 2.78– 2.82 Å [44, 87], and pyramidal Sn atoms, Σ∠(Sn) = 342°, and a trans-bent angle of 41.6°. The twist angle around the

1.3  Historical Overview

double bond in both 1 and 2 is 0°. Based on these geometries, the authors suggested that 1 and 2 result from a double donor-acceptor interaction of two singlet 1A1 ER2 monomers, in which the filled lone-pair orbital on one E(II) atom donates electrons into the vacant p orbital of the second E(II) monomer (Scheme 1.1, for E=Sn). However, 1 and 2 were stable only in the solid state, while in solution they dissociate to their corresponding monomers [87]. (tBu2MeS i)2Sn=Sn(SiMetBu2)2 synthesized in 2004 by Sekiguchi et al. [88] was the first, of only few isolated distannenes, that are stable also in solution [15, 89].

Sn

Sn

Sn

Sn

C2h Lone-pair donation Scheme 1.1  Mode of interactions of two stannylenes producing a trans-bent distannene.

In 1981, in two milestone papers, Brook and West and their coworkers reported independently the syntheses, isolation, and characterization, including by X-ray crystallography, of the first stable silene, (Me3Si)2Si=C(OSiMe3)1-Ad (3) [90–92] and disilene, Mes2Si=SiMes2 (4), [93–96], respectively. Silene 3 has a planar skeleton with a Si=C bond length of 1.764 Å, (r(Si-C) = 1.869 Å in H3CSiH3 [97]). In disilene 4, r(Si=Si) = 2.143 Å (r(Si-Si) = 2.327 Å in H3SiSiH3 [97]), and the Si centers are slightly pyramidal with Σ∠(Si) = 358° and a trans-bent angle of 12°. Since these pioneering landmark achievements, this field has witnessed an enormous progress. The key point to success is the use of large bulky substituents that protect the highly reactive E=E’ double bonds from further reactions. Stable compounds with a variety of E=E and E=E’ bonds (E, E’=Si, Ge, Sn, Pb; E’=group 13–16 elements), as well as compounds having an extended number of conjugated double bonds, i.e., heavier allenes and butadiene analogues, and heavier benzene analogues, were synthesized, isolated, and characterized and their chemistry has been reviewed extensively [16, 18, 24, 98–100]. Heavy alkyne analogues RE≡ER (E=Si, Ge, Sn) have also been isolated, but they are much less abundant than heavy doubly bonded compounds [17, 18, 24]. Unlike the linear alkynes, all known heavy alkynes have a highly trans-bent geometry [17, 18, 24]. The first digermyne ArGe≡GeAr (5) and distannyne ArSn≡SnAr (6) [Ar = 2,6-(2,6-iPr2-C6H3)2C6H3], were reported by Power and coworkers in 2002 [101]. Several digermynes and distannynes with other substituents on the aryl groups were reported by these authors in 2010 [102]. In 2006, Tokitoh et al. reported the synthesis of BbtGe≡GeBbt (7) (Bbt = 2,6-[CH(SiMe3)2]-4-[C(SiMe3)3]-C6H2) [103]. The synthesis and characterization by X-ray crystallography of the first stable disilyne RSi≡SiR, R=[SiiPr{CH(SiMe3)2}2] (8) was reported by Sekiguchi’s group in 2004 [104, 105]. Wiberg reported earlier the detection of a relatively stable disilyne, RSi≡SiR, R=SiMe(Sit-Bu3)2 (9), but its crystal structure could not be obtained [106]. A second known stable disilyne with R = Bbt (10) was reported in 2008 by Tokitoh’s group [107, 108]. In 1999, Schwarz and Apeloig reported the identification of two silynes HC≡SiX (X=F, Cl) in neutralization–reionization mass-spectrometric experiments, and demonstrated their microsecond existence under high-vacuum conditions [109]. More recently, Kato and Baceiredo reported a base stabilized silyne that was stable up to −30°C. Its X-ray structure revealed a short Si–C bond length of 1.667 Å, indicating the possible existence of a Si–C triple bond [110, 111]. Germa- [112] and stanna [113]-acetylenes were identified only as intermediates in a low-temperature matrix or by their trapping reactions [113]. To date, efforts to synthesize isolable E≡C (E=Si, Ge, Sn) triply bonded molecules were not successful. It should be emphasized that all known stable multiply bonded heavier alkene and alkyne analogues are substituted with very bulky substituents that stabilize them kinetically, preventing their dimerization and significantly slowing their other reactions. Theoretical studies in the last decade pointed to the importance of attractive noncovalent London Dispersion Forces (LDF) [114] between the bulky substituents, in stabilizing these weakly bonded multiply bonded molecules against dissociation to their tetrylene monomers [115–118]. In Table 1.2 we provide a list of characterized doubly bonded germanium compounds and some of their important geometric parameters (both homo- and heteronuclear) that were synthesized since 2010 and not covered in the reviews of Power [18] and Lee [24]. Table 1.2 also includes, where available, the results of theoretical calculations of these compounds’ structures.

7

Table 1.2  Doubly bonded organogermanium compounds, R2Ge=E’R’2, reported in 2010–2020.

Compound

Twist angle around Ge=E’

UV-Vis absorption (nm)

Ge=E’ (Å)

Σ∠(Ge); Σ∠(E’) (°)

2.270

0.3b

7.5

362, 421

[119,120]

2010

2.346

358.8

52.8

618

[121a,b]

2011, 2014

2.278

334.0

0.5

3.11d

[121b]

2.284

335.1

13.5

2.75d

2.347

358.7

55.3

2.21d

2.294

334.5 37.7b

No twist

507

[122]

2015

13.6

426

[123]

2019

Reference

Publication year

a) Digermenes 11 12

(tBuMe2Si)2Ge=Ge(SiMe2tBu)2a

(tBu2MeSi)2Ge=Ge(SiMetBu2)2a R2Ge=GeR2 (calculated)c a) R = H3Si b) R = Me3Si c) R = tBu2MeSi

13

R

R N Xyl

Si Ge

C

Xyl C

Ge Si

Si R

R

Cl

Si

Cl

R

N

R

R = Tip = 2,4,6-iPr3C6H2, Xyl = 2.6-Me2C6H3

346.53(Ge1)e

14

R2Ge2=Ge1RSiPh3 R = Tip = 2,4,6-iPr3C6H2

2.328

345.24 (Ge2)f

15

R2Ge=GeRSiMe2Cl R = Tip = 2,4,6-iPr3C6H2

–g

–

–

435

[123]

2019

16

R2Ge=GeRSiMePhCl R = Tip = 2,4,6-iPr3C6H2

–

–

–

NA

[123]

2019

17

R2Ge=GeRSiMe2Ph R = Tip = 2,4,6-iPr3C6H2

–

–

–

424

[124]

2018

18

R2Ge=GeRSiMe3 R = Tip = 2,4,6-iPr3C6H2

–

424

[124]

2018

19.9

435

[124]

2018

–

NA

[125]

2016

–

–

19

R2Ge2=Ge1RLi · 2dme R = Tip = 2,4,6-iPr3C6H2, dme = 1,2-dimethoxyethane

2.284

7.1 (Ge1)b 12.8 (Ge2)b

20

K22+[(boryl)Ge=Ge(boryl)] 2– Boryl = (HCDippN)2B, Dipp = 2,6-iPr2C6H3h

2.392

–

Compound

Ge=E’ (Å)

Σ∠(Ge); Σ∠(E’) (°)

21

R2Ge2=Ge1R(C(R’) = O) R = Tip = 2,4,6- iPr3C6H2; R’ = (a) tBu, (b) 2-methylbutan-2-yl and (c) 1-adamantyl

–

–

22

(E)-(L)HGe=GeH(L) L = N(SiiPr3)R R = 2,6-[CHPh2]-4-iPrC6H2

2.486

322.5 54.1b

2.535

344.8

23

(E)-(L)HGe=GeH(L) Dip

B N

UV-Vis absorption (nm)

Reference

Publication year

476 (Ge=Ge) and 386 (C=O)i

[123]

2019

No twist

460

[126]

2013

No twist

NA

[127]

2019

–

39.1b

N

L=

Twist angle around Ge=E’

SiMe3 N

Dip

Dip=Diisopropylphenyl toluene/18-crown-6

24

(E)-(Eind)XGe=GeX(Eind) X= (a) Br and (b) Cl

25

26

(a) 2.415 (b) 2.412

(a) 337.1, 43.29b (b) 335.9, 44.34b

No twist

(a) 406 (b) 390

[128]

2018

a) 2.509 b) 2.406

a) 44.6b b) 336.3

a) No twist

b) 449

[129,130]

2005, 2016

r(Ge=Ge) = 2.413 r(C-C) = 1.532

∠C(Bbt)-Ge-GeC(Bbt) = 133.6 39.5b

494

[131]

2015

Et Et

Et Et

Et

j

Et Et Eind

Et

ArBrGe=GeBrAr a) Ar = Bbt = 2,6-[CH(SiMe3)2]2-4[C(SiMe3)3]-C6H2 b) Ar = Tbb = 4-tBu-2,6-[CH(SiMe3)2]2-C6H2 Bbt

Bbt Ge

Ge

–

Bbt = 2,6-[CH(SiMe3)2]-4-[C(SiMe3)3]-C6H2

(Continued)

Table 1.2  (Continued)

Compound

27

Ge=E’ (Å)

2.414

Bbt

Bbt Ge

Ge

Ge

Ge

Σ∠(Ge); Σ∠(E’) (°)

∠C(Bbt)–Ge– Ge–C(Bbt) = 134.1 36.1/43.0b

Twist angle around Ge=E’

UV-Vis absorption (nm)

Reference

Publication year

–

NA

[132]

2018

Ph

28

Tbb

Tbb

Ph

r(Ge=Ge) = 2.416 r(C=C) = 1.362

351.3

–

369

[130]

2016

2.430

Ge1/Ge3 360 Ge2 334 Ge4 328

–

458, 510

[133]

2018

2.039k

286.6k,l

NA

[134,135]

2019

Ph

Tbb=4-tBu-2,6-[CH(SiMe3)2]2-C6H2

EMind

29

1

Ge EMind

Ge2

4

Ge

EMind

Ge3 EMind

Et

Et

Et Et

Me Me EMind

Me Me

b) Compounds with Ge=E’ bonds i) Ge=C bonds 30

SiMe3 Me Ge Me

NiPr2

C NiPr2

SiMe3

88.3l,m

Compound

31

Se Tbb

Ge

Ge

Ph

Tbb

R

O K

Me2Si

Ge

Me2Si

Reference

Publication year

536

[136]

2018

NA

[137,138]

2017, 2020

–

a) 442; b) 463; c) 422

[138,139]

2015, 2020

a) 314 (Ge) 359.6 (C) b) 314.5 (Ge1) 298.2 (Ge2) 359.6 ( C)

–

a) 447 b) 420

[140]

2020

351.7 (Ge), 360.0 (C)

–

368

[138,139]

2015, 2020

Σ∠(Ge); Σ∠(E’) (°)

1.921 (Ge-C) 1.375 (C-C)

339.9, 338.7

1.998n

310.5 (Ge) 359.8 (C)

b) 2.007; c) 2.036

b) 304.6 (Ge), 359.9 (C); c) 310.7 (Ge), 359.9 (C)

a) 2.055 r(C-O) = 1.252 b) 2.047 r(C-O) = 1.248

1.835 (r(C = O) = 1.388)

Twist angle around Ge=E’

∠SeGeCC = 11.6

Ph

32

33

UV-Vis absorption (nm)

Ge=E’ (Å)

Ge

Me3Si

SiMe2

18.5

SiMe2 SiMe3

Toluene/18-crown-6 o p a) R = Mes; b) R = o-Tol ; c) R = 1-Ad

34

Toluene/18-crown-6 a) R=Mes; b) R=1-Ad

35

R

Me2Si Me2Si Me3Si

O Ge

Ge

SiMe3

SiMe2 SiMe2 SiMe3

R = Mes

(Continued)

Table 1.2  (Continued)

Compound

Ge=E’ (Å)

Σ∠(Ge); Σ∠(E’) (°)

2.221

0.6b

r(Ge=Si) = 2.246 r(Ge-K) = 3.407

–

1.886

X=Cl: 359.6 (Ge), 360.0 (B)

r(Ge1-P1) = 2.266, r(Ge1-P2) = 2.272

–

Twist angle around Ge=E’

UV-Vis absorption (nm)

Reference

Publication year

359, 413

[119]

2010

495

[141]

2021

–

NA

[142]

2020

–

NA

[143]

2016

ii) Ge=Si bonds 36

(tBuMe2Si)2Ge=Si(SiMe2tBu)2 Tip

Tip

37

Si Ge

N C Xyl

K •18-crown-6

7.5 planar 4-membered ring

Si Tip

iii) Ge=E’ (E’=group 13, 15 and 16 elements) 38

Ph2 P B Ge

X

X= Cl, Br Ar*=2,6-Tip2-C6H3 (Tip=2,4,6-iPr3C6H2)

39

Dipp N

Dipp N

P1

Ge1

Ge2 P2

N Dipp

N Dipp

Dipp=2,6-iPrC6H3

40

(tBu2MeSi)2Ge=As-Mes* Mes* = 2,4,6-tBu3-C6H2

2.273

360.0

planar

390q, 450r

[144]

2018

41

Eind2Ge=O

1.647

359.8

planar

297 (IR (Ge=O) 916 cm−1)

[145,146]

2012

Compound

42 R

N

N

UV-Vis absorption (nm)

Reference

Publication year

–

NA

[147]

2009

343.6

–

NA

[148]

2011

2.077

338.5

–

NA

[149]

2004

2.095

–

–

NA

[150]

2017

2.068 (Ge-S2) = 2.234

–

–

NA

[150]

2017

Ge=E’ (Å)

Σ∠(Ge); Σ∠(E’) (°)

1.672s

333.9s

1.646

Twist angle around Ge=E’

R

Ar N Ge O N Ar

R = Me, iPr; Ar = 2,6-iPr2C6H3 43

N

N Ar N Ge O N Ar Ar = 2,6-iPr2C6H3

44

N

Ar S Ge OH

N Ar

45

Ar = 2,6-iPr2C6H3

Ar S N Ge

PCy2

N Ar

Ar = 2,6-iPr2C6H3; Cy = C6H11 46

Ar S1 N Ge

S3 S2

PCy2

N Ar

Ar = 2,6-iPr2C6H3

(Continued)

Table 1.2  (Continued)

Compound

Ar

47

Se

N Ge

UV-Vis absorption (nm)

Reference

Publication year

–

NA

[150]

2017

–

–

NA

[150]

2017

2.268, 2.290t, u

–

–

451

[151]

2017

2.312

345.3

–

383

[152]

2015

Ge=E’ (Å)

Σ∠(Ge); Σ∠(E’) (°)

2.221

–

2.163

Twist angle around Ge=E’

PCy2

N Ar Ar = 2,6-iPr2C6H3 Se

Ar

48

N Ge

P(SiMe3)2

N Ar Ar = 2,6-iPr2C6H3

c) Extended doubly bonded compounds 49

BbtGe

GeBbt Si

Bbt = 2,6-[CH(SiMe3)2]-4-[C(SiMe3)3]-C6H2

d) Germabenzenes 50

Tbb Ge

Tbb Ge

Tbb = 4-tBu-2,6-[CH(SiMe3)2]2-C6H2

Compound

51

R1 C1 Tbb

Ge

R1 C2 Ge

C3

R2

Ge=E’ (Å)

Σ∠(Ge); Σ∠(E’) (°)

r(Ge-C1) = 1.890v r(Ge-C3) = 1.866v

357.3v

Twist angle around Ge=E’

–

UV-Vis absorption (nm)

Reference

Publication year

442v

[153]

2019

Tbb

C4 R3

a) R1 = R2 = R3 = Ph b) R1 = R2 = R3 = Et c) R1 = Ph, R2 = Me, R3 = nPr Tbb = 4-tBu-2,6-[CH(SiMe3)2]2-C6H2 a

A preliminary X-ray diffraction – poor refinement; trans-bent angle; c At B3LYP/6–31G(d); d HOMO-LUMO gap in eV; e Trans-bent angle = 23.7°; f Trans-bent angle = 21.3°; g Unavailable data; X-ray crystal structure could not be obtained. Characterized as digermenes by their NMR spectra; h For previously synthesized digermene dianions, see Ref. [46] cited in Ref. [125] [Pu, L. et al. J. Am. Chem. Soc., 1998, 120, 12682–12683]. For a review, see: Wang, Y. and Robinson, G. H. Chem. Commun., 2009, 5201–5213; i For R’ = 1-adamantyl; j For earlier papers on dihalodigermenes, see references [5] and [6] cited in Ref. [128]; k Best described as a germanium/carbon ylide, with the negative charge located at the germanium atom; l Indicates the presence of an active lone pair at the germanium atom, and excludes the possibility of a Ge–C (of the three-membered carbocycle) π bonding; m The three-membered ring is oriented almost perpendicular to the germole ring; n r(K-Ge) = 3.946 Å, r(K-O) = 2.782 Å, r(C-O) = 1.241 Å; o r(K-Ge) = 3.855 Å, r(K-O) = 2.730 Å, r(C-O) = 1.236 Å; p r(K-Ge) = 3.613 Å, r(K-O) = 2.740 Å, r(C-O) = 1.231 Å; q HOMO [π(As = Ge)] – LUMO [π*(As = Ge)]; r HOMO-1 [n(As), lone pair] – LUMO [π*(As = Ge)]; s For R = Me; t Best described as R2Ge:→Si(0)←:GeR2; u ∠GeSiGe = 80.1°; v For compound 51a. b

16

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

1.4  Doubly Bonded Compounds The structures of RR’E=ERR’ (E=Si, Ge, Sn, Pb) compounds were studied and analyzed extensively by both theory and experiment over the last four decades. These studies were reviewed in many comprehensive reviews [15, 16, 18, 24, 25, 44, 45, 70, 154, 155]. The main structural and bonding aspects of these compounds, which were previously reviewed, are briefly summarized below. New insights gained more recently regarding the nature of bonding in these doubly bonded compounds have been added. The most surprising structural feature of the heavier group 14 alkene analogues, R2E=ER2, which attracted great attention, is probably their nonplanar structures (Table 1.1) [15, 16, 18, 24, 25, 44, 45, 154, 155], in sharp contrast to the planar structure of alkenes R2C=CR2. This contrast indicates that simplistic analogy between carbon and its heavier analogues is wrong, and that the heavier group 14 elements have entirely different bonding characteristics. This unexpected experimental observation was predicted by theory before many of these compounds were synthesized [156, 157], and in many cases after their synthesis, the experimental bending angles were nicely reproduced by theory [44, 45, 70, 158, 159]. The successful computational predictions in this field that preceded experiments were among the early victories of computational chemistry, which in the 1970s and 1980s still met considerable scepticism by experimentalists.

1.4.1  Bonding Models As mentioned above, for many decades the heavier group 14 elements were believed to be reluctant to form π bonds (classical “double bond rule” [79, 80]). After the first multiply bonded compounds were isolated, new questions of great theoretical relevance arose. Why, in contrast to carbon, do its heavier analogues form compounds with very different geometries than the analogous carbon compounds? For example, why are doubly bonded compounds non-planar and have trans-bent structures? Why are triply bonded compounds bent and not linear? What is the nature of these distorted π bonds? Are they “real” double bonds? These questions raised a burst of theoretical research accompanied by vibrant debate in the last four decades [18, 25, 44, 45, 70, 160–164]. Below we review the different views of theory on these intriguing fundamental questions. A major fundamental difference between elements of the first (C to F) and higher rows of the Periodic Table is their most probable valence orbital radii. For carbon, the most probable radius of the 2s orbital is relatively expanded due to Pauli repulsion from the 1s electrons, while the 2p orbital that is not shielded by an inner p shell is relatively contracted. This results in very similar radii of the 2s and 2p orbitals. Moving down group 14, the atomic np orbitals are much more expanded as a consequence of intraatomic Pauli repulsion from orthogonal inner shell (n-1)p orbitals. Consequently, the radii of the ns and np orbitals (n > 2) diverge significantly [27]. Δr = rp–rs exhibits a zigzag increase from Si to Pb, caused by d-block contraction (Ge) and relativistic effects (Sn, Pb) (Table 1.1 and Figure 1.4). The similar radii of the s and p valence orbitals of carbon (and other first row elements) facilitates hybridization. In contrast, for the heavier main group elements, hybridization is much less favorable due to the dissimilar radii of their valence ns and np orbitals. Kutzelnigg introduced the term “hybridization defects” for this phenomena [27]. The increasing “hybridization defects” down group 14 have a crucial effect on the structures and bonding of heavier group 14 compounds [70, 165]. The increase in the trans-bending angle, and in the planarization energy of the trans-bent H2E=EH2 (C2h) (Table 1.3) moving down group 14, are manifestations of increasing “hybridization defects” [27, 42, 115]. Similarly, the much higher inversion barriers of e.g., PH3 (35.1 (calculated) [166]) vs. NH3 (5.8−6.0 kcal/ mol [166–168] (calculated)) and SiH3- (26  kcal/mol (MP2/6-31+G(d)) [169] vs. CH3- (2.5 kcal/mol (CCSD(T)/ Figure 1.4  Calculated ns and np orbital radii of maximal aug-cc-PVQZ) [169]), was also rationalized by the smaller electron density of group 14 elements [27, 37, 115]. Reproduced with permission from reference [115]. Copyright (2020) American tendency for hybridization of the ns and np orbitals in the Chemical Society. heavier elements [115, 166]. The decrease in the

1.4  Doubly Bonded Compounds

dissociation energy of H2E=EH2 (C2h) to their EH2 fragments (see below, Table 1.3, section 1.4.2) was also rationalized by the increasing destabilization of heavy alkene analogues, due to the lack of effective iso-valent hybridization and the enhanced stability of the EH2 fragments [42]. The non bonding lone-pair orbital of EH2 has mainly s orbital character, thus, lowering its energy, while the E–H bonds are essentially made of non-hybridized p orbitals strengthening the E–H bonds and stabilizing the divalent hydrides (“inert pair effect”) [42, 165]. Electronegative substituents enhance “hybridization defects” in heavier p-block elements, thus destabilizing the olefin-like molecules, while their effect on the lowoxidative states (e.g., ER2) where hybridization is less important, is marginal [42, 165]. The Pauli repulsion, caused by core electrons in heavier group 14 elements, has an important effect on the E–E π and σ bonds strengths. DFT calculations by Ziegler and Jacobson have shown that the increase in core-core Pauli repulsion between the E atoms forming the E=E bond (i.e., interactions of the valence orbitals of one E atom with the inner shell orbitals of the other E atom forming the bond, termed interatomic Pauli repulsion [170, 171] or inner shell repulsion [79]), weakens the E–E σ bond and increases the interatomic E–E distance, which in turn also lowers the overlap of the p orbitals weakening the π bond [170, 171]. It is argued that the linear p overlap in the σ bonds is more effective than the sideway π overlap, making the π bonds more sensitive to changes in the bond distance. In contrast, a study of the variation of orbital overlap as a function of internuclear distance found that π bonds (not supported by an underlying σ bond) can be shorter than σ bonds and are not inherently weak [172]. Thus, it is the underlying σ bond that actually determines the E=E bond strength. Analysis of the nature of bonding in trans-bent HE≡EH (E=Si, Ge, Sn) emphasizes the importance of the E–E σ bond in stabilizing the trans-bent structures. VB calculations [173, 174] show that the σ-frame prefers trans-bending and the π bonding opposes this distortion. In HC≡CH, the destabilization of the π bond upon bending overrides the propensity of the σ-frame to distort, while in the heavier congener molecules, the stabilization of the σ-frames by trans-bending overcomes the destabilization of the total π bonding and thus results in trans-bent molecules [173, 174]. The increasing core-core repulsion down the group leads to a significant decrease in the stability of a E=E double bond as well as of a E–E single bond; thus for E2H4, E=Sn and Pb, the hydrogen dibridged isomer, HE(μ‒H)2EH becomes more stable than the trans-bent doubly bonded isomer H2E=EH2 and the mono-bridged isomer H2E(μ‒H)EH [70, 162]. For E2R4 (R=electronegative substituent) “hybridization defects” are enhanced significantly and the dibridged isomer becomes the global minimum for E=Ge, R=F, Cl, Br. For E=Ge, R=F the dibridged isomer is more stable than F2Ge=GeF2 by 18.3  kcal/mol (at BP86/def2-TZVpp) [70]. At MP2(FCI)/cc-pwCVTZ(-PP) R2Ge=GeR2, R=F, Cl, Br, I, were located as transition states [175]. At a variety of other DFT levels, F2Ge=GeF2 dissociates to two GeF2 fragments upon geometry optimization [176]. See section 1.4.2.2.2.1 for a detailed discussion on the effect of substituents on the structures and stability of various Ge2R4 isomers. The above discussion focuses on the effect of the inherent difference in the radial extension of the valence ns and np orbitals on structure and bonding. Other bonding models are discussed below. An alternative rationale for the preferred trans-bent geometries of heavy group 14 analogues of alkenes and acetylenes (R2E=ER2 and RE≡ER, respectively) is arising from second order Jahn-Teller (SOJT) effects [43, 87, 158–161, 171, 177]. The SOJT effects are caused by mixing of occupied π (HOMO) and empty σ* (LUMO+1) molecular orbitals which acquire the same symmetry (bu) upon trans-bending (Figures 1.5 and 1.6). The mixing of σ and π* orbitals is less important [171]. The extent of orbital interactions and the resulting stabilization of the trans-bent structure is inversely proportional to the energy difference between the interacting orbitals.

Figure 1.5  A schematic description of the π-σ* mixing (SOJT) upon trans-bending of R2E = ER2.

17

18

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

Figure 1.6  Orbital symmetries and orbital energies of D2h (left) and C2h (right) H2Ge=GeH2. Reprinted with permission from Ref. [159]. Copyright (2020), Royal Society of Chemistry.

The π-σ* energy separation in planar D2h H2E=EH2 decreases upon moving to the heavier elements in group 14; ΔE (eV, at ωB97X-D/def2-TZVPP) = 14.54 (E=C), 9.78 (E=Si), 9.05 (E=Ge), 7.44 (E=Sn) [159]. As a result, the orbital interaction energies and electron delocalization increase going down group 14. This is manifested also in the NBO second order stabilization energies associated with the π → σ* mixing, which increase in the order (in kcal/mol, at B3LYP/def2-TZVPP// CCSD(T)/def2-TZVPP): 3.64 (Si), 8.83 (Ge), 24.41 (Sn) [178], 47.0 (Pb) [179]. Simultaneously, the trans-bending angles widen [16, 18]. The barriers for planarization (or linearization of heavy acetylene analogues) increase down group 14 [43, 70, 159, 171], thus, For H2E=EH2 they are (in kcal/mol): 0.0 (E=C), 0.9 (E=Si), 2.7 (E=Ge), 10.1 (E=Sn) [159], 23.2 (E=Pb) [171] (see also Table 1.3), and correlate linearly with the π-σ* energy separation for heavy E-elements (Figure 1.7) [159]. The E=E Wiberg bond indices (WBIs) decrease from 2.05 (E=C) to 1.90 (E=Si), 1.79 (E=Ge), 1.54 (E=Sn), reflecting the increase in the π(E=E) → σ*(E–E) electron delocalization [178].

Figure 1.7  Planarization (inversion) barriers of H2E=EH2 vs. the π-σ* energy gap (ΔE, eV) for ethylene (D2h, ΔE=14.54) and its heavier congeners (E=Si (ΔE=9.78), Ge (ΔE=9.05), and Sn (ΔE=7.44)) in C2h symmetry. Reproduced with permission from Ref. [159]. Copyright (2020) Royal Society of Chemistry.

1.4  Doubly Bonded Compounds

Another model for understanding the structures of R4E2 compounds is based on the ground state multiplicity of the R2E fragments and their singlet-triplet energy gap (ΔEST). Carbenes, R2C:, R = alkyl, aryl, are ground state triplets (3B1), while X2C:, X = electronegative substituents, e.g., halogen, NR2, OR, are ground state singlets [180, 181]. Heavier R2E: are all ground state singlets (1A1) [182] (Figure 1.8a, Tables 1.3, 1.5 and 1.6). The reasons for this reversal in multiplicity were analysed [183]. The association of two triplet ER2 (3B1) molecules, forms a planar alkene-like structure (Figure 1.8b). In contrast, for two singlet ER2 (1A1) molecules to form a planar double bond, they have first to be excited to the triplet state, paying the cost of the singlet-triplet energy gap (ΔEST); consequently the E=E bond is weakened [184]. Instead, singlet ER2 (1A1) fragments prefer to interact via two donor-acceptor interactions, in which the lone-pair orbital of one ER2 fragment donates electrons into the empty p orbital of the other ER2 fragment, forming a trans-bent doubly bonded compound (Figure  1.8c) [70, 160–162]. Based on the trans-bent X-ray structure of the first isolated distannene [(Me3Si)2HC]2 Sn=Sn[CH(SiMe3)2]2, (2), Lappert et  al. suggested that 2 results from a donor-acceptor interaction of two singlet SnR2 monomers (Scheme 1.1) [84–87].

Figure 1.8  Bonding models of the interaction of two triplet (3B1) and singlet (1A1) R2E fragments. (a) triplet and singlet R2E; (b) Interaction of two R2E in the triplet state, forming a planar R2E=ER2; (c) Lone-pair donor-acceptor interaction of two R2E in the singlet state, forming a trans-bent R2E=ER2; (d) E–X bond-pair donation, forming a dibridged structure; (e) Lone-pair donation from X forming a dibridged structure.

19

20

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

Descending group 14, the lone-pair orbital of R2E acquires more s-character and is stabilized (“inert pair effect”) and is less prone to donate electrons to the partner R2E fragment, reducing the tendency to form an E=E bond. At the same time the E–R bonds become better donors, donating charge into the empty p orbital of the second R2E fragment, thus favoring dibridged isomers (Figure 1.8d) [70]. Substituents that enhance the “inert pair effect”, e.g., F, prefer dibridged structures over the doubly bonded isomers for the heavier element compounds [70]. Another explanation for the preference of a dibridged structure for E2R4 when R are electronegative substituents bearing lone pairs (e.g., F), is the mutual donation of the substituents’ lone pair into the empty p orbital of the tetrylene (Figure 1.8e) [185, 186]. In a series of important papers, by Carter and Goddard [184] and by Malrieu and Trinquier [160–163, 177], they suggested simple criteria that predict the occurrence of trans-bent E=E double bonds (B), the degree of their trans-bending, as well as the occurrence of other structural isomers, e.g., a dibridged isomer (C) (Scheme 1.2). These criteria, known as the “CGMT” criteria, compare ΔEST of the R2E fragments that compose E2R4 with the overall σ+π bond energy (Eσ+π) of planar R2E=ER2. According to the CGMT approach, for homonuclear R4E2 molecules, a trans-bent doubly bonded structure (B) is predicted to be more stable than the planar structure (A) or the dibridged (C), when ΔEST of ER2 is in the range given in Eq. (1.1): ¼ Eσ+π ≤ ∆EST  ≤ ½ Eσ+π 

(1.1)

When ΔEST is smaller than ¼ Eσ+π, a planar doubly bonded species is predicted, while when it is larger than half the double bond energy a dibridged isomer C is predicted. For E heavier than carbon, the double bond energy (Eσ+π, ΔEdiss) decreases and ΔEST increases, leading to trans-bent structures with increasing bending angles as E is heavier, or to dibridged structures (Table 1.3) [70].

1.4.2  Homonuclear Ge=Ge Compounds 1.4.2.1  The Parent H2Ge=GeH2 and Its Isomers: Structures and Potential Energy Surface

As emphasized above, unlike H2C=CH2 which has a planar D2h skeleton (A), calculations show that H2E=EH2 (E=Si-Pb) possess a trans-bent C2h structure B (Table 1.3). The planar structure is a transition state between two isomeric trans-bent structures B [44, 45, 70, 158, 159]. Upon moving from carbon to heavier group 14 elements, the trans-bending angle and the barrier for planarization increases from 31.4° and 0.8  kcal/mol for E=Si to 47.9° and 4.2 kcal/mol for E=Ge, reaching a bending angle θ of 53.2° and a barrier for planarization of ca. 27 kcal/mol for E=Pb [90] (Table 1.3). A SCF calculated PES of the planarization energy vs. the trans-bending angle (Figure 1.9) shows the significant increase of the barrier for planarization on moving down group 14 [87]. Simultaneously, the dissociation energy of H2E=EH2 to two EH2 (1A1) fragments is reduced, i.e., ΔEdiss is (at 0 K, in kcal/mol, at BP86/def2-TZVPP [70]): 178.8 (E=C, dissociation to two triplet CH2 fragments); 67.8 (E=Si); 51.3 (E=Ge); 28.9 (E=Sn, at LDA/TZ+p) [171]; and 10 (E=Pb, at LDA/TZ+p) [171], indicating that actually H2Pb=PbH2 dissociates at room temperature (Table 1.3). The calculated Ge=Ge bond length in H2Ge=GeH2 is 2.275–2.346  Å (depending on the computational level, Figure 1.10, Table 1.3), shorter than that of a Ge–Ge single bond length of 2.40–2.46 Å (exp. r(Ge-Ge) = 2.403 Å [187]) in H3Ge-GeH3 [188]. The calculated trans-bending angle is ca. 42–47° [43–45, 70, 188, 189]. The trans-bending angle in H2Ge=GeH2 is significantly larger than in H2Si=SiH2 (e.g., θ = 31.4°). The experimental Ge=Ge bond lengths and the trans-bending angles in isolated digermenes, R2Ge=GeR2 (R are bulky alkyl and aryl ligands), span over a large range of 2.2–2.49 Å [15, 18, 24] (Figure 1.11a) and of ca. 15–45° [18] (Σ∠Ge = 327–358°), respectively [15, 18, 24]. The bond distances in 60% of the isolated digermenes (Figure 1.11a) are in the range of those calculated for H2Ge=GeH2 (Figure 1.10). As discussed above, several additional structural isomers, C–F (Scheme 1.2), were located by calculations as minima on the E2H4 (E=C–Pb) potential energy surface (PES). The calculated relative energies of isomers B, C and F and the structural parameters of isomer B are presented in Table 1.3. The geometrical parameters of isomers B-F for E=Ge, R=H are presented in Figure 1.10. The most stable isomer for E=Si and Ge is the doubly bonded trans-bent isomer B, but for E=Sn and Pb, the most stable isomer is the trans-dibridged isomer C (Table 1.3) [44, 45, 70, 160–162]. The hydrogen-singly bridged isomer E is a minimum on the PES for E=Ge [189], Sn, and Pb [162, 163]. For Sn2H4 and Pb2H4 the calculated stability order of the isomers is: C > E > F > B, i.e., the doubly bonded isomer B is the least stable isomer [163]. The PES of Ge2H4 is relatively flat and spans over a range of ca. 14 kcal/mol compared to the PES of Si2H4 that spans over a range of 22 kcal/mol (Table 1.3) [70, 188, 189, 194]. H2Ge=GeH2 and HGe-GeH3 are nearly degenerate, with H2Ge=GeH2 being slightly favored (at most by ca. 3 kcal/mol, depending on the level of calculation, Tables 1.3 and 1.4) [70, 188, 189, 194]. The calculated barrier for the isomerization of

1.4  Doubly Bonded Compounds

Table 1.3  Calculated geometric parameters and relative energies of isomers B, C and F (ΔE) on the PES of E2H4 (E=Si, Ge, Sn, Pb), the dissociation energy of B to EH2 fragments (ΔEdiss) and the singlet-triplet energy gaps (ΔEST) of the corresponding fragments.a,b H2E=EH2 (B)

E

r(E=E) h

HEEH3(F, 1A’)

EH2

ΔEdissf

ΔE

ΔESTg

72.3 ;79.1

−15.8; −9.03k; −(8.99l)

7.9m

16.5; 19.6n; (21.0o)

HE(μ-H)2EH (C)

θ

ΔEc

ΔEdissd

ΔEe

h

C

1.322

180

(0.0)

178.7

164.7

27.3

Si

2.178

31.4

0.0 (0.8)

67.8 [100.0]

18.7

31.2h

Ge

2.300

47.9

0.0 (4.2)

51.3 [93.7]

6.8

26.9h

q

i

j

3.4p h

23.3 j

Sn

2.175

50.6

0.0 (11.5)

28.9 [74.4]

−9.4; −9.1j

40.5; 33.2

−2.1

27.3

Pb

2.922

53.2

0.0 (27.3)

10.0q [60.4]

−21.8; −23.9 j

38.5; 28.7h

−16.4 j

35.5

a

Bond lengths in Å, bond angles in degrees. Energies in kcal/mol; From Ref. [70], at BP86/def2-TZVPP, unless stated otherwise. For other calculated values see Refs. [43–45, 162, 171, 178]; c In parenthesis, the relative energy of the planar doubly bonded isomer A; d Dissociation energies: H2E=EH2 (C2h) → 2H2E: (1A1). In brackets, ΔEdiss of H2E=EH2 (planar, D2h) → 2H2E: (3B1); e Energy difference between isomers C and B; f Dissociation of C to 2H2E: (1A1); g Experimental values in parenthesis; h From Ref. [162], at CI//HF/DZ; i From Ref. [190]; j From Ref. [162] at CI/DZP; k At CASSCF-SOCI with the TZ3P(2f,2d)+2diff basis set [191]; l See references cited in Ref. [191]; m At the G1 level, from Ref. [190]; n At MRSOCI [192]; o From Ref. [193]; p At CCSD(T)/aug-cc-pVTZ//B3LYP/6-311G(d,p) [189], see Table 1.6 for values at other computational levels; q At LDA/TZ+P with nonlocal corrections [171]. b

Scheme 1.2  Possible isomers on the R4E2 potential energy surface.

H2Ge=GeH2 to HGeGeH3 at several CCSD levels of theory is 12.9–14.0 kcal/mol (Table 1.4) [195]. Similar reaction energies and barriers were calculated for the isomerization of H2Ge=SiH2 → HGeSiH3. The isomerization energy of H2Ge=SiH2 → HSiGeH3 is significantly higher than the isomerization of H2Ge=SiH2 → HGeSiH3. In part, this results from the different bond strength of the Si–H and Ge–H bonds in H2Ge=SiH2 and in the corresponding products HGeSiH3 or HSiGeH3. The

21

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

40

Figure 1.9  Variation of energy (kJ/mol) with the trans-bending angle of H2E=EH2 calculated at the SCF level. The curves for C2H4 and Si2H4 were calculated for non-adiabatic bending (i.e., with constant values for E–E, E–H bond distances and H–E–H bond angles). The curves for Ge2H4 and Sn2H4 were calculated for adiabatic bending (i.e., with re-optimized values for the E–E bond distances and H–E–H angles). Adapted with permission from Ref. [87]. Copyright (1986) Royal Chemical Society.

Si H 4

C2H

2

–1

4

∆ESCF/KJ MOL

22

30

20

10

–60

–40

–20

20

40

60 Ge H 2

4

θ/˚

–10

–20

Sn H

–30

2

(a)

1.545

4

(b)

107.8

2.275 2.278 2.310 2.311 2.346

2.305

115.6

B

Ge

Ge2H4 1Ag

H

1.596

104.9

91.8 75.1

87.7

74.4

1.771

C

Ge2H4 1A1

113.8 1.710

1.929

1.546

107.0 1.548

2.412

E Ge2H4

1A

1.543

2.528

128.2

1.595 86.7

1.598

89.0

110.9

F Ge2H4 1A'

1.540 1.548 1.556 1.569

1.534 1.539 113.3 1.545 114.1 H 1.553 113.8 2.510 1.536 1.557 114.6 2.500 1.543 114.3 2.528 2.521 1.548 2.554 1.557 Ge 1.561

D

Ge2H4 1Ag

Ge 1.537

Neutral (C2h 1Ag)

1.589

105.6

H

116.7 116.0 115.4 115.1 114.5

H

1.764

H

106.7

109.1 108.6 107.5 107.5 106.4

H 107.4 H 106.8 106.8 106.1 106.1

110.5 110.5 110.8 110.9 111.2

Ge

1.583 89.7 1.593 88.6 1.600 89.0 1.612 88.1 H 1.617 88.7

Neutral (Cs 1A')

Figure 1.10  (a) B3LYP/6-311G(d,p) optimized structures of isomers located on the G2H4 PES. Adapted with permission from Ref. [189]. Copyright (2006) Elsevier B.V., and (b) Optimized structures of trans-bent H2Ge=GeH2 and of germylgermylene, at a variety of DFT levels. Bond lengths in Å and bond angles in degrees. Adapted with permission from Ref. [188]. Copyright (2002) Wiley Periodicals.

1.4  Doubly Bonded Compounds

(a)

(b)

Distribution of Ge=Ge bond lengths in digermenes >2.50 Å 4%

2.20 –2.29 Å 14%

Distribution of Si=Si bond lengths in disilenes 2.26–2.28 Å 3%

2.23–2.25 Å 8%

2.40–2.49 Å 23%

>2.28 Å 1%

2.14–2.16 Å 34%

2.20–2.25 Å 22% 2.30 –2.39 Å 59%

2.17–2.19 Å 32%

Figure 1.11  Distribution of (a) Ge=Ge bond lengths and (b) Si=Si bond lengths in experimentally isolated digermenes and disilenes. Reprinted with permission from Ref. [25]. Copyright (2013) Elsevier.

Table 1.4  Calculated reaction energies (ΔE) and reaction barriers (ΔE#) for the H2E=E’H2 → HEE’H3 isomerization (E and E’=Ge, Si and C).a Reaction

ΔE

ΔE#

H2Si=SiH2 → HSiSiH3

74.9b

75.0b

7.9c

17.3d

2.3 to −2.3

12.9–14.0

H2Ge=SiH2 → HSiGeH3

−3.2 to −7.0; −3.6e, −4.8f, −6.3 g 14.4 to 15.7; 11.8e

H2C=CH2 → HCCH3

H2Ge=GeH2 → HGeGeH3 H2Ge=SiH2 → HGeSiH3

a

14.1f; 9.4g

–

In kcal/mol, at various CISD and CCSD levels. From Ref. [195] unless stated otherwise. For more data, see the citations in previous reviews [44, 45]; b At MP4/6-311G(d,p) [197]; c At the G1 level [198]; d MP3/6-31G(d) [199]; e At CCSD(T)/6-311(d,p)//B3LYP/6-311G(d,p), from Ref. [200]; f At CCSD(T)/cc-pVTZ (H, Si) and CCSD(T)/cc-pVTZ-pp (Ge), from Ref. [201]; g At CISD+Q, from Ref. [196].

Si–H and Ge–H bond strength in H2Ge=SiH2 are 68.5 kcal/mol vs. 57.4 kcal/mol, respectively; in HGeSiH3 the Si–H bond strength is 90.6 kcal/mol, and the Ge–H bond strength in HSiGeH3 is 84.0 kcal/mol [196]. The contrast with the extremely high barriers (75 kcal/mol) for the H2C=CH2 to HCCH3 isomerization demonstrates again that analogy between C and its heavier analogues is misleading. For Sn2R4, of the four isomers B, C, D and F, a trans-dibridged tin derivative Ar’Sn(μ-H)2SnAr’ (Ar’ = (H3C6-2,6-(C6H22,4,6-iPr3)2) [202] and a dihydrido stannylene derivative Ar”SnSnH2Ar” (Ar” = C6H-2,6-(C6H2-2,4,6-iPr3)2-3,5-iPr2) [203] were isolated and characterized by X-ray crystallography. Calculations for Ar2H2Sn2 show that stannylene isomer F becomes more favorable than the dibridged isomer C as the size of the Ar ligand increases [204]. The elusive H 2Ge=GeH2 was isolated and characterized spectroscopically only as a donor-acceptor complex, via coordination with a Lewis base (LB)/Lewis acid (LA) combination, which stabilize the reactive digermene, i.e., LB1(or LB2)→H2Ge=GeH2→LA, (52, 53), LB1=N-heterocyclic carbene, LB2=nucleophilic N-heterocyclic olefin, LA=W(CO)5 (Scheme 1.3 and Figure 1.12) [205]. A similar strategy was used to synthesize and isolate tetrachlorodigermene, LB2→Cl 2Ge=GeCl2→LA (54) [205] (Figure  1.12), and mixed E=E’ molecules, LB1→H 2Si=EH2→LA, E=Ge, Sn [206].

23

24

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

Scheme 1.3  (a) Possible resonance structures of H2Ge=GeH2 and (b) Stable digermene Lewis acid/Lewis base complex.

Figure 1.12  X-ray molecular structures of LB1→H2Ge=GeH2→W(CO)5 (52) and LB2→Cl2Ge=GeCl2→W(CO)5 (54). Reproduced with permission from Ref. [205]. Copyright (2013) American Chemical Society.

The Ge–Ge bond length in 52 is 2.421  Å which is in the range of a Ge–Ge single bond (see above). Calculations at M06-2X/cc-pVTZ of model systems of 52 and 53 where the bulky 2,6-iPr2-C6H3 substituent at N was replaced by a Me group, designated 52Me and 53Me, predict a polar Ge–Ge linkage with 60% of the charge density residing on the W atom bonded to Ge, due to electron withdrawing by the carbonyl groups. The Wiberg bond index (WBI) of the Ge–Ge bond is 0.87 and 0.91 in 52Me and 53Me, respectively. Comparison with the Ge–Ge WBI of H3Ge-GeH3 of 0.97 and that of H2Ge=GeH2 of 1.79 supports a single covalent bond in 52 and 53, and the bonding mode shown in Scheme 1.3(b). Similar to 52 and 53, also the parent H2Si-GeH2 and H2Si-SnH2 were isolated as complexes with W(CO)5 and NHC ligands [206]. WBIs calculated at B3LYP/cc-pVDZ-pp are 0.88 for the Si–Ge bond and 0.79 for the Si–Sn bond, supporting the bonding shown in Scheme 1.3(b) also for these compounds [206]. Digermene, H2Ge=GeH2 (X1Ag), and germylene HGeGeH3 (X1A’) and their fully deuterated isotopomers were observed by IR spectroscopy in GeH4 and GeD4 matrices at 12 K, upon irradiation of the matrices with energetic electrons. The digermene and D4-digermene were identified by their absorptions at 845 cm−1 (ν11), and at 1476 cm−1 (ν5), attributed by calculations to a GeH2 scissor vibration, and to the GeH2 asymmetric stretching vibration, respectively [189]. Calculations predict also two stronger bands at 2125 cm−1 (ν5, GeH2 asymmetric stretching) and at 2093 cm−1 (ν10, GeH2 symmetric stretching). The slightly less stable HGe-GeH3 (F) and its deuterated derivative were also identified by their absorption at 787 cm−1 and 558 cm−1(ν5, GeH3/GeD3 umbrella), respectively. The vibrational frequencies and IR intensities of H4Ge2 isomers B-F were calculated at

1.4  Doubly Bonded Compounds

B3LYP/6-311G(d,p) aiding in identifying the experimentally observed species [189]. Tetra-methyl digermene was detected in nitrogen and argon matrices at 5 K and were identified by Raman and IR spectroscopy [207, 208]. H-migration from the sterically hindered dihydrodigermene Ar’HGe=GeHAr’ (Ar’  =  C6H3-2,6-(2,6-iPr2C6H3)2), 55 occurs (Eq. 1.2) in the presence of PMe3, producing a base-stabilized germylene, Ar’Ge-GeH2Ar’, 56 [209]. The experimental X-ray structure of 55 exhibits a trans-bending angle of 20.5° and a Ge–Ge bond length of 2.372 Å [209], longer than the calculated values for H2Ge=GeH2 of 2.275–2.346 Å [188] (see also Table 1.3 and Figure 1.10), and it is in the upper limit of the measured bond lengths in isolated Ge=Ge compounds, of 2.20–2.45 Å (Figure 1.11) [15, 18, 24, 25]. The Ge–Ge bond length in germylene 56 is 2.530 Å, similar to the calculated Ge–Ge bond distance in HGe–GeH3 (Figure 1.10), and the C– Ge–Ge bond angle is 101.6° being larger than calculated for HGeGeH3 of 89° (Figure 1.10) [209]. Other examples of stable germylgermylenes are limited, e.g., (2,6-Mes2H3C6)GeGetBu3, an analogous germyl stannylene was isolated as well [210]. PMe3 Ar' H

Ge

Ge

H Ar'

+ PMe3

toluene

H

Ar' Ge

H

Ge



(1.2)

Ar'

55

56

1.4.2.2  Substituent Effects 1.4.2.2.1  Correlation between Structures, Bond Dissociation Energies and ΔEST

In agreement with the CGMT approach, Karni and Apeloig [211] found a linear correlation between ΔEST of HRSi (R=Li, BeH, BH2, SiH3, F, OH, NH2) and the trans-bending angles at the Si atoms in HRSi=SiH2 and its dissociation energy. Electronegative and π-donor substituents, F, OH, and NH2, increase ΔEST of the silylene fragments, and consequently, increase the trans-bending angle and reduce dramatically the dissociation energy of disilene to its fragments [211] and the energy difference between the disilenes and their dibridged isomers C (Scheme 1.2) [185]. At MP2/6-31G(d), the dibridged structure HSi(μ-H)2SiH (C) is less stable than H2Si=SiH2 (B) by 25.8 kcal/mol (18.7 kcal/mol at BP86/def2-TZVPP, Table 1.3), while the fluorine substituted dibridged isomers FSi(μ-H)2SiF and HSi(μ-F)2SiH are by only 6.1 and 9.5 kcal/mol, respectively, less stable than HFSi=SiHF [185]. Trinquier predicted computationally that F2Si=SiF2 isomerizes without a barrier to the dibridged isomer FSi(μ-F)2SiF [161]. Chen and coworkers [212] studied computationally the structures and bond dissociation energies of R2Ge=GeH2, R2Ge=SiH2 and H2Ge=SiR2 (R=H, CH3, NH2, OH, F, Cl) and also found a good linear correlation between ΔEST of GeR2 or SiR2, the trans-bending angles and the E=E’ bond dissociation energies. The data in Table 1.5 predicts that for electronegative π-donating substituents, for which ΔEST of GeR2 is significantly larger than that of GeH2, the energy required to planarize the trans-bent R2Ge=GeH2 increases significantly. Simultaneously, the Ge–Ge bond length elongates from 2.307 Å (R=H) to 2.483 Å (R=NH2), being longer than the single bond length in H3Ge–GeH3 of 2.40–2.46 Å [188] (exp. r(Ge–Ge) = 2.403 Å [187]). The trans-bending angle at the Ge atom bearing the substituents (θR) is smaller than in H2Ge=GeH2, but for electronegative π-donating substituents, θH is very large, 78–94°. The long Ge=Ge bonds that are in the range of a Ge–Ge single bond length, the large trans-bending angle θH of nearly 90°, and the significantly smaller BDE for R2Ge=GeH2, R=NH2, OH, F and Cl, relative to R=H, indicate a weak Ge–Ge bond. In our view, these geometric parameters indicate that these compounds are better described as R2Ge → GeH2 adducts, where the σ-lone pair of GeR2 donates electrons into the empty p orbital on GeH2, (57, E=Ge) while the back-donation from GeH2 is small, in contrast to a classical donor-acceptor complex (Figure  1.8c), leading to a trans-bent double bond. Similar geometry patterns are observed in a variety of NHC-stabilized divalent group 14 adducts [213–215], e.g., NHC→SiCl2 [215, 216], NHC→GeMes2 [216, 217], and NHC→GeI2 [218]. In these adducts the NHC carbene is nearly perpendicular to the plane of the ER2 unit and the C–E bond is longer than a C–E single bond. In NHC→ GeI2, r(C–Ge) = 2.102 Å [218] vs. 1.969 Å [219] in H3C–GeH3 and 1.784 Å in H2Ge=CH2 [220]. NBO calculations [213] of the NHC→SiH2 adduct suggests a Lewis structure having a C–Si polarized σ bond and a localized p(π) lone pair orbital which resides mainly (91%) on the Si atom. The calculated extent of σ-donation in NHC→SiH2 is 0.557 electrons and the SiH2→NHC back-donation is only 0.170 electrons (calculated using charge delocalization analysis (CDA) [221]). These are all indications of a donor adduct rather than a doubly bonded compound [213].

E

E

57

25

26

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

Table 1.5  Calculated singlet-triplet energy gaps (ΔEST) of R2Ge, relative energies (ΔE) of trans-bent R2Ge=GeH2 (B) and planar (A) structures, the Ge–Ge bond dissociation energy (BDE) and geometric parameters of R2Ge=GeH2, R=H, CH3, NH2, OH, F, Cl.a R

H

CH3

NH2

OH

F

Cl

ΔEST

27.5

31.5

59.2

72.3

83.5

63.6

−4.83

−6.1

−24.5

−22.6

−26.0

−20.2

BDE

−52.2

−50.1

−35.9

−29.1

−23.4

−33.5

r(Ge-Ge) (planar, A) r(Ge-Ge) (trans-bent, B) θRd θHe

2.224 2.307 43.45 43.72

2.230 2.337 30.14 59.55

2.234 2.483 12.79 94.64

2.232 2.475 17.61 92.94

2.225 2.467 28.1786.03

2.228 2.440 32.31 78.09

ΔEb

c

a

Calculated at B3LYP/6-311G(d). Energies in kcal/mol, bond length in Å, angles in degrees. From Ref. [212]; Negative values indicate that B is more stable than A. A is not a minimum on the PES; c BDE is the difference between the energy of R2Ge=GeH2 and the sum of the energies of GeH2 and GeR2 in their singlet state. The values given in Table 1.5 were derived from the reported correlation of BDE vs. ΔEST: i.e., BDE = 0.515 ΔEST −66.37 (r2 = 0.956) [212]. The individual BDEs calculated directly were not provided; d The trans-bending angle at the Ge bonded to R; e The trans-bending angle at the Ge bonded to H. b

1.4.2.2.2  Substituent Effects on Structures and Isomeric Forms of Ge2R2R’2 Compounds 1.4.2.2.2.1  Electronegative Substituents 1.4.2.2.2.1.1  Halide Substituents

Electronegative substituents significantly stabilize the singlet state of ER2, increasing ΔEST (e.g., Table 1.6, E=Ge). The large increase in ΔEST from 16.5, 23.3, and 27.3 kcal/mol for EH2 (E=Si, Ge, Sn, respectively), to 69.7, 81.5, and 80.2 kcal/ mol, for EF2 (E=Si, Ge, Sn, respectively), leads to a significant stabilization of the dibridged isomer C [70, 176, 222] and it is the most stable E2X4 isomer. The trans-bent isomer B of F2Ge=GeF2 is not a minimum on the potential energy surface [175, 176, 222]. The dibridged Ge2F4 isomer, C, was identified and characterized in matrix by IR and Raman spectroscopy [164]. Hargittai and coworkers studied computationally the PES of E2R4 (E=Si, Ge, Sn, Pb; R=F, Cl, Br, I) using the MP2(FCI)/cc-pwCVTZ(-PP) level of theory [175]. The trans-dibridged isomer, C, is the global minimum for all compounds, and it is only slightly more stable than the cis-dibridged isomer. The germylgermylene divalent isomer F is higher in energy by ca. 30  kcal/mol for R=F, but it becomes relatively more stable for R=Cl, Br, and I (Table 1.6). Structural and NBO analysis of the bonding in R3Ge–GeR for R=Br, Cl, and I, reveals that these are better described as mono-bridged isomers E. For example, in R3Ge–GeR, R=Br, Cl, and I (see atom numbering in Table 1.6), r(Ge3–R4) of 2.443, 2.596, and 2.788  Å, respectively, having Wiberg bond indices (WBIs) of 0.43, 0.46, and 0.52, respectively, are nearly identical to r(Ge1–R4) of 2.389. 2.525, and 2.711 Å and WBIs of 0.47, 0.52, and 0.58, respectively. Meanwhile, Ge1–R6 and Ge3–R5 bonds (in F), are significantly shorter, e.g., for R=Br, they are 2.284 and 2.323 Å, respectively, and for both the WBIs are 0.84 [175]. ΔEST for GeHR (R=F, Cl, Br, I) is significantly smaller than for GeR2 (Table 1.6), and according to the CGMT model the relative stability order of the isomers on the PES of (GeHR)2 is expected to be different than that of the (GeR2)2 PES. The relative energies calculated for the isomers of E2Me2Br2 shown in Figure 1.13 (the isomeric notations in this paragraph are as in Figure 1.13) show that for E=Si, (E)-dibromodimetallene (A) is the most stable isomer. The corresponding silylsilylene (E) has almost the same energy [186]. The bulkier substituted Bbt(Br)Si=Si(Br)Bbt (Bbt  =  2,6-[CH(SiMe3)2]-4[C(SiMe3)3]-C6H2)) is more stable than its isomer E, by 14 kcal/mol [186]. Similarily, dihydrodisilene Bbt(H)Si=Si(H)Bbt is more stable than the corresponding silylsilylene E by 13.2  kcal/mol, and the barrier for the isomerization of Bbt(H) Si=Si(H)Bbt to Bbt(H2)Si-SiBbt is 19.6 kcal/mol (at B3PW91/6-311+G(2df)[Si]:6-31G(d)). Bbt(H2)Si-SiBbt was suggested as an intermediate in the slow decomposition of Bbt(H)Si=Si(H)Bbt at 80° [186]. The dibridged isomers (C, D) are significantly less stable for E=Si. For E2Me2Br2, E=Ge, germylgermylene E is the most stable isomer. The doubly bonded isomer A is 5 kcal/mol higher in energy. The dibridged isomers C and D are only slightly less stable than the doubly bonded isomer A. For E=Sn, both dibridged isomers are significantly more stable than the doubly bonded isomer A (Figure 1.13) [186] as also predicted for

Table 1.6  Relative energies (kcal/mol) of isomers B-F on the Ge2R4 (R=H, F, Cl, Br, I) PES. Isomer R R

θ Ge

R Ge

R R

R

R

H F

R

Ge

Ge1

R

R6

R4

0.0 -

11.5a, 6.9b, 9.0c, 6.8d 0.0

13.6a, 8.6b, 11.6c h

i

1.8 , 1.7

R5 R2

R5 Ge1

Ge3 R2

-

3.4a, −0.24b, 2.4c; 2.8e g

i

26.6 , 29.4

GeHR ΔEST

23.3f, 26.7g

−

g

46.2g

g,j

85.0

10.4

64.1

42.0g,j

-

8.1i

57.2g,j

40.3g,j

-

i

k,l

37.5k,l

1.4

Br

-

0.0

1.9i

0.0

i

i

GeR2 ΔEST

-

0.0

2.7

4.8a, 1.9b

R4

i

-

Ge3

R6

R

At CCSD(T)/aug-cc-pVTZ//B3LYP/6-311G(d,p), from Ref. [189]; At B3LYP/6-311G(d,p), from Ref. [189]; c At CI/DZP from Ref. [162]; d At BP86/def2-TZVPP from Ref. [70]; e At BP86/DZP++ from Ref. [188]; f From Ref. [70], at BP86/def2-TZVPP; g At B3LYP/DZP++, from Ref. [223]; h At BP86/DZP++, from Ref. [176]; i At MP2(FCI)/cc-pwCVTZ(-PP) from Ref. [175]; j For values at other computational levels see Refs. [44, 45, 224]; k At B3LYP/6-311g(d,p), from Ref. [223]. b

R

Cl I a

Ge

F

R

Ge

R Ge

E

D

C

B

4.6

47.7

28

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

Figure 1.13  Calculated relative energies (kcal/mol) of E2Me2Br2 isomers at B3PW91/6-311G(3d), [TZ(2d) for Sn, and 6-31G(d) for C and H]. Energies of E-1,2-dibromo-1,2-dimethyldimetallene are set to zero). Negative relative energies indicate more stable species. Reproduced with permission from Ref. [186] Copyright (2013) Chemical Society of Japan.

Sn2H2 (Table 1.3) [162]. In fact, all known donor-free dimeric hydridostannylenes, (RHSn)2, adopt the trans-dihydrogenbridged structure RSn(μ-H)2SnR (e.g., R = 2,6-Dip2-C6H3, Dip = 2,6-iPr2-C6H3) [203, 225] or stannystannylene structure, RSnSnH2R (R  =  2,6-Tip2-C6H-3,5-iPr2 (Tip  =  2,4,6-iPr3-C6H2)) [203, 225], and not the alkene-like structure [226]. It is interesting to note that for E2Me2Br2, the Ge compounds differ from those of their closest neigbours, Si and Sn, both qualitatively (in their most stable isomer) and quantitatively (the PES is flatter for Ge than for Si or Sn (excluding isomer E)). The calculated dissociation energies (ΔEdiss) of Me(Br)E=E(Br)Me to two E(Br)Me fragments decrease as E is heavier. ΔEdiss (in kcal/mol, at B3PW91/6-311G(3d), [TZ(2d) for tin, and 6-31G(d) for C and H]) are: 37.5 (E=Si) > 23.1 (E=Ge) > 14.6 (E=Sn)[186]. These calculated dissociation trends are in agreement with experimental results that in solution dibromodisilene is stable, dibromodigermene is in equilibrium with its bromogermylene fragments [129], while for E=Sn only the Br-bridged isomer is observed [186]. Bbt(Br)Ge=Ge(Br)Bbt (25, Table 1.2) was isolated as stable orange crystals at −78°C. [129, 186] Its Ge=Ge bond length is 2.509 Å, longer than other known Ge=Ge bond length of 2.21–2.44 Å [18], and even longer than the Ge–Ge single bond of 2.463 Å in (GePh2)6 [87]. The trans-bending angle at Ge is 44.6°, more pyramidal than in other digermenes which are in the range of ca. 10–40° [18], but no twist of the double bond was observed [129]. The calculated (B3LYP/6-31+G(d)) r(Ge=Ge) and trans-bending angle in H(Br)Ge=Ge(Br)H are 2.378 Å and 53.2°, respectively, and 2.414 Å and 48.1°, respectively, in the more bulky substituted Mes(Br)Ge=Ge(Br)Mes [129]. These calculations indicate that the Ge=Ge bond in 25 is elongated due to the steric repulsion between the bulkier Bbt substituents. However, the trans-bending depends on an intrinsic property of the GeRBr germylene, e.g., its ΔEST (see above). In toluene solution, 25 is in equilibrium with its germylene fragments [129]. The synthesis and X-ray crystallography characterization of additional 1,2-dihalodigermenes, (E)–Ar(Cl) Ge=Ge(Cl)Ar, Ar = 2,6-Mes2-C6H3 [227, 228] (Mes = 2,4,6-Me3-C6H2)) (58); Ar = 2,6-Tip2-C6H3 (Tip = 2,4,6-iPr3C6H2) (59) [227], were reported by Power’s group. Very recently, Matsuo’s group reported the synthesis of (E)– Eind(R)Ge=Ge(R)Eind, R=Cl (24b) and Br (24a) (for the definition of Eind, see Table 1.2) [128]. The Ge=Ge bond length in 58, 59, 24b, and 24a are 2.443, 2.363, 2.412, and 2.414 Å, respectively, and the trans-bending angles are 39.0, 36.8, 44.3, and 43.3°, respectively. Twisting about the double bond was not observed [128]. Based on UV-Vis spectroscopy, it was concluded that in solution 24a dissociates to its germylene fragments, and 25 is in equilibrium with the GeBrBbt fragments. The different behavior of 24a and 25 was explained by dispersion effects. The computed dissociation energy (ΔG) of the Ge=Ge bond in 24a (at B3PW91-D3/6-311G(3d,f) (Ge, Br) and 6-311G(d) (H, C, Si), including dispersion) is 7.8 kcal/mol, compared to 15.3 kcal/mol in 25. Compound 24a can be regenerated from the germylenes by removal of the solvent [128]. ΔEST of Ge(SiH3)X, X=F, Cl, Br, are (at B3LYP/DZP++): 31.1, 28.6, and 27.7 kcal/mol, respectively, significantly smaller than those of Ge(CH3)X, X=F, Cl, Br, of 48.0, 43.5, and 41.7 kcal/mol, respectively (at B3LYP/DZP++), as expected for SiH3 vs. CH3 substituents [223]. Based on the CGMT model, we predict that their doubly bonded dimers e.g., X(SiR3)Ge=Ge(SiR3)X,

1.4  Doubly Bonded Compounds

X=F, Cl, Br, may be less pyramidal at the Ge atoms and with shorter and stronger Ge=Ge bonds, compared to the large trans-bending angles and long Ge=Ge bonds found in X(Ar)Ge=Ge(Ar)X (e.g., compounds 24a, 24b, 25, (Table 1.2) and 58, 59). Computational studies are required to assess this prediction. 1.4.2.2.2.1.2  Amino substituents

Calculations and experimental evidence have shown that diamino-tetrylenes (E(NR2)2) do not dimerize to form tetraaminoE=E molecules, in line with their large ΔEST values, e.g., ΔEST = 79.3 kcal/mol for Si(NH2)2 [229]. Calculations predict that (H2N)2Si=Si(NH2)2 is not a minimum on the PES, and that the dibridged H2NSi(μ-H2N)2SiNH2 should be observed [229, 230]. Following these calculations Kira and coworkers in an elegant experiment have confirmed the existence of the dibridged iPr2NSi(μ-iPr2N)2SiNiPr2 [231]. A similar behavior is found for amino substituted germanium and tin compounds. Reaction of ArECl (E=Ge, Sn, Pb) in liquid ammonia produced trans-dibridged ArE(μ-NH2)2EAr (E=Ge, Ar = Ar′ = 2,6-iPr2-C6H3 or Ar = Ar* = 2,6-(2,4,6-iPr3C6H2)2-C6H3; E=Sn, Ar=Ar*; E=Pb, Ar=Ar*) that were characterized spectroscopically [232]. E(N(SiMe3)2)2, E=Ge, Sn, are monomeric in the solid state and do not dimerize in solution [87]. Calculations at B3PW91 reveal that the Ge–Ge bond length in (Ge(N(SiMe3)2)2 is 5.841 Å, more than twice the van der Waals radius of a Ge atom (2.11 Å [233]) and it is contracted to 3.798 Å at B3PW91-D3 when attractive dispersion forces are included, still significantly longer than a Ge–Ge single bond [117]. The calculated dissociation (ΔH) energy of ((Me3Si)2N)2Ge=Ge(N(SiMe3)2)2 → 2 Ge(N(SiMe3)2)2 is −7.0  kcal/mol (i.e., exothermic) at B3PW91, but increases to 3.6  kcal/mol when dispersion forces are included. However, ΔG for dissociation (at 25°C and 1 atm.) is −12.2 kcal/mol (i.e., exergonic), indicating that monomeric species are favored under these conditions [117]. Additional experimental support for the conclusion that diaminosilylenes avoid dimerization to tetraamino-disilenes was reported by West, Apeloig, and coworkers [234], who showed that the stable N-heterocyclic silylene 60, R=tBu, does not dimerize to form disilene 61, E=E’=Si (Scheme 1.4). Instead, silylene 62, E=E’=Si, substituted by one amino group and a silyl group is formed as an intermediate, and it dimerizes to Z-diaminodisilyldisilene, 63, E=E’=Si, R=tBu, which is stabilized by two silyl groups. Calculations at B3LYP/6-31G(d) found that 63, E=E’=Si, R=Me is a minimum on the PES while the tetraaminodisilene 61, E=E’=Si, R=Me, is not. 63, E=E’=Si, R=tBu, exhibits a very long Si=Si bond of 2.289 Å (2.285 Å calc. for R=Me), an extremely large trans-bending angle of ca. 33° and a Si–Si–N twist angle of 25°. The formation of 63 and not of 61 is consistent with the much larger ΔEST of (Me2N)2Si of 68.7 kcal/mol relative to that of H3Si(Me2N)Si of 45.2 kcal/ mol (B3LYP/6-311G(d,p) [234].

Scheme 1.4  Dimerization of N-Heterocyclic tetrylenes

29

30

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

Similarly, a reaction of silylene 60, E=Si, R=tBu, with germylene 60’, E’=Ge, R=tBu, did not produce the tetraaminosubstituted germasilene 61, E=Si, E’=Ge, but instead yielded digermene 63, E=Ge, E’=Si (Scheme 1.4) substituted by two amino groups and two stabilizing silyl groups [235, 236]. In 63, E=Ge, E’=Si, R=tBu, r(Ge-Ge) is 2.45 Å [235, 236], longer than in alkyl and aryl substituted digermenes. The trans-bending angle is ca. 41° and the twist angle is 22°. Reaction of silylene 60 with Sn[N(SiMe3)2]2 yields, as a final product, the hydridodisilylstannane, 64 [235]. Calculations to study this interesting reaction are welcomed. (E)-(R2N)HGe=GeH(NR2) (22 [126], 23 [127], Table 1.2) were synthesized by hydrogenation of amido-digemynes 65 and 67, respectively (Scheme 1.5a and 1.5b respectively), and were characterized by X-ray crystalography [126, 127]. A UV-Vis spectroscopic experiment led to the conclusion that digermene 22 is in equilibrium with germylene 66, which was trapped by DMAP (4-dimethylaminopyridine) and the adduct was characterized by X-ray crystallography [126]. 68 activates H2 even at −10°C yielding amindogermylamidogermylene 69, both in the solid state and in solution. The structure of 69 was determined by X-ray crystallography (Scheme 1.5c) [237]. Based on variable temperature NMR experiments at 3–70°C in THF-d8 solution, it was concluded that in solution germylgermylene 69 is in equilibrium with digermene 70 [237]. The reaction mechanisms of addition of H2 to R2NE-ENR2 (E=Si, Ge, Sn) were studied computationally by Frenking and coworkers (for detailed PESs the reader is referred to the cited manuscripts) [225, 238]. The hydrogenation of 68 was calculated (at BP86/def2-TZVPP) to be exergonic, forming either 69 (ΔG = −7.5 kcal/mol, −9.4 kcal/mol including dispersion effects, and −8.9 kcal/mol when solvent effect is included) or 70 (ΔG = −5.6 kcal/mol, −6.6 kcal/mol including dispersion effects, and –5.7 kcal/mol, when solvent effect is included). The rate-determining free energy barrier for hydrogenation is 20.4 kcal/mol, 18.4 kcal/mol including dispersion effects, and 19.0 kcal/mol with solvent effect [225]. 69 is more stable than 70 by only 2–3 kcal/mol which is in line with the observed equilibrium between these isomers in solution. r(Ge=Ge) is 2.544 Å (70, calculated) [225], 2.486 Å (22, exp., and 2.510 Å, calc.) [126], and 2.535 Å (23, exp.) [127]. These bonds are at the longest edge of known Ge=Ge bonds, and are close to that of r(Ge-Ge) in 69 of 2.550 Å [237]. These digermenes are also highly trans-bent, i.e., by 50° (calculated for 70) [237] (54.1° measured for 22) [126], and by a considerably smaller angle of 39.1° in 23 (exp.) [127]. Computational studies are required for analyzing the nature of the bonding in these molecules.

Scheme 1.5  Synthesis of (E )-(R2N)HGe=GeH(NR2) by hydrogenation of amido-digermylenes

1.4  Doubly Bonded Compounds

1.4.2.2.2.2  Electropositive Substituents

Electropositive substituents reduce ΔEST of ER2, lower the trans-bending angle of R2E=ER2, and decrease its bond dissociation energy to the ER2 fragments. 1.4.2.2.2.2.1  Silyl Substituents

The calculated (at UB3LYP/6‑31+G(d,p)//UB3LYP/6‑31+G(d,p) + ZPE) ΔEST for (H3Si)2Si is 8.4 kcal/mol, compared to 20  kcal/mol for SiH2, and it is smaller for bulkier silyl substituents, e.g., ΔEST((Me3Si)2Si)  =  2.8 [239]. Thus, H3Si(H) Si=SiH2 [211] and (H3Si)2Si=Si(SiH3)2 [240] are both calculated to be planar. In tetrasilyldisilene, r(Si=Si)  =  2.148  Å. Isolated disilenes, with bulky silyl substituents, e.g., R3Si(R’3Si)Si=Si(SiR’3)SiR3 (R3Si=SiMe2tBu, R’3Si=SiMeiPr2) (71) [241] and (R3Si)2Si=Si(SiR3)2 (R3Si=SiMe2tBu) (72) [242] are both planar with r(Si=Si) = 2.196 Å and 2.202 Å, respectively, and the twist angles about the double bonds are 0° and 8.9°, respectively. Increasing the bulk of the substituents to SiMetBu2 (73) retains the planar geometry, but the double bond is twisted by 54.5°, elongating r(Si=Si) to 2.259 Å [243]. The calculated structure of 73 (at UBP86-D3/TZVP + D3, including dispersion corrections) [244] is in very good agreement with its X-ray structure. The significant twist of the double bond enables a facile thermal rotation around the double bond, overcoming a barrier of only 7.5 kcal/mol to access a triplet biradical that was observed by EPR spectroscopy, and its identity was supported by calculations [244]. This is the first spectroscopic observation of a triplet diradical resulting from thermal rotation around an E=E double bond, including C=C bonds. Similarly to the effect of silyl substitution on silylenes, the calculated singlet-triplet energy gap (ΔEST) of Ge(SiH3)2 is 13.8 kcal/mol (at B3LYP/DZP++) [223], significantly smaller than for GeH2 and Ge(CH3)2 for which ΔEST are 26.7 [223] (27.5 [212]) and 31.0 [223] (31.5 [212]) kcal/mol, respectively, but larger than for Si(SiH3)2 at 8.4 kcal/mol. The smaller ΔEST of Ge(SiH3)2 causes a significant decrease in the calculated trans-bending angle θ and in r(Ge=Ge); for example, in R2Ge=GeR2, R=SiH3, θ = 27.9° and r(Ge=Ge) = 2.292 Å, compared to θ = 41.14° and r(Ge=Ge) = 2.340 Å for R=CH3, and θ = 43.9° and r(Ge=Ge) = 2.309 Å for R=H (at B3LYP/6-311G(d) [245]; calculated values at other computational levels are given in Table 1.3 and in Figure 1.10). The silyl effect on geometry is exhibited also in isolated digermenes, e.g., in (iPr2M eSi)2Ge=Ge(SiMeiPr2)2, (74), (iPr3Si)2Ge=Ge(SiiPr3)2 (75), and (tBuMe2Si)2Ge=Ge(SiMe2tBu)2 (11) in which the transbending angles are: θ = 5.9, 16.4, and 7.5°, and r(Ge=Ge) = 2.268, 2.298, and 2.270 Å, respectively (the twist angle in these three compounds is 0° [119, 120]), compared to the much larger θ of 32° and longer r(Ge=Ge) of 2.347 Å in [(Me3Si)2CH]2 Ge=Ge[CH(SiMe3)2]2 (1) [86, 87], or in aryl substituted digermenes that have trans-bending angles in the range of 20–40° [16, 18]. The most significant change in geometry is observed for (tBu2MeSi)2Ge=Ge(SiMetBu2)2, (12) [121b]; i.e., θ  =  0°, r(Ge=Ge) significantly elongated to 2.346 Å, and the twist angle is very large, 52.8°, similarly to that of disilene 73. The extreme twist reduces the 4pπ–4pπ overlap, destabilizes the HOMO, and decreases the HOMO–LUMO gap (Figure 1.14), as manifested in a redshift of the UV-Vis absorption to 618 nm [121b] compared to that of 11 (421 nm) and to other isolated digermenes (412–475 nm). The partial breaking of the Ge–Ge π bond in 12 may indicate a contribution of a biradical character to the bond and a facile rotation to obtain a triplet biradical, as observed in the analogous disilene 73, but a triplet state rotated digermene analogue has not been yet reported. 12 produces stable cation- and anion-radicals by cyclic voltammetry [121b].

Figure 1.14  Optimized structure and HOMOs (B3LYP/6-31G(d)) of a set of silyl- substituted digermenes. Reprinted with permission from Ref. [121b]. Copyright (2014) John Wiley & Sons Inc.

31

32

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

1.4.2.2.2.2.2  Alkali Metal Substituents

The chemistry of heavier group 14 analogues of vinyl lithiums is a rapidly developing field. The synthesis of trans1,2-dilithio-1,2-diaryl digermene species, {[dioxane0.5-(Et2O)]LiGeC6H3-2,6-Mes2}2 (76), the first isolated compound in which the alkali metal is terminally bonded to the vinylic germanium atom, was reported by Power and coworkers (Figure 1.15) [246]. In previously known M2[ArEEAr], (M=Li+, Na+ and K+; E=Ga, Ge, Sn; Ar=[Ar’=C6H3-2,6-(C6H32,6-iPr2)2 (77) or Ar*=C6H3-2,6-(C6H2-2,4,6-iPr3)2 (78)], the metal bridges the two E–E atoms and is coordinated to the flanking aryl substituents (Figure  1.15(b)) [246, 247]. An analogous compound to 77 and 78, K22+[RGeGeR]2- (20), where potassium atoms are bridging the Ge–Ge bond and R is a N-heterocyclic borane, ((HCDippN)2B (Dipp = 2,6-iPr2C6H3)), was reported recently [125]. The X-ray structure of dilithiodigermene 76 reveals a planar digermene skeleton with a Ge–Ge bond length of 2.328 Å (a typical Ge=Ge distance), a Ge–Li distance of 2.554 Å similar to that of 2.518 Å in LiGe(SiMetBu2)3 [248] and a CGeGe bond angle of 100.2° [246]. The planarity of 76 is in agreement with theoretical expectations for a dimer of HGeLi which is a triplet ground state (3A”) with ΔEST  =  4.5  kcal/mol (B3LYP/6-311++G(d,p)), 7.3  kcal/mol (QCISD(T)/ 6-311++G(d,p)) [249]. Similarly, HSiLi is calculated (MP4SDTQ/6-31G(d)) to have a 3A” triplet ground state, by 9.7 kcal/mol lower in energy than the singlet (1A’) state [250]. DFT calculations at B3LYP/6-31G(d) show that for Li2Ge2R2, R=H or Me, trans-1,2-Li(R) GeGe(R)Li is the global minimum. The calculated r(Ge-Ge) of 2.276 (R=H) Å, and of 2.277 Å (R=Me) are shorter, and r(Ge–Li) of 2.415 Å (R=H) and 2.432 Å (R=Me), are longer than those observed experimentally. These differences probably result from the much larger size of the substituents R and the coordination of Li by the ether donors which were not included in the calculations [246]. The HOMO, HOMO-1, HOMO-2 and HOMO-3 orbitals (Figure 1.16) correspond to the π(Ge=Ge), n +lone-pair (Ge), n–lone-pair (Ge) and σ(Ge–Ge) orbitals, consistent with the existence of a Ge=Ge double bond [246]. In the dibridged structures 77, 78, 20, E=Ge. the C–Ge–Ge–C core is planar and the Ge atoms are pyramidal. The alkali metal atoms and the Ge atoms form an almost symmetric planar structure that is perpendicular to the C(or B) GeGeC(or B) plane. In 77, E=Ge, M=Li, r(Ge-Ge) = 2.455 Å, r(Ge-Li) = 2.874 Å and CGeGe bond angle = 102.9/121.8° [247]. r(Ge-Ge) is similar to a Ge–Ge single bond length. Both distances are significantly longer than those observed for 76. For R=H and Me, the Li di-bridged analogue of 77 is not a minimum on the PES, and its observation in structures of 77, 78 and 20 is attributed to stabilizing coordination of the Li+ cation to the aromatic terphenyl substituents’ rings (e.g., 77, Figure 1.15). A variety of disilenides R2Si=SiR’Li (79) were synthesized independently by the groups of Schechkewitz and Sekiguchi, and their application in the synthesis of extended and conjugated compounds was studied, both experimentally and by theory [251–256]. Lithium sileneides, R’3Si(Li)Si=C(SiR3)SiR’3 (80) [257, 258] (R’3Si=tBuMe2Si, R3Si=tBu2MeSi) and R’3Si(Li) Si=C(SiR3)(1-Ad) (R’3Si=tBuMe2Si or tBu2MeSi, and R3Si=tBu2MeSi) (81) [259a] were isolated, characterized spectroscopically, and their reactions were studied experimentally and by theory by Apeloig’s group. Very recently a lithium digermenide Tip2Ge1=Ge2(Tip)Liꞏdme (Tip  =  2,4,6-iPr3-C6H2, dme  =  1,2-dimethoxyethane) 82 was synthesized and characterized by NMR, UV-Vis spectroscopy, X-ray diffraction, and by DFT calculations [124]. Several digermenide intermediates were suggested earlier by Masamune and Weidenbruch, but their structures could not be determined [260, 261]. 82 is a contact ionpair with a Ge=Ge and Ge–Li bond lengths of 2.284 Å and 2.842 Å, respectively. The Ge=Ge bond is slightly twisted, by 19.9°, and trans-bending at Ge is small, θ(Ge2) = 12.8° and θ(Ge1) = 12°, (see also the structure of dianion 76) and significantly smaller than in Mes2Ge=GeMes2 of 33°. DFT calculations at B3LYP/6-31G(d,p) for Dip2Ge=Ge(Dip)Liꞏdme (Dip = diisopropylphenyl) (83) are in good agreement with the experimental geometry of 82. The UV-Vis spectrum of 82 was simulated by TD-DFT calculations of 83. The relevant calculated orbitals of 83 are shown in Figure 1.17. The high intensity band at 458 nm is ascribed to transitions from the HOMO, the (Ge=Ge) π orbital, to the LUMO (π*) and LUMO+1 orbitals. A minor band at 356 nm is observed as a low intensity transition in the UV-Vis spectra of 82 and 83 [124]. It stems from a transition from the n-orbital on Ge (HOMO-1) into the LUMO (π*) orbital [124]. In 2022, Apeloig’s group reported the synthesis of the first Figure 1.15  X-ray structures of 76 and 77. Drawn from the CIF files, with permission from Ref. [246]. Copyright (2004) Royal Society of Chemistry, and with permission from Ref. [247]. Copyright (2003) American Chemical Society.

1.4  Doubly Bonded Compounds

genuine germenyl lithiums (R3Si)(1-Ad)C=Ge(SiMetBu2) (Li•2L) (R3Si = tBu2MeSi, or tBuMe2Si, both with L = THF, or L = 12-crown-4), which were characterized by NMR and UV-Vis spectroscopy and by X-ray crystallography. Oxidation of the germenyl lithiums produced the first persistent germenyl radicals (R3Si)(1-Ad)C=Ge•(SiMetBu2) (R3Si = tBu2MeSi, or tBuMe2Si) which were characterized by EPR spectroscopy. These experiments were supported by DFT calculations [259b]. A cyclic heteronuclear vinyl anion analogue with a Si=Ge bond, potassium silagermenide 37, was isolated by Scheschkewitz and coworkers and was characterized by X-ray crystallography, UV-Vis spectroscopy, and DFT calculations [141]. The reaction mechanisms of heavier vinyl anions, [(CH3)2E=E’(CH3)]- (E=E’=C, Si, Ge) towards CO were studied computationally [256]. Germa-[262] and stanna-benzenyl [263] potassium analogues of phenylpotassium were recently isolated and their structures were determined by X-ray crystallography. Their electronic structure, resonance structures, and degree of aromaticity were studied computationally, and they are discussed in section 1.4.5.1.2.

Figure 1.16  Electron density surfaces for Khon-Sham orbitals of trans-MeLiGeGeLiMe calculated at B3LYP/6-31G(d). (a) HOMO (π bond), (b) HOMO-1 (n+(lone pair) + Ge–Ge σ), (c) HOMO-2 (n–(lone pair) + Ge–C σ), and (d) HOMO-3 (Ge–Ge σ + Ge–C σ). Reproduced with permission from Ref. [246]. Copyright (2004) Royal Society of Chemistry.

1.4.2.2.2.2.3  Bulky Substituents - Attractive Non-covalent London Dispersion Forces (LDF)

The synthesized stable heavier group 14 element alkene analogues R2E=ER2, are all substituted by very bulky aryl, alkyl, or silyl substituents (Table 1.2) to prevent their dimerization and slow down other reactions. The steric interactions between the bulky substituents were traditionally considered to be repulsive, lowering the dissociation energies to their ER2 fragments. However, in recent years this view was shattered. There are an increasing number of theoretical studies that convincingly show that in addition to steric repulsion, significant attractive noncovalent London dispersion forces (LDF) exist between the C–H units of the hydrocarbon groups [114]. These LDF are significant in stabilizing sterically crowded molecules [114–118]. In many cases, these attractive dispersion forces are larger than the steric repulsive forces, making the Figure 1.17  Plots of selected MOs of digermenide 83 calculated overall forces between the bulky groups attractive rather at B3LYP/6-31G(d,p). Reproduced with permission from Ref. [124]. than repulsive! Thus, the calculated energies for dissociaCopyright (2018) American Chemical Society. tion of R2E=ER2 to 2ER2 (E=Ge (1), Sn (2) and Pb, R=CH(SiMe3)2) without and with the inclusion of London dispersion effects, respectively, are (kcal/mol, at B3PW91/6-311+G(2d) and at B3PW91 with Grimme’s D3 [56, 264] dispersion correction (i.e., B3PW91-D3), ΔG at 25° C, 1 Atm): ΔG = −17.8 and 9.4; ΔE = −2.3 and 28.7 (E=Ge); ΔG = −14.8 and 7.1; ΔE = 2.1 and 26.3 (E = Sn); and ΔG = −9.9 and −1.7; ΔE = −0.6 and 15.2 (E=Pb). Thus, without the inclusion of London dispersions forces corrections all three dimetallenes will dissociate to their monomers. However, with London dispersion forces corrections, the dimers of Ge and Sn, but not of Pb, are bonded in agreement with experiments [115, 117]. The realization of the importance of LDF to the stability of E=E compounds (and other compounds [114]) have entirely changed the view of chemists on the role of bulky substituents. The importance of including the contribution of LDF corrections to the reliability of DFT calculation is shown in Figure 1.2. The contribution of the attractive and repulsive forces to the total interaction energy (ΔEint) between the two R2E fragments forming R2E=ER2 and defining its stability towards dissociation, was calculated for R2E=ER2, E=Ge and Sn; R=CH(SiMe3)2 (1 and 2), and for the smaller R=CH(SiH3)2, CH3 and H substituents, using the energy decomposition analysis (EDA, Eq. 1.3), and it is shown in Table 1.7 [118].

33

34

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

∆Eint = ∆EPauli +∆Eelstat +∆Eoi +∆Edispersion

(1.3)

Where: ΔEPauli – Pauli repulsion destabilizing energy ΔEelstat – electrostatic attraction ΔEoi – orbital interaction ΔEdispersion – attractive dispersion energy The dispersion energy ΔEdispersion was calculated using a modified D3 method denoted D3-Hess. The short and medium range dispersion contribution is included in the DFT energy, while the long-range dispersion energy is entirely missing. The Grimme’s D3 method introduces an empirical damping function to avoid double-counting of the dispersion energy at short range, and thus, the D3 calculation underestimates the total dispersion energy [118]. In order to recover the total dispersion energy, one has to combine the dispersion contribution included in the DFT calculation and the one calculated using D3. However, the dispersion energy included in the calculated energy cannot be extracted. Hobza introduced a modified damping function, analogous to Hesselmann’s damping function [265], which recovers the short-range dispersion contribution. D3-Hess are significantly more stabilizing than D3 dispersion energies (Table 1.7), and their inclusion results in a significantly higher stability of R2E=ER2 towards dissociation to its R2E monomers, i.e., the dissociation energies calculated using B3PW91-D3/6-311+G(2d) and B3LYP-D3-Hess/def2-QZVP for 1, E=Ge, R=CH(SiMe3)2, are (in kcal/mol): 28.7 [115, 117] and 47.2 [118], respectively. However, the trends in D3-Hess and D3 are similar (Table 1.7). As shown in Table 1.7, increasing the bulk of the substituent from H (a) to CH(SiMe3)2 (d) results in an increase of the Pauli repulsion and a decrease of the importance of the orbital donor-acceptor interaction energy, probably due to the elongation in the Ge–Ge distance in d. The dispersion energy works in the opposite direction and compensates for the increase in the Pauli repulsion and the decrease in the orbital interaction from a to d. The electrostatic interaction changes only slightly. It is evident that without the attractive dispersion energy, d would not exist, because (ΔEPauli + ΔEelestat + ΔEoi) is repulsive by +4.7 kcal/mol. According to the D3-Hess calculation, the dispersion energy is very significant also for the stability of H2Ge=GeH2, being ca. 40% of ΔEint, while it is unsignificant, being only 2.7 kcal/mol at Grimme’s D3 [118]. For R2Sn=SnR2, R=H (a) and its bulkier analogues (b–d), all attractive interactions, ΔEelstat, ΔEoi, and ΔEdispersion are smaller than those for R2Ge=GeR2, probably due to the longer Sn=Sn bond. However, also the repulsive Pauli energy is smaller for Sn than that for Ge. The overall ΔEint is smaller for E=Sn, reflecting its lower stability [118]. Inclusion of LDF correction raises the dissociation energy of Mes2Si=SiMes2 from 51.2  kcal/mol (without LDF) to 68.5 kcal/mol (at BP86+D), comparable to the dissociation energy of 67.7 kcal/mol calculated for H2Si=SiH2. The contribution of Grimme’s dispersion energy to the thermal stability of H2Si=SiH2 is only 1 kcal/mol, emphasizing the importance of LDF to the stability of bulky substituted molecules towards dissociation [266]. Power concluded that in sterically crowded group 14 molecules dispersion force attractions between the ligands can be more important than the element-to-element bonding, and that LDFs are a major factor responsible for their stability [117]. Table 1.7  Optimized (B97-D2/def-QZVP) Ge–Ge bond distances (Å), Wiberg Bond Indexes (WBI), total interaction energies (ΔEint), Pauli repulsion energy (ΔEpauli), electrostatic energy (ΔEelstat), orbital interaction energies (ΔEoi), Grimme D3 dispersion energies (at B3LYP/def2-QZVP) and D3-Hess dispersion energies calculated for R2Ge=GeR2, R=H, CH3, CH(SiH3)2 and CH(SiMe3)2 (a–d).a,b,c

a

ΔEdispersion D3-Hess

ΔEPauli + ΔEelstat + ΔEoi

−102.3 (39%) −2.7

−21.4 (8%)

−33.5

−141.1 (55%)

−92.8 (36%)

−4.7

−22.1 (9%)

−20.1

183.9

−114.2 (52%)

−82.2 (37%)

−12.5

−25.0 (11%)

−12.5

254.9

−155.6 (52%)

−94.6 (31%)

−28.9

−50.5 (17%)

4.7

R

r(Ge-Ge)

WBI

ΔEint

ΔEpauli

ΔEelstat

ΔEoi

a, H

2.310

1.713

−54.9d

210.1

−141.3 (53%)

b, CH3

2.344

1.511

−42.2

213.8

c, CH(SiH3)2

2.393

1.389

−37.7

d, CH(SiMe3)2

2.366

1.469

−45.8

D3

Taken from Ref. [118]; Energies in kcal/mol; c In parentheses, the contribution (in percent) to the sum of attractive energies, ΔEelstat + ΔEoi + ΔEdispersion; d Ge=Ge bond dissociation energy = 55.7 kcal/mol at B3LYP-D3-Hess/def2-QZVP [118]; 55.7 kcal/mol at CCSD(T)/aug-cc-p VTZ [118], and 51.3 kcal/mol at BP86/def2-TZVPP [70]. b

1.4  Doubly Bonded Compounds

1.4.3  Hetero-nuclear Doubly Bonded Compounds 1.4.3.1  R2Ge=CR2 , Historical Overview and Stable Compounds

Metallenes R2E=CR’2 (84) are conceptually intermediate compounds between alkenes and their homonuclear heavy element analogues. In general, these compounds are very reactive, largely due to the high polarity of the E=C bond caused by the difference in the electronegativities of the heavier group 14 elements and carbon (Table 1.1). Also, for R2E=CR’2 metallenes, calculations preceded their experimental isolation (see below). The synthesis and isolation of R2E=CR’2 (84) compounds was facilitated by using bulky substituents that prohibit their dimerization or by substituents that decrease or reverse the bond polarity [134, 267–272]. Most metallenes, unlike dimetallenes, have a planar skeleton (Tables 1.2 and 1.8 and experimental structures given in Refs. [18] and [24]). Their planar structure is consistent with the CGMT model (Eq. 1.1) [160–163, 177, 184], which predicts that when the sum of ΔEST of the R’2C and R2E that build the double bond is smaller than a half of Eσ+π (the energy of the double bond in the classical planar form), a planar doubly bonded species exists (see below). The first stable compound in this group (Me3Si)2Si=C(OSiMe3)1-Ad (3) was synthesized and characterized by Brook [90–92] in 1981, refuting the “double bond rule”. The first stable germenes were synthesized and isolated only in 1987, independently by the groups of Berndt (85) [273] and Escudié (86) groups [274, 275]. tBu Me3Si Me3Si

B C

tBu

1.827 Å GeR2 C

B

Me3Si Me3Si

tBu tBu N SiMe2 a)R=N(SiMe3)2; b)R= N tBu 85

B C

C

GeR2

1.803 Å GeMes2

B tBu

85’

86

The Ge=C bond length of 85 of 1.827 Å, is significantly longer than the values calculated for H2Ge=CH2 of e.g., 1.784 Å (at MP2/6-31G(d) [220], Table 1.8). It is however shorter by 6.7% than the Ge–C single bond length in Me4Ge of 1.958 Å [276]. The doubly bonded Ge and C centers are planar, but the Ge=C bond is twisted by an average of 35° [273]. It was suggested that 85 is stabilized by a contribution from the zwitterionic resonance structure 85’, in which negative charge is delocalized to the empty 2p orbital of the boron atoms. The contribution of resonance structure 85’ is manifested in strongly shielded B atoms in 85 (δ 11Β (85a) = 66 ppm) compared to 1,3-diboretane (δ11Β = 82 ppm), and in a much shorter GeC-B bond of 1.534 Å compared to that of the B-CSi bond of 1.628 Å [273]. In 86, r(C=Ge) = 1.803 Å, the double bond centers are planar and the twist angle is only 6°. The stability of 86 is attributed to steric stabilization due to the bulky mesityl substituents and to charge delocalization over the cyclopentadiene fragment of the fluorenylidene group [274, 275]. Theoretical analysis of the Ge=C bonding will be helpful to better understand the bonding in these compounds. Very recently, Apeloig’s group succeeded to synthesize the first known stable silyl substituted germene, (tBuMe2Si)2Ge=2-Ad (87), and to characterize it by X-ray crystallography [277]. 87 features a planar skeleton and r(Ge=C)  =  1.808  Å (1.835  Å calc. at MP2/6311+G(d,p)//B3LYP/6-311+G(d,p) (H, C, Si), MP2/SDD//B3LYP/SDD (Ge)) [277]. 87 is calculated to have a significantly reduced polarity (Δq(Ge-C) = 1.03, (NPA charges)) compared to H2Ge=CH2 (Δq(Ge-C) = 2.02) [277]. Evidence for the formation of transient (Me3Si)2Ge=2-Ad (88) was provided by trapping reactions and by isolation of its head-to-head dimer [278]. Transient (CH3)2Ge=CH2 was detected in the gas phase by electron impact mass spectrometry and in argon matrix at 12 K by Fourier transform IR spectroscopy [279]. The developments achieved in the last three decades in the synthesis, characterization, and reactions of R2E=CR2 (E=Si-Pb) were reported in many comprehensive reviews [16, 18, 23, 24, 44, 45, 99, 100, 155]. 1.4.3.2  The Parent H2Ge=CH2

H

δC

r

H

δ+ Ge

H

H 89

35

36

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

Table 1.8  Calculated structural and bonding properties of R2E=CRR’ (E=Ge, Si).a

X2E=CH2

r(E=C)

θb

q(C)c

Polarity Δq

q(E)d

π bond energye

ΣΔEST −0.5(Eσ+π)

Eσ+πf

ΣΔESTg

R2Ge=CRR’ k

H2Ge=CH2

1.784 1.778h,i 1.770j

planar Σ∠Ge = 360.0 Σ∠C=360.0

−0.77 −1.02l −0.50i

+1.22k +1.00l +0.37i

1.99k 2.02l 0.87i

40.0; 32.2m, 33.0m,n, 34.2o, 30.3p, 31±2q, 45.5r

−46.3

123, 90–104s, 108±4t

15.2

(H3Si)2Ge=CH2

1.803

planar Σ∠Ge = 360.0 Σ∠C = 360.0

−0.75k −0.52l,u

+0.41k +0.62l,u

1.16k 1.14l,u

34.7

–

–

–

FHGe=CH2

1.764i

planar Σ∠Ge = 360.0 Σ∠C = 360.0

−0.52i

+0.75i

1.27i

–

–

H2Ge=CHF

1.794i

planar Σ∠Ge = 360.0 Σ∠C = 360.0

+0.04i

+0.17i

0.13i

–

–

F2Ge=CH2

1.774 1.758v

16.4 (GeF2), 29.1 (CH2) Σ∠Ge = 358.4v, Σ∠C = 355.9v

−0.77k −0.80v

+1.87k +2.25v

2.64 3.05v

21.1

F2Ge=CHF

1.845q

Σ∠Ge = 341.9v

+0.26v

+2.04v

1.78v

–

Cl2Ge=CH2

1.759

planar Σ∠Ge = 360.0v, Σ∠C = 359.9v

v

−0.82

v

+1.97

v

2.79

Cl2Ge=CHCl

1.79 v

Σ∠Ge = 352.5v

−0.49v

+1.88v

Br2Ge=CH2

1.764

planar Σ∠Ge = 360.0v, Σ∠C = 360.0v

v

−0.81

v

Br2Ge=CHBr

1.795v

Σ∠Ge = 354v

−0.73v

H2Si=CH2

w

1.718 1.704x 1.713y

planar Σ∠Si = 360.0 Σ∠C = 360.0

k

−1.24 −1.04l −0.56i −1.30v

(H3Si)2Si=CH2

1.738w 1.730y

planar Σ∠Si = 360.0 Σ∠C = 360.0

F2Si=CH2

1.685w 1.679v 1.685y

F2Si=CHF

Cl2Si=CH2

–

–

–

109.0 96–112s

58.8 64.0s

–

–

–

–

–

–

–

2.37v

–

–

–

–

+1.80

2.61v

–

–

–

+1.7 v

2.49v

–

–

–

+2.37k +0.99l +0.32i 0.93y +2.63v

3.61k 2.03l 0.88i 3.93v

40.0, 38.5o, 35.6z, 32.75p, 35 ± 2q, 37.7aa, 54.5r, 47.1bb, 36.1cc

−56

127.5, 105s, 118.6cc, 124t

−1.18k −0.52l,u −0.06i

+0.84k +0.45l,u −0.01i +0.48y

2.02k 0.97l,u 0.05i

38.0; 42.0dd

−67.1

126.8

−3.7

planar Σ∠Si = 360.0 Σ∠C = 360.0

−1.38k −1.43v

+2.85k +3.14v +2.09y

4.23k 4.28v

26.3; 30.7dd

−4

131.2, 114–122s

61.6

1.697v

planar Σ∠Si = 360.0 Σ∠C = 360.0

−0.67v

+3.15v

3.82v

1.702w 1.696y 1.689v

planar Σ∠Si = 360.0 Σ∠C = 360.0

−1.31k −1.36v −0.58i

+2.54k +2.94v +0.59i +1.33y

3.85k 4.30v 1.17i

31.1; 35.2dd

−24.5

129.7

40.4

v

v

+4.3

–

R2Si=CRR’ 7.8

(Continued)

1.4  Doubly Bonded Compounds

Table 1.8  (Continued)

X2E=CH2

r(E=C)

θb

q(C)c

q(E)d

Polarity Δq

Cl2Si=CHCl

1.703v

planar Σ∠Si = 360.0 Σ∠C = 360.0

−1.09v

+2.93v

4.02v

Br2Si=CH2

1.693v

planar Σ∠Si = 360.0 Σ∠C = 360.0

−1.34v

+2.82v

4.16v

Br2Si=CHBr

1.705v

Planar Σ∠Si = 360.0 Σ∠C = 360.0

−1.29v

+2.82v

4.11v

π bond energye

ΣΔEST −0.5(Eσ+π)

Eσ+πf

ΣΔESTg

–

–

–

–

a From Ref. [220]. At MP4/6-31G(d)//MP2/6-31G(d); Energies of singlet-triplet splitting of the carbenoid fragments were calculated using B3LYP/6-31+G(d,p), unless stated otherwise. π bond energies are derived from rotation barriers, unless stated otherwise. Energies in kcal/mol, bond lengths in Å, bond angles in degrees and charges in electrons; b θ = trans-bent angle, Σ∠E = sum of the bond angles around E; c Charge on C in electrons; d Charge on Ge or Si in electrons; e Based on rotation energies, unless stated otherwise. Previous values are reviewed in Refs. [44, 45] and [309]; f Unless otherwise stated, the Eσ + Eπ bond energies were calculated by a thermochemical cycle based on the energy of [2+2] cycloreversion energy of 1,3-digermacyclobutanes to 2H2Ge=CH2 and 1,3-disilacyclobutanes to 2H2Si=CH2 and the rotation barriers of the corresponding metallenes. For details, see Ref. [220]; g The sum of singlet-triplet splitting of R2E: (E=Si and Ge) and H2C: ; h At B3LYP/6-311G(d,p)[219]; i Mulliken charges at B3LYP/6-311G(d,p). For germenes from Ref. [288], and for silenes from Ref. [281]; j At the LDA level [171]; k AIM charge from Ref. [220]; l NPA charges, at MP2/6-311++G(d,p)//B3LYP/6-311++G(d,p) for H, C, Si, and at MP2/SDD//B3LYP/SDD for Ge. From Ref. [277]; m From Ref. [307] at SOCI/3-21G(d); n Calculated using a thermochemical cycle (Eq. 1.7); o From Refs. [313] and [314], calculated using a dissociation thermochemical cycle: ΔEdis(E=E) = Eσ + Eπ ‒DSSE(ΕΗ2) –DSSE(Ε’H2), DSSE– divalent state stabilization energy [196]; p Rotation barrier about the E=C bond, at B3LYP/6-31G(d) [310]; q Based on a thermochemical cycle [305]; r Intrinsic bond strength derived from the electronic interaction energy only, from Ref. [171]; s From Refs. [161] and [27]; t Eπ was derived from the hydrogenation enthalpies of H2E=CH2 and Eσ is the dissociation enthalpy of H3E–CH3 (E=Ge, Si) [305]; u NPA charges for (H3Si)2E=CMe2, E=Ge and Si from Ref. [277]; v From Ref. [315] at MP2(full)/def2-TZVPP. AIM charges are given; w At MP2/6-31G(d) [287]; x At CCSD(T)/cc-pV(Q,T)Z, experimental value: 1.7039 Å [316]. For earlier computed structural parameters, see Refs. [28, 309]; y At B3LYP/6-31+G(d). q(Si) are NPA charges. From Ref. [269]; z Rotation barrier at SOCI/6-31G(d) [306]; aa At G3//MP2/6-311++G(2d,p). Derived using a thermochemical cycle [317]; bb VB calculations, from Ref. [311]; cc At MP4SDTQ/6-31G(d)//6-31G(d) [318]; dd Calculated using a thermochemical cycle [287].

1.4.3.2.1 Structure

Structural and bonding properties of the parent germene and several substituted germenes, as well as analogous silenes, are presented in Table 1.8. The calculated Ge=C bond length in H2Ge=CH2 is in the range of 1.770–1.785 Å, (Table 1.8), significantly shorter than r(Ge-C) in H3Ge-CH3 of 1.969 Å (at B3LYP/6-311G(d,p)) [219] or in Me4Ge of 1.958 Å [276]. The experimentally determined r(Ge=C) in heavily substituted germenes are in the range of 1.77–1.85 Å [18, 24]. H2Ge=CH2 is planar, in agreement with the prediction of the CGMT model. For H2Ge=CH2, [ΣΔEST  − 0.5(Eσ+π)] is −46.3 kcal/mol (Table 1.8), thus predicting a planar structure. This is mainly a result of the triplet ground state of CH2 for which ΔEST = −9.0 kcal/mol (Table 1.3), and accordingly ΣΔEST (CH2 + GeH2) is only 15.2 [220] (12.5) [161] kcal/mol, significantly smaller than 0.5(Eσ+π) of the C=Ge bond of 61.5 [220] (45–52) [161] kcal/mol. Similarly, for H2Si=CH2 [ΣΔEST − 0.5(Eσ+π)] is −56 kcal/mol also predicting a planar structure. In contrast, H2E=EH2 (E=Ge, Si), ΣΔEST is larger

37

38

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

than 0.5(Eσ+π) by 15 kcal/mol (E=Ge) [161] and by 1 kcal/mol (E=Si) [161], correctly predicting their trans-bent geometry, and a much smaller trans-bending angle for disilene than for digermene, in agreement with calculations and experiment (Table 1.3). 1.4.3.2.2  Charge Distribution

An important feature of heterometallenes such as germenes is their highly polar E+δ=C−δ double bond resulting from the difference in the electronegativities of carbon and its heavier analogues: 2.55 (C), 1.90 (Si), 2.01 (Ge), 1.96 (Sn), 2.33 (Pb) (Pauling electronegativities [31]). The total bond polarities (Δq = q(E) - q(C)) of germene, silene, and several substituted derivatives are presented in Table 1.8. The calculated charges are highly dependent on the method and level of calculation. For H2Ge=CH2, NPA and Bader’s AIM charges are similar, but they are significantly larger than the calculated Mulliken charges. However, the calculated polarity of H2Si=CH2 is significantly larger using Bader’s AIM method than calculated using the NPA method. So, while the NPA and Mulliken polarities of H2Ge=CH2 and of H2Si=CH2 are similar, at AIM the polarity of H2Si=CH2 is almost twice larger than that of H2Ge=CH2. A study of H2Si=CH2 showed that the polarization occurs mainly in the σ framework of the Si=C bond [271]. Substituents affect the polarity of metallenes and even reverse it (Table 1.8 and section 1.4.3.3) [134, 268, 269, 271, 280–286]. The C=E bond polarity strongly effects the reactivity of heterometallenes, e.g., in dimerization [220, 269, 271, 287–290] and in addition reactions [271, 281, 289–301]; the smaller the C=E bond polarity the higher is the kinetic stability of the heteroalkene. 1.4.3.2.3  Double Bond Strength

The Eσ + Eπ bond energies of Ge=C and Si=C bonds in the parent metallenes and in a variety of substituted ones are presented in Table 1.8. The methods which are used to determine these values are mentioned in the Table’s footnotes and details are given in the cited manuscripts. Eσ + Eπ of H2Ge=CH2 range between 90 to 123 kcal/mol, somewhat smaller than that of H2Si=CH2 ranging 105–127 kcal/mol (Table 1.8). Eσ + Eπ is higher than the bond dissociation energy (BDE=ΔEdis) of E=E’ double bonds to its tetrylene fragments (i.e., H2E=E’H2 → H2E: + H2E’) by the sum of divalent state stabilization energies (DSSEs) of ER2 (Eq. 1.4). Walsh [302, 303] noticed a large difference between the first and second E–R dissociation energy in SiR4 and attributed it to the divalent state stabilization of the resulting SiR2 fragments. DSSE is thus defined as the energy difference between the first and second E–R dissociation energy (Eq. 1.5) [196, 302–304]. DSSEs increase significantly moving down group 14, e.g., DSSEs, derived from heats of formation (ΔH0f, Eq. 1.6) [305] for CH2, SiH2, and GeH2 are −5.6 [304], 22.4 [305], and 26.2 [305] kcal/mol, respectively. Significantly larger DSSEs are derived as the electronegativity of the substituents increases [303, 305]; they are 71.9, 54.8, and 51.0 for GeF2, GeCl2, and GeBr2, respectively, and somewhat smaller, 67.4, 46.2, and 42.4 kcal/mol for SiF2, SiCl2, and SiBr2, respectively [305]. Thus, as DSSE increases, the double bond dissociation energy decreases. ∆Edis (E=E')  = Eσ + Eπ  − DSSE (EH2 ) − DSSE(E'H2 )

(1.4)

DSSE =∆E(ER 4 → ER 3 +R )  −  ∆E(ER 3 → ER 2 +R )

(1.5)

DSSE =  2∆H 0f (ER 3 ) − (∆H 0f (ER 4 ) + ∆H 0f (ER 2 ))

(1.6)

The BDE of H2C=CH2 is 176.8 kcal/mol, nearly twice the BDE of H3C-CH3 (86.8 kcal/mol) [171]. In contrast, the BDE of H2E=EH2, E=Si, Ge, is smaller than that of its singly bonded analogs H3E-EH3, i.e., 36.9 kcal/mol vs. 64.3 kcal/mol for E=Ge and 52.0 kcal/mol vs. 69.4 kcal/mol for E=Si [196]. This unprecedented behavior which at the time was an “eye opener” observation was attributed by Grev to the increasing stability of the divalent EH2 fragments moving down group 14 [196]. The E=C bond dissociation energies of H2E=CH2 are 85.4  kcal/mol (E=Ge) and 110.5  kcal/mol (E=Si) vs. 68.2 kcal/mol (E=Ge) and 81.6 kcal/mol (E=Si) for H3E-CH3 (at LDA/NL) [171]. The E=C double bonds are stronger than the corresponding E–C single bonds, but the ratio of their strength is smaller than two, i.e., BDE Ge=C/Ge-C = 1.25, and BDE Si=C/Si-C = 1.35. These ratios reflect the negative and small DSSE (in kcal/mol) of CH2 (−5.6 [304]) and the slightly larger DSSE of Ge (26.2 [305]) vs. Si (22.4 [305]). For the experimentalists, these theoretical values mean that there is a relatively small gain (ca. 25%) in the energy of a C=E double bond over that of a C–E single bond, in contrast to a C=C bond which is ca. twice stronger than a C–C single bond. The π bond energy (Eπ) is an important property which effects the reactivity of doubly bonded species. Table 1.8 lists the π bond energies that were derived either from the rotation barrier around the π bond or using hydrogenation thermochemical cycles (Eqs. 1.7a and 1.7b) [305–308]. Additional earlier calculated values can be found in previous reviews [44,

1.4  Doubly Bonded Compounds

45, 309]. Several drawbacks were mentioned for using rotational barriers as a measure of the π bond energy [306, 310]. First, rotation elongates the σ bond in E=E’, e.g., from 1.776 Å in planar C2v germene to 1.915 Å in the perpendicular C2v structure and to 1.922  Å in the more stable Cs pyramidal structure (at B3LYP/6-31G(d)) [310]. Rotation also causes pyramidalization at the E centers [306, 307, 310] (Scheme 1.6), which is accompanied by rehybridization of the σ bond. The Cs pyramidal structure of the rotated H2GeCH2 is by 7 kcal/mol (B3LYP/6-31G(d)) more stable than the C2v perpendicular one having planar E and C centers (Scheme 1.6) [310]. Thus, rotation energy includes other energetic changes in addition to the π bond energy.

Scheme 1.6  Calculated rotation energies of H2E=CH2 (E=C, Si, Ge, Sn) in kcal/mol (at B3LYP/6-31G(d) for E=C, Si, Ge, and at B3LYP/ LANL2DZ(d,p) for E=Sn).

Estimation of Eπ based on thermochemical hydrogenation cycles (Eq. 1.7) also suffers from the problem of geometric changes (e.g., the σ bond distance is different in E=C and E–C compounds) as well as from other approximations [311]. The calculated values of Eπ using Eq. (1.7) are also collected in Table 1.8. H2E=CH2 + H2 → H3E-CH3

∆Hhyd 

∆Hhyd = ∆Hπ(E=C)  +∆Hσ(H-H) - ∆Hσ(E-H) - ∆Hσ(C-H) 

(1.7a) (1.7b)

Eπ of H2Ge=CH2, calculated from rotation barrier or using thermochemical cycles, are in the range of ca. 30–34 kcal/mol. Eπ of H2Si=CH2 is slightly higher, in the range of 32–38 kcal/mol, and for H2Sn=CH2 it is smaller, 20.9 [307] or 26.9 [310] kcal/mol, determined from rotation barriers. Higher Eπ of 40  kcal/mol for both germene and silene was derived by Guselnikov et  al. from the thermochemical cycle of the cycloreversion of 1,3-dimetallacyclobutane [220]. Intrinsic Eπ based on electronic interactions only are significantly higher, 54.5, 45.5, and 39.0 kcal/mol for silene, germene, and stannene, respectively [171] (Table 1.8). Using a VB-based analysis, Shaik et al. [311]. achieved a clean separation of the π and σ bonds enabling the computation of the strength of the π bond without modifying the distance or the strength of the underlying σ bond. For H2Si=CH2 they report a π bond strength of 47.1 kcal/mol, similar to the intrinsic π bond energy calculated by Ziegler and Jacobsen [171]. The VB study thus supports the view that the Eπ values calculated from rotation barriers underestimate Eπ· Unfortunately, similar VB calculations for H2Ge=CH2 are not available. The E and Z geometric isomers of a stable silene (tBu2MeSi)(tBuMe2Si)Si=CH(1-Ad) (90) were synthesized and characterized spectroscopically by Apeloig et al. [312]. Its thermal Z to E isomerization was studied both experimentally and computationally. The measured activation energies for this isomerization are: Ea (activation energy) = 24.4 kcal/mol, ΔG#(Z →E) = 28.7 ± 1.4 kcal/ mol, ΔG#(E →Z) = 31.3 ± 1.6 kcal/mol. The calculated (BP86-D3BJ/def2-TZVP(-f)//BP86-D3BJ/def2-TZVP(-f)) activation barrier for rotation around the double bond of 90 is 29.6 kcal/mol, higher by 5.5 kcal/mol than the experimental value. This experimental-theoretical difference triggered further calculations, leading to an interesting insight. Thus, an unprecedented isomerization pathway that proceeds through a silyl migration- single bond rotation- back silyl migration via a silylene intermediate (Eq. 1.8a) was suggested and studied for a model silene, (1-Ad)HC=Si(SiMe3)2. The activation energy calculated for this pathway is 24.2 kcal/mol, in agreement with the measured activation energy. These calculations lead to the conclusion that the above studied reaction pathway (Eq. 1.8a) is the most probable mechanism for the E → Z isomerization of 90. The C=Si bond dissociation-recombination mechanism for the isomerization (Eq. 1.8b) was excluded due to its extremely high energy of 103 kcal/mol [312]. Similar studies for the isomerization of a germene are of great interest.

39

40

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds









(1.8a)

(1.8b)

1.4.3.2.4 Isomers

Three isomers H2E=CH2, HECH3 and HCEH3 were located as local minima on the ECH4 potential energy surface (PES). On the PES of GeCH4, H2C=GeH2 is by 7–16 kcal/mol less stable than germylene HGeCH3 (Table 1.9) and the barrier for their interconversion is ca. 34–38  kcal/mol above the germene. So, although the germene is thermodynamically less stable than the germylene, it is kinetically stable towards isomerization under ambient conditions. The carbene isomer HCGeH3, a ground state triplet, lies by 52.4 kcal/mol above the germene (Table 1.9). For comparison, silene, H2Si=CH2, is calculated to have nearly the same energy (or to be slightly more stable) as HSiCH3 (ΔE = 4.7 to −2.1, Table 1.9) and the barrier that connects them is > 25 kcal/ mol (the isomerization barrier depends strongly on the computational level [319]; an isomerization barrier of 36.3 kcal/mol was calculated at B3LYP/6-311+G(d,p)) [320]. Calculated structures (DFT) [321] of germene and methylgermylene and the transition structure that connects them are shown in Figure 1.18. Similar structures were calculated at CISD [195]. Table 1.9  Calculated reaction energies (ΔE) and reaction barriers (ΔEb) for the isomerization of R2E=CH2 to the corresponding divalent isomers (E=Ge, Si and C).a ΔE

Reaction

H2Ge=CH2 → HGeCH3

H2Ge=CH2 → HCGeH3 ( A”) 3

Me2Ge=CH2 → CH3GeC2H5

H2Si=CH2 → HSiCH3

Me2Si=CH2 → CH3SiC2H5 H2C=CH2 → HCCH3

ΔEb

−15.9 to −12.6; −10.7b; −6.9c c

52.4

34.5 to 37.5; 33.6b –

d

48.7d

−3.1

e

4.7 to −2.1

41.0e; 36.3f

14.2d

56.8d

g

75.0g

74.9

a In kcal/mol, at various CISD and CCSD levels. From Ref. [195] unless stated otherwise. For more data, see the citations in previous reviews [44, 45]; b At CCSD(T)-F12/cc-pVQZ-F12, from Ref. [321]; c At CCSD(T)/aug-cc-pVTZ, from Ref. [219]; d At MP2/6-311G(d,p) from Ref. [279]; e At various computational levels, from References [319] and [322]; f At B3LYP/6-311+G(d,p), from Ref. [320]; g At MP4/6-311G(d,p) [197].

Figure 1.18  Optimized geometric parameters calculated at B2PLYP(D3)/cc-pVTZ for: (a) germene, (b) methylgermylene and (c) the transition state between the germene and methylgermylene. Bond lengths in Å, bond angles in degrees.

1.4  Doubly Bonded Compounds

Methyl germylene (HGeCH3) was inferred from matrix isolation studies by its vibration modes that were assigned using MO calculations [219]. Recently, Kaiser and coworkers reported the experimental generation of HGeCH3 in a single collision experiment in the gas phase, via a reaction of methylidyne radicals (CH; X2Π) with germane (GeH4; X1A’) [321]. The reaction proceeds via insertion of the methylidyne radical into a Ge–H bond of the germane forming a germylmethyl (CH2GeH3; X2A’, i1) complex, which isomerizes by a 1,2-H shift to H3CGeH2 (X2A’, i2). i2 decomposes by a hydrogen atom loss to produce HGeCH3 (X1A’). The overall reaction is exergonic (computed reaction mechanism in Figure  1.19a). Statistical rate constants and branching ratios computed by Ramsberger-Kassel-Marcus (RRKM) theory predict that atomic hydrogen loss is responsible nearly exclusively for the decomposition of reaction intermediates (i1 and i2). RRKM calculations predict that under complete energy redistribution H2Ge=CH2 and HGeCH3 are formed in nearly equal amounts (52 and 48%, respectively). As the transition state for the isomerization of HGeCH3 to H2Ge=CH2 lies below the (CH + GeH4) reactants, the germylene has enough energy to overcome the interconverting barrier of 44.5 kcal/mol. The least stable isomer, HCGeH3, is predicted by RRKM not to be formed. For comparison, similar single collision reactions of methylidyne with silane, SiH4, yield eventually the thermodynamically more stable (Table 1.2) silene, H2Si=CH2 (Figure 1.19b). RRKM theory predicts a predominant formation of H2Si=CH2 (96 ± 4%) and of only 4 ± 2% HSiCH3. HCSiH3 is predicted not to be formed [323]. Irradiation of a co-condensate of Si atoms with methane at λ = 185 or 254 nm, in an Ar matrix at 10 K, yielded methylsilylene (HSiCH3). A photo equilibrium between HSiCH3 and H2Si=CH2 was established; at λ > 400 nm the equilibrium is shifted towards the silene, and at λ = 254 nm it is shifted towards the silylene [320]. Me2Ge=CH2 and its isomeric germylene CH3GeC2H5 are calculated to have a similar energy, while HGeCH3 is lower in energy than H2Ge=CH2 by 7–16 kcal/mol (Table 1.9). So, compared to H, methyl substitution at Ge favors germene over germylene. The calculated barrier for the Me2Ge=CH2 → CH3GeC2H5 isomerization, via a 1,2-methyl shift, is 48.7 kcal/mol [279], higher than the barrier of 33–37 kcal/mol for a 1,2-H-shift isomerizing H2Ge=CH2 to HGeCH3 (Table 1.9). In line with the calculations Me2Ge=CH2 was detected both in the gas phase by electron impact mass spectrometry and in solid Argon matrixes at 12 K [279]. For comparison, Me2Si=CH2 is more stable than CH3SiC2H5 by 14.2 kcal/mol and the barrier for their interconversion is 56.8 kcal/mol (Table 1.9) [279]. Transient Me2Si=CH2 was detected and identified spectroscopically in an Ar matrix using calculated harmonic vibrational frequencies (see Ref. [165] and references cited therein). 1.4.3.2.5 Dimerization

The parent metallenes and metallenes substituted by small substituents, react extremely fast by self-dimerization which prevents their isolation in ambient conditions. Evidence for their existence was presented in solid matrices and in the

Figure 1.19  Calculated (CCSD(T)-F12/cc-pVQZ-f12//B2PLYPD3/cc-pVTZ + ZPE(B2PLYPD3/cc-pVTZ) PES for the reaction of (a) methylidyne radical with germane and (b) methylidyne radical with silane. Energies are in kJ/mol. (a) Reprinted with permission from Ref. [321] and (b) from Ref. [323]. Copyright (2020) Wiley-VCH.

41

42

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

gas-phase (see above). Substitution of the E=E’ bond with bulky substituents hinders dimerization and other reactions, enabling isolation of stable metallenes (Table 1.2). Khabashesku et al. studied computationally the PES for the concerted π2s + π2s head-to-tail (HT) and head-to-head (HH) dimerization of H2Ge=CH2 and several substituted germenes, forming 1,3-digermacyclobutanes and 1,2-digermacyclobutanes, respectively [288]. According to Woodward-Hoffman rules [324] both pathways are thermally forbidden; however, it is believed that the high polarization of the Ge=C double bond leads to relaxation of these symmetry rules [77, 288, 325, 326]. The computational results in Table 1.10 predict a highly exothermic dimerization for both HT and HH pathways. The reaction exothermicity increases significantly for FHGe=CH2 and F2Ge=CH2, which are more polar than H2Ge=CH2 (Table 1.8) while the dimerization energy of the less polar H2Ge=CFH is less exothermic. The dimerization barrier for H2Ge=CH2 to produce the HT dimer is only 3.6  kcal/mol (at B3LYP/6-311G(d,p)). The HT dimerization of the polar FHGe=CH2 is predicted to be barrierless, and the HT dimerization barrier for the less polar H2Ge=CHF is larger than 6.8 kcal/mol [288]. The dimerization of H2Si=CH2 is predicted (at CCSD/DZ + P [325]) to be more exothermic than that of H2Ge=CH2, but their dimerization barriers are nearly the same (Table 1.10). In a systematic study of a series of substituted silenes, X2Si=CH2 and H2Si=CX2 (X=SiH3, Me, NH2, SH, Cl, OH, and F), Ottosson has shown that similarly to germenes, silenes with a reduced or reversed polarity are less prone to dimerize [269].

Figure 1.20  Schematic transition state structure and geometric parameters for (a) head-to-tail (HT) dimerization, and (b) head-to-head (HH) dimerization of H2E=CH2 to 1,3-dimetallacyclobutane and 1,2-digermacyclobutane, respectively. The geometric parameters for E=Ge (plain font) are calculated at B3LYP/6-311G(d,p) [288] and those for E=Si (italic font) are at CCSD/DZ+P[325]. Bond lengths in Å, angles in degrees.

Table 1.10  Calculated reaction enthalpies (ΔH) and reaction barriers (ΔH#) for the head-to-tail and head-to-head trans and cis dimerization pathways of substituted germenes.a Germene

ΔH

ΔH#

Head-to-Tail (HT)

a

ΔΔH (HH–HT)

Head-to-Head (HH)

−64.0

20.7

−1.3

MeHGe=CH2 (trans) −65.9

0.9

−63.4

21.7

−2.5

MeHGe=CH2 (cis)

−65.8

1.1

−63.0

21.8

−2.8

Me2Ge=CH2

−66.4

−1.0

−62.0

22.6

−4.4

FHGe=CH2 (trans)

−82.8

no TS

−74.7

13.6

−8.1

FHGe=CH2 (cis)

−79.2, −104.8b,e

no TS

−73.5

14.4

−5.5

H2Ge=CHF (trans)

−64.9

≥6.4

−73.7

Not found

8.8

H2Ge=CHF (cis)

−64.3

≥6.8

−73.3

Not found

9.0

In kcal/mol, at B3LYP/6-311G(d,p). From Ref. [288]; From Ref. [220], at MP4/6-31G(d)//MP2/6-31G(d); c For H2Si=CH2, at CCSD/DZ+P, from Ref. [325]; d For H2Si=CH2; e For F2Ge=CH2. b

ΔH#

3.6, 5.2c

H2Ge=CH2

−65.3, −69.2b, (−79.1c, −78.3b,d),

ΔH

1.4  Doubly Bonded Compounds

The transition state structures for the head-to-tail dimerization of both H2Si=CH2 and H2Ge=CH2 have a planar 1,3E2C2 skeleton and the E–C bond of the two reacting molecules is only slightly elongated relative to the monomer’s E=C bond length, which retain their sp2 character, i.e., a very early transition state (Figure 1.20a) [288, 325]. The head-to-head (HH) concerted dimerization pathway of germenes, forming 1,2-digermacyclobutanes, is also highly exothermic with energies slightly lower than for the HT dimerization. An exception is the dimerization of H2Ge=CHF for which the HH dimerization is by ca. 9 kcal/mol more exothermic than the HT dimerization. However, the reaction barriers are significantly higher for the HH dimerization than for the HT dimerization (Table 1.10). Thus, for H2Ge=CH2 the barrier for the HH dimerization is 20.7 kcal/mol but only 3.6–5.2 kcal/mol for the HT dimerization (Table 1.10) [288]. The transition state structure of the HH dimerization of H2Ge=CH2 (Figure 1.20b) shows that one of the germene molecules does not change much, i.e., r(Ge=C) = 1.884 Å, while in the second germene the Ge–C distance is significantly elongated to 1.971 Å, e.g., slightly shorter than the measured distances of 2.097 Å [278] in 91, and in the nearly planar 1,2-digermacyclobutane (1,1,2,2-R4Ge2C2H4, R=CH(SiMe3)2) of 2.01  Å [327], and similar to the Ge–C distance calculated (CCSD/ccTZVP) for 1,3-digermacyclobutane (H8Ge2C2) of 1.976 Å [328]. Khabashesku et al. proposed that the HH dimerization of germene is a multi-step reaction passing via a 1,4-biradical intermediate [288], in analogy to the HH dimerization of (Me3Si)2Si=CR(OSiMe3) for which the corresponding biradicals were detected [329, 330]. Bernardi et al., based on CCSD and CASSCF calculations, concluded that the preferred mechanism for HH and HT dimerization of H2C=SiH2 is via a multi-step diradical mechanism. A concerted pathway exists but has a higher barrier [331]. Based on kinetic studies, it was concluded that the photolytic cycloreversion of 1,1,3,3-Ph4E2-cyclobutane (E=Si, Ge) forming Ph2E=CH2 proceeds through a 1,2-biradical mechanism [290]. The dimerization of bis(trimethylsilyl)adamantylidene germene (91a) reported by Apeloig et  al. is the first example of an HH dimerization of a germene (Scheme 1.7). The germene was identified by trapping reactions (Scheme 1.7) and its HH dimer was characterized by X-ray crystallography [278]. A similar reaction produced the HH dimer (91, M=Si) of the analogous bis(trimethylsilyl)adamantylidene silene [332].

Scheme 1.7  Generation, HH dimerization and trapping reactions of bis(trimethylsilyl)adamantylidene germene (91a). Reproduced with permission from Ref. [278]. Copyright (1995) Royal Society of Chemistry.

A comprehensive computational study of a multi-step HH dimerization mechanism of germenes, similar to the study conducted for silenes [331], is desirable. For studies of other cycloaddition reactions to germenes, the reader is referred to Refs. [291–293, 295–299, 301, 333, 334]. 1.4.3.3  Substituted Germenes 1.4.3.3.1  Silyl and Halogen Substituents

In contrast to the comprehensive and systematic computational studies of substituent effects on the physico-chemical properties of silenes [267, 269, 271, 335], similar studies for germenes are limited, and concentrate mainly on halogen substituents. Calculated geometries and bonding properties of several substituted small germenes and a comparison with their silene analogues are shown in Table 1.8. However, note that the reported data was studied with different computational methods, making a comparison of trends difficult.

43

44

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

Silyl substituents at Ge in (H3Si)2Ge=CMe2 decrease the positive charge on Ge and reduce the Ge=C bond polarity (Δq) from 2.0 in H2Ge=CH2 to 1.14 (NPA charges, Table 1.8). The smaller polarity of the Ge=C bond results in elongation of the Ge=C bond from 1.784 Å in H2Ge=CH2 to 1.803 Å in (H3Si)2Ge=CH2 (Table 1.8). A similar trend was observed for silyl substituted silenes (Table 1.8 and Refs. [278, 281]). In contrast, the electronegative fluorine substituent at Ge in HFGe=CH2 increases the positive charge at the Ge atom and the polarity of the Ge=C, (Δq) is increased from 0.87 in H2Ge=CH2 to 1.27 (Mulliken charges, Table 1.8), and consequently r(Ge=C) shortens from 1.778 Å in H2Ge=CH2 to 1.764 Å (Table 1.8). The double bond polarity in F2Ge=CH2 increases further, causing a shortening of the Ge=C bond length (Table 1.8). When the fluorine substituent is at the C atom, the negative charge on C decreases significantly from –0.50 in H2Ge=CH2 to +0.04 in H2Ge=CHF, and the Ge becomes less positive resulting in a significantly less polar Ge=C bond (Δq is only 0.13), and is elongated to 1.794 Å (Table 1.8). In F2Ge=CHF, the carbon atom is positively charged resulting in a significantly smaller Ge=C bond polarity Δq  =  1.78 in F2Ge=CHF vs. 3.05 in F2Ge=CH2, AIM charges) [315] and a longer Ge=C bond of 1.845  Å. A similar trend is observed also for X2Ge=CHX (X=Cl, Br) (Table 1.8) [315]. In X2Ge=CH2, the Ge–C bond polarity is reduced gradually in the order X=F > Cl > Br, while the opposite is true for X2Ge=CHX for which the Ge=C polarity increases in the opposite order X=F  F). In general, the Si=C bond orders are larger than those for Ge=C, indicating stronger Si=C bonds than Ge=C bonds (Table 1.11) [315]. The E–X and C–X bond orders are larger than 1.0 (Table 1.11) indicating some double bond characters in these bonds [315]. A pronounced structural feature of F2Ge=CH2 and X2Ge=CHX (X=F, Cl, Br) is the pyramidality of their Ge center (Table 1.8). This is in agreement with the CGMT prediction and results from the significantly larger ΔEST of tetrylenes substituted by electronegative substituents, e.g., for GeF2 ΔEST = 71.2 kcal/mol [161] (compared to 23–27 kcal/mol for GeH2, Tables 1.3, 1.5, and 1.6) and ΔEST(CH2) = −9.0 kcal/mol (Table 1.3), leading in F2Ge=CH2 to a pyramidal geometry at Ge and C and to larger bending angles in F2Ge=CHF (Table 1.8, ΔEST(CHF) = 14.3 kcal/mol [337]). In contrast, their silene analogues are planar [315]. However, also the Si atom pyramidalizes as reversed polarization becomes more important, e.g., H2Si=C(NH2)2 (q(Si) = +0.25, r(Si=C) = 1.93 Å, Σ∠(Si) = 287.5°). vs. (H2N)2Si=CH2 (q(Si) = +1.86, r(Si=C) = 1.704 Å, Σ∠ (Si) = 360°) [269]. Pyramidalization in silenes was observed only when the carbon end was substituted by electronegative or π-donating substituents. For H2Si=CX2, pyramidalization at the Si atom and Si=C bond elongation increase in the order: X=F  127 (11.9, 15.0) > 125 (12.2, 13.9); and c) those with the long E–E central bond that have the smallest π-conjugation energies: 120 (14.0, 14.9) > 132 (10.6, 11.9) > 129 (9.9, 10.1). Shorter central bonds enable a better overlap between πa and π*b resulting in stronger conjugation. However, in addition to the central bond distance, the relative energy differences between the πa and π*b orbitals, Δε, and the polarity of the doubly bonded fragments also determine the degree of π-conjugation. 1.4.4.1.4  Redox Properties of E4R6 (E=Ge, Si)

The redox properties of tetragerma- and tetrasila-1,3-butadiene, E4R6, E=Ge, Si, R=Tip (108 and 111, respectively), and of the corresponding digermene (134) and disilene (135) were studied using cyclic voltammetry (CV) in THF and o-dichlorobenzene (o-DCB). In both solvents, the observed ease of oxidation followed the trend: 111 > 108 > 134 > 135 [405]. The calculated (B3LYP/6-311+G(d,p)) vertical ionization energies (IP) for E4R6 and E2R4, R=xylyl and R=H, respectively, are (in eV): Si4R6 (5.59, 7.24)  disilene. However, no obvious correlation was found between the computed electron affinities and the experimentally determined reduction potentials, which follow the trend (in THF): tetrasila-1,3-butadiene  >  disilene  >  digermene > tetragerma-1,3-butadiene. The following IPs and EAs (eV) were reported for the carbon analogue, i.e., for C2H4, IP = 10.51 (exp) [406], 10.49 (calc.) [406] and EA = −1.772 (calc.) [406], −1.55 (exp.) [407], and for C4H6, IP = 9.08 (exp.) [406], 8.803 (calc.) [406] and EA = −0.846(calc.) [406] and −0.62 (exp.) [407]. Thus, C4H6 is oxidized more easily than C2H4, similar to the trend observed in the heavier group 14 element butadienes. The smaller vertical ionization potentials of heavy group 14 element butadiene analogues relative to their corresponding alkenes reflect the conjugation in the dienes [405]. 1.4.4.2  Heavy Analogues of Allenes 1.4.4.2.1  Isolated Metallaallenes

Heavy analogues of allene, with two cumulative double bonds connected by a formally sp-hybridized tetravalent dicoordinate central atom, have unique structures, bonding, electronic properties and reactivities compared to their all-carbon analogues, again exhibiting the different behavior of carbon compounds and their heavier group 14 analogues. The syntheses and properties of isolated allene analogues were previously reviewed [18, 24, 25, 408–410]. Theoretical studies of heavy allene analogues carried out prior year 2000 were reviewed by Apeloig, Karni et al. [44], and by Frenking et al. [45]. 136–140 are group 14 heavy allene analogues isolated up to date. A summary of their important geometric parameters and spectral properties is given in Table 1.19. For data on heavy allenes 141*, with group 14 and group 15 elements, see References [20, 21, 26, 410, 411]. To our best knowledge, stable heavy allenes of the types R2C=E=CR’2, R2E=C=E’R’2 or R2E’=E=CR’2 (E, E’= heavy group 14 elements) have not been isolated to date.

63

64

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

1.4.4.2.2 Structures

Allene, R2C1=C2=C3R2, has a linear C1C2C3 skeleton, planar C1 and C3 centers and the R2C1 and R2C3 planes are perpendicular to each other. In contrast, most isolated stable metallaallenes exhibit a non-linear skeleton and pyramidality at the terminal E1 and E3 heavy elements (Table 1.19). Calculation at various theoretical levels for the parent H2E1=E2=E3H2 (E1, E2, E3=Ge or Si) show that the linear structure is not a minimum on the PES [425–429] and that they exhibit very acute E1E2E3 α angles, of 67–73° (Table 1.20). In H2E=E=EH2 (E=Si, Ge), the H2E units are nearly planar but the two H2E planes are not perpendicular to each other as in H2C=C=CH2 (ϕ1 = + 90.0°, ϕ2 = −90.0°); e.g., for H2Ge=Ge=GeH2 ϕ1 = 0.0° and ϕ2 = −109.2° [425] (ϕ1 = ∠H2E1E3H2’, ϕ2 = ∠H2E1E3H1’, Table 1.20). When one or two of the allenic skeleton atoms are carbon atoms the bending angle α is significantly larger than in the all-heavy element allenes, and it decreases going down group 14. For example, the calculated bending angle about the central atom in H2E=C=CH2 (α) follows the order: 157.2° (E=Sn)  360° (E=Si). These computational predictions for the parent systems are in

1.4  Doubly Bonded Compounds

Table 1.19  Important experimental structural and spectral parameters of isolated heavy group 14 allene analogues 136–140 and computed properties of corresponding model compounds. λmax UV-Vis (nm)b

NMR chemical shifts (δ, ppm)

Year

References

348.4 32.6d

–

235.1 (13C2)

1998

[412, 413]

172.0

357.2 16.2 d,e

–

223.6 (13C2) 13.1 (29Si)

1997

[414]

1.324

173.5

1.1d

–

225.7 (13C2) 48.4 (29Si)

1993

[415]

–

–

–

–

–

216.3 (13C2) 55.1 (29Si)

1997

[414]

136f SiCC

–

–

–

–

–

227.9 (13C2) 58.7 (29Si)

1997

[414]

136g SiCC

–

–

–

–

405

268.3 (292.6)f (13C2) −64.5 (−50.8)f (29Si1)

2001

[416]

136h SiCC

1.694

1.324

174.2

360.0

401

230.6 (13C2) 43 (29Si)

2010

[417]

137a GeGeGe

2.321

2.330

122.6

348.5

630

–

2005

[408, 418, 419]

137b GeSiGe

2.269

2.269

125.7

349.3

612

236.6 (29Si)

2005

[408, 418, 419]

137c SiGeSi

2.236

2.237

132.4

354.0

599

219.4 (29Si)

2005

[408, 419, 420]

137d SiSiSi

2.177

2.188

136.5

354.1

584

196.9 (29Si1,3) 157.0 (29Si2)

2003

[408, 419, 421]

138 GeSiGe

2.268

2.290

81.0

360.0

451

−16.46

2017

[151]

139 SiSiSi

2.179g

2.174g

164.3g

356. 8(Si1)g 357.9 (Si3)g

400

44.6 (29Si1,3)h 418.5 (29Si2)h

2011

[422]

140

2.684

2.675

156.0

344. 3 (Sn1) 346.88 (Sn3)

–

502 (119Sn1,3) 2233 (119Sn2)

1999

[423]

Allene

r1 (Å)

r2 (Å)

α°

Σ∠(E)°a

136a GeCCc

1.783

1.314

159.2

136c SiCC

1.693

1.325

136d SiCC

1.704

136e SiCC

a

The sum of bond angles at the allene terminal atoms; The longest wavelength absorption; c 136b: X-ray structure and UV-Vis absorptions are not available. Reactivity studies and 13C NMR chemical shift at 243.6 ppm of the central carbon atom validate its allenic structure [424]; d Trans-bending angle at E; e The terminal C3 is planar; f Calculated at GIAO/B3LYP/6-311+G(2df, p) for (H3Si)2Si=C=C(SiH3)2. This model compound has nearly C2 symmetry with a linear allene skeleton and planar terminal Si and C atoms. δ(29Si1) is the most up-field shifted signal of known Si=C compounds; g Calculated at B3LYP/6-31G(d). X-ray structure could not be obtained; h Calculated chemical shifts: δ(29Si1) = 72.6 ppm, δ(29Si3) = 93.1 ppm and δ(29Si2) = 471.3 ppm [422]. b

agreement with the measured geometries of 1-germaallene (136a) for which α = 159.2° and which is highly pyramidalized, Σ∠(Ge) = 348.4°, and 1-silaallenes (136c, d, and h) which are nearly linear and nearly planar at the terminal silicon atom. Also, 137a and 137b with 1,3-Ge atoms are more strongly bent than 137c and 137d with 1,3-Si atoms (Table 1.19). The pyramidality at E in all isolated metallaallenes follows the order Sn > Ge > Si, similarly to the heavier group 14 alkene analogues (Table 1.3). The larger bending angle and increasing pyramidality going down group 14 results from the increasing reluctance of heavier group 14 elements to hybridize (see Section 1.4.1) as is reflected in the increasing contribution of the p orbital to the hybridization of E on going down group 14, as calculated for H2E=C=CH2 by NBO analysis [430].

65

Table 1.20  Calculated geometry parameters (Bond lengths in Å, bond angles in degrees), Wiberg Bond Indices (WBI), relative energies (kcal/mol) of isomeric forms of H2E1E2E3H2 (E1, E2, E3=C, Si, Ge) and barriers for the ring closure of metallaallene 141 to trimetallacyclopropylidene 142.a r1 1

H2

E 1

E2 α

r2 3

E

H2'

r3

H

H H

H1'

Metallaallene (141)

a

2 r1 E2 r α E1 E3 r3

H

Trimetallacyclopropylidene (142)

E1E2E3

r1

r2

r3(E1-E3)

α

GeGeGe

2.398 [1.30]

2.398 [1.30]

2.848 [0.65]

72.9

SiSiSif

2.269 [1.33]

2.269 [1.33]

2.583 [0.56]

GeSiGeh

2.324 [1.33]

2.324 [1.33]

GeGeSi

2.402 [1.26]

SiGeSi

Σ∠(E1)

References

Φ1, ϕ2b

ΔEc

ΔE(TS)d

r1

r2

r3

α

Σ∠(E1)

Φ1, ϕ2b

ΔEe

358.4

0.0, −109.2

−30.1

5.7

2.584

2.584

2.404 [1.01]

55.4

348.2

0.0, −143.1

5.7

[425]

69.4

359.9

0.0, 115.5

−20.6

3.69 (3.0)g

2.446

2.446

2.291

55.8

351.8

0.0, 146.3

3.66 (−2.5)g

[427]

2.829 [0.68]

75.0

–

–

−23.9

–

–

–

–

–

–

–

–

[426]

2.340 [1.28]

2.724 [0.68]

70.1

–

–

−24.0

3.52

2.582

2.532

2.348 [1.02]

54.7

–

–

3.3

[426]

2.346 [1.28]

2.346 [1.28]

2.603 [0.68]

67.4

–

–

−25.0

1.39

2.503

2.503

2.331 [1.03]

55.5

–

–

0.8

[426]

SiSiGeh

2.265 [1.36]

2.324 [1.29]

2.713 [0.60]

72.5

–

–

−23.4

–

–

–

–

–

–

–

–

[426]

GeCC

1.789 [1.71]

1.307 [2.01]

3.063

162.9

346.9

73.3, −16.7

−0.63

–

–

–

–

–

–

–

–

[425]

CGeC

1.761 [1.74]

1.761 [1.74]

3.521

176.0

359.8

90.1, −86.8

−0.01

14.4

2.032

2.032

1.492 [1.10]

43.1

344.4

0.0, −142.5

−37.8

[425]

GeGeC

2.380 [1.42]

1.784 [1.73]

3.944

142.2

312.6

58.6, −121.4

−15.3

2.9

2.490

2.05

1.969 [0.90]

50.2

358.4

−2.8, 135.8

−11.7

[425]

GeCGe

1.754 [1.75]

1.754 [1.75]

3.508

180.0

360.0

90.0, −90.0

0.0

–

–

–

–

–

–

–

–

[426]

Calculated at B3LYP/6-311+G(d,p), WBIs are given in brackets, for other levels of calculation see the original paper; ϕ1 = ∠H2E1E3H2’, ϕ2 = ∠H2E1E3H1’; c Relative to the linear D2d structure which is not a minimum on the PES; d Barrier for the ring closure of metallaallene 141 to trimetallacyclopropylidene 142; e Relative to the bent metallaallene; f At B3LYP/6-31G(d,p); g At CCSD/6311+G(2df,p); h A cyclic isomer was not located on the bending PES. b

H

1.4  Doubly Bonded Compounds

H2C=E=CH2 (E=Si [430], Ge) and H2Ge=C=GeH2 are calculated to be linear and planar at the terminal atoms (Table 1.20) [425]. Based on CGMT theory, Trinquier predicted that H2Si=C=CH2 should be non-linear while H2C=Si=CH2 is expected to be linear [161]. Similarly, Lim et al. explained the different geometries of 1,2-digermaallene, which is strongly bent and highly pyramidalized (α  =  142.2°, Σ∠(Ge)  =  312.6°), and of 1,3-digermaallene, which is linear and has planar Ge centers (α = 180°, Σ∠(Ge) = 360°) [425] using a CGMT analysis. In 1,3-digermaallene, ΣΔEST of H2Ge: and :CGeH2 (the fragments of the germa-allene) is 41.7 kcal/mol, smaller than the Eσ+π bond energy of 87.0 kcal/mol, and thus these fragments interact via their triplet states and form a linear allene. In contrast, for 1,2-digermaallene, ΣΔEST(H2Ge: + :GeCH2) is 86.3 kcal/mol and the Eσ+π bond energy of only 23 kcal/mol cannot offset the high ΣΔEST of the fragments, and the fragments interact in a donor-acceptor fashion (Figure 1.8c) resulting in a bent geometry [425]. Kira et al. ascribed the origin of the bent structures in 137a–d to Jahn-Teller distortion associated with effective π-σ* mixing [419]. The Wiberg Bond Indices (WBIs) of the E1=E2 and E2=E3 bonds of H2E1=E2=E3H2 (E=Si, Ge), having an acute α bending angle, are in the region of 1.3 (Table 1.20, and references cited therein), significantly smaller than e.g., 1.85 calculated for the double bonds in linear H2Si=Si=SiH2 [427]. These relatively small WBI values indicate the presence of only a partial double bond. The WBI between the terminal E1ꞏꞏꞏꞏE3 atoms is in the range of 0.56–0.68 (Table 1.20), indicating significant bonding between E1 and E3, although smaller than in the trimetallacyclopropylidene 142, for which the WBI of the E1-E3 bond is nearly 1 (Table 1.20). NBO calculations show that the partial single bond between E1 and E3 in 141 (E=Si, Ge) arises from partial overlap of their p orbitals [426]. Interesting structural features are revealed from the X-ray structures of 137a–d substituted by bulky cycloalkyl groups. The X-ray structures of compounds 137d and 137c show a dynamic disorder which is manifested by rotation of the central Si atom in 137d and the central Ge atom in 137c around the Si•••Si axis. The central atoms are found in four positions, indicating that four structurally similar isomers exist [419, 420, 421]. Such disorder was not observed for 137a and 137b. The measured bending angle α in 137a–d is significantly wider (122–136°, Table 1.19) than α of 67–75° calculated for the analogous parent metallallenes, 141 (E1, E2, E3 =Si, Ge, R=H) (Table 1.20). The wide bending angle α in these trimetallaallenes is probably due to repulsion between the large substituents in 137a–d [419]. Another interesting feature of the geometries of 137a–d is the mutual nearly perpendicular orientation of the two C2E planes, similarly to R2C=C=CR2, in contrast to the non-perpendicular mutual orientation in the calculated analogous parent compounds 141 E1, E2, E3=Si, Ge, R=H (see Table 1.20). Veszprémi predicted, based on computations for Si3H4, that the bending angle α and the relative orientation of the R2E planes are interrelated. According to the calculations, widening α from its value at the lowest energy Cs isomer of Si3H4 (i.e., c, Scheme 1.16) results in rotation of the two terminal SiH2 fragments leading to a perpendicular orientation of the two planes, and vice versa [429]. Using NMR experiments, it was concluded that also in solution 137a–d have a bent perpendicular allenic structure [419]. The perpendicular allenic bonding character in 137c is supported by B3LYP-D3(BJ)/6-311G(d,p) calculations [431]. The optimized Si–Ge bond length is 2.256 Å and its WBI is 1.66, which is shorter than r(Si-Ge) in H2SiGeSiH2 of 2.346 Å having a WBI of 1.33 (Table 1.20), reflecting a higher contribution of double bond character in the Si=Ge bonds of 137c. The NBOs of 137c also exhibit a perpendicular allenic structure, as shown in Figure 1.28. Figure 1.28  Calculated NBOs of 2-germadisilaallene 137c, optimized at B3LYP(GD3BJ)/6-311G(d,p). BD and BD* denote bonding and antibonding orbitals, respectively. The polarity of the orbitals is given by the percentage of the electron density on Ge and Si. Occ is the occupancy of the orbital in electrons. Hydrogen atoms are omitted for clarity. Adapted with permission from Ref. [431]. Copyright (2019) American Chemical Society.

67

68

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

1.4.4.2.3  The E3R4 PES

The PES of Si3H4 studied at B3LYP/6-31G(d,p) is very complex revealing at least 16 isomers, many having hydrogenbridged structures. Eleven of these structures are within a range of only 11 kcal/mol (Scheme 1.15a) [427]. In contrast, the PES of C3H4 consists of only 6 isomers in a much larger range of 68 kcal/mol (Scheme 1.15b) [427]. The PES of Ge3H4 has not been studied yet, and it is an important computational study that should be carried out.

Scheme 1.15  Relative energies (ΔE) of singlet E3H4 isomers: (a) E=Si, at B3LYP/6-31G(d,p); (b) E=C, at B3LYP /6-31G(d). All structures are minima. Energies are from Ref. [427].

Trisilacyclopropylidene and other heavy group 14 trimetallacyclopropylidenes, 142 (E=Si, Ge) given in Table 1.20, are by only ca. 1–6 kcal/mol higher in energy than the corresponding open-chain parent metallaallenes, 141. The barriers for the transformation of 141 to 142 are 1.5–5.7 kcal/mol (Table 1.20). This implies that in practice 141 may undergo rapid rearrangements to 142 even at room temperature [427]. Due to their similar geometries and electronic structures, 141 and 142 (E1=E2=E3=Si), were suggested to be bond-stretch isomers, although their transformation requires in addition to bond stretching also rotation of the SiH2 fragments [427]. Veszprèmi and coworkers studied the PES of Si3H4 as a function of the SiSiSi bending angle. Isomeric structures of trisilaallenes were characterized by the range of the central bending angle α: α  >  100° represents sillaalenes having Si=Si double bonds and D2d (Scheme 1.16, a) or C2 symmetry (Scheme 1.16, b); in the range 80°  Si6H6 (15.6) ≈ Ge6H6 (15.3) [347]. So, planar sila- and germa-benzenes are aromatic, but less so than benzene. E6H6 ( D6h )  + 3H2E=EH2 → 3H2E=EH-EH=EH2 

(1.12)

Graphene, silicene and germanene are constructed from six-membered rings of C, Si, and Ge atoms, respectively, assembled in a honeycomb lattice. Graphene (CmHn) and its heavier group 14 element analogues silicene (SimHn) and germanene (GemHn), attract a lot of interest due to their unique electronic, mechanical and transport properties, and to their potential applications in electronics and optoelectronics [447–450]. These materials can also lead to novel chemistry [451]. Free standing silicene and germanene have not been synthesized yet, but recent studies reported their fabrication as monolayers on various substrate surfaces [450, 452–455]. Unlike graphene which has a planar geometry, silicene and germanene show puckering in each of the 6-membered rings throughout the lattice [445–448], similarly to hexasilabenzene and hexagermabenzene, which are the smallest building blocks of silicene and germanene. Such buckling makes hydrogenation facile, producing germanane, an ideal system for band gap tuning [445, 450]. Germanium-doped graphene, with Ge atoms incorporated in the six-membered carbon rings of graphene, has been achieved in a recent experiment and is a promising anode material for lithium-ion batteries [456]. 1.4.5.1.1.1  Isomers on the E6H6 PES

Unlike benzene, by far the most stable isomer on the (CH)6 PES, on the (EH)6, E=Si, Ge, Sn PES several other valence isomers, i.e., 146–148 exist, and they are more stable or have similar thermodynamic stability compared to the D6h (144) or D3d (145) E6H6 isomers. The stability of the non-benzoid 146–148 increases descending group 14 due to the increasing “hybridization defects” and the reluctance to participate in π bonding (Section 1.4.1) (Table 1.22) [44, 457, 458]. A variety of significantly lower energy cluster-like structures were located computationally on the Si6H6 PES [459]. Table 1.22 demonstrates that the most stable isomer on the E6H6 PES for E=Si, Ge, and Sn is prismane, 146, which does not have double bonds. The least stable is the Dewar benzene 148. The relative stability of the isomers may change by the electronic and steric effects of substituents (see below). At CCSD/pVDZ, the energy difference between benzvalene 147, E=Si, and prismane 146, E=Si, is negligible (Table 1.22). However, there is a large difference between the MP2/DZ(d) and the CCSD/pVDZ relative energies of 147, E=Si (Table 1.22). Hexasilaprismane, R = 2,6-diisopropylphenyl [463], hexagermaprismane, R = bis(trimethylsilyl)methyl [464], hexastannaprismane, R=tBu3Si [465], and cyclopentasilane-fused hexasilabenzvalene [466] were synthesized and isolated. Hexagerma- and hexasila-prismanes isomerized photochemically to the corresponding Dewar benzenes [458]. However, as of now, a D6h benzene type hexametallene was not isolated. For predicting the possible formation of a stable hexagermabenzene from digermynes, Vesprémi searched computationally for a substituent that can change the relative stability order of the isomers or even eliminate all other isomeric minima from the PES, and which is sufficiently bulky to protect hexagermabenzene kinetically, thus making hexagermabenzene a viable target for synthesis and isolation [467]. The effect of a large variety of substituents, including very bulky ones were benchmarked. The Gibbs free energy (calculated at B97-D/cc-pVTZ) of hexagermabenzene 145, R=CH(SiMe3)2, is by 2.9 kcal/mol higher than that for hexagermaprismane 146, R=CH(SiMe3)2, while the other isomers were not stable upon geometry optimization. However, only 146, R=CH(SiMe3)2, was synthesized. With a terphenyl 2,6-(3,5-Me2C6H3)2C6H3 substituent only hexagermabenzene remained on the PES, thus making it a suitable candidate for isolation. Using a similar

71

72

1  Computational and Theoretical Aspects of Structure and Bonding in Doubly Bonded Organogermanium Compounds

Table 1.22  Relative energies, ΔE, of valence (EH)6 isomers calculated at MP2/6-31G(d) for E=C [460] and at MP2/DZ(d) for E=Si, Ge, and Sn [461]. Values in parentheses give the energy of 145 relative to 144. For E=Si, the values in italics are at CCSD/cc-pVDZ [462]. R R E E R

E

R

E

R

R

R

E

R

R

R

E

R

E

E

146 (D3h) prismane

E (R=H)

E

R

E E

R

R E

E R

147 (C2v) benzvalene

R

R

E

E

R

144, 145 benzene

R E

E

E E

E E

R

R

E R

E E R

148 (C2v) Dewar benzene

ΔE (kcal/mol)

C

0.0

117.6

74.9

81.1

Si

0.0, 0.0

−8.1, −9.17

−2.0, −9.86

4.1, −0.23

Ge

0.0 (-9.1)

−13.5

−1.2

1.8

Sn

0.0 (-23.1)

−31.3

−11.0

−6.5

computational procedure, Vesprémi predicted that tetragermacyclobutadiene, R4Ge4, substituted by bulky fused ring substituents (R=Etind, see list of abbreviations), is a possible target for synthesis [467]. This prediction was realized recently by the synthesis and isolation of R4Ge4, 29, R=EMind by the group of Tamao (Table 1.2, and in section 1.4.5.2.3) [133]. The steric protection of the bulky substituents on Ge6R6 and Ge4R4 is presented by the schematic van der Waals structures shown in Figure 1.29 [467]. Attempting the synthesis of the missing hexasilabenzene, Schesckewitz [468, 469] synthesized and isolated a unique new hexasilabenzene isomer 149 (Scheme 1.18).

Scheme 1.18  Synthesis of hexasilabenzene isomer 149

The X-ray structure revealed a tricyclic chair-like puckered conformation with a central rhomboid Si4-ring and three types of Si atoms featuring three oxidation states, Si(II)R2, Si(I)R, and Si:(0). The NMR spectrum exhibited three δ29Si chemical shifts at 124.6, −84.8 and −89.3 ppm, which were assigned to Si(II)R2, Si(I)R, and Si:(0), respectively. The experimental data supported by the results of MO calculations suggest a cyclic delocalization of six electrons, two π and two σ electrons and a lone pair, over the central Si4 moiety, as expressed by resonance structures A ↔ B (Scheme 1.18). The electron delocalization implied in resonance structure C is supported by NICS(0) computations that indicate a strong diatropic ring current across the central Si4 fragment. The authors propose the term “dismutational aromaticity” for this unique form of aromaticity. Calculations (B3LYP/6-31G(d)) show that 149, R=Dip, is less stable than the hypothetical isomeric hexasilabenzene (145, R=Dip) and the synthesized hexasilaprismane by 4.3 and 11 kcal/mol, respectively [468].

1.4  Doubly Bonded Compounds

Figure 1.29  Calculated (B97-D/cc-pVTZ) schematic van der Waals representation of tetragermacyclobutadiene and hexagermabenzene. Adapted with permission from Ref. [467]. Copyright (2013) American Chemical Society.

Thermal, at 250°C, or photolytic rearrangement of the silicon scaffold of 149 [470] yields an aryl-substituted derivative of the calculated global minimum on the Si6H6 PES (Eq. (1.13) E’=Si) [459, 471]. R R



E

E

E' E'

R

E R

E

R

R

149, E = E’ = Si; 150, E = Si, E’ = Ge

∆ (E' = Si,Ge)

R

hν (E' = Si)

R

E' E E'

R E E

E R

R R

(1.13)

151, E = Si, E’ = Si, Ge

The germanium containing dismutational isomer 150, E=Si, E’=Ge, was synthesized, isolated and characterized by X-ray crystallography [472]. However, it rearranges, even at room temperature, to the propellane-type global minimum isomer 151 (Eq. 1.13). 150 (E=Si, E’=Sn) was not detected even at low temperatures, and it may be an intermediate in the formation of 151 (E=Si, E’=Sn) that was isolated and characterized spectroscopically [472]. 1.4.5.1.2 C6-nEnR6 , n 95%

Scheme 2.11  Synthesis and reactivity of germylborates, studied by Mochida et al.

Wagner et al. showed that trichlorosilylated germanide, [(Cl3Si)3Ge]-, could be easily obtained from GeCl4 and Cl3SiSiCl3/ [Et4N]Cl; the reaction proceeds through [GeCl3]- intermediate formation [29]. Germanide [(Cl3Si)3Ge]- gave stable 1:1 adducts with ECl3 (E = B, Ga), [(Cl3Si)3Ge–ECl3]-. In contrast, interaction with AlCl3 results in Cl- abstraction and enables isolation of (Cl3Si)4Ge (29Si NMR δ 3.8 ppm; d(Ge-Si)av 2.366 Å). Furthermore, [(Cl3Si)3Ge–GaCl3]- also reacts with excess GaCl3 with Cl- abstraction to yield dimeric [(Cl3Si)3Ge–Ga(Cl)(μ-Cl)]2 (d(Ge–Ga) 2.4071(6) Å). Formation of the B–Ge(IV) bond by formal redistribution of electron density was found by Roesky et al. by the action of t-BuLi on germylene based on the N,N-ligand (Scheme 2.12) [30]. Dipp N BH3 Ge N H Dipp

t -BuLi Et2O

Dipp N BH3 Li(OEt 2) 3 Ge N H 71% Dipp

Scheme 2.12  Germylborane studied by Roesky et al.

d(Ge-N) 1.875(4), 1.879(4) d(Ge-B) 2.016(8) 11B NMR δ - 43.68 ppm

107

108

2  Organogermanium Compounds of the Main Group Elements

Thus, in general, compounds with Ge–B bonds are usually obtained by salt metathesis reactions. According to XRD, the length of Ge–B bond is from 2.00 Å [30] to 2.18 Å [31]; the mean value is 2.08 Å. As a general rule for Main Group derivatives, introduction of sterically voluminous substituents results in an increase of bond length.

2.2.2  Organogermanium Compounds with Ge–Al Bond Nöth et al. [32] established only by NMR spectroscopy that Ph3GeLi reacted with AlCl3 in Et2O/THF/hexane resulting in an inseparable mixture of [(Ph3Ge)4-nAlCln]Li(THF)m. Furthermore, the germyl lithium reagent, Ph3GeLi, did not react with [AlMe3]2 [32, 33] unlike Ph3B (see Section 2.2.1). In contrast, Ph3GeLi reacts with H3Al∙NMe3 to give complexed lithium salts, germylaluminates, as main products (Scheme 2.13). In the case of organoaluminum chlorides, the type of product apparently depends on the order of the reagents’ mixing. Upon addition of Me2AlCl to Ph3GeLi, the corresponding germylaluminate complex, [(Ph3Ge)3AlMe]Li(OEt2)3, was formed in high yield [33]. On the contrary, reverse addition results in germylalane, Ph3GeAlMe2(OEt2), with low yield [32]. Only the presence of sterically voluminous substituents like Si(Bu-t)3 resulted in molecular germylalanes as the main products. The chemical shift δ in the 27Al NMR spectrum of (TMP)2AlGeMe2Si(Bu-t)3 is significantly shifted towards high field indicating a strong influence of the germanium atom. [(Ph3Ge)3AlMe]Li(OEt2)3 90%

d(Ge-Al)av 2.520(2) NMR δ 62 ppm

27Al

[Ph3GeAlH3]Li(THF) 4 27Al

67%

Me2AlCl

Me2AlCl

Et2O/hexane

Et2O/hexane

.

H3Al NMe3

Ph3GeLi

(reverse addition)

AlHCl2(THF)2

Et2O/THF

PhMe

NMR δ 109 ppm N

AlCl

LiGeMe2Si(Bu- t )3

2

[(Ph3Ge)3AlMe]Li(OEt 2)m +

THF

N

2

20%

d(Ge-Al) 2.515(1) 27Al NMR 160 ppm δ

[(Ph3Ge)3AlH]Li(THF)3 d(Ge-Al)av 2.540(2) 27 Al NMR δ 71 ppm AlGeMe2Si(Bu-t )3

Ph3GeAlMe2(OEt2)

27 Al 29

NMR δ 60 ppm Si NMR δ 23.3 ppm d(Ge-Al) 2.545(1)

(TMP)2AlCl Scheme 2.13  Germylalanes, studied by Nöth et al.

Power et al. studied the insertion of kinetically stabilized germylene Ar2Ge: (Ar = 2,6-Mes2C6H3) into the Al–C bond in AlMe3; germane Ar2Ge(Me)AlMe2 (d(Ge-Al) 2.485(5) Å) was formed [34]. Apparently, the reaction occurs through the initial formation of a Lewis acid-Lewis base adduct Ar2Ge∙AlMe3. In general, according to XRD, d(Ge-Al) varies within the range from 2.48 Å [34] to 2.55 Å [32]; the mean value is 2.53 Å.

2.2.3  Organogermanium Compounds with Ge–Ga Bond The reaction of sonochemically in situ prepared [“GaI”] with triphenylgermyl lithium yielded gallanes (Scheme 2.14) [35]. Ga Ph3GeGePh3

I2 ))), PhMe Li THF

["GaI"] [Ph3GeLi(THF)3]

- Ga - LiI*THF - Ge2Ph6

[(Ph3Ge)3GaI][Li(THF)2] 14% d(Ge-Ga)av 2.470(9)

+

[{(Ph3Ge) 3Ga} 2Ga][Li(THF)4] 2% d(Ge-Ga)av 2.470(9)

Scheme 2.14  Germylgallanes obtained from [“GaI”].

The action of the reducing agent [“GaI”] on germylene ArGeCl resulted in isolation of a Ga2(Ge(Cl/I)Ar)3 cluster with trigonal bipyramidal Ga2Ge3 framework (Scheme 2.15) [36]. The geometry at the Ge atom is highly distorted (the Ga–Ge–Ga angle is 76.23(3)°).

Scheme 2.15  Ga/Ge clusters obtained by Power et al.

2.3  Organogermanium Compounds with Group 14 Elements

The reaction of in situ formed [“GaI”] with (Me3Si)3GeLi∙3THF resulted in a cluster Ga22[Ge(SiMe3)3]8 (d(Ga–Ge) 2.430(2) Å, 2.428(2) Å, 2.429(3) Å, d(Ge–Si) 2.361(4)–2.384(5) Å) and the gallatetrahedrane Ga4R4 (d(Ge–Ga) 2.468(6) Å, 2.455(4) Å, d(Ge–Si) 2.35(2)–2.38(2) Å) [37]. Germylenes stabilized by the sterically voluminous groups, such as Ar2Ge: (Ar = 2,6-Mes2C6H3), inserted into the Ga–C bond in GaMe3 to give Ar2Ge(Me)GaMe2 (d(Ge–Ga) 2.4443(3) Å) [34]. For the synthesis of new germyl gallium derivatives, Baines et al. used a largely different method consisting of the interaction of digermene with anionic [NHGa]–. A compound with Ga–Ge bonding was formed, which then can be modified by the interaction with electrophiles (Scheme 2.16) [38].

Dipp Dipp N

Ga:

N Dipp

Dipp

K Mes2Ge=GeMes2 THF

N

MeI K

Ga GeMes2 N Dipp 60%

Me3SiCl

N

Ga Ge(Me)Mes2 N Dipp 97% Dipp N

Ga Ge(SiMe3)Mes2 N Dipp 67% d(Ge-Ga) av 2.4311(10)

Scheme 2.16  Germyl gallium derivatives, studied by Baines et al.

According to the XRD data, the Ge–Ga bond length varies within 2.40–2.50 Å [35] (with the mean value of 2.46 Å).

2.3  Organogermanium Compounds with Group 14 Elements The main subject of consideration in this section are organogermanium compounds with Group 14 elements (E = C, Si, Sn, Pb).

2.3.1  Organogermanium Compounds with Ge–C Bond The classical way of synthesis of compounds with Ge–C bonds includes the interaction between alkyl- or arylmetal (lithium, potassium, magnesium; mono- and dimetallic derivatives) nucleophiles with germanium electrophiles (usually, halides); the yields are high in general. The main drawback of this technique is the use of strong nucleophiles, which make it suitable for the synthesis of non-functionalized derivatives. Thus, in the presence of organic functional groups sensitive to the action of strong nucleophiles, it is required to use alternative methods (cross-coupling or further modification reactions). Unsymmetrical Grignard reagents, RMgHal, are usually used; applications of R2Mg to form Ge–C bonds are very rare [39], which is explained by the simplicity of RMgHal synthesis. At the same time, Uhl et al. elaborated a new method based on the action of Ar2Mg (Ar = Mes, 2,4,6-(Me2CH)3C6H2) reagents on GeCl4 [40]; this procedure is highly selective and makes it possible to obtain ArGeCl3 in high yields. Trace amounts of Mg present in the reaction mixture in the case of Grignard reagents can result in the so-called Ge–Ge Würtz coupling, therefore the best choice for selective introduction of an organic group to the Ge atom is application of stoichiometric amounts of RLi reagents. Typical examples of creation of the Ge–C bonds by organolithium or organomagnesium reagents are as follows: 1) the Ge–C bond in carboranyl derivatives [41, 42]; 2) the Ge–C bond in substituted cyclopentadienyl [43–46] or fluorenyl [47] compounds; 3) the Ge–CC≡C bond [48–50] including alkoxyacetylenes [51, 52], alkylacetylenes [40, 53], arylacetylenes [54–56], trimethylsilylacetylene [57], [–R2GeC≡CSiMe2C≡C(SiMe2C≡C–)n]2 (n = 0, 1) [58], branched dendrimers [59], germa[n] pericyclynes [–R2GeC≡C–]n (n = 4–6, 8, 10) and related compounds [60–64]; 4) the Ge–C(sp2) bond in substituted vinyl [65], phosphaalkene [66–68], nitrone [69], arsaalkene [70] derivatives; 5) the Ge–CAr bond [71–88] including acridan derivatives [89, 90], azobenzenes [91, 92], iminophosphoranes [93], phenols [94], molecular gyrotops [95], anthracenes [96], germoles [97, 98];

109

110

2  Organogermanium Compounds of the Main Group Elements

6) the Ge–CAr bond in heterocycles (substituted thiophenes [99–104], benzothiophenes [105], furans [73, 106], pyrazoles [107], bis(tetrathiafulvalenes) [108]; 7) the Ge–C bond in substituted ferrocenes [109–112], and related sandwich complexes [113–118]; 8) the Ge–C bond in half-sandwich (η6-Ar) complexes [119, 120]; 9) the Ge–CAlk bond [73, 121–126] including activated methylene [127–129], cyclopropyl [130], menthyl [131], benzyl, and related [132, 133] derivatives; 10) the Ge–CAllyl bond [134] including azapentadienyl [135] derivatives; 11) the Ge–C bond in fluorine-containing substituents (vinyl [136–138], aryl [139–142], alkyl [143–145], acetylenic [146] derivatives). Tacke et al. synthesized germyl amino acids in studies of C/Ge bioisosterism, using the formation of Ge–C bond by the salt metathesis reaction (Scheme 2.17) [147]. Different conditions of the ester hydrolysis (boiling aqueous HCl or LiOH, then aqueous HCl) are necessary because of the sensitivity of the phenyl derivative to acidic conditions. These amino acids were used to synthesize decapeptides.

N EtO

OEt

N

O 1) n-BuLi, THF 2) ClCH 2GeMe2 R H2 N OH 3) separation by LC 4) HCl, H2 O GeMe2R 5) Na2 CO3 (R)6) hydrolysis of R =Me, Ph the ester

O and

H 2N

OH GeMe2 R (S)R = Me

Scheme 2.17  Germanium-containing α-amino acids, studied by Tacke et al.

Subsequent alkylation and arylation of Cl3GeCH2Cl by different Grignard reagents (Alk-, Ph-, VinMgHal) with modification and resolution allowed Tacke et al. [148] to synthesize germanes with the chiral Ge atom. Dilithium [149–151] or dimagnesium [152, 153] carbon nucleophiles also react with germanium halides to give cyclic products. This method is widely used for the synthesis of substituted germoles [154] and related fused compounds (such as germafluorenes [155–161] including fluorine substituted compounds [162–164], dithienogermoles [79, 165–170], dithienogermolodithiophene [171], dipyridinogermoles [172], difuranogermoles [173, 174], and others [175–181]). These compounds or modified derivatives [182, 183] have efficient fluorescence properties with good quantum yields (up to 99% in solution) or can be used as components in solar cells [184, 185]. Tokitoh et al. investigated in detail the synthesis and properties of mono- and bis(germacyclopropa)benzenes [186–188] containing fused 3- and 6-membered rings. Such compounds were obtained by the action of gem-dilithiogermane, Tbt(Dipp)GeLi2 (Tbt = 2,4,6-[(Me3Si)2CH]C6H2) on o-dibromobenzenes. The dilithium compound Tbt(Dipp)GeLi2 also reacts with MeI to give Tbt(Dipp)GeMe2 [186]. Intermolecular Würtz synthesis (reaction of ArHal, Mg, and Alk3GeHal [189] or Ph2GeCl2 [190]) is still applied nowadays to synthesize organogermanium compounds, including introduction of several Ge atoms, but the yields are low [191]. Interesting examples of an intramolecular Würtz reaction between germanium halo and alkylbromo functional groups, mediated by Mg, are also described in the literature [149, 192]. One-pot synthesis under ultrasonic irradiation of a mixture of halogermanes, Mg, and alkyl halides (or vinyl, allyl, propargyl, aryl, benzyl halides) forming organogermanium compounds is also known [193]. Interestingly, in the case of propargyl bromide, the main product is germylallene, R3GeCH=C=CH2. A related rearranged allenic product was obtained by germylation of lithium derivatives of cyclopropene (Scheme 2.18) [194]. Me3Si Me3 Ge

CO2Et R

1) Me 3GeCl, THF 2) LDA/TMEDA

Me 3Si

CO 2Et R

1) LDA/TMEDA

EtO2 C

2) Me3 GeCl THF R=H

Me 3Si

GeMe3 GeMe3

Scheme 2.18  Organogermanium compounds obtained via metalation of 3-trimethylsilyl-3-ethoxycarbonylcyclopropene derivatives.

The heavier analogues of organolithium compounds can be also used to form Ge–C bonds (sodium derivatives of carboranes in the reaction with germanium halides [195]). As a rule, attempts to perform partial substitution of Hal atoms in GeHal4 (Hal = Cl, Br) by stoichiometric amounts of organomagnesium or organolithium compounds are rare, being usually successful in the case of voluminous substituents; otherwise, mixtures of differently substituted compounds are formed, separation of which is highly labour-intensive [131,

2.3  Organogermanium Compounds with Group 14 Elements

142, 196–198]. There are only a few reported cases in which a halogenated triarylgermane was obtained as a sole product in one step ((p-Tol)3GeCl [199, 200], (C6F5)3GeCl [141], (o-Ph2PC6H4)3GeCl [77], (3,5-(2,6-Me2C6H3)2C6H3)3GeCl [201]). Examples of monoalkylation or arylation of GeCl4 were reported using sterically voluminous groups, such as [2,4,6-(t-Bu)3C6H3]Li (Mes*Li) [202], tert-alkylmagnesium chlorides [197], (1R,2S,5R)-menthyl (Men) magnesium chloride [203], [2,6-Tip2C6H3]Li (Tip = 2,4,6-(i-Pr)3C6H2) [204], TbtLi [205], [(2-MeO–C6H3-4-R)3C]Li (R = F, CF3, OMe, t-Bu) [206, 207], [(2-MeO–C6H4)3C]Li [208]. A number of germanium monobromides can be obtained in one step by arylation of GeBr4 by ArMgBr reagents. The regioselectivity of this reaction (formation of a single product) is determined by application of sterically voluminous substituents, such as o-t-Bu-C6H4 [209]. The problem of unselective alkylation can be solved by using the strategy of protecting groups. In one of the examples, Uhl et al. described application of amides as protecting groups in Cl2Ge(NEt2)2 for the selective synthesis of dichloride (t-Bu)(t-BuCH2CH2)GeCl2 [210]. Thus, Cl is easily substituted by organolithium compounds and then amide groups are removed by HCl/Et2O [48]. Furthermore, in the case of partial alkylation or arylation, a redistribution of Hal atoms (using GeHal4 and RHal’ for generation of nucleophile) can be observed. This results in an inseparable mixture of compounds containing different Hal atoms [82, 142]. It should be noted that for complete alkylation or arylation of GeHal4 it is necessary to apply an excess of nucleophile reagent, and usually the use of 5 equiv. is sufficient, especially in the case of non-voluminous groups. Furthermore, Uhlig et al. showed that the type of substitution (for example, o-, m-, or p-Ar) plays a significant role for a number of introduced Ar groups [82]; introduction of voluminous substituents in o-position prevents full arylation; therefore, corresponding monohalides are formed. The germanium atom may contain substituents different from Alk or Ar, such as H [119, 211]. Furthermore, in RR’GeF2, especially in the presence of a sterically voluminous group (like t-Bu, Tip [66, 212], Mes [213]), it is possible to perform a single F substitution by organolithium nucleophiles. Formation of Ge–C bonds can be achieved by inversed polarity of reagents, i.e., by the reaction of carbon electrophiles (usually AlkHal) with alkali metal germyl reagents (lithium [214–217], potassium [218, 219], dilithium [220], or even ammonium [221]). Related arylation of germyl lithium compounds with thiophenes followed by oxidation is a rare example of Ge–Ar bond formation [79]. Organogermanium derivatives of other metals (Ge–M compounds) can be also used to form novel Ge–C bonds. Thus, the nickel complex of carboranes is used in such synthesis (Scheme 2.19) [222, 223].

Me 2 Ge

Me Ge 2

R

Ni Ge Me2

R

PhMe = BH =C

Ge Me 2

Me 2 Ge PEt3 PEt3

Y R

X

PhMe X

X Y

Ge Me2 Y = O=CH, N C, C C

R

Scheme 2.19  Application of Ni complexes in gemylation of alkynes and alkenes.

Germyl complexes of Pt upon the interaction with alkynes give cyclic digermanes (Scheme 2.20) [224]. Ph

R2 Ph Ge Ph Pt[PPh 3]2 C 6D6 Ge R2 R = Me, Et

R2 Ge

Ph

R2 Ge

Ph

+ Ge R2

Ph

Ge R2

Scheme 2.20  Application of Pt complexes in germyltion of alkynes.

There are several alternatives to lithium reagents known to date. The first group includes Zr derivatives, which are easily obtained using zirconation of alkenes or alkynes by Negishi reagent, [Cp2Zr] (generated in situ under the action of n-BuLi on Cp2ZrCl2), or its analogs [121, 154, 225, 226], with subsequent transmetalation by germanium halogenides (Scheme 2.21). In a similar way, the zirconium complex [Cp2Zr(η2-H2C=CH2)•PMe3] was used for ethylation of halogermanes; ClCH2CH2GePh3 was formed from ClGePh3 [227]. Intermediate [Cp2Zr(Cl)CH2CH2GePh3] was proposed.

111

112

2  Organogermanium Compounds of the Main Group Elements

R' R

R' GeCl4 R Zr Cp 2

THF

R' R

R' Ge Cl2

R

R, R' = Ar, Alk, SiMe3

Scheme 2.21  Application of Zr complexes in synthesis of halogermanes.

The second group of transmetalated reagents represents the derivatives of aluminum. Thus, Suzuki, Yamashita et al. applied aluminacyclohexadiene for synthesis of germanium chlorides, where reactions of transmetalation and oxidation of Ge(II) by germanium or aluminum chlorides occurred (Scheme 2.22) [228].

Scheme 2.22  Application of Al compounds in the synthesis of halogermanes.

The action of Bu3SnCH=CH2 on Me2GeCl2 in the presence of ACHN (1,1ʹ-azobis(cyclohexanecarbonitile)) yields H2C=CHGeMe2Cl [81, 229]. Alkynyl tin RC≡CSnEt3 compounds [230] can be electrophilically transmetalated by IGe(CF3)3 without a catalyst. Ethynyl rare earth derivatives, ((PhC≡C)2Yb(THF)n, (PhC≡C)3M(THF)) (n = 1, 4; M = Pr, Dy), react with Ph3GeCl to yield cross-coupled Ph3GeC≡CPh [231, 232]. Mochida et al. reported in situ radical alkylation of (Me3Si)3GeI by the Me2Zn (both compounds formed in situ upon the interaction of [(Me3Si)3Ge]2Zn with MeI) to give (Me3Si)3GeMe [233, 234]. In 2016, Nuckolls et al. applied p-MeS–C6H4– CH2ZnCl for benzylation of Me2GeCl2 [235]. A cadmium compound, Cd(CF3)2•glyme (glyme is DME, 1,2-dimethoxyethane), was used for alkylation of Ph3GeCl [236]. Certain difficulties emerging at the formation of C(sp3)–Ge bonds in Alk–GePh3 upon the action of germyllithium nucleophiles on alkyl halides AlkHal (including tertiary alkyls with their dehydrohalogenation) can be diminished by applying germyl zinc and magnesium reagents, Ph3GeMX (M = Zn, Mg) [237]. With germyl magnesium compounds, the reaction has a nucleophilic mechanism, whereas for Ni-catalyzed (NiBr2•diglyme, 10 mol.%) reactions with germyl zinc compounds a radical process is observed. An interesting example of alkylation of the germanium atom was reported by Sasamori, Tokitoh et al. It includes the successive action of MeLi and MeI on germylene Fc*2Ge: (Fc* = 2,5-bis(3,5-di-t-butylphenyl)-1-ferrocenyl]), which resulted in the formation of Fc*2GeMe2 [238]; the germylenoid Fc*2Ge(Me)Li was suggested to be an intermediate. The reactivity of a related germylenoid Li[Fc*GeCl2] (formed by the action of Fc*Li on :GeCl2•dioxane) is ambident, displaying reactions of dichlorogermyl anion in THF (with BuBr generating Fc*Ge(Bu)Cl2), whereas in toluene it behaves as a typical chlorogermylene (in reaction with 2,3-dimethylbutadiene giving a product of [4+1] cycloaddition; with BuBr giving Fc*Ge(Bu)ClBr, a rare compound with two different halogen atoms) [239]. Germanes PhGe(H)Cl2 and Ph3GeLi are added to the C=O group in a conjugated carbonyl compound (such as 2,6-diethyl4,8-dimethyl-1,5-dioxo-s-hydrindacene) giving expected Ge–C derivatives [240]. Germane Ph3GeH reacts in the same manner upon UV irradiation. Chernyshev et al. elaborated a gas phase (550 oC) reaction of GeCl4 with conjugated dienes in the presence of Si2Cl6 initiator. Five-membered cyclic germanes cyclo-[CH2CR=CRCH2]GeCl2 (from 1,3-butadiene, 2,3-dimethyl-1,3-butadiene) or cyclopentadiene with the GeCl3 group (from cyclopentadiene) were formed; apparently, [:GeCl2] (Scheme 2.23) is an intermediate [241]. The related interaction of GeCl4/Si2Cl6 with anisole or phenols yielded a mixture of compounds (mainly 1,1,3,3-tetrachloro-1,3-disila-2-oxaindane), also containing 1,1,3,3-tetrachloro-1,3-digerma-2-oxaindane. The proposed insertion of [:GeCl2] into the C–O bond is a key step [242].

Scheme 2.23  The formation :GeCl2 as an intermediate in gas reaction of GeCl4/Cl3SiSiCl3.

Despite the fact that the Müller-Rochow synthesis of the organogermanium chlorides has long been known, it is still applied nowadays. Thus, a six-membered 1,1,3,3,5,5-hexachloro-1,3,5-trigermacyclohexane, cyclo-[Cl2GeCH2]3 (d(Ge-Cl)

2.3  Organogermanium Compounds with Group 14 Elements

2.116(2), 2.156(2) Å), was obtained from metallic Ge, MeCl, using Cu catalyst [243]. Furthermore, the reaction of Ge, GeCl4 (germylene [:GeCl2] is proposed as an intermediate), and PhCl at 300 oC and 30  atm selectively gave PhGeCl3 [244]. Schlecht developed a selective direct synthesis of functionalized dialkylgermanes [X(CH2)n]2GeBr2 (X = CN, CO2Me, Cl; n = 2-4) from activated Ge* (generated in situ under reduction of :GeCl2•dioxane or :GeI2 by Li[Et3BH]) and X(CH2)nBr [245]. Schnepf et al. introduced [GeBr] (generated in situ by a co-condensation reaction under high vacuum at high temperature from Ge and HBr) as a reagent in modern chemistry, which is suitable for the synthesis of various organogermanium compounds. Thus, [GeBr] reacts with (2,6-(t-BuO)2C6H3)Li giving (2,6-(t-BuO)2C6H3)3GeBr (d(Ge-Br) 2.3791(6) Å) [246]. With acetone, the redox reaction of [GeBr] with condensation is observed giving hypercoordinated Br3GeCMe2CH2C(O)Me, where germanium atom is intramolecularly bonded with O atom [247]. For the synthesis of new derivatives, Weinert et al. [248] used the partial formal alkylation or arylation of GeO2, first developed by Corriu et al. Special interest in this method is explained by the chlorine-free protocol for organogermanium compounds, what can be regarded as an environmentally friendly technique. It consists of the synthesis of hypercoodinated germanates, K[Ge(OXO)3], under alcoholysis of GeO2 by 1,2-glycols (X = CH(Me)CH(Me)) with subsequent interaction with Grignard reagent. Final reduction by LiAlH4 makes it possible to obtain R3GeH (R = Ar, Alk) in preparative yields. Baines et al. used substituted catechols [249] as analogs of glycols. In this procedure, the synthesis of hypercoordinated Ge complexes, (3,5-(t-Bu)2C6H2-1,2-O2)2GeL2 (L2 = 2Py; 2 NMI, NMI = N-methyl-imidazole; TMEDA, TMEDA = N,N,N’,N’tetramethylethylenediamine), can also be achieved by application of the redox properties of the quinone/metallic Ge pair under conditions of mechanochemical synthesis. Anyway, complexed germanium alcoholates behave as pseudohalides; the subsequent reaction with Grignard reagents, RMgHal, gave R4Ge. Interestingly, changing the type of L ligand gives an opportunity to reduce the degree of alkylation of Ge atom. Functionalized derivatives of the R3Ge-CH2CN (R = Ar, Alk) type with reactive Ge–C bond, which can be intermediates in hydrogermolysis reactions, were obtained by Weinert et al. by the reaction of R3GeCl with LiCH2CN [124, 250]. These derivatives were obtaining in situ from R3GeNMe2 in MeCN in the hydrogermolysis reaction [250]. Under the treatment of Me2Ge(X)-C(Br)(SiMe3)2 by AlkLi (X=Hal, OPh), four-membered digermane, cyclo-[Me2GeC(SiMe3)2]2, is formed [251]; germene [Me2Ge=C(SiMe3)2] is proposed as an intermediate. Germanium alkoxides (usually, ethoxides or methoxides) can be used as an alternative to germanium halides. Examples of the synthesis of Ar2Ge(OR)2 [78, 252], ArGe(OR)3 [253], Ar3Ge(OR) [77, 95] (R = Me, Et) under the action of ArLi or the synthesis of Alk3Ge(OEt) under the action of AlkLi [254] have been described in the literature. Substituted diethoxygermole [255] or bis(methyltriptycil) diethoxygermane [256] were obtained under similar conditions. The action of t-BuLi on TipGe(OMe)3 [66] or substituted vinyl lithium reagents on TbtGe(OMe)3 [205] allows the substitution of one methoxy group. Furthermore, Zaitseva et al. described the action of strong nucleophiles, such as n-BuLi, on hypercoordinated germane XGe(OCH2CH2)3N (X = Br, Me3SiO) resulting in the full substitution at the Ge atom to give Ge(Bu-n)4 (61–65%) [257]. Formation of the Ge–C bond from germyl amides has been described in only several works. Thus, the action of n-BuLi on [Me2N]4Ge allows formation of [Me2N]3Ge(Bu-n) [258]. Ponomarev et al. [51] performed silylation and germylation of activated alkynes R3E–C≡C–OEt by Me3SiI or Me3GeBr, which gives substituted mono- and digermylketenes, R3E(Me3E’)C=C=O (E, E’ = Si, Ge; R = Alk, Ar). 1,3-Bisketenes RMeE[C(E’Me3)=C=O]2 [259] were obtained from RMeE[C≡COAlk]2 and Me3E’Hal (E, E’ = Ge, Si; Alk = Me, Et; Hal = I, Cl). An unusual alkynylation of Ge–Cl bond activated by Cl→AlR3 coordination was observed by Uhl et al. [40] (1,3-dyotropic rearrangement) (Scheme 2.24). Apparently, this reaction includes the formation of intermediate Ge cation, which undergoes t-Bu group migration.

Cl Ph

t-Bu t-Bu Cl Al

Al(t-Bu)2 ∆

Ge t-Bu t-Bu

PhMe

Ph

Ge t-Bu t-Bu

Cl Al t-Bu Ph

t-Bu Ph Ge

Ge t-Bu t-Bu

t-Bu

t-Bu

t-Bu

Al Cl t-Bu

t-Bu

t-Bu Ph Cl t-Bu Ge t-Bu Al Al t-Bu t-Bu Cl Ge t-Bu Ph t-Bu 69%

Scheme 2.24  Intramolecular Ge-C bond formation in Al complexes (1,3-dyotropic rearrangement).

Substitution of Hal atoms by CN in halogermanes may be performed by the action of Me3SiCN [121, 260]; removal of volatile Me3SiHal is the driving force of the interaction.

113

114

2  Organogermanium Compounds of the Main Group Elements

Hydrogermylation of alkenes and alkynes [261] attracts researchers’ attention to studying new catalysts and the mechanisms of the process. Thus, the activation of the Ge–H bond for regioselective endo-monohydrogermylation of norbornadiene (nbd) was achieved photochemically by the formation of the σ complex between Et3Ge-H and catalytic amounts of [M(CO)4(η4-diene)] (M = Mo, W; diene = cod, nbd) (μ-η2-H-Ge agostic bonding) [262, 263]. First-generation anti-Markovnikov Si- and Sn-centered dendrimers with terminal GePh3 groups were obtained by hydrogermylation of Si(CH=CH2)4 (under H2PtCl6 catalysis) and Sn(CH2CH2CH=CH2)4 (under AIBN action) [264]. Interestingly, the action of Ph3GeH on Sn(CH=CH2)4 or Sn(CH2CH=CH2)4 resulted not only in hydrogermylation but also in transmetalation giving Ph3Ge(CH2)2GePh3 and Ph3Ge(CH2)3GePh3, respectively. Gevorgyan et al. established highly diastereo- and regioselective Pt-catalyzed (PtCl2, 1 mol. %) additions of Et3GeH [265] to cyclopropenes giving substituted cyclopropanes. Hydrogermylation of the double bond is controlled by steric factors and proceeds from the most sterically available face. Regioselective β-hydrogermylation of Cl3-n(Me)nSiCH=CH2 (n  =  0–3) was studied [266]. Me3GeH was used under Pt-catalysis (H2PtCl6, 0.001–0.01 wt. %); HGeCl3•2Et2O was applied under catalyst-free conditions. In the latter case, the reaction has a mechanism of nucleophilic addition. The Marciniec group studied hydrogermylation under different conditions. Hydrogermylation of substituted alkenes with (n-Bu)3GeH was studied under Ru-catalyzed conditions (RuHCl(CO)(PCy3)2) [267]. Regioselective hydrogermylation was observed for Si-substituted derivatives, whereas for O- and C-substituted alkenes the corresponding vinylgermanes (mainly, E-) were obtained in the course of hydrogermylation-dehydrogenative germylation reactions. Furthermore, Ru-catalyzed (RuHCl(CO)(PR3)2; R = Cy, Ph) vinylgermylation [81, 267] of olefins was also catalyzed under the same conditions (Scheme 2.25). Both cases are catalyzed by active M-H and M-Ge species with cleavage of =C–H in alkenes and =C–Ge, H–Ge bonds of germanes. (n-Bu)3GeH R RuHCl(CO)(PCy ) (1 mol.%) (n-Bu)3Ge 3 2 PhMe GeR3 R' RuHCl(CO)(PCy ) (2 mol.%) 3 2 PhMe - H2C=CH2

R3Ge

R +

R' +

R + ( n-Bu) 3Ge Ge( n-Bu)3

R

R' GeR3

Scheme 2.25  Hydrogermylation of substituted alkenes catalyzed by Ru complexes.

Rhodium- (2  mol. % [Rh(GePh3)(PEt3)3]) [268] or Al-catalyzed (AlClxF3-x, x  =  0.05–0.3) [269] hydrogermylation of H2C=CH(CF3) by Ph3GeH gave expected β-addition product Ph3GeCH2CH2CF3, whereas the related reaction with Alk3GeH resulted in a mixture of Alk3GeCH2CH2CF3 and H3CCH2CF3 [268]. Reactions of R3GeH (R  =  Ph, Alk) with H2C=C(F)CF3 (10 mol. % [Rh(GePh3)(PEt3)3]) resulted in a mixture of (Z)-F3CCH=CH(GeR3) and F3CCH=CH2 [270]. It was found that thermally-induced radical hydrogermylation of H2C=CH(CH2)8CO2Me by Bu3GeH without initiators resulted in Bu3GeCH2CH2(CH2)8CO2Me [271]. Germanes R2Ge(H)Cl (in the presence of AIBN, radical mechanism) can be added to H2C=CH(OEt) giving R2Ge(Cl)CH2CH2OEt [250]. Germane Bu2Ge(H)Cl was used for β-germylation of alkenes in the presence of Et3B/dry air as a radical initiator (Scheme 2.26) [272]. Interestingly, Bu3GeH was less active under standard conditions. Germylation of diene allowed obtaining a cyclic compound with high diastereoselectivity. R R

Bu2Ge(H)Cl Et 3B (0.05 eq.)/air THF

BuLi H

Ge

chair -equatorial

R R

GeBu3 R=H cis- /tr ans- = 3:1

Scheme 2.26  Homolytic hydrogermylation of alkenes with dibutylchlorogermane.

Furthermore, in polyfunctional α,β-unsaturated compounds carbonyl groups are hydrogermylated predominantly (1,4-addition) in the presence of an unconjugated alkene group [273] (Scheme 2.27). Moreover, in the case of unconjugated unsaturated carbonyl compounds, the C=C bond with the absence of steric hindrance at the double bond takes part in radical hydrogermylation in which halogermanes are more reactive than triorganogermanes. Hydrogermylation of alkynes was catalyzed by transition metal complexes. Triethylgermane at Ru-catalyzed ([Cp*Ru(NCMe)3]PF6) hydrogermylation was trans-added to alkynes [274]. Under the same conditions, the reaction with 1,3-diynes yields 2,5-disubstituted germoles; related 2,2ʹ-bigermoles were also obtained from tetrayne (Scheme 2.28).

2.3  Organogermanium Compounds with Group 14 Elements

Ph 2Ge(H)X cat. cat = AIBN, PhCO 2OBu-t , (t -BuO)2 X = Cl, Ph

OGe(X)Ph 2 69 - 70%

O Ph 2Ge(H)X cat. Ge(X)Ph2

97 - 99%

Scheme 2.27  Radical hydrogermylation of functional ethylenic compounds.

Ph

Ph

Ph

Ph2GeH2 ClCH2CH2Cl [Cp*Ru(NCMe)3]PF6 (20 mol.%)

Ph

Ge Ge Ph

Ph

Ph Ph

56%

Scheme 2.28  Ruthenium-catalyzed hydrogermylation of polyalkynes.

Nakazawa et al. [275] reported Fe-catalyzed (CpFe(H)(CO)GeR3 or CpFe(CO)2Me, 7 mol. %) trans-hydrogermylation of terminal and internal alkynes by Alk3GeH. Terminal alkynes, RC≡CH, were selectively (E)-hydrogermylated by (n-Bu)3GeH under visible light irradiation in the presence of Mn2(CO)10 (10 mol. %) [276] giving RCH=CH(Ge(Bu-n)3). For terminal alkynes, the Rh-catalyzed hydrogermylation (5  mol. % RhCl(CO)(PPh3)2) selectively resulted in (E)-vinylgermanes (syn-addition) [277, 278]. In the case of Pd complexes, significant amounts of α-hydrogermylation of unhindered alkynes were formed in addition to major typical β-vinylgermanes (Scheme 2.29) [279].

(n-Bu)3GeH R Pd(PPh3)4 (3 mol.%) R = Alk, (CR'2) nOH THF

(n-Bu)3Ge E-

R

+

( n-Bu) 3Ge

R

Scheme 2.29  Pd-catalyzed hydrogermylation of terminal alkynes.

Fürstner et al. [280–282] found that regioselective trans-hydrogermylation (as distinguished from canonical cis-addition) of propargyl alcohols RC≡CCH(OH)R’, using Ru (1-5  mol. % of [Cp*Ru(NCMe)3]4PF6 or [Cp*RuCl]4) catalyst is determined by interligand Ru-Cl•••Ge interactions with inclusion of O–H•••Cl contacts (hydrogen bonding); α-substituted derivatives (Z)-RCH=C(GeR”3)CH(OH)R’ were formed. Fluorine-containing germanium hydrides can catalytically hydrogermylate alkynes; Pt (Karstedt’s catalyst or Pt(acac)2) and Pd (Pd(acac)2) catalysts are usually applied in these reactions [221]. Under catalyst-free conditions, the β-cis product was formed (Scheme 2.30). (C2 F5 ) 3Ge Ph

(C 2F5 )3 Ge

+

(F5C 2 )3 GeH

Ph Ph

1 mol % Pd Et2O, rt, 19 h

Ph

(F5 C2 )3 GeH rt up to 140 oC 6d

Ph Z-

Ge(C 2F5 )3

Scheme 2.30  Hydrogermylation of alkynes by (F5C2)3GeH.

Radical hydrogermylation (by AIBN or ACHN, 1,1ʹ-azobis(cyclohexanecarbonitrile) [283]; by Mn compound Li[Mn(Bu-n)3] [284]) of alkynes by Ge-H compounds was widely studied. In this case, regioselective β-substituted products with predominant (Z)-configuration were obtained. Hydrogermylation of RC≡CCF3 [285] by Ph3GeH under radical ((NH4)2S2O8; selectively (Z)-diastereomer; antiaddition) or transition metal-catalyzed (Pd(PPh3)4, 1 mol. %; selectively (E)-diastereomer; syn-addition) conditions gave α-CF3-vinylgermanes, RCH=C(CF3)GePh3 [286]. Regio- and stereoselective radical germylzincation of α-heteroatom substituted alkynes was developed by Perez-Luna et al. (Scheme 2.31) [287].

115

116

2  Organogermanium Compounds of the Main Group Elements

R'

Et2Zn R3GeH

X

[Zn] R3Ge

THF

radical germylzincation

E

E+

R3Ge

X R'

X = [N], [S], [O], [P] E = [C], [S], Hal, CF3

X R'

Scheme 2.31  Germylzincation of α-heteroatom substituted alkynes.

Enantioselective hydrogermylation of methyl methacrylate, H2C=C(Me)CO2Me, by chiral germanium bisthiophelolate (analogs of BINOL) [(RS)2]Ge(R’)H in the presence of AIBN or Et3B gave typical [(RS)2]Ge(R’)CH2CH(Me)CO2Me (where (RSH)2 = 2,2ʹ-dithiobinap, 3,3ʹ-(Me3Si)2-2,2ʹ-dithiobinap) with good diastereoselectivity, which rises with increase of the steric volume of substituents in a ligand [288, 289]. Hydrogermylation of fatty acids by PhnGeH4-n under the action of AIBN, resulting in terminal addition, was also studied [290]. In contrast, addition of highly reactive HGeCl3 to α,βunsaturated acids (acrylic, methacrylic [291], trans-glutaconic, itaconic, fumaric, citraconic [292]) took place under mild conditions without any catalysts to give products of 1,4-Michael addition. Hydrogermylation of alkynes R’C≡CR” by R3GeH is also catalyzed by Lewis acids, such as (C6F5)3B (5 mol. %) [293]. In this case, selective trans-addition was observed for Alk and Ar derivatives, resulting in (Z)-vinylgermanes; on the contrary, cis-hydrogermylation is observed for propiolates. Apparently, the ate-complex, [R3Ge]+[HB(C6F5)3]-, and the vinylgermyl cation, [R’C+=C(GeR3)R”], are trans-hydrogermylation intermediates. For activated alkynes, R’C≡CCO2R”, the zwitterionic complex [R’C+=C=C(OR”)O-(B(C6F5)3)–] and allenolate [R’(H)C=C=C(OR”)O-(B(C6F5)3)–][+GeR3] are possible intermediates. It should be noted that Rubinsztajn et al. found that B(C6F5)3 reacts with Et3Ge-H to give a mixture of compounds containing Et3GeC6F5 [294]. Various cyclohexa-2,5-dien-1-yl-substituted germanes can generate Ge-H synthons after treatment with catalytic amounts of B(C6F5)3. In the presence of alkynes or alkenes, these cascade reactions resulted in hydrogermylation [295]. Ohshima et al. investigated Fu3GeH (Fu  =  2-furyl) [106] as an activated germylating agent in hydrogermylation of internal alkenes, dienes, silyl enol ethers in the presence of Et3B (10 mol. %) [296]. In this case, it was proposed that traces of O2 resulted in radical addition. Interestingly, interaction with an excess of terminal alkynes RC≡CH [297] in the presence of Pd catalyst resulted selectively in dienylgermanes, (E,Z)-RCH=CHC(R)=CHGeFu3. Pd-catalyzed hydrogermylation of alkynes makes it possible to perform several reactions in one pot. Thus, terminal alkynes in reactions with Fu3GeH under an atmosphere of CO gave corresponding α,β-unsaturated acylgermanes in good yields (Scheme 2.32) [298].

O

R CO (1 atm), PhMe GeH [PdCl(η 3-C3H5)]2 (2.5 mol.%) 3 P(OC6H3(Bu-t)-2-4) 3 (10 mol.%)

R O

3

Ge

O

58 - 83%

Scheme 2.32  Pd-catalyzed hydrogermacarbonylation of alkynes.

Ligand-controlled Pd-catalyzed regiodivergent hydrogermylation of ynamides was also elaborated (Scheme 2.33) [299]. α,E- or β,E-Germylated enamides are selectively formed depending on the steric volume of the phosphine ligand used for stabilization of catalytically active Pd species. GePh 3 N CO2Me

MeO OMe α, E

Ph3GeH Pd(OAc)2 (1 mol.%) DPEphos (1.1 mol.%) THF

MeO N MeO

Ph3Ge

Ph3GeH

CO2Me

N CO2Me

.

Pd(dba)2 (1 mol.%) (t-Bu)2PMe HBF4 (4 mol.%) MeO Cs2CO3, TH F

42%

OMe β, E

Scheme 2.33  Pd-catalyzed hydrogermylation of ynamides.

Thiphenylgermane, Ph3GeH, was radically added to vinyloxiranes in the presence of Et3B (Scheme 2.34) [300, 301]. R O

Ph3GeH Et3B hexane

OH Ph3Ge

R 73 - 96%

Scheme 2.34  Addition of triphenylgermane to vinyloxiranes.

OR O

Ph3GeH Et3B hexane

O Ph3Ge

H 18 - 71%

73%

2.3  Organogermanium Compounds with Group 14 Elements

There are several methods of vinylation of germanes, similar to hydrogermylation of alkynes. Thus, the radical-mediated action of germanes on vinylsulfones gave vinylgermanes (Scheme 2.35) [302, 303]. Interestingly, this desulfurization reaction occurred with retention or enhancement of (E)-stereochemistry in contrast to the classical radical hydrogermylation of alkynes under similar conditions, giving β-(Z)-vinylgermanes [304].

Ph

O

X R'

S

R''

R3GeH R3Ge

AIBN PhMe X = H, F R = SiMe3, Ph, Fu, n-Bu R', R'' = H, Alk, Ar SO2Ph

R''

O

R (Me3Si)3GeH

Ge(SiMe3)3

EAIBN PhMe R AIBN C6H6

R' X

E-

R

Ge(SiMe3)3 Z-

R

Scheme 2.35  Radical-mediated germyldesulfonylation of vinyl sulfones; different stereoselectivity observed upon changing reagents.

Pd(0)-Catalyzed cross-coupling of arylbromides with arylgermanes containing at least two labile heteroatoms (usually, halides) at Ge is observed under the action of KF. Apparently, the possible intermediate includes a pentacoordinated species [73]. Furthermore, (Z)-alkenylgermanes can be selectively obtained by hydrogermylation of alkynes using ultrasonic and microwave irradiation without any catalyst or initiator [305]. Germane Et3GeH was added regio- and stereoselectively to antiaromatic pentaphenylborole without any catalyst, giving a product of formal 1,4-syn-addition (Scheme 2.36) [306]. Apparently, the key intermediate in this interaction includes Ge-H•••B bonding. Ph B

Ph Ph

Ph Ph

H

Et3GeH

Ph GeEt3 B

Ph

CH2Cl2

Ph

Ph

Ph

83%

Scheme 2.36  1,4-syn-Addition of germane Et3GeH to pentaphenylborole.

Application of germanium frustrated Lewis pair Ph2PN(Dipp)GeMe3 for germylation of alkynes was studied by Zhu et al. (Scheme 2.37) [307]; (Z)-vinylgermanes were obtained.

Dipp N

PPh2 GeMe3

MeO2C

R

R

PhMe

Dipp

CO 2Me

N PPh2 GeMe3 Z-

Scheme 2.37  Application of germanium frustrated Lewis pair for germylation of alkynes.

Oshima et al. obtained arylated or vinylated tris(furyl)germanes [308] by Pd-catalyzed cross-coupling reactions from hydrides (Scheme 2.38); vinyl halides are also involved in this interaction. These tetraarylgermanes could then be used as a source of aryl groups in coupling reactions, but in that case the Ge-containing portion goes to waste.

O

ArI GeH Pd(OAc) (5 mol. %) 2 3 dppf (5 mol. %) Cs2CO 3, DMF

O

3

GeAr

54 - 88%

Ar'I Pd2(dba)3, (2-Fu)3P n-Bu4NF

Ar-Ar'

Scheme 2.38  Synthesis of arylated tris(furyl)germanes by Pd-catalyzed cross-coupling reactions and their application.

117

118

2  Organogermanium Compounds of the Main Group Elements

Later on, Nishihara et al. performed similar cross-coupling reactions using Pd[P(Bu-t)3]2/DABCO (DABCO = 1,4-diazabicyclo[2.2.2]octane) but expanding the number of substituted compounds to include aryl iodides and germanes R4-nGeHn (n = 1–3) [161, 309]. For aryl iodides containing OH, NH2, CN, CO2R groups, the yields of the products are high. Single and multiple arylation of secondary and primary germanes is also possible. Similar conditions were applied for the synthesis of functionalized thienyl (Th) derivatives [310]. Substituted germanium hydrides, R4-nGe(CH2R’)n (n = 1–3), were obtained from R4-nGeHn using R’CHI2/Sm (R = Ar, Alk; R’ = H, Alk) [311]; the reaction occurs by insertion of Sm carbenoids into Ge–H bonds in good to high yields. In 2000, Mochida et al. described a Pd-catalyzed insertion of alkynes into the Ge–Ge bond in 3,4-benzo-1,2-germacyclobut-2-ene and its dimer (Scheme 2.39) [312]; Pd compounds with Pd–Ge bonds were intermediates.

GeEt 2 GeEt 2

Et2 Ge

Ph

Ph

Ph

Ge Et2

Pd(PPh 3) 4

Pd(PPh 3) 4

Et2 Ge

Et2 Ge

Ge Et2

Ge Et2

Scheme 2.39  Insertion of alkynes into the Ge–Ge bonds, studied by Mochida et al.

In 2001, Mochida et al. developed Pt-catalyzed ([Pt(acac)2], Pt(dba)2) (Z)-bis-germylation of alkynes with organodigermanes and cyclic oligogermanes (Scheme 2.40) [313]. Complexes with Pt–Ge bonds are intermediates in this reaction. R

R'

Me2 Ge GeMe2 Me 2Ge GeMe2 Me2 Ge GeMe2

R'

R

Me3GeGeMe 3

Me3 Ge

R, R' = H, Alk, Ar, SiMe3 , CO2Me GeMe 3 (Z)Ph Ph

Me2 Ge

GeMe2

120 oC 5 mol. % [Pt(acac) 2] Ph 120 oC 5 mol. % [Pt(acac) 2 ]

+

Me2Ge

GeMe2

Ph

Ph

Scheme 2.40  Pt-catalyzed bis-germylation of alkynes with organodigermanes and cyclic oligogermanes.

Navarro et al. described cis-addition of Me3GeGeMe3 to tolan, catalyzed by (IMe4)2Pd(η2-PhC≡CPh) (1 mol. %; IMe4 = 1,3,4,5tetramethyl-imidazol-2-ylidene), to give (Z)-Me3Ge(Ph)C=C(Ph)GeMe3 [314]. In 2005, Mochida et al. reported the insertion reaction of PhC≡CH into the Ge–Ge bond in ClMe2Ge–GeMe2Cl in the presence of Pd(dba)2 and 4-ethyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane. Moreover, metal-catalyzed (Pd[PPh3]4, Pt[PPh3]4) insertion of alkynes into 1,2-digermacyclobut-3-enes was found (Scheme 2.41) [315]. Ph

GeR2

R'

R''

R2 Ge

Ph

[Pd] or [Pt] C 6H 6

GeR2

R'

Ge R2

+

R2 Ge

Ph

Ge R2

R''

R''

R = Alk R', R'' = H, Alk, Ar, CO2Me

R'

Scheme 2.41  Metal-catalyzed insertion of alkynes into 1,2-digermacyclobut-3-enes.

1,2-Digermacyclohexa-3,5-diene and its silicon analog react with tetracyanoethylene (TCNE) to give mixtures of products including bicyclic germanes (Scheme 2.42) [316]; radical intermediates are proposed. Ph GeMe 2 O GeMe 2

Ph Ph

+

Me 2 Ge Ph Ph CN CN

Ph Ph CN Me 2 CN 9% Ge CN Ph Ph

Ph 16%

+ Ph

Ph Ge N Me2

Ph Ph

GeMe2 Ph CH2Cl2

CN NH 2

Ph Ph

GeMe2

Ph

3% d(Ge-N) 1.927(4)

Me2 Ge Ph Ph CN CN

NC

CN

NC

CN

GeMe 2 SiMe 2

Ph Ph

CH 2Cl2

Ph

Ph 15%

Ph

+

CN CN

+

Ph Ph

+ O GeMe2

Ph Ph Me 2 23% Me2Ge Si NH2 N Ph Ph

Ph NC

Ph

SiMe 2

CN

Ph

Ph 5%

4%

Scheme 2.42  Interaction of 1,2-digermacyclohexa-3,5-diene and its silicon analog with tetracyanoethylene.

Me2 Ge Ph

N Si Me2

CN CN NH 2

2.3  Organogermanium Compounds with Group 14 Elements

Highly regio-, stereo- and chemoselective carbogermanylation of allenes was observed under Pd-catalyzed conditions using Me3GeSnBu3 (Scheme 2.43) [317]. Furthermore, germylstannanes R3GeSnBu3 are used for germanylation of allyl halides under Pd catalysis (0.5 mol. % Pd2(dba)3•CHCl3) [318]. Besides, cross-coupling of Et3GeSnBu3 with RC≡CH under Pd-catalyzed conditions (Pd(dba)2; P(OCH2)3CEt) resulted in (Z)-RC(SnBu3)=CHGeEt3 [319, 320]. Ar

ArI Me3GeSnBu3

R'

Pd(dba)2(5mol.%) PhMe

R'

GeMe3 ( Z )- 82-90%

Scheme 2.43  Pd-catalyzed cross-coupling of Et3GeSnBu3 with RC≡CH.

Synthesis of substituted allylgermanes was developed by Kabalka et al. at Pd-catalyzed regio- and stereoselective crosscoupling of Baylis-Hillman adducts with digermanes (Scheme 2.44) [321]. OAc O R

O

Me3GeGeMe3 R'

Pd2(dba)3 (4 mol.%) PhMe

R

R'

(Z)-

GeMe3

R = Ar, Alk; R' = OMe, Me

64 - 85%

Scheme 2.44  Synthesis of substituted allylgermanes at Pd-catalyzed cross-coupling of Baylis-Hillman adducts with hexamethyldigermane.

Kanai et al. studied the formation of new Ge–C bonds by Pd-catalyzed C(sp2)-H ortho-activation of benzamides using a directing group followed by coupling with digermanes (Scheme 2.45) [322]. These coupling reactions admit a wide variety of functional groups, an alternative to organogermanes obtained by classical lithium or magnesium methods. 8-Aminoquinoline is a bidentate directing group for Pd coordination that can be easily removed. Silver carbonate is used as an oxidizing agent for Pd. O

O Me3GeGeMe3

N

N

Pd(OAc)2 (10 mol.%) Ag2CO3, CaSO4 R dioxane

N

GeMe3

N

R 40 - 70%

Scheme 2.45  Palladium-catalyzed C–H germanylation of benzamides.

Related Pd(II)-catalyzed regioselective γ-C(sp3)-H germylation of carboxylic acids by Me3GeGeMe3 in the presence of an auxiliary 8-aminoquinolinic directing group was studied by Maiti et al. (Scheme 2.48) [323]. The directing group is necessary to stabilize the six-membered palladacycle. O α

NHQ β

R' Q=

γ

R''

Me3GeGeMe3 Pd(OPiv) 2 (10 mol.%) 2-chloroquinoline (20 mol.%) Ag2CO 3, NaHCO3 t-BuOH

O

NHQ α

β

O

γ

R''

GeMe3

R' 51 - 69%

N α N Pd γ β R' L R''

N

Scheme 2.46  Pd-catalyzed γ-C(sp3)-H germylation of carboxylic acids in the presence of an auxiliary 8-aminoquinolinic directing group.

Similar Pd(II)-catalyzed C(sp3)-H germylation of α-amino acids was also described (Scheme 2.47) [324]; related bidentate 8-aminoquinolinic directing auxiliary groups were used. Thus, β-germyl-α-amino acids were obtained; the diastereomeric ratio is up to 20 : 1. The proposed reaction mechanism includes C-H activation, oxidative addition, and reductive elimination steps.

119

120

2  Organogermanium Compounds of the Main Group Elements Me3GeGeMe3 R α C(O)NHR'' β

R'

C-H activation

Pd(OAc) 2 O

Me3GeGeMe3 oxidative addition

N Pd N

R

BINA-PO2H (30 mol.%) Pd(OAc)2 (10 mol.%) Ag2CO3, NaHCO3, LiOAc PhMe

L

O R

O

N Pd N GeMe3

Me3Ge

R α C(O)NHR'' R = NPhth; R' = H, Alk β R'' = 8-quinolinyl R' GeMe3 36 - 76%

r eductive elimination

R

N Pd N

Me3Ge L GeMe 3

Scheme 2.47  Pd(II)-catalyzed C(sp3)-H germylation of α-amino acids.

In 2019, Shi et al. briefly described the formation of the γ-C(sp3)-Ge bond in amino acids (Scheme 2.48) [325]; quinone-type ligands and a picolinamide auxiliary are critical for the reaction. NHC(O)(Py-2) R γ H

Me3 GeGeMe3 Pd(OAc) 2 (10 mol.%) dichlone Ag 2CO3 , KHF2 DCE

CO2 R'

O

NHC(O)(Py-2) R

Cl CO2 R'

dichlone Cl

GeMe3

O

Scheme 2.48  Synthesis of Ge-containing peptides via Pd-catalyzed γ-C(sp3)–H germylation.

Zhao et al. used Pd-catalyzed germylation of oxalyl derivatives of benzylamines by Me3GeGeMe3 (Scheme 2.49) [326]; ­oxalyl amide is a directing group for ortho-metalation. R'

O N H

R

N(Pr-i)2 O

Me3GeGeMe3 Pd(OAc)2 (5 mol.%) AgOAc, K2CO3 1,4-dioxane

R'

Me3Ge

O N H

R

N(Pr-i)2 O

37 - 82%

Scheme 2.49  Pd-catalyzed germylation of oxalyl derivatives of benzylamines by Me3GeGeMe3.

A number of works concern the formation of Ge–C bonds by the cleavage of Ge–Ge bond during cross-coupling. In contrast to the above-mentioned works, the next studies are atom-economical and utilize both Ge atoms. Thus, a Pd-catalyzed three-component reaction of ArI, norbornene, and Me3GeGeMe3 (Scheme 2.50) [327] gave bisgermylated derivatives (Catellani-type reaction). Related (Z)-β-substituted vinylgermanes were obtained in the case of 2,3-dicarbomethoxy-7-oxanorbornadiene, used as a “source” of the vinyl group and undergo the retro-Diels-Alder reaction [328].

Scheme 2.50  Pd-catalyzed Catellani-type bis-germanylation of aryl iodides and norbornenes.

Bis-germylation with cyclization was developed for aryl halides containing tethered alkene under Pd catalysis (Scheme 2.51) [329].

2.3  Organogermanium Compounds with Group 14 Elements

Pd

I X

Y

X, Y = CH2, O, N(R)

Pd(OAc)2 (3 mol.%) PPh3 (6 mol.%) K3 PO4 (1.5 eq.), DMF

GeMe3

Me3Ge

Me3GeGeMe3 X

Y X

Y 87 - 94%

Scheme 2.51  Palladium-catalyzed digermanylation of alkene tethered aryl halides.

In contrast to the above-mentioned ortho-metalation, Maiti et al. developed Pd-catalyzed meta-germylation of C–H bonds in arenes by the nitrile-containing benzylsulfonate ester directing group (DG) by Me3GeGeMe3 (Scheme 2.52) [330]. An excess of silver(I) carbonate is necessary to bind AcOH, regenerating the Pd(II) active species. O S GD H

O

Me3GeGeMe3

O

Pd(OAc)2 (10 mol.%) Ac-Gly-OH (20 mol.%) Ag2CO3, Na2SO4, (F3C)2CHOH

S GD GeMe3

O DG =

42 - 92%

O NC

Scheme 2.52  Pd-catalyzed remote meta-selective C–H bond germanylation of arenes.

Germylation of aryl bromides and aryl triflates using Me3GeGeMe3 in the presence of Pd catalysts (10 mol. % Pd(OAc)2; 20 mol. % o-C6H4(OH)PPh2) gave ArGeMe3 [331]. Synthesis of acylgermanes by the Pd-catalyzed carbonylative reaction of ArI with Me3GeGeMe3 was studied in detail (Scheme 2.53) [332].

Scheme 2.53  Pd-catalyzed carbonylative germylation of ArI with Me3GeGeMe3.

Special attention was paid to catalytic (Ni, Rh, Pd) C–H bond germylation, which takes place intra- or intermolecularly. Thus, regioselectivity of nickel(0) catalytic C-H germylation or hydroarylation of fluoroarenes depends on the steric size of NHC ligands. The most voluminous IPr NHC led to selective hydroarylation, while others (IMes, IBn, iPr2Im) gave germylation products (Scheme 2.54) [333]. F F

F

GePh3

F

F

GePh3

F 5 mol.% Ni(cod)2 + NHC F F F PhMe germylation NHC=IMes, IBn, iPr2Im

F

F

GePh3 5 mol.% Ni(cod)2 + NHC PhMe NHC = IPr

F

F

GePh3 F F hydroarylation

Scheme 2.54  Influence of N-heterocyclic carbene steric bulk on selectivity in Ni-catalyzed C–H bond germylation.

Effective C(sp2)-H intramolecular germylation of arenes without auxiliary groups resulting in substituted 9-germafluorenes is achieved by application of available Rh catalysts (e.g., Wilkinson catalyst, [RhCl(PPh3)3], or [RhCl(cod)]2 with additional ligand), as performed by Murai et al. (Scheme 2.55) [334, 335] and others [310, 336]. In this variant, there is no need to apply oxidant agents, usually required in catalytic dehydrogenative coupling. There are two possible mechanisms. The first includes oxidative addition of Ge–H to the Rh(I)–H bond followed by C(sp2)–H bond activation via σ complexassisted metathesis (σ-CAM) with H2 elimination and reductive elimination of Rh(I)-H species. The second consists of the reductive elimination of H2, C–H bond cleavage and reductive elimination. Murai et al. also developed intramolecular Rh-catalyzed dehydrogenative C(sp3)–H bond germylation (Scheme 2.56) [337]. Application of bulky and electron-rich diphosphine ligands enabled effective synthesis. Tobisu et al. described C(sp3)–Ge bond activation to form benzogermoles in the reaction of 2-germylphenylboronic esters with alkynes under Rh catalysis (Scheme 2.57) [338].

121

122

2  Organogermanium Compounds of the Main Group Elements

Scheme 2.55  Rhodium-catalyzed germylative cyclization with dehydrogenation leading to 9-germafluorenes. [RhCl(cod)]2 (1.5 mol.%) Ge

H

(R)-(S)-BPPFA (4.5 mol.%) dioxane

Ph2P Ge 65%

Ph2P

Fe

NMe2

(R)-(S)-BPPFA

Scheme 2.56  Rhodium-catalyzed dehydrogenative germylation of unactivated C(sp3)–H bonds.

Scheme 2.57  Rhodium-catalyzed synthesis of germoles via the activation of C–Ge bonds.

Oshima et al. reported that hydrogen in Fu3GeH is so acidic that the corresponding germyl anion is produced under the action of even weak bases (t-BuOK, Cs2CO3). This anion is easily added to aldehydes (giving RCH(OH)GeFu3) and takes part in 1,4-addition to α,β-unsaturated carbonyl compounds (giving Fu3GeCH2CH2C(O)R) [339]. Furthermore, under Pd-catalyzed conditions (5 mol. % Pd(PPh3)4) Fu3GeH reacts with allylic esters giving substituted allyl germanes [340]. In this case, the interaction takes place without allylic rearrangement. There are several special methods for the synthesis of germylalkynes. Tri- and tetraalkynylgermanes can be obtained by the action of terminal alkynes on RGeX3 or GeX4 (X = Cl, NR’2) in the presence of ZnCl2; for halides, addition of Et3N is also required [341]. A related reaction represents Ir-catalyzed ([(Ir(μ-Cl)(CO)2)2], NEt(i-Pr)2) coupling of R3GeI [342] or R3GeCl [343] and HC≡CR’, which makes it possible to obtain R3Ge–C≡CR’. Schoenebeck et al. performed germylation of aryl sulfonium (tetrafluorothianthrenium) salts, [ArSR2]BF4, by Et3GeH to give ArGeEt3 (5 mol. % [Pd(μ-I)(P(t-Bu)3]2) [344]. In 2001, Jutzi et al. showed that germylene [(2-MeO-CH2-C6H2-4,6-(Bu-t)2)2Ge:], formed in situ, undergoes intramolecular insertion of Ge(II) into the O–Me bond, resulting in a novel five-membered ring in [2-(O-CH2)-C6H2-4,6-(Bu-t)2] Ge(Me)[C6H2-2-(CH2OMe)-4,6-(Bu-t)2] [345]. The team of Banaszak Holl showed that germylenes, R2Ge:, can activate C–H bonds in ethers, alkanes [346] and amines [347] in the presence of PhI (Scheme 2.58). Interestingly, Aldridge et al. also observed an intramolecular C-H activation for one of the methyl groups in germylene [(2,6-Me2C6H3)B(DippNCHCHNDipp)]Ge: [17].

Scheme 2.58  C−H activation of ethers, amines and alkanes by germylene−aryl halide complexes.

2.3  Organogermanium Compounds with Group 14 Elements

Germylene :Ge[CH(SiMe3)2]2 reacts with methylalkylketones (acetone, butanone, cyclopropyl methyl ketone) to give ­products of α-C-H insertion (in the presence of MgCl2) or derivatives of enolization (Scheme 2.59) [348]. Interestingly, products of enolization were exclusively formed in reactions with other types of alkylketones (cyclic, tert-butylmethyl, arylalkyl, etc.). (Me3 Si) 2CH H Ge (Me Si) CH O 3

2

14 - >99%

R

Me(R)C=O Me(R)C=O (Me3 Si) 2CH Ge: C6 H 6 or MgCl 2 ( 1 eq.) (Me3 Si) 2CH hexane PhMe

(Me3 Si) 2CH

H Ge (Me3 Si) 2CH

O

R 23 - 65%

Scheme 2.59  Insertion of germylene :Ge[CH(SiMe3)2]2 into C-H and O-H bonds in enolizable ketones.

Diarylketones (phenones) react with R2Ge: giving conjugated trienes (5-membered rings with new Ge–C and Ge–O bonds), products of formal [4+1] cycloaddition with concerted or radical mechanism (Scheme 2.60) [349]. O R

[(Me3 Si)2 HC] 2 Ge:

(Me3 Si)2 HC CH(SiMe3) 2 Ge O R

Scheme 2.60  Synthesis of conjugated trienes by the germylene cycloaddition with phenones.

The Banaszak Holl group showed that germylene [(Me3Si)2CH]2Ge: inserts into activated α-C-H bonds in nitriles, RCH2CN, giving [(Me3Si)2CH]2Ge(H)CHR(CN) [350]. On the contrary, in the case of NC(CH2)4CN, a cyclization (similar to ThorpeZiegler cyclization) with Ge–CN bond formation giving [(Me3Si)2CH]2Ge(CN)NH(cyclo-C(CH2)3CH) was observed. As the authors stated, the presence of THF and specific salts (LiCl, MgCl2, LiBr) is essential in this case. Germylenes based on different ligands, R2Ge:, actively react in the oxidative addition reaction with MeI to give R2Ge(Me)I [238, 351–355]. Related reagents, such as t-BuI [353] or primary alkyl halides (EtBr, n-PrBr, BnBr) [238, 356], also take part in this reaction. A similar thermal insertion of :GeBr2•dioxane into the C–Br bond in bromocyclopropanes [357] and in arylbromides [358] (synthesis of RGeBr3) or insertion of :GeI2 into substituted allyl iodides (synthesis of AllGeI3) [359] continues the application of the known methods for the synthesis of novel compounds. Yasuda et al. investigated the synthesis of germanium enolates, using insertion of germylene :GeBr2•dioxane into the C–Br bond in α-bromo ketones (Scheme 2.61) [360]. Tautomerization causes the shift of the equilibrium to the side of ketoform. Application of these and related germanium enolates as nucleophiles in condensation reactions with carbonyl and related compounds opens wide prospects for organic synthesis (Scheme 2.61) [361–364].

Scheme 2.61  Reductive cross-aldol reaction using bromoaldehyde, aldehyde or N-alkylimines mediated by Ge(II) compounds.

123

124

2  Organogermanium Compounds of the Main Group Elements

Germylenes (MesO)2Ge:•TMEDA [352], R2Ge•NHC (R = OBu-t, Mes; NHC = MeIiPr [365], Fc*2Ge: (Fc* = 2,5-bis(3,5-dit-butylphenyl)-1-ferrocenyl]) [238] and other compounds [366] react with 2,3-dimethylbutadiene to give products of the [4+1] cycloaddition reaction, five-membered 1-germacyclopent-3-enes. Digermenes R2Ge=GeR2 also gave the same ­products upon cleavage of the Ge=Ge bond [211]. Interestingly, germasilene [Me2(t-Bu)Si]2Ge=Si[Si(t-Bu)Me2]2 [367] or germaallene Tip(t-Bu)Ge=C=AsMes* [70] react with 2,3-dimethylbutadiene on the Ge=Si or Ge=C bond as a dienophile giving a [4+2] cycloaddition product. As is known, germylenes react with α,β-unsaturated carbonyl compounds, giving products of [4+1] cycloaddition [368]. The Yasuda’s team used interaction of :GeCl2•dioxane with α,β-unsaturated ketones to form five-membered cyclic (Z)-enolates [369]. Subsequent interaction of these enolates with aldehydes gave products of condensation with high diastereoselectivity (Scheme 2.62) [370]. Apparently, in this case the condensation includes a boat-like intermediate, which explains the stereochemistry of the reaction.

Scheme 2.62  Application of C,O-chelated germyl enolates to a diastereoselective aldol reaction.

Rivard et al. synthesized vinylgermanes from activated methylenated imidazoles, using the acidity of C-H protons or the halosilane elimination reaction (Scheme 2.63) [355]. Dipp R

N

Dipp H

R

GeCl4

N H Dipp IPrCH2 R = H , Me PhMe Me3SiOTf R

DABCO PhMe

R

N

R

N Dipp

SiMe3

H

K[N(SiMe 3) 2]

OTf

GeCl3

2 IPrCH 2

R

N

N

H

PhMe/THF

R

N H Dipp

Dipp

GeCl4 Dipp

Dipp R

PhMe R = H, R = Me

Me3 Si

N

Dipp

N

H

R=H Dipp

or

N H Dipp

H

N

R

N Dipp

R

d(C-Ge) 1.874(4), 1.884(4) d(Ge-Cl) 2.174(3), 2.204(3)

PhMe N

Dipp

Cl2 Ge

SiMe3

N H Dipp

Scheme 2.63  Synthesis of vinylgermanes from activated methylenated imidazoles.

In 2002, Sekiguchi et al. reported the synthesis of substituted germacycloprop-2-enes upon the formal [2+1] cycloaddition of in situ generated germylene (formed by the reduction of germanium dichloride) with an internal alkyne (Scheme 2.64) [371]. A related germacycloprop-2-ene is formed under thermolysis of cis,cis-1,6,7-trigerma-1,6,7-(Si(Bu-t)3)3-7-Mes-3,4-dimethylcyclo[4.1.0]hept-3-ene with tolan [372]. Interestingly, the reduction of these germacycloprop-2-enes with Li makes it possible to obtain 1,1-dilithium reagents, (R3Si)2GeLi2. Me3 Si (R3Si)2GeCl2 R 3Si = (i-Pr) 3Si, Me(Bu-t)2 Si

K reflux

SiMe3

R 3Si

MeSi

Ge

SiR3

SiMe3

Scheme 2.64  Synthesis of substituted germacycloprop-2-enes upon the formal [2+1] cycloaddition of in situ generated germylene with an internal alkyne.

In 2002, Weidenbruch et al. found that digermene Ar2Ge=GeAr2 (Ar = 2-t-Bu-C6H-4,5,6-Me3), which dissociates to germylenes [Ar2Ge:], reacts with F3CC≡CCF3 to give a three-membered ring, germirene ([2+1] cycloaddition) [373]. The complex (MesO)2Ge•TMEDA [352] reacts with internal alkyne MeO2CC≡CCO2Me. The expected germirene (formal [2+1] cycloaddition) is not stable and reacts with added HOMes. Insertion of the germylene complex into germirene results in a reactive four-membered ring with the Ge–Ge bond, which is oxidized to give a five-membered ring with a Ge–O–Ge fragment (Scheme 2.65).

2.3  Organogermanium Compounds with Group 14 Elements

Scheme 2.65  Reactivity of (MesO)2Ge:•TMEDA with alkynes, allowing to synthesize different germanes.

In 2016, Wesemann et al. showed that germylenes, stabilized by intramolecular donation by the PPh2 group, in the reaction with phenylacetylene give a cyclic germane, in which the Ph group 1,4-migrates from P+ to the Ge– atom (Scheme 2.66) [374]. Ph

Ph Ar Ge

PPh2

Ar = 2,6-Trip2C6H3

n-hexane 1h

Ar Ge Ph

PPh2

10 d

44%

Ar

Ge Ph

P cis-

Ph 24%

Scheme 2.66  Cyclic germanes obtained by interaction of alkynes with germylenes, stabilized by intramolecular donation.

In 2019, Power et al. found that kinetically stabilized germylene Ar2Ge: (Ar = 2,6-Mes2C6H3) reversibly adds to alkynes (EtC≡CEt, PhC≡CPh, Me3SiC≡CH, PhC≡CH) at an ambient temperature to give germirenes, cyclo-Ar2Ge(C(R)=C(R’)) (formal [2+1] cycloaddition) [375]. Furthermore, at room temperature germylenes Ar2Ge: (Ar  =  2,6-Mes2C6H3, 2,6-Dipp2C6H3) reversibly react with ethylene [376] to give germiranes, cyclo-[Ar2GeCH2CH2], whereas more sterically crowded Ar2Ge: also reacts with propylene. Power et al. established that diarylgermylene, Ar2Ge: (Ar  =  C6H3-2,6-(C6H2-2,4,6-Me3)2), reacts with inorganic acids (HCN, HN3, HBF4) giving oxidation products Ar2Ge(H)X (X = CN, N3, F) [377]. Apparently, in this case the initial step is the protonation of the Ge(II) centre to form a three-coordinate, cationic germanium complex; its high cationic character even makes it possible to abstract F- from BF4- [378]. Germabenzene C5H5Ge(Tbt), stabilized by the Tbt group [205], reacts with unsaturated compounds as a dienophile to give products of [3+2] and [4+2] cycloaddition (Scheme 2.67). Related 2-Tbt-2-germanaphthalene reacts with unsaturated compounds in a similar manner, resulting in the formation of new Ge–C bonds [152]. Interestingly, the reaction of 2-Tbt2-germanaphthalene with t-BuLi resulted in the addition of t-Bu anion to Ge atom. Mes

N O Ge Tbt 86%

Ph MesCNO

Ph

Ge Tbt

Ge

Tbt 69%

[4+2]

(Me3Si)2 HC

Ge

[3+2]

62%

Tbt

Tbt =

CH(SiMe3) 2

Ph

Ph

(Me3 Si)2 HC Ge

Tbt

58%

Scheme 2.67  The reactivity of germabenzene C5H5Ge(Tbt), allowing to obtain novel germanes.

Baines et al. found that addition of RLi (R = Me, Bu) to germene Mes2Ge=CHCH2Bu-t with subsequent acidification by MeOH gave Mes2Ge(R)CH2CH2Bu-t [379]. Interestingly, upon storage this digermene undergoes [2+2] cycloaddition (giving a four-membered ring, where dimerization occurs in a head-to-tail manner) and H-shift (yielding Mes2Ge(H)CH=CHBu-t) [380]. Germene Mes2Ge=CHCH2Bu-t [381] reacts with terminal alkynes HC≡CR to give different products. Thus,

125

126

2  Organogermanium Compounds of the Main Group Elements

germacyclobutenes ([2+2] cycloaddition), vinylgermanes Mes2Ge(CH=CHBu-t)CH=CHR (ene-addition) and germylacetylenes Mes2Ge(CH2CH2Bu-t)C≡CR (addition across acetylenic C–H bond) are obtained. Germylenes react with isocyanides yielding compounds with the Ge–C bond. Lee, Sekiguchi, et al. found that digermene [(t-Bu)2MeSi]2Ge=Ge[SiMe(Bu-t)2]2 reacted with t-BuNC giving [(t-Bu)2MeSi]2Ge(H)CN; extrusion of isobutylene was also observed [382]. Power et al. showed that Ar2Ge: (Ar = C6H3-2,6-(C6H2-2,4,6-Me3)2) reacted with t-BuNC, giving an adduct that underwent C–H bond activation to form Ar2Ge(H)CN [383]. In contrast, Dostal et al. found that the reaction of germylene based on the boraguanidinate ligand, R2Ge:, with t-BuNC resulted in R2Ge(CN)Bu-t [384]. Three-membered cyclic digermenes react with dichloromethane with ring expansion (Scheme 2.68) [272, 385]. 1,2-Addition of CH2Cl2 across the Ge=Ge bond to give an intermediate saturated three-membered ring is observed at the first step of this reaction. Then intramolecular insertion of activated CH2 into the Ge–Ge bond occurs. R

R R

R R CH 2Cl2

Si Ge

Ge R R R = SiMe(t-Bu)2

hexane

R

Si Cl

Ge

Ge

R

Cl Ge

Ge Cl

Cl

R

R Si

71%

R

d(Sibridge-Ge) 2.547(3), 2.475(2) d(Ge-C) 1.998(10), 1.956(10) d(Ge-Cl) 2.229(2), 2.236(2) 29

Si NMR δ 30.7 ppm

Scheme 2.68  Ring-expansion reactions in cyclic digermenes.

Addition of terminal alkynes to Mes2Ge=CR2 (CR2 = fluorenylidene) proceeds as a formal [4+2] (R’ = Alk, Ar) or [2+2] and [4+2] cycloaddition (R’ = OEt) process (Scheme 2.69) [386]. Apparently, this reaction takes place with Ge radical intermediates [387]. R'

Mes 2Ge

+

Mes 2Ge=CR2

R' = OEt

Mes 2Ge

R' R' = Alk, Ar Mes 2Ge

CR 2 =

OEt OEt

R'

Scheme 2.69  The addition of terminal alkynes to dimesitylfluorenylidenegermane.

Mehring et al. [388] found an unprecedented example of intramolecular insertion of germylene, stabilized by substituted salicylic alcohol, into an activated benzylic C–O bond to form a new Ge–CH2 bond (Scheme 2.70).

Scheme 2.70  Intramolecular C–O insertion of a germanium(II) salicyl alcoholate.

The related synthesis of germylated heterocycles includes the action of strong bases on aryl halides containing a GeCH2Cl substituent, which is accompanied by an anionic rearrangement resulting in the formation of novel Ge–C bonds (Scheme 2.71) [389]; hypercoordinated germane is proposed as an intermediate. Cl X

R

Ge

n-BuLi

Ge

THF

Z R Z = O, S, NBoc; X = Hal

Z

Ge

+

+ R

Z

R

Ge

Ge

Z

Z

R

Cl Li

Scheme 2.71  Synthesis of germylated heterocycles from aryl halides containing a GeCH2Cl substituent.

2.3  Organogermanium Compounds with Group 14 Elements

Acyl germanes, RnGe[C(O)R’]4-n (n = 0, 2, 3), are obtained in a number of ways. The classical method includes the hydrolysis of functionalized ethers PhSC(GeMe3)(R)OMe [127] giving Me3GeC(O)R; the hydrolysis of H2C=C(GeMe3)OEt [65] resulted in Me3GeC(O)Me. Other methods (Scheme 2.72) consist of the dithiane germylation followed by the removal of the protecting group [390], the action of acyl fluorides [391, 392] or acyl chlorides [393] on metalated germanes, or interaction of germylenes with acyl chlorides [394]. In these derivatives, red-shifted n→π* transition bands from enhanced σ-n conjugation are observed. Because of homolytic photoinitiated Ge–C(O)R’ bond cleavage, these derivatives found application in radical polymerization [395, 396]. Related (seleno)carbamoyl-germanes, Me3Ge-C(Se)NR2, were obtained under the action of LiC(Se)NR2 on Me3GeCl [397].

Scheme 2.72  Various methods for the synthesis of acyl germanes.

Escudié et al. found that phosphagermaallene Mes*P=C=Ge(t-Bu)Tip (Tip = 2,4,6-tri-iso-propylphenyl, Mes* = 2,4,6-tritert-butylphenyl) in reactions with alkynes formed products of [3+2] cycloaddition (between Ge=C=P and C≡C) [398]. The intermediate carbene, formed in situ, undergoes C–H insertion or is trapped by the second equivalent of alkyne (Scheme 2.73). The spirocyclic compound in solution undergoes rearrangement accompanied by the ring expansion. i-Pr

Mes *P=C=Ge(t-Bu)Tip MeO2 C

CO2 Me

H

[ 3+2] i-Pr

i-Pr

Ge t-Bu

MeO2C i-Pr

Ge t-Bu

MeO2 C

P

Mes * MeO2 C

CO2 Me

CO2Me Et2O

MeO2C Tip Ge t-Bu MeO2 C

* P Mes

CO2Me CO2 Me * P Mes

CO2 Me 81%

72% MeO2C Tip t-Bu

Ge

MeO 2 C

CO2 Me

MeO2 C

+

P CO2 Me t-Bu

Tip Ge t-Bu

t -Bu MeO2 C

CO2Me

P CO2 Me t-Bu

t-Bu

Scheme 2.73  Application of Mes*P=C=Ge(t-Bu)Tip as a 1,3-dipole for synthesis of polyciclic germanes.

Phosphagermaallene Tip(t-Bu)Ge=C=PMes* (Tip  =  2,4,6-tri-iso-propylphenyl, Mes*  =  2,4,6-tri-tert-butylphenyl) reacts with ylide Ph2S=CH2 yielding Tip(t-Bu)Ge(CH2SPh)–C(Ph)=PMes*. The reaction runs through a nucleophilic attack of the negatively charged carbon atom of ylide on the positively charged germanium atom followed by the migration of one of the phenyl groups. The reaction of Tip(t-Bu)Ge=C=PMes* with γ-butyrolactone gives a [2+2] cycloaddition by the C=O group with the Ge=C double bond, cyclo-[Tip(t-Bu)Ge-O-C(C3H6O)-C]=PMes* [399]. So et al. showed that the Ge–Cl bond of GeCl4 can insert into the C–Sn bond in coordinated stannylene to form a novel Ge–C bond (Scheme 2.74) [400].

Scheme 2.74  Reactivity of a tin(II) 1,3-benzodi(thiophosphinoyl)methanediide complex with GeCl4.

127

128

2  Organogermanium Compounds of the Main Group Elements

Power et al. found that that digermyne ArGe≡GeAr (Ar = 2,6-Dipp2C6H3) reacted with an excess of alkynes (Me3SiC≡CH, PhC≡C–C≡CPh) giving polycyclic digermanes by activation of the aromatic Dipp ring (Scheme 2.75); the reaction begins with [2+2] cycloaddition and includes diradical intermediates [401, 402]. Ar

Ar Ge

R

Ge Ar

R'

n-hexane

Ge

R

R = H; R' = SiMe3 [2+2] R = CCPh; R' = Ph

Ge

Ar R

i-Pr

R'

Ar Ge

n-hexane

R'

Pr-i

R'

R

R

Ge Ar

R

Ar Ge

R'

R

R'

Ge R'

Dipp

Scheme 2.75  Reactions of ArGe≡GeAr with alkynes.

Related ArGe≡GeAr (Ar = 2,6-Tip2C6H3; Tip = C6H2-2,4,6-i-Pr3) in the reaction with 2,3-dimethyl-1,3-butadiene yields a product of simultaneous [4+1] cycloaddition and insertion (Scheme 2.76) [403]. Treatment of Ar’Ge≡GeAr’ (Ar’ = C6H32,6-(C6H3-2,6-i-Pr2)2) with 1,4-cyclohexadiene gives a bicyclic germane, a product of C-H activation, in a mixture with other products [404]. Ar Ar Ge

Ge

Ge

Ar'

PhMe

Ar

Ge

Ge 38%

Ge Ar'

Ar

Ge

pentane - C 6H 6 - [Ar'GeH]2

Ar' 47%

Scheme 2.76  Reactions of ArGe≡GeAr with dienes.

Tokitoh, Sasamori, et al. established that digermyne TbbGe≡GeTbb (Tbb = 2,6-[CH(SiMe3)2]C6H2-4-t-Bu) reacts with acetylene through a cascade of formal cycloaddition and insertion reactions to give a bicyclic digermane, 1,4-digermabarrelene (Scheme 2.77) [405]. Cyclic digermene and 1,4-digermabenzene are proposed as intermediates. Using terminal alkynes makes it possible to isolate tricyclic digermanes, which are intermediates in the reaction of trimerization of alkynes; this is a new method of substituted arene synthesis [406]. Furthermore, interaction of the related digermyne BbtGe≡GeBbt (Bbt  =  2,6-[CH(SiMe3)2]C6H2-4-C(SiMe3)3) with ethylene at an increased pressure at room temperature resulted in bis(germiranyl)ethane; 1,2-digermacyclobut-1-ene is an isolated intermediate in this reaction, which could also react with ethylene [407].

Tbb

Ge Ge Tbb

Ge Tbb

Ge

Tbb Ge

Ge Tbb

Tbb

Ge Ge

Ar

Ar

Ar

Ge

Ar

Ar

Ge Ge bbt

(> 1 atm.) RT

Bbt Ge Bbt

Ge

Tbb =

Ge Tbb

Ge

Bu-t

(Me 3Si)2 HC

Ar Ar

Ar

(Me 3Si)2 HC

Ar Bbt

(Me 3Si)2 HC

Tbb

Ge Tbb

Ge Ge Tbb Tbb

Tbb

Tbb

Tbb

Ar

Ar Tbb

+

Ge Ge

Tbb

Ge

Ge

Bbt

Bbt

Bbt

Ge

Ge

Bbt

Bbt =

C(SiMe3 )3

(Me 3Si)2 HC

Scheme 2.77  Reactions of digermynes with acetylenes and ethylene.

In 2019, Tokitoh, Sasamori, et al. when investigating the reactivity of stable 1,4-digermabenzene found it to react reversibly with small molecules giving products of Ge(IV) (Scheme 2.78) [408].

2.3  Organogermanium Compounds with Group 14 Elements

O Et

Et

O

Ge

Ge Tbb

Tbb

Et

Et > 99%

CO2

Et Tbb Ge Et

Et

Et

Ge Tbb Et

Ge

Tbb

Et

Ge Et

Tbb

Et

> 99%

Scheme 2.78  Reactivity of 1,4-digermabenzene towards acetylene and carbon dioxide.

Organogermyl fullerenes were obtained by a number of methods. The photolysis of 1,2-digermacyclobutane cyclo[(Dis)2GeCH2CH2Ge(Dis)2] (Dis = (Me3Si)2CH) with C60 yielded two monogermyl adducts, [6,5]-germacyclopropane and [6,6]-germacyclopentane derivatives; the homolytic rupture of the Ge–Ge bond was proposed as a key stage [409]. Fullerenes C60 [410], C70 [411] and substituted fullerene C80 (i.e., Lu3N@Ih-C80) [412] react with digermirane cycloH2C(GeAr2)2 (Ar = 2,6-Et2C6H3) upon the thermal or photochemical activation, resulting in the cleavage of the Ge–Ge bond, whereas Ge-CH2Ge rings (1,4-cycloadducts) are formed. Fused germyl difullerene C60GePh2C60 was obtained by mechanochemical synthesis by mixing C60, Ph2GeCl2, and Li; the radical anion C60 was proposed as an intermediate [413]. According to XRD data, the Ge–C bond length in Ge(IV) tetracoordinated derivatives varies from 1.83 Å [414] to 2.25 Å [415]; the typical value is 1.96 Å. In general, introduction of voluminous groups increases this distance, whereas electronwithdrawing substituents diminish it.

2.3.2  Organogermanium Compounds with Ge–Si Bond As a Group 14 element, germanium is located between Si and Sn in the Periodic Table, but filling of the fourth period by d electrons results in higher electronegativity of germanium [416] which determines the chemistry of Ge-Si derivatives. One of the classical methods for making Ge–Si bonds includes the action of silicon nucleophiles (lithium, potassium salts) on germanium halides [417–419]. The reversal of reagents polarity, i.e., the reaction of germyl lithium or potassium reagents with silicon halides also resulted in the formation of Ge–Si bonds [155, 199, 367, 420–422]. Interestingly, oligosilyl substituted germanes have a forbidden band gap typical for semiconductors [422]. Furthermore, for σ-catenated compounds (p-Tol)3Ge-EMe3 (E = Si, Ge, Sn; 29Si NMR δ –10.4 ppm; d(Ge-Si) 2.3892(5)Å; 119Sn NMR δ –90.5 ppm), Zaitsev et al. [421] investigated the effect of the nature of E on the properties of compounds. Within the range of Ge-Si > Ge-Ge > Ge-Sn, the UV absorbance was found to be shifted to the red field (λ 230, 234, 240 nm, respectively); therefore, the HOMO–LUMO gap was decreased because of better conjugation. The decrease of HOMO level within this range follows also from the electrochemical oxidation data (Eox 1790, 1650, 1520 mV, respectively). Two Ge–Si bonds can be formed simultaneously using germole dianion/Me3SiCl [423] or dilithium germanium reagents/ Me3SiF [220]. Würtz-type reactions, including the interaction of germanium halides and silicon halides in the presence of an alkali metal (Li, Na) are still applied for the synthesis of simple Ge-Si derivatives [424–426]. Saito et al. obtained ArGe(SiHMe2)3 (Ar  =  2,6-Dipp2-C6H3 or 2,6-Tip2C6H3) from ArGeCl3, Me2SiHCl under the action of Mg/dibromoethane/Et3N [427]. Mochida et al. reported that a germyl zinc compound, [(Me3Si)3GeZnCl], reacts with Me3SiCl to give (Me3Si)4Ge, but in low yield (17%) [428]. Electrosynthesis makes it possible to obtain [HSiMe2]2GePh2 from HSiMe2Cl and Ph2GeCl2 [429]. Marschner et al. developed a convenient method for the synthesis of silyl substituted germanes by generation of solvated [R3GeK] nucleophiles (by selective cleavage of the R3Ge–SiMe3 bond by t-BuOK in ethereal solvents or in the presence of 18-crown-6; formation of thermodynamically stable t-BuOSiMe3 is a driving force of this interaction) which further formed various germanes by the interaction with halosilanes [219, 430–434]. Germylation of solvated [R3SiK] formed under similar conditions, by halo- [432, 435] or alkoxogermanes [436], also yields compounds with Ge–Si bonds. Interestingly, for germyl potassium reagents the order of reagent addition affects the type of products formed (Scheme 2.79) [421]. Inverse addition (addition of the potassium reagent to chloride) resulted in digermane in high yield. Direct addition (addition of chloride to the potassium reagent) gave a separable mixture of di- and trigermanes. Apparently, an unexpected trigermane product was formed because of a local excess of the potassium reagent in the course of slow interaction. (Me3 Si) 4Ge t -BuOK THF [(Me 3Si)3 GeK THF]

Ph3 GeCl THF inv er se addition Ph3 GeCl THF direct addit ion

(Me3 Si)3 GeGePh3

62%

GePh3 (Me3 Si)3 GeGePh3 + (Me3 Si) 2Ge 29 Si NMR δ -2.69 ppm 36% 32% GePh3 d(Ge-Si) 2.411(6) 29 Si NMR δ -4.35 ppm

Scheme 2.79  The products of various Ge-Si substitution under application of germylpotassium reagents.

129

130

2  Organogermanium Compounds of the Main Group Elements

Pannell et al. developed a photochemical rearrangement of linear germaoligosilanes into branched silylgermanes, ­catalyzed by iron complexes (10 mol. % CpFe(CO)2SiMe3) [437]. Thus, HSiMe2SiMe2SiMe2GeMe3 transforms into (Me3Si)3GeSiMe2H; silylene complexes of iron are proposed as intermediates. Marschner et al. investigated in detail the related rearrangement of molecular oligosilanes containing terminal germyl groups (Wagner-Meerwein rearrangement for germaoligosilanes). In the presence of Lewis acid catalyst (20 mol. % Al(Fe)Cl3, i.e., sublimed AlCl3/FeCl3 [432, 438]; 4 mol. % [Ph3C][B(C6F5)4]; [R3Si(arene)][B(C6F5)4] (R = Me, Et, i-Pr; arene = C6H5CH3, C6H6, C6H5Cl) [436]), migration of the Ge atom into the quaternary position of the molecule is observed (e.g., (Me3Si)3SiGeMe3 to (Me3Si)4Ge). In these cases, silylium ion ionic intermediates are suggested; the stabilizing β-silyl group effect is the driving force for the rearrangement. One of the non-trivial ways of simultaneously making Ge–C and Ge–Si bonds is the ortho-lithiation of fluorosilyl ­substituted arenes with the subsequent interaction with :GeCl2∙dioxane (Scheme 2.80) [439]. This synthesis was found to be successful at a strict temperature control (close to −60oC); at higher temperatures, the corresponding disilane was formed. Apparently, the corresponding germylene, [Ar2Ge:], was formed as an intermediate under arylation of :GeCl2∙dioxane. At the next stage, [Ar2Ge:] inserts into the Ar–Li bond giving a typical [Ar3GeLi] nucleophile that reacts intramolecularly with the Si–F bond. This behaviour can be caused by the low solubility of :GeCl2 under reaction conditions. Interestingly, with KF/[2,2,2]cryptand the Ge–Si bond cleavage is observed, resulting in an ambiphilic Ge anion that exhibits both nucleophilicity and electrophilicity [440].

Scheme 2.80  Formation of Ge–Si bonds in the ortho-lithiation of fluorosilyl substituted arenes with the subsequent interaction with :GeCl2∙dioxane.

Lappert et al. found that Ar2Ge: (Ar = 2,6-(Me2N)2C6H3) inserts into the Si–Cl bond in SiCl4 giving Ar2Ge(Cl)SiCl3 (d(Ge-Cl) 1.936(4) Å; d(Ge-Si) 2.3793(17) Å) [441]. As a rule, products of similar reactions are not stable to decomposition (reverse reaction), so bulky or electron donating groups should be introduced to the Ge atom. Kira et al. showed that germylene cyclo-[(Me3Si)2CCH2CH2C(SiMe3)2]Ge: (R2Ge:) also inserts into Si-Cl in SiCl4, giving R2Ge(Cl)SiCl3 (29SiSiGe NMR δ 4.2 ppm) [442]. Related kinetically stabilized silylene, cyclo-[(Me3Si)2CCH2CH2C(SiMe3)2]Si: (R2Si:), reacted with GeCl4, which makes it possible to obtain an insertion product R2Si(Cl)GeCl3 [443]. Other types of germylenes, R2Ge:, insert into R’3Si–H bonds to give R2Ge(H)SiR’3 [216, 238]. Interestingly, digermenes, RR’Ge=GeRR’, which can dissociate to germylenes, RR’Ge:, insert into Si–H bonds also giving RR’Ge(H)SiR”3 [444]. Kira, Iwamoto et al. showed that for germasilene [Me2(t-Bu)Si]2Ge=Si[Si(t-Bu)Me2]2, the equilibrium with silylgermylene [Me2(t-Bu) Si-Ge-Si(Si(t-Bu)Me2)3] is observed [367]. Upon interaction with such a germylene trapping reagent as Et3SiH, the (Me2(t-Bu)Si)-Ge(H)(SiEt3)-Si(Si(t-Bu)Me2)3 is formed. Germylenes with silyl substituents take part in the intermolecular E–H bond activation (H–H, H–NH2) at room temperature (Scheme 2.81) [17]. Similar to the case of boryl substituted germylene, in its silyl analog a small HOMO-LUMO gap (134 kJ/mol according to DFT calculations) also determines its reactivity in oxidative bond activation. Silanes, for example SiH4, can also take part in such interactions.

Mes Mes

NH 2

Ge Me3 Si Si H Me3 Si SiMe3

d(Ge-Si) 2.400(1) 74%

Mes NH 3 (1 atm.) PhMe, rt

Mes

Ge Me3 Si Si Me3 Si SiMe3

H2 (4 atm.) PhMe, rt

Mes H

Mes

Ge Me3Si Si H Me3Si SiMe3

d(Ge-Si) 2.405(1) 70%

Scheme 2.81  Insertion of germylene with silyl substituents into ammonia and hydrogen as a method of synthesis of silylgermanes.

2.3  Organogermanium Compounds with Group 14 Elements

Lee, Sekiguchi, et al. investigated the reaction of disilagermirenes with excess phenylacetylene (Scheme 2.82). Products with Ge–Si bonds were formed as a result of cascade cycloaddition and ring expansion reactions, some intermediate products of these reaction sequences can be detected by NMR spectroscopy [445, 446]. Interestingly, thermolysis of silole containing a Si=Ge fragment results in a skeletal rearrangement, giving a tricyclic compound with an elongated Ge–C bond (2.242(3) Å), where the tetracoordinated Ge(IV) atom has an unusual geometry (inverted tetrahedral geometry or “umbrella”-type configuration) [415]. R E R R

E'

Si

R E R

Ph E

E = Ge, E' = Si E = Si, E' = Ge R = SiMe( Bu-t)2

C6D6 [2+2]

R

E'

Ph R Ge Ph

Si

R R E'

R

R E

Ph R Si

R Si R

1,2 Si shif t R

R

Ph

E

C6 D6 Ph

[2+2]

R

R Ge Ph

E R R E' Si R

Si R

Si

∆

Ph

R

H

R

E'

R Si

R

Ph E = Ge, E' = Si: 63% E = Si, E' = Ge: 55%

Si R

Scheme 2.82  Reaction of disilagermirenes with phenylacetylene as a path to cylic Ge-Si derivatives.

Schnepf et al. showed that germanes with Si substituents can be obtained in a cascade reaction (including [4+2] ­intramolecular cycloaddition) from :GeCl2 under the action of S nucleophiles (Scheme 2.83) [447]. Intermediates with the three-membered ring cyclic structure and with Ge=Si double bond were proposed.

Scheme 2.83  Cascade reaction between :GeCl2 and S nucleophile, resulting to Ge-Si compounds.

Marschner et al. found that silylated germylene, stabilized additionally by complexation with PMe3, in the interaction with tolan gave germirene [448], which on heating was rearranged into cyclic germane, formed via vinylgermylene intermediate with subsequent insertion into Si–SiMe3 bond (Scheme 2.84) [449].

Scheme 2.84  Synthesis of cyclic silylgermanes by insertion of [(Me3Si)3Si]2Ge⋅PMe3 into tolan.

Related cyclic germylenes in the reaction with monosubstituted alkynes gave spirocyclic germanes, products of intramolecular germylene insertion into the cyclic Si–Si bonds (Scheme 2.85) [450].

Scheme 2.85  Synthesis of spirocyclic germanes by transannular insertion of vinyl germylenes into Si–Si bonds.

131

132

2  Organogermanium Compounds of the Main Group Elements

Typical reactivity (insertion, cycloaddition reactions) of the same cyclic disilylated germylene provides access to different Ge(IV) species (Scheme 2.86) [356].

Scheme 2.86  Reactivity of silylated germylene as a methods for the synthesis of Ge-Si derivatives.

Sasamori, Tokitoh, et al. found that cyclic digermenes activate Si–Cl bond in SiCl4 (Scheme 2.87) [451, 452]. Formally, the product of the reaction can be regarded as a double insertion of germylene R2Ge: (generated via Ge=Ge bond cleavage) into the Si–Cl bond.

Ph

SiCl4

Cl

Ge Ge R R

R

T bb: d(Ge-Cl) 2.2094(14), 2.2011(15) d(Ge–Si) 2.3734(16), 2.3938(15) 29 SiSiCl2 NMR δ 28.83 ppm

Ph

Ph

Ph

Ge

Ge

Si Cl2 trans-

R Cl > 99%

Bbt: d(Ge-Cl) 1.966(4)-2.478(3) d(Ge–Si) 2.410(4)-2.454(4) 29 SiSiCl2 NMR δ 16.03 ppm

(Me3 Si) 2HC R'

R=

(Me3 Si) 2HC R' = Bu-t (T bb), C(SiMe3 )3 (Bbt)

Scheme 2.87  Synthesis of a cyclic dichlorodigermasilane by insertion of SiCl4 into digermenes.

Scheschkewitz et al., upon the action of MeLi on Tip2Si(Cl)–Si(Tip)=Ge∙NHC (Tip = 2,4,6-i-Pr3C6H2, NHC = 1,3-diisopropyl-4, 5-dimethylimidazol-2-ylidene), obtained tetrasiladigermatricyclohexane (Scheme 2.88) [453]. Interestingly, on heating, the product rearranged to a compound with the Ge–Ge bond. Tip Tip MeLi Tip Si Si NHC PhMe Ge Cl

NHC Tip

Tip Si

Tip

Si

NHC

Ge Me

Me

Me

Ge

Ge Tip Tip

Si

Si

Tip T ip

Tip

Si

Tip

Tip Si

Si

Tip

Tip

Si

Si Ge Ge Si

Tip

Tip Tip d(Ge–Si) 2.3725(8) - 2.4596(8)

Scheme 2.88  Dimerization of a stable disilenyl germylene to tricyclic digermane.

In 2000, Mochida et al. showed that upon Pd catalysis R3GeH inserts into the Si–Si bond of 3,4-benzo-1,2-disilacyclobut2-ene (Scheme 2.89) [312]. A cyclic intermediate with the Si-Pd-Si framework is proposed.

SiMe2 SiMe2

R3GeH Pd(PPh3 )4 PhMe

Me2 Si GeR3 SiMe2H

Scheme 2.89  Pd-catalyzed insertion of R3GeH into the Si–Si bond of 3,4-benzo-1,2-disilacyclobut-2-ene.

Baines et al. investigated the mechanism of addition of aldehydes to tetramesitylgermasilene (Scheme 2.90) [454]. Radical species (such as [Mes2Si(OR)-Ge(Mes)2•], where OR = OCH(•)-cyclo-C3H3(Ph)(OMe)) are proposed as intermediates in this interaction.

2.3  Organogermanium Compounds with Group 14 Elements

Mes2 Si Mes2 Ge

1) hν Et3 SiH

GeMes2

CHO

Mes 2Ge(H)SiEt3

+

Mes Mes Mes Si Ge Mes Ph O

2) Ph

t rans-

OMe

+

Mes Mes Mes Ge Si Mes Ph O

OMe

+ other products

t rans-

Mes Mes

+

OMe

Si Ge(H)Mes 2 O

OMe Ph

Scheme 2.90  Addition of carbonyl compounds to tetramesitylgermasilene.

The Ge–Si bond length varies from 2.33 Å [432] to 2.55 Å [385] (according to XRD analysis); the typical value is 2.42 Å.

2.3.3  Organogermanium Compounds with Ge–Sn Bond Formation of Ge–Sn bond was achieved by reacting potassium salts of germyl anions with tin halides [219, 221, 430]. This classical way is highly effective and makes it possible to obtain the target compounds in high yields. Thus, in typical (Me3Si)3GeSnPh3, 29Si NMR δ is -2.7 ppm; 119Sn NMR δ -111.1 ppm [421]. The lithium stannyl anions can also be used for the synthesis of Ge-Sn compounds. Thus, in the reaction of Ph3SnLi with BrGeH3, the compound Ph3SnGeH3 (δ 119Sn -134.2 ppm; d(Ge-Sn) 2.583(7) Å) was obtained in moderate yield (50%) [455]. Interestingly, this compound can be used in fabrication of semiconductive Ge materials. Compounds with the Ge–Sn bond can be obtained by the interaction of Alk3GeH with R3SnNMe2 or Alk3GeNMe2 with R3SnH [250]; formation of gaseous HNMe2 is the driving force of the interaction. Kira, Power, et al. found that germylstannylene Ar(t-Bu3Ge)Sn: reacts with 2,3-dimethylbutadiene giving a typical fivemembered ring, stannacyclopentene (Scheme 2.91) [456].

Scheme 2.91  Reaction of germylstannylene Ar(t-Bu3Ge)Sn: with 2,3-dimethylbutadiene.

Ruzicka et al. studied the activation of the Ge–Cl bond in Ph3GeCl by C,N-chelated stannylene, R2Sn: [457]. The compound R2Sn(Cl)GePh3 was obtained in high yield (72%; 119Sn NMR δ − 167.7 ppm). Hypergermyl substituted stannylene, [(Me3Si)3Ge]2Sn: (29Si NMR δ 16.7 ppm), was studied by Klinkhammer et al. [458]. This stannylene was obtained by the transmetalation reaction of :Sn[N(SiMe3)2]2 with unsolvated [KGe(SiMe3)3]. Interestingly, a dimerization is observed in the crystal, giving distannene [(Me3Si)3Ge]2Sn=Sn[Ge(SiMe3)3]2 (d(Ge-Sn) 2.7161(7) Å, 2.7239(8) Å, d(Ge-Si) 2.408(2)–2.433(2) Å, d(Sn-Sn) 2.847(2) Å). Breher et al., in an investigation of the reactivity of [1.1.1]propellane, Ge2(SiMes2)3, found that a new Ge–Sn bond is formed under the action of H-Sn derivatives (Scheme 2.92) [459]; the reaction proceeds by a radical mechanism. Mes Mes

Mes Mes Si

Si Ge Mes Si Mes

Me 3SnH

Ge Si

Mes PhMe

Mes

H

Ge

Mes Si Mes

Ge Si

SnMe3

29

Si NMR δ -36 ppm Sn NMR δ -37 ppm

119

Mes

Mes

Scheme 2.92  Insertion of [1.1.1]propellane, Ge2(SiMes2)3, into H-Sn bond.

Schnepf et al. described the tin cluster Sn10[Ge(SiMe3)3]6 (d(Ge-Sn) 2.6617(4) Å, 2.6899(4) Å), obtained by the action of [LiGe(SiMe3)3∙2.75THF] on metastable [SnCl] [460]. According to XRD data, the bond distance between Ge and Sn atoms varies from 2.54 Å [461] to 2.73 Å [458], with 2.64 Å as the mean value.

133

134

2  Organogermanium Compounds of the Main Group Elements

2.3.4  Organogermanium Compounds with Ge–Pb Bond In 2010, Klinkhammer et al. investigated [(Me3Si)3Ge]2Pb: (29Si NMR δ 16.7 ppm; d(Ge-Pb) 2.733(2) Å, d(Ge-Si) 2.374(6)–2.396(6) Å) obtained by the transmetalation reaction of :Pb[N(SiMe3)2]2 with unsolvated [KGe(SiMe3)3] [458]. Upon interaction of [(Me3Si)3E]2Pb: with [KE’(SiMe3)3] (E, E’  =  Si, Ge), the corresponding solvated lead anions, KPb[E(SiMe3)3]3-n[E’(SiMe3)3]n (n = 0, 1), are formed. These anions can be oxidized by :Pb(OC6H3-2,6-(t-Bu)2)2 to the corresponding radicals. According to an EPR study, an increasing number of germyl substituents in the latter leads to decrease in the s-character of the singly occupied molecular orbital (SOMO) of the radical and thus to an almost planar molecular geometry. In 2019, Wesemann et al. described plumbylene Ar(H)2GePbAr’ (d(Ge-Pb) 2.7297(3) Å; 207Pb NMR δ 9183 ppm; Ar = 2,6-Trip2C6H3, Trip = 2,4,6-triisopropylphenyl; Ar′ = 2,6-Mes2C6H3), synthesized from [Li(THF)3][Ar(H)2Ge] and [Ar’PbCl]2 [462]. In the crystals, the typical value of d(Ge-Pb) for neutral plumbylgermanes is 2.73 Å [462]; for negatively charged plumbanides, the elongation of this bond is observed up to 2.84–2.87 Å [458].

2.4  Organogermanium Compounds with Group 15 Elements This section describes organogermanium compounds with N, P, and As elements. Derivatives of Sb and Bi have not been investigated after 2000.

2.4.1  Organogermanium Compounds with Ge–N Bond Germanium amides are obtained using the classical method by the action of lithium amides on germanium halides including carbodiimide [463], alkyl amides [41], alkyl boramides [464], dialkyl amides [124, 142, 200, 248, 250, 465], silazanide [466], N,N,N’,N’-tetraalkylguanidinates [467], derivatives of 2,3-diaminoquinoxaline [468], anilide [469], amidophosphaalkene [470], and other [307, 471, 472] derivatives. Furthermore, the reaction of such a strong nucleophile as LiNMe2 with hypercoordinated germane Me3SiOGe(OCH2CH2)3N results in the formation of Ge(NMe2)4 (66%) [257]. Interestingly, N-germylation is observed in the reaction of R3GeCl with an anion formed from HP(cyclo-OC6H4NH-o)2/n-BuLi [473]. Riviere-Baudet et al. under hydrolysis of germylene :Ge[N=C=NGeMes3]2 obtained germyl cyanamide Mes3GeNHCN, in equilibrium with germylcarbodiimide Mes3GeN=C=NH (d(Ge-N) 1.866(5) Å) [463]. Interestingly, Mes3GeN=C=NH can be germylated by Mes3GeCl/Et3N to give Mes3GeN=C=NGeMes3. Germyl carbodiimides cyclo-[R2GeNCN]3 can be obtained from R2GeCl2 by the action of H2NCN/Et3N or LiNCNLi [474]. Chen et al. found an interesting example of synthesis of a digermanium amide derivative, when the formation of Ge–N bonds is achieved in different ways, including salt elimination and cleavage of the N–SiMe3 bond under the action of GeCl4 (Scheme 2.93) [475]. Me3 Si Me 3Si Me2 N

NMe 2 N

Li

N Li

NMe 2 SiMe3

N Me2 N

GeCl4

N

Et2O

Me2 N

SiMe 3 N

GeCl3

N Me2 N

- LiCl

GeCl4

Me2 N

- Me3 SiCl

N Me2 N

SiMe 3

GeCl3 N GeCl2 65%

SiMe3

Scheme 2.93  Different ways of formation of Ge-N bonds in substituted β-diketiminates.

Leung et al. in a reaction of GeCl4 with monoanionic silyl substituted lithium pyridyl-1-azaallyl ligand observed not only a typical salt elimination reaction, but also an elimination of Me3SiCl with 1,3-Me3Si-shift (Scheme 2.94) [476]. SiMe Ph N N

Li

Li N

N

SiMe

SiMe Ph

GeCl THF

N

GeCl

N

SiMe Ph

N SiMe

Cl Ge

Py-2 GeCl

N

Ph

GeCl

Ph

SiM e Ph

NSiMe

Ph

N

SiMe

SiMe N

- Me SiCl

SiMe

SiMe

Scheme 2.94  Formation of Ge-N bonds in interaction of GeCl4 with lithium pyridyl-1-azaallyl ligand.

N Me Si

Ge Cl

2-Py 48%

d(Ge-Cl) 2.143(2), 2.157(2) d(Ge-N) 1.813(6)

2.4  Organogermanium Compounds with Group 15 Elements

Stephan et al. investigated germanes with phosphinimido groups, [(t-Bu)3P=N]nGeMe4-n (n = 1, 2), readily available by the salt metathesis reaction between [(t-Bu)3P=NLi] and germanium chlorides [477]. Related germanium N,N,N’,N’tetraalkylguanidinate was synthesized by two methods using N-Li and N-H/Et3N compounds (Scheme 2.95) [467]. NLi Me2N NMe2 Et2 O GeCl4

Me 2 N

NMe2

NH Me2N

N

Cl

Cl

Ge N

d(Ge-N) 1.777(2), 1.768(2) d(Ge-Cl) 2.1957(7), 2.1954(7)

NMe2

NMe2

NMe2

Et3 N/Et 2O

Scheme 2.95  Synthesis of germanium N,N,N’,N’-tetraalkylguanidinate.

Rivard et al. investigated germanium complexes based on a bulky N-heterocyclic guanidine ligand, [IPr=N]- (Scheme 2.96) [478]. Mono- and di- [IPr=N]-substituted compounds can be obtained selectively using Me3Si derivatives. Dipp

Dipp

N

GeCl4

N

N

N

N PhMe N GeCl3 Dipp 93% d(Ge-Cl) 2.1354(4) d(Ge-N) 1.7582(14)

N SiMe3 Dipp [IP r=N]SiMe3

Dipp N

Dipp

SiMe3

Dipp

N

N Dipp

N

N

N

N

Ge N Dipp Cl2 Dipp 94% d(Ge-Cl) 2.1862(5) d(Ge-N) 1.7310(11)

PhMe

Scheme 2.96  Germanium complexes with N-heterocyclic guanidine ligand.

Veith et al., in a reaction of germylene [Me2Si(OBu-t)]2N-Ge-N3 with Me3SiN3, isolated a four-membered cyclic digermane, which formed through a Ge=N intermediate upon N2 molecule extrusion (Scheme 2.97) [479]. SiMe3 [(t -BuO )Me2Si]2N Ge: N3

Me3SiN 3 PhMe -N 2

[(t -BuO )Me2Si]2N Ge NSiMe3

N3

[(t-BuO)Me2Si] 2N [2+2]

N

Ge N3

N3 Ge

N

89%

N[SiMe2(OBu-t)] 2

d(G e-NSi2 ) 1.811(2) d(G e-NSi) 1.842(2), 1.846(2) d(G e-N 3 ) 1.871(3)

SiMe 3

Scheme 2.97  Formation of four-membered cyclic digermanes in interaction of [Me2Si(OBu-t)]2N-Ge-N3 with Me3SiN3.

Roesky et al. isolated a number of germanium amides in reactions of Ge(II) compounds, based on the polydentate N,Nligand, with azide and diazomethane derivatives (Scheme 2.98); the nature of the product depends on the substituent at the Ge atom [480, 481]; the reactions occur with [1,5]-H migrations.

Scheme 2.98  Reactivity of alkylgermanium(II) compounds containing a diketiminato ligand as a way for the synthesis of Ge-N derivatives.

135

136

2  Organogermanium Compounds of the Main Group Elements

Another type of product was found by Kato et al. in the interaction of germylene, based on the N,P-ligand, with N3SiMe3 (Scheme 2.99) [366]; in this case, a silylation of N atom is also observed. Dipp

N Ge NSiMe3 R2 P P

Bu-t

Dipp N3 N Ge PR 2 = N(SiMe3 )2 R 2P P

Dipp Me 3SiN 3

N Ge: R2 P P

Me 3SiN 3

P

N N

SiMe2

Bu-t

d(Ge-N) 1.890(2) d(Ge-NN) 1.902(2), d(Ge-NSi) 1.861(2) d(Ge-P) 2.239(1)

Scheme 2.99  Interaction of germylene, based on the N,P-ligand, with N3SiMe3.

Ionkin et al. found that germylene based on the P,O-ligand in the interaction with 1-AdN3 gave products of P and Ge oxidation (Scheme 2.100) [482]. A new Ge–P bond was formed with probable isobutene elimination; migration of Ad to Ge was also observed.

F3 C

CF3 O

t -Bu

Ge P Bu-t t-Bu

CF3

Bu-t 1-AdN3

P

PhMe

O CF3 100 oC F3 C

F3C

O

P

Ge

- N 2 t-Bu P O CF3 t-Bu F3 C

N

P N t-Bu Bu-t N

F3C CF3 O

Bu-t

t -Bu

Ad

Bu-t Ge

N 39% trans-

P O CF3 F3C

+

CF3 F3C O P t-Bu t -Bu

31P Ge-P NMR δ 69.99 ppm d(Ge-P) 2.2988(15) d(Ge-O) 1.817(3), 1.820(4) d(Ge-N) 1.753(4)

Bu-t Bu-t

Ad Ge

N Ge

O

N

P

Ad

16%

O CF3 F3C

F3C

CF3 OH

+

t-Bu

N P Ad Bu-t

d(Ge-O-Ge) 1.7304(5) d(Ge-O) 1.820(2) d(Ge-N) 1.745(3)

Scheme 2.100  Interaction of germylene based on the P,O-ligand with 1-AdN3.

Hahn et al. found that bis(benzimidazoline-2-germylene) disproportionates in solution at room temperature giving Ge(IV) spirotetraamide and metallic germanium (Scheme 2.101) [483].

t-Bu

N Ge N

N CMe2 2

hexane - Ge

Ge

N

N

d(Ge-N) 1.812(2) - 1.825(2)

N

t -Bu

Bu-t

Scheme 2.101  Disproportionation of the bis(benzimidazoline-2-germylene) in solution.

Weinert et al. reported a rare example of the synthesis of Ge amide (t-Bu)3GeN(H)C(Me)=CHCN (d(Ge-N) 1.895(2) Å) by the hydrogermolysis of the sterically hindered (t-Bu)3GeNMe2 in MeCN in the presence of HGePh3 [124]. Apparently, this is an example of the Thorpe reaction. Furthermore, control experiments at the heating of Ph3GeCH2CN (usually formed in situ from Ph3GeNMe2 in MeCN under hydrogermolysis conditions) in MeCN in the presence of HNMe2 showed that germanium-containing products of dimerization and trimerization of acetonitrile were formed (Scheme 2.102) [472]. These oligomerization products are also obtained in other cases of hydrogermolysis (e.g., in the reaction of (Ph3Ge)3GeNMe2 with Ph3GeH in MeCN) [484, 485]. Ph3 Ge CH2 CN HNMe 2 Ph3 Ge CHCN dimerization

MeCN

MeCN ∆ HNMe 2

Ph3 Ge

H N

1,3 Ge shif t

Ph 3Ge CHCN

MeCN

N

∆ HNMe2

CN MeCN

HN

Scheme 2.102  Reactivity of α-germyl nitriles with acetonitrile.

N 1,5 Ge shift

GePh3 N NH

CN

Ph3Ge

N H

GePh3

N

trimerization

N

NH

2.4  Organogermanium Compounds with Group 15 Elements

Germynes ArGe≡GeAr are starting compounds for the synthesis of new germylnitrogen derivatives, four-membered (GeNH)2, in reactions with substituted azides; the dependence of the product nature on the type of azide was observed (Scheme 2.103) [486]. The reactions took place with Ge≡Ge and C–S bond cleavage; extrusion of N2, typical for reaction of azides, was observed. In reaction with (n-Bu)3SnGeN3, trans-germanium(III) diradical was proposed as intermediate, which abstracts H atoms from the solvent. For (PhS)2GeAr(N=CH2), a concerted mechanism with C–S bond cleavage was proposed. According to XRD, in (PhS)2GeAr(N=CH2) a rare π-π interaction between an Ar ring and a non-aromatic N=C double bond is observed (3.2 Å). The Ge2N2 ring is almost planar with a high degree of planarity around the N atoms.

Scheme 2.103  N−H and N=C bond formation via germanium(III) diradicaloid intermediates and C−S bond cleavage in reactions of the digermyne ArGeGeAr with azides.

Upon interaction of digermyne ArGe≡GeAr (Ar = 2,6-Dipp-C6H3) with the diazo derivative N2CHSiMe3 Power et al. found that a compound containing three different N modes (μ-bridging, single and double bonded) was formed (Scheme 2.104) [402]; two germanium atoms are bonded by μ2:η1-CHSiMe3, μ2:η1-N2CHSiMe3, and μ2:η2-N2CHSiMe3 groups. Apparently, the product structure should be presented with charge-separation resulting in betain structure. Furthermore, interaction of this digermyne, ArGe≡GeAr, with tetracyanoethylene (TCNE) resulted in the cleavage of the Ge–Ge bond and formation of a large multi-ring system with the formula ArGe[(TCNE)]3[(GeAr)]3, in which the germanium atom is presented in Ge(II) and Ge(IV) oxidation states with direct Ge–N=C bonding. N Ar

Ge Ge Ar

Ar = 2,6-DippC 6H 3

N2CHSiMe3 n-hexane

Me3SiHC N Me 3Si

Ge Ar

Ge

N

Ar 56%

N=CHSiMe3

d(Ge-N) 1.8749(17)-2.0437(16)

Scheme 2.104  Reaction of ArGe≡GeAr with N2CHSiMe3.

In 2006, West et al. showed that :GeCl2∙dioxane reacts with N,N-di-tert-butyl-1,2-diiminoethane giving a product of [4+1] cycloaddition with two new Ge–N bonds, cyclo-[(t-Bu)NCH=CHN(Bu-t)]GeCl2 (d(Ge-N)av 1.8022(17) Å, d(Ge-Cl)av 2.156(2) Å) [487]. This interaction was also applied for germylenes of different types and substituted 1,2-diiminoethanes [354, 488, 489] or their analogues [490]. Interaction of the germole dianion [cyclo-C4Ph4Ge]Li2 with t-BuN=CHCH=NBu-t proceeds through the germylene intermediate, giving spiro germane (cyclo-C4Ph4Ge)(t-BuNCH=CHNBu-t) [491]. Interestingly, Weidenbruch et al. in 2005 showed that digermenes, Ar2Ge=GeAr2, reacted with N,N-di-tert-butyl-1,2diiminoethane in the same manner, yielding products of [4+1] cycloaddition [492]. In this case, dissociation of digermene to germylenes [R2Ge:], was proposed. Kinjo et al. showed that :GeCl2∙dioxane reacts in [4+2] cycloaddition fashion with an α,β-unsaturated imine giving a five-membered ring with new Ge–C and G–N bonds (Scheme 2.105) [493].

Scheme 2.105  Interaction of :GeCl2∙dioxane with an α,β-unsaturated imine.

137

138

2  Organogermanium Compounds of the Main Group Elements

In a number of elegant works, Uhl et al. investigated hydroalumination or hydrogallation of alkynylgermanes (proceeded by the Markovnikov rule; concerted cis-addition) resulting in 1,1-disubstituted Al(Ga) and Ge alkenes [40, 48, 53, 55, 465, 494]; these compounds can be modified by subsequent introduction of nitrogen-containing functional groups. Thus, reactions with unsaturated derivatives (isocyanates, azides, nitriles) took place via the Ge-X (X=C [495], Cl [496], H [210]) σ-bond activation (Scheme 2.106). This process was favored by the activation of Ge–X bond because of intramolecular coordination of X to the Lewis acidic Al atom. Ph Ph Ph Ge N

Bu-t ( t -Bu)2Al

PhN=C=O t-Bu

Ph Ge Ph

n-hexane Al(Bu-t) 2 Ph Cl

R' t-Bu Ph N N R'N3 Ge N Al(Bu-t)2Cl R n-hexane 49% t-Bu R = CCBu-t R' = CH2-4-C6H4Bu-t d(Ge-N) 1.897(1)

R

Ge

Al(Bu-t)2

t-Bu R = Ph, CCBu-t R' = Ph, Ad

d(Ge-N) 1.984(1) Bu-t

(t-Bu) 2Al

O

R'N=C=O n-hexane or C6H6

44%

R' Ph Ge R t-Bu

N O Al(Bu-t) 2Cl 40 - 99%

Scheme 2.106  Formation of new Ge-N bonds by insertion of isocyanates and azides into Al–C and Ge–Cl bonds.

The Banaszak Holl group showed that germylene :Ge[N(SiMe3)2] reacts with 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) in tetrahydrofuran resulting in ring-opening of THF and reaction of Ge(II) with the N=N bond giving a 15-membered digermane ring [497]; Ge–O and Ge–N bonds were formed. A number of germylenes, R2Ge:, studied by Power et al. [498], Aldridge et al. [17], and Jones et al. [499] take part in activation of ammonia to give R2Ge(H)NH2. The reaction of diarylgermylene, Ar2Ge: (Ar = C6H3-2,6-(C6H2-2,4,6-Me3)2), with hydrazine and methylhydrazine gave Ar2Ge(H)NHNHR (R = H, Me) [500]. According to theoretical analysis this reaction takes place via intermolecular proton transfer mediated by the second equivalent of H2NNH2. In 2002, Weidenbruch et al. found that acetylene-linked bis(germaethene) reacts with nitrile compounds in a cascade fashion, including formal [4+2] cycloaddition between C≡C–C=Ge and C≡N bonds with subsequent [2+2] cycloaddition of Ge=C and in situ formed C=C bonds (Scheme 2.107) [501]. Ar2 Ge

Ph

Ph

Ar 2Ge

GeAr2

Ph

NCCN

Ar2 Ge

N

NCCH=CHCN

Ph

Ar2 Ge

Ph

t-Bu Ph

Ar2 Ge N

Ar =

CN CN

Scheme 2.107  Cycloaddition reactions of an acetylene-linked bis(germaethene).

The reaction of germene Mes2Ge=CR2 (CR2 = fluorenylidene) with nitriles gave different products depending on the type of nitrile [502]. With t-BuCN, a product of [2+2] cycloaddition was formed; interaction with PhCN resulted in [4+2] cycloaddition with subsequent rearomatization via [1,3]-sigmatropic H-shift, whereas the reaction with R’CH2CN (R’ = H, Ar) was accompanied with abstraction of the acidic proton (Scheme 2.108). Mes2 Ge CHR' Mes2Ge N 48%

CR2 t-BuCN t-Bu

Et2 O

CN R'CH 2CN

CR 2 H 79 - 82% Et2O

Mes 2Ge=CR 2

H Mes2Ge

PhCN

N Ph

78%

CR 2 =

d(Ge-N) 1.855(4)

Scheme 2.108  Reactivity of the germene Mes2Ge=CR2 toward nitriles as a way to Ge-N derivatives.

Related products were obtained in reactions of phosphagermaallene Tip(t-Bu)Ge=C=PMes* (Mes* = 2,4,6-(t-Bu)3C6H2) with nitriles ([2+2] cycloaddition; in the presence of acidic α-protons, ene-reaction and 1,2-addition), with imines ([2+2]

2.4  Organogermanium Compounds with Group 15 Elements

cycloaddition), nitrones and nitrile oxides ([3+2] cycloaddition) [503], and with N,N’-dicyclohexylcarbodiimide ([2+2] cycloaddition between Ge=C and C=N) [504]. Fulton et al. found that germanimine R2Ge=NMes (R2 = O(SiMe2N(Bu-t)2) behaves as a transition metal imide complex undergoing [2+2] cycloaddition with heterocumulenes ((i-Pr)N=C=N(Pr-i), RN=C=O) and protic reagents. Besides, this germanimine acts as a diene and reacts in [4+2] cycloaddition manner with PhCHO, Ph2CO, PhCH=CH2, PhC≡CH (Scheme 2.109) [505]. NHMes R2Ge Mes NHAr R2Ge

ArNH2 R'R''C Y=C= NR'

N

NR'

Y

R2Ge =N

[2+2]

X

[ 4+2]

R'' X R2Ge

R'

N

Y = i-PrN, O

Scheme 2.109  Chemical behaviour of germanimine R2Ge=NMes as a way to Ge amides.

Jambor, Turek, et al. observed spontaneous double hydrogermylation induced by N→Ge coordination in organogermanium hydrides, resulting in the formation of two Ge–N bonds and the reduction of C=N bonds in ligands (Scheme 2.110) [506]. Ar

N Ge Cl Ar

N

N Cl

K[BEt 3 H] THF

Ge

Ar

N Ge

H

H

N

Ar

Ar

Ar

34% d(Ge-N) 1.8250(19), 1.8295(18)

N

Scheme 2.110  Formation of Ge–N bonds under reduction of C=N bonds in ligands.

In 2015, Sita et al. reported the synthesis of Me3GeNCO (for the labeled compound, 15N NMR δ 31.90 ppm) by action of Me3GeCl and CO (10 psi) on [((i-Pr)N=CHMe-N(Pr-i))(Cp*)Mo-N≡N-Mo(Cp*)((i-Pr)N-CHMe=N(Pr-i))] (the labeled MoN2 complex may also be used) under photolytic conditions [507]. According to XRD analysis data, in the crystalline compounds, the d(Ge-N) varies from 1.72 Å [477] to 2.05 Å [402] (the typical value is 1.84 Å).

2.4.2  Organogermanium Compounds with Ge–P Bond Nowadays, the classical reaction of alkali metal phosphides (LiPR2) with germanium halides is still used for the formation of the Ge–P bond [41]. Pietschnig et al. synthesized Cp*Ge(Cl2)P(SnMe3)2 (31P NMR δ -242.4 ppm; d(Ge-P) 2.274(2) Å, d(Ge-Cl) 2.189(2) Å and 2.192(2) Å) using another classical approach (transmetalation by removing of Me3SnCl) in the interaction of Cp*GeCl3 with P(SnMe3)3 [508]. Catalytic heterodehydrocoupling of primary phosphines with germanes was investigated by Waterman et al. [509]. In the case of alkyl phosphines, the reaction is complicated by obtaining mixtures of compounds, including Ph2Ge(PHCy)2 (Scheme 2.111). The key intermediate in the proposed catalytic cycle includes a four-membered ring with an agostic Ge∙∙∙H∙∙∙Zr interaction. t-BuGeH3 Ph2GeH2

RPH2 t -BuGe(H2)P(H)R 5 mol. % [Zr] R = Ph, Cy C 6H 6 RPH2 Ph2Ge(H)P(H)R 5 mol. % [Zr] R = Ph, Cy C6H6

Me SiMe3 Me3Si Me3Si NN Zr N N

H R

R'' H R' Ge H

P [Zr]

Scheme 2.111  Formation of Ge-P bond under zirconium-catalyzed heterodehydrocoupling of primary phosphines with germanes.

Tetraphosphanylsilane Si(PH2)4 acted as a mild PH2-transfer reagent in the reaction with (C6F5)3GeCl giving (C6F5)3GePH2 (97% yield; d(Ge-P) 2.307(1) Å, 31P NMR δ -218.7 ppm) [510]. At the same time, the reaction of Si(PH2)4 with GeCl4 gave only unstable GeCl4-n(PH2)n. Tetraphosphorylated germane Ge(PH2)4 (31P NMR δ -194.8 ppm) as a highly unstable compound was prepared in only 2% yield by the action of (i-Bu)2AlPH2∙THF on GeCl4 [511].

139

140

2  Organogermanium Compounds of the Main Group Elements

Germylated fluorophosphaalkene (F3C)3GeP=C(F)NEt2 (59%; d(Ge-P) 2.444(2) Å; 31P NMR δ -133.4 ppm) was ­selectively obtained as a (Z)-isomer from (F3C)3GeI by abstraction of proton from HP=C(F)NEt2 in the presence of Et3N [512]. Four-membered [P2M2] heterocycles (M  =  Al, Ga, In), containing a GeMe3 group at P atoms, were obtained by ­transmetalation reactions of P(GeMe3)3 and Me2MX (X = Me, H) (Scheme 2.112) [513]. GeMe3 Al

Me3Ge

GeMe3

Me2AlH

P

Me3M

Me3Ge

P(GeMe3)3 GeMe3 PhMe P Al PhMe Me3Ge 92% - Me4Ge - Me3GeH d(Ge-P)av 2.3180(9)

M = Al (83%), 31P NMR δ -228.58 ppm P 31 GeMe3 M = Ga (85%), P NMR δ -201.61 ppm

M M

P Me3Ge

92%

M = In (68%), 31P NMR δ -224.69 ppm, d(Ge-P)av 2.3108(8)

Scheme 2.112  Transmetalation reactions of P(GeMe3)3 with Me2MX (X = Me, H).

du Mont et al. described interaction between :GeCl2∙dioxane and alkylidenediphosphanes (Me3Si)2C=P-PR2 giving bicyclic [-C(SiMe3)2-P-GeCl2-]2 (31P NMR δ 126.3 ppm), containing the P–P bond [514, 515]. Intermediate formation of the unsymmetric diphosphene [(Me3Si)2(Cl3Ge)C–P=P–C(SiMe3)2GeCl2PR2] and [Cl2Ge(PR2)2] was observed by NMR spectroscopy. Weidenbruch et al. showed that germylene Ar2Ge: (which exists as digermene Ar2Ge=GeAr2 in solid state) reacts with the P≡C bond in phosphaalkyne to give 4-membered germadiphosphacyclobutene; cascade reactions, including [2+1] cycloaddition with a subsequent 3-membered ring opening (formal [3+1] cycloaddition), were proposed (Scheme 2.113) [414].

Ar

Ge Ar

Ge Ar

t-BuC P

Ar2Ge:

Ar2Ge Ar

Ar

Ar2Ge=GeAr2

t-Bu

n-hexane

t-Bu

P

Ar

P

t-Bu

Ge P

t-Bu

Ar = 59% P d(Ge-P) 2.4351(9)

Scheme 2.113  Reaction of a diarylgermylene with a phosphaalkyne.

Digermenes react with related methylphosphaalkyne P≡CMe in an unusual way, yielding 2,3,5,6-tetraphospha-1,4-dimethylidenecyclohexane (Scheme 2.114) [516]. The proposed mechanism includes [2+1] and [2+2] cycloadditions to obtain 2,4-diphosphagermole, the rearrangement of which through a 1,3-H shift and a hydrophosphination reaction would result in the dimeric product. R2Ge=GeR2 [2+1]

P CMe R

R

P CMe

Ge

R

P

P [2+2]

R

Ge P

PP-Ge NMR δ -13.7 ppm d(Ge-P) 2.3715(7)

P

P

85%

R=

P

P

P

Ge R

R R Ge P

R

31

Et2O/PhMe

Ge P CMe R

R

1,3-H shift

SiMe3 SiMe3

R R Ge PH P

Scheme 2.114  Reaction of methylphosphaalkyne with digermenes.

Treatment of cage phosphorus compounds with Lappert’s germylene resulted in its insertion into the P–P bond (Scheme 2.115) [517]. t-Bu t-Bu t-Bu

P P P

P P

P

t-Bu

t-Bu :Ge[N(SiMe3)2]2 t-Bu t-Bu hexane

P

P P

P

Ge[N(SiMe3)2 ] 2

P P

t-Bu

60%

31

PP-Ge NMR δ 26.5 ppm

d(Ge-N) av 1.880(4) d(Ge-P) av 2.4003(12)

Scheme 2.115  Insertion of Lappert’s germylene into the P–P bond of the P6C4tBu4.

2.4  Organogermanium Compounds with Group 15 Elements

In contrast, the anionic diphosphorus complex reacted as a nucleophile with Ph3GeCl (Scheme 2.116) [518]. In solution, an exchange of the Ph3Ge group between P atoms is observed. P

P Cy2P

Ph3GeCl

X

THF

P

X

Cy2P

X = Mo(CO)Cp

GePh3

X P

X

d(Ge-P) 2.3388(7)

36%

Scheme 2.116  Reaction of the anionic diphosphorus complex [Mo2Cp2(μ-PCy2)(CO)2(μ-κ2:κ2-P2)]− with R3GeCl.

A similar germanium cage compound was prepared by the nucleophilic reaction of potassium 1,2,4-triphospholyl with GeI4 (Scheme 2.117) [519]. Apparently, in this synthesis the [(η1-P3C2(Bu-t)2)2GeI2] intermediate rapidly undergoes intramolecular [2+2] cycloaddition reaction to give four new P–C bonds; the steric effect of the large I ligand facilitates the ring coupling. t-Bu P

t-Bu P

t-Bu P

GeI4

I

I

P

P

P

K

t-Bu

t-Bu

Ge

P

P

t-Bu t-Bu

P

P

t-Bu

P

t-Bu

P P

P

P t-Bu

31

GeI2 52%

PP-Ge NMR δ 37.2 ppm d(Ge-I)av 2.5268(11) d(Ge-P)av 2.337(2)

Scheme 2.117  Synthesis of cage compound P6C4But4GeI2.

Germylenes Ar2Ge: were oxidatively inserted into the P–H bond (P–H bond activation) of PH3; the product of arene elimination was registered by NMR spectroscopy (Scheme 2.118) [520].

Ar2Ge:

PH3 (80 psi) PhMe

Ar 2Ge(H)PH2 + P NMR δ -232 ppm d(Ge-P) 2.3194(11)

31

H H P GeAr ArGe P H H

31

Ar =

Mes

Mes

P NMR δ -180 ppm

Scheme 2.118  Activation of PH3 with diarygermylene as method of Ge-P compounds synthesis.

Activation of white phosphorus P4 by kinetically stabilized germylene Ar2Ge: resulted in insertion of Ar2Ge: into one P–P bond of the P4 cage (Scheme 2.119) [521]. Interestingly, UV irradiation allows reversing this reaction to give the starting germylene (oxidative addition/reductive elimination).

Ar2Ge:

P

P4 PhMe hν

P

P Ge Ar 2

31

P 75%

PP-Ge NMR δ -135.4 ppm d(Ge-P)av 2.3471(9)

Mes Ar =

Mes

Scheme 2.119  Activation of P4 by diarylgermylenes.

Germylenes based on different ligands oxidatively inserted into the R’P–Cl bond (R’=Alk and Ar [522–524], R’=R”2N [523]) to give Ge(IV) derivatives (Scheme 2.120). Interestingly, the reaction of Lappert’s germylene, [(Me3Si)2N]2Ge:, with PCl3 gave the product with a novel P–P bond, ([(Me3Si)2N]2Ge(Cl)P(Cl))2 (d(Ge–P) 2.4086(6) Å, d(Ge-Cl) 2.1853(5) Å, d(Ge-N) 1.8373(16) Å, and 1.8386(16) Å) [523]. A range of works concerns the chemistry of Ph3GePCO. It was found that the action of ambident NaOCP on Ph3GeCl selectively resulted in the phosphaketene Ph3Ge–P=C=O isomer (31P NMR δ -344.0 ppm; 13CCO NMR δ 182.5 ppm; IR νCO 1954 cm-1) [525]. Its reduction by the action of K[HBPh3] results in the stable anion, which under methylation led to methylformyl phosphines (Scheme 2.121) [526]. Unfortunately, HC(O)P(Me)GePh3 was not isolated in pure form. Methylation by an excess of MeI made it possible to synthesize the phosphorus analog of DMF, HC(O)PMe2; therefore, Ph3Ge group can be regarded as protecting. Nucleophilic addition of KP(Bu-t)2 to Ph3GePCO gave an intermediate anion that is converted by alkylation to unsymmetrical diphosphaureas (Scheme 2.122) [527]. These ureas thermally and photochemically transformed into new organogermanium species. Interestingly, different conditions of nucleophilic silylation of Ph3GePCO resulted selectively in (E-) or (Z-) phosphaalkenes that interconvert upon UV irradiation (to E-) or heating (to Z-) [528].

141

142

2  Organogermanium Compounds of the Main Group Elements Bu-t Me 2Si

N B

N

N

Ge:

hexanes Ar

Ge

PPh 2 Ph 2PCl

N B

Cl

1:1 56% Ar 31P NMR δ -10.4 ppm d(Ge-P) 2.3125(16)/2.3107(16)

N N

31P

NMR δ -51.9 ppm d(Ge-P) 2.3306(5), 2.3357(5) d(Ge-Cl) 2.2075(5), 2.1979(5) d(Ge-N)av 1.8249(16), 1.8338(16)

R2Ge Cl P Ge Cl R2 52%

GeR 2 P Cl GeR2

PCl2 Cl

Cl2P

R2Ge: Bu-t

Ar N

N

Ar RPCl2

Ge:

N B

2:1

Ar

N N

Ge

Ar

R P

R = Ph 59 % 31P NMR δ -34.4 ppm R = tBu 59 % Ar = 31P NMR δ 44.5 ppm

N Ge B N N Cl

Cl

Ar

Ar

d(Ge-P) 2.3200(11)

Scheme 2.120  Insertion of germylenes into R’P–Cl bond. O

MeI, [Bu4N][Ph3SiF2] K[HBPh3]

Ph3GeP=C=O

Ph3B

THF

O

THF - Ph3SiF, -KI, -[Bu4N][Ph3BF]

K

THF

64%

P Me

GePh3

Ph3B O

18-crown-6

P GePh3

H

H

31

P NMR δ -46.3 ppm

31

H P GePh3 [K(18-crown-6)]

P NMR δ 23.9 (E -), 0.6 (Z-) ppm

(E -): d(Ge-P) av 2.274(4) > 99%

Scheme 2.121  Reactivity of Ph3GePCO as a method of synthesis of Ge-P derivatives. O Ph GeP=C=O

THF/PhMe

Ph SiP(Bu-t) THF Ph Si P(t-Bu)

O P

P(t-Bu) P GePh 81% K P NMR δ 26.3 ppm Ph SiCl THF

GePh

60% EP NMR δ 160.7 ppm d(Ge-P) 2.323(1)

UV

P(t-Bu)

UV GePh Ph Ge GePh GePh P(t-Bu) P + P P P C D Me Me Me -CO, -[(t-Bu) P] 62% Me r ac- and mesoNMR δ -19.7 ppm P UV GePh O P(t-Bu) P C D GePh GePh P(t-Bu) -CO P ∆ 76% GePh P(t-Bu) GePh + Ph GeP=C=O P NMR δ -36.8 ppm THF P(t-Bu)

THF

Ph GeCl

O SiPh3

∆

O

MeI

KP(Bu-t)

THF

P GePh

52% ZP NMR δ 261.4 ppm d(Ge-P) 2.348(2)

Scheme 2.122  Reactivity of Ph3GePCO with P reagents and chemical behaviour of the resulting products as a synthetic method to new germanes.

Nucleophilic addition of NHCs to electron-deficient germylphosphaketene Ph3GeP=C=O resulted in corresponding zwitterionic adducts (Scheme 2.123) [529]. Upon thermolysis these adducts undergo decarbonylation. Ar N

N Ar

Ph3GeP=C=O

Ar N

N Ar

Ar N

∆

-CO P O 91-98% GePh3 31 P NMR δ (-8.5)-9.1 ppm d(Ge-P) av 2.3471(9)

Ar = Dipp, Mes

N Ar P

GePh3

91-92%

31

P NMR δ (-155.2)-(114.7) ppm

Scheme 2.123  Interaction of NHCs with germylphosphaketene Ph3GeP=C=O.

In 2018, Roesky et al. published a work concerning germylphosphaalkene cAACP-GeCl3 obtained from the lithium phosphinidene and GeCl4 (Scheme 2.124) [530]. THF P

Li

THF Dipp N P

Li N THF Dipp THF [cAACPLi(THF) 2] 2

THF P

Li

THF Dipp N P

Li N THF Dipp THF

[cAACPLi(THF)2 ]2

Scheme 2.124  Synthesis of germylphosphaalkene cAACP-GeCl3.

GeCl4 Et 2O

P N Dipp

31P NMR 47.8 ppm δ d(Ge-P) 2.2524(5) GeCl3

85%

cAACP-GeCl3

2.5  Organogermanium Compounds with Group 16 Elements

In the solid state, the Ge–P bond length varies between 2.21 Å [366] and 2.44 Å [414] (according to XRD analysis) with 2.33 Å as the mean value.

2.4.3  Organogermanium Compounds with Ge–As Bond In 2019, Roesky et al. described cyclic Ge2N2 derivative obtained from lithium arsinoamide by salt elimination and subsequent insertion of the chlorogermylene into the As–N bond (Scheme 2.125) [531].

Scheme 2.125  Synthesis of Ge-As compounds by reaction of chlorogermylene with lithium arsinoamide.

2.5  Organogermanium Compounds with Group 16 Elements 2.5.1  Organogermanium Compounds with Ge–O Bond Germanium alkoxides, RnGe(OR’)4-n (n = 1, 2), can be obtained by the classical method from halides RnGeCl4-n by the action of R’OH in the presence of Et3N [7, 66, 205, 358, 532]; phenoxide derivatives were also synthesized under related conditions [74, 533]. The reaction of alkali metal alkoxides or related compounds with germanium halides also resulted in the formation of Ge–O bonds [213, 534–536]. The reaction of silver(I) salts with germanium halides is a special case of the formation of Ge–O bonds. Thus, Ph3GeON=NOGePh3 (d(Ge–O) 1.824(5) Å) is formed by the action of Ag2O2N2 on Ph3GeBr [537]. Germanium tetraalkoxide can be obtained using a sterically voluminous diol ligand, such as 1,1ʹ-bicyclohexyl-1,1ʹ-diol (d(Ge-O) 1.749(3)–1.751(3) Å), upon its interaction with GeCl4 in the presence of Py; the yield is extremely low (Ge=C
Ge=Si
Ge=Si< over the disilene >Si=Si 300 nm) Si

or ∆ (215 °C) R

62

R

R

Ge

Si

R

61

t

[R = SiMe Bu2]

Scheme 10.30  Synthesis of the first isolable silagermene 61.

The thermal isomerization of the isolable disilagermabicyclo[1.1.0]butane 63 formed the nearest homologue of the above-described 61, distinguished from the latter by only a CH2 unit, namely novel 1H-disilagermirene 64, featuring a more deshielded sp2 Si center resonating at 126.6 ppm (cf.: 100.7 ppm in 61) (Scheme 10.31) [44]. 64 represented the first example of an alkyl-substituted cyclopropene analogue of the heavy group 14 elements. R

R R

Ge

CH2 Si

R

63

Si

130 °C toluene

Si R

[R=SiMetBu2]

CH2–R

R

Ge

Si

R

64

Scheme 10.31  Synthesis of the alkyl-substituted 1H-disilagermirene 64.

The next three stable silagermenes, Mes2Ge=Si(SiMetBu2)2 65 [79], (tBu2MeSi)2Ge=SiMes2 66 [55], and Mes2Ge=Si(SitBu3)2 67 [80], were uniformly prepared by the coupling of 1,1-dilithioderivatives (tBu2MeSi)2SiLi2, (tBu2MeSi)2GeLi2 and (tBu3Si)2SiLi2 with diaryldichlorides Mes2GeCl2, Mes2SiCl2 and Mes2GeCl2, respectively. The silagermenes 65 and 67 exhibited unusually shielded sp2 Si centers, observed at 22.4 and 18.7 ppm, respectively. However, silagermene 66 expectedly displayed a strongly shielded double bonded Si nucleus (146.9 ppm). This striking difference was realized as a result of the specific substitution pattern (electron donating groups on Si and electron withdrawing groups on Ge) in 65 and 67, altering the inherent Geẟ–=Siẟ+ bond polarity (as in 66) into a reversed Geẟ+=Siẟ– one. Of these three silagermenes, only 67 was structurally characterized, showing peculiar features caused by the exceptionally high steric crowding of its substituents, namely: quite long [rGe =Si = 2.2769(8) Å] and remarkably twisted [τ = 24.7°] Ge=Si double bond, but perfectly planar geometry at the doubly bonded Ge and Si atoms [ΣGe = ΣSi = 360°]. On heating at 100°C, the silagermene 67 undergoes quantitative isomerization to form a symmetrically substituted isomer (E)-[Mes(tBu3Si)Ge=Si(Mes)SitBu3] [80]. A symmetrically substituted stable silagermene, (tBuMe2Si)2Ge=Si(SiMe2tBu)2 68, prepared by the reductive dehalogenation of the 1,2-dibromosilagermane Br(tBuMe2Si)2Ge–Si(SiMe2tBu)2Br with sodium in toluene, was reported by Iwamoto, Kira, and coworkers [18]. As is diagnostic for silagermenes, 68 revealed: low field resonance of the sp2 Si atom [ẟ(29Si) = 144.0 ppm], rather short (compared to other structurally characterized silagermenes) [rGe=Si = 2.2208(4) Å] and almost undistorted [τ  =  7.5°] Ge=Si double bond, and nearly planar geometry at the sp2 Si and Ge atom [θ = 0.6°].

10.2  Heavy Analogues of Alkenes

An interesting compound featuring a “H2Ge–SiH2”-fragment entrapped within the push-pull complex with both Lewis acid [W(CO)5] and Lewis base [IPr (IPr  =  1,3-bis(2,6-diisopropylphenyl)-2H-imidazol-2-ylidene)], {[(OC)5W]←:GeH2– H2Si←[:IPr]} 69, was reported by Rivard and coworkers [81]. 69 was prepared by the reaction of the nucleophilic dichlorosilylene complex IPr∙SiCl2 [82] with [GeCl2∙W(CO)5] complex forming isolable perchlorinated complex [(OC)5W∙GeCl2–Cl2Si∙IPr], which was subsequently reduced with LiAlH4 to form the final complex [(OC)5W∙GeH2– H2Si∙IPr] 69, which can be considered as the “inorganic ethylene” donor-acceptor complex. The Si nucleus in 69 resonated at a very high field [ẟ(29Si) = –71.9 ppm (triplet, 1JSi–H = 192.2 Hz)], and the presence of the Ge–H and Si–H bonds was confirmed by the IR spectroscopy (stretching vibrations at 1959 and 2140/2150 cm–1, respectively). The length of the Si–Ge bond in 69 of 2.3717(14) Å corresponds to that of the single bond, corroborated with the results of the theoretical studies (B3LYP/cc-pVDZ-pp) on the model compound (Me substituents instead of the real 2,6-iPr2-C6H3 groups on N atoms in IPr ligand): the value of WBIGe–Si of 0.88 suggested the presence of a single bond with no evidence for Ge=Si double bonding. The most recent example of isolable silagermene was reported by Scheschkewitz and coworkers. This remarkable cyclic potassium silagermenide 70, as a Si=Ge analogue of a vinyl anion, was prepared by the reduction of germylene precursor with KC8 (Scheme 10.32) [83]. The spectral and structural characteristics of 70 well agrees with those of the above-described silagermenes: characteristic low field resonance of the doubly bonded Si atom [ẟ(29Si) = 142.9 ppm (in C6D6) and 138.5 ppm (in THF-d8)], and short Ge=Si double bond [rGe =Si = 2.2590(3) Å]. The GeSi2C four-membered ring in 70 is practically planar with a negligible folding of 1.9°. Based on its X-ray and UV data, π conjugation between the endocyclic Ge=Si and exocyclic C=N double bonds in 70 is significant with the calculated value for the πGe =Si–π*C =N interaction energy of 23.6 kcal/mol. Tip

Tip NHC

Si N C

Ge:

Xyl

Ge

N C Xyl

70

Tip Tip

Tip i

Pr

Si

ECl

Ge

N C Xyl

71a,b [a: E = SiPh3;

K(18-crown-6)

Si

Cl

THF

Tip Si

THF

Si Tip

Tip

KC8 18-crown-6

E

Si Tip

N

NHC = N i

Pr

b: E = P(Ni Pr 2 )2] Scheme 10.32  Synthesis of the anionic silagermenide 70 and neutral silagermenes 71a,b.

Silagermenide 70 can be further functionalized at the anionic Ge center to form neutral Ge-substituted silagermenes 71a,b (Scheme 10.32) [83]. Both (silyl)silagermene 71a (E  =  SiPh3) and (phosphanyl)silagermene 71b (E  =  P(NiPr2)2) revealed low field sp2 Si signals [ẟ(29Si) = 136.6 ppm (singlet) and 104.5 ppm (doublet, 2JSi–P = 9.8 Hz), respectively], and short Ge=Si bonds [rGe=Si = 2.2020(2) and 2.2252(4) Å, respectively]. Similar to the case of starting material 70, the GeSi2C four-membered ring in 71a is planar (folding angle = 0.2°) and the Ge atom is insignificantly pyramidalized (ΣGe = 357.3°). Silagermenes, in which the Si=Ge double bond is incorporated into a 1,3-diene >Ge=Si–C=CGe=Si=GeSi=Ge=SiSi=Ge: unit, are discussed separately in the Sections 10.3.2, 10.4.1, and 10.5.2, respectively. 10.2.2.2.3  Germastannenes >Ge=Sn 12 kcal/mol) compared to the Berry pseudorotation (∆G# 2–8 kcal/mol) [30, 31]. When interpreting the results of NMR studies, the important question is what is the mechanism of substituents exchange in the coordination polyhedron? In the original work by Ugi et al. [38], it was shown that the implementation of the Berry pseudorotation mechanism is unlikely in cases that do not follow the rule that the most electronegative atom should occupy the axial position. However, in a number of cases, the difference in the relative electronegativity of equatorial atoms is not very significant, and therefore it is necessary to consider all factors that can affect the ligand exchange process. Thus, the answer to the question posed in each specific case can be approached from several points of view. First, analysis of the geometry of the molecule to assess potential steric factors hindering the exchange of substituents. It is very likely that these factors will lead to an increase in the activation energy of the process. Theoretically, this approach can be formalized by considering, for example, solid angles in the coordination polyhedron [39–41]; however, experimental data on the relationship between the structure and activation energy are still insufficient. Analysis of the geometry of the coordination site of both the initial structure and related molecules (with a similar coordination site structure) seems to be more fruitful for elucidating the mechanism of the dynamic process. For spirocyclic molecules, the Holmes method is used [42], in which deviations of the angles between certain planes formed by substituents in the coordination polyhedron from those in ideal SP and TBP are considered. Second, the simplified formula given in the work of Kost and Kalikhman [43], using the angles between the axial and equatorial substituents, is also often used. The nature of the change in the angles in a series of closely related spirocyclic molecules, corresponding to the one theoretically predicted for Berry pseudorotation, may allow an unambiguous conclusion about the mechanism [44]. Third, quantum chemical modeling of processes can also provide important information [45–47]. However, it should be noted that an unambiguous conclusion requires the study of an extensive potential energy surface area (more precisely, a hypersurface), which is rather difficult. A revision of the understanding of the mechanisms of substituent exchange in compounds with TBP configuration of the coordination polyhedron of silicon, phosphorus, and some transition metals [48] showed that there are only three mechanisms from the analysis of the potential energy surface: Berry pseudorotation, triple cyclic permutation (threefold cyclic permutation), and semi-twist axial-equatorial interchange. Berry’s pseudorotation and Ugi’s “turnstile” rotation are topologically equivalent [48]. Irregular mechanisms can also be implemented in compounds of group 14 elements with TBP configuration. First of all, the possibility of reversal of the configuration of silicon atom by breaking and restoration of axial bonds in a process that can be considered as SN2-like should be noted. This process can either be reversible or irreversible, depending on the effect of the solvent, the donor ability of atoms considered as attacking and leaving groups, and the nucleofugality of the leaving group. For a silicon atom with a CN 5, similar processes have been repeatedly described using the Burgi method of structural correlations [49], based on both crystallographic data [50–53] and quantum chemical calculations [54–57]. It should be noted that for germanium complexes, similar studies are virtually absent. For compounds of hexacoordinated group 14 elements, the probable mechanisms of ligand exchange at the coordination center can be divided into the same two main groups as pentacoordinate compounds: non-dissociative (regular) and dissociative (irregular) mechanisms. Dissociative (irregular) mechanisms in hexacoordinated complexes include breaking and 3

1

5

4 1

3 5 2

2 4

Figure 15.2  Mechanism of Ugi’s turnstile rotation.

Introduction and Outline

subsequent restoration of various bonds [58, 59]. Trigonal [60–62] and diagonal [63, 64] twists (Scheme 15.1) have been proposed to describe permutation processes in accordance with non-dissociative mechanisms. C D *D

M

C

D

Y

Y

*D

X

D

X

C*

X

C*

D

D*

C

D

X

Y

M

C

C*

C

*

Y

*C

D*

C

Y

* C

D*

C

M D*

Y X

a

D *D

M C*

C

C

C *D

Y X

D

*C

X * C

D

Y

X

D

C D*

D

Y

* C

M

M

X Y

D*

X

b

D D M

* D

Y

M

X

Y

D*

X

D

M

Y X

* D c

Scheme 15.1  Stereoisomerization of hexacoordinated bischelates L2MXY through the Bailar twist (a), Ray–Dutt twist (b), and the formation of a bicapped tetrahedral transition state (c).

The simple intramolecular ligand exchange processes participating directly in non-dissociative mechanisms are well known as the Bailar twist and the Ray–Dutt twist, where one triangular face of the octahedron twists relative to the opposite face. In fact, the final structures for the Bailar and Ray–Dutt twist mechanisms correspond to the octahedral species for an intramolecular mechanism, for example, (X,Y)-exchange in hexacoordinated bischelate complexes with trans-configuration of the two coordinating D atoms (Scheme 15.1). These processes are insufficient to account for observations of the dynamic NMR spectra, and a more complicated mechanism involving a bicapped tetrahedral transition state has also been proposed (Scheme 15.1) [65–67]. For the latter mechanism, the covalently bonded chelate ligands change location by twisting out of the plane formed by the axial bonds and ending in the middle of the cis-MXY moiety, while the donor atoms D are pushed out from this plane (Scheme 15.1). The topology of some of the considered mechanisms is discussed in detail in [68]. Both models for interconversion between isomers in neutral hexacoordinated bischelate species and detailed discussions of the influence of the nature of mono- and bidentate ligands on the activation barrier for dynamic processes have been reported for hypervalent 12-M-6 species containing 12 formally assignable electrons at the central atom of a group 14 element [69]. 15.1.2.3  DNMR Spectroscopy

Similar to the case of silicon complexes with their 29Si nuclei [70], the most complete and reliable information on the structure of the coordination site of hypervalent germanium complexes can be provided by 73Ge NMR spectroscopy. However, significant limitations of the 73Ge NMR method considerably narrow its possibilities for studying not only the stereochemical non-rigidity of complexes, but also their structural features in the liquid phase. There are only a few examples of using the 73Ge NMR spectroscopy to characterize complexes of hypervalent germanium [71, 72]. It should be noted that the rate of stereodynamic processes usually varies in the range from 10–1 to 10–5 s; therefore, the only method for studying them at present is NMR spectroscopy. This fact along with the development of the DNMR theory contributed to the wide practical application of the latter in various fields, including organoelement chemistry. Since the publication of the first fundamental works in this area [73, 74], a number of monographs and detailed reviews have been published on the theoretical and applied aspects of DNMR [75–87].

633

634

15  Dynamic Stereochemistry of Penta- and Hexacoordinate Germanium(IV) Complexes

As a rule, the conclusions about the stereodynamic behavior of molecules are deduced on the basis of line shape analysis of the temperature-dependent 1H NMR spectroscopy [78, 88]. Currently, two methodological approaches are being implemented to determine the activation parameters of stereodynamic processes using the DNMR method. The first one involves calculation of ∆G# at Tc using the modified Eyring equation [78]: ∆G # = ( 4.57 ×10−3 ) Tc (10.32 + log (Tc 2 / π∆ν )), where Tc is the coalescence temperature (К), and Δν is the difference of resonance frequencies of exchanging nuclei (Hz). The free energy of exchange activation ∆G# is calculated at Tc, which allows one to neglect the influence of systematic errors and determine its value with high accuracy. Indeed, in precise experiments with a temperature determination accuracy of ±0.5°С, the error in the ∆G# estimate may not exceed ±1 kcal/mol. A significant limitation of this approach is the impossibility to obtain quantitative data on the values of the enthalpy and entropy of activation. The second approach, devoid of this drawback, is based on the TLSA method for exchanging signals in the NMR spectra. With a sufficient number of experimental points, the least-squares method, despite the known limitations [83], gives a random error in estimating the ∆H# and ∆S#, comparable to the accuracy limit under the conditions of a precise experiment.

15.2  Structure of Compounds with Penta- and Hexacoordinate Germanium In monographs devoted to the properties of elements, it is customary to compare them on the basis of a number of quantitative indicators, such as relative electronegativity, atomic, covalent, and various ionic radii, as well as the lengths of the most common bonds (element–carbon, element–nitrogen, element–oxygen). Comparison of germanium with other elements of group 14, taking into account covalent, ionic, and van der Waals radii [89–91], as well as chemical properties, shows that this element is close to silicon, but still not completely analogous to it. First of all, this is manifested in the tendency of germanium to increase its coordination number (CN) to five and six, that is much less pronounced than that of silicon. According to the Cambridge Structural Database (CSD), the crystal structures of 2609 germanium compounds with a coordination number of four have been determined, in the compounds in which germanium is bonded to the atoms of the main groups 14–17 of the second and third periods, as well as to halogens, selenium, tellurium, and hydrogen atoms. Also, there are 433 germanium compounds with CN 5 and 383 with CN 6. For silicon, the numbers of publications on compounds with CN 4, 5, and 6 exceed those for germanium several times (46 572, 1351, and 1277, respectively). Thus, the number of germanium compounds with an extended coordination sphere is significantly smaller than that for silicon compounds. Depending on the nature of the substituents at the germanium atom, compounds with CN 5 and 6 can be divided into organogermanium (with a Ge–C bond) or coordination (no Ge–C bond) compounds. Despite the similarity in the structures of the coordination polyhedra, the physicochemical properties of these groups of compounds differ markedly [92–96]. There are several types of coordination polyhedra of the germanium atom. For CN 5, the most characteristic coordination polyhedron is TBP; compounds with a SP germanium atom are much less common. Note, for example, that the inclusion of the F– anion in the coordination sphere of Ge in Ge(OH)4 (3), as calculated in [97], leads to two isomeric species 3a and 3b, of which the most stable is structure 3a where the fluorine atom takes an axial position in the TBP (Figure 15.3).

F

H O

O 1.75

Ge 110.03

H

H

105.61

H

1.97 85.25 1.79

O 1.80

Ge

F 95.72

O 117.00

H

H

H

1.81 93.68

0.97

O

O

Ge

O 88.43 1.86

88.44

H

O

H

1.77

O 1.82

O

O

H

O

H

H 3

3a (TBP)

3b (SP)

Figure 15.3  Optimized geometries of Ge(OH)4 and two isomeric [FGe(OH)4]– anions. Distances are given in angstroms (rounded to the nearest 0.01) and angles are given in degrees.

Dynamic Stereochemistry and Spatial Structure

The deviation from planarity of the equatorial hydroxy groups and subsequent tilt towards the fluorine atom may reflect the tendency for intramolecular hydrogen bonding between the hydroxy groups and the fluoride ion. Note that the TBP structure of the coordination center is also typical for complexes of pentacoordinate silicon with five electron-withdrawing substituents [98]. On the other hand, attention is drawn to the fact that, structure 3b is only 4.3 kcal/mol less stable than 3a. The latter circumstance does not exclude the possibility of participation of structures 3b in the exchange processes of equatorial and axial substituents by analogy with the Berry pseudorotation, as was shown for pentacoordinate silicate anions [37, 98]. In some compounds, usually because of steric constraints, the coordination polyhedron is difficult to describe as either SP or TBP. At the same time, in virtually all compounds where the germanium atom is hexacoordinated, its coordination polyhedron corresponds to an octahedron. Based on the analysis of the structural data, it can be concluded that, in fact, a prerequisite for the stabilization of the structure with CN 5 and 6 is the involvement of the germanium atom into a ring, where germanium is bound by both covalent and coordination bonds with atoms formally considered as donors of electron pairs (usually oxygen or nitrogen). The total number of atoms in a given ring usually varies from 5 to 7. Steric repulsion violates the stability of the fourmembered ring. For a similar reason, until recently, it has been impossible to obtain cyclic structures with 8 or more atoms. At the same time, the CSD contains examples of a number of acyclic compounds that are, as a rule, adducts of halogermanes with neutral molecules such as pyridine [99, 100]. The crystal structure serves as a reference point for analyzing the structural non-rigidity of compounds. In a structurally non-rigid compound, there are always structural parameters (bond lengths, bond and torsion angles) where a change in the values does not lead to a large increase in its total energy. In turn, the change in these structural parameters is an indication for the reaction coordinate of dynamic processes. Except for the vibration of methyl and trifluorophenyl groups or the transfer of a hydrogen atom in the case of strong hydrogen bonds, virtually no reversible dynamic processes can be observed in a crystal because of the intermolecular bonding forces. Dynamic processes require weakening of intermolecular bonds and the absence of a three-dimensional periodic structure that is typical for solutions. A qualitative criterion for determining the rigidity of a structure can be the number of rings in which a germanium atom simultaneously takes part. Cyclic structures can be very rigid. For example, in atrane the germanium atom is a part of the bicycloundecane structure, where there are three five-membered rings with one common Ge–N bond. On the contrary, from the structural point of view, complexes with one chelate ring are the least rigid among cyclic structures, and virtually all dynamic processes in compounds of this type are accompanied by a significant change in the length of coordination bonds with the participation of the germanium atom. In this case, it is important to note that the structures of germanium compounds with CN 5 or 6 in crystals and in solution are different, and such differences can be quite significant. This is evidenced by the values of NMR chemical shifts, the parameters of the IR spectra, as well as the data of numerous quantum chemical calculations (see below and also in [101]. As a rule, the coordination bonds formed by a germanium atom in solution are characterized by a significantly longer length than the corresponding bonds in crystals. Some acyclic compounds in solutions are capable of completely dissociating into donor and acceptor components. Thus, a natural question arises as to how the spatial structure of a compound determines the possibility of realization of stereodynamic processes, as well as the mechanism of such process?

15.3  Dynamic Stereochemistry and Spatial Structure As noted in the previous section, the availability of information on the spatial structure of hypervalent compounds is necessary to assess the possible mechanism of dynamic processes occurring in solution. An important issue for this chapter determining the structural criteria for the feasibility of stereodynamic processes in compounds with penta- and hexacoordinated germanium. Analysis of the literature data and their comparison with the structural data from the CSD indicate that when the stereodynamic process directly affects the coordination environment of the germanium atom, two such criteria can be formulated. The first criterion is the presence of bond lengths or bond angles that can vary within wide limits without significantly affecting the total energy of the molecule in the coordination environment of the germanium atom. Complexes with the so-called soft coordination center are a special case of such structures [102]. The most typical examples of such compounds are germanium monochelate complexes containing three carbon atoms in the coordination polyhedron and a highly polarizable bond to a weakly coordinating ligand (I, OTf) [103]. In this case, a decrease in the length of the bond from germanium to one of the atoms included in the coordination environment will lead to a corresponding increase in the length of

635

636

15  Dynamic Stereochemistry of Penta- and Hexacoordinate Germanium(IV) Complexes

the opposite bond. Such a change can occur, for example, under the influence of the polarity of the medium [101]. For TBP, the geometrical parameters that can be varied without significant change of total energy are the lengths of the bonds between germanium atom and axial substituents. In the case of hexacoordination, there may be two pairs of axial bonds capable of a significant change in length. According to the results of quantum chemical calculations, a change in the Ge–O bond lengths for some germanium complexes by 0.5 Å is accompanied by a change in the total energy of molecules by no more than 10 kcal/mol [26, 104]. In this case, the change in the bond angles between the equatorial substituents occurs in a rather narrow range. The dynamic processes characteristics of such coordination sites are associated with both the dissociation and association of bonds (for example, the expansion of the coordination sphere of the germanium atom because of dimerization). There are quite a few compounds where the coordination site of germanium is soft, and in most complexes both axial bonds are weakly polarizable. Such compounds include adducts of halogermanes with electron pair donors as well as bischelate spirocyclic complexes. In these compounds, one of the bonds of the coordination environment of the germanium atom is capable of a noticeable change under the influence of external factors (polarity of the medium, specific intermolecular interactions, etc.); the lengths of the remaining bonds vary insignificantly. Dynamic changes in the structure of such compounds are usually difficult to describe using only changes in bond lengths or bond angles. This two-step process can be described taking into account changes in a whole series of bond lengths and dihedral angles. The simplest cases are Berry’s pseudorotation or Ugi’s “turnstile” rotation, discussed above; however, the proposed mechanisms, in general, can be applied with certain reservations. The second criterion is the presence of an asymmetric stereogenic center in the structure of germanium compounds with an extended coordination sphere. This criterion seems obvious, however, in the case of the germanium atom, it is critical. As noted above, the reason is the impossibility of measuring the NMR spectra on germanium nuclei in penta- and hexacoordinated compounds. For a number of tetracoordinated germanium compounds, correlations have been revealed between the chemical shifts of the 73Ge NMR signals and the chemical shifts of the 29Si NMR and 119Sn NMR signals in the corresponding organosilicon and organotin analogues [105, 106]. It seems logical to expect that the similar correlations can also take place in the case of compounds with penta- and hexacoordinated germanium. As far as we know, such correlations have not yet been revealed, leading to difficulties in the detection and observation of stereodynamic processes by the NMR spectroscopy. Commonly, it is not sufficient to use only the shape of the 1H- and 13C NMR signals to study dynamic rearrangements. Therefore, the stereodynamic processes in compounds with penta- and hexacoordinated germanium can be studied by comparing their NMR spectral features with the spectral parameters of silicon and tin compounds of similar structure. In the presence of a stereogenic center, even if it is not a germanium atom or is not directly attached to it but located in the immediate environment, the task of observing stereodynamic processes becomes somewhat easier. The structures participating in such processes have a noticeably different set of NMR signals, thus providing that the rate of the process falls within the NMR spectroscopy timescale. This is especially important for stereodynamic processes that do not directly affect the germanium atom. Based on the above-mentioned facts and the analysis of the literature data, an energy criterion for the detection and analysis of dynamic processes can be formulated. When NMR spectral parameters and quantum-chemical calculations are used, the processes, where free activation energy is limited to the range of 10–50 kcal/mol, fall into the NMR spectroscopy timescale. This interval is mainly because of the technical limitations. Quantum-chemical methods make it possible to characterize virtually any mechanism within the framework of changes in the structure and activation energy; however, without experimental data, the results of such studies cannot be verified.

15.4  Quantum-chemical Methods in the Study of Stereodynamic Processes in Compounds with Penta- and Hexacoordinate Germanium Quantum-chemical methods play a significant role in establishing and predicting the structure of complexes with a hypervalent O→Ge–X fragment and determining the probable pathways of stereochemical transformations. To date, a large number of works have been published in the field of quantum chemical studies of hypervalent compounds of group 14 elements; notably references [101, 107–116]. In this section, to discuss stereodynamic processes at the coordination site of hypervalent germanium complexes, we will use the results of quantum chemical studies that often can significantly expand our understanding of the direction and energetics of such transformations.

Quantum-chemical Methods in the Study of Stereodynamic Processes in Compounds with Penta- and Hexacoordinate Germanium

The mechanisms of many chemical reactions have been comprehensively studied by quantum chemical methods, especially for industrially important reactions involving organic and organosilicon compounds. In the case of compounds with penta- and hexacoordinate germanium, researchers often limit themselves by studying only the electronic structure of isolated molecules, while little attention is paid to the study of mechanisms of the reactions in which these molecules can be involved. This is also true for stereodynamic processes that can be parts of reactive transformations of organogermanium compounds. A complete description of the potential energy surface at a high calculation level is often difficult, since the germanium atom contains many electron shells and requires the use of extended basis sets, in turn requiring significant computational resources, especially when calculating the values of the 73Ge NMR chemical shifts. Thus, the potential error of the calculation methods in the latter case can be significantly higher than that for silicon. Note that a precise quantum chemical calculation of chemical shifts for an atom of heavier elements, for example tin, is almost impossible. Even the most accurate methods that take into account relativistic effects, which can be used to calculate chemical shifts, give an error of at least 10% [117]. These factors greatly complicate the establishment of correlation dependences between the chemical shifts of 29Si, 73Ge, and 119Sn nuclei in compounds of penta- and hexacoordinated elements. In connection with the above, the question arises: how the quantum chemical calculations can help to establish the mechanism of stereodynamic processes in solution, if, as already indicated, it is difficult to relate the results obtained with the experimentally observed physicochemical quantities? First of all, there must be an assessment of the feasibility of a particular mechanism, taking into account energy and distortion of structural parameters. For example, in the case of a fairly wide range of bischelate complexes, it was shown that the probability of permutation isomerization through the Berry pseudorotation mechanism is significantly higher than through the Ugi “turnstile” rotation mechanism. Indeed, the activation barrier and the distortions of the torsion angles in the first case are smaller [48]. Another example of the effective application of the approach under consideration are the results presented in studies on the hydrolysis mechanism of (1,1-dihydroxy)quasisilatrane (4), 1-hydroxysilatrane (5), and their germanium analogues (6 and 7) [118]. The studies carried out by the authors of the cited work are of particular interest in connection with the authors’ quantum-chemical assessment of the resistance of the Ge–O covalent bond to cleavage under the action of hydrogen bonds, which, as will be demonstrated below, can occur as a result of stereodynamic processes at the coordination center. According to the quantum chemical calculation data (B3PW91/6–311++G(df,p)), the Gibbs free energy of hydrolysis activation calculated for the isolated system “target molecule–water molecule” in the case of (1,1-dihydroxy)quasi-germatrane (6) and 1-hydroxygermatrane (7) is almost 1.5 times smaller than the silicon analogues (94.1 and 108.6 vs 123.0 aned 144.6 kJ/mol, respectively) [118]. O

N

OH Ge

HN O

O OH

Ge

O

O

OH 7

6

A similar situation is observed for the thermal effect of the reaction. Based on the fact that the limiting step of the process is the cleavage of Ge–O and Si–O bonds, it can be concluded that for compounds with penta- and hexacoordinate germanium, stereodynamic processes that proceed through cleavage of Ge–O covalent bonds will be characterized by lower energy costs compared to similar organic and coordination silicon compounds. An earlier study by Ignatiev et al. [119], carried out at a different theoretical level [B3LYP/aug-cc-pVDZ], is in agreement with conclusions of reference [118]. The authors found that the enthalpy of the hydrolysis reaction depends on the relative electronegativity of the substituents at the germanium atom, and the formation of stronger coordination bonds involving germanium decreases the barrier for breaking the Ge–O covalent bond. According to NMR data, bischelate complexes RGe(OCH2CH2NMe2)2X (R=Ph, Me; X=I, Cl) (8) can exist in either the neutral form (A) with a hexacoordinate germanium atom or the ionic form (B) where the Ge–X bond is dissociated [120]:

A

B

C

D

637

638

15  Dynamic Stereochemistry of Penta- and Hexacoordinate Germanium(IV) Complexes

According to quantum chemical calculations, the leading role in dissociation is played by the interaction of halogen X with solvent molecules (in this case, chloroform). In addition to dissociation of the Ge–X bond, the probability of dissociation of the N→Ge coordination bond was also considered. In the case of X=Cl, the difference in energy between the neutral and cationic forms is small and amounts to about 1 kcal/mol in favor of the former, consistent with experimental data on the existence of an equilibrium between these forms in solution. Note that breaking the N→Ge bond is much less energetically favorable. The energy of the RGe(OCH2CH2NMe2)2 cationic structure (C), in which one of the N→Ge bonds is absent, is on average 15 kcal/mol higher than in the same cation (B), where both coordination N→Ge bonds are present. At the same time, the energy of the RGe(OCH2CH2NMe2)2 structure (D), where both N→Ge bonds are absent, is only 1 kcal/mol higher than that of its neutral form (in the case of X=Cl). For X=I, the situation looks somewhat different. For the complex with R=Ph, the energy minimum corresponding to the neutral form could not be localized. In the case of R=Me, the cationic form is energetically more favorable by 3.1 kcal/mol in comparison with the neutral form, which agrees with the experimental data. In a molecule of bischelated dichlorogermane (L8CH2)2GeCl2 (9) with a monoanionic lactamomethyl (C,O)-chelating ligand based on enantolactam (L8H), the germanium atom is characterized by a distorted octahedral configuration with a mutual cis-position of Cl (equatorial and axial)and O (axial and equatorial) atoms and trans-position of C atoms [27].

9

In the product of the reaction of complex 9 with trifluoromethylsulfonic acid, the germanium atom is pentacoordinated, and the oxygen atoms of the chelating ligands are located in the axial positions of TBP (10). The chlorine atom is in the equatorial position; there is also a weak interaction of Ge atom with the OTf ion.

10

Thus, the relative position of the ligands in the coordination polyhedron of the germanium atom changes. This process can formally proceed either through the polytopic rearrangement of the [(L8CH2)2Ge(Cl)]+ cation (for example, Berry pseudorotation or Ugi “turnstile” rotation, Figures 15.1 and 15.2) or through a mechanism implying breaking of the O→Ge coordination bonds similar to a case described in [120]. The polytopic rearrangement of the [(L8CH2)2Ge(Cl)]+ cation is apparently unlikely because of the presence of bulky ligands with an eight-membered saturated ring. Therefore, a scheme was proposed for the transformation of dichlorogermane (9) into the bischelate cationic complex [(L8CH2)2Ge(Cl)]OTf (10) that includes two steps: dissociation of the Ge–Cl bond and cleavage of the O→Ge coordination bond with subsequent rotation of the ligand around the Ge–C bond and closure of the chelate ring. Using quantum chemical calculations (PBE/TZV), H-bonded complexes 9 with HOSO2CF3 and HCl, assumed to be intermediates in this process, have been investigated. The amount of energy required for the dissociation of H-bonded complexes into germylium ion and anion HCl2– (or anionic Cl–…HOSO2CF3 moiety) depends on the relative arrangement of Cl atoms in the coordination polyhedron of the Ge atom. In the case of the cis-arrangement of Cl atoms in the H-bonded complexes, the dissociation energy is estimated as 11.5–16.1 kcal/mol.

15.5  Neutral Complexes of Tetravalent Germanium In contrast to chelates with pentacoordinate silicon, stereochemical non-rigidity of analogous germanium derivatives containing ICB D→Ge, where D=O, N, and a number of other elements, has been studied to a much lesser extent to date [5, 8].

Neutral Complexes of Tetravalent Germanium

15.5.1  Pentacoordinate Complexes The presence of stereodynamically mobile ligands at the coordination center, as in complexes 11 and 12 with a sevenmembered lactam ring, can significantly complicate the appearance of the VT 1H NMR spectra, especially in the presence of additional stereogenic centers [121]. The stereochemical non-rigidity of the complexes under consideration was studied by the 1H DNMR spectroscopy in [121]. At 20°C, the signals of the protons of the NCH2M and MMe2 groups in the 1H NMR spectra of compounds 11 and 12 are observed as singlets. With decreasing temperature, the broadened singlet corresponding to the NCH2M group transforms into a quartet of an AB type. The free energy of activation of the process ∆G# (kcal/mol) in an equimolar mixture of CDCl3 and CD2Cl2 determined by the 1H DNMR at Тс was: 9.4 ± 0.2 (Тс = –79.1°С, NCH2Ge) for chloride 11; 9.6 ± 0.2 (Tc = –73.3°C, NCH2Sn) and 9.3 ± 0.1 (Tc = –87.8°C, SnMe2) for chloride 12.

M = Ge (11); Sn (12)

13

According to the IR spectroscopy data, the germanium atom in compound 13 is tetracoordinate (the 1500–1750 cm–1 region contains one intense absorption band ν(C=O) at 1620 cm–1, CHCl3). In the accessible temperature range (down to – 100°С), it was not possible to observe the non-equivalence of the signals of the GeMe2 groups in the 1H NMR spectra [121]. Taking into account the conformational homogeneity of the seven-membered ring at low temperatures established by 13 C NMR spectroscopy, the observed non-equivalence of the 1H signals of the indicator groups in the NMR spectra of complexes 11 and 12 is caused, in the authors’ opinion [121], by a slowdown in the inversion of the lactam ring. As a result, hydrogen atoms and methyl groups in the five-membered chelate ring appear either on one or on opposite sides of the plane of this ring with respect to the carbon atom C(5) of the seven-membered lactam: The calculated ∆G# values of chlorides 11 and 12 are close to the barrier of the inversion of the cycloheptane ring (8.1 kcal/mol) [122]. Note that diastereomers, the existence of which is because of the asymmetric conformation of the sevenmembered lactam ring and the presence of a chiral pentacoordinate silicon atom in the molecule, were observed by X-ray crystallography for N-(methyldichlorosilylmethyl)hexahydroazepin-2-one [123]. One chlorine atom in the complex occupies an axial position, while the other occupies two different (with respect to the seven-membered ring) equatorial

positions. The fact that an intramolecular chelate ring in 11 and 12, combined with the seven-membered lactam ring, leads to the anisochronism of the protons of the corresponding groups is evidenced by the absence of any stereodynamic processes in 13, where the Ge atom has a tetracoordinated structure. Structure and properties of stable neutral pentacoordinate germanium compounds substituted with three CF3 groups and the dibenzoylmethane ligand are discussed in [124]. There are two crystallographically independent molecules for complex 14 in an asymmetric unit that differ from each other by conformational arrangement of the six-membered ring with the –O–C–C–C–O– fragment and the germanium.

639

640

15  Dynamic Stereochemistry of Penta- and Hexacoordinate Germanium(IV) Complexes

According to the quantum chemical calculations (B3LYP/6-31+G* and MP2/6.31+G*//B3LYP/6-31+G*), the corresponding conformations are characterized by a difference in energies of 10.1 and 4.1 kcal/mol, respectively. The properties of highly coordinated germanium complexes of the N2O2-type tetradentate dipyrrin ligand are discussed in [125]. According to X-ray crystallography, the germanium complex 15 has a dimeric composition and a pentacoordinate structure, while the monomeric complex 16 is characterized by a hexacoordinate structure.

15

16

17

In addition, dimer 15 is readily hydrolyzed in solution to form monomer 18, although the corresponding disiloxane 17 does not react under similar conditions.

15

L = OH, H2O, n = 1, 2; 18

The authors note that germanium dipyrrin complexes exhibit intense absorption and fluorescence near the NIR region, which is more red shifted compared to silicon analogues. In the 1H NMR spectrum of dimeric complex 15 in C6D6, two pyrrole units are equivalent, despite the fact that both of them in the crystalline state are located asymmetrically in the apical and equatorial positions. The observed symmetry in solution was explained by fast exchange at the germanium atom. The non-equivalence of the two o-methyl groups of the meso-mesityl fragment may indicate that the exchange reaction proceeds not through the dissociation of the Ge–O–Ge bond, but through the Berry pseudorotation mechanism, as in the case of disiloxane 17. Experimental data on stereodynamic processes occurring at the coordination site of pentacoordinate complexes 19–23 are summarized in Table 15.1.

19 (X=OSO2CF3 (a), X = I (b))

20 (R1=H, R2=tBu (a), Ph (b)) 21 (R1=NHC6H5, R2= tBu (a), Ph (b))

Neutral Complexes of Tetravalent Germanium

22 X=Cl (a);X = Br (b);

23

Table 15.1  Activation parameters in permutation reactions for complexes 19–23. Complex

Solvent

ΔG#

ΔH#

ΔS#

kcal/mol

kcal/mol

cal/mol.K

Tс, °С

References

19а

CDCl3

10.5

–50

[126]

19b

CDCl3

10.8

–41

[126]

20a

(CD3)2CO

12.2

–23

[127]

21b

(CD3)2CO

12.4

–25

[127]

21a

CD2Cl2:CS2 (2:1) 12.1

–34

[127]

21b

CD2Cl2

–39

[127]

22a

CDCl3

>23

[103]

CD3CN

>20

[103]

11.6

CD3OD

15.7

43

[103]

22a+DMA

CD3CN

17.0

66

[103]

22b

CD3CN

17.0

67

[103]

CD3OD

15.3

36

[103]

22b+LiBr

CD3CN

13.0

–17

[103]

23

CDCl3

17.8

60

[103]

CD3OD

11.3

21

[103]

10.6

12.6

–22

–16

For the process leading to the averaging of indicator groups in complexes 20a,b and 21a,b, a mechanism without cleavage of the coordination bond was proposed, including a prototropic shift and subsequent pseudorotation at the TBP of the germanium atom (Scheme 15.2) [127]. In halides 22a,b and 23, as well as in other N-(dimethylchlorogermylmethyl)amides and lactams described to date [128, 129], the Ge atoms are pentacoordinate because of the formation of the O–Ge ICB. This is evidenced by the presence of two characteristic absorption bands of strongly coupled stretching vibrations ν(C=O) and ν(C=N) of amide fragments in the IR spectra of these compounds in the range of 1500–1700 cm–1 (see [130]). In this case, the absorption bands ν(C=O) of unchelated amide (lactam) fragments are absent, which may indicate a fairly strong O→Ge coordination. The presence of a chiral carbon label in halides 22a,b and 23 causes chemical non-equivalence of the signals of prochiral groups NCH2 and GeMe2 in the 1H NMR spectra [103]. As a result, at room temperature and below, protons of the NCH2 group appear as a multiplet of the AB system (2JHH ≈ 14 Hz) and protons of the GeMe2 group appear as two singlets of equal intensity. As the temperature rises, the NCH2 group protons quartet retains its chemical non-equivalence. At the same time, for the signals of protons of the GeMe2 group, a temperature dependence typical for permutation processes is observed: two singlets broaden in solvents with a sufficiently high donor capacity and/or upon the addition of external nucleophiles (see Table 15.1) to coalesce into a singlet with an averaged chemical shift (a decrease in temperature is accompanied by the restoration of the original picture of the spectrum). Note that, as in the case of Si analogues [131], the nature of the electronegative substituent at the central atom has a significant effect on the barrier to permutation isomerization (Table 15.1). The replacement of the Cl atom by Br causes a decrease in the ∆G# value (for example, in CDCl3 down to 17.0 kcal/mol). This can be caused by an increase in the nucleofugality of the Ge substituent and/or an increase in the Ge–Hal distance. At the same time, in CD3OD (a solvent with a higher solvating ability), the influence of the nature of halogen is practically canceled. So, bromide 22b (∆G# = 15.3 kcal/ mol) is only 0.4 kcal/mol less than for chloride 22а (Table 15.1).

641

642

15  Dynamic Stereochemistry of Penta- and Hexacoordinate Germanium(IV) Complexes

A significant decrease in ∆G# in the case of complex 23 compared to complex 22а (11.3 and 15.7 kcal/mol, respectively, Table 15.1) occurs simultaneously with a decrease in the O–Ge bond length [103]. This circumstance may indicate an increase in the contribution of the dissociative mechanism to the process of permutational isomerization at the coordination center. OH

OH

Hb

Hb O Ge

...

Ha S

pseudorotation

S R

O

Ha R

Ge

S

S O

O

O

O

prototropy

prototropy O

O

Hb

Hb O

pseudorotation

S R

Ge

O

Ha

...

Ha S

R

Ge

S

S O

O

OH

OH

Scheme 15.2  Stereoisomerization without cleavage of the coordination bond via prototropic shift and subsequent pseudorotation [127].

For halides 22a,b and 23, a decrease in the barrier to permutation isomerization is also noted when external nucleophiles (LiBr, DMA) are added in an amount close to stoichiometric (see Table 15.1). The authors [103] explain the decrease in the barrier to permutation isomerization and increased (because of an increase in the double bonding of the amide group) barrier of hindered rotation around a partially double C–N bond in DMA, when DMA is added to a solution of chloride 22а (from 17.4 to 21.0 kcal/mol), by the process leading to the formation of complex 24.

24

Attention is drawn to the significantly higher (by 5–8 kcal/mol) values of the barriers to permutation isomerization of complexes 22a,b and 23 (Table 15.1) compared to their Si analogues [5, 131], indicating a higher configurational stability of pentacoordinate Ge halides. This is consistent with the higher rigidity of the central coordination unit OGeC3Hal, established by X-ray crystallography, as compared to the isostructural coordination unit OSiC3Hal (see above and also [5]). Taking into account the large negative values of the entropy of activation of permutation isomerization ((–25)-(–28) cal/ mol.K) for complexes 22a,b and 23, by analogy with Si counterparts, a mechanism that includes the dissociation of the Ge–X (X=Hal) bond followed by effective solvation of the transition state b can be considered (Scheme 15.3) [8]. Comparison of the value of the activation barrier for permutational isomerization at the coordination center of complex 25 (∆G = 13 kcal/mol in a CD2Cl2 solution) [132] calculated at the coalescence temperature (Tc = 245 K) with analogous parameters of the above complexes with ICB O→Ge (Table 15.1), indicates a greater rigidity of the coordination center in the latter ones.

Neutral Complexes of Tetravalent Germanium

a

b

c

a

Scheme 15.3  Stereoisomerization via dissociation of the Ge–X bond (Ψ stands for the Berry’s pseudorotation).

25

15.5.2  Hexacoordinate Complexes Octahedral bischelate complexes with ICB D→M (M=Si, Ge, Sn; D is an oxygen or nitrogen donor atom in a bidentate C,Dchelating ligand) a priori can exist in the form of five diastereomers given below with an indication of the symmetry point group and mutual locations of the same coordinating atoms (the notation DtCtXc, for example, means that the pairs of atoms D and C have trans-orientation and atoms X have cis-orientation).

According to X-ray crystallography, two monodentate ligands in neutral bischelate complexes of hexacoordinate silicon [133–135], germanium, and tin [6, 129, 136] are predominantly in the cis-position with respect to each other. At the same time, there are a number of exceptions for hexacoordinate silicon complexes: bulky monodentate ligands, for example, Br and Cl atoms in complexes 26a,b [137], 27 [138], and [(AcN(Me)CH2Si(Cl)]+Cl– (28) [139] occupy sterically more favorable trans-positions relative to each other.

26 R=PhCH 2 (a), R=Ph (b)

27

According to X-ray crystallography, complexes of hexacoordinate germanium known to date are characterized by the cis-configuration of the coordination center, regardless of the nature of the monodentate ligand. This is probably because of the energetically “comfortable” cis-structure of the coordination center, in which the central germanium atom has a significantly larger size compared to the silicon atom. It was found that two stereodynamic processes can occur at the coordination center of the complexes with the O→Ge ICS: enantiomerization and diastereomerization. The exchange of ligands in the Oh of intramolecular hexacoordinate complexes

643

644

15  Dynamic Stereochemistry of Penta- and Hexacoordinate Germanium(IV) Complexes

of germanium, as well as in TBP of intramolecular pentacoordinate complexes, is slowed down because of the chelate effect. This circumstance makes it possible to use the multinuclear DNMR method to study the stereodynamic processes. 15.5.2.1 Enantiomerization

The exchange process for compounds 28–34 resulting in the interconversion of enantiomers was studied by DNMR at the temperature of coalescence of the signals for the protons of the NCH2Ge group, and the activation energies of this process were calculated according to the Eyring equation [5, 140].

(28) R1=R2=Ln, X=F: n=5 (a); n=6 (b); n=7 (c); (29) R1 =R2=Bon, X=F; (30) R1=R2=Me, X=Cl; (31) R1=R2=Ln, X=Cl: n=5 (a); n=6 (b); n=7 (c); (32) R1=R2=Bon, X=Cl; (33) R1=R2=Me, X=Br; (34) R1=R2=Ln, X=Br: n=5 (a); n=6 (b); n=7 (c)

The CH2 groups usually present in chelate ligands are appropriate for studying dynamic NMR spectra because prochiral methylene protons may interconvert and display typical coalescence phenomena. Activation parameters for permutation isomerization were studied by total line-shape analysis of the signals of protons belonging to the NCH2Ge group in the DNMR spectra with the use of the Bloch equation modified for chemical exchange processes (Table 15.2) [141–144]. Table 15.2  Activation parameters for enantiomerization of compounds 28–34 (с ~ 0.03 M). Complex

Solvent

ΔG#298, kcal/mol

ΔH#, kcal/mol

ΔS#, cal/mol⋅K

References

28a

CDCl3

12.1 ± 0.2

13.3 ± 0.2

4 ± 0.9

[143, 144]

28b

CDCl3

13.9 ± 0.2

16.6 ± 0.2

5 ± 1.2

[143, 144]

28c

CDCl3

13.6 ± 0.3

15.6 ± 0.3

5 ± 0.8

[143, 144]

29

CDCl3

15.3 ± 0.2

16.8 ± 0.2

5 ± 0.7

[143, 144]

30

CDCl3

12.0 ± 0.3

15.0 ± 0.3

10.0 ± 0.8

[143, 144]

CD3CN

11.8 ± 0.3

16.7 ± 0.3

16.5 ± 0.8

[143, 144]

(CD3)2CO

11.9 ± 0.3

17.0 ± 0.3

17.2 ± 1.1

[143, 144]

31a

CDCl3

10.0 ± 0.3

12.2 ± 0.3

7.3 ± 1.2

[143, 144]

(CD3)2CO

10.2 ± 0.2

12.0 ± 0.2

6.2 ± 0.9

[143, 144]

31b

CDCl3

12.3 ± 0.2

16.9 ± 0.2

15.3 ± 0.7

[143, 144]

31c

CD3CN

12.7 ± 0.3

16.2 ± 0.3

11.4 ± 1.2

[143, 144]

(CD3)2CO

12.2 ± 0.3

16.8 ± 0.3

15.5 ± 0.9

[143, 144]

CDCl3

12.3 ± 0.3

16.9 ± 0.2

15.3 ± 0.7

[143, 144]

12.4 ± 0.3

17.0 ± 0.3

15.5 ± 1.1

[143, 144]

CDCl3a CD3CN

12.6 ± 0.3

16.0 ± 0.3

11.5 ± 1.1

[143, 144]

(CD3)2CO

12.3 ± 0.3

16.8 ± 0.3

15.2 ± 0.9

[143, 144]

С6D5CD3

12.3 ± 0.6

17.2 ± 0.6

16.6 ± 2.0

[143, 144]

32

CDCl3

13.6 ± 0.3

14.9 ± 0.3

4.5 ± 1.2

[144]

33

CDCl3

11.9 ± 0.3

14.3 ± 0.3

8 ± 0.8

[141]

34a

CDCl3

10.0 ± 0.3

11.2 ± 0.3

4 ± 0.7

[141]

34b

CDCl3

12.1 ± 0.2

14.0 ± 0.2

5 ± 0.7

[141]

34c

CDCl3

12.6 ± 0.3

14.2 ± 0.3

6 ± 1.1

[141]

CD3CN

12.9 ± 0.3

13.8 ± 0.3

3 ± 0.7

[141]

Neutral Complexes of Tetravalent Germanium

Note that according to X-ray crystallography data, the germanium atom in complex 32 has a distorted octahedral structure in the solid phase, while in a structurally similar silicon complex, based on 29Si NMR spectroscopy data, the silicon atom in the liquid phase is pentacoordinate [144]. Depending on the structure of the bidentate ligand, the barrier to ligand exchange in bischelate hexacoordinate complexes 28–34 varies in the range 10–15 kcal/mol (Table 15.2). An increase in the size of the bidentate ligand is accompanied by an increase in the barrier of the stereodynamic process for all dihalides presented in Table 15.2 simultaneous with a decrease in the length of the O→Ge coordination bond. In different solvents, ΔG≠298 remains virtually unchanged. The positive parameter ΔS≠ is only slightly affected by the nature of the solvent (Table 15.2). The activation parameters for hexacoordinate dichlorogermanes are independent of the concentration (in the range 1–10 mol/L), excluding the intramolecular mechanism of rearrangement in this case and indicating that the intermolecular mechanism of exchange is highly improbable. Transition from difluorides to dichlorides and further to dibromides is accompanied by a decrease in the permutation barrier by an average of 1–2 kcal/mol. Among bischelates with a lactamomethyl chelate ligand, the lowest permutation barrier (ca. 10 kcal/mol) is found for the complexes of dihalogermanes 28а, 31а, and 34а, in which the ICB O→Ge, according to X-ray crystallography, is the weakest [20, 145]. One of the schemes proposed for the description of enantiomerization in bischelates 30–33 includes the formation of either a “bicapped” transition state a or reversible dissociation of the ICB O→M, which at a relatively long O→M ICB length leads to the pentacoordinate transition state b (Scheme 15.4) [141].

Scheme 15.4  Stereoisomerization of hexacoordinated bischelates (LCH2)2MX2 through the formation of a “bicapped tetrahedral” transition state.

In [142], the authors returned to the consideration of a number of controversial questions about the mechanism of permutation isomerization. In particular, it remained unclear whether or not the coordination bonds are cleaved in the course of intramolecular ligand exchange. The mechanism involving the cleavage of the Ge–Cl covalent bonds is highly improbable for covalent dichlorogermanes 30–32, as evidenced by a substantially lower electrical conductivity of their solutions in chloroform (40–260 mS⋅cm2/mol) compared to that of cation-anion complexes of the LnCH2Ge(Cl)OTf type, in which one halogen atom is replaced by the triflate group (1000–2000 mS.cm2/mol) [146]. The study of the effect of chloride ion showed that the addition of dry salts LiCl or Et3NCH2PhCl (0.05–0.2 M) at room temperature did not affect the exchange rate in germanium dichlorides, which may indicate the absence of a significant contribution from the Ge–Cl bond dissociation to the total rate of the observed process [143]. At the same time, the ionization of the Si–Cl bond in neutral hexacoordinate complexes of silicon 35 is

645

646

15  Dynamic Stereochemistry of Penta- and Hexacoordinate Germanium(IV) Complexes

characterized, despite the increase in number of particles, by the negative entropies of ∆S° = –21.8 (R=tBu), –15.0 (Me), –9.4 (CH2Ph), and –8.6 cal/mol.K (Ph) because of stabilization of the ions by solvent organization, causing formation of higher-ordered systems and hence negative entropy [147].

35

The relatively high positive entropy of activation in compounds 30–32 (Table 15.1) is in accordance with the presence of various degrees of freedom for the transition state (lower-ordered systems) in comparison with the initial state [148, 149], which may be explained by cleavage of the coordination О→Ge bond, forming a pentacoordinate species. It should be noted that the non-dissociative mechanism involving the exchange process, for example, through a trigonal-prismatic transition state, is characterized by negative entropy of activation [150], and therefore can be ruled out. Taking this into account, the authors propose a novel (L,L)-exchange mechanism between the two enantiomers of hexacoordinate bischelate complexes LnCH2MX2, including dissociation of the coordinate bond followed by formation of a pentacoordinate species, exchange of ligand positions in a trigonal bipyramidal intermediate, and reclosure of the uncoordinated chelate ligand (Scheme 15.5) [143]. H1 C O O*

H2

H2

Ge C

H1

Cl*

Cl

Cl

Cl*

H2

H1 C

Ge

O*

O*

Cl

O

H1 C

C

H2 H1

C

fast

fast

H1

H1

O O*

Ge C

B

C

H2

C

Cl Cl*

H2 H1

Cl*

Ge

H1 H2 A* = C enantiomers

A

H2

C O

slow pseudorotation

O

H2 Cl*

Ge

Cl O*

B*

C

H2 H1

Scheme 15.5  Intramolecular dissociative mechanism of two-step (L,L)-ligand-site exchange (enantiomerization) in neutral hexacoordinate bischelate complexes 30–32 through the formation of a neutral pentacoordinate intermediate and subsequent pseudorotation.

The dissociation of the О→Ge bond in hexacoordinate species A (the first step) leads to the formation of neutral pentacoordinate germanium structure B. The numerous examples of hypercoordinate germanium complexes in the solution and solid states [6] suggest that the energies associated with the conversion of six- to five-coordinate species should be small, therefore the formation of a pentacoordinate intermediate in the transition state becomes a likely possibility. According to X-ray data for stable TBP polyhedra of germanium (as other group 14 elements), the axial position in a hypervalent O→Ge–X bonding is normally occupied by the most electronegative substituent [6, 151], Cl in the case of complexes 30–32. The proposed mechanism of enantiomerization in dichlorides and its consequences may be useful for a consistent analysis of the influence of the substituent X, coordination bond lengths d(M–O), and nature of central atom M on the stereodynamic behavior in analogous hexacoordinate compounds.

Neutral Complexes of Tetravalent Germanium

15.5.2.2 Diastereomerization

In contrast to compounds of the (LnCH2)2MX2 (M=Ge, X=Cl; M=Sn, X=Cl, Br) type where only one set of signals corresponding to the cis-diastereomer was detected in the NMR spectra [136, 152, 153], the room temperature 1H- and 13C NMR spectra of dibromides 33 and 34a–c reveal the existence of two species with different integral intensities in CDCl3 (at ratios 86:14, 90:10, 64:36, and 55:45, respectively), which were assigned to two diastereomers: OcCtBrc and OtCtBrt.

33 (OcCtBrc), cis-32a-c

33 (OtCtBrt), all-trans-34a-c

Activation and thermodynamic parameters of diastereomerization are presented in Table 15.3 [141]. For dibromide complexes 33 and 34a–c, an exchange between the cis/trans-diastereomers takes place at higher temperatures (∆G#  =  15–16 kcal/mol), while enantiomerization is observed in the cis-diastereomers at lower temperatures (∆G# = 10–12 kcal/mol) (Table 15.3). On the basis of the dependence of the activation energy for cis-diastereomers on the О→Ge bond length, a regular non-dissociative mechanism of enantiomerization involving the formation of a neutral bicapped intermediate was anticipated. At the same time, the authors suggest that the alternative twist mechanism may not be excluded (Scheme 15.2) [141]. Considering the extremely short intramolecular О→Ge distance and a significant lengthening of the Ge–Br bond of two linear O→Ge–Br fragments in the crystal state, it was proposed that the epimerization process proceeds via the Ge–Br bond cleavage (Scheme 15.6).

Scheme 15.6  Dissociative mechanism of diastereomeric exchange in hexacoordinated germanium dibromides.

Data on the high electrical conductivity of dibromides in solution [6] are consistent with the dissociation of the Ge–Br bond upon dissolution of the complex, leading to the formation of a pentacoordinate germylium ion at the pre-kinetic stage (structure b). The subsequent back-side attack by the Br atom at the rate-determining step can occur as a result of

647

648

15  Dynamic Stereochemistry of Penta- and Hexacoordinate Germanium(IV) Complexes

Table 15.3  Activation and thermodynamic parameters of diastereomerization of complexes 33, 34a–c [141].

Complex

ΔG#298 kcal/mol

ΔH# kcal/mol

ΔS# cal/mol.K

ΔG°298, kcal/mol

33

16.0

7.5

–28

1.0

0

3

34a

15.8

14.5

–4

0.5

–0.9

–5

34b

15.7

13.3

–8

0.1

–1.8

–6

34c

15.5

13.6

–6

0.1

–1.9

–6

ΔH°, kcal/mol

(ΔS°), cal/mol.K

interaction either with a dibromide molecule a or with a free halide ion (b⇆bc⇆c or b⇆bd⇆d⇆dc⇆c). Note that negative values of the entropy of activation of diastereomerization can also be caused by effective solvation of the pentacoordinate cation [142]. The activation parameters of the ligand exchange in germanium diacetylacetonate complexes 35 in comparison with related silicon and tin complexes (36, 37) are considered and shown in Table 15.4.

t

t

(35) M=Ge, X=Cl, Y=Ph, R=Me; (36) M=Si: X=Cl, Y=Ph, R=Me (a); X=Y=BuO, R= Bu (b); (37) M=Sn: X=Cl, Y=Ph, R=Me (a), X=Y=Ph, R=Me (b), X=Me, Y=Cl, R=Me (c), X=Y=Cl, R=Me (d)

For the entire series of compounds, there is an almost complete coincidence of the barriers of the dynamic process, varying in the range 13–16 kcal/mol. It is noted that the lowest barrier value occurs in the case of derivatives of hexacoordinate silicon dichloride. Despite the fact that no final conclusions in favor of dissociative or non-dissociative mechanisms were made, the independence of the enantiomerization barrier on the nature of the solvent, as well as the negative entropy of activation of the process, suggest a high probability of the associative mechanism. This assumption is supported by significantly higher barriers to enantiomerization in complexes 35–37 in comparison with hexacoordinate complexes 28–34 (see above). A rare example of facile central element exchange in neutral hexacoordinate germanium and silicon complexes is presented in [154]. For example, upon the interaction of silicon complexes 38 with an excess of ZGeCl3 (Z=Me, Cl, Ph) in a chloroform solution for one hour at the boiling point or for two days at room temperature, almost the entire silicon complex is converted into a new complex identified by the authors as a germanium complex.

X

A (38) R = CF3, X = Me (a),

(39) Z = Me,

R = CF3, X = Cl(b),

(40) Z = Cl,

R = Me, X = Ph (c)

(41) Z = Ph

This conclusion was made on the basis of the disappearance of the signal in the 29Si NMR spectrum at –124 ppm and the appearance of a new signal at 12.6 ppm. The latter corresponds to MeSiCl3. Note that if methyltrichlorogermane 39 is used instead of compound 40, no exchange is observed. The authors found that the ability of the germanium atom in the complex to replace the silicon atom depends on the relative electronegativity of ligands bound to the coordination center, as well as the relative electronegativity of ligands bound to the silicon atom. Thus, GeCl4 produces dichloro complexes that have priority over monodentate ligands in the initial silicon complex (A): Me and Cl in compound 38a or Ph and Cl in complex 38с. As a result, the lower-priority ligands attached to the central silicon are replaced by the chloro-ligands and germanium. From the

Neutral Complexes of Tetravalent Germanium

Table 15.4  Activation parameters for ligand exchange in complexes 35–37 [8]. Complex

Solvent

ΔG#298, kcal/mol

ΔH#, kcal/mol

ΔS#, cal/mol K–1

35

CDCl3+ССl4

16.6

12.8

–13

36a

CDCl3+ССl4

13.1

6.4

–22

36b

C6D6

14.4

36a

CDCl3

16.6

12.2

–16

CHBr3

16.8

14.1

–9

37b

CDCl3

13.9

7.5

–22

CH2Cl2

14.1

6.8

–24

37с

CDCl3

15.3

14.1

–4

37d

CDCl3

–

–

–11

results presented in this paper, it appears that the mechanism of central element exchange is similar to that of the ligand exchange reaction. This similarity is supported by the observation that silicon–germanium exchange is essentially dominated by the same ligand priority order [155]. Apparently the bidentate chelate-forming ligands are capable of rapid bond cleavage and transfer from one central element to the other, thereby effecting complete exchange between complexes. It is likely that the dative N→Si bonds are first to cleave, and the nitrogen attacks a neighboring “heavy” element (silicon or germanium) followed by O–Si bond cleavage and complete transfer of the bidentate ligand from the silicon to the germanium. This initial process is then followed by cleavage and transfer of all the bidentate ligands from one atom (Si) to another one (Ge), and vice versa. It is important that at no point along this exchange there is a cleavage of the Si–C or Ge–C bonds.

15.5.3 Germatranes Germatranes are one of the most studied classes of pentacoordinate germanium compounds [156]. In atranes, the dynamic processes that can be realized in solution do not directly affect the coordination environment of the central germanium atom but are induced by changes in the atrane skeleton. The latter has a third-order axis of symmetry and is therefore chiral. In this regard, conformers are theoretically possible because of the inversion of α-carbon atoms of five-membered rings with a common Ge–N bond. Because of the fact that hydrogen atoms occupy either equatorial or axial positions, two triplets are observed at 3–4 ppm in the 1H NMR spectra. The mutual position of the signals does not change in the temperature range from –80 to +200°C and is practically independent of the nature of the solvent. This made it possible to conclude that the barrier to the inversion of α-carbon atoms is small (< 2 kcal/mol) [157]. Derivatives of silatranes [158], where one of the hydrogens of the α-carbon atoms is replaced by a methyl (or other) group, according to NMR spectroscopy data, exist as a mixture of diastereomers. Note that the results of NMR studies indicate the possibility of an interconversion between them in the liquid phase. Similar complexes were synthesized in the case of germatranes [159–161]. An illustrative example is germatrane 42, where one of the six hydrogen atoms of three α-carbon atoms is replaced by a phenyl group [159]. Because of the presence of a bulky phenyl group, the inversion of α-carbon atoms becomes more difficult than in unsubstituted germatranes. Thus, for 1-trimethylsiloxy-3-phenylgermatrane (42), disordering of one α-carbon atom and phenyl substituent is observed in the crystal, as well as disordering of two α-carbon atoms in adjacent rings [159].

42

This arrangement can be explained by the fact that the structure with the phenyl group and the hydrogens of the adjacent three α-carbons are maximally distant from each other, should always be the most stable. Two such diastereomers are possible. Their superposition is observed in crystal since the molecule is located on the m plane (space group Pnma), inconsistent with the atrane structure.

649

650

15  Dynamic Stereochemistry of Penta- and Hexacoordinate Germanium(IV) Complexes

In the case of 1-fluoro-3-phenylgermatrane (43), it is noted that the presence of an asymmetric carbon atom leads to difficultly in interpretation of the 1H and 13C NMR spectra; however, no disordering is observed in the crystal. In most of 1-substituted 3,7,10-trimethylgermatranes, the disordering of α-carbons and methyl groups bonded to them is observed in the crystal. The inversion process appears to be hampered; however, these compounds exist as a mixture of co-crystallizing diastereomers. For germatrane derivatives, in which both hydrogens on two of the three α-carbons are replaced with a methyl group, the inversion of the unsubstituted α-carbon can be represented as vibrations with low amplitude around the equilibrium position. Indeed, in the crystal of (1,1ʹ-((2-oxyethyl)imino)bis(2-methylpropan-2-olato))-(propan-2-olato)germanium (44), the unsubstituted five-membered ring is strongly flattened and the thermal ellipsoids of its skeletal atoms are much more elongated as compared to the neighboring carbon and oxygen atoms [162].

44

In a solution of 1-ethoxy-(2,8,9-trioxa-5-aza-3,3,6,6-tetramethyl-1-germatricyclo[3.3.3.01,5]undecane) (45), in which four methyl groups are linked to carbon atoms at positions 3 and 6, the process of epimerization was observed, consisting of the transformation of one diastereomer (Δ) to another (Λ) [163].

45

This compound crystallizes in the centrosymmetric space group P21/n, which indicates the presence of both diastereomers in the crystal. In diastereomer Δ, the germanium atom has the δ configuration, and the nitrogen atom has the S configuration. On the contrary, in the Λ diastereomer, the germanium atom has the λ configuration, and the nitrogen atom has the R configuration (Figure 15.4). Note that the 1H NMR spectrum of compound 45 obtained in CD2Cl2 solution is temperature dependent. Transition process Δ→Λ can be described as “fast” down to –30°С; however, below this point, it noticeably slows down (measurements were made down to –85°С). The coalescence temperature of signals from the CH2 groups is –60°C. 1-Bromo-(2,8,9-trioxa-5-aza-3,3,6,6-tetramethyl-1-germatricyclo[3.3.3.01,5]undecane) (46) crystallizes in the chiral space group P212121, indicating the presence of only one diastereomer in the crystal; i.e., the two diastereomers are spontaneously separated [163].

46

Neutral Complexes of Tetravalent Germanium

Figure 15.4  Optical isomerization and epimerization of compounds with the atrane fragment.

In CD2Cl2 solution, the 1H NMR spectrum of bromide 46 is also temperature-dependent; the coalescence of methylene proton signals is observed at ca. –60°С. Another example of epimerization is found in 1-[1,1ʹ,1″-(nitrilo)tris(2-methylpropan-2-olato)](3,3,3-trifluoro-2-methoxy-2-phenylpropanoato)germanium (47) [164].

47

In the synthesis of 47, two diastereomers (S, Δ) and (R, Λ) are formed; they were both studied by XRD. The authors note that epimerization in this case can proceed by two mechanisms, concerted and sequential [164]. The first mechanism implies simultaneous inversion of rings, and in the second one inversion separately occurs in each five-membered ring. The Gibbs free energy of the epimerization process determined for similar stannatranes was 39–40 kcal/mol. Quantum-chemical calculations performed using various DFT functionals indicate that the difference in energy between diastereomers did not exceed 2 kcal/mol. When using the B3LYP functional, the energy of the Δ-diastereomer is 1.02 kcal/mol less than that of the Λ-diastereomer. In the case of the BP86 functional, Λ-diastereomer is more thermodynamically favorable by 1.79 kcal/mol. An experimental estimate obtained from a comparison of equilibrium constants of the forward and reverse epimerization processes for similar stannatranes indicates that the Gibbs free energy of the Λ-diastereomer is 0.79 kcal/mol less than that of Δ-diastereomer [164].

15.5.4  Spirogermanium Bisocanes (Quasi-germatranes) The compounds considered in this section are a type of spirocyclic analogues of germatranes, reflected in the name “quasiatranes” proposed by Voronkov [165]. In a crystal, the quasi-atrane backbone, as a rule, corresponds to the symmetry group C2, but there are also examples of the Cs symmetry [166].

651

652

15  Dynamic Stereochemistry of Penta- and Hexacoordinate Germanium(IV) Complexes

As a result, in the quasi-atrane backbone, the five-membered rings in solution undergo continuous change in their conformations. Accordingly, the structure corresponding to the Cs symmetry group transforms into a structure with the C2 symmetry group. This transformation was experimentally discovered as early as 1970s for antimony complexes of similar structure and quasi-silatranes [167, 168]. Later, it was also confirmed for quasi-germatranes studied by NMR spectroscopy [169]. The nature of these processes apparently depends on the nature of the atoms in the coordination environment of germanium and differs somewhat from that in atranes. A detailed conformational analysis of quasi-germatranes was performed in a number of works [170, 171] using the B3PW91/6-311++G(df,p) quantum chemical method. The barrier to such conformational transformations is apparently low, and in compounds where germanium is bound to sulfur atoms, its value is the smallest. The latter observation is probably related to the longer Ge–S bonds as compared to Ge–N bonds. In particular, in (48) and (49), according to 13C and 1H NMR spectroscopy, the CH2CH2O and CH2CH2S fragments are equivalent, indicating establishment of dynamic equilibrium in the solution [172].

D = O, 48; S, 49

The lack of experimental data is partially compensated for by the quantum chemical calculation of various conformations of 1-fluoro-substituted quasi-silatranes [170]. Note that in the case of quasi-germatranes, another stereodynamic process is theoretically possible, i.e., the exchange of substituents at the germanium atom. However, the presence of the spirobisocane structure causes serious steric obstacles, and difficulties in recording 73Ge NMR spectra greatly complicates the detection of such a process. Practically, the only way to judge the presence or absence of such a process is to measure the NMR spectra on the nuclei of atoms directly associated with the germanium atom. In this case, the most convenient is the 19F nucleus. Thermodynamic parameters of the process can be estimated from the data available for similar quasi-silatranes. In particular, for quasi-silatrane 3-O2N-C6H4(F)Si(OCH2CH2)2NMe (50), the free activation energy of the process was found to be 13.1 kcal/mol [173]. In the case of quasi-germatranes, the activation energy can be higher since the N→Ge coordination bond should be stronger than the N→Si bond. According to quantum-chemical calculations of the series of 1-fluoro-1-methyl(aryl) quasisilatranes, the difference in the energy between isomers, in which the fluorine atom is in the axial position and the alkyl or aryl group in the equatorial position, and isomers with the reverse order of the substituents at the silicon atom does not exceed 3.4 kcal/mol [174]. This suggests that the exchange of substituents proceeds more easily in dimethyl quasi-germatranes, in which the N→Ge bond is significantly longer and weaker, compared to dihalo quasi-germatranes. For various monomers, dimers, and trimers of quasi-germatranes, the mechanism of substituent exchange at the germanium atom has been discussed in detail [175]. The presence of a dynamic process was found by the analysis of NMR spectra (broadening of signals of indicator groups). The authors believe that in the quasi-germatranes studied, a process similar to the Berry pseudorotation is accompanied by the breaking of the N→Ge bond. The answer to the question about the amount of energy consumption in this case is given by article [176]. Thus, in dimeric germanospirobis(ocanes) with a hexacoordinate germanium atom, there are two N→Ge coordination bonds and nitrogen atoms are located in the trans-position relative to each other. For such molecules, the existence of several diastereomers is possible where the relative arrangement of coordination bonds can be described as cis. In solution, the existence of these diastereomers and their transformation into each other, was confirmed by NMR spectroscopy [176]. The authors considered two possible mechanisms of isomerization. One of the mechanisms involves breaking of one of the N→Ge bonds, while the other describes the process without breaking the coordination bonds. According to quantum chemical calculations, the transition state of the isomerization process without cleavage of the N→Ge bonds is characterized by their trans-arrangement, while the octahedral coordination of the germanium atom is noticeably distorted in comparison with the most stable diastereomers. The latter are also characterized by the trans-arrangement of the N→Ge bonds (Figure 15.5). Cleavage of the N→Ge bond is accompanied by an increase in the total energy of 21.5–23.1 kcal/mol. Apparently, this process leads to an increase in the ring strength, taking into account the fact that isomerization without cleavage of the N→Ge bonds is characterized by a significantly lower barrier (13.4–21.3 kcal/mol).

Neutral Complexes of Tetravalent Germanium H N

O

O Ge O

O

N H H

H N

TS

O

O

O Ge N

N

O

O O

Ge

O O

H N Cis

H

Trans

Figure 15.5  Cis–trans isomerization in a quasi-germatrane dimer.

15.5.5  Halogermane Adducts with Donor Ligands Compounds of this type are characterized by a dynamic equilibrium between the complex and the uncoordinated ligand and halogermane. Obviously, this equilibrium is determined by the donor ability of the ligand and the acceptor ability of germanium tetrahalide. The most significant results were obtained for tetrafluorogermane complexes, since the latter are sufficiently stable towards hydrolysis and convenient for study by NMR spectroscopy. The products of the interaction of GeF4 with various neutral N-donor ligands were reported [100]. Crystal structures have been obtained for most of the complexes. In the case of complexes with cyclam (1,4,8,11-tetra-azacyclotetradecane) (51) and Me4-cyclam (1,4,8,11-tetramethyl1,4,8,11-tetra-azacyclotetradecane) (52), NMR showed the presence of two isomers in solution in a dynamic equilibrium in a 1: 1 ratio (since the ligand itself can exist in meso and DL forms, and there are several nitrogen atoms that can coordinate the germanium atom) [100].

R = H, 51; R = Me, 52

In solutions of GeF4 complexes with 2,2ʹ-dipyridyl and 1,10-phenanthroline, narrow signals of protons are observed, indicating the absence of any stereodynamic transformations on the NMR timescale. When GeF4 was replaced by GeCl4 in reaction with 1,10-phenanthroline, no visible changes occurred, while with 2,2ʹ-dipyridyl an equilibrium was found between the complex, the unbound ligand, and germanium tetrahalide. The ability of GeBr4 to bind to N-donor ligands is apparently weaker than that of GeCl4. The phenomenon of fast, on the NMR timescale, exchange was observed only in the GeBr4 complex with ethylenediamine. Similarly, reversible dissociation/association, also accompanied by the exchange of fluorine atoms, was found in solution for complexes of germanium tetrafluoride with organic phosphine oxides ([GeF4{Ph2P(O)CH2P(O)Ph2}] (53a) and [GeF4{Ph2P(O)(CH2)2P(O)Ph2}]) (53b) [177].

653

654

15  Dynamic Stereochemistry of Penta- and Hexacoordinate Germanium(IV) Complexes

53a,b n = 1, 53a, 2 53b

Here, the germanium atom is hexacoordinated in the crystal, and the complexes are monomeric (1:1 ratio of GeF4 and the ligand). Note that the germanium atom is included in a six-membered ring formed because of the O→Ge coordination bonds in 53a,b, where the conformation can be described as a “chair”. However, one cannot exclude the presence of a complex in the solution, in which the ring has a “bath” conformation. According to X-ray crystallography, the relative arrangement of the phenyl substituents at the phosphorus atom in 53a,b can be described as cis, although the trans arrangement of the corresponding substituents can also be realized. The position of equilibrium in such an association–dissociation process depends on the temperature: with decreasing temperature, not only complexes of 1:1 composition, but also 2:1 complexes were found. Dynamic equilibrium was also observed for the GeF4 complexes with thioethers Me2S(CH2)2SMe2 (54a, R=Me) and Et2S(CH2)2SEt2 (54b, R=Et) [178].

54a,b

Because of coordination with sulfur atoms in 54a,b, the crystal contains a five-membered ring with two Ge–S bonds with cisarrangement of substituents at the sulfur atom. Based on the VT 1H NMR spectra obtained in anhydrous CD2Cl2, the equilibrium at 25°C is shifted towards uncoordinated ligands and GeF4 in 54a,b. Decreasing the temperature to –50°C is accompanied by broadening of signals and the formation of a 1: 1 complex, where the pyramidal sulfur atom undergoes rapid inversion in 54a,b.

15.6  Anionic Complexes of Tetravalent Germanium Exchange between fluorine atoms in axial and equatorial positions occurs in the TBP structures of most fluorosilicates, historically the first models for studying permutation processes at the coordination center of group 14 elements [179]. The exchange rate increases with an increase in fluorine atoms in the fluorosilicate, and because of the presence of electronwithdrawing substituents in the phenyl group of aryl fluorosilicate, and decreases under the influence of sterically strained bulky substituents at the silicon atom. Experimental data on stereodynamic processes in anionic complexes of tetravalent germanium is currently very limited. A study of the structure of dianionic complexes 55–58 in solid and liquid phases was published in [180].

55

56

57

58

Cationic Complexes of Tetravalent Germanium

Structure of complexes mer-55, mer-56⋅THF, mer-57, and mer-58 was confirmed by elemental analysis (C, H, N), singlecrystal X-ray diffraction, and solid-state 29Si VACP/MAS NMR experiments (except for mer-58). In addition, all compounds were characterized by NMR studies in solution (1H, 13C, 29Si (except for mer-58)), including VT 1H NMR experiments with mer-57. The Si/Ge-analogous compounds mer-55 and mer-58 crystallize in the space group P21/c, with isotypic structures. Compounds mer-56⋅2THF and mer-57 crystallize in the space groups P1 and Pna21, respectively. The Si(Ge)-coordination polyhedra of the studied compounds are distorted octahedra, with O–Si–O (O–Ge–O) bond angles in the ranges of 85.39(5)– 95.19(5)° (85.25(5)–93.47(5)°) and 175.20(5)–179.09(5)° (176.33(5)–179.58(5)°). The Si–O (Ge–O) bond distances amount to 1.7463(12)–1.8053(14)Å (1.8523(11)–1.8970(11)Å). Generally, the Si–O and Ge–O bond lengths are similar to those reported for other dianionic hexacoordinate silicon and germanium complexes, respectively (see [181–187]). A DNMR study of the mer-58 complex was not performed; however, when discussing the obtained data, it is reasonable to take into account the results obtained in the study of a structurally similar complex mer-57 using CD2Cl2 as the solvent. In contrast to the 13C NMR spectra of mer-55, mer-56⋅2THF, and mer-58 in (CD3)2CO (four sets of resonance signals each for the salicylato(2–) ligands), only three sets of resonance signals for the 3-methylsalicylato(2–) ligands were observed in the 13C NMR spectrum of mer-57 in CD2Cl2. In addition, three resonance signals for the methyl groups of the 3-methylsalicylato(2–) ligands were observed in the 1H NMR spectrum of mer-57. These results can be interpreted in terms of configurational stability of the mer-isomer (no fac/mer-isomerization) in CD2Cl2 on the NMR timescale at room temperature. The configurational stability of the mer-57 complex in the temperature range 23–83°C in 1,1,2,2-tetrachloroethane-d2 С2D2Сl4 was also established by VT 1H NMR spectroscopy.

15.7  Cationic Complexes of Tetravalent Germanium The TBP structure both in solution and in the solid phase is typical of donor-stabilized cationic complexes 59 [188] with the so-called tweezers tridentate N,C,N-chelating ligands.

(59) R2=R3=Me,R1=H (a); R2=R3=4-tBu-2-Me-C 6H3, R1=H (b); R2 = R3 = Me, R1 = CD3 (c)

According to the NMR data, the spectral picture of 59a-c is significantly different compared to their Si-analogues for which a fast dynamic equilibrium on the NMR timescale was established (Scheme 15.7) [188]. N

N

Si

Si

N

Cl

N Cl Cl

N

Si N

Scheme 15.7  Dynamic equilibrium of the silicon-containing species in solution.

In fact, down to –90°С there is no change in the spectral pattern. This could mean either that the exchange reaction is very fast on NMR timescale or that 59a,b has a stable hypervalent structure in solution. The fact that the broadening is restricted only for the signals of the N-methyl and N-methylene groups is particularly important. Therefore, the process does not break the overall symmetry and exerts only a local effect on the groups bonded to nitrogen. A good explanation is the dynamic process of pseudorotation of the five-membered ring [188]. The ∆G# values determined at the coalescence temperature Tc are given in Table 15.5. Complexes 60–63 with two monodentate substituents differing in their nucleofugality have a structure close to ionic [189, 190]. In the TBP of these complexes, a slow (on the NMR timescale) stereodynamic process occurs, leading to averaging of the diastereotopic protons of the NCH2 groups. The ∆G# values of complexes 60–63 are characterized by high values, greater than 22–25 kcal/mol [8].

655

656

15  Dynamic Stereochemistry of Penta- and Hexacoordinate Germanium(IV) Complexes

60a X=F, Y=BF4, R1, R2= L3CH2; 60b X=Cl, Y= OTf, R1, R2= LnCH2, n =1–3; 60c M=Ge, X=Cl, Y=OClO3, R1,R2= LnCH2, n = 1, 3; 60d M=Ge, X=Cl, Y=I, 1R ,R2= L3CH2; 60e X=Cl, Y= I3, R1,R2= L3CH2; 62 M=Ge, X=Br, R1,R2= L3CH2: Y=I (a), OTf (b)

Table 15.5  Activation parameters of dissociation of ICB N→Si in complexes 59. Compound

Solvent

59a 59b

ΔG#, kcal/mol

Tc, °С

CD3OD

9.5

–70

C2D5OD

9.8

–63

CD3OD

14.1

15

The scheme of dissociation with the formation of contact ionic pairs [(LnCH2)2GeX]+Y – was proposed for complexes 59–62 based on the dependence of electrical conductivity on concentration (in the range of 0.5–11 mmol/L) [6, 8]. The mechanism of permutational isomerization presented for complexes 59–62 in Scheme 15.8 includes at the first step (not shown) the dissociation of the Ge–Y bond, followed by a five-step pseudo-rotation at the pentacoordinate germanium (an asterisk indicates the reference atom) [8]. O3 C4

Ge

C5 X1

O2

Ge

*

O3

O3

C4

C5 O2

X1

C5

C4

C

*

Ge

5

X1 *

2 *O

Ge

C4 O3 *

X1

4

C

O2

O3

Ge

O3 C5 O2

X1

X1

Ge

C4 C5

O2

Scheme 15.8  Permutational isomerization of hexacoordinated germanium complexes 59–62 in solution.

Quantum-chemical calculations (DFT, PBE) performed for complexes 59–62 in accordance with this scheme demonstrate that the experimental and theoretical (25 kcal/mol) ∆G# values are close. The replacement of the chlorine atom in complexes 59–62 with a bromine dramatically lowers the barrier to permutational isomerization. Indeed, in cationic bromide-iodide [(L7CH2)2Ge(Br)]I (62а) and bromide-triflate [(L7CH2)2Ge(Br)] OTf (62b) complexes obtained by the reaction of dibromide 32с with Me3SiI and Me3SiOTf, permutation proceeds with a significantly lower barrier (7–9 kcal/mol, Table 15.6) compared to the corresponding fluorides and chlorides [191]. Such difference suggests that the mechanisms of stereodynamic processes in complexes [(LCH2)2GeBr]+X– and [(LCH2)2GeCl]+X– are different. Negative ΔS# values suggest that the transition state is more ordered and compact than the initial molecule, typical of multistep mechanisms requiring considerable reorganization of the molecular structure. Table 15.6  Activation parameters of ligand exchange in complexes 62a and 62b (с ~ 0.1 M). Compound

Solvent

ΔG#, kcal/mol

ΔH#, kcal/mol

ΔS#, cal/mol.K

Тс, °С

62a

CDCl3 + CD2Cl2

7.7 ± 0.1

0.5 ± 0.1

–24 ± 3

–103

ca.  0.01 M

C6D5CD3

15.4 ± 0.1

12 ± 0.2

–8 ± 2

40

62b

CDCl3 + CD2Cl2

8.9 ± 0.1

3.4 ± 0.4

–18 ± 3

–73

Cationic Complexes of Tetravalent Germanium

Poor solubility in most organic solvents limited the possibility of studying the effects of both the nature of the solvent and the concentration of complexes 62a, 62b on the ∆G# value. Only for compound 62a at a sufficiently low concentration (less than 0.01 M) was it possible to obtain the 1H NMR spectrum in the non-polar aprotic solvent C6D5CD3. An increase in temperature to 90°С was accompanied by broadening and subsequent coalescence of the components of the AB system quartet (2JHH = 13.9 Hz) into a singlet. The ∆G# value in this case turned out to be significantly higher than the corresponding value obtained using an equimolar mixture of CDCl3 and CD2Cl2 as a solvent (Table 15.6). To describe the stereodynamic transformations at the coordination center of complexes 62a, 62b, a dissociation–association mechanism has been proposed, including the following main steps (Scheme 15.9):

Ψ

Scheme 15.9  Dissociation–association mechanism of stereodynamic transformation in bis(O–Ge) – chelates (62a) and (62b).

At the first step of the process, on dissolution of complex A, dissociation takes place with the formation of monodentate ligand X and pentacoordinate ionic intermediate B (step 1). Subsequent interaction of intermediate B with the starting complex A proceeds with the formation of a Br-bridged intermediate C (step 2). Structurally similar intermediates with a bridging halogen atom were proposed earlier to describe stereodynamic transformations in compounds with pentacoordinate silicon [131] and tin [192, 193]. Subsequent dissociation of intermediate C (step 3) leads to hexacoordinate dibromide D with the trans-arrangement of the Br atoms. This assumption is supported by the high stability of the trans-diastereomers of hexacoordinate germanium dibromides in solution at room temperature in slow dynamic equilibrium with cisdiastereomers, see above [144]. At the last step, as a result of the dissociation of the Br atom, a pentacoordinate ion B′ with a reversed configuration of the germanium atom is formed. According to the data presented in [27], the Berry pseudorotation and Ugi “turnstile” rotation for the cation B seem unlikely. The change in the relative position of the substituents at the germanium atom is achieved, most likely, as a result of the dissociation of the O→Ge coordination bonds. Based on the quantum chemical calculations with the PBE0/6-311G(d, p) basis set, the stability of the cationic dimer C, consisting of units A and B, was studied. Its dissociation energy is about 20 kcal/mol; the cationic dimer C is stabilized by the weak van der Waals interactions with the participation of hydrogen atoms of the side methylene groups and halogen atoms. Thus, in the dimer C, only one of the two germanium atoms (upper-right one) remains pentacoordinated while another is hexacoordinated. In other words, the central bromine atom is not coordinated to the upper-right germanium atom.

657

658

15  Dynamic Stereochemistry of Penta- and Hexacoordinate Germanium(IV) Complexes

15.8 Conclusions In this chapter, the data on dynamics and stereochemistry of a wide range of neutral, anionic, and cationic complexes of tetravalent germanium are presented. The set of experimental data, obtained by multinuclear NMR, DNMR spectroscopy, and X-ray crystallography, jointly with quantum-chemical studies, has significantly expanded the understanding of the nature and mechanisms of stereodynamic processes. Current advances in the study of stereodynamic processes include data on structural rearrangements in the coordination unit of the germanium atom, energy of coordination bonds, and activation energies. This requires information about set of factors, including those that affect the direction and mechanism of external stereodynamic processes, the solvent nature, the presence of nucleophiles and/or the asymmetric center, the cyclic/acyclic structures of molecules, and the structural features of mono- and bidentate ligands. Because of the difficulties in recording 73Ge NMR spectra, as well as in the synthesis of organogermanium compounds (high price, low availability, and low hydrolytic stability), the data on the discussed stereodynamic processes are rather scarce. We hope that the readers of this chapter will be convinced that further research in this intriguing area of organometallic chemistry will not only provide answers to remaining questions but will also pose new exciting questions for researchers.

Acknowledgments The authors wish to thank all their colleagues who have been involved in the chemistry of hypervalent compounds of germanium and whose names have been cited in the text, particularly to our teacher, Professor Yuri Baukov. We thank Dr. Dmitry Tarasenko, Dr. Alexei Nikolin, and Dr. Tania Shmigol for their assistance in the preparation of the manuscript. We are also grateful to RFBR (Russian Foundation for Basic Research) for the financial support (project nos. 13-03-01084, 14-03-00995, 17-03-01211). The Ministry of Science and Higher Education of the Russian Federation is also kindly acknowledged.

References 1 Carey, F.A. and Sundberg, R.J. (1984). Advanced Organic Chemistry, Part A, Structure and Mechanisms, 2e. New York: Plenum Press. 2 Wolf, C. (2007). Dynamic Stereochemistry of Chiral Compounds: Principles and Applications. Washington: RSC Publishing. 3 von Zelewsky, A. (1996). Stereochemistry of Coordination Compounds. New York: Wiley. 4 Gielen, M. (1982). Chirality, static and dynamic stereochemistry of organotin compounds. Top. Curr. Chem. 104: 57–105. 5 Negrebetsky, V.V. and Baukov, Y.I. (1997). Dynamic stereochemistry of hypervalent silicon, germanium and tin compounds containing amidomethyl C,O-chelating ligands. Russ. Chem. Bull. 46 (11): 1807–1831. 6 Baukov, Y.I. and Tandura, S.N. (2002). Hypervalent compounds of organic germanium, tin and lead derivatives. In: The Chemistry of Organic Germanium, Tin and Lead Compounds, 2 (ed. Z. Rappoport), 963–1239. Chichester: Wiley, Ch. 16. 7 Konovalov, A.I., Antipin, I.S., Burilov, V.A., Madzhidov, T.I., Kurbangaliev, A.P. et al. (2018). Modern trends of organic chemistry in Russian Universities. Russ. J. Org. Chem. 54 (2): 157–371. 8 Negrebetsky, V.V., Tandura, S.N., and Baukov, Y.I. (2009). Stereochemical non-rigidity of complexes of hypercoordinate Group 14 elements. Russ. Chem. Rev. 78 (1): 21–51. 9 Gielen, E.M. and Tiekink, E.R.T. (2005). Metallotherapeutic Drugs and Metal-based Diagnostic Agents. The Use of Metals in Medicine, 598. Chichester: Wiley. 10 Makovec, T. (2019). Cisplatin and beyond: Molecular mechanisms of action and drug resistance development in cancer chemotherapy. Radiol. Oncol. 53 (2): 148–158. 11 Aldossary, S.A. (2019). Review on pharmacology of cisplatin: Clinical use, toxicity and mechanism of resistance of cisplatin. Biomed. Pharmacol. J. 12 (1): 7–15. 12 Harvey, J.D., Baker, H.A., Mercer, E., Budhathoki-Uprety, J., and Heller, D.A. (2017). Control of carbon nanotube solvatochromic response to chemotherapeutic agents. ACS Appl. Mater. Interfaces 9 (43): 37947–37953. 13 Galea, A.M. and Murray, V. (2008). The anti-tumour agent, cisplatin, and its clinically ineffective isomer, transplatin, produce unique gene expression profiles in human cells. Cancer Inform. 6: 315–355. 14 Kishimoto, T., Yoshikawa, Y., Yoshikawa, K., and Komeda, S. (2020). Different effects of cisplatin and transplatin on the higher-order structure of DNA and gene expression. Int. J Mol. Sci. 21 (1): 34.

References

15 Nakamura, T., Shimada, Y., Takeda, T., Sato, K., Akiba, M., and Fukaya, H. (2015). Organogermanium compounds, Ge-132, forms complexes with adrenaline, ATP and other physiological cis-diol compounds. Future Med. Chem. 7 (10): 1233–1246. 16 Baramov, T., Keijzer, K., Irran, E., Mosker, E., Baik, M.H., and Sussmuth, R. (2013). Synthesis and structural characterization of hexacoordinate silicon, germanium, and titanium complexes of the E. Coli siderophore enterobactin. Chem. Eur. J. 19 (32): 10536–10542. 17 Airapetyan, D.V., Murasheva, T.P., Bylikin, S.Y., Korlyukov, A.A., Shipov, A.G. et al. (2012). Trichloro- and methyldichlorogermyl monochelates and dibromo- and dichlorogermyl bischelates derived from N,N-disubstituted amides of 2-hydroxycarboxylic acids. Russ. Chem. Bull. 61 (3): 642–651. 18 Kramarova, E.P., Shipov, A.G., Negrebetsky, V.V., Bylikin, S.Y., Komissarov, E.A. et al. (2007). Donor-stabilized germylium cations. Bis{[bis((O→Ge)-chelato)-bis(N,N-dimethylcarbamoylmethyloxy)]chloromethylgermylium} hexachlorodimercurate, the first bis-chelate cationic complex of pentacoordinate germanium 2-hydroxycarboxamides. Russ. Chem. Bull. 56 (9): 1932–1933. 19 Shipov, A.G., Gruner, S.V., Korlyukov, A.A., Kramarova, E.P., Murasheva, T.P. et al. (2010). Tribromogermyl monochelates - derivatives of N,N-disubstituted 2-hydroxycarboxylic amides. Russ. Chem. Bull. 59 (4): 761–770. 20 Ovchinnikov, Y.E., Pogozhikh, S.A., Khrustalev, V.N., Bylikin, S.Y., Negrebetsky, V.V. et al. (2000). Crystal and molecular structure of difluorides and dibromides of hexacoordinated germanium containing lactamomethyl C,O-chelating ligands. Russ. Chem. Bull. 49 (10): 1775–1781. 21 Bylikin, S.Y., Kramarova, E.P., Shipov, A.G., Negrebetskii, V.V., and Baukov, Y.I. (2001). Synthesis and thermal stability of N-(Chlorodimethylgermylmethyl)-N-methyl-O-(trimethylsilyl)salicylamide. Russ. J. Gen. Chem. 71 (8): 1323–1324. 22 Bylikin, S.Y., Kramarova, E.P., Pogozhikh, S.A., Shipov, A.G., Negrebetsky, V.V. et al. (2003). Synthesis and molecular and crystal structures of new cation-anionic hypervalent germanium complexes with 1,1,3,3-tetrakis(O–Ge)chelato-1,1,3,3tetra(lactamomethyl)-1,3-digermoxane dications. Russ. Chem. Bull. 52 (8): 1720–1729. 23 Bylikin, S.Y., Shipov, A.G., Kramarova, E.P., Artamkina, O.B., Negrebetskii, V.V., and Baukov, Y.I. (2004). Reactivity of N-(chlorodimethylgermyl)methyl and N-(chlorodimethylsilyl)methyl derivatives of lactams and amides toward Grignard reagent. Russ. J. Gen. Chem. 74 (9): 1356–1358. 24 Shipov, A.G., Kramarova, E.P., Murasheva, T.P., Negrebetsky, V.V., Bylikin, S.Y. et al. (2005). Hypercoordinate germanium compounds with new five-membered monoanionic (O,O)-chelate ligands, the derivatives of 2-hydroxycarboxylic acids amides. Russ. Chem. Bull. 54 (9): 2233–2234. 25 Baukov, Y.I., Korlyukov, A.A., Kramarova, E.P., Shipov, A.G., Bylikin, S.Y. et al. (2008). Hexacoordinate germanium mixed bischelates with the GeCO3Cl2 ligand environment. Arkivoc 2008 (Part (iv)): 80–89. 26 Korlyukov, A.A., Lyssenko, K.A., Baukov, Y.I., and Bylikin, S.Y. (2013). Chemical bonding in 1-(chlorodimethylstannylmethyl)-2-piperidone and its Si and Ge analogues. General trends and O→M (M = Si, Ge, Sn) coordination bond energy. J. Mol. Struct. 1051: 49–55. 27 Korlyukov, A.A., Komissarov, E.A., Kramarova, E.P., Shipov, A.G., Negrebetsky, V.V. et al. (2016). Donor-stabilized germylium cations. To the scheme of formation of bis(chelate) germylium ions using the complexes with lactamomethyl (С,О)-chelate ligand (enantolactam derivatives) as an example. Russ. Chem. Bull. 65 (11): 2583–2593. 28 Mertens, R.T., Parkin, S., and Awuah, S.G. (2020). Exploring six-coordinate germanium(IV)-diketonate complexes as anticancer agents. Inorg. Chim. Acta 503: 119375. 29 Muetterties, E.L. (1969). Topological representation of stereoisomerism. I. Polytopal rearrangements. J. Am. Chem. Soc. 91 (7): 1636–1643. 30 Berry, R.S. (1960). Correlation of rates of intramolecular tunneling processes, with application to some Group V compounds. J. Chem. Phys. 32 (3): 933–938. 31 Westmoreland, T.D. (2008). Symmetry control of chemical reactions: Applications to the Berry pseudorotation of fivecoordinate transition metal complexes. Inorg. Chim. Acta 361 (4): 1187–1191. 32 Holmes, R.R. (1984). Pentacoordinated molecules. 53. Influence of d-orbital occupancy on the geometry of pentacoordinated molecules. J. Am. Chem. Soc. 106 (13): 3745–3750. 33 Tacke, R., Pülm, M., and Wagner, B. (1999). Zwitterionic pentacoordinate silicon compounds. Adv. Organomet. Chem. 44: 221–273. 34 Windus, T.L., Gordon, M.S., Burggraf, L.W., and Davis, L.P. (1991). A new twist on pseudorotation. J. Am. Chem. Soc. 113 (11): 4356–4357. 35 Gordon, M.S., Windus, T.L., Burggraf, L.W., and Davis, L.P. (1990). Theoretical study of pseudorotation of pentacoordinated silicon anions: The prototypical SiH5–. J. Am. Chem. Soc. 112 (20): 7167–7171. 36 Cramer, C.J. and Squires, R.R. (1995). Quantum chemical conformational analysis of dihydroxysiliconate ion [H3Si(OH)2–]. An exceptionally fluxional pentacoordinate [10-Si-5] system. J. Am. Chem. Soc. 117 (36): 9285–9290.

659

660

15  Dynamic Stereochemistry of Penta- and Hexacoordinate Germanium(IV) Complexes

37 Ugi, I., Marquarding, D., Klusacek, H., Gokel, G., and Gillespie, P. (1970). Chemistry and logical structures. Angew. Chem. Int. Ed. Engl. 9 (9): 703–730. 38 Ugi, I., Marquarding, D., Klusacek, H., Gillespie, P., and Ramirez, F. (1971). Berry pseudorotation and turnstile rotation. Acc. Chem. Res. 4 (8): 288–296. 39 Ovchinnikov, Y.E., Struchkov, Y.T., Shipov, A.G., Smirnova, L.S., Baukov, Y.I., and Bylikin, S.Y. (1994). Modeling of hexacoordinated transition-states of germanium based on the crystal-structures of germanium-substituted Bis(lactamo-NMethyl)germanes L2GeX2 and L2GeXY (X, Y=Hal, OTf, BF4, I3). Mendeleev Commun. 4 (5): 178–180. 40 Vologzhanina, A.V., Korlyukov, A.A., and Antipin, M.Y. (2008). Special features of intermolecular bonding A·D (A = Si, Ge and D = nucleophile) in crystal structures. Acta Crystallogr. Sect. B: Struct. Sci. 64 (4): 448–455. 41 Guzei, I.A. and Wendt, M. (2006). An improved method for the computation of ligand steric effects based on solid angles. Dalton Trans. 33: 3991–3999. 42 Holmes, R.R. and Deiters, J.A. (1977). Structural distortions of cyclic phosphoranes and the Berry exchange coordinate. A quantitative description. J. Am. Chem. Soc. 99 (10): 3318–3326. 43 Kost, D. and Kalikhman, I. (2009). Hypercoordinate silicon complexes based on hydrazide ligands. A remarkably flexible molecular system. Acc. Chem. Res. 42 (2): 303–314. 44 Bassindale, A.R., Sohail, M., Taylor, P.G., Korlyukov, A.A., and Arkhipov, D.E. (2010). Four independent structures of a pentacoordinate silicon species at different points on the Berry pseudorotation pathway. Chem. Commun. 46 (19): 3274–3276. 45 Couzijn, E.P.A., Schakel, M., de Kanter, F.J.J., Ehlers, A.W., Lutz, M. et al. (2004). Dynamic configurational isomerism of a stable pentaorganosilicate. Angew. Chem. Int. Ed. 43 (26): 3440–3442. 46 Couzijn, E.P.A., van den Engel, D.W.F., Slootweg, J.C., de Kanter, F.J.J., Ehlers, A.W. et al. (2009). Configurationally rigid pentaorganosilicates. J. Am. Chem. Soc. 131 (10): 3741–3751. 47 Windus, T.L., Gordon, M.S., Davis, L.P., and Burggraf, L.W. (1994). Theoretical study of pseudorotation of pentacoordinated silicon anions: SiH5–nXn– (X = F, Cl). J. Am. Chem. Soc. 116 (8): 3568–3579. 48 Couzijn, E.P.A., Slootweg, J.C., Ehlers, A.W., and Lammertsma, K. (2010). Stereomutation of pentavalent compounds: Validating the berry pseudorotation, redressing Ugi’s turnstile rotation, and revealing the two- and three-arm turnstiles. J. Am. Chem. Soc. 132 (51): 18127–18140. 49 Bürgi, H.B. (1975). Stereochemistry of reaction paths as determined from crystal structure data-A relationship between structure and energy. Angew. Chem. Int. Ed. Engl. 14 (7): 460–473. 50 Macharashvili, A.A., Shklover, V.E., Struchkov, Y.T., Oleneva, G.I., Kramarova, E.P. et al. (1988). Model of the SN2 nucleophilic substitution at the Si atom: X-ray structural study of N-(halogenodimethylsilylmethyl) lactams. J. Chem. Soc. Chem. Commun. 683–685. 51 Bassindale, A.R., Parker, D.J., Taylor, P.G., Auner, N., and Herrschaft, B. (2003). Modelling SN2 nucleophilic substitution at silicon by structural correlation with X-ray crystallography and NMR spectroscopy. J. Organomet. Chem. 667 (1–2): 66–72. 52 Bassindale, A.R., Baukov, Y.I., Borbaruah, M., Glynn, S.J., Negrebetsky, V.V. et al. (2003). Modelling nucleophilic substitution at silicon using solution 19F-NMR chemical shift, 1JSi-F and 2JC-F coupling constant data of pentacoordinate silicon compounds. Correlation with other magnetic nuclei and X-ray structures. J. Organomet. Chem. 669 (1–2): 154–163. 53 Bassindale, A. (2001). Modelling nucleophilic substitution at silicon using hypervalent silicon compounds based on urea ligands. J. Organomet. Chem. 619 (1–2): 132–140. 54 Korlyukov, A.A., Lyssenko, K.A., and Antipin, M.Y. (2002). Chemical bonding in isolated molecules and crystals of zwitterionic pentacoordinate silicon chelates. Russ. Chem. Bull. 51 (8): 1423–1432. 55 Sidorkin, V.F., Vladimirov, V.V., Voronkov, M.G., and Pestunovich, V.A. (1991). Modelling of the pathway of SN2 intramolecular substitution at the silicon(IV) atom. J. Mol. Struct. 228: 1–9. 56 Nikolin, A.A., Kuznetsova, O.V., Arkhipov, D.E., Kramarova, E.P., Shipov, A.G. et al. (2013). Intra- and intermolecular coordination reactions in solution involving pentacoordinated fluorosilanes based on proline. Russ. Chem. Bull. 62 (8): 1892–1899. 57 Korlyukov, A.A., Arkhipov, D.E., Volodin, A.D., Negrebetskii, V.V., Nikolin, A.A. et al. (2019). Mono-C,O-chelated bromoand triflatosilanes with an amino acid moiety: Salts or covalently bonded complexes? Russ. Chem. Bull. 68 (1): 137–148. 58 Klemperer, W.G. (1972). Topological representation of permutational isomerization reactions. J. Am. Chem. Soc. 94 (20): 6940–6944. 59 Farnham, W.B. and Whitney, J.F. (1984). Stereomutation at hexacoordinate silicon by a ligand-dissociation process. J. Am. Chem. Soc. 106 (14): 3992–3994. 60 Bailar, J.C. (1958). Some problems in the stereochemistry of coordination compounds. J. Inorg. Nucl. Chem. 8: 165–175.

References

61 Rây, N.K.D. (1943). Kinetics and mechanism of racemization of optically active cobaltic trisbiguanide complex. J. Indian Chem. Soc. 20: 81–92. 62 Bickley, D.G. and Serpone, N. (1976). Configurational rearrangements in cis-M(AA)2X2, cis-M(AA)2XY, and cis-M(AB)2X2 complexes. I. A permutational analysis. Inorg. Chem. 15 (4): 948–954. 63 Kost, D., Krivonos, S., and Kalihman, I. (1997). Ligand exchange mechanism in novel hexacoordinate silicon complexes. In: Organosilicon Chemistry III, from Molecules to Materials (ed. N. Auner and J. Weis), 435–445. Weinheim: VCH, Ch. 73. 64 Kalikhman, I., Kingston, V., Girshberg, O., and Kost, D. (2001). Neutral hexacoordinate silicon tris-chelates: Structure and stereodynamics. Organometallics 20 (22): 4713–4720. 65 Breliere, C., Carre, F., Corriu, R.J.P., Poirier, M., Royo, G., and Zwecker, J. (1989). Hexacoordinated silicon species: A possible model for reaction intermediates. 1. X-ray determination of the geometry in the solid state. Organometallics 8 (7): 1831–1833. 66 Corriu, R.J., Mix, A., and Lanneau, G.F. (1998). Intramolecular nitrogen ligand stabilization of phenyl-imidazolidine derived silicon species. J. Organomet. Chem. 570 (2): 183–193. 67 Kost, D., Kalikhman, I., Krivonos, S., Stalke, D., and Kottke, T. (1998). Crystal structures and stereodynamics of neutral hexacoordinate silicon chelates: Use of an optically active ligand for assignment of an intramolecular ligand exchange process. J. Am. Chem. Soc. 120 (17): 4209–4214. 68 Mislow, K. (1970). Role of pseudorotation in the stereochemistry of nucleophilic displacement reactions. Acc. Chem. Res. 3 (10): 321–331. 69 Akiba, K. (1998). Hypervalent compounds. In: Chemistry of Hypervalent Compounds (ed. K. Akiba), 1. New York: Wiley-VCH. 70 Wrackmeyer, B. (2006). Applications of 29Si NMR parameters. Annu. Rep. NMR Spectrosc. 57 1–49. 71 Kupče, E., Ignatovich, L.M., and Lukevics, E. (1989). 73Ge NMR of hexacoordinate organogermanium compounds. J. Organomet. Chem. 372 (2): 189–191. 72 Kupče, Ē. and Lukevics, E. (1988). The 73Ge chemical shift of hexacoordinated Germanium in the Ge(NCS)62– Anion. J. Magn. Reson. 79 (2): 325–327. 73 Gutowsky, H.S., McCall, D.W., and Slichter, C.P. (1953). Nuclear magnetic resonance multiplets in liquids. J. Chem. Phys. 21 (2): 279–292. 74 Gutowsky, H.S. and Saika, A. (1953). Dissociation, chemical exchange, and the proton magnetic resonance in some aqueous electrolytes. J. Chem. Phys. 21 (10): 1688–1694. 75 Binsch, G. and Kessler, H. (1980). The kinetic and mechanistic evaluation of NMR spectra. Angew. Chem. Int. Ed. Engl. 19 (6): 411–428. 76 Sack, R.A. (1958). A contribution to the theory of the exchange narrowing of spectral lines. Mol. Phys. 1 (2): 163–167. 77 Stewart, W.E. and Siddall, T.H. (1970). Nuclear magnetic resonance studies of amides. Chem. Rev. 70 (5): 517–551. 78 Jackman, L.M. (1975). Dynamic Nuclear Magnetic Resonance Spectroscopy. New York: Academic Press. 79 Kessler, H. (1970). Nachweis gehinderter rotationen und inversionen durch NMR-spektroskopie. Angew. Chem. 82 (6): 237–253. 80 Kaplan, J.I. and Fraenkel, G. (1980). NMR of Chemically Exchanging Systems. New York: Academic Press. 81 Sandström, J. (1982). Dynamic NMR Spectroscopy. London: Academic Press. 82 Bain, A.D. (2003). Chemical exchange in NMR. Prog. Nucl. Magn. Reson. Spectrosc. 43 (3–4): 63–103. 83 Aganov, A.V., Klochkov, V.V., and Samitov, Y.Y. (1985). New aspects of the application of nuclear magnetic resonance to the study of chemical exchange processes. Russ. Chem. Rev. 54 (10): 931–947. 84 Derome, A. (1992). Modern NMR Techniques for Chemistry Research. Oxford: Pergamon Press. 85 Higinbotham, J. and Marshall, J. (2001). NMR lineshapes and lineshape fitting procedures. Annu. Rep. NMR Spectrosc. 43: 59–120. 86 Kolehmainen, E. (2003). Novel application of dynamic NMR in organic chemistry. Annu. Rep. NMR Spectrosc. 49: 1–41. 87 Bányai, I. (2018). Dynamic NMR for coordination chemistry. New J. Chem. 42 (10): 7569–7581. 88 Binsch, G. (1968). The study of intramolecular rate processes by dynamic nuclear magnetic resonance. Top. Stereochem. 3 97–192. 89 Pyykkö, P. and Atsumi, M. (2009). Molecular single-bond covalent radii for elements 1–118. Chem. Eur. J. 15 (1): 186–197. 90 Shannon, R.D. (1976). Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Cryst. A. 32 (5): 751–767. 91 Rowland, R.S. and Taylor, R. (1996). Intermolecular nonbonded contact distances in organic crystal structures: Comparison with distances expected from van der Waals Radii. J. Phys. Chem. 100 (18): 7384–7391. 92 Meyer, R.J. (1961). Germanium. Berlin, Heidelberg: Springer.

661

662

15  Dynamic Stereochemistry of Penta- and Hexacoordinate Germanium(IV) Complexes

93 Glockling, F. (1988). Ge Organogermanium Compounds: Part 1. GeR4 Compounds and Ge(CH3)3R Compounds up to Cyclic Alkyl Groups (ed. U. Krüerke and C. Siebert). Berlin: Springer. 94 Glockling, F. (1988). Ge Organogermanium Compounds: Part 2. Ge(CH3)3R and Ge(C2H5)3R Compounds (ed. U. Krüerke). Berlin: Springer. 95 Glocking, F., Krüerke, U., and Satgé, J. (1990). Ge Organogermanium Compounds: Part 3. Tetraorganogermanium Compounds from Ge(C3H7)3R′ to GeRR′R″R‴, Germacyclic Compounds, and Organogermanium Compounds with LowCoordinate Germanium Atoms (ed. U. Krüerke and C. Siebert). Berlin: Springer. 96 Drake, J.E., Siebert, C., and Wöbke, B. (1994). Ge Organogermanium Compounds: Part 4. Compounds with GermaniumHydrogen Bonds (ed. U. Krüerke, C. Siebert, and B. Wöbke). Berlin: Springer. 97 Morgon, N.H., Xavier, L.A., and Riveros, J.M. (2000). Gas-phase nucleophilic reactions of Ge(OCH3)4: Experimental and computational characterization of pentacoordinated Ge anions. J. Mass. Spectrom. 195-196: 363–375. 98 Gordon, M.S., Davis, L.P., and Burggraf, L.W. (1992). Theoretical studies of hypervalent silicon anions. In: Advances in Gas Phase Ion Chemistry, 1 (ed. N.G. Adams and L.M. Babcock), 203–223. Greenwich: JAI Press. 99 Bechstein, O., Ziemer, B., Hass, D., Trojanov, S.I., Rybakov, V.B., and Maso, G.N. (1990). Halogenaustausch an Siliciumtetrahalogeniden. XIII. Zur Struktur und Reaktivitat von Siliciumtetrahalogenid-Pyridin-Komplexen. Z. Anorg. Allg. Chem. 582 (1): 211–216. 100 Cheng, F., Davis, M.F., Hector, A.L., Levason, W., Reid, G. et al. (2007). Synthesis, spectroscopic and structural systematics of complexes of germanium(IV) halides (GeX4, X = F, Cl, Br or I) with Mono-, Bi- and Tri-dentate and macrocyclic nitrogen donor ligands. Eur. J. Inorg. Chem. 2007 (31): 4897–4905. 101 Korlyukov, A.A. (2015). Coordination compounds of tetravalent silicon, germanium and tin: The structure, chemical bonding and intermolecular interactions in them. Russ. Chem. Rev. 84 (4): 422–440. 102 Sidorkin, V.F., Pestunovich, V.A., and Voronkov, M.G. (1995). The structure and properties of hypervalent silicon. Compounds with a Soft Coordination Center. In: Progress in Organosilicon Chemistry (ed. B. Marciniec). New York: Gordon & Breach. 103 Bylikin, S.Y., Pogozhikh, S.A., Khrustalev, V.N., Negrebetsky, V.V., Shipov, A.G. et al. (2000). Synthesis, crystal and molecular structures, and stereochemical nonrigidity of N-(chlorodimethylgermylmethyl)- and N-(bromodimethylgermylmethyl)-N-[(S)-1-phenylethyl]acetamides and N-(chlorodimethylgermylmethyl)-4-phenyl-2pyrrolidone. Russ. Chem. Bull. 49 (1): 140–147. 104 Bylikin, S.Y., Shipov, A.G., Kramarova, E.P., Negrebetsky, V.V., Korlyukov, A.A. et al. (2009). O,O-Monochelate complexes of silicon and germanium halides: The derivatives of l-mandelic N,N-dimethylamide. J. Organomet. Chem. 694 (2): 244–248. 105 Watkinson, P.J. and Mackay, K.M. (1984). On the relation between germanium-73 and other main group IV element NMR chemical shifts. J. Organomet. Chem. 275 (1): 39–42. 106 Benedetti, M., De Castro, F., and Fanizzi, F.P. (2017). 73Ge, 119Sn and 207Pb: General cooperative effects of single atom ligands on the NMR signals observed in tetrahedral [MXnY4–n] (M = Ge, Sn, Pb; 1 ≤ n ≤ 4; X, Y = Cl, Br, I) coordination compounds of heavier XIV group elements. Dalton Trans. 46 (9): 2855–2860. 107 Frolov, Y.L., Abramov, A.V., Chipanina, N.N., Aksamentova, T.N., and Voronkov, M.G. (1999). Quantum chemical study of silicon pentacoordination in (Aroyloxymethyl)trifluorosilanes and their analogs. J. Struct. Chem. 40 (6): 849–854. 108 Lazarev, I.M., Ratovskii, G.V., Brodskaya, E.I., and Lazareva, N.F. (2003). Ab initio calculations of the geometry, electronic structure, and vibrational spectrum of (Acetoxymethyl)triflouorosilane. Russ. J. Gen. Chem. 73 (7): 1065–1071. 109 Lazarev, I.M., Ratovskii, G.V., Brodskaya, E.I., and Lazareva, N.F. (2004). Ab initio study of the structure and vibration spectrum of N-Methyl-N-(trifluorosilylmethyl)acetamide. Russ. J. Gen. Chem. 74 (1): 66–73. 110 Fujimoto, H., Arita, N., and Tamao, K. (1992). Theoretical study of ligand substitution reactions in pentacoordinated silicon species. Organometallics 11 (9): 3035–3041. 111 Belogolova, E.F. and Sidorkin, V.F. (2003). Geometry of intramolecular silicon complexes: An ab initio estimation of the sensitivity to the effect of medium. (Aroyloxymethyl)trifluorosilanes. Russ. Chem. Bull. 52 (7): 1472–1476. 112 Sidorkin, V.F., Belogolova, E.F., and Pestunovich, V.A. (2001). Ab initio study of medium effects on the geometries of the compounds of trigonal-bipyramidal silicon with coordination center ClSiC3O. J. Mol. Struct. 538 (1–3): 59–65. 113 Belogolova, E.F. and Sidorkin, V.F. (2004). Ab initio and DFT study of the structure of pentacoordinated silicon compounds: Si-substituted (O–Si)dimethyl(N-acetylacetamidomethyl)silanes. J. Mol. Struct. 668 (2–3): 139–145. 114 Sidorkin, V.F., Belogolova, E.F., and Pestunovich, V.A. (1998). Quantum-chemical study of pentacoordinated silicon compounds: Si-substituted (O−Si) dimethyl(N-acetylacetamidomethyl)silanes. Russ. Chem. Bull. 47 (2): 225–230. 115 Milov, A.A., Minyaev, R.M., and Minkin, V.I. (2003). Hypervalent intramolecular X-N (X = C, Si, Ge) coordination in atranes: Quantum-chemical study. Russ. J. Org. Chem. 39 (3): 340–347.

References

116 Milov, A.A., Minyaev, R.M., and Minkin, V.I. (2004). Quantum-chemical investigation of the hyprevalent intramolecular coordination X←N (X = C, Si, Ge) in quasimonocyclic models of IVa Group atranes. Russ. J. Org. Chem. 40 (2): 261–268. 117 Demissie, T.B. (2017). Relativistic effects on the NMR parameters of Si, Ge, Sn, and Pb alkynyl compounds: Scalar versus spin-orbit effects. J. Chem. Phys. 147 (17): 174301. 118 Vereshchagina, Y.A., Chachkov, D.V., Alimova, A.Z., and Ishmaeva, E.A. (2016). Mechanism of the hydrolysis reactions of 1-hydroxysilatrane and 1-hydroxygermatrane, 2,2-dihydroxysilocane and 2,2-dihydroxygermocane. Phosphorus Sulfur Silicon Relat. Elem. 191 (3): 496–501. 119 Ignatyev, I.S., Montejo, M., Rodriguez Ortega, P.G., Kochina, T.A., and López González, J.J. (2015). DFT study of the hydrolysis reaction in atranes and ocanes: The influence of transannular bonding. J. Mol. Model. 22 (1): 1–14. 120 Portnyagin, I.A., Lunin, V.V., and Nechaev, M.S. (2007). New organogermanium cations [RGe(OCH2CH2NMe2)2]+ with intramolecular N→Ge coordination bonds. Russ. Chem. Bull. 56 (5): 926–934. 121 Negrebetskii, V.V. and Baukov, Y.I. (1998). The hindered inversion of the seven-membered ring in N-(chlorodimethylgermylmethyl)- and N-(chlorodimethylstannylmethyl)hexahydroazepin-2-ones. Russ. Chem. Bull. 47 (11): 2307–2309. 122 Mozzchukhin, A.O., Macharashvili, A.A., Shklover, V.E., Struchkov, Y.T., Shipov, A.G. et al. (1991). Crystal and molecular structures of (O-Ge)-chelate 1-(dimethylchlorogermylmethyl)pyrrolidone-2 and 1-(dimethylchlorogermylmethyl) piperidone-2, and of (N-Ge)-chelate O-(dimethylchlorogermylmethyl)-δ-valerolactim and 2-(chlorodimethylgermylmethylthio)-pyrrolidine-1. Structural correlations between derivatives of four- and fivecoordinated germanium. J. Organomet. Chem. 408 (3): 305–322. 123 Struchkov, Y.T., Ovchinnikov, Y.E., Shipov, A.G., and Baukov, Y.I. (1995). Influence of a valent environment on the hypervalent bond of a silicon atom. Russ. Chem. Bull. 44 (9): 1705–1710. 124 Bombicz, P., Kovács, I., Nyulászi, L., Szieberth, D., and Terleczky, P. (2010). Neutral pentacoordinate group 14 compounds with β-diketonato ligands. Organometallics 29 (5): 1100–1106. 125 Yamamura, M., Takizawa, H., Sakamoto, N., and Nabeshima, T. (2013). Monomeric and dimeric red/NIR-fluorescent dipyrrin–germanium complexes: Facile monomer–dimer interconversion driven by acid/base addition. Tetrahedron Lett. 54 (51): 7049–7052. 126 Cosledan, F., Castel, A., Rivière, P., Satgé, J., Veith, M., and Huch, V. (1998). Synthesis, structural characterization, and reactivity of (8-methoxynaphthyl)hydridogermanium triflates and iodides. Organometallics 17 (11): 2222–2227. 127 Takeuchi, Y., Nishikawa, M., Tanaka, K., and Yamamoto, G. (2001). Dynamic behavior of pentacoordinate organogermanium carboxylic acids and their pyridinium salts. Heteroatom Chem. 12 (5): 451–456. 128 Baukov, Y.I., Shipov, A.G., Ovchinnikov, Y.E., and Struchkov, Y.T. (1994). Synthesis, structure and chemical properties of hypervalent germanium compounds, derivatives of lactams with a fragment. Russ. Chem. Bull. 43 (6): 917–928. 129 Baines, K.M. and Stibbs, W.G. (1995). The molecular structure of organogermanium compounds. Coord. Chem. Rev. 145: 157–200. 130 Kalikhman, I.D., Albanov, A.I., Bannikova, O.B., Belousova, L.I., Pestunovich, S.V. et al. (1989). Formation of O- and N-(dimethylchlorogermyl)methylated products by reaction of dimethyl(chloromethyl)chlorogermane with N-trimethylsilylated derivatives of amides and lactams. Organomet. Chem. USSR 2 (3): 326–330 (Engl. Transl.). 131 Negrebetsky, V.V., Shipov, A.G., Kramarova, E.P., Negrebetsky, V.V., and Baukov, Y.I. (1997). Stereochemical non-rigidity of N-(dimethylhalogenosilylmethyl)-N-(1-phenylethyl) acetamides in solutions. J. Organomet. Chem. 530 (1–2): 1–12. 132 Breliere, C., Carre, F., Corriu, R.J.P., De Saxce, A., Poirier, M., and Royo, G. (1981). Pentacoordinate silicon and germanium derivatives: Molecular structure. J. Organomet. Chem. 205 (1): 1–3. 133 Kost, D. and Kalikhman, I. (1998). Hypervalent silicon compounds. In: The Chemistry of Organosilicon Compounds, 2 (ed. Z. Rappoport and Y. Apeloig), 1339–1445. Chichester: Wiley, Ch. 23. 134 Klebe, G. and Tran Qui, D. (1984). Chiral and hexacoordinated chloro(methyl)bis(8-quinolinolato)silicon(IV), Si(CH3) (C9H6NO)2Cl. Acta Crystallogr. C 40 (3): 476–479. 135 Chuit, C., Corriu, R.J.P., and Reye, C. (1998). Structure and reactivity of hypercoordinate silicon species. In: Chemistry of Hypervalent Compounds (ed. K. Akiba), 81–146. New York: Wiley-VCH, Ch. 4. 136 Jastrzebski, J.T.B.H. and Van Koten, G. (1993). Intramolecular coordination in organotin chemistry. Adv. Organomet. Chem. 35: 241–294. 137 Kalikhman, I., Gostevskii, B., Girshberg, O., Krivonos, S., and Kost, D. (2002). Donor-stabilized silyl cations 4: N-isopropylidene hydrazides, novel bidentate ligands for penta- and hexacoordinate silicon chelates1. Organometallics 21 (12): 2551–2554. 138 Kost, D., Gostevskii, B., and Kalikhman, I. (2007). Silicon rehybridization and molecular rearrangements in hypercoordinate silicon dichelates. Pure Appl. Chem. 79 (6): 1125–1134.

663

664

15  Dynamic Stereochemistry of Penta- and Hexacoordinate Germanium(IV) Complexes

139 Kramarova, E.P., Korlyukov, A.A., Bylikin, S.Y., Shipov, A.G., Baukov, Y.I., and Kost, D. (2004). Ionic structure of the bischelate dichlorosilane stabilized by O→Si coordination, the product of reaction of N-methyl-N-trimethylsilylacetamide with (ClCH2)2SiCl2. Russ. Chem. Bull. 53 (5): 1135–1136. 140 Glasstone, S., Laidler, K.J., and Eyring, H. (1941). The Theory of Rate Processes: The Kinetics of Chemical Reactions, Viscosity, Diffusion and Electrochemical Phenomena. New York: McGraw Hill. 141 Negrebetsky, V.V., Bylikin, S.Y., Shipov, A.G., Baukov, Y.I., Bassindale, A.R., and Taylor, P.G. (2003). Stereochemical rearrangements of dibromides of hexacoordinated germanium containing amidomethyl and lactamomethyl chelate ligands. J. Organomet. Chem. 678 (1–2): 39–47. 142 Shumsky, A.N., Tandura, S.N., Ugrak, B.I., Negrebetsky, V.V., Baukov, Y.I., and Kolesnikov, S.P. (2004). Stereochemical nonrigidity of (O-Ge)-chelate bis[(2-oxo-1-hexahydroazepinyl)methyl]dichlorogermane: Substantiation of the dissociative mechanism of chelating ligand exchange. Russ. Chem. Bull. 53 (3): 551–553. 143 Tandura, S.N., Shumsky, A.N., Ugrak, B.I., Negrebetsky, V.V., Bylikin, S.Y., and Kolesnikov, S.P. (2005). Stereochemical flexibility of Bis(amidomethyl)dichlorogermanes. A novel dissociative mechanism of ligand exchange in neutral hexacoordinated bischelate complexes. Organometallics 24 (22): 5227–5240. 144 Negrebetsky, V.V., Kramarova, E.P., Arkhipov, D.E., Korlyukov, A.A., Shipov, A.G., and Baukov, Y.I. (2015). Synthesis, structure, and stereochemical non-rigidity of bis[(2,2-dimethyl-4-oxo-2H-benzo[e][1,3]oxazin-3(4H)-yl)methyl] dichlorosilane and -germane. Russ. Chem. Bull. 64 (8): 1808–1813. 145 Kramarova, E.P., Oleneva, G.I., Shipov, A.G., Baukov, Y.I., Mozzhukhin, A.O. et al. (1991). Reaction of bis(chloromethyl) dichlorogermane with (N-trimethylsilyl)lactams – A route to new types of compounds of hexacoordinate germanium. Crystal and molecular structures of the (O - Ge)-chelates bis[1-(2-oxopyrrolidinyl)methyl]dichlorogermane, bis[1-(2oxopiperidyl)methyl]dichlorogermane, and bis[1-(2-oxohexahydroazepinyl)methyl]dichlorogermane. Organomet. Chem. USSR 4: 496–499 (Engl. Transl.). 146 Bylikin, S.Y., Shipov, A.G., Kramarova, E.P., Negrebetsky, V.V., Smirnova, L.S. et al. (1997). Features of nucleophilic substitution at six-coordinate germanium atom in (O-Ge)Bischelate Bis(lactamomethyl)dichloro-germanes. Russ. J. Gen. Chem. 67 (11): 1742–1756. 147 Kost, D., Kalikhman, I., Kingston, V., and Gostevskii, B. (2002). Low-temperature ionization of neutral hexacoordinate to ionic pentacoordinate silicon complexes. Unusual temperature, solvent and substituent effects. J. Phys. Org. Chem. 15 (12): 831–834. 148 Hoffmann, R.W. (1976). Aufklärung von Reaktionsmechanismen. Stuttgart: GTV. 149 Minkin, V.I., Olekhnovich, L.P., and Zhdanov, Y.A. (1977). Molecular Design of Tautomeric Systems, 271. Rostov on Don: Rostov University. 150 Hoffmann, R., Howell, J.M., and Rossi, A.R. (1976). Bicapped tetrahedral, trigonal prismatic, and octahedral alternatives in main and transition group six-coordination. J. Am. Chem. Soc. 98 (9): 2484–2492. 151 Tandura, S.N., Voronkov, M.G., and Alekseev, N.V. (1986). Molecular and electronic structure of penta- and hexacoordinate silicon compounds. Top. Curr. Chem. 131 99–189. 152 Bylikin, S.Y., Shipov, A.G., Negrebetsky, V.V., Smirnova, L.S., Baukov, Y.I. et al. (1996). Synthesis, crystal structure and stereochemical non-rigidity of (O-Sn)-bischelated bis(lactamomethyl)dichlorostannanes. Russ. Chem. Bull. 45 (11): 2627–2638. 153 Ovchinnikov, Y.E., Pogozhikh, S.A., Razumovskaya, I.V., Bylikin, S.Y., Shipov, A.G. et al. (1999). Synthesis and crystal and molecular structures of six-coordinate tin dibromides and diiodides containing lactamomethyl C,O-chelating ligands. Russ. Chem. Bull. 48 (10): 1964–1974. 154 Yakubovich, S., Kalikhman, I., and Kost, D. (2010). Facile central-element exchange in neutral hexacoordinated germanium and silicon complexes; synthesis and characterization of germanium complexes. Dalton Trans. 39 (39): 9241–9244. 155 Sergani, S., Kalikhman, I., Yakubovich, S., and Kost, D. (2007). Complete intermolecular ligand crossover in hexacoordinate silicon dichelates. Establishment of ligand priority order. Organometallics 26 (24): 5799–5802. 156 Karlov, S.S. and Zaitseva, G.S. (2001). Germatranes and their analogs. Synthesis, structure, and reactivity. Chem. Heterocycl. Compd. 37: 1325–1357. 157 Sidorkin, V.F., Pestunovich, V.A., and Voronkov, M.G. (1980). The physical chemistry of silatranes. Russ. Chem. Rev. 49 (5): 414–427. 158 Pestunovich, V., Kirpichenko, S., and Voronkov, M. (2009). Silatranes and their tricyclic analogs. Patai’s Chemistry of Functional Groups (eds. Z. Rappoport, Y. Apeloig), 1–91. Hoboken: John Wiley and Sons.

References

159 Zaitseva, G.S., Karlov, S.S., Pen’kovoy, G.V., Churakov, A.V., Howard, J.A.K. et al. (1999). Synthesis and Characterisation of 1-[9-(H, Me3Si, Me3Ge, Me3Sn)-9-Fluorenyl]-3,7,10-trimethylgermatranes. The Crystal Structure Analysis of 1-(9-Fluorenyl)-3,7,10-trimethylgermatrane. Z. Anorg. Allg. Chem. 625 (4): 655–660. 160 Selina, A.A., Karlov, S.S., Gauchenova, E.V., Churakov, A.V., Kuz’mina, L.G. et al. (2004). Bromination of silatranyl-, germatranyl-, silyl-, and germyl-phenylacetylenes. Heteroatom Chem. 15 (1): 43–56. 161 Shutov, P.L., Karlov, S.S., Harms, K., Churakov, A.V., Howard, J.A.K. et al. (2002). Synthesis and characterization of group 14 1-Haloazametallatranes. Eur. J. Inorg. Chem. 11: 2784–2788. 162 Moon, J.H., Kim, S.H., Lee, K.M., You, T.S., Do, Y., and Kim, Y. (2011). Synthesis and X-ray diffraction analysis/crystal structure of new germatranes containing methyl substituents in three five-membered chelating rings. Polyhedron 30 (13): 2333–2338. 163 Glowacki, B., Lutter, M., Alnasr, H., Seymen, R., Hiller, W., and Jurkschat, K. (2017). Introducing stereogenic centers to group XIV metallatranes. Inorg. Chem. 56 (9): 4937–4949. 164 Glowacki, B., Lutter, M., Hiller, W., and Jurkschat, K. (2019). Control of Λ- and Δ-isomerization of the atrane cages in group XIV metallatranes by chiral axial substituents. Inorg. Chem. 58 (7): 4244–4252. 165 Voronkov, M.G. (2007). Not silocanes and silocins but quasisilatranes. Chem. Heterocycl. Compd. 43 (7): 938–940. 166 Lukevics, E., Belyakov, S., and Pudova, O. (1996). A new pathway for the synthesis of 1,3-dioxa-6-aza-2-germacyclooctanes: Molecular structure of 2,2-di(2-thienyl)-6-methyl-1,3-dioxa-6-aza-2-germacyclooctane. J. Organomet. Chem. 523 (1): 41–45. 167 Cea-Olivares, R., Muñoz-Hernández, M.A., Hernández-Ortega, S., and Silvestru, C. (1995). On the 5-thia-(substituted)-1oxa-4,6-dithia-5-stibocanes. Synthesis and characterization of O(CH2CH2S)2SbS2P(OC2H5)2 and crystal and molecular structure of (CH2CH2S)2SbSCH2CH2OCH2CH2SSb(SCH2CH2)2O, a compound with two different eight-membered ring conformations. Inorg. Chim. Acta. 236 (1–2): 31–36. 168 Kupče, E., Liepiņš, E., and Lukevics, E. (1983). The conflict between the conformational properties of the eight-membered heterocycle and trigonal bipyramid of five-coordinated silicon in 1,3-dioxa-6-aza-2-silacyclooctanes. J. Organomet. Chem. 248 (2): 131–147. 169 Lermontova, E.K., Selina, A.A., Karlov, S.S., Churakov, A.V., Howard, J.A.K. et al. (2006). The transannular interaction germanium–nitrogen in germocanes: The influence of substituents. J. Organomet. Chem. 691 (26): 5710–5724. 170 Vereshchagina, Y.A., Alimova, A.Z., Chachkov, D.V., Ishmaeva, E.A., and Kochina, T.A. (2015). Polarity and structure of 1,1-dihalo-2,8-dioxa-5-azagermocanes. Russ. J. Org. Chem. 51 (5): 750–752. 171 Vereshchagina, Y.A., Alimova, A.Z., Chachkov, D.V., Ishmaeva, E.A., Kochina, T.A., and Voronkov, M.G. (2014). Polarity and structure of 1,3,5,11-tetraoxa-8-aza-4-germaspiro [3,7]undecan-2-one. Russ. J. Org. Chem. 50 (8): 1225–1226. 172 de Velasco, D.O.-G., Sánchez-Jiménez, R., Hernández-Ortega, S., Toscano, R.A., and García-Montalvo, V. (2010). Study on the transannular bond formation and hypercoordination in tin and germanium spirometallocanes. Polyhedron 29 (12): 2435–2439. 173 Voronkov, M.G., Grebneva, E.A., Albanov, A.I., Zel’bst, E.A., Trofimova, O.M., Vasil’ev, A.D., and Timofeeva, E.N. (2014). Neutral pentacoordinate silicon complexes with SiO2FC skeleton: Synthesis, structural characterization and stereodynamical behavior. J. Organomet. Chem. 768: 10–14. 174 Aksamentova, T.N., Chipanina, N.N., Grebneva, E.A., Timofeeva, E.N., Bolgova, Y.I., and Trofimova, O.M. (2016). DFT study of the 1-aryl-1-fluoro-5-methylquasisilatrane stereoisomers. NBO and QTAIM analysis. Comput. Theor. Chem. 1091: 78–84. 175 Chen, D.H. and Chiang, H.C. (1995). Chemical specific cycloaddition of diethanolamine to germanium dioxide. Polyhedron 14 (5): 687–691. 176 Karlov, S.S., Lermontova, E.K., Zabalov, M.V., Selina, A.A., Churakov, A.V., et al. (2005). Synthesis, X-ray diffraction studies, and DFT calculations on hexacoordinated germanium derivatives: The case of germaspirobis(ocanes). Inorg. Chem. 44 (13): 4879–4886. 177 George, K., Hector, A.L., Levason, W., Reid, G., Sanderson, G. et al. (2011). Hypervalent neutral O-donor ligand complexes of silicon tetrafluoride, comparisons with other group 14 tetrafluorides and a search for soft donor ligand complexes. Dalton Trans. 40 (7): 1584. 178 Davis, M.F., Levason, W., Reid, G., Webster, M., and Zhang, W. (2008). The first examples of germanium tetrafluoride and tin tetrafluoride complexes with soft thioether coordination—synthesis, properties and crystal structures. Dalton Trans. 533–538. 179 Klanberg, F. and Muetterties, E.L. (1968). Nuclear magnetic resonance studies on pentacoordinate silicon fluorides. Inorg. Chem. 7 (1): 155–160.

665

666

15  Dynamic Stereochemistry of Penta- and Hexacoordinate Germanium(IV) Complexes

180 Seiler, O., Burschka, C., Penka, M., and Tacke, R. (2002). Dianionic complexes with hexacoordinate silicon(IV) or germanium(IV) and three bidentate ligands of the salicylato(2–) type: Syntheses and structural characterization in the solid state and in solution. Silicon Chem. 1 (5/6): 355–365. 181 Flynn, J.J. and Boer, F.P. (1969). Structural studies of hexacoordinate silicon. Tris(o-phenylenedioxy) siliconate. J. Am. Chem. Soc. 91 (21): 5756–5761. 182 Sackerer, D. and Nagorsen, G. (1977). Die Kristallstruktur von Bis(athylendiamin)kupfer(II)-tris(brenzcatechino)silicat. Z. Anorg. Allg. Chem. 437 (1): 188–192. 183 Jorgensen, N. and Weakley, T.J.R. (1980). The crystal structure of potassium trioxalatogermanate(IV) monohydrate. J. Chem. Soc. Dalton Trans. 2051–2052. 184 Balkus, K.J., Gabrielova, I.S., and Bott, S.G. (1995). Tris(oxalato) complexes of silicon as precursors to porous silicate materials: Synthesis and structure. Inorg. Chem. 34 (23): 5776–5780. 185 Hahn, F.E., Keck, M., and Raymond, K.N. (1995). Catecholate complexes of silicon: Synthesis and molecular and crystal structures of [Si(cat)2]2THF and Li2[Si(cat)3]3.5dme (cat = Catecholate Dianion). Inorg. Chem. 34 (6): 1402–1407. 186 Tacke, R., Stewart, A., Becht, J., Burschka, C., and Richter, I. (2000). Di[(hydroxyalkyl)dimethylammonium] tris[benzene1,2-diolato(2–)]silicates and their germanium analogs: Syntheses, crystal structure analyses, and NMR studies. Can. J. Chem. 78 (11): 1380–1387. 187 Biller, A., Burschka, C., Penka, M., and Tacke, R. (2002). Dianionic complexes with hexacoordinate silicon(IV) or germanium(IV) and three bidentate ligands of the hydroximato(2−) type: Syntheses and structural characterization in the solid state. Inorg. Chem. 41 (15): 3901–3908. 188 Benin, V.A., Martin, J.C., and Willcott, M.R. (1997). Solution and solid state studies of some new silicon and germanium compounds stabilized by tridentate ligands. Tetrahedron 53 (29): 10133–10154. 189 Kramarova, E.P., Pogozhikh, S.A., Shipov, A.G., Negrebetsky, V.V., Tandura, S.N. et al. (2001). Bis(O-Si)-chelate fluorobis(2oxohexahydroazepinomethyl)silyliumtetrafluoroborate, a cation-anionic complex containing a silylium ionstabilized by O→Si coordination. Russ. Chem. Bull. 50 (2): 331–332. 190 Shipov, A.G., Kramarova, E.P., Pogozhikh, S.A., Negrebetskii, V.V., Smirnova, L.S. et al. (2007). Synthesis and molecular and crystal structures of mono-and bis-chelate hypercoordinate silicon compounds containing the C,O-chelating 2,2-dimethyl-4-oxo-2,3-dihydro-1,3-oxazin-3-ylmethyl ligand. Russ. Chem. Bull. 56 (3): 461–474. 191 Negrebetsky, V.V., Korlyukov, A.A., Bylikin, S.Y., Kramarova, E.P., and Baukov, Y.I. (2020). Synthesis, molecular and crystal structure, and stereochemical non-rigidity of (O→Ge)-Bischelate bis[1-(2-oxoperhydroazepinyl)methyl] bromogermanium iodide and triflate. J. Organomet. Chem. 916: 121244. 192 Van Koten, G., Jastrzebski, J.T.B.H., Noltes, J.G., Pontenagel, W.M.G.F., Kroon, J., and Spek, A.L. (1978). Synthesis of C,Sn-chiral triorganotin halides via C-chiral arylcopper and arylgoldlithium intermediates. Crystal and molecular structure of one [2-[1-(S)-(dimethylamino)ethyl]phenyl]methylphenyltin bromide diastereomer. J. Am. Chem. Soc. 100 (16): 5021–5028. 193 Jang, M. and Janzen, A.F. (1994). Ligand exchange in adducts of triphenyltin fluoride. J. Fluor. Chem. 66 (2): 129–135.

667

16 X-ray Crystallography of Organogermanium Compounds Catherine Hemmert and Heinz Gornitzka This chapter is based on 6412 structures extracted from the Cambridge Crystallographic Data Center (CSD version 5.41 (November 2019)) and covers the literature from 2000 to 2019.

16.1  Statistics Concerning Distances between Germanium and P-block Elements These data are taken directly from the database without corrections. We distinguished between single, double, triple, aromatic and delocalized bonds, and π interactions. We used minimal, maximal, and mean (or average) values, as well as the standard deviation, and the statistically more robust median value. Hits corresponds to the number of structures concerned and not to the number of bonds.

16.2  Detailed Analysis of the Statistics In the following section, a more detailed analysis of the raw data in Table 16.1 is presented considering the oxidation states of germanium and the number of atoms bonded. For the statistics, all data of the database are considered but examples are limited to those of the last 20  years (2000–2019). In most cases, we couldn’t present all publications and we therefore limited our choice to some representative examples. Only the names of the corresponding authors are mentioned, representing naturally all authors involved in the presented work.

16.2.1  Germanium and Group 1 Elements Ge–Li  hits: 27, min: 2.224, max: 3.065, average: 2.674, standard deviation: 0.143, median: 2.666

O

O Li Li

Me(tBu)2Si

Ge Li

Me(tBu)2Si

Si(tBu)2Me Li

O

Li1

Si(tBu)2Me

Ge

tBuMe2Si H

O Ge

tBuMe2Si

Li

O O

O

Li2

In the average/median range, Sekiguchi reported in 2002 a dilithiated germane Li1 [1] containing two pentacoordinated germanium atoms forming a four-membered Ge–Li–Ge–Li cycle with distances of 2.664 and 2.709 Å. Each germanium atom coordinates a third lithium atom (Ge–Li = 2.649 Å). In 2010, Iwamoto and Kira presented a more classical fourbonded germanium-lithium compound Li2 [2] of type (tBuMe2Si)2Ge(H)Li(THF)3 with a Ge–Li distance of 2.675 Å. Short Ge–Li distances have been published by Sekiguchi in 2002 in a nonsolvated, monomeric, and nearly planar structure of tris[di-tert-butyl(methyl)silyl]germyllithium Li3 [3] (Ge–Li = 2.518 Å) and by Kinjo in 2018 presenting a dianionic digermanium species Li4 [4] (Ge–Li = 2.224 and 2.501 Å). Organogermanium Compounds: Theory, Experiment, and Applications, Volume 2, First Edition. Edited by Vladimir Ya. Lee. © 2023 John Wiley & Sons, Inc. Published 2023 by John Wiley & Sons, Inc.

668

16  X-ray Crystallography of Organogermanium Compounds

Table 16.1  Statistics concerning bonds between germanium and main-group-elements. Hits

Minimal

Maximal

Mean

Standard deviation

Median

With Group 1 elements

Ge–Li

27

2.224

3.065

2.674

0.143

2.666

GeLi π

7

2.569

2.897

2.691

0.096

2.688

Ge–Na

5

2.974

3.16

3.083

0.064

3.098

Ge–K

33

3.255

4.042

3.544

0.151

3.522

GeK π

4

3.301

3.899

3.469

0.196

3.406

GeRb

7

3.441

3.898

3.686

0.114

3.681

GeCs

2

3.673

4.072

3.812

0.121

3.776

GeMg

8

2.607

2.766

2.686

0.051

2.701

GeCa

4

3.022

3.229

3.085

0.078

3.053

GeSr

2

3.123

3.197

3.15

0.033

3.14

106

1.971

2.444

2.153

0.078

2.143

2

2.336

2.739

GeAl

8

2.450

2.563

2.520

0.031

2.525

GeGa

15

2.390

2.592

2.473

0.047

2.468

GeIn

5

2.617

3.078

2.843

0.122

2.862

GeTl

2

2.905

3.107

3.039

0.072

3.07

Ge–C

2981

1.715

2.579

1.968

0.049

1.961

Ge=C

44

1.761

2.189

1.889

0.081

1.878

GeC arom

32

1.728

2.114

1.908

0.06

1.921

GeC deloc

2

1.893

1.942

1.915

0.02

1.915

47

1.899

3.408

2.48

0.248

2.467

With Group 2 elements

With Group 13 elements

Ge–B GeB π

With Group 14 elements

GeC π C=Ge=C

1

1.882

Ge–Si

411

2.274

2.767

2.410

0.048

2.397

Ge=Si

13

2.214

2.276

2.237

0.015

2.232

GeSi arom

4

2.293

2.322

2.306

0.012

2.304

Si=Ge=Si

7

2.226

2.237

2.231

0.004

2.229

Ge–Ge

659

2.052

3.435

2.594

0.107

2.579

Ge=Ge

65

2.212

2.535

2.338

0.075

2.321

Ge≡Ge

3

2.206

2.357

2.250

0.061

2.226

GeGe arom

5

2.312

2.374

2.341

0.018

2.334

GeGe deloc

2

2.322

2.368

2.337

0.021

2.330

Ge=Ge=Ge

1

2.321

2.330

Ge–Sn

49

2.567

3.092

2.709

0.104

2.702

Ge=Sn

1

2.506

2.684

2.595

0.126

2.595

GePb

8

2.599

2.868

2.740

0.075

2.732 (Continued)

 

Table 16.1  (Continued) Hits

Minimal

Maximal

Mean

Standard deviation

Median

Ge–N

1667

1.726

3.182

1.961

0.122

1.936

Ge=N

11

1.669

1.782

1.702

0.029

1.701

3

1.938

1.966

1.949

0.015

1.944

With Group 15 elements

GeN arom GeN deloc

1

2.071

2.111

2.088

0.012

2.086

Ge–P

213

2.218

3.196

2.411

0.104

2.410

Ge=P

6

2.137

2.267

2.206

0.058

2.194

GeP π

1

2.413

2.421

2.417

0.004

2.416

Ge–As

15

2.394

2.635

2.504

0.052

2.497

Ge=As

2

2.273

2.362

2.317

0.063

2.317

GeSb

8

2.578

2.796

2.693

0.052

2.690

GeBi

7

2.662

2.935

2.764

0.075

2.743

Ge–O

1591

1.548

3.300

1.807

0.116

1.772

Ge=O

30

1.625

1.843

1.722

0.048

1.717

With Group 16 elements

O=Ge=O

1

1.760

1.761

Ge–S

439

2.066

3.055

2.237

0.087

2.229

Ge=S

52

2.048

2.218

2.098

0.037

2.087

2

2.156

2.179

2.168

0.005

2.168

Ge–Se

171

2.216

2.860

2.362

0.063

2.360

Ge=Se

53

2.172

2.327

2.227

0.035

2.219

S=Ge=S

Se=Ge=Se

1

2.285

2.327

Ge–Te

44

2.415

2.894

2.576

0.065

2.581

Ge=Te

12

2.384

2.480

2.443

0.027

2.453

Ge–F

122

1.600

1.991

1.789

0.041

1.784

Ge–Cl

815

2.572

2.985

2.236

0.092

2.224

Ge–Br

137

2.245

3.133

2.432

0.132

2.415

74

2.483

3.217

2.647

0.146

2.595

With Group 17 elements

Ge–I

GeX any bond; Ge–X single bond; Ge=X double bond; Ge≡X triple bond; GeX arom – aromatic system; GeX π–π interaction; GeX deloc – delocalized system.

Ph N

Ph

Me(tBu)2 Si

O Si(tBu)2 Me

Me(tBu)2 Si

Ph

Li

Ge N

Li3

Ph Li4

N

Ge

N

Ph

Ph

N

N

Li Ge

Mes Mes

Mes

Mes

Li

O

N N

Ph Ph

669

670

16  X-ray Crystallography of Organogermanium Compounds iPr Si

O O

tBuSi O

Ph N Ge

Si

O

iPr

Li

iPr Ge

O iPr

N

Si

Ge

iPr

O

Ph

iPr

iPr iPr

Li

N

Ph

iPr

iPr iPr

iPr

Ge

iPr

Li

iPr iPr

Li5

iPr

Li6

iPr

Ge

Ph

C

C

3

Ge

Ge

3

Li(Et2O)3

Li7

Li(DME)2

Li8

A long Ge–Li distance of 2.904 Å has been presented by Veith in 2000 where an anionic germanium is stabilized by three nitrogen atoms, Li5 [5]. In 2003, Power presented Li2ArGeGeAr Li6 [6], with two lithium atoms coordinated side-on by a multiple bonded Ge2-unit (Ge–Li = 2.874 and 2.892 Å). The longest distance (3.066 Å) has been reported by Leung in 2006 corresponding to a [{(PhC≡C)3Ge}3GeLi(Et2O)3] Li7 [7]. In 2018, Scheschkewitz showed a structure of digermenide Li8 [8] with a σ-coordinated lithium atom (Ge–Li = 2.842 Å). Ge–Li π  hits: 7, min: 2.569, max: 2.897, average: 2.691, standard deviation: 0.096, median: 2.688 The germole dianion complex Li9 [9] has been reported in 2000 by Tilley. In this this sandwich complex the short Ge–Li distances are 2.569 and 2.592 Å. In the same year, Boudjouk published the lithium complex of a germaindenyl dianion L10 [10] with significantly longer Ge–Li distances (2.694 and 2.716 Å). A slightly longer Ge–Li distance of 2.760 Å is present in the structure of a heavier LiCp analogue bearing an anionic GeSi2C2 ring system Li11 [11], presented by Sekiguchi in 2005. In 2010, Driess and Jones published a Li(THF) fragment coordinated by an essentially planar GeNC3 Cp-type heterocycle Li12 [12] in an η5-fashion with Li-Ge = 2.595 Å. The longest Ge–Li distance (2.897 Å) could be observed in the lithiated germylidenide anion Li13 [13].

Li

Hf *Cp

Me2N

Hf

Ge

Cp*

Ge Li

Li

Ph

Ge

O

O

Li

Li9

Li10

Me2N

Li(THF)

NMe2 Ph Ph

Ge

Si Si

Si(tBu)2Me Si(tBu)2Me

Si(tBu)2Me

N

Li(THF)

Ge

tBu Ar

N

Ge

tBu

N tBu

NMe2

Li11

Li12

Li tBu

tBu

tBu N

Li Ge N

Li13

Ge–Na  hits: 5, min: 2.974, max: 3.16, average: 3.083, standard deviation: 0.064, median: 3.098 iPr

iPr

iPr

iPr iPr

iPr

Ge

Na

Na

Ge

iPr iPr

H

H

Ge iPr iPr

iPr

iPr

iPr

Na1

Na

iPr Ge iPr

iPr

Na

Na

Na2

Ge Na

Na

Na3

Only five structures showing germanium-sodium bonds are in the database, three of them have been published the last twenty years. In 2003, an anionic Ge2 unit bridged by one sodium cation Na1 [6] (Ge–Na = 3.139 and 3.160 Å), and in 2004, a dianionic Ge2 unit bridged by two sodium cations Na2 [14] (Ge–Na = 3.052 and 3.122 Å) have been reported by Power. Ten years later, West presented a 9-germafluorenyl dianion in contact with four sodium cations Na3 [15] in η1-, η3-, and η5-modes with distances from 2.929 to 3.061 Å. Ge–K  hits: 33, min: 3.255, max: 4.042, average: 3.544, standard deviation: 0.151, median: 3.522 Perfectly fitting with the average/median range are two structures reported in 2003 by Power showing potassium bridging Ge2 units, one with one potassium cation K1 (Ge–K = 3.533 Å) and one with two potassium cations K2 (Ge–K = 3.557 Å)[6].

16.2  Detailed Analysis of the Statistics

The same author published in 2004 a very short Ge–K distance of 3.285 Å observed in the potassium analogue (K3) of Na2 [14]. Some other short contacts have been published the following years. In 2005, Ruhlandt-Senge studied monomeric solvent-stabilized potassium complexes of (Me3Si)3Ge anions K4 [16] (Ge–K = 3.373 and 3.399 Å). A dimeric solvent-free of the same motif K5 [17] shows Ge–K distances in the range 3.348–3.396 Å. In a related compound, where one Me3Si group on the germanium atom of K4 has been replaced by a silatrane K6 [18], the Ge–K distance is of 3.339 Å. In 2007, Baines published a Ge–K distance of 3.399 Å in a germylene adduct of a potassium gallate K7 [19]. In 2008, Driess gave an example of a dimeric half-sandwich complex K8 interconnected via intermolecular Ge–K dative bonds of 3.575 Å [20]. In the same publication, a dipotassium salt of the first “heavy” cyclobutadiene-like dianion consisting of a Ge4 core K9 [20] is presented (Ge–K = 3.355 and 3.477 Å). Müller published in 2018 a dipotassium complex of a germole dianion K10 [21] in which the germanium atom is connected to two potassium cations in a η1-mode with distances of 3.429 and 3.455 Å and to two other potassium cations in η5-modes leading to a coordination polymer. The shortest Ge–K distance (3.256 Å) has been found in [(cAAC)KGeTip]2 K11 [22], presented in 2018 by Stalke, Koley, and Roesky. iPr

iPr

iPr Ge

iPr

iPr iPr

K

K3

iPr

K2

iPr

K

K6

iPr

iPr

K1

Ge

O

H Ge iPr iPr

iPr

SiMe3

Si

N

iPr Ge iPr H K

K

K

Ge

iPr

iPr

O

iPr

iPr Ge iPr

K

SiMe3

O

iPr

iPr

Ge

K

iPr

iPr

Ge

Ar N iPr

iPr

N Ga N

K

Ge

Mes iPr

R N Mes iPr Ga

Ge

K N

N

N

Ge

N

iPr

R

SiMe 3 N

R

SiMe3

N

K

Ge

Ge

iPr

iPr

N

Ge

iPr

K

iPr

iPr K

K

iPr

iPr

K

K10

K9

K7

K8

Ge K

K iPr

K

Ge

Ge

iPr

K

iPr

N Ar

R

iPr

iPr

Ge

K11

Pri

K(18-c-6) K(18-c-6)

Ge

Ge

Ge

Ge Ge Ge Ge

Ge

K12

Ge Ge Ge Ge

Ge

Ge Ge Ge Ge

Ge Ge

K13

Ge n

(18-c-6)K

The longest Ge–K distances in the single-bond search concern complexes containing Ge9 polyanions, where potassium cations cap faces of three germanium atoms. This has been observed by Guloy in 2000 with a 3.826 Å distance in K12 [23] and by Fässler in 2011 with a distance of 3.889 Å for K13 [24]. Ge–K π  hits: 4, min: 3.301, max: 3.899, average: 3.469, standard deviation: 0.196, median: 3.406

K

K

K Ge tBu Ge

Ge

K

K

K14

K15

671

672

16  X-ray Crystallography of Organogermanium Compounds

Three of the four structures in the database dealing with potassium in contact with a germanium involving π system have been published the last twenty years. In 2002, West synthesized the potassium complex of the 9-germafluorenyl dianion K14 [25]. Two different coordinating η5-modes have been observed in the solid state with Ge–K distances between 3.327 and 3.872 Å. The Ge–K distance in the η5-coordinated potassium cation in K8 [20] is 3.449 Å. The only six-membered ring system has been reported by Tokitoh in 2017. In the germabenzenylpotassium K15 [26], the potassium is η6-coordinated by the anionic aromatic system with a distance of 3.899 Å between germanium and the cation. Ge–Rb  hits: 7, min: 3.441, max: 3.898, average: 3.686, standard deviation: 0.114, median: 3.681 The only example concerning a η1–Ge–Rb bond Rb1 [16] has been presented in 2005 by Ruhlandt-Senge. In the [Rb(18-c-6) Ge(SiMe3)3], the Ge–Rb distance is 3.480 Å. Five structures are based on Ge9-type polyanions. Korber published in 2006 a [Ge9]3– trianion where one rubidium is in a η4- and another in a η3-mode Rb2 [27] with distances between 3.483 and 3.867 Å. The same author presented in 2018 a complex of a [HGe9]3– trianion Rb3 [28]. In this complex, the rubidium cation is connected η4-like with the basal square plane of the cage. The Rb-Ge distances range 3.584–3.740 Å. Fässler published in 2013 an Ge9 analogous cage containing different numbers of germanium and silicon atoms Rb4 [29] (Ge–Rb between 3.550 and 3.782 Å). In the same year, the same author presented a rubidium complex of a [(MesCu)2(Si4–xGex)]4– [x = 2.2(1)] anion Rb5 [30]. In this complex, the rubidium cations cap different faces of the Si/Ge tetrahedron with distances from 3.483 to 4.027 Å.

Ge–Cs   hits: 2, min: 3.673, max: 4.072, average: 3.812, standard deviation: 0.121, median: 3.776 Only one cesium–germanium contact has been published between 2000 and 2019. It comes from Korber in 2006 and concerns a [Ge9]3– trianion Cs1 [27], where a cesium is coordinated in a η3-mode with a distances range of 3.676-2.330 Å.

16.2.2  Germanium and Group 2 Elements Ge–Mg  hits: 8, min: 2.607, max: 2.766, average: 2.686, standard deviation: 0.051, median: 2.701 In 2004, Sekiguchi published a magnesium salt of 1,3-disila-2,4-digermabicyclo[1.1.0]butane-2,4-diide Mg1 [31] with Ge–Mg distances of 2.626 and 2.766 Å. In 2011, Castel presented a THF-stabilized bis[tris(trimethylsilyl)germyl]magnesium Mg2 [32] with Ge–Mg distances (2.679 Å) in the same range. Very similar to this is a cyclic complex Mg3 [33] reported by Baumgartner and Marschner in 2016. A long distance of 2.747 Å has been observed in the polycyclic system Mg4 [34]. This complex could be seen as a neutral imine-stabilized N,N’-germylene coordinating a magnesium complex. Imine-stabilized germylidenide anion complex Mg5 [35] shows the shortest Ge–Mg bond with 2.607 Å (So 2014), demonstrating the charge effect on the distances. Two more classical examples, Mg6 [36] and Mg7 [37], show Ge–Mg distances 2.701 and 2.641 Å, respectively.

16.2  Detailed Analysis of the Statistics

Ge–Ca  hits: 4, min: 3.022, max: 3.229, average: 3.085, standard deviation: 0.078, median: 3.053 Ge–Sr  hits: 2, min: 3.123, max: 3.197, average: 3.150, standard deviation: 0.033, median: 3.140 Ge–Ba  hits: 1, min: 3.398, max: 3.431, average: 3.415, standard deviation: 0.023, median: 3.415 tBu

M Me(tBu)2Si

Ge

Ge Si

N

Si(t Bu)2 Me N

Si

Me(tBu)2Si

N

Si(t Bu)2 Me

M = Ca(THF)4 or Sr(THF)4 Ca1

Ge

Ar

Ge

Ca

Ca(THF)3

N tBu

Ar

Ca3

Ca4

Sr1

Complexes concerning the higher homologues of Group 2 metals will be described in one paragraph. In 2004, Sekiguchi published calcium and strontium complexes of 1,3-disila-2,4-digermabicyclo-[1.1.0]butane-2,4-diide Ca1 and Sr1 [31] with Ge–M distances of 3.037/3.039 Å in the case of calcium and 3.123/3.132 Å for strontium. In the same year, Ruhlandt-Senge presented THF-stabilized bis[tris(trimethylsilyl)germyl]calcium Ca2 (Ge–Ca  =  3.022/3.067  Å), -strontium Sr2 (Ge– Sr = 3.147/3.197 Å) and -barium Ba1 (Ge–Ba = 3.398/3.431 Å) complexes [38]. A long Ge–Ca distance of 3.229 Å has been observed in a calcium complex of germylidendiide dianion radical Ca3 [35] (So 2014). Another interesting complex contains a Cp*2Ca unit stabilized by a N-heterocyclic germylene Ca4 [39] (Ge–Ca = 3.117 Å).

16.2.3  Germanium and Group 13 Elements Ge–B  hits: 106, min: 1.971, max: 2.444, average: 2.153, standard deviation: 0.078, median: 2.143

B Ph

R' R

GeH R

B

Ph

Ge BB

BB

B B B

Ph

B1: R = R' = Et B2: R = R' = Et or R = Ph, R' = Me

B

B Ge B B

H

B B B

H

H

B B B Ge BB

B

BB

H B B

2B B B

B3 and B10

B B

B

B

Ge B B

Ph

M

tBu

B

Ge B

B

B B B

2-

Ge

B

B

iPr

Ge B B B

B B B B

E

N N iPr

iPr

Si

OSiMe3

Ge

B

B4: E = BH, M = Pt, Ni B5: E = BH, M = Fe, Ru B6: E = CH, M = Ir B7: E = BH, M = Ag B8: E = BH, M = Ag B9: E = BH, M = Cu

iPr

BH 3

N

O

N

Ge

X Si

X = H, Cl tBu

B11

tBu

N

N

B tBu

H

B12

Ph

Analyzing examples of molecules showing Ge–B distances in the average and median range, it seems that they could not be considered as classical Ge–B single bonds. Two examples correspond to R3Ge–BPh3 anions (B1 [40] and B2 [41]) with Ge–B distances of 2.145-2.152 Å. Seven other publications from Wesemann between 2009 and 2014 deal with germadodecaborate type structures. In all cases, some Ge–B distances between 2.141 and 2.153 Å are present (B3 [42] and B4 [43] in 2009; B5 [44], B6 [45], B7 [46] and B8 [47] in 2011; B9 [48] in 2014). In this type of structure, a very long distance of 2.409 Å could also be observed B10 [42]. Very short Ge–B distances have been observed in NHC-stabilized Ge(II) compounds B11 [49] (1.978 to 2.034 Å) and silylene-stabilized boryl(silyl)germylene B12 [50] (1.971 Å). In order to obtain a more accurate information on classical Ge–B single bonds, we refined our research by limiting the number of atoms bound to each element, permitting differentiation between Ge(II) and Ge(IV) boranes. In the field of Ge(IV) boranes, Nöth published in 2001 [51] a series of compounds containing a Ph3Ge unit combined with a borazine B13 (Ge–B = 2.124 Å), a (bisamino)diborane B14 (Ge–B = 2.097/2.098 Å), and a piperidino(chloro)borane B15 (Ge–B = 2.105 Å), or two Ph3Ge units with a piperidinoborane B16 (Ge–B = 2.160/2.118 Å). The same author published in 2003 compounds containing benzodiaza- and benzodioxa-borolidinyl fragments B17 [52] with Ge–B distances of 2.061 and 2.054  Å,

673

674

16  X-ray Crystallography of Organogermanium Compounds

respectively. In 2016, Aldridge reported borylgermanes B18, B19 and B20 [53] (Ge–B  =  2.045-2.094  Å), and Westcott described B21 [54] with a distance of 2.073 Å. Wesemann presented in 2017 borylgermane B22 [55] with a Ge–B distance of 2.059 Å. Concerning Ge(II), Jones, Kaltsoyannis, Mountford, and Aldridge published in 2012 germylene B23 [56] with a Ge–B distance of 2.141 Å, and in 2016 Aldridge presented B24 and B25 [53] (Ge–B = 2.127/2.129 Å). iPr

Me N

B

Ph3Ge

NMe 2

B N

N

iPr

Ph3Ge

B

B

iPr

B

E Ph3Ge

GePh 3

B

NMe2

Ph3Ge

Cl

Me

N

B

B

Ph3Ge

E

Ph3Ge

B15

B14

B13

N

E = NiPr or O B17

B16

O Mes

iPr

iPr

iPr

iPr N

N

Ge

B N

iPr

B H

N

N(SiMe3)2

iPr

iPr

H

Ge H

B

iPr

iPr

N(SiMe3)2

iPr

N

N

B

Ge H

Ge

B N

N

B19

iPr

B18

iPr

iPr

iPr

iPr

H

iPr B

N

SiMe 3 Ge

B

.NMe

3

Ge H

H PPh2

B22

N

B23

iPr

N Ge

Ge

B

N(SiMe3)2 N

iPr

Ar

iPr

iPr

iPr

iPr

Ge–B 

iPr

N

Ar

B O

Ph

iPr

N

iPr

Ar

O

B20

iPr

iPr

N

B21

R

R = H, I, SiH3, BH2 iPr

GePh3

O

B25

B24

π hits: 2, min: 2.336, max: 2.739, average: 2.604, standard deviation: 0.232, median: 2.736

B

SiMe3

N(SiMe3) 2

B

Ge (Me 3Si)2N

B

Ge B26

N

SiMe 3

B27

Herberich published in 2001 the bent germanium sandwich complex B26 [57]. The boron atoms, integrated in the anionic six-membered ring systems, have distances of 2.736 and 2.739 Å to the η6-coordinated germanium atom. In 2018, Albers and Müller reported a η5-aminoborole half-sandwich complex of germanium(II) B27 [58] with a Ge–B distance of 2.336 Å. Ge–Al  hits: 8, min: 2.450, max: 2.563, average: 2.520, standard deviation: 0.031, median: 2.525 The first germylaluminate Al1 [59] has been published in 2001 by Nöth showing Ge–Al distances in the range of 2.511 to 2.526 Å. The same author [60] followed in 2009 with another aluminate Al2 (Ge–Al = 2.532 - 2.548 Å) and two neutral triorganogermylalanes Al3 (Ge–Al = 2.515 Å) and Al4 (Ge–Al = 2.545 Å). Power presented in 2015 another non-solvated germylalane Al5 [61] with a slightly shorter Ge–Al distance of 2.485  Å. Apeloig and Driess published in 2019 a

16.2  Detailed Analysis of the Statistics

Ph

tBu OEt2 [(Ph3 Ge)3 AlR]-

Ph3 Ge

Al

N Al

Me

Al1 : R = Me Al2 : R = H

Me

Me

Ge

Me

Al 3

Si N tBu

Mes

Mes

Me

Ge

SiMe3

N

N

Mes

Al

tBu

Al 5

Ge

(AlBr3 )n

n = 1,2 Si N tBu N Al6 Ph

Me

Mes

Al 4

O

Me

silylene-stabilized Ge(0) center coordinating by one or two AlBr3 Lewis acids Al6 [50], giving Ge–Al distances of 2.486 and 2.501/2.563 Å, respectively. Ge–Ga  hits: 15, min: 2.390, max: 2.592, average: 2.473, standard deviation: 0.047, median: 2.468 In 2000 and 2001, Linti published neutral and anionic gallanes including tris(triorganylgermyl)gallanes. The neutral species are a gallatetrahedrane Ga1 with Ge–Ga distances in the median range (2.455 and 2.468 Å) and Ga22R8-cluster Ga2 with distances between 2.428 and 2.430 Å [62]. In the anionic systems Ga3 and Ga4 [63], the distances are between 2.410 and 2.494 Å. In 2004, Power presented Ga2 compound Ga5 [64] with slightly longer Ge–Ga distances of 2.493 Å. Long Ge– Ga distances have been observed in anion Ga6 (2.540 Å) and in neutral Ga7 (2.516 Å)[65]. The same gallium fragment has been used by Baines in 2007 to isolate anion Ga8 and the corresponding neutral complex Ga9 [19] with Ge–Ga distances of 2.460 and 2.431 Å, respectively. In 2013, Fischer and Frenking reported Ge2 and Ge4 species stabilized by N-heterocyclic gallylene [66]. While the Ge2 species Ga10 shows very short Ge–Ga distances of 2.390 and 2.411 Å, the Ge4 species Ga11 shows values (2.475 and 2.506 Å) close the average range. A distance of 2.444 Å is present in the classical triorganylgermylgallane Ga12 [61]. The coordination of a GaCl3 by a bis(NHC)-stabilized germylone Ga13 [67] leads to a long distance of Mes

Ge(SiMe3) 3 GePh 3 Ga

Ge(SiMe3)3

Ga

Ga Ga Ga Ge(SiMe3)3

(Me3Si)3Ge

Ph 3Ge

GePh 3

Ph3Ge Ga

Ph3Ge Ph3Ge

Mes Ga Ga

Ge

Ga4

I

Ge

Mes

GePh3 GePh3

Ga3

Ga1

Ga

Ga

I GePh3

Mes

Mes

Ge Mes

I

I

Ga5 iPr

iPr iPr

N Ga N

iPr

N Ge

iPr

iPr N

N

CH(SiMe3)2

iPr

N

iPr

Ga6

Ga N

iPr

iPr

iPr

iPr

N

N(iPr) 2

Ge

Ga

CH(SiMe3) 2

iPr

iPr iPr

Ge iPr

iPr

N Ga

Mes

N

SiMe 3 Ge

iPr

Mes

Mes

Mes

Pri

Pri

iPr

Ga9

Ga8

Ga7

N iPr

Mes iPr

iPr iPr iPr Ge

N

Ge iPr iPr

iPr

iPr

N

N

Ge iPr

Mes

N

Ge

Mes

Ga

Ga

iPr

Ga10

iPr Ge

N

Ga

Ga N

iPr

iPr

N

N

Ge

iPr

Ga11

Me

N

Ge

N N

GaCl3

iPr

Mes iPr

iPr

GaMe2 Ge

Ga12

iPr

Ga13

675

676

16  X-ray Crystallography of Organogermanium Compounds

2.520 Å. In 2019, Wagner showed the difference between the anion [(Me3Si)3Ge–GaCl3]– Ga14 (Ge–Ga = 2.460 Å) and the corresponding neutral (Me3Si)3Ge–GaCl2 Ga15 (Ge–Ga = 2.407 Å)[68]. Ge–In  hits: 5, min: 2.617, max: 3.078, average: 2.843, standard deviation: 0.122, median: 2.862 Only two publications of the last 20 years deal with germanium-indium compounds. One of them concerns [Ge9]4– Zintl ions In1 to In3 [69]. The Ge–In distances in In1 are between 2.824 and 3.002 Å, in In2 in the range of 2.824-2.864 Å, and in In3 they are significantly shorter (2.667/2.669 Å). The second publication, presented by Sen in 2018, concerns a totally different topic, In4 [70] represents a germylene coordinating InX3 (X = Cl, Br). The Ge–In distances are slightly shorter in the chlorine system (2.617 Å) in comparison with the bromine system (2.624 Å).

Ge Ge Ge Ge Ge Ge

Ge

Ge

Ge

Ge

Ge

Ge Ge

Ge

In Ge

Ge

Ge

Ph

5-

Ge

Ge Ge

Ge Ge Ge

Ge Ge

Ge Ge

Ge

Ge

Ph3In

In2

Ge

Ge

Ge Ge

tBu Ph

Ge Ge InPh3

In3

N Ge N tBu

In4

InX3 N(SiMe3)2 X = Cl, Br

Si(SiMe3 )3

3-

Ge Ge Ge Ge Ge

Ge

hits: 2, min: 2.905, max: 3.107, average: 3.039, standard deviation: 0.072, median: 3.070

Ge

Ge

Ge

4-

Ge

4-

Ge Ge Ge

Ge Ge Ge Ge

In1

Ge–Tl 

In

Ge

Ge Ge Ge Ge Ge

Ge

Ge

Tl

(Me3 Si)3Si

Tl1

Ge

Ge Ge

Tl

Tl2

Si(SiMe3)3

All examples discussing the germanium-thallium contacts have been published by Sevov in 2011 and 2014. Both are dealing with Ge9-clusters. In [Ge9Tl]3– Tl1 [71] the Ge–Tl distances range from 2.905 to 2.997  Å. In the neutral species Ge9{Si(SiMe3)}3Tl Tl2 [72], the Ge–Tl distances are significantly longer, ranging from 3.068 to 3.102 Å.

16.2.4  Germanium and Group 14 Elements C–Ge–C  hits: 25, min: 1.896, max: 2.183, average: 2.021, standard deviation: 0.039, median: 2.030 Ge–C  hits: 2981, min: 1.715, max: 2.579, average: 1.968, standard deviation: 0.049, median: 1.961 A large number of molecules containing germanium–carbon contacts have been analyzed by structure determination. In order to have an information on classical bond distances, we first selected germanium atoms bonded to the same groups leading to distinct selections for GeC2 and GeC4. R"

R

R'

R

R"

Ge R"

R

R' R

Ge

R"

C1 R=1-naphthyl, R'=R"=H C2 R=2,6-diisopropylphenyl, R=R"=H C3 R=Mes with R'=H, R"=iPr; R'=Cl, R"=H; R'=SiMe3, R"=H C4 R=2,4,6-triisopropylphenyl, R=R"'=H

N

N C5

16.2  Detailed Analysis of the Statistics

Five publications since 2000 give examples of symmetrical acyclic two-coordinated germanium GeC2 compounds. In four of them, highly sterical demanding aromatic groups have been used to stabilize a two-coordinated Ge(II) species. In 2001, Schmidbaur used 2,6-bis(1-naphthyl)phenyl to stabilize the germylene C1 [73] with Ge–C distances of 2.030 and 2.036 Å. Power published in 2006 C2 [74] (Ge–C = 2.033 and 2.048 Å), in 2015, in collaboration with Herber and Fischer, C3 [75] (Ge–C = 2.020 to 2.053 Å) and in 2018 with Grimme C4 [76] (Ge–C = 2.039 and 2.043 Å). In 2013, Roesky, Zhu, Stalke, and Andrara presented a two-coordinated Ge(0) species stabilized by two cAAC-ligands leading to significantly shorter Ge–C distances of 1.939-1.954 Å in C5 [77]. R3 R2

R R

Ge

R

R4

R

C6 R= CH2SPh C7 R= C C Ph

R2 R3

R1 R5

R 1 R 5 Ge R R 4 R5 1

2

R4

R 5 R1

R3 R2

N

Ar O

Ge

Ar

O

Ar

N

C8 R1=R2=R4=R5=H; R3=COOH C9 R1=R5=OMe,OEt; R2=R3=R4=H C10 R1=R4=R5=H; R2=Me; R3=H,Me R1=R3=R5=H; R2=R4=Me R1=R4=R5=H; R2-R3=-CHCHCHCH C11 R1=R2=R4=R5=H; R3=CCH

Ar O

C12

C Ge

R 2 R3 O

N

C

R4

C

C

N

C13

For the GeC4 system we found 9 publications. In 2004, Strohmann showed examples of -CH2SPh substituted compounds C6 [78] with Ge–C distances of 1.968–1.995 Å. One year later, Churakov presented the structure of tetrakis(phenylethynyl) germanium(IV) C7 [79] (Ge–C = 1.886 Å). In 2008, Lambert published a metal-organic framework C8 [80] containing GeAr4 units (Ge–C  =  1.944–1.948  Å). Other examples of teraarylgermanium(IV) were given by Yoder in 2010 C9 [81] (Ge–C = 1.947-1.957 Å), by Uhlig in 2017 C10 [82] (Ge–C = 1.940–1.960 Å) and by Freudenberger and Bunz in 2018 C11 [83] (Ge–C = 1.950 Å). Longer Ge–C distances can be observed in the case of tetraacylgermanes as reported by Haas in collaboration with Gescheidt and Stueger in 2017 C12 [84] (Ge–C  =  2.011–2.050  Å). Even longer distances have been observed in C13 [85], a Ge(II) dication stabilized by isocyanides (Ge–C = 2.033–2.065 Å). Some examples with very long Ge–C distances are summarized in the next paragraph. In 2000, Leung published a pyridine-coordinated dialkylgermaneselone C14 [86] with a Ge–C distance of 2.283  Å. A bit shorter (2.242  Å) is the example C15 [87] presented by Sekiguchi in 2002. Leung reported in 2010 a sulfur coordinated alkylgermanium(II) chloride C16 [88] and in 2013 a cyclic GeCGeC system C17 [89], with Ge–C distances of 2.153 and 2.162 Å, respectively. A GeCl2 coordinated by a Wittig reagent C18 [90] shows a Ge–C distance of 2.154 Å. Molecules with anionic germanium centers have been published by Wesemann in 2016 C19 [91] and in 2017 C20 [55] with distances of 2.150 and 2.161 Å, respectively. The last example of long distances comes from Frenking and Jones, in 2019 they presented C21 [37] with a Ge–C distance of 2.163 Å.

Se Me3Si Ph

C

Ge N

R 3Si

N C

Ph SiMe3

C14

R 3Si

Si

Ge

Si SiR 3 C

R3Si

Ph

C15

Ph

S

H C

Ph2P

Ge

Ph2P

C16

N

Cl

N

Ph3P

Ge

S R2P

GeCl2

Ge S

CMe2

PR 2 S

C18

Ar

Ge

H PPh2

X

H R

C19 X = CH2 C20 X = O

R 2N

Ge Ge

NR2

C21

C17

A very short Ge–C distance (1.761 Å) found in the search of Ge–C single bonds is in reality a double bond reported by Rit and Aldridge 2016 [92].

677

678

16  X-ray Crystallography of Organogermanium Compounds

Ge–C(NHC) 

hits: 143, min: 1.847, max: 2.339, average: 2.052, standard deviation: 0.056, median: 2.054

iPr

iPr

HGe

N3

C

Ge

Ge

Se

GaCl3

Cl

iPr

iPr

C22

C23

N

Ad

N

t Bu

N

Ge(Fe(CO)4)m

N

N

Ad

N

n

C30

R = iPr, n = 1, m = 3 R = iPr, Me, n = 2, m = 2

N

N

Ar

GaCl3

Ge

N

N N

N

Ar

C27

Ar

C28

C25

R

R

Mes

Ge

R

N

N

N

Ge N

R M = Cr, Mo, W

C31

N Ge

C26

t Bu

N

Ge

GeH2

R

N

Ge

N

N

Mes

Ar

N

R Ph

iPr

R

Ar

Si

Ge

N

N

R

Si C

iPr

R

Ar

N

Ge

C24

R

N

iPr

N

Ar

Si

iPr

N

Fe(CO)3

N

N

iPr

N

Si

iPr

N

N

Ar

R

R

N

N

N

N

iPr

iPr

X2 X1

N R

X1 = X2 = F, Cl, Br, OtBu, NCS, Mes X1 = Cl, X2 = OTf X1 = Cl, X2 = Ge(Cl)-Ge(Cl)Mes 2

R

C32

C33

M(CO)5

C29

A very interesting class of germanium-carbon bonds concerns N-heterocyclic carbene stabilized germanium centers. Four examples fit perfectly with the average/median range. A bis(NHC)GeN3 cation C22 [93] with a Ge–C distance of 2.050– 2.054 Å has been published by Driess in 2014, and two years later the same author, in collaboration with Apeloig, presented C23 [67], a bis(NHC)Ge=Se complex coordinating GaCl3, with a distance of 2.052  Å. Two other examples come from Scheschkewitz in 2014 and in collaboration with Yldiz in 2016, C24 [94] with a Ge–C distance of 2.053 Å and C25 [95] with Ge–C = 2.056 Å, respectively. Using NHC-ligands it was possible to stabilize Ge(0). The first example has been shown by Jones, Stasch, and Frenking, an NHC stabilized digermanium(0) C26 [96] with a Ge–C distance of 2.030 Å. In 2013, Driess published a Ge(0) stabilized by a bis(NHC) ligand C27 [97] (Ge–C = 1.961 and 1.965 Å). In 2016, Apeloig and Driess coordinated GaCl3 with C27 giving C28 [67] with longer Ge–C distances (2.033 and 2.043 Å). In 2018, Rzepa, Scheschkewitz, and Jana observed the same phenomenon coordinating Fe(CO)4 units by a Ge(0) stabilized by mono NHC ligands C29 [98] (Ge–C  =  2.067–2.096  Å). The shortest Ge–C contacts involving NHC systems have been observed by Kinjo with Ge(0) complexes. In 2014, the imino-NHC-germanium(0) complex C30 [99] has been described and in 2016, this complex has been used to coordinate M(CO)5 (M = Cr, Mo, W) units C31 [100]. The Ge–C distances are 1.887 Å for C30 and 1.847– 1.859 Å in the case of the transition metal complexes C31. The longest distances between germanium and NHC carbon atoms have been observed in Ge(II) compounds. A distance of 2.339 Å has been reported by Hitchcock and Lappert in 2000 for C32 [101] and Baines published a series of NHC stabilized Ge(II) compounds C33 [102] with distances ranging 2.068–2.147 Å. Ge–C(carboranes)  hits: 34, min: 1.922, max: 2.579, average: 2.182, standard deviation: 0.207, median: 2.138

B B

Me2 Ge

B B B B

B B B B

C

X

C

Y Ge Me2

X-Y = Ni(PEt3)2 or CH-R X-Y = RC=CR'

C34

B B

B B B B

B B B

C C

B

C35

GeMe 2

GeMe2

B B

B B B B

B B B B

C C

Me 2 Ge

C

C Ge Me2

C36

B B B B

B B B B

B B

B B

B C B

C B B B

C37

B

SMe2

B

Ge(Cl)Me2 B

B

Ge B B B

B B B B

C38

C

B

B

B

R

B B B B

B B B B

C C

C NR' GeXn n = 1 or 3

C39 R = tBu C40 R = NHR"

A special paragraph deals with compounds where the carbon atom bonded to germanium is a part of a carborane, because of the special environment of such carbon atoms. Ko published in 2001, and in collaboration with Kang in 2002 series of compounds containing two germanium atoms C34 to C36 [103-105]. The Ge–C distances are between 1.974 and

16.2  Detailed Analysis of the Statistics

2.012  Å in  the case of organic linkers between the two germanium atoms and slightly longer (2.028–2.035  Å) in the nickel complex. Another example of a Ge(IV)-carborane system C37 [106] has been reported by Hosmane in 2008, and the Ge–C has a distance of 2.121 Å. A η5-bonded germanium atom has been observed by Wesemann in 2011 in the anion C38 and corresponding metal complexes B6 [45]. The Ge–C distance is 2.203 Å in the anion and between 2.155 and 2.197 Å in the metal complexes. Xie used an imine to stabilize the germanium group in C39 [107] (2017, Ge–C = 1.956– 2.067 Å), while Edelmann used an amidinate C40 [108,109] (2017 and 2019, Ge–C = 1.923–2.041 Å). Ge=C  hits: 44, min: 1.761, max: 2.189, average: 1.889, standard deviation: 0.081, median: 1.878 Classical germanium carbon double bonds are slightly shorter than the average values from the database. The first classical example comes from Sekiguchi in 2002 with the Ge=C double bond being a part of a bicyclic system C41 [87]. The Ge–C distance is 1.859 Å. Escudié presented in 2010 a phosphagermaallene C42 [110] with the shortest Ge–C double bond of 1.761 Å. In the same year, he published in collaboration with Gornitzka germene C43 [111] with a distance of 1.806 Å. A Ge–C distance of 1.835 Å has been observed by Haas for germene C44 [112] in 2015. The next two examples fit very well with the average and median range. In 2005, Leung analyzed manganese complex C45 [113] with a Ge=C distance 1.885 Å, and in 2011 So presented C46 [114] (Ge–C = 1.882 Å). In both examples, the authors discuss a delocalized system, meaning that this is not a classical double bond.

tBu H

R 3Si

SiR3 Si O Ph Si

Ph

Ge R 3Si R Si 3

Ar

Ar Ge C

P

tBu

Me3 Si Me3 Si

Ge Mes

H

C41

Mes

C42

Me2 Si

SiMe2 OSiMe3 Ge

Ge Me2 Si

Me3SiN

PPh2

Ph2 P

SiMe2 Ar

Ge

Me3 SiN

S

Ph2P

S

Ge

Mn OC CO

C44

C43

Ph2P S

C45

PPh2

S

PPh2

C46

t Bu

Ge–Carom 

hits: 32, min: 1.728, max: 2.114, average: 1.908, standard deviation: 0.060, median: 1.92

This paragraph concerns GeC-units being a part of an aromatic system. Tilley published in 2000 a dianionic “Cp” system C47 [9,115] with Ge–C distances between 1.917 and 1.958 Å, and in 2002 in collaboration with Rheingold a ferrocene analogue C48 [116] showing shorter Ge–C bonds (1.892 and 1.896 Å). A hafnium complex of a silicon substituted dianionic system C47 has been presented by Müller in 2018 [117] (Ge–C = 1.954 and 1.961 Å). A monoanionic GeNC3 “Cp” system C49 has been developed by Driess in 2008 [20] and in collaboration with Jones in 2010 [12] giving short Ge–C distances of 1.887 and 1.896  Å, respectively. Sekiguchi published in 2005 [11] and in collaboration with Frenking in 2007 [118] an R R'

R' Fe

R

Ge

C47

R'

Ge R R

C48

Ge R

R'

Ge

N

C49

Ph

Si R

Si

R

Ge

Ph

Ph Ge

R

C50

N Ge

C51

Ge

R Ge

NR

Ar

C52

C53

Ge

C54

t Bu

Ar

R

C55

2 Ge

Ge

C56

C57

Ar

Ge

C58

R

anionic GeSi2C2 system C50 with Ge–C distances of 1.930 and 1.924 Å, respectively. Aromatic benzannulated germole dianion C51 [10] has been published by Boudjouk in 2000 (Ge–C = 1.948–1.987 Å). An analogous system containing an imine side-arm C52 [13], showing a similar Ge–C distance (1.946 Å), has been presented by So in 2016. Introduction of a germanium atom in a six-membered benzene analogue C53 [119] lead to Ge–C distances of 1.827/1.829 Å, as shown by Tokitoh in 2002. The same author used such system to complex chromium and molybdenum in 2003 [120] and tungsten in 2007 [121] giving slightly longer Ge–C distances (1.859–1.893  Å). In 2017, Tokitoh presented the corresponding

679

680

16  X-ray Crystallography of Organogermanium Compounds

anionic system C54 [26] showing longer distances (1.900–1.945 Å) than the neutral compound. In 2-germanaphthalene C55, presented by Tokitoh in 2001 [122], 2003 [123], and 2008 [124], short Ge–C distances of 1.803–1.876  Å could be observed. Aromatic 9-germafluorenyl dianions C56 [25], published by West in 2002, show long Ge–C distances in the range 1.935–1.988 Å. In neutral 9-germaanthracene C57 and 9-germaphenanthrene C58 [125], the Ge–C distances are between 1.792 and 1.886 Å. Ge–C π  hits: 47, min: 1.889, max: 3.408, average: 2.480, standard deviation: 0.248, median: 2.467 R

R

R'

R

R R

R Ge

R

R' R

C59

SiMe3 Ge

GeH

Cl

Fc

Ge

C60

Ge

Fc

Ge

C62

C61

Fe

Me2Si

Ge

B

Ge

Me2Si

Ge

C64

C63

C65

NR 2

SiMe3

C66

Different types of carbon-containing π systems are discussed separately. The first one is a classical “Cp” type system. In 2000, Lawless published a sandwich complex of germanium(II) C59 [126] with R=Me and R’  =  SiMe2Cl. The Ge–C distances are between 2.499 and 2.530  Å. The corresponding Cp* derivative has been published independently by Weidenbruch [127] and by Schnepf in 2006 [128]. The structures are very similar with Ge–C distances 2.397–2.645 Å. In 2013, Schnepf presented a derivative with R=CH2C6H4-iPr showing slightly longer Ge–C distances (2.412–2.668 Å)[129]. The Cp*GeCl structure C60 [130] has been analyzed by Banaszak Holl in 2010. The replacement of one Cp* by a Cl anion led to shorter Ge–C distances between 2.213 and 2.596 Å. In the Cp*Ge cation C61 [131], published by Rivière in 2002, the germanium is more symmetrically bonded to the Cp* with distances in the range 2.258–2.308 Å. The same author published in 2000 a p-phenylene-bridged germanocene C62 [132] (Ge–C = 2.274–2.712 Å). Rivière and Castel published in 2004 ferrocene substituted germanocenes C63 and C64 [133] with Ge–C distances between 2.343 and 2.691 Å. Schäfer presented in 2017 a disilane bridged germanocene derivative C65 [134] and in 2019 the corresponding Cp* analogue [135]. The Ge–C distances are slightly longer in the Cp derivative (Ge–C  =  2.249–2.264  Å) in comparison to the Cp* one (Ge–C = 2.209–2.229 Å). A bit different is a neutral η5-aminoborole complex of germanium(II) C66 [58], presented in 2018 by Albers and Müller (Ge–C = 2.194–2.209 Å).

B

N(SiMe3)2

Ge

C67

P P

B

N(SiMe3) 2

Ge Ar

SiR3

tBu

R 3Si tBu

Ge

C68

Ge R 3Si

SiR3

R 3Si

SiR3

X

Ge Ge Ge

SiR 3

Ge

C70

C71

Me3Si

C72 Ph R 3Si

Ph Ge

SiR3

Ph

Ph

R 2Si

Ge

Me2Si

C74

C75

SiR2

R Ar

SiMe2

C76

SiMe3

Ge

Ar

C69

Ge SiMe3

Ph

HfCp2

Me3Si

C73

HfCp2 X = Fe(CO)4, W(CO)5 or BAr3

R' Ge

Ar

R, R' = CH 2CH 3 R, R' = H, Ph

Another cyclic system, aminoboratabenzene, has been used by Heberich in 2001 to coordinate germanium C67 [57]. In this complex, the Ge–C distances vary between 2.320 and 2.939 Å. The germanium 1,3-diphosphacyclobutadienyl complex C68 [136] has been published by Francis in 2003 (Ge–C = 2.204 Å). The symmetric germapyramidane C69 [137] has been presented by Lee, Sekiguchi, and Minkin in 2016, showing Ge–C distances between 2.133 and 2.143 Å. Sekiguchi published in 2003 1,4,5-trigermabicyclo[2.1.0]pent-2-en-5-ylium C70 [138] showing contacts between one germanium atom and a CC-π system (Ge–C = 2.254 and 2.415 Å). Germanium atoms η3-coordinated by a π-bound cot ring C71 [139] with Ge–C distances between 2.207 and 2.239 Å has been presented by Power in 2011. A germylene stabilized by homoconjugation C72 and some metal and boron complexes C73 have been synthesized by Müller in 2016 [140] and 2018 [117,141]. Both are very similar with Ge–C distances in the range of 2.248–2.322  Å. An interaction between a CC triple bond and a germylene

16.2  Detailed Analysis of the Statistics

unit C74 [142] has been shown by Baumgartner and Marschner in 2015. This system leads to very short Ge–C contacts of 1.968 and 1.970 Å. The same authors in collaboration with Müller published in 2016 a bicyclic analogue C75 [33] with a very short Ge–C distance of 1.958 Å. Even shorter distances have been observed by Power in 2019 in complexes C76 [143] with distances between 1.914 and 1.928 Å. Ge–Si  hits: 411, min: 2.274, max: 2.767, average: 2.410, standard deviation: 0.048, median: 2.397

R

Me

R

R3 Si

Si R

Ge

Ge

Me2 Si

Mes

Mes Si Ge Ge Si Si Mes Mes Mes Mes

Si Si

R

R3 Si

SiR3

Si1

Me2 Si

Me

Ge Ge SiR3

Si2

Me2 Si

Ge

SiMe3

Me3 Si

Mg(THF) 2 Me2 Si Ge SiMe3 Me3 Si

Si7

Me3 Si

Si Ge

Me3Si

Si3

Me3 Si

Me2 Si O Ge Si O Me2

SiMe 3

Me3Si

HfCp2 (PMe3 )

SiMe2

Ge

Ge

Me 2Si

C

SiMe2

Ge Ge SiMe3

Me3Si Si Ge SiMe3 Me3 Si

SiMe3

Si5 SiMe3 SiMe 3 Si SiMe 3 Ge Si SiMe 3 Si Si Me2 Me2

Ph Ph

Mes

Si9

Si8

Me3 Si

Ph

Si(SiMe 3) 3

Si6

SiMe3 O

Ge Ge SiMe3

Me3 Si

Si4

Me 2Si

Me3 Si

SiMe 3

Ph

SiMe3

Me3 Si

R

Me R

Si

R

Ge Si

R

R Si Si Ge R Me

Ar Ar

Si11 R

Si10

Ar Ge Ge SiPh3

Si12

Ge SiMe3 Me3 Si

Me3Si

Ge SiPh3

Ge SiMe3

SiMe3

Me3 Si

(Me3 Si)3 Si

R3 Si Pb Ge(SiMe3 )3

R3 Si

(Me3 Si)3Si

Si14

Si13

(Me 3Si) 3Si Ge Li(THF)3

R3 Si

Si15

Si16

Ge

Ge

M Ge

Ge Ge Ge Ge Ge (Me3 Si) 3Si

Si(SiMe 3)3

Si17

M = Ni(PPh3), Pt(PPh3), R = Et M = Mo(CO)3, R =

Because of the large number of structures containing a Ge–Si single bond, only few examples are presented fitting perfectly with the median value. Ge–Si distances in the range 2.376–2.399 Å have been observed in structures Si1–Si17. Sekiguchi published the cyclic system Si1 [144] in 2005 and Lee/Sekiguchi Si2 in 2007 [145]. A bicyclic system Si3 [146] has been presented by Breher in 2011. Baumgartner and Marschner published the hafnium complex Si4 and the cyclic tetragermane Si5 in 2009 [147]. The same authors published Si6 [142] in 2015 and Si7 and Si8 in 2016 [33]. A six-membered Ge2Si4 ring Si9 [112] has been presented by Haas in 2015. Spiro-germanium compound Si10 [148] has been described by Baumgartner, Marschner, Albers, and Müller in 2018. Scheschkewitz published polycyclic Ge2Si4 system Si11 [149] in 2016, and silylated digermene Si12 [150] in 2019. The more classical system Si13 [151] has been presented by Baumgartner and Marschner in 2014. Anionic systems have been presented by Ruhlandt-Senge Si14 [16] in 2005, by Hinderberger/Klinkhammer Si15 [17] in 2010, and by Iwamoto/Kira Si16 [2] in 2010. Metal containing Ge9-clusters Si17 have been published by Schnepf in 2011 [152] and by Fässler in 2018 [153]. Mes Ge R 3Si

Ge

SiR 3

Ge

SiR3

= -CH 2 -CH=C(Me)-CH2 or -CH=C(Ph)Si18

MetBu2 Si

R Ge SiMetBu2

MetBu2 Si

(L)

R R

Si

Ge Si Si

(Li)

L = CH3CN Si19

R Si Ge R

Si20

The longest Ge–Si single bond distances of the last 20 years have been presented by Sekiguchi in 2000 in bicyclic system Si18 [154] with Ge–Si distances in the range 2.478–2.553 Å. Distances between 2.504 and 2.561 Å have been observed in anion Si19 [155] with a very slight elongation between the free anion and the solvent coordinated form. An anionic Si/Ge cluster Si20 [156], published by Scheschkewitz in 2019 gave Ge–Si distances of 2.418–2.480 Å for both forms, a contact ionpair and a solvent separated ion-pair.

681

682

16  X-ray Crystallography of Organogermanium Compounds

Ge=Si  hits: 13, min: 2.214, max: 2.279, average: 2.237, standard deviation: 0.015, median: 2.232 SiR3 R 3Si

Si

Ge

Me 3Si

SiR3

Si

SiMe3 Ge

SiR3

Ph

Me3Si

Me3Si Si

SiMe3

Si21

SiMe3

Me 3Si

Ge Me3Si

SiMe 3 Si

SiMe3

Me3 Si

Si22

Me3 Si

Ge

SiMe3

SiMe 3

Si

Si

Me3Si

Fe(CO)4

R 2ClSi

Ar

Si

Ge

Ar

NHC

Ge

Ar

NHC

SiMe3

Si23

Si24

Si25

Only five publications since 2000 deal with Ge=Si double bonds. Sekiguchi published in 2000 a five-membered ring system containing such a double bond Si21 [157] with a distance of 2.250 Å. Kira presented heteroallenes Si22 [158] and Si23 [159] in two publications and the 2-germadisilalallene shows slightly shorter distances (2.237 Å) than the 1,3-digermasilaallene (2.269 Å). Scheschkewitz reported in 2013 a NHC stabilized Ge(II) silagermenylidene Si24 [160] and in 2014 an iron complex of such species Si25 [94]. The iron complex shows a shorter (2.248 Å) double bond than the iron-free form (2.276 Å). Ge–Siarom 

hits: 4, min: 2.293, max: 2.322, average: 2.306, standard deviation: 0.012, median: 2.304

R R

Ge

Si Si

R

Ph Si26

The only examples containing a Ge–Si unit as a part of an aromatic system have been presented by Sekiguchi in 2005 [11] and in their collaboration with Frenking in 2007 [118]. The lithium complex of Si26 has a Ge–Si distance of 2.322 Å and the sandwich complex formed with Cp*Fe gives 2.304 Å. Ge–Ge  hits: 659, min: 2.052, max: 3.435, average: 2.594, standard deviation: 0.107, median: 2.579

2

H Ar

Ge

Ge Ar H

Ge1

SiMe3 SiMe3 K

Ge

Ge

K

SiMe3 SiMe3

Ge2

Me2 P Pt P Me2

Ph 2 Ge GePh2 Ge Ph2

Ge3

R2 Ge R Ge

R2 R Ge Ge Me2Si SiMe2 Ge Ge Si R Si R Me2 Me2

Ge4

Ph tBu

N

NR 2 N

tBu

Ge (Me3Si)2N

Ge5

GeCl2

Ge R2N

Ge Ge

Ge NR 2

NR 2

Ge6

The median/average range for Ge–Ge single bonds is 2.57–2.59 Å. Here are some examples fitting perfectly well within this range. Power published in 2004 dianion Ge1 [14] with a Ge–Ge distance of 2.578 Å. Marschner presented in 2005 and in 2010 dianionic Ge–Ge system Ge2 with distances of 2.581 [161] and 2.574 Å [162], respectively. A PtGe3 metallocyclic compound Ge3 [163] with one of the Ge–Ge distances being 2.578  Å has been reported by Osakada in 2009. Tricyclic silagermane Ge4 [164] has been evidenced by Baumgartner, Marschner, and Müller in 2013 (Ge–Ge = 2.257 Å). The complexation of GeCl2 by an amidinato germylene lead to Ge5 [165] with a Ge–Ge distance of 2.587 Å (So, 2017). The last example for this range is an amido germanium(I) cluster Ge6 [37] presented by Frenking and Jones in 2019. Very long Ge–Ge distances, longer than 3  Å, have been observed exclusively in germanium clusters. In 2004, Sevov reported anionic systems Ge7–Ge10 [166] with distances between 3.018 and 3.218 Å for R=GeR’3 and SnR’3. In 2007, the same author published G10 with R=tBu [167] and Ge9 with R=Fc–CH=CH– [168], with distances of 3.213 and 3.091 Å, respectively. An unsubstituted Ge9 cluster Ge11 [27] has been presented by Korber in 2006 (Ge–Ge  =  3.200–4.046  Å). Goicoechea prepared in 2008 a polyanion containing four cluster units bridged by mercury cations Ge12 [169]. Ge–Ge distances between 3.069 and 3.185 Å are present in this structure. In 2010, the same author showed an iron complex of a

16.2  Detailed Analysis of the Statistics 3

Ge Ge Ge Ge Ge Ge

Ge

Ge

3

Ge

Ge

R

Ge7

Ge

R

Ge

Ge Ge Ge

Ge

Ge

R

Ge

Ge8

Ge

Ge

R

Ge

Ge

R

Ge9

Ge

Ge Ge Ge Ge

Ge Ge

Ge

Ge

Ge

R

Ge Ge

Ge Ge Ge Ge

Ge10

Ge

Ge

10

Ge

Ge Ge Ge Ge

Ge Ge Ge Ge

Ge

Ge

Ge

Hg Ge

Ge

Ge

Ge

Ge

Ge Hg

Ge

Ge

Ge

Ge

Ge Hg

Ge

OC

Ge

Ge Ge Ge Ge

Ge Ge Ge Ge

Ge

Ge

CO

Si(SiMe3 )3

3

Ge

CO

Fe

Ge Ge Ge Ge

Ge Ge Ge Ge Ge

Ge

Ge12

Ge

Ge

Ge11

Ge Ge

3

Ge

4

Ge

Ge Ge Ge Ge

Ge Ge Ge Ge

Ge

2

Ge

Ge

(Me3 Si)3 Si

Ge

Ge

Ge

Ge

Si(SiMe3 )3

R

Ge13

Ge14

Ge8 cluster Ge13 [170] with a long distance of 3.342 Å. A neutral system Ge14 [171] has been reported by Fässler in 2018 with distances of 3.080 and 3.124 Å.

Pt

Ge Ge Cl

W Ge Ge W

B

Ge

B B

B

B Ge B B

B

R3Si

Ge

Ge Ge Ge

R3Si

Ge20

Ge B B B

BB

B

Ge

Ad

N

SiR 3

R2N

Ge21

B

N

Ar

Ar

B B B B B

B

B

B B

B B

B Ge B

B B

BB

B B B

Ge B

B

B

B B

B

NHR2

N

Ge Ge

N

B

Ge19

Ar

N

2

Ge

B B B

Ge18

Ar

SiR 3

B

B

Ge17

Ge16

E Fc Ar Ge Ge Ar Fc

B

Ge B

Me

Me

2

Ge

BB

B B

Ge

Cl

Ge15

Ge

B

Ge

NR 2

R2N

Ge Ge

Ge NR2

NR 2

Ge22

Ge23

For short Ge–Ge distances, shorter than 2.4 Å, different motifs are possible. In 2005, Filippou reported the shortest Ge–Ge single bond (2.362 Å) observed in tungsten complex Ge15 [172]. Germaplatinacycle Ge16 [173] with a Ge–Ge distance of 2.396 Å between the germafluorene units has been described by Braddock-Wilking in 2009. Totally different in motif are the digerma-closo-dodecaborate Ge17 and digerma-closo-dodecaborane G18 [42] presented by Wesemann in 2009 (Ge– Ge = 2.364 and 2.397 Å) or the germa-closo-dodecaborate Ge19 [174] (Ge–Ge = 2.329 Å) reported in 2015. Sasamori and Tokitoh published in 2012 sulfur and selenium bridged compounds Ge20 [175] with Ge–Ge distances of 2.359 and 2.370 Å, respectively. Lee, Sekiguchi, and Minkin presented pentagermapyramidane Ge21 [176] in 2015 with Ge–Ge bond lengths within the Ge4 base ranging 2.327–2.399 Å. In 2018, azadigermiridene Ge22 [177] was published by Ketkov and Dostal with a Ge–Ge single bond of 2.374 Å. Other very short Ge–Ge single bonds could be found in the database for the last twenty years from Power (2003 [6], 2005 [178], and 2010 [179]), Tokitoh (2006 [180]) and Tobita (2014 [181]), but in all cases these are not single but multiple Ge–Ge bonds. Ge=Ge  hits: 65, min: 2.212, max: 2.535, average: 2.338, standard deviation: 0.075, median: 2.321

Ar Ar Ar

Ge

Se

Ge

Ge24

Ph

Ar

Ar Ge Ge Ar

Ar

Ar

Ge Ar Ge Ge Ar NAr

Ge25

R

R

Ar N

Ge Ge R

Ge26

N Ar Ge

R

Ge

Ar N

Ge27

But

N

N tBu Si N(SiMe3)2 Ge

N Ar

Ge

(Me3Si)2N

Ge28

But

Si N N

R

Cl

R

Si ArN tBu

Ph

R Si

Ge Ge Si R

Me3Si NAr

SiMe3 Me3Si

SiMe3

Ge Ge Ge

Si R

Ge29

R

Ar

R Ge Ge

R Cl

Me3Si

SiMe3 Me3Si SiMe3

Ge30

Ge31

Ar

683

684

16  X-ray Crystallography of Organogermanium Compounds

Here are some examples of classical Ge–Ge double bonds. A five-membered Ge4Se cycle Ge24 containing a Ge–Ge double bond with a distance of 2.298 Å and four-membered ring Ge25 (Ge–Ge = 2.281 Å) have been published by Weidenbruch in 2003 [182]. In 2006, the same author presented a digermene of type Ge26 [183] with R = 2,5-tBu2C6H3 and a Ge–Ge bond length of 2.364 Å. One year later, Baines discussed another Ge26 [184] derivative with R = mesityl (Ge–Ge = 2.286 Å) and Iwamoto and Kira [2] reported an analogous compound with R = SiR’3 with an even shorter Ge–Ge distance of 2.270 Å in 2010. A Ge=Ge unit containing two Ge(II) atoms stabilized by NHC ligands Ge27 [96] with a slightly longer Ge–Ge distance of 2.349 Å has been presented by Jones, Stasch, and Frenking in 2009, while Yim and So used silylene groups to obtain Ge28 [185] in 2014 (Ge–Ge = 2.352 Å). In 2015, Scheschkewitz published Ge29 [186] with a distance of 2.294 Å. In the database, an example of Power in 2002 shows a very short Ge–Ge distance of 2.285 Å, and in reality, it is a triple bond, so it will be discussed later on. The next three examples contain structures with Ge=Ge bond lengths perfectly fitting with the average and median range. In the trigermaallene Ge30 [158] from Kira (2005), the Ge–Ge distances are 2.321 and 2.330 Å, respectively. Power gave in 2010 example Ge31 with R = CCSiMe3 and CC(tBu) [187] and distances of 2.332 and 2.324 Å, respectively. Sasamori and Tokitoh published in 2012 a Ge31 with a ferrocene containing substituent [175] (Ge–Ge = 2.332 Å). R2N Ar

R2 N

Br

NR 2 R

Ge Ge

Ge Ge Br

H

Ge Ge

H

Ar

NR2 R

Ge32

R Si

Ge33

Ge34

Ge Ge

R

Ge35

Significantly longer Ge–Ge distances (2.503 Å) have been observed by Tokitoh in 2005 in dibromodigermene Ge32 [188]. Dihydrodigermenes Ge33 [37, 189] have been presented by Frenking and Jones in 2013 and 2019 with Ge–Ge bond lengths of 2.486 and 2.535 Å, respectively. The same authors presented in 2015 cyclic system Ge34 [190] with a Ge–Ge of 2.518 Å. The shortest Ge–Ge double bond (2.243 Å) has been published in 2005 by Sekiguchi, being a part of cyclic digermene Ge35 [144]. Ge≡Ge  hits: 3, min: 2.206, max: 2.357, average: 2.250, standard deviation: 0.061, median: 2.226

The first example of a digermanium analogue of an alkyne Ge36 has been reported by Power in 2002 [191]. He stabilized the system using very bulky aromatic substituents and he observed a bond length of 2.285 Å. In 2006, Tokitoh [180] followed with two other examples using other bulky aromatic groups leading to distances of 2.206 and 2.226 Å. A much longer distance (2.357 Å) has been observed by Frenking and Jones in 2013 [189] and calculations suggest the presence of a double bond and a biradicaloid character. Ge–Gearom 

hits: 5, min: 2.312, max: 2.374, average: 2.341, standard deviation: 0.018, median: 2.334 R 3Si

SiBu3

Ge

Ge + Ge Ge Bu3 Si

Ge SiBu3

Ge37

SiR3 2-

Ge Ge

R3Si

SiR3 Ge38

16.2  Detailed Analysis of the Statistics

The only two examples concerning Ge2 units in aromatic systems of the last 20 years have been published by Sekiguchi. Cationic cyclotrigermenium salts Ge37 [192] show Ge–Ge distances between 3.331 and 3.340 Å, while tetragermacyclobutadiene dianion ligand Ge38 [193] in iron and cobalt complexes give Ge–Ge distances from 2.354 to 3.374 Å. Ge–Ge deloc 

hits: 2, min: 2.322, max: 2.368, average: 2.337, standard deviation: 0.021, median: 2.330

Ar Ar

Ge Ge Ar Ge Ge Ar

Ar

Ge39

The only example until 2020 showing a Ge3 unit within a delocalized system has been reported by Weidenbruch in 2000. The anion Ge39 [194] has Ge–Ge distances of 2.368 Å in the delocalized area. Ge–Sn  hits: 49, min: 2.567, max: 3.092, average: 2.709, standard deviation: 0.104, median: 2.702

Ar Ar

Sn Ge(t Bu)3 Sn Ge(t Bu)3

Sn3 Ar 2Sn

Sn2

Sn

Ge(SnMe3 )4

Sn1

GeAr2

O

(Me3Si) 3Ge

ArGe

O

Sn4

Sn Sn

(Me3Si)3 Ge

Ge(SiMe3 )3 Ge(SiMe3 )3

Sn10

Me 2 Sn (Me3 Si) 2Ge Ge(SiMe3 )2

Sn Sn GeAr Sn

(Me3 Si)3 Ge

Sn5

NHC

SnR 3

Me 2Si

Sn6 Sn(SiMe3) 3 Ge Sn(SiMe3) 3

H 3 Ge

SiMe2

Ge(C 2F5 ) 3

Sn12

SnPh 3

Sn8

Sn7

Ph3 Sn

Bu3 Sn

Ar H

Ge Sn H

Ar

H N Ge

Ar Ge

N H

SnBu3

Sn9

Ar'

Sn13

Sn11

Here are some classical examples containing Ge–Sn single bonds. In 2001, Sakamoto and Kira presented Sn1 and Sn2 [195]. The Sn(II) compound showed a slightly longer Ge–Sn distance than the Sn(IV) compound with 2.722 against 2.707 Å. The highly symmetrical tetrastannylgermane Sn3 [196] has been studied by Dinnebier in 2002 at different temperatures and the bond length vary from 2.58 to 2.62 Å. The same structure has been published by Groy [197] in 2008 giving distances from 2.591 and 2.595 Å. In the oxygen containing cyclic compound Sn4 [198], presented by Weidenbruch in 2003, the Ge– Sn distance is 2.635 Å. The bond distances in germanium-tin cluster Sn5 [199] (Power 2003) are in the range 2.704 and 2.763 Å, perfectly fitting with the average value. In 2005, Baumgartner reported distances of 2.589 and 2.611 Å for R = SiMe3 and R  =  iPr, respectively, in Sn6 [200]. In the same year, Marschner published digermylstannane Sn7 [161] (Ge– Sn = 2.612 Å). In 2009, Tice studied Sn8 [201] with a Ge–Sn = 2.583 Å and Power Sn9 [202] with bond lengths of 2.636 and 2.666 Å. Hinderberger and Klinkhammer observed significantly longer distances (2.716 and 2.724 Å) in 2010 in Sn10 [17]. In 2011, Escudié and Castel presented carbene stabilized bis(hyperstannyl)germylene Sn11 [203] with Ge–Sn distances of 2.686 and 2.703 Å. Hoge reported in 2017 germylstannane Sn12 [204] (Ge–Sn = 2.634 Å) and Wesemann in 2019 germyl aryl stannylene Sn13 [205] (Ge–Sn = 2.669 Å). In nearly all these compounds, the Ge–Sn bond lengths are shorter than the average database values. Very long Ge–Sn distances can be observed in cluster compounds, as Ge9-containing systems described by Sevov in 2004 [166] (Ge–Sn = 2.940 and 2.985 Å) and 2014 [72] (Ge–Sn = 3.092–3.534 Å) or by Fässler in 2016 [206] (Ge–Sn = 2.975 and 3.151  Å), as well as in metal-centered deltahedral Zintl ions [{Ni@Sn8(μ-Ge)1/2}2]4– presented by Sevov in 2011 [207] (Ge–Sn = 3.026–3.061 Å). Ge=Sn  hits: 1, 2.507 Only one example could be found in database concerning a Ge–Sn double bond. Weidenbruch [198] used bulky aromatic substituents in order to stabilize this double bond with a Ge–Sn distance of 2.507 Å, which has been published in 2003.

685

686

16  X-ray Crystallography of Organogermanium Compounds

Ge–Pb  hits: 8, min: 2.599, max: 2.868, average: 2.740, standard deviation: 0.075, median: 2.732 (Me3Si)3 Ge

Ge(SiMe3 )3

Ge(SiMe3 )3

Pb

Ar

Pb E(SiMe3) 3

H

E(SiMe3) 3

Pb1

Ar'

H Pb3

E = Si or Ge

Pb2

Ge Pb

Examples containing Ge–Pb bonds are scarce. Only two publications concerning such compounds could be found within the last twenty years. In 2010, Hinderberger and Klinkhammer [17] presented the symmetrical plumbylene Pb1 with a Ge–Pb = 2.733 Å and anions Pb2 with distances between 2.840 and 2.870 Å. Wesemann published in 2019 an asymmetrical plumbylene Pb3 [205] with a Ge–Pb bond length of 2.730 Å.

16.2.5  Germanium and Group 15 Elements Ge–N  hits: 1667, min: 1.726, max: 3.182, average: 1.961, standard deviation: 0.122, median: 1.936 H B

Ge N3

N N3 Ge N3 N 3 N3

N3

Ph2P

F3C

N N SiMe3

Ge N

Ar

N

MoCp(CO)3

GeN(SiMe3)2

Me3Si

Ge Ge L N

Ar

Ar

N14

SiMe 3

Ge

PPh2 Cl N Ge N Ge Ar Ph2 P

N16

N Ge N

Ge Ph

AlClR'2

N17

Ar

Ar

R

B R 2N

R'

PR 2

PPh 2 N

N20

B

N

PPh2

Ar

W

N Ge N Ar

Ar

N

N

N

Ge

Ge

Ge

N

N

Cl Ar

N18

R

R

Ge N

N22

Ph2P Ge

PR 2 N Ar

N21

N SiMe 3 Ge N SiMe3

N13

NR 2

Cl N

N

Ar Cl Ge N

Ar

N19

Ge N N N

R

N

N12

Ph N

R

N7

N Ge N

N11

O

N Ge

Cl

Ar

N10

N

Me 3Si

N SiMe 3 N Ge N SiMe3 Cl

N15

Ge

N N N Ge Cl Cl Ph

N

N

Cl

Me3Si Me3Si

N

CF3

L

Ph

N6 Ar

Ar

Ge

N5

N

SiMe3

Ph Cl

N

Ge Cl N 3 N3

N4

N9

N

NMe2

Ge(NCS) 2

Ar

N

Et2N

N

N

CF3

N

SiMe 2OR

H Ge

Cl Me2N Cl

F C MoCp(CO) 3 3

N F 3C

N

Ge N(SiMe3)2

SiMe 2

N3

N

N8

R 2N

O

CF3

N

Ge

Ge R

NSiMe 3

Ph2P Me3SiN

Me2Si

N2

N1

O

N

N

N N N

N3

NEt 2

SiMe2 OR

N N N

Ge

Ar Cl

N23

Ar

N

Ph2P

Ge Ge PPh2

N

Ar

Ge PPh2 Cl

N24

Considering the large number of structures containing Ge–N single bonds in the first paragraph only, some examples with Ge–N bond lengths perfectly fitting with the average/median range (1.936–1.961 Å) are presented. In 2000, Filippou presented azidogermanate(IV) compounds N1 [208], N2 and [Ge(N3)6]2– [209]. Bis(amino)germylene N3 [210] has been presented by Veith in 2001. Couret isolated in 2002 nitrogen-stabilized germanium(II) compound N4 [211]. The same year Sobota presented diazido-dichloro-germanium(IV) N5 [212]. Leung [213] published in 2005 N6 and N7 with R=[C(py)=C(Ph)N(SiMe3)2]. The same author presented in 2006 N7 with R = Br7, in 2009 a dimer without R forming a digermane [214], in 2012 a MoCp(CO)4 complex of the dimer [215]. In 2014, the same author followed with a Fe(CO)4 complex of the dimer [216] and a dimer with a N2Ph2 linker between the two germanium atoms, in 2016 the linker has been formed by Se and Te [217] and in 2013 Leung published N8 [218]. Another Mo complex N9 [219] has been presented by Breher in 2006. In 2008, Ge(II) species have been stabilized by a NHC-ligand N10 [102] or by a nitrogen-donor N11 [220], reported by Baines and by Driess, respectively. In 2009, Karlov presented dimer N12 [221] and in 2013 in collaboration with

16.2  Detailed Analysis of the Statistics

Zaitsev a tungsten complex of the monomer N13 [222]. In 2014, Nembenna published three coordinated Ge(II) N14 [223] and in 2015 Frenking and Jones N-donor stabilized amido-digermyne N15 [190]. In 2016, Khan showed N16 [224], Uhl N17 [225] and Dostal N18 [226]. Tsai published in 2017 tetranuclear N19 [227]. In 2018, Goicoechea presented phosphine stabilized N20 [228] and Moret the anion N21 [229] in 2019. The same year Cabeza reported phosphine stabilized Ge(II) N22 [230] and the reaction product with AuCl. In 2019, two-coordinated amido-chloro Ge(II) N23 [231] was isolated by Hinz and tetranuclear compound N24 [232] by So. R1

R1

N Ge N R2

R2

N25 In order to obtain a more precise idea of Ge–N single bonds, we selected some non-stabilized symmetrical bis(amido)germylenes of type N25. In this class of molecules, the Ge–N distances vary between 1.819 and 1.908 Å. Schnepf reported in 2006 distances of 1.908 and 1.897 Å for R1 = R2 = SiMe2iPr [233]. In 2010, Power found 1.896 Å for R1 = H and R2 = Ar [234]. Zaitsev, Sundermeyer and Karlov reported in 2012 a fluorinated system with R1 = R2 = C6F5 and Ge–N = 1.871 and 1.877 Å [235]. Two compounds have been presented by Jones in 2013, one with R1 = Ar and R2 = Ar’ Ge–N = 1.888 Å) and another one with R1 = Ar and R2 = CHPh2 (Ge–N = 1.864 and 1.870 Å) [236]. In 2015, Rivard observed a very short Ge–N distance of 1.819 Å in the case of the implication of a guanidine ligand [237]. Khan combined R1 = Ar with R2 = PPh2 in 2016 and observed 1.859 and 1.970 Å for Ge–N [224]. Roesky published in 2019 R1 = Ph and R2 = AsMes2 (Ge–N = 1.879 Å) [238].

Ar

N

N

Ar

Fe

N26

N

O

O O

O O

N27

N

Me 2N

R N

N29

R

Me 2N

Ge

O

OO

N33

R'

Ge

Ge

N

OO

N31

Cl N N

Ge

O

O

Ge

Cl

N32

N

N30

O Ge

NMe2

N

R Ge O O R

Ph

N

N

NMe2 GeCl2

R

N

O

O

NEt2

N28

Ge

R'

R'

O

O

R

Ge N(SiMe3 )2

GeCl3

O

Ph

NEt 2

NMe2 GeCl3

N

Cl

N34

N35

N36

The longest bond lengths found for Ge–N single bonds in the database are all donor-bonds. In this category is N26 [239] presented by Roesky in 2001 with a Ge–N distance of 2.419 Å. A distance of 2.583 Å has been observed by Lorenz for N27 [240] in 2002. Couret observed in 2002 two different lengths (2.390 and 2.699 Å) in N28 [211]. Driess found in 2004 2.389 and 2.409 Å for compound N29 [241]. Karlov published in 2006 Ge(IV) compounds N30 [242] (Ge–N = 2.739 and 3.182 Å) and in 2012 dimers of the corresponding Ge(II) compounds N31 [243] (Ge–N = 2.315–2.584 Å). Baines observed in 2008 2.524 Å between the nitrogen atoms and a cryptand encapsulated Ge(II) dication N32 [244]. Reid published in 2010 [245] GeCl2 coordinated by TMEDA N33 (Ge–N = 2.458 Å) and a GeCl cation coordinated by a PMDETA N34 (Ge–N = 2.324 Å). In 2017 Schulz [246] presented N35 and N36 with distances of 2.360/2.433 and 2.328 Å, respectively. Ph2 P

Me (tBu )3P

N Ge N Me

N37

P(tBu) 3

Me3 SiN Me3 SiO

Ph 2 N P Ge

Ge N

OSiMe3

F3 C

NSiMe3 P PPh2 Ph 2

N38

CF3 O

R P

Ge R2 P

N

O

N39 Ar

CF3 CF3

N

GeCl3 N

N42

Ar

R 2P

Cl N

Ge

N N N

Cl

N43

Ar N O Ad Ge O Ge

Ar

N N

CF3

N

N40

Ar

N

Ar

F3 C

Ar

Ad

PR2

Fe(CO)4

N N Ge N(SiMe3 )2

O CF3

CF3

N Ar

N41

687

688

16  X-ray Crystallography of Organogermanium Compounds

Short Ge–N single bonds with distances