Organotransition Metal Chemistry [1 ed.] 9789350431696, 9788184888362

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ORGANOTRANSITION METAL CHEMISTRY

"This page is Intentionally Left Blank"

ORGANOTRANSITION METAL CHEMISTRY

Dr. G.R. CHATWAL M.Sc. Ph.D. Formerly, Reader in Chemistry

OVAL SINGH COLLEGE

(University of Delhi) New Oelhi-110003. Edited by

M. ARORA

K4)iI Cflimalaya Gpublishing'House [J

Q MUMBAI Q DELHI Q NAGPUR q BANGLURU Q HYDERABAD Q CHENNAI Q .PUNE LUCKNOW Q AHMEDABAD Q ERNAKULAM [J BHUBANESWAR Q KOLKATA Q INDORE

Author Stored in a. retrieval system, or transmitted in any form or by any means, electronic, mechanical. photocopying, recording and/or otherwise without the prior written permission of the author.

ISBN

: 978-81-84888-36-2

First Edition: 2010

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Indore

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Contents Chapter 1 Introduction

1.1 - 1.37

Chapter 2 Ligands

2.1 - 2.13

Chapter 3 Cyclopentadienyl Complex

3.1 - 3.6

Chapter 4 Alkyls and Aryls of Transition Metals

4.1 - 4.23

Chapter 5 Compounds having Transition Metal-Carbon Multiple Bonds

5.1 - 5.20

Chapter 6 Transition Metal x-Complexes

6.1 - 6.65

Chapter 7 . Transition Metal Compounds with Bond to Hydrogen

7.1 - 7.15

Chapter 8 Homogeneous Catalysis

8.1 - 8.42

Chapter 9 Fischer Carbene Complexes

9.1 - 9.5

Chapter 10 Oxidative Coupling and Reductive Elimination

10.1 - 10.5

Chapter 11

Oxidative Addition

11.1 - 11.5

Chapter 12

Migratory Insertion Reactions

12.1 - 12.10

Chapter 13

Coupling Reactions

13.1 - 13.7

Chapter 14

Reaction of Organometallic Compounds

14.1 - 14.25

Chapter 15

Fluxionality

15.1 - 15.3

Chapter 16

Fluxional Organometallic

Appendix

Co~pounds

16.1 - 16.11 17.1 - 17.22

CHAPTEI

INTRODUCTION .. I

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Definition

The term organometallic is usually restricted to those compounds which are having at least one direct metal-carbon bond. This bond can be simple covalent as in lead tetraethyl Pb(C 2H 5)4 or 7t dative as in ferrocene [Fe(T\5 - C5H 5)2] or even predominantly ionic as in ethylsodium. Na+C 2H 5-. On this basis compounds like metal alkoxides [e.g., aluminium triethoxide Al(OC 2H 5)3]' metal amides [e.g., LiN(CH 3)2]' chelate complexes (e.g., metal acetylacetonates) or the metal salts of carboxylic acids are considered as organometallic compounds although they contain both metal atoms and organic groups, since none of these systems contains a direct metal-carbon bond. These compounds are organic derivatives of metals but not organometallic compounds. (May be alkyl, alkene, fM=TC~ alkyne, aromatic,

(May be main-group element, transition element, lanthanide or actinide)

heterocyclic, carbonyl)

[linkage may be ionic, covalent (cr, 1t, 7t) localised or delocalised] The above definition can be further refined by saying that the term organometallic is restricted to those compounds in which an organic group is attached through carbon or to an atom which is less electronegative than carbon. Thus, organic compounds of the lighter halogens, and chalcogens are excluded from the field of organometallic chemistry but not the organic derivatives of boron, silicon and arsenic because these are considered as metalloids. Organophosphorous compounds are so diversified and numerous that it would be impractical to deal with them within the limits of organometallic chemistry and it is more appropriate to have an independent branch of organophosphorus chemistry. Practically all elements, except at present the noble gases form compounds with element-carbon compounds. In the above definition of organometallic the term 'metal' includes all those elements 'which are less electronegative than carbon. (1.1)

Introduction

1.2

Table 1.1 : Electronegativities of Some Common Elements on Pauling's scale

Li

Be

B

C

N

1.0

1.5

2.0

2.5

Na

Mg

Al

Si

0.9

1.2

1.5

1.8

3.0 P 2.1

K

Ca

Ga

Ge

0.8

1.0

1.6

1.8

0 3.5

F 4.0

S

Ci

2.5

3.0

As

Se

Br

2.0

2.4

2.8

The electronegativity of carbon is 2.5. The electronegativities of Si(1.8), B(2.0), As(2.0), Ge(1.8) etc., have been lower than that of C. Thus the compounds in which metals or metalloids like Si, B, As, Ge etc., bonded directly to cabon atom are known as organometallic compounds. Some examples of organometallic compounds are given as follows:

CHa 'CH B-CH-CH / 2 3

,2

/CH 2 CHa Dimethylberyllium

CH

/f\2"

black

bimetallic with two m-H bridges and a fulvalene bridging ligand (structure shown later) very air sensitive, paramagnetic bimetallic with 1'\1, 1'\ S-C 5H 4 bridges and terminal hydrides (structure shown later).

173

very air sensitive several bimetallic isomers with fulvalene and 1'\ 1, 1'\S bridges and terminal hydrides (structures shown later), diamagnetic, air-sensitive.

"W(C 5H S)2"

yellow green

Mn(C sH 5)2

brown

173

air-sensitive and easily hydrolyzed, interesting highspin to low-spin interconversion

Fe(C sHs)2

orange

173

air-stable, can be oxidized to blue-green [Fe(C 5HS)2r which, in turn, is a good "inert" oxidizing agent.

CO(C SHJ2

purple-black

174

air-sensitive, paramagnetic 1ge-complex, can be oxidized to the air-stable 18e-yellow [Co(C sHJ2r

Ni(C sHs)2

green

173

20e-complex, slow oxidation in air to the labile, orange cation [Ni(C sH s)2r

same as Mo

Metal Chemistry

2.11

Structural Features The parallel sandwich structures have the following structural features: Distances (A)

M_c~Icp--_cp c-c~

c-c

M

M-C

Cp····Cp

Fe

2.04

3.29

1.42

[Fer

2.07

3.40

1.40

Ru

2.19

3.64

1.43

Os

2.19

3.61

1.45

Co

2.10

3.44

1.41

[Cor

2.03

3.24

1.42

Ni

2.18

3.63

1.41

Note the various trends in the bond distances. The changes in the neutral Fe, Co, Ni metallocenes are a direct result of going from lSe- (Fe) to 1ge- (Co) to 20e- (Ni) counts. The extra electrons for the Co and Ni complexes are going into M-Cp antibonding orbitals, which are delocalized and progressively weaken the M-Cp bonding, leading to the increase in bond distances. This in spite of the fact that the metal's covalent radius is decreasing as one goes from Fe ----.. Ni (effective atomic number contraction effect).

Problem: Explain why the Fe-C distance lengthens for [CPzFe]+, while the Co-C distance shortens for [Cp 2 Co]+. Oxidation of CP20s does not produce a simple cationic monomer as seen for Co and Fe. Instead one gets dimerization to produce the bimetallic complex that has an Os-Os bond (3.04 A).

Problem : Is this complex para- or diamagnetic? The simple neutral bis-Cp complexes of the early transition metals are quite different because they are in very low +2 oxidation states (very electron-rich) and quite unsaturated. Thus, they are very reactive towards oxidative addition and other reactions. "Nb(C 5H 5)2", for example, is nominally a 15e- complex with a highly reactive d 3 Nb electronic configuration. Two molecules of niobocene react with one another via C-H bond activation (oxidative addition) to produce the structure shown to the right. Note that two of the Cp rings are dianionic forming both a traditional anionic ..,5 6e-1t-type donor to one metal, while bridging over and acting as an anionic 2e-cr-donor to the other metal center. "Ti(C 5H 5)2", is nominally a 14e-complex with a highly reactive d 2 electronic configuration. Two molecules of titanocene also react with one another via C-H bond activation (oxidative addition) to produce a bimetallic complex that may well look just like the niobium complex just discussed. But it has a further reaction (perhaps due to steric crowding brought

Ligands

2.12

on by the smaller Ti centers) leading to the coupling of the two cr-bound Cp's to produce C-C bound bis-Cp and the complex shown below. The more · sterically crowded pentamethyl-Cp (Cp*) complex simply does a hydride abstraction and stops atthe complex also shown below : Me

. Me . Me

Me~Me

Me-WCH,

Me~Me TiM

Me~Me

Me~e

Ti- Ti = 2.99 A

Ti::

. *M: Me

Me Me The "Mo(C 5H 5)2" and "W(C 5H 5)2" complexes might appear to have a "reasonable" 16e-count, but they are quite reactive, like their early transition metal cousins, and also self react with one another via C-H bond activations to produce several isomeric bin:letallic complexes shown below.

Problem : Electron-count the following complex. What does the arrow between the two Mo atoms indicates? It is not a covalent Mo-Mo bond. What name for this type of bonding would you. use?

n . M\~O

~

Mo-Mo

~H'?fjJ

~ 3.19A

2.13

Metal Chemistry Cp Variants These have special bonding properties important in substitution reactions.

fulvalenediyl (2-)

indenyl (-)

fluol'enyl (-)

~ ~o --

6edonor

2.45A.

©>

azulene

Azulene is neutral, so TJ5-coordination of the C 5 ring only provides 5e-, to get 6eone needs to use one of the C7 ring carbon 7t-orbitals.

MO Comparison of Cp- vs. Arene Ligands Benzene-Metal Complex

------1t*

Cyclopentadienyl-Metal Complex

_/-1t ~1t

90 M

"This page is Intentionally Left Blank"

•

CHAPTEI

CYCLOPENTADIENYL COMPLEXES II I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I II

3.1. Introduction The cyclopentadienyl (Cp) ligand is a mono anionic ligand with the formula C5H 5. The first characterized example of a cyclopentadienyl complex was ferrocene, CP2Fe, which has an iron atom "sandwiched" between two planar Cp rings as shown on the right. For this reason, bis(cyclopentadienyl) complexes are sometimes called "sandwich compounds" or metallocenes. Some metallocene derivatives, such as CP2 TiCI 2, have their Cp rings tilted with respect to each other and are called ''bent metallocenes". The discovery of ferrocene in 1951 and its structural elucidation by two separate research groups the following year marked the birth of contemporary organometallic chemistry. This revolutionary advance in organometallic chemistry was recognized with a Nobel Prize in Chemistry in 1973. Thousands of Cp complexes have since been characterized, and several of these form the basis for important industrial processes.

3.2. General Properties of Cyclopentadienyl Complexes All of the first row transition metal metallocenes, CP2M, have been synthesized except for titanocene, CP2Ti, which has the unusual fulvalene-hydride structure shown below. As you can see in the following table, Cp ligands can stabilize metals in a variety of d-electron counts (as well as oxidation states other than 2+).

CP2V

CP2 Cr

Cp#n

Cp~e

CP2 CO

Cp~i

Color

purple

red

amber

orange

purple

green

m.p.

162

172

193

173

173

173

d count#

15

16

17

18

19 -

20

unpaired

3

2

5/1

0

1

2

2.28

2.17

2.38

2.06

2.12

2.20

M-C distanc (Angstroms)

(3.1)

Cyclopentadienyl Complexes

3.2

3.3,. Bonding in Cp .Complexes ?

.

The normal bonding mode for Cp is 1'\5 (pentahapto), for which several different resonance structures can be drawn for the bonding of an 1'\5_Cp ligand to a transition metal complex. The one on the right makes it easy to remember that a Cp ligand donates either 5 or 6 electrons to a transition metal complex (depending on which electron counting formalism you use) as it looks like one alkyl ligand + two alkene ligands.

q>

9'

M M While some of these forms are important in special cases (see ring slips below), a molecular orbital (MO) diagram best describes the bonding in a Cp complex. Consider the five MO's of a Cp ligand. If we have two of these, we can ·add and subtract various orbital combinations to generate the MO diagram for a metallocene.

e2

(2 nodal surfaces)

e1

(1 nodal surfaces)

(0 nodal surfaces)

The lowest energy orbital, al' does not have· any favorable overlap with any of the metal d-orbitals. It has little interaction with the d Z2 because the ligand p-orbitals lie on the d Z2 conical nodal plane. The e1g set of degenerate orbitals overlaps quite well with the dxz and dyZ orbitals on the metal, forming a strong set of pi-bonds. The e1u interaction between the metal Px and Py also gives some stabilization. Although the metal d X2 -y2 and dry can overlap with the e2g orbitals on the ligand, the degree of overlap is not very large and these levels are essentially non-bonding.

Metal Chemistry

3.3 M

2 Cp :

=

IT

~

I e1u \ ......l ....--1t-~\

4p

_ _ _ ......

.. .,.....

~

.... ".... l :

//

~-

/~

l,·l

a 2u

'r \ \

ezu

'~ ~

: l/'

~\

1.\. ,,(;

4s

n1. . t

!

\

~\

\

alit ..',, ====:: ,\

I

~~.." l

,: ",\ \ l ,:

\ \~ ........ n\\\ ..........

---~\~~V!

\ .... \ J..,'

:~\

\\ X \r/ \'

:t. , \ '\ . . .

GIg'

3d e Ig, e2g

'

I

"

~/

IJ \

\l/\ f...... I,

,: :' ,:

','I I,

..

\

,:=

\ \ \ '\ \\ \

~~'

\

..........

\ .~' \

..

1

"l"\~'

... '\~ e1u \ \\ \ -==-:i/\ .... ,....\,.. ~\.... " ~ ====t.l::.:r"y"y" ..... "'---\:' \ \ \ •~ ,........' a \, \ \,. \ \ \" \ \ ....\ 'Jg,\ I

I~

~

\

\

......

\\ \ \ \ "

•

II

I

\ \

~'"

e2g

1

\

\

1

\ \ \ \

\

II.

\ '\1\

I

I''''

,

\\'\

\

!~ \ \

\\ \:~

\

,

\

\

••

\ ,

\

\~

..

\

\ \\ \ \ ,

'\.\

\

•

,

II

\

1\. . ;;;

\ \ \~ ...........':..... \\,' , .' \ ' .... ' 1,t{ \ \\~ . ,~' \\ \\\\\ \\ "" \ \ e 1u , , " ' , ':. 11 " ,

\ 1\

\\\\JLJ;. / \\\ .. \\\ \\\ ',\\ e1g

II

II

\\'.\

\\ 1 1111 LlldL ':'~ .... ~ . ':.\ a21~ ........::.... .. , :::........ ..

.

...

GIg

The MO diagram for generic metallocenes, CP2M is shown above. Notice that the Cp orbitals fill the six lowest orbitals. The next five unoccupied MO's shown in the box have little or no bonding character, which explains our observation above that metallocenes are known for a variety of d-electron counts. As one can infer from the MO description, most Cp complexes display completely delocalized bonding with equivalent C-C bond lengths in the C 5 ring. The carboncarbon bond distance of 143.3(6) pm in the Cp ring of ferrocene is somewhat longer than that observed in Na(TMEDA)Cp (C-C distance of 138 pm) and other aromatic systems such as benzene (C-C distance = 135 pm). The longer C-C distance in ferrocene is the result of n-backbonding from filled d-orbitals on Fe to the antibonding molecular orbitals on Cpo While the C 5 ring of ferr~cene is planar, the hydrogens are bent downwards towards the metal by about 5 degrees. This can be ascribed to a canting of the p-orbitals on carbon which permits better overlap of the ligand MO's with the metal orbitals (see the MO diagram above).

3.4

Cyclopentadienyl Complexes

The success of Cp ligands in contemporary organotransition metal chemistry can be traced to several key features: D The M-Cp bond dissociation is large (ferrocene is stable to 400 degrees C). D The ligand tends not to get involved chemically (although it certainly can from. time to time). D The ligand blocks several coordination sites. D It has excellent NMR properties.

3.4. Cp Derivatives A variety of substituted Cp derivatives are known, many of which have desirable steric, electronic or spectroscopic properties. The pentamethylcyclopentadienyl ligand, C5Me 5 is one of the best known of these and has the designation Cp*. The Cp* is sterically more demgmding than Cp, allowing the isolation of Cp* complexes for which the Cp analogs are unknown or are kinetically unstable. In decamethylferrocene, Cp* 2Fe, the methyl groups are actually tilted above the C 5 plane by 3.4 degrees due to steric . interactions. The methyl groups on Cp* are electron donors, so this results in more electron density at the metal than for the analogous Cp complex. This can be observed spectroscopically by looking at carbonyl stretching frequencies of Cp/Cp* carbonyl complexes. The increased donation of the Cp* ligand results in greater 1t-backbonding which is apparent from a red shift of approximately 50 cm-1 in the IR spectrum. Likewise, electrochemical measurements indicate that Cp* complexes are more easily oxidized than their Cp analogs by approximately 0.5 V. A variety of other symmetric and asymmetric alkylated Cp derivatives have been prepared. For example, MeCp, C 5H 4Me, which is sometimes called Cp', is often used to impart subtle electronic differences to a complex or to make crystallization easier. Another important class of Cp derivatives are indenyl complexes. These contain a benzene ring fused to the Cp ring. As is discussed below, indenyl ligands can undergo a ring slippage which results in accelerated reaction rates.

() N-N

/ "ML" R-Bf§9'

Finally, it is worth mention here of a ligand that has similar electronic donation properties to Cpo Tris(pyrazolyl)borate, commonly called Tp, is also a monoanionic ligand (B bears a negative charge) that donates six electrons to the metal center. Tp complexes are quite versatile and a wide variety of Tp derivatives with differing steric and electronic properties have been synthesized. Spectroscopic Features

In the IH NMR spectrum, Cp rings appear as singlets in the range of 4.0 to 5.5 ppm. The barrier to rotation around the M-ring centroid vector is quite low (approximately 1 kcallmol), so even though the molecule may possess low symmetry, all five ring

Metal Chemistry

3.5

protons are equivalent on the NMR time scale. In the l3C NMR, the carbons are also equivalent and typically appear between 80 and 95 ppm. The methyl groups of a Cp* ligand typically appear at 1-2 ppm in the lH NMR and 20-30 ppm in the l3C NMR.

3.5. Synthesis of Cp Complexes Almost all syntheses of Cp complexes start from readily available (and inexpensive) dicyclopentadiene (b.p. 170 C). Upon heating, this undergoes a retro Diels-Alder reaction ("cracking") to afford free cyclopentadiene (b.p. 46 C) which can be distilled off as it is formed. The CpH can then be employed in several ways: 1. Deprotonation of CpH using a strong base such as n-butyllithium, potassium hydride or alkali metal :

Na/THF

•

acidic hydrocarbon pKa = 18

---+.

CpMLn + NaX

2. Reduction of CpH by a metal precursor with elimination of H 2. This reaction may occur through a radical pathway. Fe(CO)5 +

0

N

aITH~

[CpFe(CO)J2

via

9