Reactivity of Transition Metal Organometallics L. J...

L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics

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Reactivity of Transition Metal Organometallics L. J. Farrugia MSc Core Course C5

Text books : Inorganic Chemistry - Housecroft & Sharpe Ch 23 - the very basics

Inorganic Chemistry - Shriver & Atkins Ch 16 - the very basics

This course assumes familiarity with the Level-2 and Level-3 courses on Organometallic Chemistry, and this is covered in the above texts

Organometallics - Elschenbroich & Salzer (library) - much more useful

Topic 1 - Introduction to Cyclopentadienyl Compounds

First part of course covers the role of the ligand cyclopentadienyl (Cp) and its derivatives. Cp is one of the most important ligands in organometallics after CO. A considerable percentage of organometallic compounds contain this ligand - it is also a good ligand for main group metals and the f-block metals (lanthanides & actinides).

Cyclopentadiene Cyclopentadiene complex (4 e donor ligand) very rare as a ligand

The anion C5H5

- is a very useful synthetic reagent. It is usually treated as equivalent to occupying THREE coordination sites, so that C5H5 ≡ 3(CO). In electron counting terms, it can be treated as either as a 6-e donor ANION or a 5-e donor NEUTRAL molecule. The latter is the recommended approach because it is simpler (do not need to worry about oxidation levels). 1.1 Bonding in cyclopentadienyl compounds

Cp has 5 π electrons in the 5 out-of-plane p-orbitals on the C atoms. These 5 orbitals combine as a1 + e1 + e2 under five-fold symmetry. With a single Cp ring,

H

H

H

H

Fe

OC CO

CO

H

H acidic H atom

Na / -H2

Na+

planar aromatic (6-pi electron) dienyl anion

-


2

inspection shows the possible combinations with the metal d-orbitals are shown below

Cp orbitals Metal orbitals Symmetry of bond a1 pz dz2, s σ e1 dxz dyz px py π e2 dx2-y2 dxy δ

For two Cp rings in a metallocene M(C5H5)2 the rings may be either staggered or eclipsed - Fe(C5H5)2 is eclipsed but Co(C5H5)2 and Ni(C5H5)2 are staggered.

The bonds can also be divided into sigma, pi and delta symmetry as shown in OHP #1 OHP # 2 shows the formal MO interaction diagram for ferrocene - Fe(C5H5)2 The main points to remember (i) the 9 filled orbitals have 18 electrons - hence the rule ! (ii) the LUMO is a doubly degenerate π* orbital, so Ni(C5H5)2 is paramagnetic


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This shows how the metal and ring π-orbitals match by symmetry. The resultant MO scheme for ferrocene shows the way that the basis orbitals having the same symmetry can combine to give new orbitals (NOT necessary to remember the MO scheme) The filling of the nine bonding orbitals in ferrocene explains the high stability of this compound. The mixing of metal and Cp orbitals indicates a strong covalent character to the transition metal - cyclopentadienyl bond. In Co(C5H5)2 ONE electron fills the e*1g while in Ni(C5H5)2 there are TWO unpaired electrons - hence both compounds are paramagnetic


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Co(C5H5)2 readily loses an electron to give the cobalticinium cation [Co(C5H5)2]+

an 18e cation. Similarly Ni(C5H5)2 loses one electron to give a 19e cation, but further oxidation results in decomposition. 1.2 Structural types of Cp compounds (i) Metallocenes MCp2 These are known for the metals Ti, V, Cr , Mn, Co and Ni. Staggered or eclipsed rings leads to D5h or D5d symmetry with virtually NO barrier to rotation of the Cp ring about the metal-Cp axis. So all Cp protons appear as equivalent in the 1H NMR spectrum V, Cr, Fe, Co and Ni give the "classic" sandwich compounds illustrated above. The exceptions are


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(a) Titanocene "TiCp2"

This is really a fulvalene complex made by reducing Cp2TiCl2. It contains bridging hydrides and a Ti-Ti bond - gives 16 e Ti atoms. Real "titanocene" TiCp*2 has recently been made but is extremely reactive. (b) Manganocene MnCp2 This is ionic at low temperature, with a polymeric chain like structure

Above 159oC it becomes isomorphous with ferrocene, so must adopt a sandwich structure. All the sandwich metallocenes apart from ferrocene are paramagnetic

No. unpaired electrons Cp2V 3e Cp2Cr 2e Cp2Mn 5e high spin Mn2+ Cp2Co 1e Cp2Ni 2e

(ii) 'bent' metallocenes These have non-parallel rings due to the presence of other ligands. Some examples are

18 e 16 e 17 e 18 e

W

H

H

Ti

Cl

Cl

V CO Re H


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(iii) half-sandwich compounds - piano stool compounds These have one Cp ring and a variety of other ligands. Some examples are

4-legged stool 3-legged stool Chem-3 lab 2-legged stool The 17 electron species CpMo(CO)3 and CpFe(CO)2 are not stable as such, but dimerise to give the familiar compounds with a Mo-Mo or Fe-Fe metal-metal bond. This is typical behaviour of odd-electron organometallic species. (iv) other types of bonding mode

So far only the so-called eta-5 η5-C5H5 bonding mode has been illustrated, where (in principle) all five C atoms are equally bonded to the transition metal. However there are other possibilities, most common are η3-C5H5 and η

1-C5H5

η1-C5H5 η3-C5H5

1 e donor - like alkyl group 3e donor - like allyl group

One good example is Mo(NO)Cp3 - with "normal" η

5-C5H5 bonding modes, electron counting give a 24 electron compound (!!) - simply not possible. In fact it is an example of a compound containing all three types. η1-C5H5 ligands are usually fluxional. Consider the compound (C5H5)4Ti - again cannot be 4 η5-C5H5 bonding modes because it would be a 24 e compound. In this case it has two η5-C5H5 and two η

1-C5H5 ligands.

V Mn Ru Co

OCCO

COOC

OCCO

COCl PPh3

PPh3

OC CO

H

M

M

H

Mo

ON

Ti


7

At the very lowest temperature measured, the 1H NMR spectrum shows a singlet for the two (equivalent) η5-C5H5 groups and an AA'BB'C multiplet for the two (equivalent) η1-C5H5 groups (three different H environments). This compound shows two fluxional processes (i) migration of metal atom around the η1-C5H5 group via 1,2 shifts (ring whizzing) (ii) exchange of the η1-C5H5 and η

5-C5H5 groups Process (i) is a very low energy process, which is frozen out only at the lowest temperatures. Process (ii) occurs at higher temperatures ~ room temperature. The net result of (i) and (ii) is that all 20 protons appear equivalent at the highest temperatures measured. Process (i) could in principle occur also by 1,3 shifts or random shifts. How can we tell which ? We can if it is possible to assign the protons of the AA'BB' signal

The resulting exchanges are :

a→c a→b i.e. both a type protons exchange b→a b→b i.e. only half of b type protons exchange c→a

So... the rate of broadening for the a type protons is twice as fast as for the b type protons The actual spectrum of (η5-C5H5)(η

1-C5H5)Fe(CO)2 which exhibits a similar exchange is shown below.

bb

aa

M c

b b

a

c

M

a

ab

cb

M a

1, 2 shift


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1.3 Oxidation states in Cp compounds Since Cp is formally charged with a -ve charge, each Cp ligand present in a compound contributes a charge of +1 towards the formal oxidation state of the metal. Hence a metal must be in a +ve oxidation level. The only exception would be with +ve charged ligands - NO+ is the only common example. CpNi(NO) has a zerovalent Ni atom. In terms of its ability to stabilize oxidation states, Cp is comfortable with both low and high oxidation states of the metal (unlike many pi-acid ligands like CO which are only found for low oxidation states). In general Cp is a good sigma- and pi-donor, but a poor pi-acceptor. Substituting H for Me on the Cp ring makes it an even better sigma-donor - Cp* is the common symbol for C5Me5 .

Re

OCCO

CO

Re

OO

O

H2O2

CO

Re (+1) Re(+7) - oxo complex


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Topic 2 - Reactivity of Cyclopentadienyl-type Ligands Very often Cp is a spectator ligand (i.e. it does not take part in the reaction and it is unchanged at the end). Under some circumstances however the ring will react. Ferrocene Fe(C5H5)2 has been the most studied (because it is a stable 18 e metallocene) but ruthenocene Ru(C5H5)2 and osmocene Os(C5H5)2 will give similar reactions - they are in the same periodic group ! However, most Cp compounds will not survive the reaction conditions described below. 2.1 Electrophilic substitution at ring This is a very facile process, occuring some 3×106 times faster than with benzene.

E+ is a general electrophile, see E/S p 328-330. A concrete example is a Friedel-Crafts acylation

The mechanism probably involves exo attach at the ring C-atom, followed by a movement of the proton down to the metal.

The hydride intermediate can be made by treating FeCp2 with very strong acids. The hydride signal in the 1H NMR spectrum comes aroound -2.1 ppm, which is typical of metal hydrides. This reaction does not work if the electrophile is an oxidising electrophile as many are such as NO2

+. These reagents oxidise the complex to give the

Fe

E+

Fe + Fe

EE

H

- H+

Fe

CH3COCl/AlCl3

Fe

C

O

Fe

C

O

C

O

+

Fe Fe

EE

H Fe

E

H+

+


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ferricinium cation [FeCp2]+ and the +ve charge now on the rings makes

electrophilic attack very difficult. Another example and indication of the high reactivity of FeCp2 is that the Vilsmeier reaction works

This reaction only works well with highly activated aromatics such as amines (anilines) and phenols. Because of the high reactivity of the Cp rings, there is little of the selectivity observed in aromatic compounds.

2.2 Reactivity of attached organic groups Organic functional groups attached to Cp undergo many of the most common reaction types, e.g. below (non-oxidising conditions)

Fe

(i) POCl3

Fe

CO

H

(ii) H2O

C

O

HMePhN

+

substituted formamides

Fe

Et

4.2

1.0

for Friedel-Crafts acylation

CH3COCl/AlCl3

1.4

Fe

C

O

H

Fe

PhCOMe/base

H

O Ph

H

Fe

H

R

H

Ph3P=CHRWittig ylid

Fe

C

O

CH3

Fe

RCHO/base

O

H

H

R

aldol condensation


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2.3 Metallation of ferrocene Further substituents may be introduced by the very useful reaction with butyl lithium

Note that it is not possible to introduce the NO2 group directly by nitration as the electrophile NO2

+ is oxidising 2.4 Stabilization of αααα-carbonium ions

The hydrolysis of the aceto substituted ferrocene proceeeds seven times faster than the solvolysis of Ph3C-OAc (this is a classic example of SN1 hydrolysis via the stable carbonium ion Ph3C+ the trityl ion). So we conclude that the carbonium ion intermediate is more stable that the trityl cation. Reactions involving vinylic substituents also proceed through α-carbonium ions, e.g. the reaction with CH3CO2H

FeFe

Fe

FeFe

PPh2Cl

Li

BuLiFe

CO2H

Fe

NO2

CO2

N2O4

Fe/HCl

NH2OMe

NH2 2 BuLi

Li

Li

PPh2

PPh2

Fe

PPh2

PPh2

Fe(CO)4Fe(CO)5

Fe FeH2O/H+

OAc

RH

R

H

+

OH-

Fe

OH

RH

carbonium ion intermediate

Fe FeH2O/H+

CH3

H

+

OAc-

Fe

OAc

CH3H

carbonium ion intermediate


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The carbonium ion can actually be isolated and crystallised as the BF4- or PF6

- salts. The crystal structures show clear evidence of an "interaction" between the exo- C-C bond and the Fe atom - "bends" towards the Fe atom.

There is also NMR evidence for the presence of a π-bond between the exo-C atoms and the Fe atom. Uses 57Fe-13C coupling constants, which was observed to be 1.5 Hz in the above cation. Clearly indicates that the bond between the carbonium carbon and the Fe is nearer to a π-bond.

Type of bond J(57Fe-13C) /Hz

σ Fe-C ~ 9 π Fe-C 1.5 - 4.5

2.5 The "indenyl" effect and ring slippage Indene is a bicyclic hydrocarbon closely related to cyclopentadiene

Indenyl forms complexes which are similar to Cp, but substitution reactions are much faster (up to 108 times !!). Example

(Ind)Rh(CO)2 + PPh3 → (Ind)Rh(CO)(PPh3) + CO This is an SN2 reaction, i.e. 2nd order and associative. Therefore the rate determining step involved addition of the phosphine Example

(Cp)Mn(CO)3 + PBu3 → no reaction (Ind)Mn(CO)3 + PBu3 → (Ind)Mn(CO)2(PBu3) + CO (Flr)Mn(CO)3 + PBu3 → (Flr)Mn(CO)2(PBu3) + CO

Fluorenyl (Flr) is 250 times faster than the indenyl reaction

Fe

Ph

Ph+

overall 6 e donor

Fe

160 deg

crystal structure

HH

base

indenyl anion Ind


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What is the reason for this ? - ring slippage shown in the mechanism below

The C5 ring is moving from eta-5 to eta-3 back to eta-5 coordination to the rhodium atom. The driving force is that the benzene ring can become fully aromatic when the ring is slipped. This is even more so in fluorenyl. Evidence for ring slippage The ring slipped complex is usually a reactive intermediate and is not observed as a stable compound. However in the following reaction the ring slipped compound can be isolated

(Ind)Ir(PMe2Ph)2 + PMe2Ph → (Ind)Ir(PMe2Ph)3 18 e 20 e ??

Fluorenyl anion Flr

Rh

OC CO

Rh

OC CO

eta-5 eta-3

Rh

OC

eta-3

PPh3

CO

PPh3

Rh

OC PPh3

eta-5

- CO

18 e16 e 18 e

18 e


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The X-ray structure shows that only 3 C atoms of the C5 ring are coordinated to the iridium

The eta-3 coordination means that the bond between the Ir and the indenyl group is weakened and it may be displaced by a further mole of phosphine. The displacement of Cp or Cp related ligands is highly unusual ! 2.6 Heterocycles as Cp analogues The CH group in Cp may be replaced by hetero-atoms to give heterocycles

N (P, As etc) ≡ CH (isolectronic)

C4H4N

- N pyrrolyl C4H4P

- P phospholyl C4H4As

- As arsolyl These anions form π-complexes which are directly analogous to Cp

Ir

PhMe2PPMe2Ph

PMe2Ph

18 e

PMe2Ph

[Ir(PMe2Ph)4]+ Ind-

16 e

N

CpFe(CO)2I +N

N

N

FeFe

OCOC

heat

- 2CO

Aza - ferrocenesigma-complex

[CpFe(CO)2]2 +P

Ph

150 C

P

Fe

Phospha - ferrocene

+ 2P

Li

P

P

FeFeCl2


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The P (and indeed N) atoms have lone pairs and so are capable of acting as ligands in their own right. May be thought of as derivitised phosphine ligands.

Thiophene C4H4S has one extra electron and so behaves as a six-electron donor analogous to benzene

Boroles C4H4BR have one less electron and so behave as four-electron donors. So compare Cp2Fe(CO)4 with (C4H4BMe)2Co(CO)4 - isostructural and iso-electronic compounds.

There are a large number of heterocycles which can act as ligands (see E/S p 376-385 for other examples). The replacement of CH by P for example can go the whole way.

All the P atoms are equivalent and give a singlet at 153 ppm in the 31P NMR spectrum.

P

P

Fe+ 2 Fe(CO)4(THF)

Source of 16 e "Fe(CO)4"

P

P

Fe

Fe(CO)4

Fe(CO)4

S

Cr(CO)3Cr(CO)3

S

Fe Fe

C

O

CO

OC CO

BMe

BMe

Co Co

C

O

CO

OC CO

[Cp*Fe(CO)2]2 +

150 C

PP

PP

P

Fe

Penta phospha - ferrocene

P4


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Topic 3 - Stabilisation of Unstable Molecules by Complexation Transition metals have the remarkable ability to stabilise unstable, unknown or highly reactive organic (and inorganic) molecules. This is because coordination to a metal changes the electron density in the ligand. This facet of organometallic chemistry will be illustrated by a few pertinent examples. 3.1 Unstable molecule - Cyclobutadiene This is a highly strained and unstable molecule. It is non-aromatic, i.e. doesn't obey the (4n+2) π-electron Hückel rule. In fact it is anti-aromatic and is destabilised by a square-planar arrangement of C atoms. This is easily seen by thinking about the π-orbitals in this symmetry (D4h)

The four π-electrons thus give rise to an expected triplet state, which is subject to Jahn-Teller distortion. In fact C4H4 is not square but rectangular with localised double bonds and D2h symmetry,

In 1955, Longuett-Higgins and Orgel predicted theoretically that the aromatic square form of C4H4 would be stabilised by complexation to a transition-metal. The synthesis of such a complex was soon accomplished.

v antibonding slightly antibonding bonding

ψ−4 ψ−3, ψ−2 ψ−1

ψ−1

ψ−3, ψ−2

ψ−4

H H

HH t-Bu H

t-But-Bu

highly reactive stable because of steric restraints


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In 1959 the nickel complex of tetramethylcyclobutadiene was synthesised and the X-ray crystal structure determined. The Ni(CO)4 also abstracts the two chlorine atoms and NiCl2 is a by-product. The C-C distances are ~ 1.45Å which is intermediate between a single and double bond. The first complex of the parent unsubstituted cyclobutadiene was made in a similar way

This compound is a low melting yellow crystalline complex with a signal at 3.91 ppm in the 1H NMR spectrum and two ν(CO) stretches in the IR spectrum, consistent with the pseudo three-fold symmetry. This reaction also proceeds by halogen abstraction and it is a general reaction for the synthesis of complexes of cyclobutadiene (and substituted cyclobutadienes)

The anion [Mo(CO)5]2- is a powerful nucleophile. Another synthetic route for

cyclobutadiene complexes is through the dimerisation of alkynes. This is a thermally forbidden reaction (Woodward-Hoffman rules) but with the mediation of a transition metal, this may be overcome

3.1.1 Physical properties In complexes the ring is essentially square-planar. If all distances are approximately equal, this indicates bond delocalisation and a fully conjugated π-system. Some structures however show partial double bond localisation. The ring has aromatic properties, for example the 1H NMR signals of the CH ring protons are in the range 4 - 6 ppm, similar to Cp protons.

H3C CH3

CH3H3C

Cl

Cl+ Ni(CO)4

Ni Ni

Cl

ClCl

Cl

+ Fe2(CO)9

Cl

Cl

Fe

OC COCO

+ FeCl2 + 6 CO

2 Na+ [Mo(CO)5]2-

Cl

Cl

Mo

OC CO

COOC

CoCo

COCO

RC CR+ 2

R

RR

R


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3.1.2 Chemical reactivity 3.1.2.1 Spectator role The cyclobutadiene ligand is often a "spectator" ligand in much the same way as Cp

In the latter complex the butadiene ligand act as steric "plugs" preventing access to the metal atom 3.1.2.2 Reactivity at ring Electrophilic substitution occurs very readily and in a very similar fashion to Cp. For example the Vilsmeier reaction gives aldehyde substitued ring product

Organic functional groups also show the same reactivity as with Cp rings.

Likewise the stabilisation of the α-carbonium ion occurs in a similar fashion to Cp

Fe(CO)3 Fe(CO)2(PPh3)

+ PPh3

Fe Fe

CO

C

O

CO

18 e with triple FeFe bond

hv

Fe(CO)3 Fe(CO)3

N C

H

O

Ph

Me H2O

POCl3+

O

H

Fe(CO)3

O

H

Fe(CO)3 Fe(CO)3

Fe(CO)3

CH2OH CH2Cl

CH3

BH4- HCl

LiAlH4


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3.1.2.3 Release of free cyclobutadiene Low temperature oxidation with mild oxidants such as Cerium(+4) oxidises the Fe atom and releases free cyclobutadiene. This is highly reactive and it is prepared in situ with the reactive substrate. This reaction may be used in organic synthesis. A nice example is the preparation of cubane in E/S p315

2. Unstable molecule - Trimethylene methane This molecule C4H6 is an isomer of butadiene. It is extremely unstable in the free state since it doesn't obey the conventional rules of bonding.

Fe(CO)3

CH2Cl

Fe(CO)3

+

Fe(CO)3

Fe(CO)3

CH2+ CH2

CH2OH

SbCl5 HCl

OH-

H2C C

CH2

CH2

butadiene trimethylene methane


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Stable complexes of this ligand (and derivatives) can be easily made, in an extension of the routes used to make the cyclobutadiene complexes.

FeCl2 is formed as a byproduct from halide abstraction. The four C atoms are not quite coplanar so the shape is a little like an umbrella, with the exterior CH2 groups bending towards the Fe atom. The whole ligand acts as a four electron donor

The crystal structure of this complex is shown above. The six H atoms are equivalent and the complex adopts a staggered geometry, as shown by a view down the 3-fold axis

How is this odd molecule bonded to the transition metal ? Need to consider the lowest lying π-orbitals

H2C CCH2Cl

CH2Cl+ Fe2(CO)9

H2CCH2

Fe(CO)3

CH2

Fe

COOC

CO

H H

H

HH

H

e symmetry non-bonding

a1 symmetry bonding


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The a1 is the σ-donor orbital and the e set are the π-acceptor orbitals. The Fe(CO)3 fragment has a similar set (as far as symmetry is concerned) of orbitals which provide a perfect match

The bonding is thus the "classical" σ-donotion from ligand and π-back-donation from metal. This orbital approach provides a delocalised view of the bonding and avoids the problems with the conventional view of localised bonds. The coordination geometry of the Fe atoms is roughly octahedral and there is a high barrier to rotation about the 3-fold axis. This barrier cannot be observed in the Fe(CO)3 complex, because all 6 protons are equivalent anyway. In order to observe any barrier, we need to make them chemically inequivalent. So choose an ML3 fragment which does not have 3-fold symmetry

All six H's are now chemically inequivalent (the compound is chiral) and even at elevated temperatures, there are 6 signals between -0.1 and 3.5 ppm in the 1H NMR spectrum, showing that if rotation occurs that it must have a high barrier. 3. Unstable molecule - Benzyne This molecule is highly strained and hence very reactive, but is stable in an inert matrix at 8o K. The alkyne group should be linear - hence the strain. Benzyne can be made in situ by abstraction of HCl from chlorobenzene, using a very strong base such as sodamide NaNH2. The generated benzyne may be trapped using a diene such as cyclopentadiene by the Diels-Alder reaction

high lying sigma acceptor

sp hybrid

high lying pi-acceptor and low lying pi-donor

derived from eg

IrCl(CO)(PPh3) + H2CCH2SiMe3

CH2Cl

Ir

PPh3OC

Cl

H H

H

HHH

- SiMe3Cl

Benzyne


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Simple and stable benzyne complexes of transition metals are known, for example

Benzyne has obvious similarities to alkynes in its complexes. Quite a few are known for cluster compounds

Os3(CO)12 + C6H6 → H2Os3(CO)9(C6H4) The alkyne "dances" around the three Os-Os edges

4. Unstable molecule - E2 (E=As,Sb, Bi) analogues of N2 While N2 is the stable form of elemental nitrogen, the corresponding molecules P2, As2, Sb2 and Bi2 are not stable at room temperature. While P2, As2, Sb2 have been observed in mass spectra at high temperatures, Bi2 is wholly unknown. However, when coordinated to transition metals, these molecules may be isolated at room temperature.

The Bi2 ligand with a formal Bi≡Bi triple bond is analogous to alkynes RC≡CR

Ta

H3C

H3CCH3

Ta

H3C

H3C

heat

- CH4

(OC)3Co Co(CO)3

CH

CH

(OC)3Co Co(CO)3

Bi

Bi


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The "Mercedes Benzenes" have a similarly E2 group (E=As, Sb, Bi) coordinated to three M(CO)5 groups (M = Mo, W). They are made by the simple reaction

ECl3 + [M2(CO)10]2- → E2[M(CO)5]3

The structures of typical examples are shown below, both schematically and in 3D.

Mercedes Benzene


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Topic 4 - Reactivity of Coordinated Olefin Complexes 4.1 Introduction The coordination of ligands to metals can, in general, quite substantially change not only the stability (as shown in previous topic) but also the reactivity of the ligand. The reason for this is that coordination to a metal changes the electron distribution within the ligand. As an example to show why this happens, consider the case of butadiene

The ligand π-orbitals (frontier orbitals) are the most important in bonding to the metal. Assuming the conformation found in complexes, we get

(OC)3Fefree ligand

ENERGY

LUMO

Ψ-4

Ψ-3

Ψ-2

Ψ-1

(δ)

(π)

(π)

(σ)

π-orbitals of butadiene

(+)

(+)

(+)

(+)

(+)(+)

(+)

(+)

(+)

(+)

(-)(-)

(-)

(-)

(-)

(-)

coefficients are either 0.37 or 0.60

HOMO


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The coefficients of each basis p-orbital on the C atoms tells us qualitatively about the bonding/antibonding nature of each of the orbitals • Orbital Ψ-1 provides π-bonding between all atoms, but more so between the

two inner C-atoms. • Orbital Ψ-2 provides π-bonding between the inner and outer C-atoms, but is

antibonding between the two inner atoms. • Orbital Ψ-3 provides π-bonding between the two inner C-atoms, but is

antibonding between the inner and outer C-atoms. • Orbital Ψ-4 is antibonding between all C-atoms but is not occupied or used. For the ground state, the bonding is the sum of Ψ-1 and Ψ-2, which leads to a higher π-bond order between the inner and outer atoms (as expected from conventional ideas). For the excited state, the partial population of Ψ-3 and partial depopulation of Ψ-2 leads to an inversion of π-bond orders compared with the ground state. This is the simple Hückel picture of bonding. Coordination of butadiene to a metal fragment such as Fe(CO)3 uses the same orbitals as previously shown

Ψ-1

Ψ-2

Ψ-3

Ψ-4

π-bond order bond lengths

0.89

0.44 1.48Å

1.34Å

ground state

Ψ-1

Ψ-2

Ψ-3

Ψ-4

π-bond order bond lengths

0.44

0.72 1.39Å

1.45Å

excited state

high lying sigma acceptor

sp hybrid

high lying pi-acceptor and low lying pi-donor

derived from eg


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These orbitals interact with the ligand orbitals to give delocalised bonds of σ- and π-symmetry (the δ-orbital is too high lying and has no symmetry match with any metal orbital). The synergic bonding between metal and ligand redistributes the electron population of the butadiene in a similar fashion as that explained above for the excited state. Thus qualitatively we can see that : (a) changes in the relative populations of the frontier orbitals of the ligand can change the bonding density in the ligand. Some examples show the effects on the bond lengths.

The arrows show the direction of flow of electrons and the numbers are the bond lengths in Ă units. The Fe fragment is electron rich and the Zr fragment is electron deficient. In the case of the Fe compound there is thus a small population of the LUMO of butadiene, while in the Zr compound there is a small withdrawal from the HOMO of butadiene. In terms of the valence bond approach, it is possible to view the bonding as arising from two extreme canonical forms

Early transition metals such as Zr which are electron deficient are more like B, while later transition metals such as Fe are electron rich and more like A (b) The orbital approach also allows us to rationalise the site of attack of the incoming nucleophile. The LUMO of butadiene has most of the wavefunction on the outer C-atoms and the lone pair of the nucleophile will seek this, hence leading to preferential attack at the outer carbon atom. The reaction of nucleophile with electrophile is a Lewis acid-base interaction.

(OC)3Fe

1.41

1.45

Cp2Zr

1.45

1.40

MLn MLn

A B

Nu

better overlap here than at the inner C atom


27

A number of MO studies of individual complexes has led to a set of simple rules for determining the site of nucleophilic attack at unsaturated hydrocarbon ligands in cationic metal complexes. 4.2 Green/Mingos/Davies Rules (GMD) for Nucleophilic Addition Normally, unsaturated hydrocarbons are not at all susceptible to nucleophilic addition. On coordination into a CATIONIC metal complex this changes, and attack by nucleophiles to give addition products is well known. The increase in susceptibility to nucleophilic attack arises because there is a net flow of electrons from the ligand to the metal (i.e akin to introducing electron withdrawing substituents). An example from organic chemistry of this enhancement is the attack on the bromonium ion by the relatively weak nucleophile Br- during the bromination of alkenes.

The nucleophilic additions to transition metal complexes are generally very regio-specific, i.e only one out of a possible number of products are formed. For attack on 18-electron organometallic cations, where reactions are kinetically controlled, the most favourable position for nucleophilic attack is given by the GMD rules. Some definitions are needed first. Hydrocarbon ligands described as even or odd and as open or closed Even n = 2,3,4 ...... Odd n = 3,5,7 .... refers to the η number, i.e. how many C-atoms of ligand are

bonded to the metal Open not cyclically conjugated Closed cyclically conjugated Rule1 Nucleophilic attack prefers EVEN polyenes with no unpaired

electrons in their HOMO.Cyclo-C4R4 only common example) EVEN before C4R4 before ODD

Rule2 OPEN before CLOSED

Rule3 For even open polyenes, attack occurs at terminal C-atom.

For odd open polyenes, attack occurs at terminal C-atom only if MLn is strongly electron withdrawing

Br

Br-


28

Rules are best illustrated by examples

Even before odd. NOTE that the nucleophile attacks from the EXO position

Open before closed Attack occurs at terminal C-atom of conjugated system

M M M

eta-5 eta-5 eta-5

odd closed odd open odd open

M M M

even closed even open even open

eta-6 eta-4 eta-4

Fe

eta-5 odd

eta-6 even

FeFe

H

Et

cyclohexadienyl

Et

H

EtMgCl

Et

Rh

eta-5 closed

eta-5 open

Rh

H

H

NaBH4

H

Rh


29

Rules need to be applied sequentially. Consider the case of the Mo+ cation shown below when treated with hydride source (BH4

-)

Rule 1 even before odd - therefore allyl NOT attacked Rule 2 open before closed - therefore butadiene attacked Rule3 terminal C-atom - therefore product the methylallyl complex Another way of expressing Rules 1 & 2 is that the order of reactivity of unsaturated hydrocarbons coordinated to cations is as shown below

It can be seen that Cp is the least reactive - this explains it's stability and role as a "spectator" ligand. It may be used to design complexes for nucleophilic addition to even ligands and allyl and pentadienyl ligands. CAVEAT Cations which contain only ONE unsaturated hydrocarbon ligand and at least ONE carbonyl ligand may undergo nucleophilic attack at the CO. This most often occurs with nucleophiles having heteroatoms as the nucleophilic site (e.g. methoxide -OCH3) and when choice is between a CO and an eta-5 ligand.

When there is more than one unsaturated ligand, the normal MGD rules apply again, e.g. for [(C5H5)Mo(CO)3(C2H4)]

+ attack occurs at the ethene ligand.

Mo

eta-6 even closed

eta-3 odd open eta-4 even open

Mo

Me

BH4

Os

OCCO

CO

Os

OCCO

O

OMe

OMe

Os

OCCO

CO

slow isomerisation

MeO


30

Topic 5 - Carbene and Carbyne Complexes Textbooks : E/S pages 210 - 220 5.1 Introduction Carbene :CH2 is a highly reactive and unstable molecule. Carbene itself can be made by the thermal decomposition of diazomethane

CH2N2 → N2 + :CH2 → (CH2)n polymerises to "polythene" Chlorocarbene is also easily made

CHCl3 + strong base → :CCl2 (reactive intermediate) Carbenes may be stabilised by coordination to transition metals, as in this

example of the formation of a carbene complex directly from carbene. This reaction is the organometallic analogue of the reaction of carbenes with alkenes to give cyclopropanes. Carbenes are thus further example of unstable molecules which are stabilised by coordination to transition metals. Relatively recently, stable carbenes at room temperature have been synthesised by Arduengo - contain N-heterocycles.

The terms carbene complex and alkylidene complex and also carbyne complex and alkylidyne complex are used interchangeably. 5.2 Fischer Synthesis of Carbene Complexes Carbene complexes were first made in 1964 by E.O. Fischer who subsequently won a Nobel Prize for his work. The synthesis is based on the nucleophilic addition of heteroatom nucleophiles to coordinated CO (see last part of Topic 4).

Rh Rh

OC

CO

+ CH2N2 Rh Rh

OC

COCH2

NNRR


31

The lithium intermediate is unstable and methylation with Me+ reagents result in thermally stable and isolable carbene complexes. Many examples of this type are now known, most with OR or NR2 substituents. These have several resonance forms.

The latter canonical form bears a positive charge on the carbene carbon, and so may be expected to be electrophilic at this atom. 5.3 Schrock Synthesis of Carbene Complexes This is the other very important route to carbene complexes, of quite a different character to those made by Fischer. The reaction was the treatment of the

neopentyl (Np) alkyl complex of tantalum Np3TaCl2 with two moles of the lithium reagent LiNp. The original purpose of the experiment was the synthesise TaNp5 but the actual product was much more interesting (as is often the case in organometallic chemistry). The first stage involves an α-deprotonation step, whereby a proton from one neopentyl group is transferred to another neopentyl group. This releases the volatile hydrocarbon neopentane (tetramethyl methane) which is one of the driving forces for this reaction. This α-deprotonation of alkyl groups ONLY occurs with very sterically bulky groups and is unusual. A related reaction forming carbenes is that of hydride abstraction from methyl groups

WOC

OC CO

CO

CO

CO

WOC

OC CO

CO

CO

C

WOC

OC CO

CO

CO

C

R OLi R OCH3

(i) (ii)

(i) nucleophilic addition to CO with LiR(ii) methylation using Me3O+ BF4-

LnM C

OR

R

LnM C

OR

R

LnM C

OR

R

(Me3CH2)3Ta

Cl

Cl

TaNp5(Np)2ClTa

CH2CMe3

HC

H

CMe3

Np2Ta

Cl

C

CMe3

H

- CMe4

LiNp

LiNpTaNp3 C

H

CMe3

Np = neopentyl = Me3CCH2-


32

The two research groups of Fischer and Schrock have developed the chemistry of these carbenes, which are intrinsically different. The most important differences lie in their reactivity. • Fischer carbenes have π-donor hetero-substituents and are electrophilic at

the carbene • Schrock carbenes have alkyl (or H) substituents and are nucleophilic at the

carbene 5.4 Evidence for Multiple Bonding 1. X-ray Structures. These show (i) planar trigonal sp2 C

(ii) M=C bonds is shorter than a single bond, but not as short as M-CO bond

2. 13C NMR chemical shifts are in the region 200-400 ppm - cited as evidence for a δ(+) charge on C 3. MO picture predicts a barrier to free rotation in Fischer carbenes, which is observed in NMR spectra. Also the observed inequivalence of CR2 groups in Schrock carbenes in the NMR spectra also indicates a very high barrier to rotation about M=C bond

Re

L

ONCH3

Re

L

ONCH2

Ph3C BF4 + Ph3CH


33

5.5 Why the distinction between Fischer and Schrock carbenes ? The table below lists their general properties and differences Property Fischer Schrock

Nature of carbene carbon Electrophilic Nucleophilic Typical R group π-donor (e.g. -OR) Alkyl, H Typical metal Mo(0), Fe(0) Tav(V), W(VI) Typical ancilliary ligands Good π-acceptors - CO Cl, Cp, alkyl Electron count 2e 2e Oxidation state change on addition of CR2 to metal

0 +2

Actually not easy to provide a convincing explanation for the difference. The MO scheme suggests that while both ligands are effective σ-donors • the Fischer carbene is a π-acid • the Schrock carbene is a π-base


34

5.6 Reactivity of Fischer carbenes Fischer carbenes are readily susceptible to nucleophilic substitution

The analogy between metal carbenes and organic ketones is quite strong. For another example, compare their reactions with P-ylides:

Another important reaction is with alkenes to give metallocycles, which in turn decompose to give re-arranged alkenes. This is the so called "alkene metathesis" reaction, which is of major commercial importance. The resultant olefin has the =CPh(OMe) group from the carbene complex and the =CH2 group from the starting CH2=CH(OR) alkene. This re-arrangement involves breaking a very strong C=C double bond, and is impossible without the presence of the organometalic carbene catalyst

(OC)5Cr

CH3

OCH3

(OC)5Cr

CH3

NHEt

(OC)5Cr

CH3

Ph

NH2Et

+ MeOH

+ MeOH

PhLi

(OC)5Cr

CH3

Ph

OCH3Mechanism involves the intermediate

Similar to amminolysis of esters

O

CH3

OCH3

NH2R O

CH3

OCH3

O

CH3

NHR

NHR

H

(OC)5W

Ph

OCH3

H2C

Ph

OCH3Ph3P=CH2+ (CO)5WPPh3

compare with Wittig reaction

O

Ph

Ph

H2C

Ph

PhPh3P=CH2+ OPPh3


35

Carbene complexes are excellent catalysts for the general alkene metathesis reaction

R2C=CR'2 + R''2C=CR'''2 → R2C=CR''2 + R'2C=CR'''2 etc 5.7 Reactivity of Schrock carbenes Schrock carbenes behave much like ylides

They are easily attacked by electrophiles (i.e. behave as nucleophiles) - two example reactions are

One of the most useful carbene compounds is Tebbe's reagent.

(OC)5Cr

Ph

OCH3

OR

(OC)5Cr

C

OR

Ph OCH3

(OC)5Cr

C

Ph OCH3

OR

(OC)5Cr

H

OR

Ph

OCH3

+


36

The reagent is released by the treatment with ammine bases NR3. It behaves as a carbene transfer reagent, and has a number of specialised uses.

"Cp2Ti=CH2" + RC(O)OR' → R(OR')C=CH2 This has the useful advantage over Wittig's reagents in the same reaction in that it works !!. When used with enolisable ketones it does not result in racemisation.

While the distinction between Fischer and Schrock carbenes is a useful one, it should be realised that this distinction is not rigid, but merely represents two extremes. An example of an "in-between" carbene is Roper's carbene based on osmium, which shows both types of reactivity, depending on the substrate. SO2 here is acting as an electrophile, while CO is acting as a nucleophile. The "in-between" character of Roper's carbene is sensible in terms of the descriptions given in the Table, as there are both π-donors (chloride ligand) and π-acceptors (nitrosyl ligand) and the carbene carbon has no π-donor substituents. 5.8 Carbyne complexes Carbyne complexes have a CR group attached to a metal atom, with a triple M≡C bond. Their chemistry is closely related to those of carbenes :


37

The evidence for a triple bond comes from crystal structures, which show a very short M-C bond. Moreover the M-C-R angle is close to 180o indicative of linear sp hybridisation. The bonding of the C atom to a metal is quite similar to that of CO, with a sigma-donation and pi-back donation to the (unhybridised) p-orbitals of the carbyne atom

The reactions of carbyne complexes show some similarities with carbene complexes. Thus alkyne metathesis with carbyne complexes as catalysts is possible.


38

This is usually only possible with alkyl or aryl R groups. Carbyne complexes may be thought of as "half-inorganic" analogues of alkynes

For instance the analogy is obvious by comparing these reactions

The PtW2 compound can also be viewed as a cluster of 3 metals with two bridging CR groups

In fact, carbynes (or alklidynes) are very common as bridging ligands. One particular well know series of very stable compounds can be easily made Co2(CO)8 + CRX3 (X=Cl, Br, I) → CoX2 + Co3(µ3-CR)(CO)9 - "Fred" A huge variety of derivatives with different R groups have been made and extensive chemistry is known. The structure of the simplest CH derivative is shown. The three cobalt atoms and the bridging carbon make up a tetrahedron


39

Using the reactions described above it was possible to synthesise a related compound with three different metal atoms. Because the vertices of the tetrahedron were all different, the compound is chiral and it is possible to resolve these chiral clusters by making derivatives with homo-chiral phosphines PRR'R''. They have proved to be some of the most chiral molecules made (i.e. have the highest molar rotation coefficients).

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