L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
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).
H
H H
Fe CO H OC CO Cyclopentadiene Cyclopentadiene complex (4 e donor ligand) very rare as a ligand
Na / H2 H - Na+ H acidic H atom planar aromatic (6 pi electron) dienyl anion The anion C 5H5 is a very useful synthetic reagent. It is usually treated as equivalent to occupying THREE coordination sites, so that C 5H5 ≡ 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 a 1 + e 1 + e 2 under five fold symmetry. With a single Cp ring,
1 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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(C 5H5)2 the rings may be either staggered or eclipsed - Fe(C 5H5)2 is eclipsed but Co(C 5H5)2 and Ni(C 5H5)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(C 5H5)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(C 5H5)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(C 5H5)2 ONE electron fills the e* 1g while in Ni(C 5H5)2 there are TWO unpaired electrons hence both compounds are paramagnetic
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+ Co(C 5H5)2 readily loses an electron to give the cobalticinium cation [Co(C 5H5)2] an 18e cation. Similarly Ni(C 5H5)2 loses one electron to give a 19e cation, but further oxidation results in decomposition.
1.2 Structural types of Cp compounds (i) Metallocenes MCp 2
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 "TiCp 2"
This is really a fulvalene complex made by reducing Cp 2TiCl 2. 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 MnCp 2 This is ionic at low temperature, with a polymeric chain like structure
Above 159 oC it becomes isomorphous with ferrocene, so must adopt a sandwich structure. All the sandwich metallocenes apart from ferrocene are paramagnetic No. unpaired electrons Cp 2V 3e Cp 2Cr 2e 2+ Cp 2Mn 5e high spin Mn Cp 2Co 1e Cp 2Ni 2e (ii) 'bent' metallocenes These have non parallel rings due to the presence of other ligands. Some examples are
H Cl W Ti V CO Re H H Cl
18 e 16 e 17 e 18 e
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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
OC V CO Mn Ru Co PPh3 OC CO Cl CO OC CO PPh3 OC CO 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 5 So far only the so called eta 5 η C5H5 bonding mode has been illustrated, where (in principle) all five C atoms are equally bonded to the transition metal. However 3 1 there are other possibilities, most common are η C5H5 and η C5H5 H M
M
1 3 η C5H5 η C5H5 1 e donor like alkyl group 3e donor like allyl group
H
Mo
ON
5 One good example is Mo(NO)Cp 3 with "normal" η 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. 1 η C5H5 ligands are usually fluxional. Consider the compound (C 5H5)4Ti again 5 cannot be 4 η C5H5 bonding modes because it would be a 24 e compound. In 5 1 this case it has two η C5H5 and two η C5H5 ligands.
Ti
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At the very lowest temperature measured, the 1H NMR spectrum shows a singlet 5 for the two (equivalent) η C5H5 groups and an AA'BB'C multiplet for the two 1 (equivalent) η C5H5 groups (three different H environments). This compound shows two fluxional processes 1 (i) migration of metal atom around the η C5H5 group via 1,2 shifts (ring whizzing) 1 5 (ii) exchange of the η C5H5 and η 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
M c c M a M a a a b c
a
b b b b b a
1, 2 shift 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 5 1 The actual spectrum of ( η C5H5)( η C5H5)Fe(CO) 2 which exhibits a similar exchange is shown below.
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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 C 5Me 5 .
H2O2
CO Re Re O CO O OC O 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(C 5H5)2 has been the most studied (because it is a stable 18 e metallocene) but ruthenocene Ru(C 5H5)2 and osmocene Os(C 5H5)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 ×10 6 times faster than with benzene.
E E E+
Fe Fe + H Fe H+
E+ is a general electrophile, see E/S p 328 330. A concrete example is a Friedel Crafts acylation
O O C C CH3COCl/AlCl3 Fe Fe Fe + O C
The mechanism probably involves exo attach at the ring C atom, followed by a movement of the proton down to the metal.
E + E E + Fe H Fe H Fe
The hydride intermediate can be made by treating FeCp 2 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 NO 2 . These reagents oxidise the complex to give the
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+ ferricinium cation [FeCp 2] and the +ve charge now on the rings makes electrophilic attack very difficult. Another example and indication of the high reactivity of FeCp 2 is that the Vilsmeier reaction works
O O (i) POCl C 3 H + C Fe Fe MePhN H (ii) H2O
substituted formamides 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.
4.2 1.4
Et for Friedel Crafts acylation
Fe CH3COCl/AlCl3
1.0 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)
O H C H H Fe Fe PhCOMe/base O Ph
aldol condensation H
H Fe R Ph3P=CHR Wittig ylid
O O C
CH3 Fe Fe H RCHO/base H R
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2.3 Metallation of ferrocene Further substituents may be introduced by the very useful reaction with butyl lithium
Li CO2H CO2 Fe Fe Fe BuLi
N2O4
NH2OMe
NO2 2 BuLi NH2 Fe/HCl Fe Fe
Li PPh2 PPh2
Fe Fe Fe Fe(CO)4 PPh2Cl Fe(CO)5 Li PPh2 PPh2 Note that it is not possible to introduce the NO 2 group directly by nitration as the + electrophile NO 2 is oxidising 2.4 Stabilization of ααα-carbonium ions OAc + R OH
R H H O/H+ H OH R Fe 2 H Fe Fe
carbonium ion intermediate The hydrolysis of the aceto substituted ferrocene proceeeds seven times faster than the solvolysis of Ph 3C OAc (this is a classic example of S N1 hydrolysis via the stable carbonium ion Ph 3C+ 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 CH 3CO 2H
+ CH3 OAc
H OAc CH3 H2O/H+ H Fe Fe Fe
carbonium ion intermediate 11 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics
The carbonium ion can actually be isolated and crystallised as the BF 4 or PF 6 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.
Ph overall 6 e donor Ph Fe + 160 deg
Fe crystal structure There is also NMR evidence for the presence of a π bond between the exo C atoms and the Fe atom. Uses 57 Fe 13 C 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(57 Fe-13 C) /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
base
H H indenyl anion Ind Indenyl forms complexes which are similar to Cp, but substitution reactions are much faster (up to 10 8 times !!). Example (Ind)Rh(CO) 2 + PPh 3 → (Ind)Rh(CO)(PPh 3) + 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 + PBu 3 → no reaction (Ind)Mn(CO) 3 + PBu 3 → (Ind)Mn(CO) 2(PBu 3) + CO (Flr)Mn(CO) 3 + PBu 3 → (Flr)Mn(CO) 2(PBu 3) + CO Fluorenyl (Flr) is 250 times faster than the indenyl reaction
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Fluorenyl anion Flr What is the reason for this ? ring slippage shown in the mechanism below eta 5 eta 3 eta 3
PPh3
Rh Rh Rh CO OC CO OC CO OC PPh3 18 e 18 e 16 e eta 5
- CO
Rh
OC PPh3 18 e 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(PMe 2Ph) 2 + PMe 2Ph → (Ind)Ir(PMe 2Ph) 3 18 e 20 e ??
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The X ray structure shows that only 3 C atoms of the C 5 ring are coordinated to the iridium
PMe2Ph
[Ir(PMe2Ph)4]+ Ind Ir
PMe2Ph 16 e PhMe2P PMe2Ph
18 e 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)
N C4H4N N pyrrolyl C4H4P P phospholyl C4H4As As arsolyl These anions form π complexes which are directly analogous to Cp
N heat CpFe(CO)2I + Fe Fe N 2CO OC N OC sigma complex Aza ferrocene
Ph 150 C P [CpFe(CO)2]2 + Fe
P
Phospha ferrocene
Li P P FeCl2 + 2 Fe
P
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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.
P P Fe(CO)4 + 2 Fe(CO)4(THF) Fe Fe Source of 16 e "Fe(CO)4"
P P Fe(CO)4 Thiophene C 4H4S has one extra electron and so behaves as a six electron donor analogous to benzene
S S
Cr(CO)3 Cr(CO)3 Boroles C 4H4BR have one less electron and so behave as four electron donors. So compare Cp 2Fe(CO) 4 with (C 4H4BMe) 2Co(CO) 4 isostructural and iso electronic compounds.
O BMe O C C BMe Fe Fe Co Co
OC C CO OC C CO O O 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.
150 C [Cp*Fe(CO) ] 2 2 + P4 Fe P P P P P Penta phospha ferrocene All the P atoms are equivalent and give a singlet at 153 ppm in the 31 P NMR spectrum.
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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 )
bonding v antibonding slightly antibonding ψ−4 ψ−3, ψ−2 ψ−1
ψ−4
ψ−3, ψ−2
ψ−1 The four π electrons thus give rise to an expected triplet state, which is subject to Jahn Teller distortion. In fact C 4H4 is not square but rectangular with localised double bonds and D2h symmetry, H H t Bu t Bu
HH t Bu H highly reactive stable because of steric restraints In 1955, Longuett Higgins and Orgel predicted theoretically that the aromatic square form of C 4H4 would be stabilised by complexation to a transition metal. The synthesis of such a complex was soon accomplished.
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H C 3 CH3 Cl Cl Cl Ni Ni + Ni(CO)4 Cl Cl Cl H3C CH3 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 NiCl 2 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
Cl
+ FeCl + 6 CO + Fe2(CO)9 2 Fe Cl CO OC CO 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) Cl
+ 2 2 Na [Mo(CO)5] Mo OC CO Cl OC CO 2 The anion [Mo(CO) 5] 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 R R
CO CO R + 2 RC CR R Co Co
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.
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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
+ PPh3
Fe(CO)3 Fe(CO)2(PPh3) hv
O C
Fe Fe
C C O O 18 e with triple FeFe bond 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 O H
Ph H POCl3 + NC Me O H2O Fe(CO)3 Fe(CO)3 Organic functional groups also show the same reactivity as with Cp rings. O CH2OH CH2Cl H BH4 HCl
Fe(CO) Fe(CO) Fe(CO)3 3 3
LiAlH4
CH3
Fe(CO)3 Likewise the stabilisation of the α carbonium ion occurs in a similar fashion to Cp
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CH2+ CH2 CH2Cl SbCl 5 HCl +
Fe(CO) Fe(CO) Fe(CO)3 3 3 OH
CH2OH
Fe(CO)3 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 C 4H6 is an isomer of butadiene. It is extremely unstable in the free state since it doesn't obey the conventional rules of bonding.
CH2 H2CC
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.
CH2Cl CH2 H2C H2CC + Fe2(CO)9 CH2 CH2Cl Fe (CO)3 FeCl 2 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 CH 2 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
HH
OC CO
Fe
H H CO H H How is this odd molecule bonded to the transition metal ? Need to consider the lowest lying π orbitals
e symmetry non bonding
a1 symmetry bonding
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The a 1 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
high lying high lying pi acceptor and low lying pi donor sigma acceptor derived from eg sp hybrid 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 ML 3 fragment which does not have 3 fold symmetry H H
OC PPh3 CH2SiMe3 SiMe3Cl IrCl(CO)(PPh3) + H2C Ir
CH2Cl H Cl H H H 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 8 o 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 NaNH 2. The generated benzyne may be trapped using a diene such as cyclopentadiene by the Diels Alder reaction
Benzyne
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Simple and stable benzyne complexes of transition metals are known, for example
heat
H3C Ta CH4 H3C Ta CH3
H3C H3C
Benzyne has obvious similarities to alkynes in its complexes. Quite a few are known for cluster compounds Os 3(CO) 12 + C 6H6 → H 2Os 3(CO) 9(C 6H4) The alkyne "dances" around the three Os Os edges
4. Unstable molecule - E 2 (E=As,Sb, Bi) analogues of N 2 While N 2 is the stable form of elemental nitrogen, the corresponding molecules P2, As 2, Sb 2 and Bi 2 are not stable at room temperature. While P 2, As 2, Sb 2 have been observed in mass spectra at high temperatures, Bi 2 is wholly unknown. However, when coordinated to transition metals, these molecules may be isolated at room temperature.
(OC)3Co Co(CO)3 (OC)3Co Co(CO)3
CH Bi
C Bi H The Bi 2 ligand with a formal Bi ≡Bi triple bond is analogous to alkynes RC ≡CR
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The "Mercedes Benzenes" have a similarly E 2 group (E=As, Sb, Bi) coordinated to three M(CO) 5 groups (M = Mo, W). They are made by the simple reaction 2 ECl 3 + [M 2(CO) 10 ] → E 2[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
free ligand (OC)3Fe The ligand π orbitals (frontier orbitals) are the most important in bonding to the metal. Assuming the conformation found in complexes, we get
ENERGY
( ) (+) Ψ 4 (δ) (+) ( )
( ) (+) Ψ 3 LUMO (π) ( ) (+)
( ) ( ) 2 Ψ HOMO (π) (+) (+)
(+) (+)
Ψ 1 (σ) (+) (+)
π orbitals of butadiene coefficients are either 0.37 or 0.60
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Ψ 4 0.44 1.48Å Ψ 3 0.89 1.34Å Ψ 2 π bond order bond lengths Ψ 1
ground state
Ψ 4 0.72 1.39Å Ψ 3 0.44 1.45Å Ψ 2 π bond order bond lengths Ψ 1
excited state 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
high lying high lying pi acceptor and low lying pi donor sigma acceptor derived from eg sp hybrid
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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. 1.41 1.45 1.45 1.40
(OC)3Fe Cp2Zr 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
MLn MLn
A B 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. Nu
better overlap here than at the inner C atom
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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.
Br
Br
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 C 4R4 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 ML n is strongly electron withdrawing
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eta 5 eta 5 eta 5
MMM
odd closed odd open odd open
MMM
even closed even open even open
eta 6 eta 4 eta 4 Rules are best illustrated by examples
eta 5 odd Et
H EtMgCl Fe Fe Fe
Et cyclohexadienyl
H eta 6 even
Et Even before odd. NOTE that the nucleophile attacks from the EXO position eta 5 closed H
H
NaBH4 Rh Rh Rh H
eta 5 open Open before closed Attack occurs at terminal C atom of conjugated system
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Rules need to be applied sequentially. Consider the case of the Mo+ cation shown below when treated with hydride source (BH 4 )
eta 6 even closed
BH4
Mo Mo
eta 3 odd open eta 4 even open Me 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 OCH 3) and when choice is between a CO and an eta 5 ligand. MeO
OMe slow isomerisation
Os Os O Os CO CO OC OC OC CO CO OMe CO When there is more than one unsaturated ligand, the normal MGD rules apply + again, e.g. for [(C 5H5)Mo(CO) 3(C 2H4)] attack occurs at the ethene ligand.
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Topic 5 - Carbene and Carbyne Complexes Textbooks : E/S pages 210 220
5.1 Introduction Carbene :CH 2 is a highly reactive and unstable molecule. Carbene itself can be made by the thermal decomposition of diazomethane CH 2N2 → N 2 + :CH 2 → (CH 2)n polymerises to "polythene" Chlorocarbene is also easily made CHCl 3 + strong base → :CCl 2 (reactive intermediate) Carbenes may be stabilised by coordination to transition metals, as in this O OC C
Rh Rh + CH2N2 Rh Rh C O C H2 CO 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.
R R N N
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).
30 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics CO CO CO OC CO (i)OC CO (ii) OC CO W W W OC CO OC CO OC CO CO C C
R OLi R OCH3
(i) nucleophilic addition to CO with LiR (ii) methylation using Me3O+ BF4 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 NR 2 substituents. These have several resonance forms.
OR OR OR LnM C LnM C LnM C
R R R 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
CMe3 HC Cl LiNp (Np)2ClTa H TaNp5 (Me3CH2)3Ta C Cl H2CMe3
Np = neopentyl = Me3CCH2 CMe4 CMe3 H LiNp C H TaNp3 C Np2Ta CMe3 Cl neopentyl (Np) alkyl complex of tantalum Np 3TaCl 2 with two moles of the lithium reagent LiNp. The original purpose of the experiment was the synthesise TaNp 5 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
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Ph3C BF4 + Ph3CH
Re Re ON ON CH3 CH2 L L 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 sp 2 C (ii) M=C bonds is shorter than a single bond, but not as short as M CO bond
2. 13 C 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 CR 2 groups in Schrock carbenes in the NMR spectra also indicates a very high barrier to rotation about M=C bond
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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 0 +2 on addition of CR 2 to metal
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
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5.6 Reactivity of Fischer carbenes Fischer carbenes are readily susceptible to nucleophilic substitution
NHEt OCH3 NH2Et (OC)5Cr (OC)5Cr + MeOH
CH3 PhLi CH3
Ph
(OC)5Cr + MeOH
CH3 Ph Mechanism involves the intermediate (OC) Cr 5 OCH3
CH3
Similar to amminolysis of esters
OCH3 OCH3 NHR O NH R O H 2 N O CH HR 3 CH3 CH3 The analogy between metal carbenes and organic ketones is quite strong. For another example, compare their reactions with P ylides: OCH OCH 3 Ph3P=CH2 3
(OC)5W H2C + (CO)5WPPh3 Ph Ph
compare with Wittig reaction
Ph Ph3P=CH2 Ph O + OPPh H2C 3
Ph Ph 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 =CH 2 group from the starting CH 2=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
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OR
OCH3 (OC) Cr (OC)5Cr 5
Ph C OR
Ph OCH3
OR
OR OCH3
(OC)5Cr + (OC)5Cr Ph H C
Ph OCH3 Carbene complexes are excellent catalysts for the general alkene metathesis reaction R2C=CR' 2 + R'' 2C=CR''' 2 → R 2C=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.
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The reagent is released by the treatment with ammine bases NR 3. It behaves as a carbene transfer reagent, and has a number of specialised uses. "Cp 2Ti=CH 2" + RC(O)OR' → R(OR')C=CH 2 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. SO 2 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 :
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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 180 o 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.
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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 PtW 2 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 Co 2(CO) 8 + CRX 3 (X=Cl, Br, I) → CoX 2 + Co 3( 3 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
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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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