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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 Cyclopentadiene CO Cyclopentadiene complex (4 e donor ligand) very rare as a ligand H H Na / -H2 - Na+ acidic H atom planar aromatic (6-pi electron) dienyl anion 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 5e 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, 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 a1 e1 e2 Metal orbitals pz dz2, s dxz dyz px py dx2-y2 dxy Symmetry of bond σ π 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 2 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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 3 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics + 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 4 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics (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 3e 2e 5e 1e 2e Cp2V Cp2Cr Cp2Mn Cp2Co Cp2Ni high spin Mn2+ (ii) 'bent' metallocenes These have non-parallel rings due to the presence of other ligands. Some examples are H W Cl Ti H 18 e V CO Re Cl 16 e 17 e 5 18 e H L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics (iii) half-sandwich compounds - piano stool compounds These have one Cp ring and a variety of other ligands. Some examples are V OC CO OC CO 4-legged stool Mn Ru Co PPh3 CO Cl PPh3 OC CO 3-legged stool Chem-3 lab OC CO 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 η1-C5H5 H M M η1-C5H5 1 e donor - like alkyl group η3-C5H5 3e donor - like allyl group H Mo ON One good example is Mo(NO)Cp3 - with "normal" η5-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 η1-C5H5 ligands. Ti 6 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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 η5-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 M c M a a a a c b 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 The actual spectrum of (η5-C5H5)(η1-C5H5)Fe(CO)2 exchange is shown below. 7 which exhibits a similar L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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 pidonor, 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 . H2O2 CO Re CO Re O OC O CO Re(+7) oxo complex Re (+1) 8 O L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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 E+ Fe E H Fe + Fe - H+ E+ is a general electrophile, see E/S p 328-330. A concrete example is a FriedelCrafts 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. + Fe E E H + Fe H E Fe 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 9 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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 O O C (i) POCl3 H C + Fe MePhN Fe (ii) H2O H 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) H O C H H Fe Fe PhCOMe/base O Ph aldol condensation H H Fe R Ph3P=CHR Wittig ylid O O C CH3 Fe H Fe RCHO/base H R 10 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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 N 2O4 NH2OMe 2 BuLi NH2 NO2 Fe/HCl Fe Fe Li PPh2 Fe PPh2 Fe Fe Fe(CO) 4 Fe(CO) 5 PPh2Cl Li PPh2 PPh2 Note that it is not possible to introduce the NO2 group directly by nitration as the electrophile NO2+ is oxidising 2.4 Stabilization of α-carbonium ions OAc Fe H + R R H H2O/H+ OH OH- Fe H Fe R carbonium ion intermediate 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 + Fe CH3 H H2O/H+ Fe OAc OAcFe carbonium ion intermediate 11 H CH3 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics The carbonium ion can actually be isolated and crystallised as the BF4- or PF6salts. 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 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. J(57Fe-13C) /Hz ~9 1.5 - 4.5 Type of bond σ Fe-C π Fe-C 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 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 12 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics Fluorenyl anion Flr What is the reason for this ? - ring slippage shown in the mechanism below eta-5 eta-3 eta-3 PPh3 Rh OC 18 e Rh CO OC Rh OC CO 18 e 16 e CO PPh3 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(PMe2Ph)2 + PMe2Ph → (Ind)Ir(PMe2Ph)3 18 e 20 e ?? 13 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics The X-ray structure shows that only 3 C atoms of the C5 ring are coordinated to the iridium PMe2Ph [Ir(PMe2Ph)4]+ Ind- Ir 16 e PMe2Ph 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 C4H4NC4H4PC4H4As- N P As pyrrolyl phospholyl arsolyl These anions form π-complexes which are directly analogous to Cp N CpFe(CO)2I heat + Fe - 2CO N N OC OC sigma-complex Aza - ferrocene Ph [CpFe(CO)2]2 P 150 C + Fe P Phospha - ferrocene Li P P FeCl2 Fe +2 Fe P 14 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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 P Fe(CO)4 + 2 Fe(CO)4(THF) Fe Fe Source of 16 e "Fe(CO)4" P Thiophene C4H4S has one extra electron and so behaves as a six-electron donor analogous to benzene S S Cr(CO)3 Cr(CO)3 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 isoelectronic compounds. BMe O C O C Fe OC Fe C O Co OC CO BMe Co C O CO 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 spectrum. 15 31 P NMR L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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 slightly antibonding v 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 C4H4 is not square but rectangular with localised double bonds and D2h symmetry, H H t-Bu H t-Bu H highly reactive t-Bu H stable because of steric restraints 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. 16 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics H3C CH3 Cl Cl Cl Ni + Ni(CO)4 Cl H3C Ni Cl Cl 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 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 Cl + FeCl2 + Fe2(CO)9 + 6 CO Fe Cl OC CO 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 Na [Mo(CO)5] 2- OC OC Cl Mo CO CO 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 R CO Co R CO + 2 RC CR R Co R 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 πsystem. 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. 17 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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 + N Me Fe(CO)3 H POCl3 O H2O C Fe(CO)3 Organic functional groups also show the same reactivity as with Cp rings. O CH2OH H BH4- Fe(CO)3 CH2Cl HCl Fe(CO)3 Fe(CO)3 LiAlH4 CH3 Fe(CO)3 Likewise the stabilisation of the α-carbonium ion occurs in a similar fashion to Cp 18 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics CH2+ CH2Cl CH2 SbCl5 Fe(CO)3 HCl + Fe(CO)3 Fe(CO)3 OHCH2OH 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 C4H6 is an isomer of butadiene. It is extremely unstable in the free state since it doesn't obey the conventional rules of bonding. H2C C CH2 CH2 butadiene trimethylene methane 19 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics Stable complexes of this ligand (and derivatives) can be easily made, in an extension of the routes used to make the cyclobutadiene complexes. CH2Cl H2C CH2 H2C + Fe2(CO)9 C CH2 CH2Cl Fe (CO)3 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 H H 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 20 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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 high lying sigma acceptor high lying pi-acceptor and low lying pi-donor 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 ML3 fragment which does not have 3-fold symmetry H H CH2SiMe3 IrCl(CO)(PPh3) OC - SiMe3Cl PPh3 Ir + H2C CH2Cl H H H Cl 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 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 Benzyne 21 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics Simple and stable benzyne complexes of transition metals are known, for example heat H3C Ta CH3 H3C - CH4 Ta H3C H3C 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. Co(CO)3 (OC)3Co Co(CO)3 (OC)3Co CH Bi C H Bi The Bi2 ligand with a formal Bi≡Bi triple bond is analogous to alkynes RC≡CR 22 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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 23 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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) Ψ-2 HOMO (π) (+) (-) (-) (-) (+) (+) (+) (+) Ψ-1 (+) (+) π-orbitals of butadiene coefficients are either 0.37 or 0.60 24 (σ) L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics Ψ-4 0.44 Ψ-3 1.48Å 1.34Å 0.89 Ψ-2 π-bond order Ψ-1 bond lengths ground state Ψ-4 0.72 Ψ-3 1.39Å 1.45Å 0.44 Ψ-2 Ψ-1 π-bond order bond lengths 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 Ψ2 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 sigma acceptor high lying pi-acceptor and low lying pi-donor derived from eg sp hybrid 25 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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. 1.45 1.41 1.40 1.45 Cp2Zr (OC)3Fe 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 B A 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 26 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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. BrBr The nucleophilic additions to transition metal complexes are generally very regiospecific, 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 27 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics eta-5 eta-5 M eta-5 M odd closed odd open odd open M M M even closed eta-6 M even open even open eta-4 eta-4 Rules are best illustrated by examples eta-5 odd Et H EtMgCl Fe Fe Et Fe cyclohexadienyl H eta-6 even Et Even before odd. NOTE that the nucleophile attacks from the EXO position eta-5 closed H H Rh NaBH4 Rh Rh H eta-5 open Open before closed Attack occurs at terminal C-atom of conjugated system 28 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics Rules need to be applied sequentially. Consider the case of the Mo+ cation shown below when treated with hydride source (BH4-) eta-6 even closed BH4 Mo Mo eta-3 odd open eta-4 even open Me Rule 1 Rule 2 Rule3 even before odd - therefore allyl NOT attacked open before closed - therefore butadiene attacked 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. MeO OMe slow isomerisation Os Os Os O CO OC CO CO OC CO OMe OC CO 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. 29 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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 OC O C Rh Rh + CH2N2 Rh C O Rh 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 N N R 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 OC CO W OC CO CO (i) OC OC (ii) CO CO OC OC CO CO CO W W CO C C R R OLi 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 NR2 substituents. These have several resonance forms. OR OR LnM LnM C OR C LnM R R C 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 Cl TaNp5 CMe3 HC LiNp (Np)2ClTa (Me3CH2)3Ta H C H2CMe3 Cl Np = neopentyl = Me3CCH2H TaNp3 C CMe3 CMe3 C LiNp - CMe4 H Np2Ta Cl 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 31 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics + Ph3CH Ph3C BF 4 Re ON L Re CH3 ON L CH2 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 32 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 5.5 Why the distinction between Fischer and Schrock carbenes ? The table below lists their general properties and differences Property Nature of carbene carbon Typical R group Typical metal Typical ancilliary ligands Electron count Oxidation state change on addition of CR2 to metal Fischer Electrophilic π-donor (e.g. -OR) Mo(0), Fe(0) Good π-acceptors - CO 2e 0 Schrock Nucleophilic Alkyl, H Tav(V), W(VI) Cl, Cp, alkyl 2e +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 33 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 5.6 Reactivity of Fischer carbenes Fischer carbenes are readily susceptible to nucleophilic substitution OCH3 NHEt NH2Et (OC)5Cr (OC)5Cr CH3 + MeOH CH3 PhLi Ph (OC)5Cr + MeOH CH3 Ph Mechanism involves the intermediate (OC)5Cr OCH3 CH3 Similar to amminolysis of esters OCH3 O OCH3 H O NH2R N HR CH3 CH3 NHR O CH3 The analogy between metal carbenes and organic ketones is quite strong. For another example, compare their reactions with P-ylides: OCH3 OCH3 Ph3P=CH2 H2C (OC)5W + (CO)5WPPh3 Ph Ph compare with Wittig reaction Ph Ph3P=CH2 O Ph + OPPh3 H2C 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 =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 34 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics OR OCH3 (OC)5Cr (OC)5Cr C Ph OR Ph OCH3 OR OR OCH3 (OC)5Cr + (OC)5Cr H Ph C Ph OCH3 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. 35 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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 : 36 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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. 37 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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 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 38 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 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). 39