Mark J. Winter

The sequential linking of alkynes at dichromium and dimolybdenum centres; X-ray crystal structure of [Cr2(CO)(µ-C4Ph4)(η-C5H5)2]

In heptane at reflux, the alkynes RCCR (R = Ph, H, or CO2Me) react with [Cr2(CO)4(η-C5H5)2] to give complexes [Cr2(CO)(µ-C4R4)(η-C5H5)2]. An X-ray diffraction study on the product from PhCCPh shows that the crystals of [Cr2(CO)(µ-C4Ph4)(η-C5H5)2] when grown from dichloromethane–hexane incorporate ¼CH2Cl2 per molecule of complex and are orthorhombic, with Z= 8 in a unit cell of dimensions a= 19.569(4), b= 19.731(5), c= 16.637(2)A, and space group Pbcn(no. 60). The structure has been solved by heavy-atom methods from 3 166 data for which l 3.0σ(l), collected on a four-circle diffractometer, and refined to R 0.066. The axis of the molecule comprises a (η-C5H5)CrCr(η-C5H5) moiety which is non-linear, with the cyclopentadienyl rings in an unsymmetrical trans relationship to one another, and with a carbonyl ligand semi-bridging in a plane which is effectively a mirror plane for the whole molecule. Two PhCCPh molecules have joined to form a four-carbon chain, of which the two terminal atoms form a quasi-tetrahedral group with the two chromium atoms [CrCr 2.337(2), Cr–C 2.025(7) mean, C ⋯ C(non-bonded)ca. 2.7 A], and the two central atoms are π-bonded to that Cr atom which does not carry the carbonyl ligand. The two metal atoms lie on opposite sides of the plane through the four-carbon portion of the CrC4Ph4 ring. The alkynes RCCH (R = Ph or Me) react with [Cr2(CO)4(η-C5H5)2] to yield as major products the complexes [Cr2(CO)(η-C4H2R2)(η-C5H5)2] in which there is a head-to-tail arrangement of the Ph or Me groups in the C4Cr rings. However, with phenylacetylene the isomer containing the ring system [graphic omitted]Ph was also detected. The compounds [Mo2(CO)n(η-C5H5)2](n= 4 or 6) react with PhCCPh in octane at reflux to give [Mo2(CO)(µ-C4Ph4)(η-C5H5)2], and similarly [Mo2(CO)4(η-HC2H)(η-C5H5)2] affords [Mo2(HC2H)(PhC2Ph)2(η-C5H5)2], a ‘fly-over’ complex containing a six-carbon chain bridging a MoMo bond. Reaction of [Mo2(CO)4(η-HC2H)(η-C5H5)2] with RCCR (R = CO2Me) yields the complexes [Mo2(CO)2(µ-C6H2R4)(η-C5H5)2](two isomers) and [Mo2(η-C8H2R6)(η-C5H5)2](two isomers) in which C6 and C8 chains bridge Mo–Mo and MoMo bonds respectively. The structures of these species were deduced from 1H and 13C n.m.r. spectra. Reaction of [Mo2(CO)4(µ-RC2R)(η-C5H5)2] with RCCR (R = CO2Me) gives [Mo2(CO)2(µ-C6R6)(η-C5H5)2] and [Mo2(µ-C8R8)(η-C5H5)2](two isomers), characterised inter alia by their 1H and 13C n.m.r. spectra. Treatment of [Mo2(HC2H)(PhC2Ph)2(η-C5H5)2] with MeO2CCCCo2Me gives two isomers of compositon [Mo2(HC2H)(PhC2Ph)2(MeO2CC2CO2Me)(η-C5H5)2], thereby establishing that the complexes with four linearly linked alkynes are formed from the species with three such linked groups. Possible mechanisms for carbon-chain growth on the dichromium and dimolybdenum centres are discussed and are related to the changes in multiplicities of the metal–metal bonds.

Ditropylmolybdenum complexes by the sodium reduction of [Mo(CO)3(η-C7H7)][BF4]. X-Ray crystal structure of the ditropyl complex “Mo(CO)3”2(η6,η′6-C14H14)

Treatment of [Mo(CO) 3 (η-C 7 H 7 )][BF 4 ] with either sodium amalgam or sodium naphthalide results in a reductive dimerisation to the ditropyl complex “Mo(CO) 3 ” 2 (η 6 ,η′ 6 -C 14 H 14 ). The results of an X-ray crystallographic study confirm that the hydrogen atoms of the linking carbon atoms are both endo with respect to molybdenum. The reaction of Mo(CO) 6 with ditropyl, C 14 H 14 , leads to all three possible isomers of “Mo(CO) 3 ” 2 (η 6 ,η′ 6 -C 14 -H 14 ). The structure of each is assigned by its 1 H NMR spectrum.

Protonation of μ-alkyne-dimolybdenum complexes: X-ray structure of [Mo2{OC(O)CF3}(CO)4(μ-CHCH2)(η-C5H5)2]

Protonation of the μ-ethyne complex [Mo 2 (CO) 4 (μ-CHCH)(η-C 5 H 5 ) 2 ] with HBF 4 yields the fluxional μ-vinyl cation [Mo 2 (CO) 4 (μ-CHCH 2 )(η-C 5 H 5 ) 2 ] + which is attacked at molybdenum by various nucleophiles X − (X = OC(O)CF 3 , OC(O)Me, Cl, Br) to produce [Mo 2 (X)(CO) 4 (μ-CHCH 2 )(η-C 5 H 5 ) 2 ]; the structure of [Mo 2 {OC(O)CF 3 }(CO) 4 (μ-CHCH 2 )(η-C 5 H 5 ) 2 ] has been determined by X-ray diffraction.

Acetylene linkage at a di-metal centre: X-ray crystal structures of [Cr2(CO)(η-C5H5)2(PhC2Ph)2] and [Mo2(η-C5H5)2{(MeO2CC2CO2Me)-(HC2H)(MeO2CC2CO2Me)2}]

A sequence of acetylene linking at a di-metal centre has been identified; X-ray diffraction studies have established the structures of di-chromium and di-molybdenum complexes containing two and four acetylene molecules, respectively.

The addition of isonitriles to [{M(CO)2(η-C5H5)}2](M = Mo or W). Syntheses and properties of [M2(µ-η2-CNR)(CO)4(η-C5H5)2](M = Mo or W, R = Me or But) and [{Mo(CNR)(CO)2(η-C5H5)}2](R = Me or But). X-Ray crystal structures of [Mo2(µ-η2-CNBut)(CO)4(η-C5H5)2] and [{Mo(CNBut)(CO)2(η-C5H5)}2]

Addition of CNR (R = Me or But) to the MM complex [{M(CO)2(η-C5H5)}2](M = Mo or W) results in addition of one isonitrile across the MM bond and the formation of [M2(µ-η2-CNR)(CO)4(η-C5H5)2]. The But molybdenum derivative crystallises as black platey needles [orthorhombic, a= 31.75(3), b= 8.171(11), c= 15.722(17)A; R converged to 0.0357 for 1 775 independent reflections for which I/σ(I) > 3.0]. The molecule consists of two Mo(CO)2(η-C5H5) groups linked by a Mo–Mo single bond and bridged by the CNBut group in a µ-η2 four-electron donor fashion. The bonding is viewed as donation of the carbon long pair to one molybdenum atom and donation of CN π-electron density to the second. Prolonged reaction times of [Mo2(µ-η2-CNR)(CO)4(η-C5H5)2] with excess CNR results in cleavage of the CN–Mo π-donor interaction and formation of the symmetrically disubstituted complex [{Mo(CNR)(CO)2(η-C5H5)}2]. The But derivative crystallises as dark red prisms [monoclinic, a= 10.354(7), b= 11.672(5), c= 10.793(5)A, β= 103.68(4)°; R converged to 0.0519 for 1 102 independent reflections for which I/σ(I) > 3.0]. The molecule consists of two Mo(CNBut)(CO)2(η-C5H5) groups linked through a Mo–Mo single bond with a crystallographically imposed centre of symmetry midway along the Mo–Mo bond. Decarbonylation by thermolysis of this compound results in the formation of [Mo2(CNBut)(µ-η2-CNBut)(CO)3(η-C5H5)2], containing one terminal and one bridging isonitrile. Reductive cleavage of [{Mo(CNR)(CO)2(η-C5H5)}2] with sodium amalgam gives the anions [Mo(CNR)(CO)2(η-C5H5)]– which react with SnPh3Cl to give the tin complexes [Mo(SnPh3)(CNR)(CO)2(η-C5H5)], as mixtures of cis and trans isomers.

Reactions of metallabromoalkanes [M{(CH2)nBr}(CO)3(η-C5H5)](M = Mo or W, n= 3 or 4) with transition metal anions. Dimetalla-alkane and cyclic carbene complex formation. Crystal and molecular structures of [(η-C5H5)(OC)3Mo{(CH2)4}Mo(CO)3(η-C5H5)] and [(η-C5H5)(OC)3MoW(CO)2{C–O–(CH2)2CH2}(η-C5H5)]

Reaction of Br[CH2]3Br with excess [Mo(CO)3(η-C5H5)]– leads to the spectroscopically characterised [(η-C5H5)(OC)3MoMo(CO)2{[graphic omitted]H2}(η-C5H5)] containing a cyclic carbene ligand. This species is formed via the intermediate [Mo{(CH2)3Br}(CO)3(η-C5H5)]. The related complex [(η-C5H5)(OC)3MoW(CO)2{[graphic omitted]H2}(η-C5H5)] is formed during reaction of [Mo{(CH2)3Br}(CO)3(η-C5H5)] with [W(CO)3(η-C5H5)]– and has been characterised by X-ray crystallography. This molecule crystallises as maroon needles, monoclinic, a= 8.017(6), b= 15.026(16), c= 7.981 (3)A, β= 92.88(5)°; R converged to 0.0645 for 1 256 independent reflections for which I/σ(I) > 3.0. The molecule contains a Mo–W single bond of 3.239(4)A, the cyclic carbene ligand is bonded to tungsten rather than molybdenum and is situated trans to the Mo–W bond regarding the co-ordination polyhedra of both metals as square-based pyramidal. Reaction of [M(CO)3(η-C5H5)]– with I[CH2]nI leads to [(η-C5H5)(OC)3M{(CH2)n}M(CO)3(η-C5H5)](M = Mo, n= 4; M = W, n= 3 or 4). The structure of one member of this class [(η-C5H5)(OC)3Mo{(CH2)4}Mo(CO)3(η-C5H5)] has been determined by X-ray crystallography. The molecule crystallises as yellow elongated plates, triclinic, a= 6.978 2(16), b= 7.005 1(10), c= 11.315 0(25)A, α= 102.378(15), β= 95.806(18), γ= 105.909(15)°; R converged to 0.0263 for 1 646 independent reflections. The molecule contains a crystallographically imposed centre of symmetry at the centre of the tetramethylene chain and the molecular structure may be compared to that of the known [(η-C5H5)(OC)2Fe{(CH2)4}Fe(CO)2-(ηC5H5)].

Cyclic carbene complexes of molybdenum and tungsten. Crystal and molecular structure of [Mol(CO)2{CO(CH2)2CH2}(η-C5H5)]

The cyclic carbene complex [Mol(CO)2{[graphic omitted]H2}(η-C5H5)] is formed during reaction of [Mo(CO)3(η-C5H5)]– with I[CH2]3l or of [Mo{(CH2)3Br}(CO)3(η-C5H5)] with Lil. Crystals of this complex are Monoclinic, space group P21/c, with a= 6.372(2), b= 14.168(5), c= 14.390(3)A, β= 98.12(2)°, and Z= 4. The molecule contains mutually trans carbonyl ligands and a cyclic carbene ligand in which all but one of the ring carbon atoms are coplanar with the metal. Similarly, reactions of [Mo{(CH2)3Br}(CO)3(η-C5H5)] with SPh– or CN– provide [MoX(CO)2{[graphic omitted]H2}(η-C5H5)](X = SPh or CN). The cyclisation is tolerant to functionality on the cyclopentadienyl ring, thus reaction of [Mo(CO)3(η-C5H4R)]– with I[CH2]3l produces [Mol(CO)2{[graphic omitted]H2}(η-C5H4R)][R = Me, C(O)Me, or SiMe3]. The tungsten analogue [W{(CH2)3Br}(CO)3(η-C5H5)] undergoes carbene formations with l– and CN– to form [WX(CO)2{[graphic omitted]H2}(η-C5H5)](X = l or CN). Formation of the carbene complexes is likely to proceed by migration of the (CH2)3Br alkyl group to an adjacent CO group together with co-ordination of the incoming anion. This is followed by a ring closure reaction eliminating Br– and isomerisation so that in all cases the two CO groups are mutually trans in the isolated products. While [Mol(CO)2{[graphic omitted]H2}(η-C5H5)] does not react with transition-metal anions to form dinuclear carbene complexes by elimination of l–, the salt [Mo(CO)2(PPh3){[graphic omitted]H2}(η-C5H5)]Br reacts with [W(CO)3(η-C5H5)]– to give the unusual acyl complex [(η-C5H5)(Ph3P)(OC)2MoC(O)(CH2)3W(CO)3(η-C5H5)].

Synthetic and structural studies on organomolybdenum and organotunsten complexes containing bismuth. X-Ray crystal structures of [BiCl{Mo(CO)3(η-C5H5)}2] and [BiCl{Mo(CO)2(CNBut)(η-C5H5)}2]

The reaction between BiCl3 and 3 equivalents of Na[Mo(CO)3(η-C5H5)] affords the trimolybdenum–bismuth complex [Bi{Mo(CO)3(η-C5H5)}3], (1), which has been characterised by spectroscopic methods. The corresponding reaction involving 2 equivalents of Na[Mo(CO)3(η-C5H5)] affords the disubstituted product [BiCl{Mo(CO)3(η-C5H5)}2], (3), which has been characterised by X-ray diffraction. The molecular structure comprises a trigonal pyramidal bismuth atom bonded to one chlorine and two Mo(CO)3(η-C5H5) fragments and containing a stereochemically active lone pair at the apex of the pyramid. The two crystallographically independent molecules have similar conformations but a weak intermolecular Bi ⋯ Cl interaction, which is present for both molecules, is significantly different in each case. Treatment of a solution of compound (1) with 0.5 equivalent of BiCl3 affords (3) and when pure samples of (3) are treated with 1 equivalent of BiCl3 a redistribution reaction is also observed forming [BiCl2{Mo(CO)3(η-C5H5)}]. A similar range of compounds has been synthesised containing the methylcyclopentadienyl ligand. The analogous tungsten chemistry has also been investigated and similar tri-, di-, and mono-tungsten–bismuth compounds have been synthesised. Slight differences between the molybdenum and tungsten systems are observed, however, under the reaction conditions employed and these are discussed in terms of the possible mechanisms involved. Finally, the reactions between BiCl3 and 2 equivalents of the carbonyl-substituted anions [Mo(CO)2L(η-C5H5)]–(L = ButNC, MeNC, or PPh3) have been examined. These result in the dimolybdenum–bismuth complexes [BiCl{Mo(CO)2L(η-C5H5)}2], L = ButNC (15), MeNC (16), or PPh3(17). The first two complexes exist as mixtures of isomers, viz, cis, trans and trans,trans with regard to the stereochemistry at the molybdenum centres, whilst the latter exists as the trans,trans form. The cis,trans form of (15) was characterised by X-ray diffraction. The structure is similar to that observed for compound (3) except that no short intermolecular Bi ⋯ Cl interactions are present. Spectroscopic and analytical data for the complexes are presented.

Dimolybdenum complexes containing bridging silyl-substituted acetylenes: crystal and molecular structure of [Mo2(CO)4(µ-Me3SiC2SiMe3)(η-C5H5)2]

Reaction of [Mo2(CO)4(η-C5H5)2] with the alkyne Me3SiC2SiMe3 in toluene at reflux affords the complex [Mo2(CO)4(µ-Me3SiC2SiMe3)(η-C5H5)2]. In hexane, i.r. measurements reveal that the compound exists as an isomeric mixture, the species present having structures with or without semi-bridging CO ligands, in addition to terminally bonded CO groups. The simplicity of the n.m.r. spectra (1H and 13C) shows a rapid interconversion between isomers at room temperature, but low solubility of the complex prevented measurement of limiting spectra. The structure in the solid state was established by an X-ray diffraction study which shows that the molecule has C2 symmetry in the crystal, and that the carbonyl ligands are all terminal. The alkyne ligand is bonded transversely with respect to the Mo–Mo bond [2.952(1)A], forming a tetrahedrane type Mo2C2 core. The trimethylsilyl ligands bend back from the C–C bond at an angle of 137°. The Mo(CO)2(η-C5H5) moieties are in a skew (trans) relationship to the metal–metal bond; the actual configuration adopted in the crystal arises largely as a result of steric factors. Thus the structure in the solid is different from that established previously for compounds [Mo2(CO)4(µ-alkyne)(η-C5H5)2] which have been shown to have semi-bridging CO ligands. Reaction of [Mo2(CO)4(µ-HC2H)(η-C5H5)2] with an excess of the alkynes R3SiC2H (SiR3= SiMePh2, SiMe3, or SiEt3) in octane at reflux affords the complexes [Mo2(CO)4(µ-R3SiC2H)(η-C5H5)2]. The spectroscopic properties of these compounds are in accord with structures containing a semi-bridging CO ligand. The compound [Mo2(CO)4(µ-Me3SiC2SiMe3)(η-C5H5)2] reacts with C2(CO2Me)2 to afford the complexes [Mo2(CO)2{C6(SiMe3)2(CO2Me)4}(η-C5H5)2] and [Mo2{C6(CO2Me)2(SiMe3)2(CO2Me)2}(η-C5H5)2].

Carbene formation from CpM(SnPh3)(CO)2 (M = Fe, Ru). Crystal structures of CpFeI(CO){C(OEt)Ph}, CpRuI(CO){C(OEt)Ph} and CpRuI(CO){C(NHMe)Ph}

Addition of LiPh followed by [Et3O][BF4] to CpM(SnPh3)(CO)2 (M = Fe, Ru) gives the carbene complexes CpM(SnPh3)(CO){C(OEt)Ph}. These undergo aminolysis on treatment with EtNH2 or MeNH2 to form the amino carbenes CpFe(SnPh3)(CO){C(NHEt)Ph} and CpRu(SnPh3)(CO){C(NHMe)Ph}, respectively. All four compounds react with iodine, the SnPh3 group being replaced by I to give the new carbenes CpMI(CO){C(X)Ph} (M = Fe, Ru; X = OEt; M = Fe, X = NHEt; M = Ru, X = NHMe). The IR and NMR spectra show that the amino carbenes exist as single isomers while the ethoxy carbenes form fluxional isomeric mixtures. The compounds CpFeI(CO){C(OEt)Ph}, CpRuI(CO){C(OEt)Ph} and CpRuI(CO){C(NHMe)Ph} have been studied by X-ray diffraction. In each case the geometry about the metal is essentially conventional, with the carbene groups eclipsed by the MCO groups.