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

Abstract

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)Å, 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 Å], 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.