Reactions of co-ordinated ligands. Part 58. The reaction of dimolybdenum µ-σ:η2-(4e)-vinylidene complexes with proton sources and diazomethane; synthesis and crystal structure of the MoMo bonded complex [Mo2{µ-σ:η2-CH2C(Ph)CH2}(CO)3(η-C5H5)2][CF3SO3]

Abstract

Protonation (CF₃CO₂H) of the ‘side-on’ bonded dinuclear vinylidene complexes [Mo₂{µ-σ:η²-(4e)-CCR₂}(CO)₄(η-C₅H₅)₂](R = H or Me) afforded the bridged µ-vinyl complexes [Mo₂(OC(O)CF₃}(µ-CHCR₂)(CO)₄(η-C₅H₅)₂], whereas a similar reaction of one of the isomers of [Mo₂{µ-σ:η²-(4e)-CC(Ph)Me}(CO)₄(η-C₅H₅)₂] afforded the isomerie complexes [Mo,{OC(O)CF₃}{µ-CHC(Me)Ph}(CO)₄(η-C₅H₅)₂] and [Mo₂{OC(O)CF₃}{µ-CHC(Ph)Me}(CO)₄(η-C₅H₅)₂]. Deuterium-labelling experiments and extended-Hückel molecular-orbital (EHMO) calculations suggest that these reactions involve delivery of a proton by CF₃CO₂H to the α-carbon of the vinylidene complex in a direction perpendicular to the plane containing the β-carbon substituents. Protonation of the α-carbon is also implicated in the reaction of [Mo₂{µ-σ:η²-(4e)-CCMe₂}(CO)₄(η-C₅H₅)₂] with HBF₄·Et₂O or CF₃SO₃H where the products are the unusual cationic species [Mo₂{µ-σ:η²-CH₂C(Me)CH₂}(CO)₃(η-C₅H₅)₂][X](X = BF₄ or CF₃SO₃). The same cations are also produced by α protonation of the µ-allylidene complex [Mo₂{µ-σ:η³-CHC(Me)CH₂}(CO)₄(η-C₅H₅)₂]. Similarly, protonation (CF₃SO₃H) of [Mo₂{µ-σ:η³-CHC(Ph)CH₂)(CO)₄(η-C₅H₅)₂](formed on thermolysis of [Mo₂{µ-σ:η²-(4e)-CC(Ph)-Me}(CO)₄(η-C₅H₅)₂] afforded the X-ray crystallographically identified complex [Mo₂{µ-σ:η²-CH₂C-(Ph)CH₂)(CO)₃(η- C₅H₅)₂][CF₃SO₃]. In the solid state the complex conta ins an unusual bonding mode for a dinuclear allyl complex, the CH₂C(Ph)CH₂ fragment being bonded viaη² co-ordination to one molybdenum atom and a three-centre two-electron interaction between one of the CH₂ groups of the allyl and both metal atoms, which are bonded to each other by a formal triple bond. In solution this cation and the related methyl-substituted species show dynamic behaviour resulting in equivalencing on the NMR time-scale of the two anti-and two syn-protons of the allyl ligand. The reaction of diazomethane with [Mo₂{µ-σ:η²-(4e)-CCR₂)(CO)₄(η-C₅H₅)J (R = H or Me) has also been studied. The reactions lead respectively to the formation of the µ-allene complexes [Mo₂(µ-η²CH₂CCR₂)(CO)₄(η-C₅H₅)₂]. A similar reaction between CH₂N₂ and the unsymmetrical labile ‘side-on’ bonded vinylidene [Mo₂{µ-σ:η²-(4e)-CC(Ph)H}(CO)₄(η-C₅H₅)₂] gave [Mo₂{µ-η²:η²-CH₂CC(Ph)H}(CO)₄(η-C₅H₅)₂, whereas, in contrast, [Mo₂{µ-σ:η²-(4e)-CC(Ph)Me}(CO)₄(η-C₅H₅)₂] affords the diastereo-isomers [Mo₂{µ-η²;η²-CH₂CC(Ph)Me}(CO)₄(η-C₅H₅)₂] and [Mo₂{µ-η²-:η²-CH₂CC(Me)Ph}-(CO)₄(η-C₅H₅)₂]. The EHMO calculations implicate dipolar intermediates in these reactions formed by nucleophilic attack under frontier-orbital control on the lowest unoccupied molecular orbital of the vinylidene ligand located on the α-carbon.