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 MoMo bonded complex [Mo2{µ-σ:η2-CH2C(Ph)CH2}(CO)3(η-C5H5)2][CF3SO3]

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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-η2CH2 CCR2)(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-CH2 CC(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-CH2 CC(Ph)Me}(CO)4(η-C5H5)2] and [Mo2{µ-η2-:η2-CH2 CC(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.

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