Reactions of Co-ordinated Ligands. Part 58. The Reaction of Dimolybdenum μ-?:η2-(4e)-Vinylidene Complexes with Proton Sources and Diazomethane; Synthesis and Crystal Structure of the MoMo Bonded Complex 2-CH2C(Ph)CH2>(CO)3(η-C5H5)2>

Article abstract of DOI:10.1039/DT9940003569

Protonation (CF3CO2H) of the 'side-on' bonded dinuclear vinylidene complexes 2-(4e)-C=CR2>(CO)4(η-C5H5)2> (R = H or Me) afforded the bridged μ-vinyl complexes (μ-CH=CR2)(CO)4(η-C5H5)2>, whereas a similar reaction of one of the isomers of 2-(4e)-C=C(Ph)Me>(CO)4(η-C5H5)2> afforded the isomeric complexes <μ-CH=C(Me)Ph>(CO)4(η-C5H5)2> and <μ-CH=C(Ph)Me>(CO)4(η-C5H5)2>.Deuterium-labelling experiments and extended-Hueckel 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 2-(4e)-C=CMe2>(CO)4(η-C5H5)2> with HBF4*Et2O or CF3SO3H where the products are the unusual cationic species 2-CH2C(Me)CH2>(CO)3(η-C5H5)2> (X = BF4 or CF3SO3).The same cations are also produced by α protonation of the μ-allylidene complex 3-CHC(Me)CH2>(CO)4(η-C5H5)2>.Similarly, protonation (CF3SO3H) of 3-CHC(Ph)CH2>(CO)4(η-C5H5)2> (formed on thermolysis of 2-(4e)-C=C(Ph)Me>(CO)4(η-C5H5)2> afforded the X-ray crystallographically identified complex 2-CH2C(Ph)CH2>(CO)3(η-C5H5)2>.In the solid state the complex contains an unusual bonding mode for a dinuclear allyl complex, the CH2C(Ph)CH2 fragment being bonded via η² 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 2-(4e)-C=CR2>(CO)4(η-C5H5)2> (R = H or Me) has also been studied.The reactions lead respectively to the formation of the μ-allene complexes 2:η²-CH2=C=CR2)(CO)4(η-C5H5)2>.A similar reaction between CH2N2 and the unsymmetrical labile 'side-on' bonded vinylidene 2-(4e)-C=C(Ph)H>(CO)4(η-C5H5)2> gave 2:η²-CH2=C=C(Ph)H>(CO)4(η-C5H5)2>, whereas, in contrast, 2-(4e)-C=C(Ph)Me>(CO)4(η-C5H5)2> affords the diastereoisomers 2:η²-CH2=C=C(Ph)Me>(CO)4(η-C5H5)2> and 2:η²-CH2=C=C(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.