Site-selective substitution and vinylidene–alkylidyne–vinylidene interconversion in dimolybdenum–ruthenium clusters

Abstract

The thermal reaction of the dimolybdenum–ruthenium vinylidene clusters [Mo₂Ru(µ₃-C=CHR)(CO)₇ (η-C₅H₅)₂] (R = H 1a, Me 1b, Ph 1c or CO₂Me 1d) with the tertiary phosphine PPh₂Me afforded the monosubstituted complexes [Mo₂Ru(µ₃-C=CHR)(CO)₆ (PPh₂Me)(η-C₅H₅)₂] 2a–2d in excellent yields. The products exist as single isomers in which site-selective substitution of a CO ligand has occurred exclusively at the ruthenium atom. The crystal structure of 2b has been determined by X-ray diffraction; the geometry of the vinylidene ligand is identical to that previously found in the parent compound 1b, but a slightly different pattern of semi-bridging carbonyls is present. The reactions of 1a and 1b with diphenylphosphine are more complex. Depending on the stoichiometry and conditions employed, three different types of cluster have been isolated: the substitution product [Mo₂Ru(µ₃-C=CH₂)(CO) ₆(PPh₂H)(η-C₅H₅)₂ ] 2e; the 46-electron monophosphido compounds [Mo₂Ru(µ₃-CCH₂R)( µ-PPh₂)(CO)₅(η-C₅H ₅)₂] 3a, 3b and the saturated bis(phosphido) species [Mo₂Ru(µ₃-C=CHR)(µ- PPh₂)₂(CO)₄(η-C₅H ₅)₂] 4a, 4b. Separate experiments have established that these products are formed sequentially, thus demonstrating the conversion of the initial vinylidene into an alkylidyne ligand and then back to a vinylidene. The structures of 3b and 4a have been confirmed by X-ray diffraction, showing that the phosphido groups bridge one or both of the Mo–Ru edges respectively.

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