Pentabenzylcyclopentadienyl molybdenum and tungsten hydrides: Syntheses, structures and electrochemistry of [MHCpBz(CO)2(L)] (L = CO, PMe3, PPh3)

Complexes [MHCpBz(CO)2(PR3)] (R = CH3, M = Mo (1); M = W (2); R = Ph, M = Mo (3); CpBz = C5(CH2Ph)5) were prepared by thermal decarbonylation of the corresponding [MHCpBz(CO)3] in the presence of trimethyl- or triphenyl-phosphine. In solution the NMR spectra of all compounds show the presence of cis and trans isomers that interconvert at room temperature. In the solid state the molecular structures obtained for compounds 1 and 2 correspond to the trans isomers, while for 3 the cis isomer is present. The electrochemistry of [MoHCpBz(CO)2(PMe3)] (1), [MoHCpBz(CO)3] (5), [WHCpBz(CO)3] (6), [WCpBz(CO)3]2 (7), and [MCpBz(CO)3(CH3CN)]BF4 (8), is described. The cleavage of M–H bonds takes place upon oxidation or reduction. Cations [MCpBz(CO)2L(CH3CN)]+ form in solvent-assisted M–H bond breaking upon oxidation of [MHCpBz(CO)2L] (L = PMe3, CO). Reduction of [MHCpBz(CO)3] gives [MCpBz(CO)3]− and H2. The presence of one PMe3 ligand lowers the reduction potential and precludes the observation of reduction waves.