Synthesis and characterisation of hydrido molybdenum and tungsten complexes having a hemilabile tridentate Si,Si,O-ligand: observation of stepwise hydrosilylation of a nitrile to form an N-silylimine on the metal centre

Photoinduced decarbonylation of Cp*M(CO)(3)Me (M = Mo and W, Cp* = η(5)-C(5)Me(5)) in the presence of xantsilH(2) xantsil = (9,9-dimethylxanthene-4,5-diyl)bis(dimethylsilyl)] in pentane gave bis(silyl)hydrido complexes Cp*M(κ(2)Si,Si-xantsil)(CO)(2)(H) (1a: M = Mo and 1b: M = W) through two-fold Si-H oxidative addition and methane elimination. Further irradiation of 1a,b in toluene afforded tridentate xantsil complexes Cp*M(κ(3)Si,Si,O-xantsil)(CO)(H) (2a: M = Mo and 2b: M = W) via CO dissociation. Reactions of complexes 2a,b with nitriles led to stoichiometric hydrosilylation at the Ctriple bond, length as m-dash]N triple bond. Thus, reaction of 2a,b with t-BuCN at room temperature afforded N-silyliminoacyl complexes 3a,b, through insertion of a nitrile into the M-Si bond, and the products slowly isomerised to the corresponding N-silylimine complexes 4a,bvia intramolecular hydrogen migration. On the other hand, reaction of 2a,b with PhCN afforded N-silylimine complexes 5a,b directly. The molecular structures of 1a, 3a and 5b were determined by X-ray crystallography, revealing that complex 3a has a 3-centre-2-electron (3c-2e) Mo-Si-H bond.