Preparation and hydrosilylation activity of a molybdenum carbonyl complex that features a pentadentate bis(imino)pyridine ligand

Attempts to prepare low-valent molybdenum complexes that feature a pentadentate 2,6-bis(imino)pyridine (or pyridine diimine, PDI) chelate allowed for the isolation of two different products. Refluxing Mo(CO)₆ with the pyridine-substituted PDI ligand, ^PyEtPDI, resulted in carbonyl ligand substitution and formation of the respective bis(ligand) compound ( ^PyEtPDI)₂Mo (1). This complex was investigated by single-crystal X-ray diffraction, and density functional theory calculations indicated that 1 possesses a Mo(0) center that back-bonds into the π-orbitals of the unreduced PDI ligands. Heating an equimolar solution of Mo(CO) ₆ and the phosphine-substituted PDI ligand, ^Ph2PPrPDI, to 120 °C allowed for the preparation of (^Ph2PPrPDI)Mo(CO) (2), which is supported by a κ⁵-N,N,N,P,P-^Ph2PPrPDI chelate. Notably, 1 and 2 have been found to catalyze the hydrosilylation of benzaldehyde at 90°C, and the optimization of 2-catalyzed aldehyde hydrosilylation at this temperature afforded turnover frequencies of up to 330 h^-1. Considering additional experimental observations, the potential mechanism of 2-mediated carbonyl hydrosilylation is discussed.