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 (<b>1</b>). This complex was investigated by single-crystal X-ray diffraction, and density functional theory calculations indicated that <b>1</b> 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) (<b>2</b>), which is supported by a κ⁵-<i>N</i>,<i>N</i>,<i>N</i>,<i>P</i>,<i>P</i>-^Ph2PPrPDI chelate. Notably, <b>1</b> and <b>2</b> have been found to catalyze the hydrosilylation of benzaldehyde at 90 °C, and the optimization of <b>2</b>-catalyzed aldehyde hydrosilylation at this temperature afforded turnover frequencies of up to 330 h^–1. Considering additional experimental observations, the potential mechanism of <b>2</b>-mediated carbonyl hydrosilylation is discussed