Expanding the role of oxo-molybdenum(VI) catalysts: a DFT interpretation of X–H activation leading to reduction or oxidation

Organometallic complexes of transition metals in high oxidation states, namely Re(V) and Mo(VI), have been shown experimentally to catalyze a variety of reduction reactions, such as hydrosilylation of ketones, alkyne hydrogenation, sulfoxide reduction (deoxygenation), etc, often under mild conditions and with high yields and selectivity. These recently found reactions apparently contrast with the traditional oxidation of alkenes in the presence of an oxygen source. We describe a series of DFT calculations, showing that most X–H bonds are activated by [MoO₂Cl₂], forming a hydride complex, which is the catalytic active species in catalysis. This occurs by a [2 + 2] addition of the X–H bond to the MoO bond of the Mo(VI) complex. While hydrosilylation is particularly effective, hydrogenation is deactivated in many systems by the reduction of the catalyst, rather than the substrate, to a stable Mo(IV) complex, and P–H activation in HP(O)(OR)₂ molecules takes place through a different route. The activation of O–H from HOOR differs significantly, since the high electronegativity of oxygen results in an “inverse” addition with formation of OH and a complex of OOR.