Metal-Dependent Strengthening and Weakening of M-H and M-C Bonds by an Oxo Ligand: Thermal Gas-Phase Activation of Methane by [OMH] + and [MH] + (M=Mo, Ti).

The thermal gas-phase reactions of methane with [OMoH]+ and [MoH]+ were investigated by using electrospray-ionization mass spectrometry (ESI-MS) complemented by quantum-chemical calculations. In contrast to the inertness of [MoH]+ towards methane, [OMoH]+ activates the C-H bond to form the ionic product [OMo(CH3 )]+ concomitantly with the liberation of H2 . The origin of the varying reactivities is traced back to a different influence of the oxo ligand on the Mo-C and Mo-H bonds. While the presence of this ligand weakens both the Ti-H and the Ti-CH3 bonds, both the Mo-H and Mo-CH3 bonds are strengthened. The more pronounced strengthening of the Mo-CH3 bond compared to the Mo-H bond favors the exothermicity of the reaction of [OMoH]+ with CH4 .