Density functional study of C-H and O-H bond activation by transition metal d(0)-oxo complexes .1. Thermodynamic considerations

Density functional methods have been applied to the thermochemistry of methanol C-H and O-H bond activation by group 5 through group 8 transition metal (TM)-oxo model complexes. Two channels have been investigated for both the C-H and O-H bond cleavage: i.e., abstraction of the hydrogen by the TM-oxo complexes and addition of the C-H or O-H bond to the M=O linkage. Structures of all species involved in these processes were fully optimized and further confirmed to be energy minimum points on the potential energy surfaces by frequency calculations. It is shown that the reaction enthalpies for all TM systems under: investigation follows the order O-H hydrogen abstraction > C-H hydrogen abstraction > C-H bond addition > O-H bond addition. The O-H addition process was found for most of the systems to be a thermodynamically feasible channel. On the other hand, abstraction of a hydrogen from the O-H bond is for all but the Fe systems too endothermic to proceed. For the hydrogen abstraction channel, endothermicity of the reactions increases down a given TM triad and decreases from left to right within a transition series. The opposite trend was found for the C-H or O-H bond addition processes. A detailed analysis of the theoretical MO-H, M-OCH3, and M-CH2OH bond energies revealed that the energy gap between the HOMO and LUMO of the TM d(0)-oxo complexes is responsible for the periodic trends in the calculated reaction enthalpies. Our thermochemical estimates were further used to explain why the preferred pathway for O-H and C-H activation by TM oxo complexes changes with the position of the metal in the periodic table.