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 trans ition metal (TM)-oxo model complexes. Two channels have been investiga ted for both the C-H and O-H bond cleavage: i.e., abstraction of the h ydrogen 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 processe s were fully optimized and further confirmed to be energy minimum poin ts on the potential energy surfaces by frequency calculations. It is s hown that the reaction enthalpies for all TM systems under: investigat ion follows the order O-H hydrogen abstraction > C-H hydrogen abstract ion > 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 c hannel. 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 hyd rogen abstraction channel, endothermicity of the reactions increases d own a given TM triad and decreases from left to right within a transit ion series. The opposite trend was found for the C-H or O-H bond addit ion processes. A detailed analysis of the theoretical MO-H, M-OCH3, an d 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 estim ates were further used to explain why the preferred pathway for O-H an d C-H activation by TM oxo complexes changes with the position of the metal in the periodic table.