Alkane activation by silica-supported Group VB metal hydrides. A quantum-chemical study

A quantum-chemical study of alkane hydrogenolysis and metathesis on silica-supported Group VB metal hydrides (M = V, Nb, Ta) was carried out. Using a model fragment with the beta-crystobalite structure permits a correct description of the properties of the grafted organometallic complexes. Alkane hydrogenolysis and metathesis reactions involve (i) pi-complexes and carbene complexes of M-V and (ii) alkyl derivatives of M-III as possible intermediates. The character of the potential energy surfaces of the key alkane transformations points to an important role of triplet states in the catalytic processes involving niobium and vanadium. An alternative low-energy reaction pathway characterized by a change in the spin state was found.