Methane C-H bond activation by niobium oxides: Theoretical analyses of the bonding and reactivity properties of Nbo(m)(n+) (m=1, 2; n=0, 1, 2)

摘要

The catalytic properties of NbOmn+ (m = 1, 2; n = 0, 1, 2) on the methane C-H bond activation were investigated using B3LYP and CCSD(T) calculations. The spin-orbit relativistic effects were evaluated by means of ZORA approximations. The main results indicate that in agreement with the previous experimental findings only the neutral NbO monoxide is kinetically and thermodynamically feasible via the oxidative addition, whereas the remaining oxides and dioxides show better theoretical predictions in the hydrogen abstraction pathways. In these reactions, a known mechanism described as "Oxidative Hydrogen Migration (OHM)" was found to be more stable for the neutral and singly charged niobium-oxo species, while competitive direct H abstraction (DHA) and OHM pathways were obtained for the doubly charged niobium oxides, with slightly preferential conditions observed in DHA. Overall, the increase of charge on the metal center and the presence of the oxo ligands favors the initial electrostatic interaction between niobium oxides and methane, decreasing the activation barrier heights in the H abstraction pathways due to the electron acceptor nature of oxo ligands. Among the investigated oxides, NbO2+ and NbO22+ show the best performances in the methane activation process, with activation barriers computed to be around 1.5-5.9 kcal mol(-1). (C) 2015 Elsevier B.V. All rights reserved.