Ligand and substituent effects in methane activation by mercury(II) complexes

This paper reports a computational study of methane activation by the HgN and HgF bonds of the Hg-II complexes FHgNZ (where NZ is NH2, NO2, N-3, NC, NCO, NCS, NO, NS, N2H). The present study probes the thermodynamic and kinetic effects of activating ligand (L-a) modification for a given spectator ligand (L-s) and vice versa. The wide range of calculated kinetic and thermodynamic values indicates a degree of sensitivity in methane activation by Hg-II complexes to the ligand environment. The trends show that [2(sigma) + 2(sigma)] activation of of a methane C-H bond by Hg-II complexes is thermodynamically favored by a more basic activating ligand and a less basic spectator ligand. The lowest methane activation barriers are observed for an activating ligand with variable denticity (i.e. NO2) that can provide extra stabilization for the methane activation transition state versus the ground state. The implications of the results for experimental Hg-II methane conversion catalysts are discussed. (C) 1998 Elsevier Science B.V.