Homogeneous conversion of methane to methanol.: 1.: Catalytic activation and functionalization of methane by cis-platin in sulfuric acid:: A density functional study of the thermochemistry

The recent announcement by Periana et al. (Science 1998, 280, 560) of 70% one-pass homogeneous catalysis of methane-to-methanol :conversion with high selectivity in sulfuric acid solution under moderate conditions represents an important advance in the selective oxidation of alkanes, an-area of considerable current interest and activity. The conversion is catalyzed by bis(2,2' bipyrimidine)Pt(II)Cli. In this work, the thermodynamics of the activation and functionalization steps of the related cis-platin-catalyzed process in H2SO4 are calculated using density functional techniques, including the calculation of solvation free energies by a dielectric continuum method. It is concluded that electrophilic attack by CH4 on an intermediate which may:be regarded as a tetracoordinate solvated analogue of a gas-phase, T-shaped, three-coordinate Pt(LI) species, followed by oxidation of the resulting methyl complex to a methyl bisulfate eater, is thermodynamically feasible. This is in general accord with the mechanism proposed by Periana et al., but now, on the basis of the computational predictions, the nature of the active catalyst, as well as that of the intermediates, can be more precisely defined. While the alternative mechanism of oxidative addition does not appear to be thermodynamically feasible when using Pt(II) catalysts, catalysis by a Pt(IV) species is predicted to be, on thermodynamic grounds, a viable alternative pathway.