Homogeneous conversion of methane to methanol. 1. Catalytic activation andfunctionalization 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% o ne-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 cons iderable current interest and activity. The conversion is catalyzed by bis( 2,2' bipyrimidine)Pt(II)Cli. In this work, the thermodynamics of the activa tion and functionalization steps of the related cis-platin-catalyzed proces s in H2SO4 are calculated using density functional techniques, including th e calculation of solvation free energies by a dielectric continuum method. It is concluded that electrophilic attack by CH4 on an intermediate which m ay:be regarded as a tetracoordinate solvated analogue of a gas-phase, T-sha ped, three-coordinate Pt(LI) species, followed by oxidation of the resultin g 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 a ctive catalyst, as well as that of the intermediates, can be more precisely defined. While the alternative mechanism of oxidative addition does not ap pear to be thermodynamically feasible when using Pt(II) catalysts, catalysi s by a Pt(IV) species is predicted to be, on thermodynamic grounds, a viabl e alternative pathway.