Homogeneous conversion of methane to methanol.: 2.: Catalytic activation of methane by cis- and trans-platin:: A density functional study of the Shilov type reaction

The C-H activation of methane catalyzed by cis- and trans-platin in aqueous solution has been studied by density functional based computational methods. By analogy with the Shilov reaction, the initial step is the replacement of an ammonia ligand by methane, followed by the formation of a methyl complex and the elimination of a proton. The computations utilize the B3LYP hybrid functionals, effective core potentials, and double-zeta to polarized double-zeta basis sets and include solvation effects by a dielectric continuum method. In contrast with the Shilov reaction studied by Siegbahn and Crabtree (J. Am. Chem. Sec. 1996, 118, 4442), in the platins the replacement of an ammonia ligand by methane is found to be effectively rate determining, in that the energy barriers to C-H activation are comparable with those of the initial substitution reaction, viz. similar to 34 and 44 kcal/mol;for cis- and trans-platin, respectively. Several reaction pathways for C-H activation and subsequent proton elimination were identified. For cis-platin the energy barriers associated with the oxidative addition and sigma-bond metathesis type mechanisms were found to be comparable, while for trans-platin oxidative addition is predicted to be strongly preferred over sigma-bond metathesis,which, interestingly,; also proceeds through a Pt(IV) methyl hydride complex as reaction intermediate. In line with accepted ideas on trans influence, the methyl and hydride ligands in the Pt(IV) complexes that arise in the oxidative addition;reactions were always found to be cis to each other. On the basis of the population analyses on the Pt(TV) complexes it is suggested that the Pt-H and Pt-CH3 bonds are best described as covalent bonds and, further, that the preference of the hydride and methyl anions to be cis to each other is a consequence of such covalent bonding. In light of these findings, the energies of several methyl Pt(IV) hydride bisulfate complexes were also recalculated, with CH3 and H placed cis to each other. The revised results provide evidence for the thermodynamic feasibility of oxidative addition of methane to catalysts such as [Pt(NH3)(2)(OSO3H)(2)] Or [Pt(NH3)(2)(OSO3H) (H2SO4)](+).