A detailed theoretical study of the mechanism and energetics of methane to methanol conversion by cisplatin and catalytica

The conversion of methane to methanol by dichloro(eta(2)-{2,2'-bipyrimidyl})platinum(II) [Pt(Bpym)Cl-2] (Catalytica) and Pt(NH3)(2)Cl-2 (cisplatin) [Science 1998, 280, 560] has been studied using hybrid density functional theory in conjunction with the conductor-like polarizable continuum solvent model (CPCM). We have determined the full potential energy profiles for plausible catalytic pathways along the three major phases of the catalytic cycle, namely, (a) C-H activation, (b) Pt(II) to Pt(IV) oxidation, and (c) functionalization. For Catalytica, oxidation of Pt(II) to Pt(IV) is the highest barrier step for all active catalytic forms considered. Oxidation of Pt(II) to Pt(IV) is significantly easier for cisplatin compared to Catalytica, explaining the faster catalytic transformation by cisplatin. Our calculations suggest that the oxidation barrier is significantly affected by the ligand environment on the Pt center of the catalyst. We predict that monoprotonation of the bipyrimidine ring of Catalytica significantly affects the oxidation process only if catalysis proceeds through electrophilic C-H activation cis to the protonated pyrimidine ring. We also determine a full potential energy profile for catalytic conversion by cisplatin proceeding through oxidative C-H addition, subsequent deprotonation, followed by oxidation of Pt(II) to Pt(IV), and then functionalization.