Acceptor Pincer Chemistry of Osmium: Catalytic Alkane Dehydrogenation by (^CF₃PCP)Os(cod)(H)

Syntheses of osmium analogues of acceptor pincer (^CF₃PCP)­Ru­(II) systems are reported. Treatment of [Et₄N]₂OsCl₆ with ^CF₃PCPH at 130 °C in ethanol in the presence of Et₃N gave the coordinatively saturated anionic carbonyl complex HNEt₃⁺[(^CF₃PCP)­Os­(CO)­Cl₂]⁻, which subsequently may be converted to <i>cis</i>-(^CF₃PCP)­Os­(CO)₂Cl or <i>cis</i>-(^CF₃PCP)­Os­(CO)₂H by reaction with Me₃SiOTf or (Et₃Si)₂(μ-H)⁺B­(C₆F₅)₄^–, respectively. (^CF₃PCP)­Os­(cod)H was obtained in modest yields by thermolysis of Os­(cod)­(η³-2-methylallyl)₂ with ^CF₃PCPH in neat cod under 3 atm of H₂ at 130 °C. The alkane dehydrogenation activity of (^CF₃PCP)­Os­(cod)H was examined: under identical conditions to previously studied (^CF₃PCP)­Ru­(cod)­H (1:1 cyclooctane/<i>tert</i>-butylethylene, 200 °C), the initial turnover rate for cyclooctene production was 1520 h^–1, 75% the rate observed for the ruthenium analogue, but with significantly enhanced catalyst lifetime. Acceptorless cyclodecane dehydrogenation under reflux conditions gave 125 turnovers of cyclodecenes in one hour. Spectroscopic evidence on the nature of the catalyst resting state and catalyst thermal and air stability is presented