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

Syntheses of osmium analogues of acceptor pincer ((PCP)-P-CF3)Ru-(II) systems are reported. Treatment of [Et4N](2)OsCl6 with (PCPH)-P-CF3 at 130 degrees C in ethanol in the presence of Et3N gave the coordinatively saturated anionic carbonyl complex HNEt3+[((PCP)-P-CF3)Os(CO)Cl-2](-), which subsequently may be converted to cis-(cF(3)PCP)Os(CO)(2)Cl or cis-(cl(3)PCP)Os(CO)(2)H by reaction with Me3SiOTf or (Et3Si)(2)(mu-H)(+)(C6F5)(4)-, respectively. ((PCP)-P-CF3) Os (cod)H was obtained in modest yields by thermolysis of Os(cod)(r/3-2-methyla1ly1)2 with c8313CPH in neat cod under 3 atm of H-2 at 130 degrees C. The alkane dehydrogenation activity of (cF(3)PCP)Os(cod)H was examined: under identical conditions to previously studied (c83PCP)Ru(cod)H (1:1 cyclooctane/tert-butylethylene, 200 C), the initial turnover rate for cyclooctene production was 1520 11-1, 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.