Mechanistic studies of the transfer dehydrogenation of cyclooctane catalyzed by iridium bis(phosphinite) p-XPCP pincer complexes

Reaction of bis(phosphinite) PCP iridium pincer complexes (p-XPCP)IrHCl (5a-f) [X = MeO (5a), Me (5b), H (5c), F (5d), C6F5 (5e), Ar-F(= 3,5-bis(trifluoromethyl)phenyl) (5f)] with NaOtBu in neat cyclooctane (COA) generates 1:1 mixtures of the respective (p-XPCP)IrH2 complexes 4a-f and the cyclooctene (COE) olefin complexes (p-XPCP)lr(COE) (6a-f) at 23 degreesC. At higher temperatures, complexes 4 and 6 are equilibrated because of the degenerate transfer dehydrogenation of COA with free COE (6 + COA = 4 + 2COE), as was shown by temperature-dependent equilibrium constants and spin saturation transfer experiments at 80 degreesC. At this temperature, the COE complexes 6 exchange with free COE on the NMR time scale with the more electron-deficient complexes 6 exchanging COE faster. The exchange is dissociative and zero order in [COE]. Further analysis reveals that the stoichiometric hydrogenation of COE by complex 4f, and thus the separated back reaction 4f + 2COE --> 6f + COA proceeds at temperatures as low as - 100 degreesC with the intermediacy of two isomeric complexes (p-(ArPCP)-P-F)lr(H)(2)(COE) (8f, 8f'). COE deuteration with the perdeuterated complex 4f-d(38) at -100 degreesC results in hydrogen incorporation into the hydridic sites of complexes 8f,8f'-d(38) but not in the hydriclic sites of complex 4f-d(38), thus rendering COE migratory insertion in complexes 8f,8f' reversible and COE coordination by complex 4f rate-determining for the overall COE deuteration.