Preparation, X-ray structure, and reactivity of an osmium-hydroxo complex stabilized by an N-heterocyclic carbene ligand:: A base-free catalytic precursor for hydrogen transfer from 2-propanol to aldehydes

Complex [(eta(6)-p-cymene)OsCl(IPr)]OTf (1; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene; OTf = CF3SO3) reacts with NaOH to give [(eta(6)-p-cymene)Os(OH)(IPr)]OTf (2), which under hydrogen atmosphere affords [(eta(6)-p-cymene)OsH3(IPr)]OTf (3). The hydride ligands of 3 undergo exchange coupling, decreasing the H-H coupling constant from 206 to 74 Hz in the temperature range 213-178 K. Complex 3 is also obtained in a two-step procedure involving the reaction of 2 with NaBH4 to give (eta(6)-p-cymene)OsH2(IPr) (4) and the protonation of the latter with HOTf. Attempts to obtain 4 directly from 1 or (eta(6)-p-cymene)OSCl2(IPr) (5) are unsuccessful. In both reactions (eta(6)-p-cymene)OsHCl(IPr) (6) is formed. Complex 2 also reacts with methanol and 2-propanol. At -30 degrees C, the reaction with the first of them leads to [(eta(6)-p-cymene)Os(OMe)(IPr)]OTf (7), while with the second one, [(eta(6)-p-cymene)OsH{kappa(1)-OC(CH3)(2)}(IPr)]OTf (8) is obtained. Complex 2 is a catalyst precursor for the hydrogen transfer from 2-propanol to aldehydes, which does not need a base. The obtained TOF values are between 950 and 3500 h(-1). The X-ray structures of 2 and 6 are also reported.