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

摘要

Complex [(η6-p-cymene)OsCl(IPr)]OTf (1; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene; OTf = CF3SO 3) reacts with NaOH to give [(η6-p-cymene)Os(OH)(IPr)] OTf (2), which under hydrogen atmosphere affords [(η-p-cymene)OsH 3(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 (η6-p-cymene)OsH 2(rPr) (4) and the protonation of the latter with HOTf. Attempts to obtain 4 directly from 1 or (η6-p-cymene)OsCl2(IPr) (5) are unsuccessful. In both reactions (η6-η-cymene) OsHCl(rPr) (6) is formed. Complex 2 also reacts with methanol and 2-propanol. At -30 °C, the reaction with the first of them leads to [(η6-p- cymene)Os(OMe)(IPr)]OTf (7), while with the second one, [(η6-p- cymene)OsH{κ;1-OC(CH3)2J(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. © 2008 American Chemical Society.