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