The reversible transformations between ketones and alcohols via sequential hydrogenation dehydrogenation reactions are efficiently achieved using a single precatalyst HRu(bMepi)(PPh3)(2) (bMepi = 1,3-bis(6'-methyl-2'-pyridylimino)isoindolate). The catalytic mechanism of HRu(bMepi)(PPh3)(2) mediated acceptorless alcohol dehydro-genation (AAD) has been investigated by a series of kinetic and isotopic labeling studies, isolation of intermediates, and evaluation of Ru(b4Rpi)(PPh3)(2)Cl (R = H, Me, Cl, OMe, OH) complexes. Two limiting dehydrogenation scenarios are interrogated: inner-sphere beta-H elimination and outer-sphere bifunctional double hydrogen transfer. Isotopic labeling experiments demonstrated that the proton and hydride transfer in a stepwise manner. Catalyst modifications suggest that the imine group on the bMepi pincer scaffold is not necessary for catalytic alcohol dehydrogenation. Evaluation of the kinetic experiments and catalyst modifications suggests a pathway whereby HRu(bMepi)(PPh3)(2) operates via the inner-sphere beta-H elimination mechanism. Following a single PPh3 dissociation, an alcohol substrate can bind and undergo proton transfer followed by a turnover-limiting beta-H elimination step. Analysis of the Eyring plot established activation parameters for the beta-H elimination reaction as Delta H-double dagger = 15(1) kcal/mol and Delta S-double dagger = -41(3) eu. AAD reactions using a series of Ru(b4Rpi)(PPh3)(2)Cl complexes indicated that the ortho-substituted methyl groups of bMepi slightly impede catalytic activity, and electronic modifications of the pincer scaffold have a minimal effect on the reaction rate.
