Efficient Catalyst for Acceptor less Alcohol Dehydrogenation: Interplay of Theoretical and Experimental Studies

The promoterless AAD (acceptorless alcohol dehydrogenation) reaction mediated by an iridium catalyst Cp*Ir(bpyO) 1-Ir (Cp* = pentamethylcyclopentadienyl, bpyO = alpha,alpha'-bipyridonate) has been theoretically investigated with the density functional theory. The reaction occurs through three steps, including alcohol dehydrogenation, formation of dihydrogen complex, and H-2 elimination from the iridium center. In the first two steps, the metal center and the bpyO ligand work cooperatively via the aromatization/dearomatization process of the bpyO ligand. The second step is rate-determining, where the Delta G(0 not equal) and Delta G(0) values are 23.9 and 13.9 kcal/mol, respectively. Our calculations demonstrate that the aromatization of the bpyO ligand as well as the charge transfer (CT) from the Cp* ligand to the iridium center plays important roles in stabilizing the transition state of the rate-determining step. We have theoretically and experimentally examined the 4d rhodium analogue Cp*Rh(bpyO) 1-Rh and found that it exhibits similar activity to that of 1-Ir. On the basis of those results, a new catalyst (HMB)Ru(bpyO) 1-Ru (HMB = hexamethylbenzene) is designed both theoretically and experimentally, where a cheaper and more abundant 4d ruthenium element is employed with the HMB and bpyO ligands. Theoretical calculations certainly show that 1-Ru is active for the promoterless AAD reaction via the same reaction mechanism as that of the reaction by 1-Ir. The experiments also demonstrate that 1-Ru is as efficient as 1-Ir for the AAD reaction.