Efficient Catalyst for Acceptorless 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 = α,α′-bipyridonate) has been theoretically investigated with the density functional theory. The reaction occurs through three steps, including alcohol dehydrogenation, formation of dihydrogen complex, and H₂ 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 ΔG^0≠ and ΔG⁰ 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.