A computational mechanistic study of pH-dependent alcohol dehydrogenation catalyzed by a novel [C,N] or [C,C] cyclometalated Cp*Ir complex in aqueous solution

Abstract

Developing efficient dehydrogenation is critical to understanding organic hydride hydrogen storage. The catalytic mechanism of the pH-dependent acceptorless-alcohol-dehydrogenation in aqueous solution catalyzed by a novel [C,N] cyclometalated Cp*Ir-complex, [Ir^III(Cp*)-(4-(1H-pyrazol-1-yl-κN²)benzoic acid-κC³)(H₂O)]₂SO₄, has been investigated using density functional theory (DFT) with the M06 dispersion-corrected functional. Using water as the solvent with liberation of dihydrogen represents a safe and clean process for such oxidations. The overall catalytic cycle has been fully characterized. The pre-catalyst A_Ir first reacts with the ethanol in basic solution to generate an active hydride complex D_Irvia an inner-sphere mechanism, involving the hemi-decoordination of [C,N] ligand followed by the β-H elimination. Subsequently, the complex D_Ir interacts with the protons in acid solution to generate H₂ molecules, which is a downhill process nearly without an energy barrier. The present theoretical results have shown that both the hydroxyl in basic solution and the proton in acidic solution play a crucial role in promoting the whole catalytic cycle. Therefore, our results theoretically demonstrated a significant dependence of the reaction system studied on pH value. The present study also predicts that the F_Ir (at the first triplet excited state, T₁) formed from D_Ir under laser excitation can catalyze the dehydrogenation of ethanol. Remarkably, the replacement of Ir by Ru may yield an efficient catalyst in the present system.