The mechanism of aqueous methanol dehydrogenation to yield carbon dioxide, either producing H₂ in the absence of acceptor or transferring hydrogen to acetophenone to yield 1-phenylethanol, has been elucidated by a combination of DFT calculations, which includes solvating MeOH molecules, and NMR/kinetics experimental investigations.

Abstract

The mechanisms of the Cp*Ir^III(bpyOO)-catalyzed (bpyOO=bidentate (NN) doubly deprotonated 2,2′-bipyridine-6,6′-diol) acceptorless methanol dehydrogenation and acetophenone transfer hydrogenation by methanol under basic conditions have been explored by the combination of ¹H NMR, kinetics, and DFT computational studies. During dehydrogenation of methanol and of its dehydrogenated derivatives, the presence of two iridium hydride species (anionic [Cp*Ir(bpyOO)H]⁻, C* and neutral [Cp*Ir(bpyOOH)H], D*), which interconvert depending on pH, was detected. The DFT studies on a Cp model system highlighted three interrelated catalytic cycles of methanol, formaldehyde and formic acid dehydrogenation, all leading to the same hydride intermediates C and D. The dehydrogenation of methanol prefers a direct β-hydride transfer pathway from the methoxide ion to Ir, rather than the classical β-hydride elimination pathway from a coordinated methoxide ligand, but an alternative bifunctional H⁺/H⁻ transfer with involvement of a ligand O atom may become competitive at lower pH. The transfer hydrogenation of acetophenone using methanol as hydrogen source features species C* as resting state, with the acetophenone reduction being rate-determining and following the reverse pathway of methanol oxidation, with a first-order acetophenone decay and a kinetic isotope effect of 2.36±0.09.