Mechanistic Insights of the Ir‐bipyridonate Catalyzed Aqueous Methanol Dehydrogenation and Transfer Dehydrogenation to Acetophenone: Experimental and DFT Study

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 1 H 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.