Hydrogen Generation from Alcohols Catalyzed by Ruthenium Triphenylphosphine Complexes: Multiple Reaction Pathways

We report a comprehensive density functional theory (DFT) study of the mechanism of the methanol dehydrogenation reaction catalyzed by [RuH2(H-2)(PPh3)(3)]. Using the B97-D dispersion-corrected functional, four pathways have been fully characterized, which differ in the way the critical beta-hydrogen transfer step is brought about (e.g., by prior dissociation of one PPh3 ligand). All these pathways are found to be competitive (Delta G double dagger = 27.0-32.1 kcal/mol at 150 degrees C) and strongly interlocked. The reaction can thus follow multiple reaction channels, a feature which is expected to be at the origin of the good kinetics of this system. Our results also point to the active role of PPh3 ligands, which undergo significant conformational changes as the reaction occurs, and provide insights into the role of the base, which acts as a co-catalyst by facilitating proton transfers within active species. Activation barriers decrease on going from methanol to ethanol and 2-propanol substrates, in accord with experiment.