An In-Depth Mechanistic Study of Ru Catalysed Aqueous Methanol Dehydrogenation and Prospects for Future Catalyst Design

Herein we provide mechanistic insights into the dehydrogenation of aqueous methanol catalysed by the [Ru(trop₂dae)] complex (which is in-situ generated from [Ru(trop₂dad)]), established by density functional theory based molecular dynamics (DFT-MD) and static DFT calculations incorporating explicit solvent molecules. The aqueous solvent proved to participate actively in various stages of the catalytic cycle including the catalyst activation process, and the key reaction steps involving C-H activation and hydrogen production. The aqueous solvent forms an integral part of the reactive system for the C-H activation steps in the [Ru(trop₂dae)] system, with strong hydrogen bond interactions with the anionic oxygen (RO^-, R = CH₃, CH₂OH, HCO) and hydride moieties formed along the reaction pathway. In contrast to the [Ru(trop₂dad)] catalyst, C-H activation and hydrogen production does not proceed via a metal-ligand cooperative pathway for the [Ru(trop₂dae)] system. The pK_a of the coordinated amine donors in these complexes provides a rationale for the divergent reactivity, and the obtained mechanistic information provides new guidelines for the rational design of active and additive free catalytic systems for aqueous methanol dehydrogenation.