Unravelling the Mechanism of Basic Aqueous Methanol Dehydrogenation Catalyzed by Ru–PNP Pincer Complexes

Abstract

Ruthenium PNP complex 1a (RuH(CO)Cl(HN(C₂H₄Pi-Pr₂)₂)) represents a state-of-the-art catalyst for low-temperature (<100 °C) aqueous methanol dehydrogenation to H₂ and CO₂. Herein, we describe an investigation that combines experiment, spectroscopy, and theory to provide a mechanistic rationale for this process. During catalysis, the presence of two anionic resting states was revealed, Ru-dihydride (3^-) and Ru-monohydride (4^-) that are deprotonated at nitrogen in the pincer ligand backbone. DFT calculations showed that O- and CH- coordination modes of methoxide to ruthenium compete, and form complexes 4^- and 3^-, respectively. Not only does the reaction rate increase with increasing KOH, but the ratio of 3^-/4^- increases, demonstrating that the "inner-sphere" C-H cleavage, via C-H coordination of methoxide to Ru, is promoted by base. Protonation of 3^- liberates H₂ gas and formaldehyde, the latter of which is rapidly consumed by KOH to give the corresponding gem-diolate and provides the overall driving force for the reaction. Full MeOH reforming is achieved through the corresponding steps that start from the gem-diolate and formate. Theoretical studies into the mechanism of the catalyst Me-1a (N-methylated 1a) revealed that C-H coordination to Ru sets-up C-H cleavage and hydride delivery; a process that is also promoted by base, as observed experimentally. However, in this case, Ru-dihydride Me-3 is much more stable to protonation and can even be observed under neutral conditions. The greater stability of Me-3 rationalizes the lower rates of Me-1a compared to 1a, and also explains why the reaction rate then drops with increasing KOH concentration.