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

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.