On the catalytic activity of [RuH₂(PPh₃)3(CO)] (PPh₃ = triphenylphosphine) in ruthenium-catalysed generation of hydrogen from alcohol: a combined experimental and DFT study

Using Density Functional Theory calculations (at the B97-D2//BP86 level) and measurements of kinetic isotope effects, we explored the mechanism of [RuH₂(PPh₃)₃(CO)] (22) in catalytic acceptor-less dehydrogenation of methanol to formaldehyde. 22 is found to exhibit a similar activity as the previously studied [RuH₂(H₂)(PPh₃)₃] (1b) complex. On the computed pathway, η² → η¹ slippage of Ru-bound formaldehyde prior to decoordination is indicated to be rate-limiting, consistent with the low k_H/k_D KIE of 1.3 measured for this reaction. We also explored computationally the possibility of achieving complete dehydrogenation of methanol (into CO₂ and H₂), through subsequent decarbonylation of formaldehyde and water-gas shift reaction of the resulting carbonyl complex. Complete pathways of this kind are traced for 22 and for [RuH₂(PPh₃)₂(CO)₂]. An alternative mechanism, involving a gem-diol intermediate (obtained upon attack of OH^- to coordinated formaldehyde), has also been investigated. All these pathways turned out to be unfavourable kinetically, in keeping with the lack of CO₂ evolution experimentally observed in this system. Our calculations show that the reactions are hampered by the low electrophilicities of the CO and HCHO ligands, making OH^- uptake unfavourable. Consequently, the subsequent intermediates are too high-lying on the reaction profiles, thus leading to high kinetic barriers and preventing full dehydrogenation of methanol to occur by this kind of mechanism.