Rapid and selective formic acid dehydrogenation catalysis by molecular ruthenium hydrides supported by rigid PCcarbeneP pincer ligands

A series of four molecular ruthenium hydrido complexes supported by previously reported rigid PCcarbeneP pincer ligand frameworks were evaluated as formic acid dehydrogenation (FAD) catalysts. The ligands in the complexes LRRu(H)X (R = H, NMe2; X = Cl, kappa-2-O2CH) differ in the electron richness by substitution on the aryl groups linking the di-iso-propylphosphine arms to the central carbene donor. We find that only the unsubstituted (R = H) chloro and formato complexes are effective catalyst precursors; the NMe2 substituted derivatives decompose under catalytic conditions. However, the two compounds LHRu(H)X are highly active (TOF = 1300-4200 h-1), long lived (TON up to 122 000) and selective (dihydrogen and carbon dioxide are the sole products) at 21˚C with no base additives necessary in 13M formic acid in water/dioxane. These performance metrics compare well with state of the art catalysts operating under ambient conditions. Mechanistic experiments support a simple two-step mechanism involving rate limiting protonolysis of the Ru-H by formic acid to release H2 and rapid loss of CO2 via -elimination from the resulting formato complex. This second step is facilitated kinetically because the donor properties of the X ligands lower the barrier to formation of the 1 binding mode for the formato ligand, needed to access the -elimination transition state for CO2 loss. Structural and DFT studies support this notion.