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

Abstract

A series of four molecular ruthenium hydrido complexes supported by previously reported rigid PC_carbeneP pincer ligand frameworks were evaluated as formic acid dehydrogenation (FAD) catalysts. The ligands in the complexes L_RRu(H)X (R = H, NMe₂; X = Cl, κ²-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 L_HRu(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 13 M 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 CO₂via β-elimination from the resulting formato complex.