Ru-nitrosyl complex salts as efficient catalysts for the reversible CO2 hydrogenation/FA dehydrogenation in ionic liquids

We demonstrate that bench-stable ruthenium-PNP nitrosyl complex salts with different counteranions (Cl-, BF4-, BPh4-, PF6-, and OTs-) and a ruthenium-POP nitrosyl complex are competent (pre-)catalysts for the CO2 hydrogenation to formic acid (FA) at low temperature in ionic liquids. Only little effect of variation of the counteranion was observed and weakly basic ionic liquids such as EMIM OAc, BMIM OAc, and EMIM HCO2 were suitable for this transformation, affording conversions up to 94 mol% formic acid compared to the ionic liquid (FA/IL) and turnover numbers (TONs) up to 1305. Importantly, the same catalytic system was also efficient for the dehydrogenation of formic acid back to CO2 and H2, affording conversions up to >95% (949 TON) after 3 h at 95 °C. To investigate the application of such protocol for hydrogen storage and transportation purposes, hydrogenation/dehydrogenation cycles were performed, showing that this new catalytic system can promote up to 10 reversible CO2 hydrogenation/FA dehydrogenation cycles before losing its activity.