Ru-Nitrosyl Complex Salts as Efficient Catalysts for the Reversible CO ₂ 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 temperatures in ionic liquids. Only a minor 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 protocols 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.