Formic acid dehydrogenation using Ruthenium-POP pincer complexes in ionic liquids

Abstract

Formic acid is one of the most promising candidates for the long-term storage of hydrogen in liquid form. Herein, we present a new collection of ruthenium pincer complexes of the general formula [RuHCl(POP)(PPh₃)] using commercially available or easy-to-synthesize tridentate xantphos-type POP pincer ligands. We applied these complexes in the dehydrogenation of formic acid to CO₂ and H₂ using the ionic liquid BMIM OAc as solvent under reflux-free, mild conditions. The best performing catalyst with respect to maximum turnover frequency, the literature-known complex [RuHCl(xantphos)(PPh₃)], produced a turnover frequency of 4525 h^− 1 with 74% conversion after 10 minutes at 90°C and complete conversion (> 98%) occurring within 3 hours. On the other hand, the best overall performing catalyst, the novel complex [RuHCl(iPr-dbfphos)(PPh₃)], facilitated full conversion within 1 hour leading to an overall turnover frequency of 1009 h^− 1. Moreover, catalytic activity was observed at temperatures as low as 60°C. Finally, Arrhenius plots suggest activation energies between 50.4 and 63.7 kJ mol^− 1. Only CO₂ and H₂ are observed in the gas phase, with no CO detected.