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

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)(PPh 3 )] using commercially available or easy-to-synthesize tridentate xantphos-type POP pincer ligands. We applied these complexes in the dehydrogenation of formic acid to CO 2 and H 2 using the ionic liquid BMIM OAc (1-butyl-3-methylimidazolium acetate) as solvent under mild, reflux-free conditions. The best performing catalyst with respect to maximum turnover frequency, the literature-known complex [RuHCl(xantphos)(PPh 3 )] Ru-1 , produced a maximum turnover frequency of 4525 h −1 with 74% conversion after 10 min at 90 °C and complete conversion (> 98%) occurring within 3 h. On the other hand, the best overall performing catalyst, the novel complex [RuHCl( i Pr-dbfphos)(PPh 3 )] Ru-2 , facilitated full conversion within 1 h leading to an overall turnover frequency of 1009 h −1 . Moreover, catalytic activity was observed at temperatures as low as 60 °C. Only CO 2 and H 2 are observed in the gas phase, with no CO detected. High-resolution mass spectrometry suggests the presence of N -heterocyclic carbene complexes in the reaction mixture.