Formic acid dehydrogenation using Ruthenium-POP pincer complexes in ionic liquids
Alexander Tobias Nikol, Brenda Rabell, Mike Steffen Bernhard Jørgensen, René Wugt Larsen, Martin Nielsen
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)(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.