Dehydrogenation of Formic Acid Catalyzed by Water-Soluble Ruthenium Complexes: X-ray Crystal Structure of a Diruthenium Complex

Abstract

Dehydrogenation of formic acid over various Ru-arene complexes containing N-donor chelating ligands was investigated in H2O and isolated and characterized several important catalytic intermediate species to elucidate the reaction pathway for formic acid dehydrogenation. Among the studied complexes, Ru-arene complexes, namely [(η6-C6H6)Ru(κ2-NpyNH2-AmQ)Cl]+ (C-2), [(η6-C10H14)Ru(κ2-NpyNH2-AmQ)Cl]+ (C-3) and [(η6-C6H6)Ru(κ2-NpyNHMe-MAmQ)Cl]+ (C-4) [AmQ = 8-aminoquinoline and MAmQ = 8-(N-methylamino)quinoline] were proved to be the efficient catalysts for formic acid dehydrogenation at 90 °C, even in the absence of base. With an initial TOF of 940 h–1, complex C-4 displayed the highest catalytic activity for formic acid dehydrogenation in H2O and it can be recycled up to 5 times with a TON of 2248. Effect of temperature, pH, formic acid and catalyst concentration on the reaction kinetics were also investigated in detail. Extensive mechanistic investigations using mass spectrometry and NMR evidenced the formation of a coordinatively unsaturated species [(η6-C6H6)Ru(κ2-NpyNH-AmQ)]+ (C-2A)/[(η6-C6H6)Ru(κ2-NpyNMe-MAmQ)]+ (C-4A) as the active component during the catalytic dehydrogenation of formic acid. We further characterized the dimer-form of C-2A, possibly the catalyst resting state, by single-crystal X-ray crystallography.