Highly active and long lived homogeneous catalyst for the dehydrogenation of dimethylamine borane starting with ruthenium(III) acetylacetonate and oleylamine precatalyst

Abstract

Herein, we report the results of our study on the homogeneous catalytic dehydrogenation of dimethylamine borane in the presence of Ru(acac)3 and oleylamine (OAm) in toluene solution. The precatalyst system comprising Ru(acac)3 and 3 equivalent OAm shows the high catalytic activity in hydrogen generation from dimethylamine borane solution. Monitoring the hydrogen evolution shows the presence of an induction period during which the homogeneous catalyst is formed, and then the hydrogen evolution continues almost linearly until the complete dehydrogenation of dimethylamine borane. UV-vis spectroscopic observation implies on the formation of a ruthenium(II) species. Using UV-vis, FTIR, Mass, 1H and 13C NMR spectroscopies we could identify the presence of trans-[Ru(acac)2(OAm)2] in the solid residue obtained after the catalytic reaction. This complex might be a stable form of the active catalyst as it shows similar catalytic activity in dehydrogenation of dimethylamine borane as before. The precatalyst system comprising Ru(acac)3 and 3 equivalent OAm provides 15,000 turnovers over 8 days with a constant turnover frequency of TOF = 77.8 h-1 in hydrogen generation from the dehydrogenation of dimethylamine borane in toluene solution at 60.0 ± 0.1 °C. Adding 3 equivalent OAm per Ru into the precatalyst solution increases the catalytic activity by a factor of ∼5 and the lifetime of the catalyst by a factor of >9 compared to the ones obtained by using sole Ru(acac)3. Kinetics of catalytic dehydrogenation of dimethylamine borane starting with Ru(acac)3 and 3 equivalent OAm in toluene solution were studied depending on catalyst concentration, substrate concentration and temperature. The catalytic reaction was found to be first-order with respect to each of both Ru(acac)3 and dimethylamine borane concentrations. The activation energy for the catalytic dehydrogenation is Ea = 58 ± 2 kJ/mol. © 2015 Published by Elsevier B.V.