Mechanistic Insights into Formic Acid Dehydrogenation and Carbon dioxide Amidation Using Electrophilic Ru(II)-Complexes

The [RuCl(dppe)2][OTf] (1) complex dehydrogenates formic acid under ambient conditions and results in the formation of trans-[Ru(η2-H2)Cl(dppe)2][OTf] (2) and trans-[Ru(η2-H2)H(dppe)2][OTf] (3) complexes. Addition of sodium formate to this reaction mixture increased the rate of formic acid dehydrogenation and complex 3 was obtained as the final product. Furthermore, complex 1 dehydrogenates formic acid catalytically in the presence of Hunig base. After several catalytic cycles, quantitative amounts of H2 and CO2 were produced at 298 K. The proposed formate bound intermediates cis-[2-Ru(HCO2)(dppe)2] were too unstable to be observable (or isolable), however, an analogous cis-[Ru(2-CF3CO2)(dppe)2][OTf] complex (6) was synthesized and characterized. This complex also dehydrogenates formic acid and led to the formation of complex 3. Based on NMR spectroscopic studies and other related chemical reactions, a plausible mechanism for formic acid dehydrogenation using complex 1 has been proposed. Moreover, 13C NMR spectral data on transfer hydrogenation of CO2 using complex 1 in presence of tert-butyl amine-borane (TBAB) as a secondary hydrogen source resulted in the amidation of CO2 to tert-butyl formamide.