Ruthenium‐Catalyzed Hydrogenation of Carbon Dioxide to Formic Acid in Alcohols

Abstract

AbstractCatalytic hydrogenation of CO2 to formic acid with the (solvento)metal hydride complex, TpRu(PPh3)(CH3CN)H [Tp = hydrotris(pyrazolyl)borate], in various alcohols was studied. High‐pressure NMR monitoring of the catalytic reaction in non‐acidic methanol shows that the observable intermediate is a formate complex resulting from CO2 insertion into the Ru−H bond and is stabilized by the hydrogen‐bonding interaction between the formato ligand and a methanol molecule. However, in the case of the acidic alcohol, CF3CH2OH, the observable intermediates are [TpRu(PPh3)(CH3CN)2]+CF3CH2O− and the alkyl carbonate complex, TpRu(PPh3)(η2‐O2COCH2CF3), which are formed by the reaction of CO2 with the alkoxide species, TpRu(PPh3)(CH3CN)(OCH2CF3), generated by a very facile reaction between TpRu(PPh3)(CH3CN)H and CF3CH2OH. We propose that the productive catalytic cycles of the reactions conducted in a variety of alcohols are similar to the one we formulated for the catalytic hydrogenation of CO2 in hydrous THF. The formic acid is produced by the transfer of a hydride and a proton from the transient alcohol hydride intermediate, TpRu(PPh3)(ROH)H, to an approaching CO2 molecule. The activity of TpRu(PPh3)(CH3CN)H is higher in CF3CH2OH than in methanol and other non‐acidic alcohols and it is probably due to the enhanced electrophilicity of the carbon atom of CO2, which results from the strong interaction between the proton of the highly acidic alcohol in TpRu(PPh3)(CF3CH2OH)H and an oxygen atom of CO2. The electrophilic carbon atom of CO2 could in turn abstract the hydride from Ru−H in a more facile manner. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)