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

Catalytic hydrogenation of CO 2 to formic acid with the (solvento)metal hydride complex, TpRu(PPh 3 )(CH 3 CN)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 CO 2 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, CF 3 CH 2 OH, the observable intermediates are [TpRu(PPh 3 )(CH 3 CN) 2 ] + CF 3 CH 2 O − and the alkyl carbonate complex, TpRu(PPh 3 )(η 2 ‐O 2 COCH 2 CF 3 ), which are formed by the reaction of CO 2 with the alkoxide species, TpRu(PPh 3 )(CH 3 CN)(OCH 2 CF 3 ), generated by a very facile reaction between TpRu(PPh 3 )(CH 3 CN)H and CF 3 CH 2 OH. 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 CO 2 in hydrous THF. The formic acid is produced by the transfer of a hydride and a proton from the transient alcohol hydride intermediate, TpRu(PPh 3 )(ROH)H, to an approaching CO 2 molecule. The activity of TpRu(PPh 3 )(CH 3 CN)H is higher in CF 3 CH 2 OH than in methanol and other non‐acidic alcohols and it is probably due to the enhanced electrophilicity of the carbon atom of CO 2 , which results from the strong interaction between the proton of the highly acidic alcohol in TpRu(PPh 3 )(CF 3 CH 2 OH)H and an oxygen atom of CO 2 . The electrophilic carbon atom of CO 2 could in turn abstract the hydride from Ru−H in a more facile manner. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)