Ruthenium-catalyzed hydrogenation of carbon dioxide to formic acid in alcohols

摘要

Catalytic hydrogenation of CO 2 to formic acid with the (solvento)metal hydride complex, TpRu(PPh 3)(CH 3CN)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 3CH 2OH, the observable intermediates are [TpRu(PPh 3)(CH 3CN) 2] +CF 3CH 2O - and the alkyl carbonate complex, TpRu(PPh 3)(η 2O 2COCH 2CF 3), which are formed by the reaction of CO 2 with the alkoxide species, TpRu(PPh 3)(CH 3CN)(OCH 2CF 3), generated by a very facile reaction between TpRu(PPh 3)(CH 3CN)H and CF 3CH 2OH. 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 3CN)H is higher in CF 3CH 2OH 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 3CH 2OH)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.