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

Abstract

Catalytic 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)

Recent advances in catalytic hydrogenation of carbon dioxide

“…The intermediate transfers a hydride and a proton simultaneously to CO 2 to yield formic acid and itself is converted to a transient hydroxo species, which then associates a H 2 molecule to regenerate TpRu(PPh 3 )(H 2 O)H. The same group subsequently studied the effect of alcohol on the same catalyst. 266 The key species of the catalytic process, TpRu(PPh 3 )(ROH)H, is the alcohol analogue of the aquo hydride complex. Among the alcohols, pronounced promoting effect of CF 3 CH 2 OH is resulted from the enhanced electrophilicity of the carbon atom of CO 2 , which is due to the strong interaction between an oxygen atom of CO 2 and the highly acidic hydrogen in the intermediate.…”

Homogeneous Hydrogenation of Carbon Dioxide

“…There is no reason why this mechanism should not also be possible for CO 2 hydrogenation (Scheme 17.8 b) [6,43,68], although it has not yet been possible to demonstrate that this mechanism operates for any of the catalysts shown in Table 17.1. The concerted mechanism for CO 2 hydrogenation was more recently suggested by Lau's group [43,44] to explain the catalytic activity of the complex TpRuH(-MeCN)(PPh 3 ) in the presence of water, methanol and the acidic alcohol CF 3 CH 2 OH, the last of which offers the greatest rates of reaction. Pomelli et al [68] evaluated the possibility that bound formic acid could serve as the proton source in such a mechanism for RhH(diphosphine) catalyst, but found that the energy of the intermediates in the conventional insertion pathway were lower.…”

Fluorinated Alcohols as Solvents, Cosolvents and Additives in Homogeneous Catalysis

“…The hydrogenation of carbon dioxide with Tp-Ru(PPh 3 )(MeCN)H [Tp = hydrotris(pyrazolyl)borate] as a catalyst was investigated (Table 5). 28 Among the different solvents tested, TFE was found to be superior (entry 4).…”