A trace amount of alcohol cocatalyst and a stoichiometric amount of base are required during the hydrogenation of CO2 to formic acid catalyzed by ruthenium trimethylphosphine complexes. Variation of the choice of alcohol and base causes wide variation in the rate of reaction. Acidic, nonbulky alcohols and triflic acid increase the rate of hydrogenation an order of magnitude above that which can be obtained with traditionally used methanol or water. Similarly, use of DBU rather than NEt3 increases the rate of reaction by an order of magnitude. Turnover frequencies up to 95 000 h-1 have now been obtained, and even higher rates should be possible using the cocatalyst and amine combinations identified herein. Preliminary in situ NMR spectroscopic observations are described, and the possible roles of the alcohol and base are discussed.
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