Formic acid is an important chemical product as well as a major player in synthetic chemistry as an acid, a reductant, and a carbon source.

Hydrogenation of carbon dioxide has been widely investigated using transition-metal complexes, especially with rhodium, ruthenium, and iridium catalysts. Noyori and co-workers carried out the reaction in supercritical carbon dioxide using a Ru(II) catalyst and obtained ammonium formate, alkyl formate, and formamide in high yields. For example, the highest turnover frequency (TOF) (95,000 h^-1) has been achieved using RuCl(OAc)(PMe₃₎₄, which is soluble in supercritical CO₂. Himeda reported a highly active cationic Cp*Ir(III) catalyst containing phenanthroline derivatives as ligands. The maximum turnover number (TON) of the hydrogenation reached 222,000 (the highest TON reported to date) in a basic aqueous solution.

—Tanaka et al.

[The turnover frequency is the number of molecules of a given product made (turnover) per catalytic site per unit time.]

The researchers performed catalytic hydrogenation of carbon dioxide in aqueous potassium hydroxide using a newly synthesized isopropyl-substituted PNP-pincer iridium trihydride complex as a catalyst. Potassium formate was obtained with turnover numbers up to 3,500,000 and a turnover frequency of 150,000 h^-1, both of which are the highest values reported to date.

Resources

• Ryo Tanaka, Makoto Yamashita and Kyoko Nozaki (2009) Catalytic Hydrogenation of Carbon Dioxide Using Ir(III)-Pincer Complexes. J. Am. Chem. Soc., Article ASAP doi: 10.1021/ja903574e