Among several greenhouse gases such as water vapor, methane, ozone, and nitrous oxide, anthropogenic carbon dioxide contributes to global warming most significantly. Chemists have considered recycling carbon dioxide as a renewable and environmentally friendly source of carbon to reduce our dependence on petrochemicals. However, there are only a few industrial processes that utilize CO₂ because of its high kinetic and thermodynamic stability with high oxidation state of carbon.

The best way to overcome such difficulties could be transition metal-based catalysis. In recent years, many methods for catalytic conversion of CO₂ have been developed. Among them, hydrogenation of CO₂ to formic acid is an attractive approach, as formic acid serves as a versatile carbon source through many transformations in organic chemistry. Ru, Rh, Ir, and Fe complexes have been reported to be active for homogeneous catalytic hydrogenation of CO₂. A PNP pincertype Ir (III) catalyst developed by Tanaka et al. exhibited the highest TON of 3,500,000 to date [1]. Despite this remarkable achievement, more economical and biorelevant metal-based catalytic systems are required. Fe-based CO₂ hydrogenation catalysts recently developed independently by Beller [2] and Milstein [3] were highly attractive, but TONs are in the range of 508~5104, which requires significant improvement.

More promising solution to global energy problem is the hydrogenation of CO₂ to produce methanol. Methanol is considered an excellent fuel for fuel cells and combustion engines [4]. Hydrogenation of CO₂ to methanol has favorable thermodynamics, but high activation energy barriers should be overcome by appropriate catalysts. Heterogeneous catalysts have been actively investigated with some success, but still the conditions are too harsh with the requirement of high temperature and high pressure of CO₂ and hydrogen. Homogeneous direct hydrogenation of CO₂ to methanol is less efficient than heterogeneous catalysis and has been pursued especially with Ru catalysts [5,6]. Recently developed indirect routes to methanol from carbonates, readily available from CO₂, are organic chemists’ notable approaches to the problem [7,8].

In summary, hydrogenation of chemically stable carbon dioxide is a highly attractive solution to the recycling of the carbon source. New concepts and approaches for the development of highly efficient and economical CO₂ hydrogenation catalysts are long-awaited for the practical utilization of CO₂ as a renewable carbon source.