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A rhenium(I) tricarbonyl diimine complex with a N,N-dimethylformamide ligand captures one CO2 molecule in the presence of triethanolamine (TEOA), giving fac-[Re(I)(bpy)(CO)3{R2N-CH2CH2O-COO}] (bpy = 2,2'-bipyridine, R = CH2CH2OH). This could be a predominant complex in various photocatalytic CO2 reduction reactions using [Re(I)(N^N)(CO)3X](n+) (N^N = diimine ligand; X = monodentate ligand; n = 0, 1) type complexes in the presence of TEOA.

Previously undescribed supramolecules constructed with various ratios of two kinds of Ru(II) complexes-a photosensitizer and a catalyst-were synthesized. These complexes can photocatalyze the reduction of CO 2 to formic acid with high selectivity and durability using a wide range of wavelengths of visible light and NADH model compounds as electron donors in a mixed solution of dimethylformamide-triethanolamine. Using a higher ratio of the photosensitizer unit to the catalyst unit led to a higher yield of formic acid. In particular, of the reported photocatalysts, a trinuclear complex with two photosensitizer units and one catalyst unit photocatalyzed CO 2 reduction (Φ HCOOH ¼ 0.061, TON HCOOH ¼ 671) with the fastest reaction rate (TOF HCOOH ¼ 11.6 min −1 ). On the other hand, photocatalyses of a mixed system containing two kinds of model mononuclear Ru(II) complexes, and supramolecules with a higher ratio of the catalyst unit were much less efficient, and black oligomers and polymers were produced from the Ru complexes during photocatalytic reactions, which reduced the yield of formic acid. The photocatalytic formation of formic acid using the supramolecules described herein proceeds via two sequential processes: the photochemical reduction of the photosensitizer unit by NADH model compounds and intramolecular electron transfer to the catalyst unit. R ecently, global warming and shortages of fossil fuels and carbon resources have become serious issues. The development of technologies to convert CO 2 into useful organic compounds using sunlight as an energy source would serve as an ideal solution to these problems.Formic acid, which is the two-electron reduction product of CO 2 , has recently attracted attention as a storage source of H 2 (1, 2). Formic acid itself is an important chemical. It has been employed as a preservative and an insecticide and is also a useful acid, reducing agent, and source of carbon in synthetic chemical industries.Only a few photocatalysts for the selective formation of formic acid from CO 2 have been reported (3-8). Although oligo(p-phenylenes) (3) or a mixed system of phenazine and Co cyclam (4) successfully photocatalyzed the reduction of CO 2 to formic acid, these systems cannot work with visible light. It has been reported that ½RuðbpyÞ 2 ðCOÞ 2 2þ (bpy ¼ 2,2′-bipyridine) acted as a catalyst for reducing CO 2 (5-8). Under basic conditions, a mixed system of this complex with ½RuðbpyÞ 3 2þ as a redox photosensitizer photocatalyzed the reduction of CO 2 to formic acid with high selectivity (6, 8). However, this photocatalytic system is limited by instability as evidenced by the fact that the catalyst decomposed following prolonged irradiation and generated black precipitates.We have recently developed a unique architecture for constructing visible-light-driven supramolecular photocatalysts, consisting of a ½RuðN ∧ NÞ 3 2þ (N ∧ N ¼ a diimine ligand)-type complex as a photosensitizer and a Re(I) diimine complex as a catalyst (9-12). These supramolecules can selectively photocatalyze ...

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