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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 ...

Photocatalytic reduction of CO2 into energy-rich compounds utilizing solar light as an energy source is expected to provide a solution to serious problems of the shortage of fossil resources and global warming. In this perspective, we summarize advances in supramolecular photocatalysts for the reduction of CO2, of which photosensitizer and catalyst units are connected via a bridging ligand. The first successful Ru­(II)–Re­(I) supramolecular photocatalysts reported in 2005 indicated molecular architecture for developing efficient supramolecular photocatalysts for CO2 reduction to CO with high selectivity and durability. On the basis of this architecture, both the bridging ligands and Re­(I) catalyst unit were optimized to increase the photocatalytic activity. In addition, the compositional units of supramolecular photocatalytic systems were modified: (1) Ir­(III) and Os­(II) complexes, free- and metallo-porphyrins, and chlorophyll functioned as alternative or better photosensitizer units in comparison to the Ru­(II) complexes, (2) Ru­(II) carbonyl complexes reduced CO2 giving HCOOH selectively, and (3) dihydrobenzoimidazole derivatives were suitable sacrificial electron donors for evaluating the potential of supramolecular photocatalytic systems. These research studies have provided efficient photocatalytic systems for CO2 reduction with high selectivity, durability, and reaction rate under visible-light irradiation.

A novel molecular photocatalytic system with not only high reduction ability of CO but also high capture ability of CO has been developed using a Ru(II)-Re(I) dinuclear complex as a photocatalyst. By using this photocatalytic system, CO of 10% concentration could be selectively converted to CO with almost same photocatalysis to that under a pure CO atmosphere (TON > 1000, ΦCO > 0.4). Even 0.5% concentration of CO was reduced with 60% initial efficiency of CO formation by using the same system compared to that using pure CO (TON > 200). The Re(I) catalyst unit in the photocatalyst can efficiently capture CO, which proceeds CO insertion to the Re-O bond, and then reduce the captured CO by using an electron supplied from the photochemically reduced Ru photosensitizer unit.

New Ru(II)-Re(I) supramolecular photocatalysts with a rhenium(I) biscarbonyl complex as a catalyst unit were synthesized. They photocatalyzed CO2 reduction to CO using a wide-range of visible light, and their photocatalytic abilities were strongly affected by the phosphorus ligands on the Re site. Especially, Ru-Re(FPh), with two P(p-FPh)3 ligands, exhibited tremendous photocatalytic properties, i.e. TN(CO) = 207 and phi(CO) = 0.15, and, in addition, this is one of the fastest-operating photocatalysts for CO2 reduction to CO, with TF(CO) = 281 h(-1). We also clarified a balance of transferred electrons in this photocatalytic reaction and found that the two electrons necessary for CO formation were provided by two sequential reductive quenching processes of the excited Ru photosensitizer unit by the reductant BNAH.

Photocatalytic reduction of CO2 to formic acid with high efficiency, durability, and rate.

Rapid water oxidation catalysis is observed following electrochemical oxidation of [Ru(II)(tpy)(bpz)(OH)](+) to [Ru(V)(tpy)(bpz)(O)](3+) in basic solutions with added buffers. Under these conditions, water oxidation is dominated by base-assisted Atom Proton Transfer (APT) and direct reaction with OH(-). More importantly, we report here that the Ru(IV)═O(2+) form of the catalyst, produced by 1e(-) oxidation of [Ru(II)(tpy)(bpz)(OH2)](2+) to Ru(III) followed by disproportionation to [Ru(IV)(tpy)(bpz)(O)](2+) and [Ru(II)(tpy)(bpz)(OH2)](2+), is also a competent water oxidation catalyst. The rate of water oxidation by [Ru(IV)(tpy)(bpz)(O)](2+) is greatly accelerated with added PO4(3-) with a turnover frequency of 5.4 s(-1) reached at pH 11.6 with 1 M PO4(3-) at an overpotential of only 180 mV.

The novel supramolecular complexes, which are composed of an [Os(5dmb)2(BL)](2+)-type complex (5dmb = 5,5'-dimethyl-2,2'-bipyridine; BL = 1,2-bis(4'-methyl-[2,2'-bipyridin]-4-yl)ethane) as a photosensitizer and cis,trans-[Re(BL)(CO)2{P(p-X-C6H4)3}2](+)-type complexes (X = F, Cl) as a catalyst, have been synthesized. They photocatalyzed selective reduction of CO2 to CO under red-light irradiation (λ > 620 nm). The photocatalytic abilities were affected by the phosphine ligands on the Re unit, and the supramolecule with P(p-Cl-C6H4)3 ligands exhibited better photocatalysis (ΦCO = 0.12, TONCO = 1138, TOFCO = 3.3 min(-1)). The detailed studies clarified the electron balance and material balance; i.e., one molecule of the sacrificial electron donor (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH)) donated two electrons, one molecule of CO2 accepted the two electrons, and another CO2 molecule served as an "O(2-)" acceptor to give each molecule of the two-electron oxidized compound of BIH, CO, and HCO3(-).