Photocatalytic CO ₂ reduction with high turnover frequency and selectivity of formic acid formation using Ru(II) multinuclear complexes

Abstract

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 ...

Full‐Spectrum Solar‐Light‐Activated Photocatalysts for Light–Chemical Energy Conversion

“…Since the work reported by Lehn et al in 1983, [92] many kinds of metal complexes that contain Re or Ru have been studied to realize the selective formation of CO or HCOOH. [93] These metal complexes play the role of co-catalyst and supply the proper lowest unoccupied molecular orbital (LUMO) to reduce CO 2 with the electron transported from the photocatalyst/photosensitizer. For instance, Maeda et al [90] constructed different Ru complexes (trans(Cl)-[Ru(bpyX 2 )(CO) 2 Cl 2 ] (bpyX 2 = 2,2′-bipyridine with substituents X in the 4-positions, X = H, CH 3 , PO 3 H 2 , or CH 2 PO 3 H 2 ) with graphitic C 3 N 4 .…”

Supramolecular activation of a molecular photocatalyst

“…Recent examples of intramolecular photocatalysts include water oxidation catalysts, water reduction catalysts and carbon dioxide reducing catalysts. [2][3][4][5][6][7][8][9][10][11] In direct comparison with intermolecular [12][13][14][15][16] and heterogeneous photocatalysts, [17][18][19][20] intramolecular photocatalysis, however, often suffer from lower catalytic activity. It is therefore of paramount importance to learn more about the factors determining the catalytic activity of intramolecular photocatalysts.…”

Product Selectivity in Homogeneous Artificial Photosynthesis Using [(bpy)Rh(Cp*)X]n+-Based Catalysts

“…This was ascribed to fast back electron transfers as a consequence of the rather slow decomposition kinetics of (BNA)2 [23,24,115]. In contrast to the findings of the Ishitani group, where BNAH and (BNA)2 act as competing excited state quenchers during the photocatalysis, several other reports indicate that the dimeric species can also serve as an effective electron donor.…”

“…Therefore, they hypothesized that the dimeric species (BNA) 2 , despite its higher reduction potential in comparison to the monomeric BNAH, represents the endpoint of the photooxidation pathway. This was ascribed to fast back electron transfers as a consequence of the rather slow decomposition kinetics of (BNA) 2 [23,24,115].…”

“…Impressive selectivities and TONs (turnover numbers) have also recently been obtained with a cobalt cryptate as the catalyst [22]. In contrast to the mentioned Re complexes, the Ru-based catalyst of the general composition [(NN)Ru(CO)2Cl2] or [(NN)2RuX2] (X = CO, Cl, DMF; NN = N,N-chelating diimine ligand) shows a selectivity towards formate production in the majority of cases, although sometimes CO can also be obtained as the major product [8,[23][24][25].…”