Architecture of Supramolecular Metal Complexes for Photocatalytic CO₂ Reduction:  Ruthenium−Rhenium Bi- and Tetranuclear Complexes

Abstract

We study the electrochemical, spectroscopic, and photocatalytic properties of a series of Ru(II)−Re(I) binuclear complexes linked by bridging ligands 1,3-bis(4′-methyl-[2,2′]bipyridinyl-4-yl)propan-2-ol (bpyC 3 bpy) and 4-methyl-4′-[1,10]phenanthroline- [5,6-d]imidazol-2-yl)bipyridine (mfibpy) and a tetranuclear complex in which three [Re(CO) 3 Cl] moieties are coordinated to the central Ru using the bpyC 3 bpy ligands. In the bpyC 3 bpy binuclear complexes, 4,4′-dimethyl-2,2′-bipyridine (dmb) and 4,4′-bis(trifluoromethyl)-2,2′-bipyridine ({CF 3 } 2 bpy), as well as 2,2′-bipyridine (bpy), were used as peripheral ligands on the Ru moiety. Greatly improved photocatalytic activities were obtained only in the cases of [Ru{bpyC 3 bpyRe(CO) 3 Cl} 3 ] 2+ (RuRe 3 ) and the binuclear complex [(dmb) 2 Ru(bpyC 3 bpy)Re-(CO) 3 Cl] 2+ (d 2 Ru−Re), while photocatalytic responses were extended further into the visible region. The excited state of ruthenium in all Ru−Re complexes was efficiently quenched by 1-benzyl-1,4-dihydronicotinamide (BNAH). Following reductive quenching in the case of d 2 Ru−Re, generation of the one-electron-reduced (OER) species, for which the added electron resides on the Ru-bound bpy end of the bridging ligand bpyC 3 bpy, was confirmed by transient absorption spectroscopy. The reduced Re moiety was produced via a relatively slow intramolecular electron transfer, from the reduced Ru-bound bpy to the Re site, occurring at an exchange rate (∆G ∼ 0). Electron transfer need not be rapid, since the rate-determining process is reduction of CO 2 with the OER species of the Re site. Comparison of these results with those for other bimetallic systems gives us more general architectural pointers for constructing supramolecular photocatalysts for CO 2 reduction.

Highly efficient photochemical HCOOH production from CO2 and water using an inorganic system

“…There have been a lot of attempts to realize similar reactions to natural photosynthesis using mainly organic complexes. Carbon monoxide (CO) [1][2][3][4] and formic acid (HCOOH) 5 have been successfully produced from CO 2 by light illumination. Research interest has recently extended to certain types of hybrid materials, [6][7][8][9] and reactor systems have been been reconsidered in connection with the shape of cataysts.…”

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

“…The requirements for constructing efficient supramolecular photocatalysts with Ru(II) and Re(I) complexes involves two key principles: First, in the triplet metal-to-ligand-charge-transfer ( 3 MLCT) excited state of the Ru(II) unit, the excited electron should be located at the bridging ligand. Second, a nonconjugated bridging ligand should be used because a conjugated system in the bridge ligand lowers the reducing power of the catalyst unit (9). The efficiencies and the stability of these supramolecular photocatalysts (Φ CO ¼ 0.12-0.15, TON CO ∼ 200) are much greater than those of the other reported supramolecular-type photocatalysts for both CO 2 reduction (13-16) and H 2 evolution (16)(17)(18)(19).…”

“…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)(10)(11)(12). These supramolecules can selectively photocatalyze the reduction of CO 2 to CO with high efficiency.…”

Conversion of CO2 via Visible Light Promoted Homogeneous Redox Catalysis

“…Therefore, strong electronic communication between the photosensitizer moiety and the reduction site lessens the photocatalytic activity even though it accelerates the electron transfer between them. [53]. Another consideration for supramolecular systems is adjustment of the distance between the covalently tethered metal centers.…”