The Puzzling Question about the Origin of the Second Electron in the Molecular Photocatalytic Reduction of CO₂

Abstract

Herein, a new supramolecular photocatalyst RuRe3 containing three Re(dmb)CO 3 Cl (dmb = 4,4'-dimethyl-2,2'-bipyridine) (Re) building blocks connected through an ethylene bridge to one [Ru(dmb) 3 ] 2 + -unit (Ru) is presented. We investigated the photophysical properties of this novel tetranuclear complex and compared these to compounds with one (RuRe) and two (RuRe2) catalytic units. Under irradiation, all three photocatalysts exhibit high activity and photostability for the reduction of CO 2 to CO, with RuRe3 achieving the highest turnover number (11800) reported to date for a Re(I)/Ru(II)containing homogeneous catalyst. This tetranuclear complex is especially superior at small catalyst concentrations, which is attributed to an efficient second electron transfer via an intramolecular mechanism. Intermolecular electron transfer from small and mobile Re to RuRe motifs are found to also increase the catalytic performance of the system to a similar level (turnover number = 12100). These synergistic effects are attributed to an improved catalytic cycle, stabilizing the bi-and tetrametallic complexes by providing the electrons quickly and effectively. Since the second electron provision is not finally clarified for molecular systems until today, our photocatalytic studies present important insights into this crucial step. Further, these investigations should be considered for the design and synthesis of new and efficient supramolecular CO 2 -reducing photocatalysts.

Bimetallic Re(i)/Pt(ii)/Pd(ii) complexes: study of synthesis and properties

“…[27][28][29] Several bimetallic Ru(II)-Re(I) polypyridyl complexes, where the metal ions are connected through various bridging ligands, have been reported and demonstrated to have potential applications as efficient photocatalysts for CO 2 fixation. [30][31][32] By comparison, bimetallic compounds that contain a Re(I) unit and other metal ions such as Pt(II) and Pd(II) have not been investigated extensively, and only a few examples are known in the literature. 33,34 In a previous study, we reported the ligand L1 (Chart 1) that has two different binding units, i.e., 2-(2 0 -pyridyl)benzimidazolyl (py-im) and 2-(2 0 -pyridyl)indolyl (py-in) moieties, linked together by a benzene ring.…”

“…27–29 Several bimetallic Ru( ii )–Re( i ) polypyridyl complexes, where the metal ions are connected through various bridging ligands, have been reported and demonstrated to have potential applications as efficient photocatalysts for CO 2 fixation. 30–32 By comparison, bimetallic compounds that contain a Re( i ) unit and other metal ions such as Pt( ii ) and Pd( ii ) have not been investigated extensively, and only a few examples are known in the literature. 33,34…”