This article describes the results of an IR spectroelectrochemical study of the electrocatalytic reduction of carbon dioxide using the complexes [Re(CO) 3 (bpy)L] n (bpy ) 2,2′-bipyridine; n ) 0, L ) Cl -, CF 3 SO 3 -; n ) +1, L ) CH 3 CN, P(OEt) 3 ) as catalyst precursors. The study was performed for the first time with an optically transparent thin-layer electrochemical (OTTLE) cell. The results confirm unambiguously the catalytic activity of the reduced fivecoordinate complexes, the radical [Re(CO) 3 (bpy)] • and the anion [Re(CO) 3 (bpy)] -. The catalytic behavior of these species could be investigated separately for the first time due to the application of complexes other than those with L ) halide, whose catalytic routes may involve simultaneously both radical and anionic catalysis depending on the solvent used. The complex [Re(CO) 3 (bpy)Cl], so far the most studied catalyst precursor, upon one-electron reduction gives the corresponding radical-anion [Re(CO) 3 (bpy)Cl] •-, which was previously believed to react directly with CO 2 . By contrast, this study demonstrates its stability toward attack by CO 2 , which may only take place after dissociation of the chloride ligand. This conclusion also applies to other six-coordinate radicals [Re(CO) 3 (bpy)L] • (L ) CH 3 CN (in CH 3 CN) and P(OEt) 3 ) whose catalytic route requires subsequent one-electron reduction to produce the anionic catalyst [Re(CO) 3 (bpy)] -(the 2e pathway). The catalytic route of [Re(CO) 3 (bpy)Cl] in CH 3 CN therefore deviates from that of the related [Re(CO) 3 (dmbpy)Cl], the other complex studied by IR (reflectance) spectroelectrochemistry, with the more basic ligand, 4,4′-dimethyl-2,2′-bipyridine (dmbpy). The latter complex tends to form the fivecoordinate radicals [Re(CO) 3 (dmbpy)] • , capable of CO 2 reduction (the 1e pathway), even in CH 3 CN, hence eliminating the possibility of the 2e pathway via the anion [Re(CO) 3 (dmbpy)] -, which operates in the case of the 2,2′-bipyridine complex. For [Re(CO) , the 1e catalytic route becomes possible in weakly coordinating THF, due to the instability of the radical [Re(CO) 3 (bpy)(THF)] • . The inherent stability of the radical [Re(CO) 3 (bpy){P(OEt) 3 }] • was found convenient for the investigation of the 2e pathway via [Re(CO) 3 (bpy)] -. The main, spectroscopically observed products of the CO 2 reduction are, independent of the 1e and 2e catalytic routes, CO, CO 3 2-, and free CO 2 H -. The latter product is formed via one-electron reduction of the radical anion [Re(CO) 3 (bpy)(CO 2 H)] •-, which is the main byproduct in the catalytic cycle.

Spectroelectrochemical (IR,UV/VIS) Determination of the Reduction Pathways for a Series of [Re(CO)C3(alpha-diimine)LE']E0E/E+ (LE'=Halide, Otf, THF, MeCN, n-PrCN, PPhC3C' P(OMe)C3) Complexes Stor, G.J.; Hartl, F.; van Outersterp, J.W.M.; Stufkens, D.J.

Efficient photocyclization from a low-lying triplet state is reported for a photochromic dithienylperfluorocyclopentene with Ru(bpy)3 units attached via a phenylene linker to the thiophene rings. The ring-closure reaction in the nanosecond domain is sensitized by the metal complexes. Upon photoexcitation into the lowest Ru-to-bpy 1MLCT state followed by intersystem crossing to emitting 3MLCT states, photoreactive 3IL states are populated by an efficient energy-transfer process. The involvement of these 3IL states explains the quantum yield of the photocyclization, which is independent of the excitation wavelength but decreases strongly in the presence of dioxygen. This behavior differs substantially from the photocyclization of the nonemissive dithienylperfluorocyclopentene free ligand, which occurs from the lowest 1IL state on a picosecond time scale and is insensitive to oxygen quenching. Cyclic voltammetric studies have also been performed to gain further insight into the energetics of the system. The very high photocyclization quantum yields, far above 0.5 in both cases, are ascribed to the strong steric repulsion between the bulky substituents on the dithienylperfluorocyclopentene bridge bearing the chelating bipyridine sites or the Ru(bpy)3 moieties, forcing the system to adopt nearly exclusively the reactive antiparallel conformation. In contrast, replacement of both Ru(II) centers by Os(II) completely prevents the photocyclization reaction upon light excitation into the low-lying Os-to-bpy 1MLCT state. The photoreaction can only be triggered by optical population of the higher lying 1IL excited state of the central photochromic unit, but its yield is low due to efficient energy transfer to the luminescent lowest 3MLCT state.