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.

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We report herein the mechanism of the photochemical ligand substitution reactions of a series of fac-[Re(X(2)bpy)(CO)(3)(PR(3))](+) complexes (1) and the properties of their triplet ligand-field ((3)LF) excited states. The reason for the photostability of the rhenium complexes [Re(X(2)bpy)(CO)(3)(py)](+) (3) and [Re(X(2)bpy)(CO)(3)Cl] (4) was also investigated. Irradiation of an acetonitrile solution of 1 selectively gave the biscarbonyl complexes cis,trans-[Re(X(2)bpy)(CO)(2)(PR(3))(CH(3)CN)](+) (2). Isotope experiments clearly showed that the CO ligand trans to the PR(3) ligand was selectively substituted. The photochemical reactions proceeded via a dissociative mechanism from the (3)LF excited state. The thermodynamical data for the (3)LF excited states of complexes 1 and the corrective nonradiative decay rate constants for the triplet metal-to-ligand charge-transfer ((3)MLCT) states were obtained from temperature-dependence data for the emission lifetimes and for the quantum yields of the photochemical reactions and the emission. Comparison of 1 with [Re(X(2)bpy)(CO)(3)(py)](+) (3) and [Re(X(2)bpy)(CO)(3)Cl] (4) indicated that the (3)LF states of some 3- and 4-type complexes are probably accessible from the (3)MLCT state even at ambient temperature, but these complexes were stable to irradiation at 365 nm. The photostability of 3 and 4, in contrast to 1, can be explained by differences in the trans effects of the PR(3), py, and Cl(-) ligands.