Electrocatalytic reduction of CO2 using the complexes [Re(bpy)(CO)(3)L](n) (n=+1, L=P(OEt)(3), CH3CN; n=0, L=Cl-, Otf(-); bpy=2,2'-bipyridine; Otf(-)=CF3SO3) as catalyst precursors: Infrared spectroelectrochemical investigation

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-, CF3SO3-; n = +1, L = CH3CN, 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 five-coordinate 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-axion [Re(CO)(3)(bpy)Cl](.-), which was previously believed to react directly with CO2. By contrast, this study demonstrates its stability toward attack by CO2, 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 = CH3CN (in CH3CN) 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 CH3CN 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 five-coordinate radicals [Re(CO)(3)(dmbpy)](.), capable of CO2 reduction (the 1e pathway), even in CH3CN, 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)(3)(bpy)L](n) (n = 0, L = Cl-, CF3SO3-; n = +1, L = CH3CN), 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 CO2 reduction are, independent of the 1e and 2e catalytic routes, CO, CO32-, and free CO2H-. The latter product is formed via one-electron reduction of the radical anion [Re(CO)(3)(bpy)(CO2H)](.-), which is the main byproduct in the catalytic cycle.