Electrocatalytic Reduction of CO2 Using the Complexes [Re(bpy)(CO)3L]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

Abstract

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 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-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 five-coordinate 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) 3 (bpy)L] n ( n = 0, L = Cl -, CF 3 SO 3 -; n = +1, L = CH 3 CN), 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.