ELECTROCATALYTIC REDUCTION OF CO2 USING THE COMPLEXES [RE(BPY)(CO)(3)L](N) (N=-, OTF(-) BPY=2,2'-BIPYRIDINE OTF(-)=CF3SO3) AS CATALYST PRECURSORS - INFRARED SPECTROELECTROCHEMICAL INVESTIGATION(1, L=P(OET)(3),CH3CN N=0, L=CL)

This article describes the results of an IR spectroelectrochemical stu dy of the electrocatalytic reduction of carbon dioxide using the compl exes [Re(CO)(3)(bpy)L](n) (bpy = 2,2'-bipyridine; n = 0, L = Cl-, CF3S O3-; n = +1, L = CH3CN, P(OEt)(3)) as catalyst precursors. The study w as performed for the first time with an optically transparent thin-lay er electrochemical (OTTLE) cell. The results confirm unambiguously the catalytic activity of the reduced five-coordinate complexes, the radi cal [Re(CO)(3)(bpy)](.) and the anion [Re(CO)(3)(bpy)](-). The catalyt ic 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 radi cal and anionic catalysis depending on the solvent used. The complex [ Re(CO)(3)(bpy)Cl], so far the most studied catalyst precursor, upon on e-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 C O2, which may only take place after dissociation of the chloride ligan d. This conclusion also applies to other six-coordinate radicals [Re(C O)(3)(bpy)L](.) (L = CH3CN (in CH3CN) and P(OEt)(3)) whose catalytic r oute 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 relat ed [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-coord inate 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 cas e 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 poss ible in weakly coordinating THF, due to the instability of the radical [Re(CO)(3)(bpy)(THF)](.). The inherent stability of the radical [Re(C O)(3)(bpy){P(OEt)(3)}](.) was found convenient for the investigation o f 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.