| The electrocatalytic reduction of carbon dioxide (CO 2) to carbon monoxide (CO) is explored for both rhenium and manganese complexes. Electrochemistry, X-ray crystallography, Infrared spectroelectrochemistry, and stopped-flow kinetics are employed in order to identify catalysts and probe their mechanism and selectivity.;Two catalysts in particular, Re(bipy-tBu)(CO)3 (L) and Mn(bipy-tBu)(CO)3L (where bipy- tBu = 4,4'-di-tert-butyl-2,2'-bipyridine and L = Cl-, Br- or (MeCN)(OTf)-), were studied extensively and displayed high activity, Faradaic efficiency, and selectivity for the reduction of CO2 to CO. The Re-Cl catalyst exhibits a turnover frequency of >200 s-1, one of the fastest reported rates for a catalyst with appreciable turnover number. The Mn catalysts, when Bronsted acid sources are added to the electrochemical solution, exhibit current densities rivaling those of the Re-Cl catalyst. Amazingly, these catalysts showed high selectivity for CO2 in both dry solvents and those with significant amounts of Bronsted acid added. Stopped-flow UV-Vis kinetics experiments showed that the reaction of the active form of the catalysts, [M(bipy-tBu)(CO)3]-1, is ca. 50 times faster with CO2 than they do with protons. Stopped-flow IR kinetics experiments comparing the reactions [Re(bipy- tBu)(CO)3]-1 with CO2 and [Re(bipy)(CO) 3]-1 with CO2 shows, that at equal CO 2 concentrations, the bipy-tBu analog reacts ten times faster than the bipy analog. CO2 also appears to react with both complexes via a concerted, two-electron oxidative addition of CO2 to the metal center.;The heterogenization of these catalysts was also explored with limited success. Intercalation, covalent bonds to gold, and covalent bonds to p-Si were all demonstrated, but none displayed activity towards the reduction of CO2. Future experiments are suggested to solve this issue. |
