Electrocatalytic reduction of carbon dioxide to carbon monoxide by rhenium and manganese polypyridyl catalysts.

摘要

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.