Re(bipy-tBu)(CO)3Cl−improved Catalytic Activity for Reduction of Carbon Dioxide: IR-Spectroelectrochemical and Mechanistic Studies

Five Re(bipy)(CO)(3)Cl complexes were prepared and studied where bipy was 4,4′-dicarboxyl-2,2′-bipyridine (1), 2,2′-bipyridine (2), 4,4′-dimethyl-2,2′-bipyridine (3), 4,4′-di-tert-butyl-2,2′-bipyridine (4), and 4,4′-dimethoxy-2,2′-bipyridine (5). From this group, a significantly improved catalyst, Re(bipy-tBu)(CO)(3)Cl (4), for the reduction of carbon dioxide to carbon monoxide was found. The complex shows two one-electron reductions under argon, one reversible at -1445 mV (vs SCE), and one irreversible at -1830 mV. Under CO(2) the second irreversible wave displays a large catalytic enhancement in current. Diffusion coefficients were determined using the Levich-Koutecky method (1.1 × 10(-5) cm(2)/s for the neutral complex and 8.1 × 10(-6) cm(2)/s for the singly reduced species), and a second order rate constant for the electrochemical reduction with CO(2) of 1000 M(-1) s(-1) was measured. Bulk electrolysis studies were performed to measure Faradaic efficiencies for the primary gaseous products, η(CO) = 99 ± 2% in acetonitrile.