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 CO2 the second irreversible wave displays a large catalytic enhancement in current. Diffusion coefficients were determined using the Levich-Koutecky method (1.1 X 10(-5) cm(2)/s for the neutral complex and 8.1 x 10(-6) cm(2)/s for the singly reduced species), and a second order rate constant for the electrochemical reduction with CO2 of 1000 M-1 s(-1) was measured. Bulk electrolysis studies were performed to measure Faradaic efficiencies for the primary gaseous products, eta(CO) = 99 +/- 2% in acetonitrile.