AN INSITU INFRARED STUDY OF CO2 REDUCTION CATALYZED BY RHENIUM TRICARBONYL BIPYRIDYL DERIVATIVES

The electrochemical reduction of CO2 by derivatives of rhenium carbonyl bipyridyl at carbon cathodes in acetonitrile solution is shown to take place by different routes depending on the concentration of water in the system. In the absence of water and CO2, reduction of fac-[Re(dmbipy)(CO)3Cl] (dmbipy = 4,4'-dimethyl-2, 2'-bipyridyl) leads first to fac-[Re(dmbipy.-)(CO)3Cl] followed by elimination of Cl- and partial dimerisation of the resultant five-co-ordinate species to give both fac-[Re(dmbipy)(CO)2] and [{Re(dmbipy)(CO)3}2]. Further reduction gives rise to fac-[Re(dmbipy.-)(CO)3], which is the final product. Cathodic reduction of [Re(dmbipy)(CO)3Cl] in the presence of CO2 and absence of water leads to direct attack of CO2 on fac-[Re(dmbipy.-)(CO)3Cl] to give [Re(dmbipy)(CO)3(CO2H)] E, which can undergo further reduction to give [Re(dmbipy.-)(CO)2(CO2H)]. By contrast, in the presence of H2O, the complex E can be protonated to yield [Re(dmbipy)(CO)3(CO2H2)]+ which is attacked by acetonitrile to yield [Re(dmbipy)(CO)3(MeCN)]+ and CO, this latter complex being identified unambiguously by comparison with the species formed in the oxidation of the original rhenium complex.