An in situ infrared study of CO2 reduction catalysed by rhenium tricarbonyl bipyridyl derivatives

Abstract

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)₃] 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.