Reduction of CO₂ using a Rhenium Bipyridine Complex Containing Ancillary BODIPY Moieties

The reduction of carbon dioxide to chemical fuels such as carbon monoxide is an important challenge in the field of renewable energy conversion. Given the thermodynamic stability of carbon dioxide, it is difficult to efficiently activate this substrate in a selective fashion and the development of new electrocatalysts for CO₂ reduction is of prime importance. To this end, we have prepared and studied a new fac-Re^I(CO)₃ complex supported by a bipyridine ligand containing ancillary BODIPY moieties ([Re(BB2)(CO)). Voltammetry experiments revealed that this system displays a rich redox chemistry under N₂, as [Re(BB2)(CO) can be reduced by up to four electrons at modest potentials. These redox events have been characterized as the Re^I/0 couple, and three ligand based reductions - two of which are localized on the BODIPY units. The ability of the BB2 ligand to serve as a non-innocent redox reservoir is manifest in an enhanced electrocatalysis with CO₂ as compared to an unsubstituted Re-bipyridine complex lacking BODIPY units ([Re(bpy)(CO)). The second order rate constant for reduction of CO₂ by [Re(BB2)(CO) was measured to be k = 3400 M^-1s^-1 at an applied potential of -2.0 V versus SCE, which is roughly three times greater than the corresponding unsubstituted Re-bipyridine homologue. Photophysical and photochemical studies were also carried out to determine if [Re(BB2)(CO) was a competent platform for CO₂ reduction using visible light. These experiments showed that this complex supports unusual excited state dynamics that precludes efficient CO₂ reduction and are distinct from those that are typically observed for fac-Re^I(CO)₃ complexes.