Direct observation of the reduction of carbon dioxide by rhenium bipyridine catalysts

Abstract

In order to further efforts in synthesis and catalysis, the mechanisms of catalysts must be completely understood. The Re(bpy)(CO)₃Cl molecular catalysts are some of the most robust and well-characterized CO₂ reduction catalysts known to date. Stopped-flow infrared spectroscopy is reported as a technique for studying the kinetics and mechanisms of the reactions of catalytically-relevant [Re(bpy-R)(CO)₃]⁻ anions (R = tBu or H) with CO₂/H⁺. [Re(bpy-tBu)(CO)₃]⁻ reacts approximately ten times faster with CO₂ than does [Re(bpy)(CO)₃]⁻. These reactions occur via a direct two-electron oxidative addition of CO₂ to the metal center and result in the formation of an intermediate CO₂ reduction product, Re(bpy-R)(CO)₃(CO₂H). This is the first in situ identification of this key intermediate. Evidence for this Re–CO₂H species includes isotopic labeling studies, stopped-flow experiments of the kinetics of its formation in the presence of proton sources, comparison with genuine Re(bpy)(CO)₃(CO₂H), and DFT calculations.