Activation of carbon dioxide by electrochemical reduction of molybdenum hexacarbonyl in aprotic solvent: a combined IR spectroelectrochemical and DFT calculation study

Abstract

The mechanism of carbon dioxide (CO₂) activation by the electrochemical reduction of molybdenum hexacarbonyl (Mo(CO)₆) in dry organic solvent was reinvestigated using IR spectroelectrochemistry (IR-SEC) combined with density functional theory (DFT) calculations. Cyclic voltammetry (CV) and IR-SEC experiments, carried out under inert atmosphere, confirmed that the stable pentacarbonyl dianion [Mo(CO)₅]²⁻ is readily formed at the reduction potential of the hexacarbonyl parent complex. In addition, IR-SEC monitoring of the reduction of Mo(CO)₆ in CO₂-saturated solution showed an absorption band ascribed to the formation of bicarbonate (HCO₃⁻), but no signs for the formation of formate (HCO₂⁻) or oxalate (C₂O₄²⁻). These experimental results were rationalized by DFT calculations on the coordination mode of CO₂ to [Mo(CO)₅]²⁻. Indeed, no stable structure could be calculated for the η¹-OCO isomer, whereas the optimized structure of the η²-CO₂ isomer was calculated to be energetically less stable than that of the η¹-CO₂ isomer, the latter being identified as a key intermediate for the selective formation of carbon monoxide (CO) and water (H₂O) upon O-protonation of the CO₂-adduct. This catalytic behavior is discussed here in terms of Mulliken atomic charge redistribution over the CO₂ binding and activation processes, and compared with what was previously reported for tetracarbonyl Mo-diimine complexes, where diimine ligands display “redox non-innocent” properties.