A unique proton coupled electron transfer pathway for electrochemical reduction of acetophenone in the ionic liquid [BMIM][BF₄] under a carbon dioxide atmosphere

The mechanism of electrochemical reduction of acetophenone in 1-butyl-3-methylimidazolium tetrafluroborate ([BMIM][BF₄]) under nitrogen (N₂) and carbon dioxide (CO₂) atmospheres have been investigated using transient voltammetry, steady-state voltammetry, bulk electrolysis and numerical simulation. Under a N₂ atmosphere, acetophenone undergoes a one-electron reduction to the radical anion followed by rapid dimerization reactions with an apparent rate constant of 1.0 × 10⁶ M⁻¹ s⁻¹. In contrast, under a CO₂ atmosphere, the electrochemical reduction of acetophenone is an overall two-electron transfer chemically irreversible process with the final electrolysis product being 1-phenylethanol, instead of the anticipated 2-hydroxy-2-phenylpropionic acid resulting from an electrocarboxylation reaction. A proton coupled electron transfer pathway leading to the formation of 1-phenylethanol requires the presence of a sufficiently strong proton donor which is not available in neat [BMIM][BF₄]. However, the presence of CO₂ enhances the C-2 hydrogen donating ability of [BMIM]⁺ due to strong complex formation between the deprotonated form of [BMIM]⁺, N-heterocyclic carbene, and CO₂, resulting in a thermodynamically favorable proton coupled electron transfer pathway.