Electrochemical oxidation of dicarbonylcyclopentadienylcobalt(I) in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ionic liquid

The electrochemical oxidation of the 18-electron half-sandwich organometallic complex [Co(η⁵-C₅H₅)(CO)₂], 1, has been studied in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide, [bmpyr][NTf₂], and in dichloromethane, CH₂Cl₂, containing [bmpyr][NTf₂] as the supporting electrolyte. In the ionic liquid, the 17-electron cation 1⁺ is generated under voltammetric conditions via a process that has a reversible mid-point potential (E_m) of 0.22 V vs. Fc^0/+ (E_m = 0.23 V vs. Fc^0/+ in CH₂Cl₂/[bmpyr][NTf₂]). 1⁺ reacts with 1 to form the cationic dimer [Co₂(η⁵-C₅H₅)₂(CO)₄]⁺, 2⁺, which reacts with [NTf₂]⁻ to form cobaltocenium, Cc⁺, and [Co(NTf₂)_n]⁽ⁿ⁻²⁾⁻ (with n = 2, 4, or 6) complexes as the products of bulk electrolysis. A second voltammetric oxidation process at more positive potential is associated with the oxidation of 2⁺ (E_m = 0.62 V vs. Fc^0/+ in [bmpyr][NTf₂] and 0.72 V vs. Fc^0/+ in CH₂Cl₂/[bmpyr][NTf₂]) to form 2²⁺, which accelerates the formation of Cc⁺ and [Co(NTf₂)_n]⁽ⁿ⁻²⁾⁻ complexes, which are reduced to cobaltocene and cobalt metal under conditions of cyclic voltammetry and bulk electrolysis. Mechanistic details and the reaction products have been characterised by cyclic voltammetry, chronocoulometry, bulk electrolysis, and FT-IR, UV–Vis and ESR spectroelectrochemical measurements. This investigation reveals how ionic liquid media may influence the electrochemical oxidation mechanism of half-sandwich metal and other complexes.