Two-electron-transfer redox systems.: Part 3.: Electrochemical reduction of N,N′-dialkyl-4,5-dimethylimidazolium-2-dithiocarboxylates to 1,1-dithiolate dianions in THF.: Steric modulation of potential ordering by substituents

Three CS2 adducts of imidazol-2-ylidenes, N,N'-dialkyl-4,5-dimethylimidazolium-2-dithiocarboxylates (alkyl=CH3, C2H5, i-C3H7), have been reduced electrochemically at a glassy carbon electrode in THF. For the methyl and the ethyl compounds two reduction peaks are observed in their cyclic voltammograms, while the isopropyl derivative exhibits a single signal. Chronocoulometric results and numerical simulations show that the formal potentials for the two electron transfer steps are 'compressed', and in the isopropyl case even 'inverted'. This behavior is due to a steric interaction between the alkyl substituents and the semi-occupied orbital in the respective monoanion. It is consistent with the fact that during the two-electron reduction a considerable structural change occurs, most likely during the second electron-transfer step.