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-dimethylimidazol ium-2-dithiocarboxylates (alkyl=CH3, C2H5, i-C3H7), have been reduced elect rochemically at a glassy carbon electrode in THF. For the methyl and the et hyl compounds two reduction peaks are observed in their cyclic voltammogram s, while the isopropyl derivative exhibits a single signal. Chronocoulometr ic results and numerical simulations show that the formal potentials for th e two electron transfer steps are 'compressed', and in the isopropyl case e ven 'inverted'. This behavior is due to a steric interaction between the al kyl substituents and the semi-occupied orbital in the respective monoanion. It is consistent with the fact that during the two-electron reduction a co nsiderable structural change occurs, most likely during the second electron -transfer step.