Studies on the nuances of the electrochemically induced room temperature isomerization of cis-stilbene in acetonitrile and ionic liquids.

Electrochemical reduction of cis-stilbene occurs by two well-resolved one-electron reduction steps in acetonitrile with (n-Bu)4NPF6 as the supporting electrolyte and in N-butyl-N-methylpyrrolidinium (Pyrr1,4(+)) and (trimethylamine)(dimethylethylamine)-dihydroborate bis(trifluoromethylsulfonyl)amide (NTf2(-)) ionic liquids (ILs). Mechanistic details of the electroreduction have been probed by dc and Fourier transformed ac voltammetry, simulation of the voltammetry, bulk electrolysis, and EPR spectroscopy. The first one-electron reduction induces fast cis to trans isomerization in CH3CN and ILs, most likely occurring via disproportionation of cis-stilbene radical anions and fast transformation of the cis-dianion to the trans-configuration. The second reduction process is chemically irreversible in CH3CN due to protonation of the dianion but chemically reversible in highly aprotic ILs under high cis-stilbene concentration conditions. Increase of the (n-Bu)4NPF6 supporting electrolyte concentration (0.01-1.0 M) in CH3CN induces substantial positive shifts in the potentials for reduction of cis-stilbene, consistent with strong ion pairing of the anion radical and dianion with (n-Bu)4N(+). However, protection by ion pairing against protonation of the stilbene dianions or electrochemically induced cis-trans-stilbene isomerization is not achieved. Differences in electrode kinetics and reversible potentials for cis-stilbene(0/•-) and trans-stilbene(0/•-) processes are less pronounced in the Pyrr1,4-NTf2 ionic liquid than in the molecular solvent acetonitrile.