The Observation of Dianions Generated by Electrochemical Reduction of trans-Stilbenes in Ionic Liquids at Room Temperature

Three highly aprotic bis(trifluoromethylsulfonyl)-amide (NTf2-) based ionic liquids (ILs) containing the cations trihexyl(tetradecyl)phosphonium (P-6,6,6,14(+)), N-butyl-N-methylpyrrolidinium (Pyrr(4,1)(+)), and (trimethylamine)(dimethylethylammine)dihydroborate ((N-111)(N-112)BH2+) have been examined as media for room temperature voltammetric detection of highly basic stilbene dianions electrochemically generated by the reduction of trans-stilbene (t-Stb) and its derivatives (4-methoxy-, 2-methoxy-, 4,4'-dimethyl-, and 4-chloromethyl-). Transient and steady-state data in the ILs were compared with results obtained in the molecular solvent acetonitrile. In all media examined, the t-Stb(0/center dot-) process is chemically and electrochemically reversible with a heterogeneous charge transfer rate constant in CH3CN of 1.5 cm s(-1), as determined by Fourier transformed AC voltammetry. However, further reduction to the dianion was always irreversible in this molecular but weakly acidic solvent. On the other hand, a substantial level of chemical reversibility for the reduction of t-Stb(center dot-) to t-Stb(2-) on the time scale of cyclic voltammetry is achieved when the concentration of trans-stilbene, [t-Stb], appreciably exceeds the concentration of adventitious water or other proton sources. In particular, these conditions are met when [t-Stb] >= 0.1 M in thoroughly dehydrated and purified ILs, while in the presence of CH3CN, t-Stb(2-) still suffers fast irreversible protonation under these stilbene concentration conditions. The E-0/center dot-(0) values (vs Fc(0/+)) for substituted trans-stilbenes in acetonitrile and (N-111)(N-112)BH2-NTf2 do not differ substantially, nor do the E-0/center dot-(0) and E center dot-/(2-) differences or other aspects of the voltammetric behavior.