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

摘要

Three highly aprotic bis(trifluoromethylsulfonyl)- amide (NTf_2~-) 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))BH_2~+) 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/·-) process is chemically and electrochemically reversible with a heterogeneous charge transfer rate constant in CH_3CN 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~(·-) 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 CH_3CN, t-Stb~(2-) still suffers fast irreversible protonation under these stilbene concentration conditions. The E_(0/·-)~0 values (vs Fc~(0/+)) for substituted trans-stilbenes in acetonitrile and (N_(111))(N_(112))BH_2-NTf_2 do not differ substantially, nor do the E_(0/·-)~0 and E_(·-/2-)~0 differences or other aspects of the voltammetric behavior.