Behavior of Electrogenerated Bases in Room-Temperature Ionic Liquids

Abstract

The reductive electrochemistry of substituted benzophenones in the aprotic room-temperature ionic liquid (RTIL) 1-butyl-1-methylpyrrolidinium bistriflimide occurs via two consecutive one-electron processes leading to the radical anion and dianion, respectively. The radical anion exhibited electrochemical reversibility at all time-scales whereas the dianion exhibited reversibility at potential sweep rates of >or=10 V s(-1), collectively indicating the absence of strong ion-paring with the RTIL cation. In contrast, reduction in 1-butyl-3-methylimidazolium bistriflimide is complicated by proton-transfer from the [Bmim] cation. At low potential sweep rates, reduction involves a single two-electron process characteristic of either an electrochemical, chemical, electrochemical (ECE) or disproportion-type (DISP1) mechanism. The rate of radical anion protonation in [Bmim] is governed by basicity and conforms to the Hammett free-energy relation. At higher potential sweep rates in [Bmim][NTf2], reduction occurs via two consecutive one-electron processes, giving rise to the partially reversible generation of the radical anion and the irreversible generation of the dianion, respectively. Also, the redox potentials for the reversible parent/radical anion couples were found to be a linear function of Hammett substituent constants in both RTIL media and exhibited effectively equivalent solvent-dependent reaction constants, which are similar to those for reduction in polar molecular solvents such as acetonitrile or alcohols.