Conductivity and Redox Potentials of Ionic Liquid Trihalogen Monoanions [X3]−, [XY2]−, and [BrF4]− (X=Cl, Br, I and Y=Cl, Br)

Abstract

The ionic liquid (IL) trihalogen monoanions [N2221][X3]− and [N2221][XY2]− ([N2221]+=triethylmethylammonium, X=Cl, Br, I, Y=Cl, Br) were investigated electrochemically via temperature dependent conductance and cyclic voltammetry (CV) measurements. The polyhalogen monoanions were measured both as neat salts and as double salts in 1-butyl-1-methyl-pyrrolidinium trifluoromethane-sulfonate ([BMP][OTf], [X3]−/[XY2]− 0.5 M). Lighter IL trihalogen monoanions displayed higher conductivities than their heavier homologues, with [Cl3]− being 1.1 and 3.7 times greater than [Br3]− and [I3]−, respectively. The addition of [BMP][OTf] reduced the conductivity significantly. Within the group of polyhalogen monoanions, the oxidation potential develops in the series [Cl3]−>[BrCl2]−>[Br3]−>[IBr2]−>[ICl2]−>[I3]−. The redox potential of the interhalogen monoanions was found to be primarily determined by the central halogen, I in [ICl2]− and [IBr2]−, and Br in [BrCl2]−. Additionally, tetrafluorobromate(III) ([N2221]+[BrF4]−) was analyzed via CV in MeCN at 0 °C, yielding a single reversible redox process ([BrF2]−/[BrF4]−).