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 ]^- ).