Ferrocenyl-phosphonium ionic liquids – synthesis, characterisation and electrochemistry

Abstract

New unsymmetrically substituted ferrocenyl-phosphonium ionic liquids (ILs) [FcPR₂R′]NTf₂5a–j are synthesized by two or three step syntheses starting from ferrocene, Fc = (C₅H₅)Fe(C₅H₄); R = Me, ⁿBu, ⁿHex, Ph; R′ = Me, ⁿPr, ⁿBu, Ph; NTf₂ = N(SO₂CF₃)₂. The selective synthesis of alkyl phosphines FcPR₂via a Friedel–Crafts phosphorylation is highlighted as an alternative for the standard protocol commonly used for ferrocenyl arylphosphines involving lithiation of FcH followed by phosphorylation. The influence of the P-substituents on thermal stability, electrochemical potential, chemical shift, and UV-Vis absorption behavior of the ILs is studied. The phosphonium group acts both as an ionic tag and as an electron-withdrawing substituent directly bound at the Cp-ring position. Therefore the title compounds are attractive for further studies to use them as tunable redox mediators for (photo)electrochemical devices such as dye sensitized solar cells (DSSCs) or redox flow batteries.