Electrochemically stable fluorohydrogenate ionic liquids based on quaternary phosphonium cations

Fluorohydrogenate ionic liquids of quaternary phosphonium cations, tri-n-butylmethylphosphonium (P-4441) fluorohydrogenate, tetra-n-butylphosphonium (P-4444) fluorohydrogenate, and tri-n-butyl-n-octyl-phosphonium (P-4448) fluorohydrogenate, have been synthesized by the metatheses of anhydrous hydrogen fluoride and the corresponding phosphonium chloride precursors. All the obtained salts have melting points below room-temperature with a vacuum-stable composition of P-444m(FH)(2.3)F (m = 1, 4, and 8) and were characterized by density, conductivity, and viscosity measurements. Linear sweep voltammetry with a glassy carbon working electrode shows that the P-444m(FH)(2.3)Fs have wide electrochemical windows exceeding 5.2 V. In particular, P-4441(FH)(2.3)F has an electrochemical window of 6.0 V, which is the widest among fluorohydrogenate ionic liquids reported to date. The thermal stability of these ionic liquids is also improved compared to the salts of N-heterocyclic ammonium cations. (c) 2009 Elsevier B.V. All rights reserved.