Na[B(hfip)4] (hfip = OC(H)(CF3)2): a weakly coordinating anion salt and its first application to prepare ionic liquids

Abstract

The fast, high yield synthesis and full characterization of Na[B(hfip)₄] (hfip: OC(H)(CF₃)₂) from NaBH₄ and hexafluoroisopropanol (hfipH) is presented. By anion metathesis, five [B(hfip)₄]⁻ salts with classical/functionalized ionic liquid (IL) cations with melting points between 0 ([C₆MIM]⁺[B(hfip)₄]⁻) and 113 °C ([C₄MMorph]⁺[B(hfip)₄]⁻) were prepared. Four of these qualify as ILs and one as room temperature IL (RTIL). The properties of the borate anion [B(hfip)₄]⁻ and its aluminum analogue [Al(hfip)₄]⁻ were compared based on the available structural information from XRD. Viscosities (10.3 (90 °C) to 855 (0 °C) mPa s⁻¹) and conductivities (0.603 (30 °C) to 4.844 (90 °C) mS cm⁻¹) were measured between 0 and 90 °C, and described by the Vogel–Fulcher–Tammann (VFT) equations. The properties of the [B(hfip)₄]⁻ ILs were analyzed in the context of the anion-dependent molecular volume V_m-viscosity-/conductivity-correlations, also in comparison to ILs with [BF₄]⁻/[PF₆]⁻, [N(CN)₂]⁻, [Tf₂N]⁻ and [Al(hfip)₄]⁻ counterions. The viscosities and conductivities of [B(hfip)₄]⁻ ILs are slightly inferior to [Al(hfip)₄]⁻ ILs, similar to/better than all other anions given above. According to the Walden plots, the ionicity of the [B(hfip)₄]⁻ ILs may at least be classified as “good”. By sharp contrast to the [Al(hfip)₄]⁻ ILs, the [B(hfip)₄]⁻ ILs have good stability against humidity/water. Thus, handling of [B(hfip)₄]⁻ ILs in an open laboratory atmosphere over hours and days is allowed and further facilitates the use of this new IL class.