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)(CF(3))(2)) from NaBH(4) 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 ([C(6)MIM](+)[B(hfip)(4)](-)) and 113 °C ([C(4)MMorph](+)[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 [BF(4)](-)/[PF(6)](-), [N(CN)(2)](-), [Tf(2)N](-) 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.

Towards Weakly Coordinating Anions with the Extremely Electron Withdrawing Perfluoropyridinoxy Ligand –OC5F4N

“…The solid‐state structure of 10· THF is monomeric with a four‐coordinate tetrahedral boron center. The sodium cation is bound to the anion in a manner that resembles that of the related [Na(THF) 2 ][B{OCH(CF 3 ) 2 } 4 ] . It has four Na–O interactions, two from the –OC 5 F 4 N ligands and two from the THF solvent molecules, and four Na–F interactions from the ortho ‐fluorine atoms of each –OC 5 F 4 N ligand bound to boron.…”

Tunable Aryl Alkyl Ionic Liquids with Weakly Coordinating Tetrakis((1,1,1,3,3,3‐hexafluoropropan‐2‐yl)oxy)borate [B(hfip)4] Anions

“…The imidazolium salts with borate [B(hfip) 4 ] − anions were prepared using the sodium borate Na[B(hfip) 4 ], as reported by Krossing et al. For comparison, the corresponding ILs with one of the most commonly used anions, [NTf 2 ] − , were also synthesized.…”

“…Recent reports from the Krossing group on their new anions raised the question whether the combination of new cations with new anions will lead to ILs with specific, interesting properties. As the AlR 4 − anions turned out to be very moisture sensitive, we focused on the newer fluorinated tetrakis((1,1,1,3,3,3‐hexafluoropropan‐2‐yl)oxy)borate [B(hfip) 4 ] − anions. In this paper we present the results of the new ionic liquids with 1‐aryl 3‐alkyl imidazolium cations and the weakly coordinating [B(hfip) 4 ] − anion as shown in Figure .…”

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

“…This class of anion was first reported by Strauss in 1996, [119][120][121] and has been complemented by our group since 1999 (Table 4). [125][126][127][128] Thev ery weak coordinative strength of the [Al(OR F ) 4 ] À anions gives rise to electrolytes possessing high ionicities, which means that the extent of ion pairing is very low and results in more free ions to contribute to charge transport. Moreover, these desirable properties of aluminate salts have been exploited in an umber of applications.I na ddition, as eries of related borates [B(OR) 4 ] À (where R = C(H)(CF 3 ) 2 (OR HF ) and CH 2 CF 3 (tfe)) have also been published and investigated as WCAsf or ionic liquids.T hese borates typically demonstrate higher melting points and viscosities than their aluminate counterparts,t hus making them more applicable for battery electrolytes than ionic liquids.…”