Synthesis of Room‐Temperature Ionic Liquids with the Weakly Coordinating [Al(OR^F)₄]⁻ Anion (R^F=C(H)(CF₃)₂) and the Determination of Their Principal Physical Properties

Abstract

A large series of ionic liquids (ILs) based on the weakly coordinating alkoxyaluminate [Al(hfip)(4)](-) (hfip: hexafluoroisopropoxy) with classical as well as functionalized cations were prepared, and their principal physical properties determined. Melting points are between 0 ([C(4)MMIM][Al(hfip)(4)]) and 69 °C ([C(3)MPip][Al(hfip)(4)]); three qualify as room-temperature ILs (RTILs). Crystal structures for six ILs were determined; their structural parameters and anion-cation contacts are compared here with known ILs, with a special focus on their influence on physical properties. Moreover, the biodegradability of the compounds was investigated by using the closed-bottle and the manometric respirometry test. Temperature-dependent viscosities and conductivities were measured between 0 and 80 °C, and described by either the Vogel-Fulcher-Tammann (VFT) or the Arrhenius equations. Moreover, conductivities and viscosities were investigated in the context of the molecular volume, V(m). Physical property-V(m) correlations were carried out for various temperatures, and the temperature dependence of the molecular volume was analyzed by using crystal structure data and DFT calculations. The IL ionicity was investigated by Walden plots; according to this analysis, [Al(hfip)(4)](-) ILs may be classified as "very good to good ILs"; while [C(2)MIM][Al(hfip)(4)] is a better IL than [C(2)MIM][NTf(2)]. The dielectric constants of ten [Al(hfip)(4)](-) ILs were determined, and are unexpectedly high (ε(r)=11.5 to 16.8). This could be rationalized by considering additional calculated dipole moments of the structures frozen in the solid state by DFT. The determination of hydrogen gas solubility in [Al(hfip)(4)](-) RTILs by high-pressure NMR spectroscopy revealed very high hydrogen solubilities at 25 °C and 1 atm. These results indicate the significant potential of this class of ILs in manifold applications.

[HMIM][Br9]: a Room-temperature Ionic Liquid Based on a Polybromide Anion

“…In diluted solutions of different imidazolium salts in dichloromethane they found the resonance of this hydrogen atom shifted to higher field the weaker these C-2-H···X interactions are. In contrast to Spange et al, we investigated in our case the pure ionic liquids and compared the proton NMR spectra of [ [23]), which are typical weakly coordinating anions.…”

Communication: Are hydrodynamic models suited for describing the reorientational dynamics of ions in ionic liquids? A case study of methylimidazolium tetra(hexafluoroisopropoxy)aluminates

“…23 Experimental crystal structure determinations of several of the ILs are in good quantitative agreement with the calculated volumes V cat . 8,9 The low coupling factors ͑Table II͒ signal almost frictionless rotation, which in the spirit of hydrodynamic theory corresponds to almost perfect slip conditions. This conclusion is corroborated by data by Buchner and collaborators 4 for some ILs with the same cations, but different anions, which we reanalyzed along the lines considered here.…”

“…7,8 Table I defines the homologues considered here and lists some important solvent properties. 9 The most attractive features in the present context are the low viscosity compared to other ILs and the small viscosity changes along the homologous cation series ͑ =9-12 mPa s −1 at 343 K͒. Both properties make this class of ILs ideal systems for testing hydrodynamic models.…”

“…Details are given elsewhere. 9 We used a coaxial reflection technique described recently 3 to determine the microwave dielectric response up to 20 GHz of the ILs at ͑343.1Ϯ 0.2͒ K, which is well above their melting temperatures ͑Table I͒. The dielectric response to an oscillating electric field of frequency can be described by the complex dielectric function 10,11 …”

Development of low viscous ionic liquids: the dependence of the viscosity on the mass of the ions

“…The N,Ndialkyl-imidazolium cation provides a very good delocalization of the positive charge. For this reason, imidazolium ionic liquids with large weakly coordinating anions like the tris(pentafluoroethyl)trifluorophosphate ion (FAP-anion) [6,7] or the tetrakis(perfluoroalkoxy)-aluminate ions [8] have low viscosities in comparison to ionic liquids with [PF 6 ] − or [BF 4 ] − ions. By dissociation of ion-pairs, the distance between the anion and the cation increases.…”