Synthesis of Room-Temperature Ionic Liquids with the Weakly Coordinating [Al(ORF)4]- Anion (RF=C(H)(CF3)2) and the Determination of Their Principal Physical Properties

A large series of ionic liqs. (ILs) based on the weakly coordinating alkoxyaluminate [Al(hfip)4]- (hfip: hexafluoroisopropoxy) with classical as well as functionalized cations were prepd., and their principal phys. properties detd. M.ps. are between 0 ([C4MMIM][Al(hfip)4]) and 69 °C ([C3MPip][Al(hfip)4]); three qualify as room-temp. ILs (RTILs). Crystal structures for six ILs were detd.; their structural parameters and anion-cation contacts are compared here with known ILs, with a special focus on their influence on phys. properties. Moreover, the biodegradability of the compds. was investigated by using the closed-bottle and the manometric respirometry test. Temp.-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 mol. vol., Vm. Phys. property-Vm correlations were carried out for various temps., and the temp. dependence of the mol. vol. was analyzed by using crystal structure data and DFT calcns. The IL ionicity was investigated by Walden plots; according to this anal., [Al(hfip)4]- ILs may be classified as "very good to good ILs"; while [C2MIM][Al(hfip)4] is a better IL than [C2MIM][NTf2]. The dielec. consts. of ten [Al(hfip)4]- ILs were detd., and are unexpectedly high (εr=11.5 to 16.8). This could be rationalized by considering addnl. calcd. dipole moments of the structures frozen in the solid state by DFT. The detn. of hydrogen gas soly. 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.