DOI: 10.1002/cphc.201200028 Preparation of [Al(hfip) 4 ]  -Based Ionic Liquids with Siloxane-Functionalized Cations and Their Physical Properties in Comparison with Their [Tf 2 N]  Analogues** Safak Bulut, [a] M. A. Ab Rani, [b] Tom Welton, [b] Paul D. Lickiss, [b] and Ingo Krossing* [a] 1. Introduction Ionic liquids (ILs) are generally defined as salts with melting temperatures below 100 8C. They attracted much recent atten- tion as solvents, due to their extremely low vapor pressures as well as interesting transport and electrochemical properties. Accordingly, considerable work was put into the determination of their polarity by means of solvatochromic polarity indica- tors, [1] for example, in terms of the solvatochromic comparison method (SCM) introduced by Kamlet and Taft. [2] Currently ILs find useful applications in electrochemistry, [3] solar and fuel cells, [4] nanoscience, [5] and others. [6] Weakly coordinating anions (WCA) are suitable counterions for the synthesis of new IL salts with low melting points and other optimized physical properties. [7–9] Of those, [Al(hfip) 4 ]  is a WCA we have been using since 1999. [10, 11] ILs of this anion with ammonium, imidazolium, pyrrolidinium, pyridinium, mor- pholinium, and piperidinium cations led to interesting proper- ties, for example, low viscosity, high conductivity, and very good dihydrogen solubility. [7, 9] The Li[Al(hfip) 4 ] starting materi- al [11–14] is formed in a simple high yielding procedure per- formed in scales up to 250 g per batch [13] and [Al(hfip) 4 ]  has a high electrochemical stability (> 5.0 V vs. Li/Li + ). [15] Thus, it appeared logical to extend the available [Al(hfip) 4 ]  ILs to fur- ther cation classes. Inert siloxane side chains both on imidazoli- um as well as pyrrolidinium residues led to low viscosity ILs which also were further investigated in detail with the help of quantum chemistry. [16] Herein we present the preparation and characterization of [Al(hfip) 4 ]  ILs with two siloxane-functionalized cations and the determination of their temperature-dependent viscosities and conductivities in comparison to those of further three [Tf 2 N]  ILs with the same or closely related cations, which were deter- mined in our laboratories for the first time. The synthesis of the three [Tf 2 N]  ILs has been published earlier. [2, 16] 2. Synthesis and Characterization In the first step [Eq. (1)], Li[Al(hfip) 4 ] was synthesized by react- ing pure Li[AlH 4 ] with dry hexafluoroisopropanol in dry n- hexane. [12] Li½AlH 4 þ 4 ðCF 3 Þ 2 CHOH abs: hexane reflux, 56h  !Li½AlðhfipÞ 4 þ 4H 2 ð1Þ The reaction works well with yields exceeding 95 % for the optimized procedure. [13, 14] In the anion metathesis according to Equation (2), lithium was exchanged for substituted imidazoli- um and pyrrolidinium cations (Scheme 1). ½CATCl þ Li½AlðhfipÞ 4 !½CAT½AlðhfipÞ 4 þ LiCl ð2Þ Anion metathesis took place in dry CH 2 Cl 2 ; after stirring at room temperature for 24 h. The mixture was filtered through siliceous earth (which was the best of all tested filter materials to eliminate the LiCl precipitate). Before filtration, the siliceous earth in the Schlenk frit was rinsed several times with CH 2 Cl 2 and dried in vacuum for 3–4 h with heating to 350–500 8C. After filtration a clear solution was obtained. Since the [Al- (hfip) 4 ]  anion is water-sensitive, the samples could not be Two new ionic liquids (ILs) with siloxane-functionalized cations and the weakly coordinating tetraalkoxyaluminate [Al(hfip) 4 ]  (hfip = hexafluoroisopropoxy) are prepared and characterized by nuclear magnetic resonance (NMR), infrared (IR) and Raman spectroscopy. With melting points below 0 8C they qualify as room temperature ILs (RTILs). Their temperature-dependent viscosities and conductivities, together with those of two [Tf 2 N]  ILs with the same cations and a further siloxane-func- tionalized [Tf 2 N]  IL, are measured between 0 and 80 8C, and all are described by the Vogel–Fulcher–Tammann (VFT) equa- tions. We note that the [Al(hfip) 4 ]  ILs have lower viscosities than their [Tf 2 N]  analogues at all measured temperatures and higher conductivities at room temperature. [a] Dr. S. Bulut, Prof. Dr. I. Krossing Albert-Ludwigs-Universität Freiburg Institut für Anorganische und Allgemeine Chemie and Freiburger Materialforschungszentrum (FMF) Albertstr. 21, 79104 Freiburg (Germany) Fax: (+ 49) 761 203 6001 E-mail : krossing@uni-freiburg.de Homepage: http://portal.uni-freiburg.de/molchem [b] M. A. Ab Rani, Prof. Dr. T. Welton, Prof. Dr. P. D. Lickiss Department of Chemistry Imperial College London SW7 2AZ (UK) [**] (hfip=(CF 3 ) 2 CHO) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201200028. ChemPhysChem 0000, 00, 1 – 5  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim &1& These are not the final page numbers! ÞÞ