Tetraalkylammonium Salts of Weakly Coordinating Aluminates: Ionic Liquids, Materials for Electrochemical Applications and Useful Compounds for Anion Investigation

Abstract

In this study, we investigated the tetraalkylammonium salts of the weakly coordinating fluorinated alkoxyaluminates [pftb](-) ([Al(O(C(CF(3))(3))(4)](-)), [hfip](-) ([Al(OC(H)(CF(3))(2))(4)](-)) and [hftb](-) ([Al(OC(CH(3))(CF(3))(2))(4)](-)) in order to obtain information on their undisturbed spectral and structural properties, as well as to study their electrochemical behavior (i.e., conductivities in non-polar solvents and electrochemical windows). Several of the compounds qualify as ionic liquids with melting points as low as 42 degrees C for [NBu(4)](+)[hfip](-). Simple and almost quantitative metathesis reactions yielding these materials in high purity were developed. These [NR(4)](+) salts serve as model compounds for undisturbed anions and their vibrational spectra--together with simulated spectra based on quantum chemical DFT calculations--were used for the clear assignment of the anion bands. Besides, the ion volumes of the anions (V(ion)([pftb](-)) = 0.736 nm(3), V(ion)([hftb](-)) = 0.658 nm(3), V(ion)([hfip](-)) = 0.577 nm(3)) and their decomposition pathways in the mass spectrometric measurements have been established. The salts are highly soluble in non-polar solvents (up to 1.09 mol L(-1) are possible for [NBu(4)](+)[hftb](-) in CH(2)Cl(2) and 0.41 mol L(-1) for [NBu(4)](+)[hfip](-) in CHCl(3)) and show higher molar conductivities if compared to [NBu(4)](+)[PF(6)](-). The electrochemical windows of CH(2)Cl(2), CH(3)CN and 1,2-F(2)C(6)H(4) using the [NBu(4)](+) aluminate electrolytes are up to +0.5 V/-0.7 V larger than those using the standard [NBu(4)](+)[PF(6)](-).

Is Universal, Simple Melting Point Prediction Possible?

“…The exceptions are two perfluorinated aluminates for which our group could demonstrate reversible, high-temperature fusion. [55] The decomposition temperatures of halides are generally found to be amongst the lowest of all anions. [49,56] Therefore, we did not include monoatomic ions in our survey.…”

The Future of Main Group Chemistry

“…[63] A cogent example is the ring expansion observed in the formation of [Cu(cyclo-S 12 )] þ (22) from the reaction of cyclo-S 8 with [Cu(1,2-F 2 C 6 H 4 ) 2 ] þ [Al{OC(CF 3 ) 3 } 4 ] À . [64] In addition to their use as partners for the stabilization of unusual, highly electrophilic main group species, the use of known WCAs and the development of new examples of WCAs are likely to play an influential role in fields such as catalysis, as counter-ions for electrophilic transition-metal centers, in ionic liquids to generate low viscosities, [65] and in lithium batteries.…”

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.…”

“…This new class of ILs is of considerable interest for applications because the weakly basic anion can stabilize reactive cations, e.g., formed in chemical or electrochemical reactions. 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.…”