Journal article

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

In this study, we investigated the tetraalkylammonium salts of the weakly coordinating fluorinated alkoxyaluminates [pftb](-) ([Al(O(C(CF3)(3))(4)](-)) [hfip](-) ([Al(OC(H)-(CF3)(2))(4)](-)) and [hftb](-) ([Al(OC(CH3)(-) (CF3)(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 [NBU4](+)[hfip](-). Simple and quantitative metathesis reactions most yielding these materials in high purity were developed. These [NR4](+) 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 [NBu4](+)[hftb](-) in CH2Cl2 and 0.41 mol L-1 for [NBu4](+)[hfip](-) in CHCl3) and show higher molar conductivities if compared to [NBu4](+)[PF6](-). The electrochemical windows of CH2Cl2, CH3CN and 1,2-F2C6H4 using the [NBu4](+) aluminate electrolytes are up to +0.5 V/-0.7 V larger than those using the standard [NBu4](+)[PF6](-).


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