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Keywords:

  • density functional calculations;
  • electrochemistry;
  • ionic liquids;
  • tetraalkylammonium cations;
  • weakly coordinating anions (WCAs)

Abstract

Thumbnail image of graphical abstract

Weak and robust? Tetraalkylammonium salts of weakly coordinating fluorinated alkoxyaluminates are easily accessible, chemically robust materials that show interesting physico-chemical properties like low melting points, high electrochemical stability and electric conductivity in weakly polar solvents such as CH2Cl2, Ph-F and toluene.

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 °C for [NBu4]+[hfip]. Simple and almost quantitative metathesis reactions 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 (Vion([pftb])=0.736 nm3, Vion([hftb])=0.658 nm3, Vion([hfip])=0.577 nm3) 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].