Dalton Transactions Blog

Fluorides of aluminium are used in the aluminium industry and also as catalysts for ozone friendly alternatives to chlorofluorocarbons (CFC’s). Previously, crystal structures of some fluoroaluminate anions have been solved by the introduction of either organic or metal cations. But until now, the crystal structure of a particular fluoroaluminate anion, the bioctahedral  Al₂F₉^3- anion had not been reported.

The crystal structure of this anion has recently been solved by Rika Hagiwara and colleagues at Kyoto University. They first synthesized the compound, [C₁₈MIm][AlF₄] (1-methyl-3-octadecylimidazolium tetrafluoroaluminate) and discovered that slow evaporation of the chloroform solution gave transluscent crystals of [C₁₈MIm]₃[Al₂F₉](CH₂Cl₂)_n. It is thought that this species was formed by the reaction below:

3[C₁₈MIm][AlF₄] + nCH₂Cl₂ → [C₁₈MIm]₃[Al₂F₉](CH₂Cl₂)_n + AlF₃

The Al₂F₉^3- anion has two octahedral AlF₆ units which are face-sharing and has D_3h symmetry.

To find out more, read the full Dalton Transactions article:

The first crystallographic example of a face-sharing fluoroaluminate anion Al₂F₉³⁻
Fei Xu, Kazuhiko Matsumoto and Rika Hagiwara