Chloride Ion Affinity (CIA)

Typically the strength of a strong Lewis acid, is classified by its – usually calculated – FIA value (FIA = fluoride ion affinity).^[1] In analogy to the FIA, the chloride ion affinity CIA of any Lewis acid A

allows for a quantitative ordering of the Lewis acidity with respect to chloride as a base (Table 1). In comparison to the FIA, the CIA values are lower but (almost) the same ion affinity trends were observed. The CIA of Al(OR^F)₃ is with 356 (giving [ClAl(OR^F)₃]^–) and 436 kJ mol^–1 (giving [μCl]^– = [(^FRO)₃AlClAl(OR^F)₃]^–) in the typical range of very strong Lewis acids and also exhibits a higher chloride ion affinity than SbF₅. Thus, also with respect to gaseous SbF₅, Al(OR^F)₃ and AlBr₃ a Lewis Superacid.^[2]

Table 1: Overview on the calculated CIA-values of classical Lewis acids and Al(OR^F)₃. The FIA-values of the acids, with respect to the homologous fluoride complexes,^[1] are included in parentheses. CIA (FIA) values are given in kJ mol^–1. 
^[a] Reference system at G3MP2 level at 298 K.

Literature

[1] Lutz O. Müller, Daniel Himmel, Julia Stauffer, Gunther Steinfeld, John Slattery, Gustavo Santiso-Quiñones, Volker Brecht, Ingo Krossing* (2008): „Simple Access to the Non-Oxidizing Lewis Superacid Ph F→Al(OR^F)3 (R^F = C(CF₃)₃).”, Angew. Chem. 120, 7772-7776.

[2] Anne Kraft, Jennifer Beck, Ingo Krossing: „Facile Access to the Pnictocenium Ions [Cp*ECl]+ (E = P, As) and [Cp*2P]+. Chloride Ion Affinity of Al(ORF)3.“ Chem. Eur. J. In press.