Influence of Counter Anions on Inclusion Complexation of p-Sulfonatocalix[6]arene with 1-Butyl-3-methylimidazolium Salts (Ionic Liquids)

Using the inclusion complexation of methylene blue with p-sulfonatocalix[6]arene (Calix-S6) as a chemical indicator, the association constants of inclusion complexes of Calix-S6 with visible-spectroscopically inert 1-butyl-3-methylimidazolium salts (ionic liquids (IL): counter anions BF₄^-, PF₆^-, and Cl^-) were determined. It was found that the experimentally observed association constants of IL with Calix-S6 in a water-methanol mixture increased in the order of counter anion Cl^- < BF₄^- < PF₆^-. The ¹⁹F NMR signals of counter anions BF₄^- and PF₆^- shift upfield upon addition of Calix-S6. From the ¹⁹F NMR chemical shifts, we suggested that Calix-S6 forms weak 1:1 complexes with the BF₄^- and PF₆^- anions. The binding constants and their thermodynamic parameters for the complexation of the BF₄^- and PF₆^- anions with Calix-S6 were evaluated in a water-methanol mixture (6:4 (V/V)) at 298 K: K_A = 26.2 M^-1 for BF₄^- and 106 M^-1 for PF₆^-. Based on the results, the characteristics for the complexation of the counter anions with Calix-S6 were discussed and the inclusion constants of Calix-S6 with the cationic ring moiety of 1-butyl-3-methyl-imidazolium salts were determined.