Water Structure Recovery in Chaotropic Anion Recognition: High-Affinity Binding of Dodecaborate Clusters to γ-Cyclodextrin**

Dodecaborate anions of the type B(12)X(12)(2−) and B(12)X(11)Y(2−) (X=H, Cl, Br, I and Y=OH, SH, NH(3)(+), NR(3)(+)) form strong (K(a) up to 10(6) L mol(−1), for B(12)Br(12)(2−)) inclusion complexes with γ-cyclodextrin (γ-CD). The micromolar affinities reached are the highest known for this native CD. The complexation exhibits highly negative enthalpies (up to −25 kcal mol(−1)) and entropies (TΔS up to −18.4 kcal mol(−1), both for B(12)I(12)(2−)), which position these guests at the bottom end of the well-known enthalpy-entropy correlation for CDs. The high driving force can be traced back to a chaotropic effect, according to which chaotropic anions have an intrinsic affinity to hydrophobic cavities in aqueous solution. In line with this argument, salting-in effects revealed dodecaborates as superchaotropic dianions.