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

Abstract

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.