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

Dodecaborate anions of the type B12X122- and B12X11Y2- (X=H, Cl, Br, I and Y=OH, SH, NH3+, NR3+) form strong (K-a up to 10(6)Lmol(-1), for B12Br122-) inclusion complexes with -cyclodextrin (-CD). The micromolar affinities reached are the highest known for this native CD. The complexation exhibits highly negative enthalpies (up to -25kcalmol(-1)) and entropies (TS up to -18.4kcalmol(-1), both for B12I122-), 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.