Face‐Fusion of Icosahedral Boron Hydride Increases Affinity to γ‐Cyclodextrin: closo,closo‐[B(21)H(18)](−) as an Anion with Very Low Free Energy of Dehydration

The supramolecular recognition of closo,closo‐[B(21)H(18)](−) by cyclodextrins (CDs) has been studied in aqueous solution by isothermal titration calorimetry and nuclear magnetic resonance spectroscopy. These solution studies follow up on previous mass‐spectrometric measurements and computations, which indicated the formation and stability of CD ⋅ B(21)H(18) (−) complexes in the gas phase. The thermodynamic signature of solution‐phase binding is exceptional, the association constant for the γ‐CD complex with B(21)H(18) (−) reaches 1.8×10(6) M(−1), which is on the same order of magnitude as the so far highest observed value for the complex between γ‐CD and a metallacarborane. The nature of the intermolecular interaction is also examined by quantum‐mechanical computational protocols. These suggest that the desolvation penalty, which is particularly low for the B(21)H(18) (−) anion, is the decisive factor for its high binding strength. The results further suggest that the elliptical macropolyhedral boron hydride is another example of a CD binder, whose extraordinary binding affinity is driven by the chaotropic effect, which describes the intrinsic affinity of large polarizable and weakly solvated chaotropic anions to hydrophobic cavities and surfaces in aqueous solution.