Carba‐ <i>closo</i> ‐dodecaborates as Superchaotropic Anions

ABSTRACT The host‐guest chemistry of the 1‐carba‐ closo ‐dodecaborate anion (CB 11 H 12 – ) and its 12‐monosubstituted derivatives (R = I, Ph, Et, Bu, and OH) with cyclodextrins (CDs) has been studied, a class of weakly coordinating polyhedral boron cluster anions that have already found applications in catalysis and as ionic liquids. These mono‐anions bind surprisingly tightly ( K a ≈ 10 5 M −1 ) to the hydrophobic cavity of CDs, particularly the smallest α‐CD and β‐CD homologues. This binding propensity closely resembles the binding of larger boron clusters, decaborates and dodecaborates (B 10 X 10 2– and B 12 X 12 2– , with X = Cl, Br, and I) as well as metallacarboranes (Co(C 2 B 9 H 11 ) 2 – ), which has been attributed to the superchaotropic properties of these large, charge‐dispersed anions. The thermodynamic signature of the encapsulation of 1‐carba‐ closo ‐dodecaborates by CDs is consistent with the chaotropic effect as dominant driving force, that is, the binding is strongly enthalpically driven (Δ H down to –19 kcal mol −1 ). The binding of the parent CB 11 H 12 – mono‐anion exceeds the binding of the isosteric B 12 H 12 2– di‐anion to the three CD homologues by 2‐3 orders of magnitude, which indicates a higher chaotropicity, as expected when an ion has a lower charge density. The results position CB 11 H 12 – as the smallest superchaotropic ion known to date, an assignment that was confirmed through its strong effect on the cloud point of a nonionic polymer (hydroxypropylcellulose). The CB 11 H 12 – ion was subsequently also tested as a chaotropic transmembrane carrier, and it was found to be active in delivering hydrophilic peptides (Arg 8 and phalloidin) through lipid bilayer membranes, in artificial liposomes as well as live CHO‐K1 cells. The binding and transport activities of the different superchaotropic boron cluster anions correlate well with their volume and surface charge density, as descriptors of their chaotropicity.