Chiral hemicucurbit[8]uril as an anion receptor: selectivity to size, shape and charge distribution

Abstract

Chiral (all-R)-cyclohexanohemicucurbit[8]uril binds anions in a 1 : 1 ratio in pure methanol like a molecular Pac-Man™ with remarkable selectivity based on the size, shape and charge distribution of the anion.

Templating Effects in the Dynamic Chemistry of Cucurbiturils and Hemicucurbiturils

“…[12], cycHC figures are based on the crystal structures CCDC 945664 from ref. [74] Therefore, formation of chiral cycHCs proceeds according to the route B, whereas water solubility equilibrium of route C on Scheme 5 can also be effective in reactions conducted in water (Table 3, lines 3, 8, 13, 14). [15].…”

“…a In heterogeneous reaction conditions solubility equilibrium through pathway C is marked effective, where it cannot be unequivocally concluded whether the product solubility only influences its isolation or also the product formation . Thermodynamic parameters for the binding of anions with substituted HCs [74,75] have proven that anion binding in protonic media is entropically disfavoured, which is why at elevated temperatures the binding of anions is suppressed. [74] Therefore, formation of chiral cycHCs proceeds according to the route B, whereas water solubility equilibrium of route C on Scheme 5 can also be effective in reactions conducted in water (Table 3, lines 3, 8, 13, 14).…”

“…The diastereomers of the latter, the (S,S)-or (R,R)-cycHC [n] [15,72,74,84] are ball shaped hosts, contrasting to the cylindrical or bamboo stem shapes of the (R,S)-isomers. The cyclohexano groups on each side of the cycHC[n] lean together, creating a round cavity bordered by narrow portals.…”

“…The larger 8-membered homologue (R,R)-cycHC [8] on the other hand holds a cavity sufficiently large to encapsulate a variety of guest molecules ( Figure 16). [15,74] The guest anions are fixed within (R,R)-cycHC [8] through multiple CÀH···anion host-guest interactions. Large octahedral anion SbF 6 À that ideally fits the size and shape of the (R,R)-cycHC [8] cavity is able to form sixteen simultaneous CÀH···F hydrogen bonds.…”

“…Large octahedral anion SbF 6 À that ideally fits the size and shape of the (R,R)-cycHC [8] cavity is able to form sixteen simultaneous CÀH···F hydrogen bonds. [74] Smaller tetrahedral anions form fewer simultaneous hydrogen bonds to the host and are therefore disordered in the crystal structure. [74] Comparing the dimensions of (R,R)-cycHC [8] in all of its reported anion inclusion complexes, it transpires that the cavity size of (R,R)-cycHC [8] changes very little between the complexes with different anions (Table 6).…”