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

Sandra Kaabel, Jasper Adamson, Filip Topić, Anniina Kiesilä, Elina Kalenius, Mario Öeren, Mart Reimund, Elena Prigorchenko, Aivar Lõokene, Hans J. Reich, Kari Rissanen*, Riina Aav*

*Corresponding author for this work

Research output: Contribution to journalArticleScientificpeer-review

59 Citations (Scopus)

Abstract

A novel eight-membered macrocycle of the hemicucurbit[n]uril family, chiral (all-R)-cyclohexanohemicucurbit[8]uril (cycHC[8])The name cyclohexylhemicucurbituril, previously used for these macrocycles, is changed in accordance with the IUPAC nomenclature for fused cycles, as the cyclohexane substituents are fused with the parent hemicucurbituril. binds anions in a purely protic solvent with remarkable selectivity. The cycHC[8] portals open and close to fully encapsulate anions in a 1 : 1 ratio, resembling a molecular Pac-Man™. Comprehensive gas, solution and solid phase studies prove that the binding is governed by the size, shape and charge distribution of the bound anion. Gas phase studies show an order of SbF6 ≈ PF6 > ReO4 > ClO4 > SCN > BF4 > HSO4 > CF3SO3 for anion complexation strength. An extensive crystallographic study reveals the preferred orientations of the anions within the octahedral cavity of cycHC[8] and highlights the importance of the size- and shape-matching between the anion and the receptor cavity. The solution studies show the strongest binding of the ideally fitting SbF6 anion, with an association constant of 2.5 × 105 M−1 in pure methanol. The symmetric, receptor cavity-matching charge distribution of the anions results in drastically stronger binding than in the case of anions with asymmetric charge distribution. Isothermal titration calorimetry (ITC) reveals the complexation to be exothermic and enthalpy-driven. The DFT calculations and VT-NMR studies confirmed that the complexation proceeds through a pre-complex formation while the exchange of methanol solvent with the anion is the rate-limiting step. The octameric cycHC[8] offers a unique example of template-controlled design of an electroneutral host for binding large anions in a competitive polar solvent.

Original languageEnglish
Pages (from-to)2184-2190
Number of pages7
JournalChemical Science
Volume8
Issue number3
DOIs
Publication statusPublished - 2017
MoE publication typeA1 Journal article-refereed

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