Chiral hemicucurbit[8]uril as an anion receptor: selectivity to size, shape and charge distributionElectronic supplementary information (ESI) available: MS, NMR, dynamic NMR and computational details and a DFT-based video of complexation. CCDC 1514736-1514741, 1521388. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6sc05058a

• Main Authors: Kaabel, Sandra, Adamson, Jasper, Topi, Filip, Kiesilä, Anniina, Kalenius, Elina, Öeren, Mario, Reimund, Mart, Prigorchenko, Elena, Lõokene, Aivar, Reich, Hans J, Rissanen, Kari, Aav, Riina
• Format: Journal Article
• Language: English
• Published: 28.02.2017
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/c6sc05058a

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 SbF 6 − PF 6 − > ReO 4 − > ClO 4 − > SCN − > BF 4 − > HSO 4 − > CF 3 SO 3 − 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 SbF 6 − anion, with an association constant of 2.5 × 10 5 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. 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.