Cryptand- and tripod-type thiourea derivatives 4b and 5a-d, which have binding functionalities at the homobenzylic positions, were synthesized as possible neutral receptors toward anions with an expectation that the three binding sites work cooperatively to bind an anion selectively. 1 H NMR spectroscopic monitoring of the titration of cryptand 4b with CH 3 CO 2 - , Cl - , and F - in CDCl 2 CDCl 2 at 100°C showed that the binding constants were considerably smaller than those of tripodal thiourea 5a, presumably owing to the presence of strong intramolecular hydrogen bonding in 4b. Complexation constants of tripodal receptors 5a-d with H 2 PO 4 - , CH 3 CO 2 - , Cl - , and Br - anions were evaluated by 1 H NMR and/or UV/Vis spectroscopic analysis of the titration in DMSO. Though tripodal receptors 5a,b undergo complexation with phosphate anion in a 1:1 stoichiometry, their association constants were not as large as simple reference compound 14 probably because of the steric hindrance around the binding sites and the large entropy cost for cooperative binding. Receptor 5c exhibits complexation in a 1:2 stoichiometry with H 2 PO 4 - and CH 3 CO 2 - , whereas it forms 1:1 complexes with chloride and bromide anions because of the subtle balance between the steric hindrance and the binding ability. However, by increasing the acidity of the thiourea functionality, receptor 5d exhibited remarkably enhanced binding ability and selectivity toward H 2 PO 4 - compared to those of reference compound 15 presumably through cooperative complexation of the three binding sites to the guest anion.
- Host-guest systems
- Hydrogen bonds