Synthesis and Anion‐Selective Complexation of Homobenzylic Tripodal Thiourea Derivatives

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. 1H NMR spectroscopic monitoring of the titration of cryptand 4b with CH3CO2–, Cl–, and F– in CDCl2CDCl2 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 H2PO4–, CH3CO2–, Cl–, and Br– anions were evaluated by 1H 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 H2PO4– and CH3CO2–, 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 H2PO4– compared to those of reference compound 15 presumably through cooperative complexation of the three binding sites to the guest anion. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)