Hydrogen-bond forming ionophore for highly efficient transport of phosphate anions across the nitrobenzene-water interface

Thiourea-based hydrogen-bond forming ionophore 2, alpha,alpha'-bis(N'-p-nitrophenylthioureylene)-m-xylene, is synthesized and investigated by using ion transfer polarography for the facilitated transfers of H2PO4-, HPO42- and Cl- across the nitrobenzene-water interface. Bis-thiourea 2 has a significant ability to assist H2PO4- transfer across the interface whereas its counterpart, N-(p-nitrophenyl)-N'-propylthiourea (ionophore 3), cannot facilitate the transfer of this hydrophilic anion. The H2PO4- transfer assisted by 2 is based on the formation of a 2:1 complex between H2PO4- and ionophore, and the transfer reaction is more stable by over -12 kJ mol(-1) than the case of 3. The stabilization of the H2PO4- transfer for 2 is even stronger by -11 kJ mol(-1) than that for bis-thiourea 1, 2,7-di-t-butyl-4,5-bis(N'-butylthioureylene)-9,9-dimethylxanthene, which forms a 1:1 complex through the formation of four hydrogen bonds. Bis-thiourea 2 is also able to facilitate transfers of HPO42- and Cl- by the formation of 1:1 complex. As compared to bis-thiourea 1, HPO42- transfer by 2 is significantly stabilized by -27 to -31 kJ mol(-1) while the stabilization of the Cl- transfer is relatively moderate (-6.1 kJ mol(-1)). These binding properties of bis-thiourea 2 are discussed for the design of phosphate-selective ionophores for use in two-phase distribution systems such as ion-selective electrodes.