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

Thiourea-based hydrogen-bond forming ionophore 2, α,α′-bis(N′-p-nitrophenylthioureylene)-m-xylene, is synthesized and investigated by using ion transfer polarography for the facilitated transfers of H₂PO₄^-, HPO₄^2- 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 HPO₄^2- and Cl^- by the formation of 1:1 complex. As compared to bis-thiourea 1, HPO₄^2- 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.