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.

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Applicability of a thiourea-based hydrogen-bonding ionophore 2, N,N′-bis(p-nitrophenyl)thiourea, for amperometric anion sensors is presented. Chloride transfer across the nitrobenzene (NB)–water interface facilitated by this ionophore is studied by using ion transfer polarography. When 2 is present in the organic phase, a well-defined wave can be observed, indicating that the chloride transfer across the NB–water interface is facilitated by 2 via formation of hydrogen bonds. The analysis of ion transfer polarograms shows that the chloride transfer is assisted by 1 : 1 complexation between 2 and chloride, and that the transfer process is reversible and controlled by diffusion of 2 from the bulk NB phase to the interface. Despite its simple binding mode based on two point interactions, mono-thiourea 2 has a strong ability to facilitate chloride transfer, which is comparable to that obtained for a highly preorganized bis-thiourea ionophore 1, 2,7-di-t-butyl-4,5-bis(N′-butylthioureylene)-9,9-dimethylxanthene, in which four hydrogen bonds are involved in the chloride binding.

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A hydrogen bond-forming bis-thiourea 3 can effectively facilitate the transfer of various hydrophilic anions, such as Cl−, CH3COO−, H2PO4−, HPO42− and SO42−, across the nitrobenzene–water interface, suggesting possible use of 3 for multianalyte detection with an amperometric sensing mode.