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.

Due to the large differences in physical properties compared to bulk solutions, 1,2 studies on chemistry at the liquid-liquid (L-L) interface are of growing interest. Unique reactions of molecules (adsorbed) at the L-L interface have been reported in various systems, including photoinduced electron transfer reactions, 3 organic synthesis, 4 electron transfer, 5 ion transfer, 6 phasetransfer catalysis, 7 ion-pair extraction, 8 chemical oscillation, 9 and molecular recognition. [10][11][12][13] Among them, our particular interest here focuses on hydrogen bond-mediated complexations at the L-L interface for specific analyte recognition. 11-13We have recently studied electrochemical anion transfer across the L-L interface as facilitated by abiotic hydrogenbonding ionophores. 12The ionophore-assisted anion transfer process was successfully analyzed for the first time, 12a indicating that, in spite of significant interference from anion hydration, complementary hydrogen bonding at phase boundaries is indeed effective for analyte recognition; in the bulk organic phase, the binding behavior of this type of ionophore is quite sensitive to environmental changes, and the trace presence of water does cause serious interference in the binding events. 14 Another interesting feature of hydrogen bondforming ionophores we found is that, at the L-L interface, they are able to selectively recognize H2PO4 -over hydrophobic anions such as Cl -, Br -, ClO4 -and CH3COO -in the aqueous phase, and this is accompanied by the interfacial adsorption of the complex. 13 Such complexation-induced interfacial adsorption behaviors of ionophores are expected to offer a novel approach to the sensing of very hydrophilic anions, which are difficult to detect by conventional ionophore-based chemical sensors. 15These studies therefore suggest that the solvating environment at the interface is suitable for the appearance of unique functions of hydrogen bond-forming ionophores, and the examination of their interfacial binding behaviors would provide a potential basis for the development of ionophorebased chemical sensors.In this report, we investigate facilitated SO4 2-transfers by hydrogen bond-forming ionophores across the nitrobenzene (NB)-water interface by using polarography with a dropping electrolyte electrode. From the examination of three kinds of ionophores that have thiourea groups 16 as hydrogen bond forming sites (Fig. 1), bis-thiourea 1, α,α′-bis(N′-pnitrophenylthioureylene)-m-xylene, is found to strongly facilitate the SO4 2-transfer. Interestingly, the SO4 2-transfer assisted by 1 is indeed based on the formation of a 1:2 complex between SO4 2-and ionophore even under the condition of [SO4 H NMR binding studies reveal the predominant formation of a 1:1 complex with SO4 2-in the bulk NB phase. The facilitated transfer of SO4 2-with bis-thiourea 1 is also compared to that of HPO4 2-and H2PO4 -across the NB-water interface, which was previously shown to be assisted by the same ionophore through the formation of the 1:1 and 2:1 (anion to ...