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Thiourea derivatives (46 aroylthioureas) having different substituents close to the sulfur atom were synthesized and their ionophore potential in ion selective electrodes (ISEs) was examined. Structural considerations were taken into account based on the corresponding heavy-metal ISE parameters. As ionophores, some 1-furoyl-3-substituted thioureas (series 2) gave the best results in Pb(), Hg() and Cd() ISEs. The strong intramolecular hydrogen bond in series 2 allows ligand interaction only through the C᎐ ᎐ S group. Substituents on the furan and phenyl rings give rise to low solubility in the membrane plasticizer. 3-Alkyl substituted furoylthioureas improve solubility but enhance oxidative processes with chain length. New X-ray diffraction (XRD) structures and theoretical DFT calculations were considered in the analysis of the substituent influence on the selectivity of ISEs. These new ionophores have advantages because of their stability, simple synthesis and easy modification of the sulfur binding ability resulting from substitution.

We have studied the electronic and structural properties responsible for the molecular recognition that the 1,3-diphenyl-thiourea ionophore (L) experiences by the Hg2+ cation. The theoretical data was obtained for the bare L and for the [L−HgOH]+ and [L−Hg−L]2+ coordination compounds involved in the reaction mechanism in reported sensors. Calculations were performed with the Gaussian-98 program at the B3LYP/6-31G** level. A LANL2DZ pseudopotential was used for Hg. The calculated electrostatic potential of the ionophore has a maximum on the S atom and, with less intensity, on the Z-phenyl group; consistently, high negative charges occur on those sites. The two highest occupied molecular orbitals are also mainly located on the sulfur-Z-phenyl atoms. Thus, this region defines the active site. These results account for the softness of the S atom that is bonded to Hg2+ in the complexes. The metal−sulfur interaction is key in the observed selectivity of Hg2+ by L. However, a significant Z-phenyl-Hg2+ bonding was also found. This rather unexpected result suggests that Z-phenyl is crucial in the recognition of Hg2+. In fact, in [L−Hg−L]2+, two S atoms and two Z-phenyl groups carry the Hg2+ ion.