Synthesis and Binding Properties of 1,3,5-Tris(2-arylthiomethyl)mesitylene Bull. Korean Chem. Soc. 2008, Vol. 29, No. 2 417 Synthesis and Binding Properties of 1,3,5-Tris(2-arylthiomethyl)mesitylene: A Selective Ag (I) Ionophore Hong-Seok Kim, * Seon-Yun Bae, Ki Soo Kim, Jun-Hyeak Choi, Heung Jin Choi, Jun Ho Shim, † Geun Sig Cha, † and Hakhyun Nam †,* Department of Applied Chemistry, Kyungpook National University, Daegu 702-701, Korea. * E-mail: kimhs@knu.ac.kr † Chemical Sensor Research Group, Department of Chemistry, Kwangwoon University, Seoul 139-701, Korea * E-mail: namh@daisy.kw.ac.kr Received November 22, 2007 The efficient synthesis of four mesitylene-based receptors 1-4 and their potentiometric response characteristics to alkali metal, alkaline earth metal, and transition metal ions, under various pH conditions are outlined. Receptor 1-based electrode exhibited more sensitive response to Ag + ion (49 mV/decade of range from 10 −6 to 10 −2 M) than the 2-based electrode (47 mV/decade of range from 3 × 10 −5 to 10 −2 M), while the 3- and 4-based ones revealed sub-Nernstian below 40 mV/pAg + . All electrodes showed substantial responses to Ag + ion under acidic condition, but there was almost nil-response to other transition metal ions (Fe 2+ , Co 2+ , Zn 2+ , Ni 2+ , Pb 2+ , Cd 2+ , Cu 2+ and Hg 2+ ). The association constant of receptor 1 toward Ag + ion, measured by 1 H NMR titration, showed the largest value (200 M −1 ) among the tested receptors. The results were interpreted with semi empirically-modeled structures. Key Words : Tripodal receptor, Mesitylene, NMR titration, ISE, Silver ion selective Introduction Selective metal ion recognition by neutral receptors has attracted increasing interest in recent years because of their significant importance and potential applications in physio- logical, environmental, and supramolecular chemistry. 1 Silver (I) is one of the main transition metals used in electrical and electronic applications, photographic film industries and drugs. Due to their antibacterial properties, silver compounds have been used for dental and pharma- ceutical purposes. Therefore, the design of Ag + -selective ionophores and the construction of ion-selective electrodes (ISEs) have attracted much attention regarding environ- mental and clinical analyses. 2,3 Various ionophores including calix[4]arenes, 4 crown ether derivatives, 5 benzene-based tripodal derivatives, 6 and steroidal tweezers 7 have been used for the selective detection of Ag + ions by introducing soft heteroatoms such as N and S, as an electron donor to metal cations. Rigid benzene-platforms have been widely used as building blocks for extended, well-defined molecular archi- tecture and as a scaffold of synthetic receptors that show high selectivity toward cations, anions and organic mole- cules. 8,9 Raymond et al. synthesized tripodal ionophores by attaching three catechol units to mesitylene and found that the binding constant of tris(catechol)mesitylene for Fe (III) is higher than that of the natural enderrobactin. 10 By incorporating heterocyclic units such as pyrazole, indole, and pyridine into a benzene motif, complexation studies with Pd (II), 11 Fe (II), 12 and Cu (I) 13 have been performed. Recently, we have designed new receptors that contain thiazole moieties. 14 The ISEs based on solvent polymeric membranes (plasticized PVC) doped with these ionophores, exhibited large cation selectivity. To better understand the cation recognition of these ionophores, we introduced benzothiazole, 15 benzoxazole, 16 pyridine, and pyrimidine units into a mesitylene-platform (compounds 1-4 in Scheme 1), and compared their binding properties to soft heavy metal ions. We described herein an efficient synthesis of four different mesitylene-based receptors 1-4 and their response characteristics to various mono- and divalent cations (alkali metal, alkaline earth metal, and transition metal ions). The cation binding characteristics of 1-4 are also investigated with PVC-based ISE membrane; their sensitivity and selectivity to various transition metal cations and alkali and alkaline earth metal cations were understood structurally in conjunction with solution NMR results. Experimental Section General. Melting points were measured on Thomas Hoover Melting point apparatus and are uncorrected. 1 H and 13 C NMR spectra were recorded on a Varian Unity Spectro- meter ( 1 H, 300 MHz; 13 C, 75 MHz) in a CDCl 3 solution with TMS as an internal standard. 1 H NMR titrations were run at 45 mM concentrations, with aliquots of a 0.25 M (nBu) 4 N + X − salts solution added, on a Bruker AM-400 spectrometer. Elemental analyses were performed at the Center for Scientific Instruments, Kyungpook National University. Flash column chromatography was performed with Merck silica gel 60 (70-230 mesh) (Art 7734). All reactions were carried out under an atmosphere of argon. 2- Mercaptobenzothiazole, 2-mercaptobenzoxazole, 2-mercapto- pyridine, 2-mercaptopyrimidine, and cesium carbonate were purchased from Aldrich Chemical Co and used without further purification. 1,3,5-Tris(bromomethyl)mesitylene was prepared according to the literature procedure. 17