Molecular design of crown ethers. 19. Synthesis of novel disulfide- and diselenide-bridged bis(benzo-12-crown-4)s and their Ag⁺-selective electrode properties

Two novel heteroatom-bridged bis(benzo-12-crown-4 ether)s, i.e, bis(2- nitro-4,5-(1,4,7,10-tetraoxa-decamethylene)phenyl) disulfide (4) and diselenide (5), have been synthesized and characterized by elemental analysis and mass, IR, UV, and ¹H NMR spectroscopy. An X-ray crystallographic structure was obtained for 4. Ion-selective electrodes (ISE) for Ag⁺, containing 4 and 5 in PVC membrane as neutral carriers, were prepared, and their selectivity coefficients for Ag⁺ (K(Ag,M)/(pot)) were determined against other heavy metal ions, alkali and alkaline-earth metal ions, and ammonium ion using the matched potential method. These ISEs showed excellent Ag⁺ selectivities, log K(Ag,M)/(pot) ≤ -4.0, against most of the interfering cations examined, except for Hg⁺ (log K(Ag,M)/(pot) ≥ -1.2). These K(Ag,M)/(pot) values are comparable to those reported for the representative Ag⁺-selective thioethers 6 and 7, revealing that both disulfide and diselenide functionalities in 4 and 5 are equally effective Ag⁺-selective binding sites as the 1,7-dithia-4-oxa functionality in 6 and 7, irrespective of the different atom type and relative position of the sulfur/selenium donors in the ligands. Also discussed are the steric and electronic effects of the nitro groups in 4 and 5 on the Nernstian slopes obtained with the 4- and 5-based ISEs.