Further studies on the potentiometric salicylate response of polymeric membranes doped with tin(IV)-tetraphenylporphyrins

Abstract

The chemistry leading to the selective potentiometric response toward salicylate of polymer membranes doped with 5,10,15,20-tetraphenyl(porphyrinato)tin(IV) dichloride (Sn(TPP)Cl2) is examined via 119Sn NMR, 3H2O uptake, spectrophotometric, and solution conductometric techniques. The response properties of ion-selective electrodes prepared with such membranes suggest a complex mechanism of anion binding within the membrane phase. In the pure organic phase, spectroscopic data indicate that salicylate binds directly to one or both axial coordination sites of Sn(IV), displacing the initial chloride ligands. In the presence of water, however, both chloride and salicylate are displaced by water molecules, resulting in a dipositively charged metalloporphyrincation. Furthermore, conductance measurements point to the formation of salicylate--porphyrin complexes with greater than 2:1 stoichiometry. The results of these studies are discussed in conjunction with the anomalous cationic emf response of Sn(TPP)Cl2-based membranes at high concentrations of salicylate. A response mechanism is proposed which involves outer-sphere coordination of salicylate to a diaquo ligated metalloporphyrin as a key step in the observed anion extraction/equilibrium with the membrane phase.