Complexes of tetra-tert-butyl-tetraazaporphine with Al(III) and Zr(IV) cations as fluoride selective ionophores

In this work, complexes of Zr(IV) and Al(III) cations with 2,7,12,17-tetra-tert-butyl-5,10,15,20-tetraazaporphine (TAP) were tested as ionophores in plasticized PVC membranes of ion-selective electrodes. It was found that both tested ionophores show enhanced affinity towards fluoride anion. High fluoride selectivity was observed in the presence of anionic or cationic additives in the membrane, which indicates that proposed compounds work according to charged or neutral carrier mechanism, depending on membrane composition and pretreatment.tert-Butyl substituents, present in the structure of tested compounds, were supposed to prevent formation of ionophore dimers within the membrane phase. This process was found to be responsible for some unfavorable potentiometric properties of electrodes based on complexes of Zr(IV) and Al(III) cations with porphyrins (compounds closely related to tetra-tert-butyl-5,10,15,20-tetraazaporphine). As it was shown using spectrophotometrical measurements, Al(III)–TAP was not susceptible to dimerization, while dimer formation was observed for Zr(IV)–TAP. In full agreement with these observations, electrodes with membranes containing Al(III)–TAP responded in near-Nernstian and fast manner towards fluoride anion, while the employment of Zr(IV)–TAP as ionophore resulted in super-Nernstian and sluggish response. Plasticized PVC membranes doped with Al(III)–TAP and 20 mol% of lipophilic anionic additives shown remarkable F− selectivity, with selectivity coefficients, log KF−,Y−pot., as follows: −4.4 (Y−Br−), −4.3 (Cl−), −4.2 (NO3−), −3.6 (SCN−), −2.9 (ClO4−).