Abstract
The electrochemical properties of diazophenylcalix[4]arenes bearing ortho-carboxyl group (o-CAC) and ortho-ester group (o-EAC), respectively, in the presence of various metal ions were investigated by voltammetry in CH3CN. o-CAC and o-EAC showed voltammetric changes toward divalent metal ions and no significant changes with monovalent alkali metal ions. However, o-CAC preferentially binds with alkaline earth and transition metal ions, whereas no significant changes in voltammetric signals are observed in o-EAC with alkaline earth metal ions. o-EAC only binds with other transition metal ions. This can be explained on metal ion complexation-induced release of proton from the azophenol to the quinone-hydrazone tautomer followed by internal complexation of the metal ion with aid of nitrogen atoms and ortho-carbonyl groups in the diazophenylazocalix[4]arenes.
Highlights
Macrocyclic receptors have been synthesized and received much attention to be endowed with superior molecular recognition properties used in chemical sensors [1, 2]
The voltammetric behavior of ortho-carboxyl azocalix[4]arene (o-CAC) or ortho-ester azocalix[4]arene (o-EAC) is complicated because the reductions of the azo groups of o-CAC or o-EAC produce highly proton-sensitive intermediates so that azophenol itself acts as a proton source
More attention has been paid to the oxidation wave rather than reduction waves of o-CAC or o-EAC
Summary
Macrocyclic receptors have been synthesized and received much attention to be endowed with superior molecular recognition properties used in chemical sensors [1, 2]. Azocalixarene bearing azophenol units acting as a chromogenic center are attractive for their interesting aspects of complexation with alkali, alkaline earth, and transition metal ions disclosed by studying the optical behavior of chromophoric units [4,5,6,7,8]. As one of our efforts, we have developed a simple qualitative analysis protocol to screen alkali, alkaline earth, and transition metal ions using azocalix[4]arene derivatives. Our group reported colorimetric discrimination system towards alkali, alkaline earth, and transition metal ions using azophenylcalix[4]arenes bearing ortho-carboxyl group (o-CAC) and ortho-ester group (oEAC) (Scheme 1) [9]. Present study reports o-CAC exhibiting a selective response to alkaline earth and transition metal ions and o-EAC to transition metal ions in acetonitrile solution (CH3CN) by electrochemical methods. One can discriminate almost all metal ions by simple electroanalytical methods
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