Tetrazoles and para-Substituted Phenylazo-Coupled Calix[4]arenes as Highly Sensitive Chromogenic Sensors for Ca(2+)

Abstract

Calix[4]arenes 3a (R = OMe) and 3b (R = NO(2)) with 5,17bis[4-(4-substituted-phenyl)azo] and 25,27-bisoxymethyltetrazole groups were synthesized by 1,3-dipolar cycloaddition of oxyacetonitrile azocalix[4]arenes 2a and 2b activated with trimethylsilyl azide. UV/Vis screening of 3a and 3b with 14 metal ions showed that 3a (with p-methoxyphenylazo substituent) was a highly chromogenic sensor to Ca(2+), whereas 3b (with p-nitrophenylazo substituent) showed color changes toward Ca(2+), Ba(2+), and Ph(2+). Job plot experiments revealed 1:1 binding stoichiometry for each of the complexes. The association constants for 3a center dot Ca(2+), 3b center dot Ca(2+), 3b center dot Ba(2+), and 3b center dot Pb(2+) were determined by Benesi-Hildebrand plots. On the basis of (1)H NMR titration results, Ca(2+) was bound to the two partially deprotonated hydroxy azophenol groups and one of the two tetrazole groups of 3a and 3b. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)