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

Calix[4]arenes 3a (R = OMe) and 3b (R = NO 2 ) with 5,17‐bis[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 Pb 2+ . Job plot experiments revealed 1:1 binding stoichiometry for each of the complexes. The association constants for 3a· Ca 2+ , 3b· Ca 2+ , 3b· Ba 2+ , and 3b· 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 .(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)