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 5,17bis[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 Pb ²⁺ . Job plot experiments revealed 1:1 binding stoichiometry for each of the complexes, The association constants for 3ȧCa ²⁺ , 3b-Ca ²⁺ , 3b-Ba ²⁺ , and 3ḃPb 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.