Synthesis of Upper-Rim Allyl- and <i>p-</i>Methoxyphenylazocalix[4]arenes and Their Efficiencies in Chromogenic Sensing of Hg²⁺ Ion

A series of upper-rim p-allyl and p-methoxyphenylazocalix[4]arenes (6, 8, 9a,b, and 10a,b) were synthesized and shown to exhibit substantial color changes upon complexation with Hg2+ ion. Both the upper-rim p-allyl- and p-methoxyphenylazo groups on calix[4]arenes are proven to be key components in the recognition of Hg2+ ion. Job's plots revealed 1:1 binding stoichiometry for all these p-allyl- and p-arylazo-coupled calix[4]arenes with Hg2+ ions and Benesi−Hilderbrand plots were used for determination of their association constants. Our results also demonstrated that two p-methoxyphenylazo groups prefer to bind Hg2+ in a distal orientation rather than a proximal one, and if there are three p-methoxyphenylazo groups, the third flanking p-methoxyphenylazo group plays a role in disturbing the binding of the two distal diazo groups. Furthermore, it should be noted that triazocalix[4]arenes (6, 9a, and 9b) responded to all 14 metal ions without showing much preference among the eight transition-metal ions screened in this work (Cr3+, Ni2+, Cu2+, Ag+, Cd2+, Hg+, Hg2+, and Pb2+).