Synthesis of Upper-Rim Sulfanylpropyl- and p- Methoxyphenylazo-Substituted Calix[4]arenes as Chromogenic Sensors for Hg 2+ and Ag + Ions.

A series of calix[4]arenes with upper-rim sulfanylpropyl and p -methoxyphenylazo groups (compounds 8 - 10 ) were synthesized and found to be effective chromogenic sensors for selectively detecting Hg 2+ , Hg + , and Ag + ions among 18 screened metal perchlorates. In comparison to previously reported diallyl- and dithioacetoxypropyl-substituted calix[4]arenes ( 5 , 6 , 14 , 15 , and 16 ) and the newly synthesized compound 7 , the distal (5,17)-disulfanylpropyl-substituted di- p -methoxyphenylazocalix[4]arene 9 demonstrated superior performance with a limit of detection of 0.028 μM for Hg 2+ ions in a chloroform/methanol (v/v = 399/1) cosolvent. Job's plot revealed 1:1 binding stoichiometry for all these upper-rim sulfanylpropyl- and p- methoxyphenylazo-substituted calix[4]arenes 8 - 10 with Hg 2+ ions, and Benesi-Hildebrand plots from ultraviolet/visible (UV-vis) titration spectra were used for the determination of their association constants. Our findings indicated that the distal orientation of two p -methoxyphenylazo and two sulfanylpropyl groups in calix[4]arenes 8 - 10 is more favorable for binding Hg 2+ ions than the proximal (5,11-) orientation; moreover, the adjacent sulfanylpropyl groups exhibited superior coordination as ligands compared to the allyl and thioacetoxypropyl groups. Notably, compounds 8 - 10 displayed a comparable trend in their association with Ag + ions, albeit with 1 order of magnitude lower binding constants and a distinct binding mode compared to Hg 2+ ions. UV-vis spectroscopy, Job's plots, high-resolution mass spectrometry, and 1 H nuclear magnetic resonance titration studies are presented and discussed.