Diazo-coupled calix[4]arenes for qualitative analytical screening of metal ions

New chromogenic diazophenylcalix4]arenes 1 and 2 were synthesized in cone conformation. Compound 1 with the ortho-carboxyl groups in CH₃CN solution preferentially binds with alkaline earth and transition metal ions, whereas no significant changes in absorption spectra are observed in the presence of alkali metal ions. While 2 with the ortho-ester groups shows selective complexation properties towards transition metal ions over alkali and alkaline earth metal ions. The detection of metal ions gives rise to bathochromic shifts in the absorption spectra (from orange/yellow to red), which is clearly visible even to the naked eye. According to the selective color changes using both receptors upon cation complexation, one can set up a qualitative analytical routine to screen alkali, alkaline earth, and transition metal ions.