A sinapic acid–calix[4]arene hybrid selectively binds Pb2+ over Hg2+ and Cd2+

The binding affinity for Pb2+, Cd2+ and Hg2+ of the sinapic acid–calix[4]arene hybrid 2, having four sinapyl pendants at the upper rim, has been investigated via an UV–Vis study. Compound 2 has better complexing ability than the monomeric p-phenetidine derivative 1. This highlights that the clustering of sinapyl units in a basket-like structure, dictated by the calixarene scaffold, greatly enhances the complexing properties. Ligand 2 forms complexes even with Hg2+, which is not complexed by 1 at all; the complexes formed by 2 with Pb2+ and Cd2+ are much stronger than the analogous complexes formed by 1. The UV–Vis investigation shows that the hybrid 2 markedly favors Pb2+ over Cd2+ and Hg2+. Information on the structural properties of the complex species was obtained by 1H NMR spectroscopy. NMR data show that all three metal ions are placed into the cavity consisting of the calixarene scaffold and the sinapyl pendants, though their binding affects the coordinating regions to a different extent.