Five Cd(II) Complexes Decorated with Pyridine-2,6-dicarboxamide and Salicylaldimine Units Showing Selective Recognition and Removal of Cu(II) Ions

A racemic ligand (<b>L</b>^{<b><i>P</i></b>}/<b>L</b>^{<b><i>M</i></b>}) incorporating pyridine-2,6-dicarboxamide and salicylaldimine units was synthesized by extending our previous work. Five Cd(II) complexes [Cd₂<b>L</b>₂I₄]·2DMF (<b>1</b>), [Cd₂<b>L</b>₂Br₄]·2DMF (<b>2</b>), {[Cd<b>L</b>Cl₂]·MeOH·H₂O}_<i>n</i> (<b>3</b>), {[Cd₂<b>L</b>₂(NO₃)₄(DMF)(MeCN)]·2H₂O}_<i>n</i> (<b>4</b>), and {[Cd<b>L</b>₂(H₂O)₂]·2ClO₄·2H₂O·2MeOH}_<i>n</i> (<b>5</b>), were synthesized by combining this ligand with the respective CdX₂ salts (X = I^-, Br^-, Cl^-, NO₃^-, and ClO₄^-). The structural diversity of these complexes revealed the chiral self-discrimination or self-recognition process of racemic ligand <b>L</b> in coordinating to Cd(II) salts. Furthermore, the selective ion recognition of these complexes was examined by UV-vis spectra. The results showed that all the Cd(II) complexes displayed selective ion recognition of Cu²⁺ in DMF due to the formation of polynuclear Cd(II)-Cu(II) complexes. Experiments on the removal of Cu²⁺ by the Cd(II) complexes were carried out in aqueous solution, which showed the complexes selectively removed Cu²⁺ from aqueous solutions containing various metal ions.