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 (L^P/L^M) incorporating pyridine-2,6-dicarboxamide and salicylaldimine units was synthesized by extending our previous work. Five Cd(II) complexes [Cd₂L₂I₄]·2DMF (1), [Cd₂L₂Br₄]·2DMF (2), {[CdLCl₂]·MeOH·H₂O}_n (3), {[Cd₂L₂(NO₃)₄(DMF)(MeCN)]·2H₂O}_n (4), and {[CdL₂(H₂O)₂]·2ClO₄·2H₂O·2MeOH}_n (5), 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 L 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.