Synthesis, structure and metal ion coordination of novel benzodiazamacrocyclic ligands bearing pyridyl and picolinate pendant side-arms

Abstract

Novel derivatives of benzodiaza-15-crown-5 (B15C5) and benzodiaza-18-crown-6 (B18C6) ether containing pyridyl (Py) and picolinate (Pic) pendant arms at amine nitrogen atoms of the macrocycle residue were synthesized. The protonation constants of the studied compounds and stability constants of their complexes with a range of heavy and transition metal ions (Cu²⁺, Zn²⁺, Ni²⁺, Cd²⁺, Pb²⁺) were determined in aqueous solutions by the potentiometric method. The obtained data showed that all ligands form rather stable complexes with the studied cations of different compositions; however, compounds with picolinate groups were found to be more efficient complexons. NMR studies revealed that in the case of B15C5 type ligands the complex geometry depends on the metal ion size: small radius metal ions (like Zn²⁺) are placed in the center of the crown ether cavity and the coordination by the pendant groups occurs at opposite sides of the macrocyclic plane, whereas binding of large cations (such as Cd²⁺ and Pb²⁺) results in a distorted structure of the complexes. Additional information concerning the structure of complexes was obtained from the analysis of X-ray-diffraction data.