Toxic metal complexes of macrocyclic cyclen molecule – synthesis, structure and complexing properties

Toxic metal (Cd²⁺, Hg²⁺, Pb²⁺, and Ag⁺) complexes with the tetradentate macrocyclic ligand - cyclen (1,4,7,10-tetraazacyclododecane, [12]aneN₄, L) were prepared and studied in the solid state by IR, X-ray diffraction, elemental and thermal analysis. Diffraction results have yielded three molecular structures, [Cd([12]ane-κ⁴N^1,4,7,10)(NO₃)₂)] (1), [Hg([12]ane-κ⁴N^1,4,7,10)(NO₃-κ²O,O`)]NO₃ (2), [Pb₂([12]ane-κ⁴N^1,4,7,10)₂][Pb(NO₃)₆] (3) and one polymeric structure {[Ag₂([12]ane-κ³N^1,4,7)(μ₂-[12]aneN¹⁰)](NO₃)₂∙2H₂O)}_n (4) featuring a unique coordination mode not observed before with cyclen as a ligand. The monodentate (1) and chelate (with small bite angle 50.3(3)°, (2) coordination modes of nitrate ligands were confirmed. Stereochemically active 6s² lone pair was suggested in 3 and DFT results confirmed no significant metal–metal covalent bond. The stability constants of the complexes with Cd²⁺ and Pb²⁺ ions were determined by potentiometric methods in aqueous solutions. Additionally, the structures of complexes in solution were observed by ¹H NMR. Both methods confirm similar cyclen complexing properties toward Zn²⁺ biometal and Cd²⁺, Pb²⁺ toxic metals.