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-κ4N1,4,7,10)(NO3)2)] (1), [Hg([12]ane-κ4N1,4,7,10)(NO3-κ2O,O`)]NO3 (2), [Pb2([12]ane-κ4N1,4,7,10)2][Pb(NO3)6] (3) and one polymeric structure {[Ag2([12]ane-κ3N1,4,7)(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.