Homo- and heteropolynuclear Ni2+ and Cu2+ complexes of polytopic ligands, consisting of a tren unit substituted with three 12-membered tetraazamacrocycles

Abstract

Two new polytopic ligands L¹ and L² have been synthesized. They consist of a central tren unit to which three 1,4,7,10-tetraazacyclododecane rings are attached via an ethylene and a trimethylene bridge, respectively. The complexation properties of L¹ and L² towards Cu²⁺ and Ni²⁺ were studied by potentiometric pH titration, UV-Vis, EPR spectroscopy and kinetic techniques. As a comparison, the Cu²⁺ and Ni²⁺ complexes with L³ (1-(N-methyl-2-aminoethyl-1,4,7,10-tetraazacyclododecane)) were also investigated. The crystal structures of [CuL³H(H₂O)](ClO₄)₃ and [NiL³Cl](ClO₄) were solved and show that the side chain in its protonated form is not involved in coordination, whereas deprotonated it binds to the metal ion. The thermodynamically stable 3 : 1 complexes of L¹ or L² have a metal ion in the three macrocyclic units. However, when three equivalents of Cu²⁺ are added to L¹ or L² the metal ion first binds to the tren unit and only then to the macrocycles. The kinetics of the different steps of complexation have been studied and a mechanism is proposed.