Tren centered tris-macrocycles as polytopic ligands for Cu(ii) and Ni(ii)

Abstract

Two novel symmetric polytopic ligands L¹ and L² have been synthesized. They are composed of three 1,4,8,11-tetraazacyclotetradecane macrocycles which are connected to a central tren moiety via an ethylene and a trimethylene bridge, respectively. The complexation potential and the speciation diagrams of L¹ and L² towards Cu²⁺ and Ni²⁺ were determined by spectrophotometric and potentiometric titrations. Insight into the geometry of the Cu²⁺ complexes is provided by UV-VIS and EPR spectroscopy. The simplified ligands L³ and L⁴ are utilized as references for an aminoethyl- and a tren-substituted tetraaza macrocycle to help assign the EPR spectra of the polytopic ligands L¹ and L². At a metal-to-ligand ratio of 3 : 1, the metal cations are preferentially bound to the tetraaza macrocycles of L¹ and L² in a square planar geometry. At high pH values, a nitrogen atom of the tren moiety in L¹ serves as an additional ligand in an axial position leading to a square pyramidal coordination around Cu²⁺, whereas in L² no such geometry change is observed. At a metal-to-ligand ratio of 4 : 1, the additional metal cation resides in the central tren moiety of L¹ and L². However, in contrast to the typical trigonal bipyramidal geometry found in the [Cutren]²⁺ complex, the fourth Cu²⁺ has a square pyramidal coordination caused by the interaction with the Cu²⁺ cations in the macrocycles (as evidenced by EPR spectra). Since the sequence of metal complexation is such that the first three metal ions always bind to the three macrocycles of L¹ and L² and the fourth to the tren unit, it is possible to prepare heteronuclear complexes such as [Cu₃NiL]⁸⁺ or [Ni₃CuL]⁸⁺, which can be unambiguously identified by their spectral properties.