Homo-and heterodinuclear complexes of the tris(catecholamide) derivative of a tetraazamacrocycle with Fe3+, Cu2+ and Zn2+ metal ions

Abstract

The new ditopic catecholamide 3,7,11-tris-{N-[3,4-(dihydroxybenzoyl)-aminopropyl]} derivative of a 14-membered tetraazamacrocycle containing pyridine (H₆L¹) has been synthesized. The protonation constants of (L¹)^6– and the stability constants of its mono-, homo- and hetero-dinuclear complexes with Fe³⁺, Cu²⁺ and Zn²⁺ metal ions were determined at 298.2 K and ionic strength 0.10 mol dm^–3 in KNO₃. The large overall basicity of the ligand was ascribed to the very high protonation constants of the catecholate groups, and its acid–base behaviour was correlated with the presence of tertiary nitrogen atoms and secondary amide functions. The UV-vis spectrum of the red solution of [FeL¹]^3– complex exhibits the LMCT band of catecholate to iron(III), and its EPR spectrum revealed a typical isotropic signal of a rhombic distorted ferric centre in a high-spin state and E/D ≈ 0.31, both characteristic of a tris-catecholate octahedral environment. The ligand forms with copper(II) and zinc(II) ions mono- and dinuclear protonated complexes and their stability constants were determined, except for the [ML¹]^4– complexes as the last proton is released at very high pH. Electronic spectroscopic studies of the copper complexes revealed the involvement of catecholate groups in the coordination to the metal centre in the mono- and dinuclear copper(II) complexes. This information together with the determined stability constants indicated that the copper(II) ion can be involved in both types of coordination site of the ligand with comparable binding affinity. The EPR spectrum of [Cu₂L¹]^2– showed a well resolved seven-line hyperfine pattern of copper(II) dinuclear species typical of a paramagnetic triplet spin state with weak coupling between the two metal centres. Thermodynamically stable heterodinuclear complexes, [CuFeH_hL¹]^h–1 (h = 0–3) and [CuZnH_hL¹]^h–2 (h = 0–4), were formed as expected from a ditopic ligand having two dissimilar coordination sites. At physiological pH, the [CuFeL¹]^– complex is formed at ≈100%. The formation of the [CuFeH_hL¹]^h–1 complexes in solution was supported by electronic spectroscopic measurements. The data indicated the specific coordination of each metal centre at the dissimilar sites of the ligand, the iron(III) bound to the oxygen donors of the catecholate arms and the copper(II) coordinated to the amine donors of the macrocyclic ring. The two metal centres are weakly coupled, due to the fairly large distance between them.