Imidazolate bridged Cu(II)–Cu(II) and Cu(II)–Zn(II) complexes of a terpyridinophane azamacrocycle: a solution and solid state study

The dinuclear Cu²⁺ and Zn²⁺ as well as the mixed Cu²⁺–Zn²⁺ complexes of a 5,5″-pentaazaterpyridinophane ligand (L) are able to incorporate imidazolate (Im⁻) as a bridging ligand. The crystal structure of [Cu₂L(Im)(Br)(H₂O)](CF₃SO₃)₂·3H₂O (1) shows one copper coordinated by the three pyridine nitrogens of the terpyridine unit, one nitrogen of the imidazolate bridge (Im⁻) and one bromide anion occupying the axial position of a distorted square pyramid. The second copper atom is coordinated by the remaining imidazolate nitrogen, the three secondary nitrogens at the centre of the polyamine bridge and one water molecule that occupies the axial position. Magnetic measurements have been performed in the 2.0–300.0 K temperature range. Experimental data could be satisfactorily reproduced by using an isotropic exchange model H = –JS₁S₂ with J = −52.3 cm⁻¹ and g = 2.09. Potentiometric studies have provided details of the speciation and stability constants for the mixed Cu²⁺–L–HIm, Zn²⁺–L–HIm (HIm = imidazole) and Cu²⁺–Zn²⁺–L–HIm systems. The apparent stability constant obtained at pH = 9 for the addition of imidazole to the dinuclear Cu²⁺ complexes is one of the highest so far reported (log K = 7.5). UV-Vis spectroscopy and paramagnetic NMR data show that imidazole coordinates to the Cu²⁺ ions as a bridging imidazolate ligand from pH 5 to 10. Electrochemical reduction of the Cu²⁺–Zn²⁺–L complex occurs in two successive one-electron per copper ion quasi-reversible steps. The formal potential of the Cu²⁺–Zn²⁺–L/Cu⁺–Zn²⁺–L couple is close to that of SOD. The IC₅₀ values measured at pH 7.8 by means of the nitro blue tetrazolium method show significant SOD activity for the dinuclear Cu²⁺ complexes (IC₅₀ = 2.5 µM) and moderate activity for the Cu²⁺–Zn²⁺ mixed systems (IC₅₀ = 30 µM).