Potentiometric and spectroscopic study of copper(II) and nickel(II) complexes of trans-dioxopentaaza macrocycles in aqueous solution

Abstract

The trans-dioxopentaaza macrocycles 2,6-dioxo-1,4,7,10,13-pentaazacyclopentadecane (L¹) and 2,6-dioxo-1,4,7,11,14-pentaazacycloheptadecane (L²) have been prepared. Their protonation and metal-binding properties with Cu²⁺ and Ni²⁺ have been investigated in 0.10 mol dm^–3 KNO₃ (aq) at 25.0 °C by potentiometric pH-metry, calorimetry (in part), electronic absorption and IR spectroscopy. Metal-ion complexation promotes the deprotonation and the co-ordination of the amide nitrogens, resulting in neutral complexes with four nitrogen donors and a MLH_–2 stoichiometry at pH 8. Additional complexes with stoichiometry MHL, ML and MLH_–1 complete the complexation set. The overall stability constants of the complexes were calculated. The electronic spectra support a tetragonal co-ordination of Cu²⁺ and a square-planar co-ordination of Ni²⁺ in the MLH_–2 complexes leaving one amino group unco-ordinated. It is suggested that the planar trigonal nature of the co-ordinated amido nitrogens in trans position to each other in the equatorial plane around Ni²⁺ or Cu²⁺ structurally prevents the fifth nitrogen donor from binding at an axial site. The NiL²H_–2 complex exhibits a yellow-to-blue conversion equilibrium.