Metal complexes of a 12-membered tetraaza macrocycle containing pyridine and N-carboxymethyl groups

Abstract

The protonation constants of the macrocycle H ₃ L ¹ [3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-3,6,9- triacetic acid] and the stability constants of its complexes formed with Mg ²⁺ , Ca ²⁺ , Mn ²⁺ , Co ²⁺ , Ni ²⁺ , Cu ²⁺ , Zn ²⁺ , Cd ²⁺ , Pb ²⁺ , Ga ³⁺ , Fe ³⁺ and In ³⁺ were determined by potentiometric methods, at 25 °C and ionic strength 0.10 mol dm ^-3 in tetramethylammonium nitrate. This macrocycle is not a selective ligand for the first-row transition divalent metal ions, exhibiting similar stability constants for all of them. The Irving–Williams order of stability is not obeyed, the complexes of Co ²⁺ and Zn ²⁺ having higher stability constants than those of Cu ²⁺ and Ni ²⁺ . The stability constants of alkaline-earth-metal ion complexes decrease with increasing ionic radius of the metal; however, those of Mg ²⁺ and Ca ²⁺ are of the same order. The stability of the iron(III) complex is lower than expected (log K _ML = 21.77). Based on spectroscopic measurements in solution (electronic and EPR) and the magnetic moments of the complexes, some explanations of the peculiarities of the formation of these complexes are proposed. The low value of the stability constant for the nickel(II) compared to that of the cobalt(II) complex is explained by the five-co-ordinate geometry adopted in each case. To explain the low stability of the copper(II) complex it is proposed that the ligand only co-ordinates via three of the four nitrogen donor atoms, the co-ordination being completed by two oxygen atoms of the acetate groups. The use of [FeL ¹ ] as a model for iron proteins is proposed.