Metal complexes of a tetraaza macrocycle with N-carboxymethyl groups as pendant arms

Abstract

The protonation constants of H ₂ L ¹ (7-methyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene- 3,11-diacetic acid) and stability constants of its complexes with Mg ²⁺ , Ca 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. This macrocycle is selective exhibiting a very high stability constant for Cu ²⁺ (log K _ML = 21.61), while for the remaining complexes of the first-row transition-metal ions the stability decreases sharply. The calcium complex has very low stability (log K _ML = 3.74), but that of Mg ²⁺ is higher (log K _ML = 5.30) and those of trivalent metal ions (Ga ³⁺ , Fe ³⁺ and In ³⁺ ) have very low stability, the value for Fe ³⁺ (log K _ML = 20.64) being lower than that of Cu ²⁺ . Spectroscopic studies (electronic and EPR) in aqueous solution have shown that H ₂ L ¹ upon complexation (with Co ²⁺ , Fe ³⁺ and possibly Cu ²⁺ ) exhibits a tendency to adopt a folded conformation with the nitrogen atom of the macrocycle opposed to the pyridine in the axial position, the basal plane being formed by the three other nitrogen atoms of the ring and the other carboxylate in a square-pyramidal arrangement for the metal ion. The complex [Fe(L ¹ )Br]·4H ₂ O shows a temperature-dependent magnetic behaviour with µ _eff ranging from 3.58 µ _B at 292 K to 1.70 µ _B at 3.1 K.