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

Abstract

The protonation constants of H₂L¹ (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 Mg²⁺, Ca²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺, Ga³⁺, Fe³⁺ and In³⁺ were determined by potentiometric methods. This macrocycle is selective exhibiting a very high stability constant for Cu²⁺ (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 Mg²⁺ is higher (log K_ML = 5.30) and those of trivalent metal ions (Ga³⁺, Fe³⁺ and In³⁺) have very low stability, the value for Fe³⁺ (log K_ML = 20.64) being lower than that of Cu²⁺. Spectroscopic studies (electronic and EPR) in aqueous solution have shown that H₂L¹ upon complexation (with Co²⁺, Fe³⁺ and possibly Cu²⁺) 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(L¹)Br]·4H₂O 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.

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