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

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 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-LM = 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 H(2)L(1) 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(L(1))Br]. 4H(2)O shows a temperature-dependent magnetic behaviour with mu(eff) ranging from 3.58 mu(B) at 292 K to 1.70 mu(B) at 3.1 K.