Tris(carboxymethyl)oxatriazamacrocycles and their metal complexes†

Abstract

The protonation constants of H₃L¹ [4,8,12-tris(carboxymethyl)-1-oxa-4,8,12-triazacyclotetradecane] and of H₃L² [4,7,11-tris(carboxymethyl)-1-oxa-4,7,11-triazacyclotridecane] and the stability constants of the complexes formed by both with alkaline-earth metal ions, divalent first-row transition-metal ions, Cd²⁺, Pb²⁺ and Fe³⁺ were determined by potentiometric methods, at 25 °C and ionic strength 0.10 mol dm^–3 in tetramethylammonium nitrate. The co-ordination properties of both ligands for alkaline-earth and divalent first-row transition-metal ions are quite different. The 13-membered macrocycle, (L²)^3–, exhibits fairly high stability constants but unsatisfactory selectivity with the first-row transition-metal ions. The 14-membered ligand, (L¹)^3–, is very selective for the same series of metal ions, the difference in stability between its complexes of Cu²⁺ and Mn²⁺ being 10.96 log units and that of the complexes of Cu²⁺ and Zn²⁺ 7.29 log units, the constant of the complex of Mn²⁺ is sufficiently high for its possible quantitative determination by (L¹)^3–, at pH values higher than 7. However, the metal complexation behaviour of the (L¹)^3– is similar to that of the corresponding bis(carboxymethyl) derivative, therefore the difficult preparation of H₃L¹ does not compensate the benefits. Increase in size of the macrocycle cavity leads to a sharp decrease in stability of complexes of metal ions involved mainly in electrostatic interactions (the alkaline-earth metal ions, Mn²⁺ and Pb²⁺). Cobalt(II) complexes also undergo a significant decrease in stability with increase in cavity size and the constant for [ZnL¹]^– is much lower than that of [ZnL²]^–. However, the complexes of Cu²⁺ and Ni²⁺ with both macrocycles have about the same values of the stability constants. To explain these results it is proposed that all the donor atoms of the ligands are involved in co-ordination to metal ions which form complexes mainly by electrostatic interactions, although the distances and principally the orientation of the lone pairs of electrons are gradually more disfavoured for co-ordination with increasing cavity size. Complexes of the first-row transition-metal ions undergo the same effects, each being able to choose the donor atoms from the ligands most appropriate for their strict co-ordination preferences. So Co²⁺, Ni²⁺ and Cu²⁺ adopt five- or six-co-ordination with these potentially seven-co-ordinate ligands, as shown by electronic and EPR spectroscopic measurements in solution and the magnetic moments of the complexes.