1-Oxa-4,8,12-triazacyclotetradecane-4,12-diacetic acid (H2L2): studies on protonation and metal complexation; crystal structure of [CuL2]·5H2O

Abstract

A new N-diacetate derivative of an asymmetrical 14-membered oxatriaza macrocycle, 1-oxa-4,8,12-triazacyclotetradecane-4,12-diacetic acid (H₂L²), has been synthesised from the parent amine by the method of complete carboxymethylation followed by anion-exchange chromatography. Its protonation constants have been determined by potentiometric and ¹H NMR techniques (at 25.0 °C and I= 0.10 mol dm^–3). The first two protonations occur at the nitrogen atoms, the third at a carboxylate group and not at the last nitrogen atom. The stability constants of the complexes formed by this ligand with various first-series transition metal ions, Zn²⁺, Cd²⁺, Pb²⁺ and Ca²⁺ were determined by potentiometric measurements. The value for the copper complex (and also for the nickel one) is very high while the complexes of Ca²⁺, Mn²⁺ and Pb²⁺ have relatively low stability. An X-ray structure determination of the complex [CuL²]·5H₂O has been performed in order to understand these values. Crystals are monoclinic, space group P2₁/c, a= 11.19(3), b= 12.19(2), c= 16.22(6)Å, β= 110.16(9)°. The structure was refined to a final R= 0.089 for 3656 reflections with |F_o| 2σ|F_o|. The co-ordination geometry around the copper atom can be described as a distorted octahedron in which the equatorial plane is defined by the three nitrogen atoms of the ring and an oxygen atom of one of the carboxylate groups of the ligand. Two oxygen atoms occupy the axial positions, one from the remaining carboxylate group and the other from the macrocyclic ring. Hence, in the crystalline state (and probably also in solution) all the donor atoms co-ordinate to the metal ion, the ligand being in a folded, very strained conformation. The low stability of the complexes of the larger metal ions (Ca²⁺, Mn²⁺ and Pb²⁺) could be understood if one of the nitrogen-donor atoms of the ring were not involved in the co-ordination.