Methyl pyridine derivatives of 14-membered tetraaza macrocycles. A new host with high selectivity for cadmium†

Abstract

Two N-(2-pyridylmethyl) derivatives of a 14-membered tetraaza macrocycle containing pyridine have been synthesized, L¹ and L². The protonation constants of these compounds and the stability constants of complexes of both ligands with Co²⁺ to Zn²⁺, Cd²⁺, Pb²⁺, Fe³⁺ and In³⁺ were determined by potentiometric methods at 25 °C and ionic strength 0.10 mol dm^–3 in KNO₃. The values of the protonation constants for L¹ and L² are quite different. The metal complexes of both ligands present lower values of stability constants than expected. The only exception is the case of the Cd²⁺ complex with L² which exhibits a particularly high stability constant, indicating that this ligand has a remarkable selectivity for this metal ion relative to zinc. The Cd²⁺ complex seems to be the only one to bond to all the donor atoms of the ligand or at least to more than the other metal ions studied. The pM values determined at physiological pH, which take into account the competition of the protons for the ligand, have shown that L² is the most selective compound for cadmium relative to zinc. The single crystal structure of [CuL²][ClO₄]₂ determined by X-ray diffraction has shown that the co-ordination geometry around the copper atom can be described approximately as a distorted square pyramid. To achieve this geometric arrangement the macrocycle adopts a folded conformation. Furthermore the nitrogen atom of one free pyridylmethyl pendant arm is directed towards the copper centre leading to a distance between these two atoms of 3.304(11) Å, which suggests a weak bonding interaction consistent with a [5+1] co-ordination.