Dioxadiaza- and trioxadiaza-macrocycles containing one dibenzofuran unit selective for cadmium

Abstract

The binding properties of dioxadiaza- ([17](DBF)N₂O₂) and trioxadiaza- ([22](DBF)N₂O₃), macrocyclic ligands containing a rigid dibenzofuran group (DBF), to metal cations and structural studies of their metal complexes have been carried out. The protonation constants of these two ligands and the stability constants of their complexes with Ca²⁺, Ba²⁺, and Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺ and Cd²⁺, were determined at 298.2 K in methanol–water (1 : 1, v/v), and at ionic strength 0.10 mol dm⁻³ in KNO₃. The values of the protonation constants of both ligands are similar, indicating that no cavity size effect is observed. Only mononuclear complexes of these ligands with the divalent metal ions studied were found, and their stability constants are lower than expected, especially for the complexes of the macrocycle with smaller cavity size. However, the Cd²⁺ complex with [17](DBF)N₂O₂ exhibits the highest value of stability constant for the whole series of metal ions studied, indicating that this ligand reveals a remarkable selectivity for cadmium(II) in the presence of all the metal ions studied, except copper(II), indicating that this ligand reveals a remarkable selectivity for cadmium(II) in the presence of the mentioned metal ions. The crystal structures of H₂[17](DBF)N₂O₃²⁺ (diprotonated form of the ligand) and of its cadmium complex were determined by X-ray diffraction. The Cd²⁺ ion fits exactly inside the macrocyclic cavity exhibiting coordination number eight by coordination to all the donor atoms of the ligand, and additionally to two oxygen atoms from one nitrate anion and one oxygen atom from a water molecule. The nickel(II) and copper(II) complexes with the two ligands were further studied by UV-vis-NIR and the copper(II) complexes also by EPR spectroscopic techniques in solution indicating square-pyramidal structures and suggesting that only one nitrogen and oxygen donors of the ligands are bound to the metal. However an additional weak interaction of the second nitrogen cannot be ruled out.