New dioxadiaza- and trioxadiaza-macrocycles containing one dibenzofuran unit with two amino pendant arms: synthesis, protonation and complexation studies

Abstract

New dioxadiaza- and trioxadiaza-macrocycles containing one rigid dibenzofuran unit (DBF) and N-(2-aminoethyl) pendant arms were synthesized, N,N′-bis(2-aminoethyl)-[17](DBF)N₂O₂ (L¹) and N,N′-bis(2-aminoethyl)-[22](DBF)N₂O₃ (L²), respectively. The binding properties of both macrocycles to metal ions and structural studies of their metal complexes were carried out. The protonation constants of both compounds and the stability constants of their complexes with Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, and Pb²⁺ were determined at 298.2 K, in aqueous solutions, and at ionic strength 0.10 mol dm⁻³ in KNO₃. Mononuclear complexes with both ligands were formed, and dinuclear complexes were only found for L². The thermodynamic binding affinities of the metal complexes of L² are lower than those of L¹ as expected, but the Pb²⁺ complexes of both macrocycles exhibit close stability constant values. On the other hand, the binding affinities of Cd²⁺ and Pb²⁺ for L¹ are very high, when compared to those of Co²⁺, Ni²⁺ and Zn²⁺. These interesting properties were explained by the presence of the rigid DBF moiety in the backbone of the macrocycle and to the special match between the macrocyclic cavity size and the studied larger metal ions. To elucidate the adopted structures of complexes in solution, the nickel(II) and copper(II) complexes with both ligands were further studied by UV-vis-NIR spectroscopy in DMSO–H₂O 1 : 1 (v/v) solution. The copper(II) complexes were also studied by EPR spectroscopy in the same mixture of solvents. The crystal structure of the copper complex of L¹ was also determined. The copper(II) displays an octahedral geometry, the four nitrogen atoms forming the equatorial plane and two oxygen atoms, one from the DBF unit and the other one from the ether oxygen, in axial positions. One of the ether oxygens of the macrocycle is out of the coordination sphere. Our results led us to suggest that this geometry is also adopted by the Co²⁺ to Zn²⁺ complexes, and only the larger Cd²⁺ and Pb²⁺ manage to form complexes with the involvement of all the oxygen atoms of the macrocyclic backbone.