Bis- and tris-(3-aminopropyl) derivatives of 14-membered tetraazamacrocycles containing pyridine: synthesis, protonation and complexation studies

Abstract

New N-(3-aminopropyl) (L¹, L²) and (2-cyanoethyl) (L³, L⁴) derivatives of a 14-membered tetraazamacrocycle containing pyridine have been synthesized. The protonation constants of L¹ and L² and the stability constants of their complexes with Ni²⁺, Cu²⁺, Zn²⁺ and Cd²⁺ metal ions were determined in aqueous solutions by potentiometry, at 298.2 K and ionic strength 0.10 mol dm⁻³ in KNO₃. Both compounds have high overall basicity due to the presence of the aminopropyl arms. Their copper(II) complexes exhibit very high stability constants, which sharply decrease for the complexes of the other studied metal ions, as usually happens with polyamine ligands. Mono- and dinuclear complexes are formed with L² as well as with L¹, but the latter exhibits mononuclear complexes with slightly higher K_ML values while the dinuclear complexes of L² are thermodynamically more stable. The presence of these species in solution was supported by UV-VIS-NIR and EPR spectroscopic data. The single crystal structures of [Cu(H₂L²)(ClO₄)]³⁺ and [CoL³Cl]⁺ revealed that the metal centres are surrounded by the four nitrogen atoms of the macrocycle and one monodentate ligand, adopting distorted square pyramidal geometries. In the [CoL³Cl]⁺ complex, the macrocycle adopts a folded arrangement with the nitrogen atom opposite to the pyridine at the axial position while in the [Cu(H₂L²)(ClO₄)]³⁺ complex, the macrocycle adopts a planar conformation with the three aminopropyl arms located at the same side of the macrocyclic plane.