Properties of a new 4-imidazolyl derivative of a 14-membered tetraazamacrocyclic chelating agent

Abstract

A new bis-N,N′-(5-methylimidazol-4-ylmethyl) derivative of a 14-membered tetraazamacrocycle, L¹, has been synthesized. The protonation constants of this compound and the stability constants of its complexes with divalent first-row transition metal ions and Fe³⁺ were determined at 298.2 K in aqueous 0.10 mol dm⁻³ KNO₃. Compound L¹ exhibits high overall basicity, which is mainly conferred by the imidazolyl groups. The complexes of the divalent first row-transition metal ions of L¹ follow the Irving–Williams order of stability with the maximum for Cu²⁺ as expected, but a steep fall of constants is verified for the Mn²⁺, Fe²⁺ and Co²⁺, in one side, and for the Zn²⁺ complexes, in the other side. Additionally, L¹ shows a large affinity for Fe³⁺, and the relative stability constants for its Cd²⁺ and Pb²⁺ complexes indicate that L¹ may be useful for the complexometric determination of these two toxic metal ions in solutions containing both metal ions. These studies together with NMR, UV-vis and EPR spectroscopic data indicated the presence of mononuclear complexes, which adopt distorted pyramidal or octahedral geometries depending on the metal centre. The X-ray crystal structure of [Cu(HL¹)](PF₆)₂(NO₃)·H₂O showed that the coordination sphere of the copper centre can be described as a distorted square pyramid with the basal plane defined by three nitrogen donors of the macrocycle backbone and one nitrogen atom from one imidazolyl pendant arm. The apical position is occupied by the nitrogen atom of the macrocycle trans to the pyridine ring. To achieve this coordination environment, the macrocycle is folded along the axis defined by the two N atoms contiguous to the pyridine ring. The free methylimidazolyl arm points away from the metal centre leading to an intramolecular Cu⋯N distance of 5.155(1) Å.