Cyclam derivatives containing three acetate pendant arms: synthesis, acid–base, metal complexation and structural studies

Abstract

The ligands 1,4,8,11-tetraazacyclotetradecane-1,4,8-triacetic-11-methylphosphonic acid (H₅te3a1p) and 1,4,8,11-tetraazacyclotetradecane-1,4,8-triacetic acid (H₃te3a) were synthesized, the former one for the first time. The syntheses of these ligands were achieved from reactions on 1,4,8,11-tetraazacyclotetradecane-1,4,8-tris(carbamoylmethyl) hydroiodide (te3am·HI), and compounds (Hte3am)⁺, 1, and (H₇te3a1p)²⁺, 4, were characterized by X-ray diffraction. Structures of two other compounds resulting from side-reactions, (H₂te2lac)²⁺, 2, and (H₄te2a2p^OEt2)²⁺, 3, were also determined by X-ray diffraction. Potentiometric titrations of H₅te3a1p and H₃te3a were performed at 298.2 K and ionic strength 0.10 mol dm⁻³ in NMe₄NO₃ to determine their protonation constants. ¹H and ³¹P NMR titrations of H₅te3a1p were carried out in order to determine the very high first protonation constant of this ligand and to elucidate the sequence of protonation. Potentiometric studies of the two ligands with Ca²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ metal ions performed in the same experimental conditions showed that the complexes of H₅te3a1p present very high thermodynamic stability while complexes of H₃te3a, particularly Co²⁺ and Zn²⁺, are even more stable. ³¹P NMR spectra of the cadmium(II) complex of H₅te3a1p showed that the phosphonate moiety was coordinated to the metal ion. The UV-vis-NIR spectroscopic data and magnetic moment values of Co²⁺ and Ni²⁺ complexes of H₅te3a1p and H₃te3a together with the EPR of the corresponding Cu²⁺ complexes indicated that all these complexes adopt distorted octahedral coordination geometries in solution. This was confirmed by the single crystal structure of [Cu₂(Hte3a)(H₂O)₃Cl]Cl_0.5(ClO₄)_0.5·2H₂O that showed two distorted octahedral copper centres bridged by a N-acetate pendant arm with a Cu⋯Cu distance of 4.890(1) Å. The first one is encapsulated into the macrocyclic cavity surrounded by four nitrogen and two oxygen donors from the macrocycle, whereas the second one is on the periphery of the macrocycle and is coordinated to two oxygen atoms of one acetate pendant arm in chelating fashion, one chloride and three water molecules.