Coordination Behavior of 1,4-Disubstituted Cyclen Endowed with Phosphonate, Phosphonate Monoethylester, and...

Three 1,4,7,10-tetraazacyclododecane-based ligands disubstituted in 1,4-positions with phosphonic acid, phosphonate monoethyl-ester, and H-phosphinic acid pendant arms, 1,4-H₄do2p, 1,4-H₂do2p^OEt, and 1,4-H₂Bn₂do2p^H, were synthesized and their coordination to selected metal ions, Mg(II), Ca(II), Mn(II), Zn(II), Cu(II), Eu(III), Gd(III), and Tb(III), was investigated. The solid-state structure of the phosphonate ligand, 1,4-H₄do2p, was determined by single-crystal X-ray diffraction. Protonation constants of the ligands and stability constants of their complexes were obtained by potentiometry, and their values are comparable to those of previously studied analogous 1,7-disubstitued cyclen derivatives. The Gd(III) complex of 1,4-H₄do2p is ~1 order of magnitude more stable than the Gd(III) complex of the 1,7-analogue, probably due to the disubstituted ethylenediamine-like structural motif in 1,4-H₄do2p enabling more efficient wrapping of the metal ion. Stability of Gd(III)-1,4-H₂do2p^OEt and Gd(III)-H₂Bn₂do2p^H complexes is low and the constants cannot be determined due to precipitation of the metal hydroxide. Protonations of the Cu(II), Zn(II), and Gd(III) complexes probably takes place on the coordinated phosphonate groups. Complexes of Mn(II) and alkali-earth metal ions are significantly less stable and are not formed in acidic solutions. Potential presence of water molecule(s) in the coordination spheres of the Mn(II) and Ln(III) complexes was studied by variable-temperature NMR experiments. The Mn(II) complexes of the ligands are not hydrated. The Gd(III)-1,4-H₄do2p complex undergoes hydration equilibrium between mono- and bis-hydrated species. Presence of two-species equilibrium was confirmed by UV-Vis spectroscopy of the Eu(III)-1,4-H₄do2p complex and hydration states were also determined by luminescence measurements of the Eu(III)/Tb(III)-1,4-H₄do2p complexes.