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Keywords:

  • Macrocyclic ligands;
  • Copper;
  • Thermodynamics;
  • Kinetics;
  • Phosphonate complexes

Abstract

Three compounds that are based on cyclen and contain three methylphosphonate pendant arms ― (1,4,7,10-tetraazacyclododecane-1,4,7-triyl)tris(methylene)triphosphonic acid (H6do3p), 3-{4,7,10-tris[(dihydroxyphosphoryl)methyl]-1,4,7,10-tetraazacyclododecan-1-yl}propanoic acid (H7do3p1pr) and [10-(3-hydroxypropyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl]tris(methylene)triphosphonic acid (H6do3p1ol) ― were synthesized and characterized. X-ray crystal structures were determined for H6do3p and for the complex [Cu(H2O)6]2+[Cu(H2O)(H4dotp)]2– of a related ligand H8dotp [H8dotp = (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetrakis(methylene)tetraphosphonic acid]. They show the copper(II) centre coordinated only to the four amines of the macrocycle and one water molecule in the apical position. The acid–base properties of the three compounds were studied in aqueous solution by potentiometry and 31P NMR spectroscopy. All ligands exhibit very high basicity and their protonation schemes are dominated by proton relocations between the basic sites and intramolecular hydrogen bonding. The thermodynamic stability constants for complexes of the three ligands with Cu2+ and Zn2+ metal ions were determined by potentiometry and exhibit very high values for the complexes of Cu2+. UV/Vis spectroscopy was used to assess the acid-assisted dissociation of the Cu2+ complexes and showed that the dissociation rates are faster than for the corresponding complex of H8dotp, whereas the [Cu(do3p1ol)]4– complex is the most inert one in this series.