title: H4octapa: highly stable complexation of lanthanide(III) ions and copper(II).
abstract: The acyclic ligand octapa(4-) (H4octapa = 6,6'-((ethane-1,2-diylbis((carboxymethyl)azanediyl))bis(methylene))dipicolinic acid) forms stable complexes with the Ln(3+) ions in aqueous solution. The stability constants determined for the complexes with La(3+), Gd(3+), and Lu(3+) using relaxometric methods are log KLaL = 20.13(7), log KGdL = 20.23(4), and log KLuL = 20.49(5) (I = 0.15 M NaCl). High stability constants were also determined for the complexes formed with divalent metal ions such as Zn(2+) and Cu(2+) (log KZnL = 18.91(3) and log KCuL = 22.08(2)). UV-visible and NMR spectroscopic studies and density functional theory (DFT) calculations point to hexadentate binding of the ligand to Zn(2+) and Cu(2+), the donor atoms of the acetate groups of the ligand remaining uncoordinated. The complexes formed with the Ln(3+) ions are nine-coordinated thanks to the octadentate binding of the ligand and the presence of a coordinated water molecule. The stability constants of the complexes formed with the Ln(3+) ions do not change significantly across the lanthanide series. A DFT investigation shows that this is the result of a subtle balance between the increased binding energies across the 4f period, which contribute to an increasing complex stability, and the parallel increase of the absolute values of the hydration free energies of the Ln(3+) ions. In the case of the [Ln(octapa)(H2O)](-) complexes the interaction between the amine nitrogen atoms of the ligand and the Ln(3+) ions is weakened along the lanthanide series, and therefore the increased electrostatic interaction does not overcome the increasing hydration energies. A detailed kinetic study of the dissociation of the [Gd(octapa)(H2O)](-) complex in the presence of Cu(2+) shows that the metal-assisted pathway is the main responsible for complex dissociation at pH 7.4 and physiological [Cu(2+)] concentration (1 μM).
authors.affiliation: Department of Inorganic and Analytical Chemistry, University of Debrecen , Egyetem tér 1, H-4010 Debrecen, Hungary.
authors.forename: Ferenc Krisztián
authors.initials: FK
authors.lastname: Kálmán
authors.forename: Andrea
authors.initials: A
authors.lastname: Végh
authors.forename: Martín
authors.initials: M
authors.lastname: Regueiro-Figueroa
authors.forename: Éva
authors.initials: É
authors.lastname: Tóth
authors.forename: Carlos
authors.initials: C
authors.lastname: Platas-Iglesias
authors.forename: Gyula
authors.initials: G
authors.lastname: Tircsó
mesh_terms: D002614:Chelating Agents / Q000138:chemical synthesis / Q000737:chemistry*; D003300:Copper / Q000737:chemistry*; D007477:Ions / Q000737:chemistry; D007700:Kinetics; D028581:Lanthanoid Series Elements / Q000737:chemistry*; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D010848:Picolinic Acids / Q000737:chemistry*; D011789:Quantum Theory
chemical_list: C000602656:6,6'-((ethane-1,2-diylbis((carboxymethyl)azanediyl))bis-(methylene))dipicolinic acid); D002614:Chelating Agents; D007477:Ions; D028581:Lanthanoid Series Elements; D008024:Ligands; D009942:Organometallic Compounds; D010848:Picolinic Acids; D003300:Copper