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Unusual Coordination Behaviour of a Phosphonate‐ and Pyridine‐Containing Ligand in a Stable Lanthanide Complex
Author(s) -
Gałęzowska Joanna,
Janicki Rafał,
Kozłowski Henryk,
Mondry Anna,
Młynarz Piotr,
Szyrwiel Łukasz
Publication year - 2010
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201000058
Subject(s) - chemistry , lanthanide , protonation , pyridine , phosphonate , ligand (biochemistry) , crystallography , ethylenediamine , coordination complex , crystal structure , medicinal chemistry , stereochemistry , inorganic chemistry , ion , metal , organic chemistry , biochemistry , receptor
An experimental study of the coordination of lanthanide ions with {ethane‐1,2‐diylbis[imino(pyridin‐2‐ylmethanediyl)]}bis(phosphonic acid), a ligand containing phosphonate and pyridine arms attached to the well‐known ethylenediamine scaffold, is reported. The X‐ray crystal structure of an anionic, octacoordinate Eu III complex of the formula [C(NH 2 )] 6 [Eu 2 (L) 2 (CO 3 ) 2 ] · 8H 2 O has been obtained and found to contain cyclic dimeric complex anions in which the coordination environment of each Eu III ion is composed of three oxygen atoms from phosphonate groups, two nitrogen atoms from imino groups and one nitrogen atom from a pyridine group, a combination that has seldom been observed for Ln III complexes. In order to check the coordination behaviour of the ligand, an investigation of the complexes in solution has been performed, which revealed a different coordination mode. The protonation constants of the free ligand and the stability constants of their complexes with selected lanthanide ions (La III , Sm III , Eu III , Gd III , Tb III , Ho III and Lu III ) have been determined potentiometricaly at 25 °C and an ionic strength of 0.1 M (KCl). The results obtained show the formation of monomeric, protonated ([LnH 2 L], [LnHL]) and nonprotonated ([LnL]) species in the pH range studied (2–11). The determined stability constants (log β LnL ) are relatively high, varying between 14.96 and 16.42. The Eu III L system was also fully characterized by means of UV/Vis spectroscopy, which indicated that the coordination process starts above pH 3 and remains constant in the range pH 4–10. In contrast to the crystal structure, solution NMR and luminescence studies suggested that the pyridine moieties are not present in the Ln III coordination sphere. These results suggest that this ligand–unsaturated metal ion coordination sphere may provide a potential binding site for other target molecules.