Lower denticity leading to higher stability: Structural and solution studies of Ln(III)-OBETA complexes

Roberto Negri, Zsolt Baranyai, Lorenzo Tei, Giovanni B. Giovenzana, Carlos Platas-Iglesias, Attila C. Bényei, Judit Bodnár, Adrienn Vágner, Mauro Botta

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Abstract

The heptadentate ligand OBETA (2,2′-oxybis(ethylamine)-N,N,N′,N′-tetraacetic acid) was reported to form complexes with Ln3+ ions more stable than those formed by the octadentate and more popular congener EGTA (ethylene glycol O,O′-bis(ethylamine)-N,N,N′,N′-tetraacetic acid). The structural features leading to this puzzling coordination paradox were investigated by X-ray diffraction, solution state NMR, molecular modeling, and relaxometric studies. The stability constant of Gd(OBETA) (log KGdL = 19.37, 0.1 M KCl) is 2 orders of magnitude higher than that of the higher denticity analogue Gd(EGTA) (log KGdL = 17.66, 0.1 M KCl). The half-lives (t1/2) for the dissociation reactions of Gd(OBETA) and Gd(EGTA) ([Cu2+]tot = 0.2 mM, [Cit3-]tot = 0.5 mM, [PO43-]tot = 1.0 mM, and [CO32-]tot = 25 mM at pH = 7.4 and 25 °C in 0.1 M KCl solution) are 6.8 and 0.63 h, respectively, reflecting the much higher inertness of Gd(OBETA) near physiological conditions. NMR studies and DFT calculations using the B3LYP functional and a large-core ECP indicate that the [Gd(OBETA)(H2O)2]- complex most likely exists in solution as the δ( λ λ)(δδδδ)A/A(δδ)( λ λ λλ)A enantiomeric pair, with an activation free energy for the enantiomerization process of ∼40 kJ·mol-1. The metal ion is nine-coordinate by seven donor atoms of the ligand and two inner-sphere water molecules. The X-ray crystal structure of [C(NH2)3]3[Lu(OBETA)(CO3)]·2H2O is in agreement with the predictions of DFT calculations, the two coordinated water molecules being replaced by a bidentate carbonate anion. The 1H NMRD and 17O NMR study revealed that the two inner-sphere water molecules in Gd(OBETA) are endowed with a relatively fast water exchange rate (kex298 = 13 × 106 s-1). The higher thermodynamic stability and inertness of Ln(OBETA) complexes, peaking in the center of the 4f series, combined with the presence of two coordinated water molecules suggests that Gd(OBETA) is a promising paramagnetic probe for MRI applications.

Original languageEnglish
Pages (from-to)12499-12511
Number of pages13
JournalInorganic Chemistry
Volume53
Issue number23
DOIs
Publication statusPublished - Dec 1 2014

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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    Negri, R., Baranyai, Z., Tei, L., Giovenzana, G. B., Platas-Iglesias, C., Bényei, A. C., Bodnár, J., Vágner, A., & Botta, M. (2014). Lower denticity leading to higher stability: Structural and solution studies of Ln(III)-OBETA complexes. Inorganic Chemistry, 53(23), 12499-12511. https://doi.org/10.1021/ic5020225