Lower Denticity Leading to Higher Stability: Structural and Solution Studies of Ln(III)–OBETA Complexes

• Published in: Inorganic chemistry Vol. 53; no. 23; pp. 12499 - 12511
• Main Authors: Negri, Roberto, Baranyai, Zsolt, Tei, Lorenzo, Giovenzana, Giovanni B, Platas-Iglesias, Carlos, Bényei, Attila C, Bodnár, Judit, Vágner, Adrienn, Botta, Mauro
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 01-12-2014
• Subjects: Crystallography, X-Ray; Ligands; Magnetic Resonance Spectroscopy; Models, Molecular; Molecular Structure; Organometallic Compounds - chemistry
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic5020225

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 K GdL = 19.37, 0.1 M KCl) is 2 orders of magnitude higher than that of the higher denticity analogue Gd(EGTA) (log K GdL = 17.66, 0.1 M KCl). The half-lives (t 1/2) for the dissociation reactions of Gd(OBETA) and Gd(EGTA) ([Cu2+]tot = 0.2 mM, [Cit3–]tot = 0.5 mM, [PO4 3–]tot = 1.0 mM, and [CO3 2–]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(λλ)(δδδδ)A/Λ(δδ)(λλλλ)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 (k ex 298 = 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.