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

详细信息    查看全文

• 作者：Roberto Negri ; Zsolt Baranyai ; Lorenzo Tei ; Giovanni B. Giovenzana ; Carlos Platas-Iglesias ; Attila C. B茅nyei ; Judit Bodn谩r ; Adrienn V谩gner ; Mauro Botta
• 刊名：Inorganic Chemistry
• 出版年：2014
• 出版时间：December 1, 2014
• 年：2014
• 卷：53
• 期：23
• 页码：12499-12511
• 全文大小：606K
• ISSN：1520-510X

文摘

The heptadentate ligand OBETA (2,2鈥?oxybis(ethylamine)-N,N,N鈥?N鈥?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鈥?bis(ethylamine)-N,N,N鈥?N鈥?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, [Cit^{3鈥?/sup>]_tot = 0.5 mM, [PO₄3鈥?/sup>]_tot = 1.0 mM, and [CO₃2鈥?/sup>]_tot = 25 mM at pH = 7.4 and 25 掳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]鈭?/sup> complex most likely exists in solution as the 螖(位位)(未未未未)_A/螞(未未)(位位位位)_A enantiomeric pair, with an activation free energy for the enantiomerization process of 鈭?0 kJ路mol鈥?. 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)]路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_ex298 = 13 脳 106 s鈥?). 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.}