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• 作者：Edina Balogh ; Zhenjie He ; Wenyuan Hsieh ; Shuang Liu ; É ; va T&oacute ; th
• 刊名：Inorganic Chemistry
• 出版年：2007
• 出版时间：January 8, 2007
• 年：2007
• 卷：46
• 期：1
• 页码：238 - 250
• 全文大小：260K
• 年卷期：v.46,no.1(January 8, 2007)
• ISSN：1520-510X

文摘

Mn²⁺ has five unpaired d-electrons, a long electronic relaxation time, and labile water exchange, all of which makeit an attractive candidate for contrast agent application in medical magnetic resonance imaging. In the quest forstable and nonlabile Mn²⁺ complexes, we explored a novel dimeric triazacyclononane-based ligand bearing carboxylatefunctional groups, H₄ENOTA. The protonation constants of the ligand and the stability constants of the complexesformed with some endogenously important metals (Ca²⁺, Cu²⁺, Zn²⁺), as well as with Mn²⁺ and Ce³⁺, have beenassessed by NMR methods, potentiometry, and UV-vis spectrophotometry. Overall, the thermodynamic stability ofthe complexes is lower as compared to that of the corresponding NOTA analogues (H₃NOTA, 1,4,7-triaazacyclononane-1,4,7-triacetic acid). The crystal structure of Mn₂(ENOTA)(H₂O)·5H₂O contains two six-coordinatedMn²⁺, in addition to the three amine nitrogens and the two oxygens from the pendent monodentate carboxylategroups, and one water (Mn2) or one bridging carboxylate oxygen (Mn1) completes the coordination sphere of themetal ion. In an aqueous solution, this bridging carboxylate is replaced by a water molecule, as evidenced by the¹⁷O chemical shifts and proton relaxivity data that point to monohydration for both metal ions in the dinuclearcomplex. A variable-temperature and -pressure ¹⁷O NMR study has been performed on [Mn₂(ENOTA)(H₂O)₂] toassess the rate and, for the first time on a Mn²⁺ chelate, also the mechanism of the water exchange. The innersphere water is slightly more labile in [Mn₂(ENOTA)(H₂O)₂] (isubscribe/journals/inocaj/46/i01/eqn/ic0616582e10001.gif"> = 5.5 × 10⁷ s^-1) than in the aqua ion (2.1 × 10⁷s^-1, Merbach, A. E.; et al. Inorg. Chem. 1980, 19, 3696). The water exchange proceeds via an almost limitingassociative mechanism, as evidenced by the large negative activation volume (images/gifchars/Delta.gif" BORDER=0 >V^{images/entities/thermod.gif">} = -10.7 cm³ mol^-1). Theproton relaxivities measured on [Mn₂(ENOTA)(H₂O)₂] show a low-field dispersion at ~0.1 MHz arising from acontact interaction between the Mn^II electron spin and the water proton nuclear spins.