Complexation of [Gd(DTTA–Me)(H2O)2]− by Fluoride and Its Consequences to Water Exchange

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• 作者：Shima Karimi ; Gail Hunter ; Loïck Moriggi ; Carlos Platas-Iglesias ; Lothar Helm
• 刊名：Inorganic Chemistry
• 出版年：2016
• 出版时间：June 20, 2016
• 年：2016
• 卷：55
• 期：12
• 页码：6231-6239
• 全文大小：462K
• 年卷期：0
• ISSN：1520-510X

文摘

The displacement of water molecule(s) from the inner coordination sphere of [Gd(DTTA–Me)(H₂O)₂]⁻ (DTTA = ethylenetriamine-N,N,N″,N″-tetraacetate) by fluoride has been studied by multinuclear NMR relaxation (¹H, ¹⁷O, ¹⁹F) and DFT calculations. Fluoride anions can replace only one of the coordinated water molecules. The thermodynamic stability constant (K_GdLF,298⁰ = 11.6 ± 0.3) and thermodynamic parameters characterizing the formation of [Gd(DTTA–Me)(H₂O)F]^2– were determined (ΔH⁰ = +6.3 ± 0.1 kJ mol^–1; ΔS⁰ = +41.5 ± 3.4 J mol^–1 K^–1; ΔV⁰ = +4.5 ± 1.2 cm³ mol^–1). Fluoride binding causes a marked acceleration of the water exchange, which is seven times faster for [Gd(DTTA–Me)(H₂O)F]^2– (k_ex,1²⁹⁸ = 177 × 10⁶ s^–1) than for [Gd(DTTA–Me)(H₂O)₂]⁻ (k_ex,2²⁹⁸ = 24.6 × 10⁶ s^–1). Water exchange on both compounds is faster than formation of the fluoride complex. The analysis of the Gd–O_water distances, electron density, and electron localization function (ELF) at the bond critical points using DFT calculations reveals that F^– binding weakens the Gd–O_water bonds, thereby facilitating the departure of the coordinated water molecule following a dissociative mechanism. The water exchange on both Gd(DTTA–Me) complexes follow dissociative reaction pathways as shown by the positive activation volumes ΔV^⧧ = +8 ± 2 cm³ mol^–1 and +15 ± 4 cm³ mol^–1 for the bis-aqua complex and the monofluoro complex, respectively.