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Understanding the Effect of the Electron Spin Relaxation on the Relaxivities of Mn(II) Complexes with Triazacyclononane Derivatives
Inorganic Chemistry ( IF 4.3 ) Pub Date : 2021-10-07 , DOI: 10.1021/acs.inorgchem.1c02057 Rocío Uzal-Varela 1 , Laura Valencia 2 , Daniela Lalli 3 , Marcelino Maneiro 4 , David Esteban-Gómez 1 , Carlos Platas-Iglesias 1 , Mauro Botta 3 , Aurora Rodríguez-Rodríguez 1
Inorganic Chemistry ( IF 4.3 ) Pub Date : 2021-10-07 , DOI: 10.1021/acs.inorgchem.1c02057 Rocío Uzal-Varela 1 , Laura Valencia 2 , Daniela Lalli 3 , Marcelino Maneiro 4 , David Esteban-Gómez 1 , Carlos Platas-Iglesias 1 , Mauro Botta 3 , Aurora Rodríguez-Rodríguez 1
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Investigating the relaxation of water 1H nuclei induced by paramagnetic Mn(II) complexes is important to understand the mechanisms that control the efficiency of contrast agents used in diagnostic magnetic resonance imaging (MRI). Herein, a series of potentially hexadentate triazacyclononane (TACN) derivatives containing different pendant arms were designed to explore the relaxation of the electron spin in the corresponding Mn(II) complexes by using a combination of 1H NMR relaxometry and theoretical calculations. These ligands include 1,4,7-triazacyclononane-1,4,7-triacetic acid (H3NOTA) and three derivatives in which an acetate group is replaced by sulfonamide (H3NO2ASAm), amide (H2NO2AM), or pyridyl (H2NO2APy) pendants. The analogue of H3NOTA containing three propionate pendant arms (H3NOTPrA) was also investigated. The X-ray structure of the derivative containing two acetate groups and a sulfonamide pendant arm [Mn(NO2ASAm)]− evidenced six-coordination of the ligand to the metal ion, with the coordination polyhedron being close to a trigonal prism. The relaxivities of all complexes at 20 MHz and 25 °C (1.1–1.3 mM–1 s–1) are typical of systems that lack water molecules coordinated to the metal ion. The nuclear magnetic relaxation profiles evidence significant differences in the relaxivities of the complexes at low fields (<1 MHz), which are associated with different spin relaxation rates. The zero field splitting (ZFS) parameters calculated by using DFT and CASSCF methods show that electronic relaxation is relatively insensitive to the nature of the donor atoms. However, the twist angle of the two tripodal faces that delineate the coordination polyhedron, defined by the N atoms of the TACN unit (lower face) and the donor atoms of the pendant arms (upper face), has an important effect in the ZFS parameters. A twist angle close to the ideal value for an octahedral coordination (60°), such as that in [Mn(NOTPrA)]−, leads to a small ZFS energy, whereas this value increases as the coordination polyhedron approaches to a trigonal prism.
中文翻译：
了解电子自旋弛豫对具有三氮杂环壬烷衍生物的 Mn(II) 配合物弛豫率的影响
研究顺磁性 Mn(II) 复合物诱导的水1 H 核的弛豫对于了解控制用于诊断磁共振成像 (MRI) 的造影剂效率的机制很重要。在此，设计了一系列包含不同悬臂的潜在六齿三氮杂环壬烷 (TACN) 衍生物，通过结合1 H NMR 弛豫和理论计算来探索相应 Mn(II) 配合物中电子自旋的弛豫。这些配体包括 1,4,7-三氮杂环壬烷-1,4,7-三乙酸 (H 3 NOTA) 和三个衍生物，其中乙酸基被磺酰胺 (H 3 NO2ASAm)、酰胺 (H 2 NO2AM) 或吡啶基（H2 NO2APy) 吊坠。还研究了包含三个丙酸悬垂臂（H 3 NOTPrA）的 H 3 NOTA类似物。含有两个乙酸酯基团和一个磺酰胺悬臂 [Mn(NO2ASAm)] -的衍生物的 X 射线结构证明了配体与金属离子的六配位，配位多面体接近三棱柱。所有配合物在 20 MHz 和 25 °C (1.1–1.3 mM –1 s –1) 是缺乏与金属离子配位的水分子的典型系统。核磁弛豫曲线证明复合物在低场 (<1 MHz) 下的弛豫率存在显着差异，这与不同的自旋弛豫率有关。使用 DFT 和 CASSCF 方法计算的零场分裂 (ZFS) 参数表明电子弛豫对供体原子的性质相对不敏感。然而，由 TACN 单元的 N 原子（下表面）和悬臂的供体原子（上表面）定义的描绘配位多面体的两个三足面的扭曲角对 ZFS 参数有重要影响. 接近八面体配位（60°）的理想值的扭曲角，例如 [Mn(NOTPrA)] -, 导致小的 ZFS 能量，而该值随着配位多面体接近三棱柱而增加。
更新日期：2021-10-18
中文翻译：
了解电子自旋弛豫对具有三氮杂环壬烷衍生物的 Mn(II) 配合物弛豫率的影响
研究顺磁性 Mn(II) 复合物诱导的水1 H 核的弛豫对于了解控制用于诊断磁共振成像 (MRI) 的造影剂效率的机制很重要。在此，设计了一系列包含不同悬臂的潜在六齿三氮杂环壬烷 (TACN) 衍生物，通过结合1 H NMR 弛豫和理论计算来探索相应 Mn(II) 配合物中电子自旋的弛豫。这些配体包括 1,4,7-三氮杂环壬烷-1,4,7-三乙酸 (H 3 NOTA) 和三个衍生物，其中乙酸基被磺酰胺 (H 3 NO2ASAm)、酰胺 (H 2 NO2AM) 或吡啶基（H2 NO2APy) 吊坠。还研究了包含三个丙酸悬垂臂（H 3 NOTPrA）的 H 3 NOTA类似物。含有两个乙酸酯基团和一个磺酰胺悬臂 [Mn(NO2ASAm)] -的衍生物的 X 射线结构证明了配体与金属离子的六配位，配位多面体接近三棱柱。所有配合物在 20 MHz 和 25 °C (1.1–1.3 mM –1 s –1) 是缺乏与金属离子配位的水分子的典型系统。核磁弛豫曲线证明复合物在低场 (<1 MHz) 下的弛豫率存在显着差异，这与不同的自旋弛豫率有关。使用 DFT 和 CASSCF 方法计算的零场分裂 (ZFS) 参数表明电子弛豫对供体原子的性质相对不敏感。然而，由 TACN 单元的 N 原子（下表面）和悬臂的供体原子（上表面）定义的描绘配位多面体的两个三足面的扭曲角对 ZFS 参数有重要影响. 接近八面体配位（60°）的理想值的扭曲角，例如 [Mn(NOTPrA)] -, 导致小的 ZFS 能量，而该值随着配位多面体接近三棱柱而增加。
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