摘要/Abstract

以金属冠醚[15-MC_Ni（II）-5]和卤素离子分别作为Tb（III）在赤道平面和轴向上的配体，成功地组装了三例混合过渡金属-稀土配合物{TbNi₅X₂}（X=F，Cl，Br），其中TbNi₅F₂为首例双氟端基配位的稀土配合物.X射线单晶衍射分析结果表明，配合物中Tb（III）离子采用具有高轴次对称性的五角双锥（D_5h）配位方式，表明利用金属冠醚策略可以定向构筑含五角双锥稀土基元的混合过渡金属-稀土配合物.交流磁化率的测试结果表明，Tb（III）轴向的电荷分布对配合物的慢磁弛豫行为有着重要的影响.依次削弱轴向配体X^-的电负性，在1 kOe外场下{TbNi₅F₂}和{TbNi₅Cl₂}分别表现出场诱导的单分子磁体和弱的慢磁弛豫行为，而{TbNi₅Br₂}则由于快速的磁量子隧穿而不表现出慢的磁弛豫行为.
关键词: 稀土, 铽离子, 金属冠醚, 慢磁弛豫, 单分子磁体
Single-molecule magnets (SMMs), exhibiting magnetic bistability and slow magnetization relaxation, have fascinated scientific community for their promising applications in data storage and information processing. Great development has been achieved in lanthanide-based SMMs due to the unquenched orbital momentum and strong anisotropy of lanthanide ions. According to the crystal-field theory, the magnetic anisotropy of lanthanide ions arises from crystal-field splitting. Appropriate arrangement of coordination environment of lanthanide ion (including the local symmetry as well as the charge distribution) is key to design high-performance SMMs. However, it still remains a huge challenge to generate lanthanide-containing compounds with certain coordination environment. Taking advantage of metallacrown (MC) approach, herein a series of 3d-4f complexes {TbNi₅X₂} (X=F, Cl, Br) were successfully isolated via solvothermal reactions. To obtain these complexes, a mixture of stoichiometric metal salt and quinaldichdroxamic acid with excess of pyridine derivative was dissolved in methanol and then heated at 75℃ for 2 d. X-ray single-crystal diffraction analysis indicated that the Tb(III) site equatorially coordinates with[15-MC_Ni(II)-5], whilst is axially capped by halide ions. As a result, the lanthanide ion possesses high axiality with a pentagonal bipyramid geometry (D_5h). Alternative current magnetic susceptibility data revealed that the electrostatic interactions between f-electrons and ligand electrons play an important role in modulating the magnetic relaxation dynamics. Maximizing the axial charge density in {TbNi₅F₂} where the[F-Ln-F]⁺ moiety is firstly reported in lanthanide chemistry, the oblate Tb(III) is placed in a judicious crystal field. The out-of-phase signal of {TbNi₅F₂} shows obvious temperature and frequency dependence under 1 kOe applied dc field. Additionally, the slow magnetization relaxation of {TbNi₅F₂} can be fitted by the power law or Arrhenius plot with reversal barrier of 19.0 K. By lowering the electrostatic interactions of axial ligation, the out-of-phase signal significantly weakens in {TbNi₅Cl₂} and even vanishes in {TbNi₅Br₂}. The decline of magnetic anisotropy in {TbNi₅Cl₂} and {TbNi₅Br₂} accelerates the fast quantum tunneling of magnetization. The results demonstrate for the first time that the Off/Part/On slow magnetization relaxation can be modulated via the improvement of electronegativity of axial ligands.
Key words: lanthanide, Tb (III) ion, metallacrown, slow magnetic relaxation, single-molecule magnets


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