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