Modulation of the magnetic properties of mononuclear Dy(iii) complexes by tuning the coordination geometry and local symmetry

Abstract

Precise control of the crystal field and local symmetry around the paramagnetic spin center is crucial for the design and synthesis of single-molecule magnets (SMMs). Herein, three mononuclear Dy(III)-based complexes, [Dy(L^N6)(CH₃COO)₂](BPh₄)(CH₂Cl₂) (1), [Dy(L^N6)(2,6-Cl-4-NO₂-PhO)(H₂O)₂]₂(PF₆)₂(H₂O)(2,6-Cl-4-NO₂-PhO)₂ (2) and [Dy(L^N6)(2,6-Cl-4-NO₂-PhO)₂](BPh₄)(CH₂Cl₂)₂ (3) (L^N6 = N6-hexagonal plane accomplished by a neutral Schiff base ligand formed from 2,6-diacetylpyridine and ethylenediamine), are successfully isolated. In these complexes, the Dy(III) centers are coordinated with six neutral N atoms from a nonrigid equatorial ligand, while different oxygen-bearing ligands are arranged at the axial positions of the central ions by gradual regularization of the axial ligands. As a result, Dy(III) ions in the three complexes exhibit various coordination geometries, forming a ten-coordinate tetradecahedron for 1, a nine-coordinate muffin configuration for 2 and a distorted eight-coordinate hexagonal bipyramid for 3. Magnetic studies reveal that all complexes exhibit no SIM behaviour under zero dc field, due to the predominant quantum tunneling of magnetization (QTM), which can be effectively suppressed by additional dc fields. Experiments, coupled with theoretical calculations, demonstrate that varying local symmetries and coordination geometries are synergistically responsible for the disparities of QTM and uniaxial anisotropy, resulting in notably different magnetic properties.