Enhancing the Magnetic Behaviors of Dy2 Complexes by Modulating the Crystal Field Environment with Different μ-O Bridging Ligands

Four similar dinuclear lanthanide complexes have been synthesized by linking two [Ln(hfac)_2-3] units (hfac stands for hexafluoroacetylacetone) with different <i>μ</i>-O bridging ligands. The 2,2'-bipyridine-N-oxide ligand (bmpo) constructed two centrosymmetric complexes [Ln₂(hfac)₆(bmpo)₂] (Ln = Dy(<b>1</b>), Tb(<b>2</b>)), with nine-coordinated Ln^III ions showing <i>C</i>_s low symmetry, while the ligand di(2-pyridyl)methanediol (py₂C(OH)₂) formed another two compounds [Ln₂(hfac)₄(py₂C(OH)O)₂] (Ln = Dy(<b>3</b>), Tb(<b>4</b>)), with two kinds of eight-coordinated Ln^III ions exhibiting improved symmetries of <i>D</i>_4d and <i>D</i>_2d. Magnetic analysis reveals that Dy₂ complex <b>1</b> shows intramolecular antiferromagnetic coupling (<i>J</i> = -1.07 cm^-1) and no relaxation process above 2.0 K even in a 1000 Oe dc field, owing to the low symmetry of Dy^III ions, while the similar Dy₂ complex <b>3</b> with improved Dy^III symmetry shows ferromagnetic coupling (<i>J</i> = 1.17 cm^-1), which induces a 1000 Oe dc field-induced two-step magnetization relaxation processes with effective energy barrier <i>U</i>_eff = 47.4 K and 25.2 K for the slow relaxation and fast relaxation processes, respectively. This study proves again that the improved symmetry combined with intramolecular ferromagnetic interactions, both mediated by bridging ligands, can enhance the Dy^III anisotropy, further quench the quantum tunneling of the magnetization, and finally, enhance the magnetic behavior of Ln^III-based systems.