Series of Benzoquinone-Bridged Dicobalt(II) Single-Molecule Magnets

Abstract

Mononuclear complexes within a particular coordination geometry have been well recognized for high-performance single-molecule magnets (SMMs), while the incorporation of such well-defined geometric ions into multinuclear complexes remains less explored. Using the rigid 2-(di(1<i>H</i>-pyrazol-1-yl)methyl)-6-(1<i>H</i>-pyrazol-1-yl)pyridine (PyPz₃) ligand, here, we prepared a series of benzoquinone-bridged dicobalt(II) SMMs [{(PyPz₃)Co}₂(L)][PF₆]₂, (<b>1</b>, L = 2,5-dioxo-1,4-benzoquinone (dhbq^2-); <b>2</b>, L = chloranilate (CA^2-); and <b>3</b>, L = bromanilate (BA^2-)), in which each Co(II) center adopts a distorted trigonal prismatic (TPR) geometry and the distortion increases with the sizes of 3,6-substituent groups (H (<b>1</b>) < Cl (<b>2</b>) < Br (<b>3</b>)). Accordingly, the magnetic study revealed that the axial anisotropy parameter (<i>D</i>) of the Co ions decreased from -78.5 to -56.5 cm^-1 in <b>1</b>-<b>3</b>, while the rhombic one (<i>E</i>) increased significantly. As a result, <b>1</b> exhibited slow relaxation of magnetization under a zero dc field, while both <b>2</b> and <b>3</b> showed only the field-induced SMM behaviors, likely due to the increased rhombic anisotropy that leads to the serious quantum tunneling of the magnetization. Our study demonstrated that the relaxation dynamics and performances of a multinuclear complex are strongly dependent on the coordination geometry of the local metal ions, which may be engineered by modifying the substituent groups.