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

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 H -pyrazol-1-yl)methyl)-6-(1 H -pyrazol-1-yl)pyridine (PyPz 3 ) ligand, here, we prepared a series of benzoquinone-bridged dicobalt(II) SMMs [{(PyPz 3 )Co} 2 (L)][PF 6 ] 2 , ( 1 , L = 2,5-dioxo-1,4-benzoquinone (dhbq 2- ); 2 , L = chloranilate (CA 2- ); and 3 , 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 ( 1 ) < Cl ( 2 ) < Br ( 3 )). Accordingly, the magnetic study revealed that the axial anisotropy parameter ( D ) of the Co ions decreased from -78.5 to -56.5 cm -1 in 1 - 3 , while the rhombic one ( E ) increased significantly. As a result, 1 exhibited slow relaxation of magnetization under a zero dc field, while both 2 and 3 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.