Magnetic Anisotropy from Easy-Plane to Easy-Axial in Square Pyramidal Cobalt(II) Single-Ion Magnets

By elaborately employing a rigid tetrazo macrocyclic ligand, both the designed Co(II) complexes [Co(12-TMC)Py](BF4)(2) (1) and [Co(12-TMC)DMF](BF4)Cl (2) (12-TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane; Py = pyridine; and DMF = N,N-dimethylformamide) adopt the distorted square pyramidal geometry with the identical equatorial site but a flexible axial ligand environment. Such a molecular design enables the realization of field-induced single-ion magnets. Importantly, the designed single-molecule magnets display axial ligand-mediated magnetic anisotropy. The axially elongated geometry in complex 1 is devoted to stabilizing a positive zero-field splitting parameter (D), while an axially compressed one in complex 2 tends to stabilize a negative D. Such an understanding was further validated by a series of complexes with the same square pyramidal geometry but different axial ligands such as Cl- and NCO-. Therefore, this study provides a new example of the delicate modification of magnetic behavior and presents fundamental insights into magneto-structural correlation.