High-coordinate CoII and FeII compounds constructed from an asymmetric tetradentate ligand show slow magnetic relaxation behavior

Abstract

Using an asymmetric tetradentate ligand N-((6-(1H-pyrazol-1-yl)pyridin-2-yl)methylene)benzohydrazide (pypzbeyz), two high-coordinate 3d transition metal compounds [Co^II(pypzbeyz)(NO₃)₂] (1) and [Fe^II(pypzbeyz)₂](BF₄)₂(CH₃CN) (2) have been synthesized and characterized by structural and magnetic measurements. X-ray crystallographic analyses revealed that compound 1 is seven-coordinate with a distorted pentagonal bipyramidal geometry (pseudo-D_5h) and compound 2 is eight-coordinate with a triangular dodecahedral geometry (pseudo-D_2d). Direct current (dc) magnetic susceptibilities revealed that compound 1 shows easy-plane magnetic anisotropy (D = +29.9 cm⁻¹, E = 0.31 cm⁻¹) and 2 shows easy-axis magnetic anisotropy (D = −6.6 cm⁻¹, E = 0.02 cm⁻¹). Alternating current (ac) magnetic measurements indicate that both compounds exhibit field-induced slow magnetic relaxation behavior. Furthermore, ab initio calculations also demonstrate that compound 1 presents a strong easy-plane magnetic anisotropy and 2 presents an easy-axis magnetic anisotropy, which are further identified by the calculated orientations of the local magnetic axes. These results demonstrate an effective way to achieve the targeted synthesis of high-coordinate 3d SIMs.