Correlating magnetic anisotropy with the subtle coordination geometry variation of a series of cobalt(II)-sulfonamide complexes

Systematic substitution on the N-(pyridine-2-ylmethyl)-sulfonamide ligand leads to the subtle variation of the CoN₄ coordination geometry in a series of cobalt(II) complexes sharing the common formula of Co[R₁(C₆N₂H₅)R₂]₂, where R₁ = H, R₂ = 4-tert-butylphenylsulfonyl (^tBuphs) 1, R₂ = 5-(dimethylamino)naphthalen-1-ylsulfonyl (DNps) 2, R₂ = mesitylsulfonyl (Ms) 3, R₂ = tosyl (Tos) 4, and R₂ = naphthalen-1-ylsulfonyl (Nps) 5; R₁ = Me, R₂ = ^tBuphs 6. Magnetic studies show that the axial zero-field splitting parameter (D) is subtlely correlated with the coordination geometric variation subjected to the peripheral substituted groups. Specifically, the distortion from the ideal tetrahedral geometry (T_d symmetry) to the seesaw geometry (D_2d symmetry) increases uniaxial magnetic anisotropy. The degree of distortion measured by the continuous symmetry measure (CSM) shows that a narrow interval of CSM (6–7), which corresponds to 14–15 degree deviation from the standard tetrahedron, is ideal for maximising the D value in this coordination geometry, while the direction of the D tensor is less sensitive to such a structural variation.