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[R-1(C6N2H5)R-2](2), where R-1 = H, R-2 = 4-tert-butylphenylsulfonyl ((t)Buphs) 1, R-2 = 5-(dimethylamino)naphthalen-1-ylsulfonyl (DNps) 2, R-2 = mesitylsulfonyl (Ms) 3, R-2 = tosyl (Tos) 4, and R-2 = naphthalen-1-ylsulfonyl (Nps) 5; R-1 = Me, R-2 = (t)Buphs 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.