Slow magnetic relaxation in mononuclear octa-coordinate Fe(II) and Co(II) complexes from a Bpybox ligand.

Two 3d transition metal mononuclear complexes, [(FeL)(ClO)]·CHCN (1) and (CoL)(ClO)·2CHCN (2), have been prepared from a rigid tetradentate bpybox (L = 6,6'-bis(2,5-dihydrooxazol-4-yl)-2,2'-bipyridine) ligand. Single crystal X-ray diffraction analyses together with the help of calculations show that both compounds are octa-coordinate. Direct current magnetic studies reveal their significant magnetic anisotropy. Impressively, field-induced relaxation of magnetism is observed in the two complexes and the apparent anisotropy barriers are 14.1 K for 1 and 21.6 K for 2, respectively. Theoretical calculations reveal that two Fe(II) centers in 1 have small negative values of -4.897 and -4.825 cm and relatively small values of 0.646 and 0.830 cm, indicating a uniaxial magnetic anisotropy. In contrast, the and values in the Co(II) center of 2 are 46.42 cm and 11.51 cm, featuring a rhombic anisotropy. This work demonstrates that field-induced slow magnetic relaxation in 3d transition metal complexes with high coordination numbers can be manipulated through rigid ligand design.