Remote substituents effect on the valence and magnetic behavior in mononuclear cobalt complexes

Four Co(Ⅱ/Ⅲ) mononuclear complexes [Co^Ⅲ(H₂L1)](NO₃)·H₂O (1), [Co^Ⅲ(H₂L2)](NO₃) (2), [Co^II(H₂L3)] (3), [Co^II(H₂L4)] (4) (H₂L = N,N’-bis(2‑hydroxy-5-methylbenzyl)-1,4-bis(3-iminopropyl)-piperazine, H₂L2 = N,N’-bis[(2-hydroxybenzilideneamino)-propyl]-piperazine, H₂L3 = N,N’-bis(2‑hydroxy-5-bromobenzyl)-1,4-bis(3-iminopropyl)-piperazine, H₂L4 = N,N’-bis(2‑hydroxy-5-nitrobenzyl)-1,4-bis(3-iminopropyl)-piperazine) based on hexadentate Schiff base ligands were synthesized and charactered. Influenced by the electron-donating Me and H groups, the paramagnetic Co(II) ions in 1 and 2 become diamagnetic Co(Ⅲ) ions, forming a six-coordinate octahedral coordination geometry. For complexes 3 and 4, the Co(Ⅱ) ions remain the same because of the electron-withdrawing Br and NO₂ groups, forming a trigonal prismatic coordination geometry. Magnetic studies show that 3 is a field-induced SMM and 4 is a zero field SMM, which proves that the remote substituents have an important effect on the oxidation states of cobalt ion and the magnetic properties of the system. Ab initio calculations indicate that complexes 3 and 4 possess similar axial magnetic anisotropy parameters. However, complex 3 represents much stronger rhombicity, which can be ignored in 4. Moreover, the transition magnetic moment within the ground KD of 3 is much higher for 3 compared to 4, suggesting more significant QTM process in 3, which is consistent with no peaks in ac susceptibility measurement under zero dc magnetic field. For complex 4, when the intermolecular antiferromagnetic exchange was considered through the mean-field approximation, the better agreement between calculated and experimental plots were obtained. This result further proves their existing significant magnetic anisotropy and the reliability of these parameters.