Syntheses,Crystal Structures And Fine-Tuned Properties Of Octahedral Cobalt(?) Single-Ion Magnets With Substituted Benzoic Acid/?-Diketone

This paper aims at exploring the ligand-field effect on the magnetic anisotropy and effective energy barrier of octahedral cobalt(?)single-ion magnets,which provide theoretical basis of the magneto-structural correlations of single-ion magnets.Herein,based on cobalt(?)as spin center,the hexa-coordinated Co(II)cores in complexes display octahedral geometries were designed and constructed.By fine-tuning the coordination microenvironment of metal ions results in a nonnegligible manipulation of their magnetic anisotropy and dynamic magnetic behavior of Co(?)SIMs,the magneto-structural correlations and magnetic relaxation mechanism were discussed by combined with theoretical calculation.Firstly,two mononuclear cobalt(?)complexes,with the formulas[Co(2,6-dfba)2(bpp)2(H2O)2]n(1)and[Co(2,6-dfba)2(bpe)2(H2O)2]n(2)(2,6-Hdfba=2,6-difluorobenzoic acid,bpp=1,3-bis(4pyridyl)propane,bpe-1,2-bis(4-pyridyl)ethylene),have been synthesized by combining Co(II)ions with benzoate derivatives and two homogeneous N-donor ligands,respectively.Constrained by the analogous CoN2O4 coordination spheres,the discretely hexa-coordinated Co(II)cores in both complexes display stretched octahedral geometries.Ideally,the variables of ligand modification should be as few as possible in order to establish a conclusive trend of the magnetic anisotropy upon the change of these variables.The equatorial environments in both complexes are identical,whereas the axial sites are finely modulated by the different chemical natures of the terminal N-donor ligands.The combined analyses of the magnetic data,the high-frequency electron paramagnetic resonance(HF-EPR)and the ab initio calculations unveil large easy-plane magnetic anisotropies for both complexes(D=+53.19 and+65.67 cm-1 for 1 and 2,respectively),which function as field-induced single-ion magnets(SIMs)with effective barriers(Ueff)of 45.34(1)and 57.97 K(2).Secondly,in methanol solvent,a tetranuclear cubane-type complex[Co4(ntfa)4(CH3O)4(CH3OH)4](3)that features a {Co4O4} core and a mononuclear complex[Co(ntfa)2(CH3OH)2](4)have been prepared by adjusting the ratio of the ?-diketonate and Co(?)ions.When two coordinated CH3OH molecules in 4 are replaced by 2,2'-bipyridyl(bpy),5,5'-dimethyl-2,2'-bipyridyl(5,5-(CH3)2-bpy)and 6,6'-dimethyl-2,2'-bipyridyl(6,6-(CH3)2-bpy)ligands,forming the configurations of 5-7.[Co(ntfa)2(bpy)2](5),[Co(ntfa)2(5,5-(CH3)2-bpy)2](6)and[Co(ntfa)2(6,6-(CH3)2-bpy)2](7).X-ray crystallography showed that complexes 4-7 are mononuclear with distorted octahedral geometries around the Co? ions.HF-EPR measurements and theoretical calculations demonstrate that 4 has easy-axis magnetic anisotropies(D=-60.48 cm-1),whereas the easy-plane magnetic anisotropies(+72.85 cm-1,+35.71 cm-1,+51.28 cm-1)were confirmed in complexes 5-7.Magnetic investigations evidenced single-ion magnet behavior in all complexes with energy barriers Ueff of 30.28 K(3),29.29 K(4),29.74 K(5),13.03 K(6)and 25.81 K(7)under 2000 Oe.