Effects of strong coordination bonds at the axial or equatorial positions on magnetic relaxation for pentagonal bipyramidal dysprosium(III) single-ion magnets

Three pentagonal bipyramidal mononuclear Dy(III) complexes based on amino-substituted nitrophenol and tetradentate amide ligands of formulas [Dy(Hbpen)(OPhNO2NH2Cl)Cl-2] (1), [Dy(Hbpen)(OPhNO2NH2)Cl-2] (2) and [Dy(Hbpen)(OPhNO2NH2Cl)(3)] (3) (Hbpen = N,N'-bis(2-pyridylmethyl)-ethylenediamine, OPhNO2NH2Cl = 2-amino-6-chloro-4-nitrophenol, and OPhNO2NH2 = 2-amino-4-nitrophenol) were isolated. X-ray diffraction studies illustrate that complexes 1 and 2 with one strongly coordinating phenol ligand at their equatorial positions have a similar structure except for a slight difference in the chloride substituent of the phenol ligand. Complex 3 possesses the same equatorial coordination as 1 but its apical positions are occupied by two other phenol ligands. Magnetic studies show that 1 and 2 are zero-field single-ion magnets (SIMs), and 3 exhibits field-induced SIM behavior. Upon removing the chloride substituent groups from the phenol ligand, the effective energy barrier enhances from 233.7 K (1) to 362.7 K (2) under external dc fields. The stronger quantum tunneling of magnetization observed for 3 in comparison with 1 shows the destructive influence of a strong phenoxyl oxygen ligand field contributing to the transverse component on the magnetic properties. A comparison of complex 2 and the reported Dy(III) analogue [Dy(Hbpen)Cl(OPhBr2NO2)(2)] with two phenol ligands (2,6-dibromo-4-nitrophenol) in the axial direction leads to the conclusion that the magnetic anisotropy is strongly dependent on the Dy-O-phenoxyl bond lengths. The results provide direct information vital to understanding how the strong coordination environment at the axial or equatorial positions influences the SIM behavior.

Automated summary

Three pentagonal bipyramidal mononuclear Dy(III) complexes based on amino-substituted nitrophenol and tetradentate amide ligands were synthesized and studied for their structural and magnetic properties. The substitution of the phenol ligand significantly affects the magnetic properties, and the Dy-O-phenoxyl bond lengths greatly influence the magnetic anisotropy.