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.