Enhancing the energy barrier by replacing the counterions in two holmium(III)-pentagonal bipyramidal single-ion magnets

摘要

Based on a phosphine oxide ligand, HMPA (hexamethylphosphoric triamide), two mononuclear Ho-III-pentagonal bipyramidal complexes were synthesized with the formulas [Ho(HMPA)(2)(H2O)(5)](2)Cl-6 center dot 2HMPA center dot 2H(2)O (1) and [Ho(HMPA)(2)(H2O)(5)]Br-3 center dot 2HMPA (2). Single-crystal X-ray diffraction results show that all Ho-III ions in both the two complexes are hepta-coordinated and are located in pentagonal bipyramidal {HoO7} coordination polyhedrons constructed by two axial HMPA ligands and five equatorial water molecules. However, due to the employment of different halide ions as counterions, the second coordination sphere surrounding each [Ho(HMPA)(2)(H2O)(5)](3+) moiety is different in the two complexes: in 1, three Cl- ions, one water molecule and one HMPA ligand; in 2, three Br- ions and two HMPA ligands. Ac magnetic susceptibilities under zero dc field show that both the two complexes are single-ion magnets with effective energy barriers of 290 K and 320 K for 1 and 2, respectively. Compared with 1, the enhancement in the energy barrier of 2 is believed to be induced mainly by the change in the second coordination sphere rather than the minor differences in the {HoO7} polyhedrons.