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

Based on a phosphine oxide ligand, HMPA (hexamethylphosphoric triamide), two mononuclear Ho-pentagonal bipyramidal complexes were synthesized with the formulas [Ho(HMPA)(HO)]Cl·2HMPA·2HO (1) and [Ho(HMPA)(HO)]Br·2HMPA (2). Single-crystal X-ray diffraction results show that all Ho 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)(HO)] 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.

A new air- and moisture-stable pentagonal-bipyramidal Dy single-ion magnet based on the HMPA ligand.

Based on the HMPA ligand, a new air- and moisture-stable pentagonal-bipyramidal DyIII single-ion magnet [Dy(HMPA)2(H2O)5]2·Br6·2HMPA·2H2O (1) was prepared and characterized. Single-crystal X-ray tests showed the two crystallographically independent DyIII ions located in a pentagonal-bipyramidal coordination sphere with strong axial HMPA ligands and weak equatorial water molecules relating to strong unaxial anisotropy. Direct-current and alternating-current magnetic susceptibilities were measured and showed that 1 exhibited slow relaxation of magnetization up to 36 K (1000 Hz) with an energy barrier of 556 K and blocking temperature of 7 K (defined by the peak temperature of zero-field cooling data).