In this work, a large excess of electrostatic repulsion, arising from the axial ligands, over that from the equatorial ligands is taken as the design strategy for high performance pentagonal bipyramidal (PBP) DyIII single-ion magnets (SIMs).

It is crucial to promote axiality to enhance easy-axis magnetic anisotropy. Two mononuclear Dy compounds, in which each Dy is nine-coordinated, namely, [(CHNO)Dy(NO)(HO)]·CHOH (1) and [(CHNO)Dy(NO)(CHOH)]·HO (2) (HL = N-(2-pyridoyl)-4-pyridinecarboxamidrazone), have been prepared through controlling the amount of CHOH and HO solvents. Geometry modulations were realized by interchanging coordinated and lattice solvents, thus achieving a structure closer to the configuration of a capped square antiprism for 2 compared to that for 1. Notably, magnetic studies revealed that compound 1 displays no slow relaxation of magnetization while compound 2 exhibits single-molecule magnet (SMM) behaviour in the absence of a static magnetic field, with the highest energy barrier (203.11 K) of nine-coordinated Dy SMMs. Ab initio calculations were performed to elucidate such a distinct performance, demonstrating that the combination of the larger charge distribution along the magnetic axis and the lower charge distribution in the equatorial plane generates a strong easy-axis ligand field to enhance magnetic properties, which is further associated with the structural symmetry. In addition, a possible coordination mode is proposed to realize high axis anisotropy for nine-coordinated Dy compounds. This work presents an effective method to modulate the dynamic magnetic relaxation of the Dy SMMs through interchange between coordinated and lattice solvents.