Fine-Tuning the Anisotropies of Air-Stable Single-Molecule Magnets Based on Macrocycle Ligands

Air-stable single-molecule magnets (SMMs) can be obtained by confining Dy^III ion in a D₆ coordination environment; however, most of the current efforts were focused on modifying the rigidity of the macrocycle ligand. Herein, we attempt to assemble air-stable SMMs based on macrocycles with a replaceable coordination site. By using an in situ 1 + 1 Schiff-base reaction of dialdehyde with diamine, three air-stable SMMs have been obtained in which one of the equatorial coordination sites can be varied from -NH- (for Dy-NH), -O- (for Dy-O), and -NMe- (for Dy-NMe). Complex Dy-NH shows a less distorted D₆ symmetry and an anisotropy energy barrier of 1270 K. For complex Dy-O, the coordination site of -O- gives a relatively longer coordination bond but a comparable energy barrier in contrast with that of Dy-NH. In the case of complex Dy-NMe, although the -NMe-group gives a very long coordination bond, the large steric effect on the -NMe- group enforces a larger distortion of the D₆ coordination geometry, resulting in the fast quantum tunneling of the magnetization that shortcuts the thermal relaxation process; therefore, Dy-NMe shows a lower energy barrier. This study provides a new strategy for modifying the coordinate site on the equatorial plane of D₆ symmetry to fine-tune the structure and magnetic anisotropy of SMMs.