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 <i>D</i>_6<i>h</i> 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 <b>Dy-NH</b>), -O- (for <b>Dy-O</b>), and -NMe- (for <b>Dy-NMe</b>). Complex <b>Dy-NH</b> shows a less distorted <i>D</i>_6<i>h</i> symmetry and an anisotropy energy barrier of 1270 K. For complex <b>Dy-O</b>, the coordination site of -O- gives a relatively longer coordination bond but a comparable energy barrier in contrast with that of <b>Dy-NH</b>. In the case of complex <b>Dy-NMe</b>, although the -NMe-group gives a very long coordination bond, the large steric effect on the -NMe- group enforces a larger distortion of the <i>D</i>_6<i>h</i> coordination geometry, resulting in the fast quantum tunneling of the magnetization that shortcuts the thermal relaxation process; therefore, <b>Dy-NMe</b> shows a lower energy barrier. This study provides a new strategy for modifying the coordinate site on the equatorial plane of <i>D</i>_6<i>h</i> symmetry to fine-tune the structure and magnetic anisotropy of SMMs.