Million-fold Relaxation Time Enhancement across a Series of Phosphino-Supported Erbium Single-Molecule Magnets

Maintaining strong magnetic anisotropy in the presence of collective spin interactions has become a defining challenge in the advancement of single-molecule magnet (SMM) research. Herein we demonstrate effective decoupling of these often competing design goals in a series of new phosphino-supported SMMs containing the anisotropic [Er(COT)]⁺ (COT^2- = cyclooctatetraene dianion) subunit. Across this series, a magnetic nuclearity increase from 1 to 2 and subsequent optimization of the relative local anisotropy axis orientation results in dramatic improvements to the long time scale behavior. Specifically, we observe a 6 orders of magnitude increase in relaxation time at 2 K and a consequent open magnetic hysteresis up to 6 K. This drastic scaling of the magnetic dynamics tracks monotonically with the introduction and approach to parallel of the angle between intramolecular anisotropy axes. These results illustrate the powerful implications of fully controlling direction and magnitude of anisotropy in the design of scalable SMMs.