Magnetic Hysteresis at 31 K in a Four-Coordinate Dysprosium(III) Amide Complex

The state of the art in single-molecule magnet (SMM) design is dominated by charged dysprosium sandwich complexes with mono- or dianionic π-ligands, which exhibit some of the highest open-loop magnetic hysteresis temperatures (<i>T</i>_H), the highest 100 s blocking temperatures (<i>T</i>_B100), and some of the largest barriers to magnetic reversal (<i>U</i>_eff). Architectures that leverage more charge-dense ligands, such as amides or alkoxides, with axial coordination and weak equatorial interactions can generate larger crystal fields (CFs), and hence larger <i>U</i>_eff values; however, this is often not accompanied by correspondingly high <i>T</i>_H or <i>T</i>_B100 values. Here, we report a four-coordinate dysprosium(III) SMM, [Dy{Me₂Si(NSi<i>i</i>Pr₃)₂}₂{K(toluene)₂}]_<i>n</i> (<b>1Dy</b>), with charge-dense amide donors but a coordination environment that is between axial and tetrahedral in structure. Nevertheless, <b>1Dy</b> displays open-loop magnetic hysteresis up to 31 K using sweep rates of 22 Oe s^-1. The coercive field (<i>H</i>_C) is 1.65 T at 1.8 K, and <i>T</i>_B100 = 10.4 K. <i>Ab initio</i> calculations show the four-coordinate geometry imparts a strong axial CF as the ⁶H_15/2 spin-orbit multiplet is split over 1958 K (1361 cm^-1). The experimental <i>U</i>_eff (742 K, 516 cm^-1) is smaller and resides near the energy of the third excited Kramers doublet. <i>Ab initio</i> spin dynamics calculations show that the fitted barrier height at high <i>T</i> (>45 K) is 559 cm^-1. Non-Kramers ion analogues <b>1Tb</b> and <b>1Ho</b> show waist-restricted hysteresis and short relaxation times at all temperatures. The magnetic properties of <b>1Dy</b> place it highly among monometallic lanthanide SMMs using an alternative design strategy, which is synthetically simple and may be further refined.