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 (T_H), the highest 100 s blocking temperatures (T_B100), and some of the largest barriers to magnetic reversal (U_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 U_eff values; however, this is often not accompanied by correspondingly high T_H or T_B100 values. Here, we report a four-coordinate dysprosium(III) SMM, [Dy{Me₂Si(NSiiPr₃)₂}₂{K(toluene)₂}]_n (1Dy), with charge-dense amide donors but a coordination environment that is between axial and tetrahedral in structure. Nevertheless, 1Dy displays open-loop magnetic hysteresis up to 31 K using sweep rates of 22 Oe s^-1. The coercive field (H_C) is 1.65 T at 1.8 K, and T_B100 = 10.4 K. Ab initio 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 U_eff (742 K, 516 cm^-1) is smaller and resides near the energy of the third excited Kramers doublet. Ab initio spin dynamics calculations show that the fitted barrier height at high T (>45 K) is 559 cm^-1. Non-Kramers ion analogues 1Tb and 1Ho show waist-restricted hysteresis and short relaxation times at all temperatures. The magnetic properties of 1Dy place it highly among monometallic lanthanide SMMs using an alternative design strategy, which is synthetically simple and may be further refined.