A Linear Dysprosium(II) Metallocene with a High Effective Energy Barrier and Magnetic Hysteresis up to 70 Kelvin

Dysprosium in the oxidation state +3 is ubiquitous in studies of single-molecule magnets (SMMs). In contrast, SMMs based on lanthanides in the oxidation state +2 are rare, and examples with both a high effective energy barrier to reversal of the magnetization and hysteresis with coercivity at high temperatures are extremely uncommon. Here, we show that one-electron reduction of the dysprosium(III) complex [(η⁵-C₅^<i>i</i>Pr₅)Dy(η⁵-Cp*)(BH₄)] (Cp* = C₅Me₅) with KC₈ generates the linear dysprosium(II) metallocene [(η⁵-C₅^<i>i</i>Pr₅)Dy(η⁵-Cp*)]. Magnetic measurements, ultraviolet/visible/near infrared (UV/vis/NIR) spectroscopy and theoretical calculations show that the dysprosium(II) center in [(η⁵-C₅^<i>i</i>Pr₅)Dy(η⁵-Cp*)] adopts a 4f⁹ (6s/5d)¹ configuration. Coupling of the 4f and 5d electrons in [(η⁵-C₅^<i>i</i>Pr₅)Dy(η⁵-Cp*)] results in an effective magnetic moment of 11.38 μ_B at 217 K, equaling the highest magnetic moment recorded for a mononuclear complex. AC and DC magnetic measurements establish the SMM properties of [(η⁵-C₅^<i>i</i>Pr₅)Dy(η⁵-Cp*)], including an energy barrier of 1551 cm^-1, the largest yet reported for a divalent lanthanide SMM, and a 100-s blocking temperature of 62 K. Magnetic hysteresis measurements produce loops that remain open up to 70 K. Multireference calculations reveal that the eas<i>y</i>-axis of magnetization in the ground doublet of [(η⁵-C₅^<i>i</i>Pr₅)Dy(η⁵-Cp*)] coincides with the molecular symmetry axis, and that this doublet has extremely strong axial character. The strong axiality results in Orbach relaxation probably occurring via the second-excited doublet. The SMM parameters and the theoretical insight suggest that the high effective barrier and blocking temperature are linked to the strong and perfectly axial crystal field experienced by the dysprosium(II) center.