High blocking temperatures for DyScS endohedral fullerene single-molecule magnets

Abstract

Dy-based single-molecule magnets (SMMs) are of great interest due to their ability to exhibit very large thermal barriers to relaxation and therefore high blocking temperatures. One interesting line of investigation is Dy-encapsulating endohedral clusterfullerenes, in which a carbon cage protects magnetic Dy³⁺ ions against decoherence by environmental noise and allows for the stabilization of bonding and magnetic interactions that would be difficult to achieve in other molecular architectures. Recent studies of such materials have focused on clusters with two Dy atoms, since ferromagnetic exchange between Dy atoms is known to reduce the rate of magnetic relaxation via quantum tunneling. Here, two new dysprosium-containing mixed-metallic sulfide clusterfullerenes, DyScS@C _s(6)-C₈₂ and DyScS@C _3v(8)-C₈₂, have been successfully synthesized, isolated and characterized by mass spectrometry, Vis-NIR, cyclic voltammetry, single crystal X-ray diffractometry, and magnetic measurements. Crystallographic analyses show that the conformation of the encapsulated cluster inside the fullerene cages is notably different than in the Dy₂X@C _s(6)-C₈₂ and Dy₂X@C _3v(8)-C₈₂ (X = S, O) analogues. Remarkably, both isomers of DyScS@C₈₂ show open magnetic hysteresis and slow magnetic relaxation, even at zero field. Their magnetic blocking temperatures are around 7.3 K, which are among the highest values reported for clusterfullerene SMMs. The SMM properties of DyScS@C₈₂ far outperform those of the dilanthanide analogues Dy₂S@C₈₂, in contrast to the trend observed for carbide and nitride Dy clusterfullerenes.

Fig. 1. (a) HPLC chromatograms of purified DyScS@C₈₂ (I, II) on a Buckyprep column with toluene as the eluent at a flow rate of 4 mL min⁻¹; (b) the positive mode MALDI-TOF mass spectra and expansions of the experimental and theoretical isotopic distributions of DyScS@C₈₂ (I, II).

Fig. 2. ORTEP drawing of (a) DyScS@C_s(6)–C₈₂·Ni^II(OEP) and (b) DyScS@C_3v(8)–C₈₂·Ni^II(OEP) with 10% thermal ellipsoids, respectively. Only the predominant DyScS clusters are shown, whereas minor sites and solvent molecules are omitted for clarity.

Fig. 3. Perspective drawings show (a) the predominant sites of the DyScS cluster within the C_s(6)–C₈₂ cage; (b) the predominant sites of the DyScS cluster within the C_3v(8)–C₈₂ cage; (c) relative positions of two predominant sites of the DyScS cluster in DyScS@C_s(6)–C₈₂; (d) relative positions of two predominant sites of the DyScS cluster in DyScS@C_3v(8)–C₈₂. The DyScS unit is modeled with the major site shown in orange and the second major site shown in blue. The metal atoms labeled with ‘i’ are generated by the crystallographic operation.

Fig. 4. Histogram of Dy–S bond lengths reported in the Cambridge Structural Database (CSD). The orange rectangle shows the range of Dy–S bond lengths observed in the two isomers of DyScS@C₈₂.

Fig. 5. Both isomers of DyScS@C₈₂ show magnetic hysteresis and irreversibility at low temperatures, consistent with single-molecule magnet behavior. The main panel shows magnetic hysteresis loops taken at 2 K with a slow field sweep rate of 2.5 mT s⁻¹. The inset shows magnetization vs. temperature under zero-field cooled (ZFC) and field-cooled (FC) conditions, in each case taken upon warming at a rate of 5 K min⁻¹ under an applied field of 0.3 T.

Fig. 6. Temperature-dependence of the magnetic hysteresis for (a) DyScS@C_s(6)–C₈₂ and (b) DyScS@C_3v(8)–C₈₂, collected with a field sweep rate of 10 mT s⁻¹. (c) The coercive field (H_C) for each isomer as a function of temperature.

Fig. 7. Characterization of magnetic relaxation times of DyScS@C_s(6)–C₈₂ (left panels) and DyScS@C_3v(8)–C₈₂ (right panels) via DC magnetometry. (a) and (b) show representative magnetic relaxation experiments, where the magnetization M is monitored as a function of time t after the application and subsequent ramp down of a 5 T magnetic field to a target field of either 0 T or 0.3 T. The colored lines indicate fits to stretched exponential functions, which are used to extract the relaxation times. (c) and (d) show the relaxation times extracted using these curves at temperatures ranging from 8 K to 1.8 K, and for applied fields of 0 T and 0.3 T.