Selective arc-discharge synthesis of Dy2S-clusterfullerenes and their isomer-dependent single molecule magnetism

Abstract

A method for the selective synthesis of sulfide clusterfullerenes Dy₂S@C_2n is developed. Addition of methane to the reactive atmosphere reduces the formation of empty fullerenes in the arc-discharge synthesis, whereas the use of Dy₂S₃ as a source of metal and sulfur affords sulfide clusterfullerenes as the main fullerene products along with smaller amounts of carbide clusterfullerenes. Two isomers of Dy₂S@C₈₂ with C_s(6) and C_3v(8) cage symmetry, Dy₂S@C₇₂-C_s(10528), and a carbide clusterfullerene Dy₂C₂@C₈₂-C_s(6) were isolated. The molecular structure of both Dy₂S@C₈₂ isomers was elucidated by single-crystal X-ray diffraction. SQUID magnetometry demonstrates that all of these clusterfullerenes exhibit hysteresis of magnetization, with Dy₂S@C₈₂-C_3v(8) being the strongest single molecule magnet in the series. DC- and AC-susceptibility measurements were used to determine magnetization relaxation times in the temperature range from 1.6 K to 70 K. Unprecedented magnetization relaxation dynamics with three consequent Orbach processes and energy barriers of 10.5, 48, and 1232 K are determined for Dy₂S@C₈₂-C_3v(8). Dy₂S@C₈₂-C_s(6) exhibits faster relaxation of magnetization with two barriers of 15.2 and 523 K. Ab initio calculations were used to interpret experimental data and compare the Dy-sulfide clusterfullerenes to other Dy-clusterfullerenes. The smallest and largest barriers are ascribed to the exchange/dipolar barrier and relaxation via crystal-field states, respectively, whereas an intermediate energy barrier of 48 K in Dy₂S@C₈₂-C_3v(8) is assigned to the local phonon mode, corresponding to the librational motion of the Dy₂S cluster inside the carbon cage.

Fig. 1. HPLC traces of the extracts obtained in the arc-discharge synthesis with Dy₂S₃ and different amounts of methane in the reactor atmosphere. Block letters in the lower trace denote the main clusterfullerene fractions, corresponding to (A) Dy₂S@C₇₂, (B) Dy₃N@C₈₀, (C) Dy₂S@C₈₂-I/Dy₂C₂@C₈₂-I, and (D) Dy₂S@C₈₂-II. Note that a different intensity is used for each curve.

Fig. 2. (a–c) UV-vis-NIR absorption spectra of Dy-clusterfullerenes in toluene: (a) Dy₂C₂@C₈₂-C_s(6), (b) Dy₂S@C₈₂-C_s(6), and (c) Dy₂S@C₈₂-C_3v(8); insets show the LDI-TOF mass-spectra for each clusterfullerene (positive ion mode). (d–f) DFT-optimized molecular structures of (d) Y₂C₂@C₈₂-C_s(6), (e) Y₂S@C₈₂-C_s(6) and (f) Y₂S@C₈₂-C_3v(8); Y atoms are green, S atoms are yellow, and carbon atoms are light gray; C₈₂-C_s(6) and C₈₂-C_3v(8) cages are related via Stones–Wales transformations of the two C–C bonds highlighted in red.

Fig. 3. (a, b) Born–Oppenheimer molecular dynamics simulations of (a) Y₂S@C₈₂-C_s(6) and (b) Y₂S@C₈₂-C_3v(8) at the PBE/DZVP level, T = 300 K, propagation time 100 ps. Displacements of carbon atoms are not shown. In (a), the symmetry plane of the C₈₂-C_s(6) cage is perpendicular to the paper (in Fig. 2e, the plane is parallel to the paper). In (b), the molecule is viewed along the C₃ axis of the C₈₂-C_3v(8) cage (which lies in the plane of the paper in Fig. 2f).

Fig. 4. UV-vis-NIR absorption spectrum (left) and positive-ion LDI mass-spectrum (right) of Dy₂S@C₇₂. The molecular structure of Dy₂S@C₇₂-C_s(10528) is shown in the middle, and adjacent pentagon pairs are highlighted in black.

Fig. 5. (a) Relative orientation of the Ni^II(OEP) and Dy₂S@C₈₂ molecules in the Dy₂S@C₈₂-C_s(6)·Ni^II(OEP)·2C₇H₈ cocrystal; only one orientation of the C₈₂-C_s(6) cage together with the major site of the Dy₂S cluster are shown, solvent molecules are omitted for clarity; (b) major site of the Dy₂S cluster within the C_s(6)-C₈₂ cage. Selected geometry parameters: Dy1–S1, 2.465(5) Å; Dy2–S1, 2.518(5) Å; Dy1–S1–Dy2, 98.3(2)°. (c) Relative orientation of the Ni^II(OEP) and Dy₂S@C₈₂ molecules in the Dy₂S@C₈₂-C_3v(8)·Ni^II(OEP)·2C₇H₈ cocrystal; only one orientation of the C₈₂-C_3v(8) cage together with the major site of the Dy₂S cluster are shown, solvent molecules are omitted for clarity; (d) major site of the Dy₂S cluster within the C₈₂-C_3v(8) cage. Selected geometry parameters: Dy2–S1, 2.437(11) Å; Dy4–S1, 2.511(9) Å; Dy2–S1–Dy4, 94.4(2)°. Displacement parameters are shown at the 30% probability level.

Fig. 6. Magnetization curves for (a) Dy₂S@C₈₂-C_s(6), (b) Dy₂S@C₈₂-C_3v(8), (c) Dy₂C₂@C₈₂-C_s(6), and (d) Dy₂S@C₇₂-C_s(10528) measured at T = 1.8–5 K with the magnetic field sweep rate of 8.33 mT s^–1. The inset in each panel zooms into the region near zero-field. In (b), determination of the blocking temperature of Dy₂S@C₈₂-C_3v(8) as the peak in the susceptibility of the zero-field-cooled (ZFC) sample is also shown (magnetic field 0.2 T, temperature sweep rate 5 K min^–1).

Fig. 7. χ″ of Dy₂S@C₈₂-C_s measured at different temperatures as a function of AC frequency. Dots are experimental points, lines are results of the fit with a generalized Debye model.

Fig. 8. Magnetization relaxation times of (a) Dy₂C₂@C₈₂-C_s and Dy₂S@C₈₂-C_s and (b) Dy₂S@C₈₂-C_3v. Dots are experimental points, red lines are results of a global fit with three Orbach processes; green, magenta, and brown lines represent contributions of individual Orbach processes. For Dy₂C₂@C₈₂-C_s with a limited number of data points, a single Orbach process was considered (blue line). Insets show enhancement of the high-temperature range for Dy₂S@C₈₂-C_s and Dy₂S@C₈₂-C_3v. Fitting of the magnetization relaxation of Dy₂S@C₈₂-C_s with two Orbach processes and one Raman process is shown in the ESI.

Fig. 9. (a) Endohedral clusters in selected clusterfullerenes and magnetic anisotropy axes (shown as red lines) for each Dy center according to ab initio calculations. Color code: Dy – green, Sc – magenta, Ti – cyan, S – yellow, C – gray, N – blue, and O – red; carbon cages are omitted for clarity. The compounds not studied experimentally are marked in gray. (b) Computed energies of CF states in different clusterfullerenes (when the molecule has two Dy ions, the energies for each center are given in blue and gray).

Fig. 10. (a) Low-energy part of the Raman spectra of Dy₂S@C₈₂-C_3v (top) and Dy₂S@C₈₂-C_s (bottom) compared to DFT-computed vibrational frequencies of individual molecules (black lines). (b) Atomic displacements of the two vibrations of the Dy₂S cluster in Dy₂S@C₈₂-C_3v.