Shape-adaptive single-molecule magnetism and hysteresis up to 14 K in oxide clusterfullerenes Dy₂O@C₇₂ and Dy₂O@C₇₄ with fused pentagon pairs and flexible Dy–(μ₂-O)–Dy angle

Abstract

Dysprosium oxide clusterfullerenes Dy₂O@C_s(10528)–C₇₂ and Dy₂O@C₂(13333)–C₇₄ are synthesized and characterized by single-crystal X-ray diffraction. Carbon cages of both molecules feature two adjacent pentagon pairs. These pentalene units determine positions of endohedral Dy ions hence the shape of the Dy₂O cluster, which is bent in Dy₂O@C₇₂ but linear in Dy₂O@C₇₄. Both compounds show slow relaxation of magnetization and magnetic hysteresis. Nearly complete cancelation of ferromagnetic dipolar and antiferromagnetic exchange Dy⋯Dy interactions leads to unusual magnetic properties. Dy₂O@C₇₄ exhibits zero-field quantum tunneling of magnetization and magnetic hysteresis up to 14 K, the highest temperature among Dy-clusterfullerenes.

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Introduction

Endohedral metallofullerenes (EMFs) comprising metal and non-metal atoms in their endohedral species are known as clusterfullerenes.¹ Enhanced electrostatic interactions between endohedral lanthanides (Ln) and non-metal ions (X^q−), such as N³⁻, S²⁻, or O²⁻, reinforced by exceptionally short Ln–X bonds result in a strong magnetic anisotropy of Ln ions and lead to the single-molecule magnetism in many Ln-clusterfullerenes.² Especially Dy-clusterfullerenes are known as robust single molecule magnets (SMMs) with many favourable properties, including thermal and chemical stability, reasonably high blocking temperatures of magnetization, and magnetic hysteresis in bulk samples as well as in monolayers on different substrates.³ Thus, with the improvement of EMF-SMMs, the tremendous progress achieved over the last two decades in the Ln-SMM field⁴ has a good chance to further advance to the state of device-oriented functional molecular materials.

Oxide clusterfullerenes⁵ with endohedral Ln₂O clusters were predicted to have the largest ligand-field splitting in the whole EMF family^3g,6 and thus are viable synthetic targets as prospective SMM candidates. Indeed, our recent study of three isomers of Dy₂O@C₈₂ revealed their unique magnetic properties.⁷ The pronounced magnetic anisotropy is combined in these molecules with antiferromagnetic (AFM) exchange interactions, which are the strongest through all dinuclear Dy complexes with non-radical bridging ligands. In this Communication, we report that the encapsulation of the Dy₂O cluster in smaller carbon cages has a profound influence on the magnetic properties and leads to the Dy₂O@C₇₄ compound with the highest temperature of magnetic hysteresis among clusterfullerene-SMMs.

Results and discussion

Synthesis, molecular and electronic structure

New Dy-oxide clusterfullerenes Dy₂O@C₇₂ and Dy₂O@C₇₄ were synthesized by arc-discharge evaporation of Dy₂O₃-filled graphite rods in He/CO₂ atmosphere (200/20 Torr) and isolated by multi-step high-performance liquid chromatography (HPLC) as described in ESI.† Basic characterization of the purified Dy₂O@C₇₂ and Dy₂O@C₇₄, including their HPLC profiles, high-resolution laser desorption ionization mass spectra with characteristic isotopic patterns, and UV-vis-NIR absorption spectra are surveyed in Fig. 1.

Fig. 1 Characterization of isolated Dy₂O@C₇₂ and Dy₂O@C₇₄: (a) HPLC profiles (BuckyPrep column, toluene as eluent, flow rate 4 mL min⁻¹; solvent peak appears at 6 min); (b) LDI mass-spectra in a positive-ion mode; (c) UV-Vis-NIR absorption spectra in toluene (the inset shows magnification of the low-energy range).

The molecular structures of Dy₂O@C₇₂ and Dy₂O@C₇₄ were elucidated by single-crystal X-ray diffraction of co-crystals with Ni^II(OEP) revealing C_s(10528)–C₇₂ and C₂(13333)–C₇₄ carbon cages (Fig. 2a and b).§ Both fullerenes violate the Isolated Pentagon Rule (IPR) and have two pairs of adjacent pentagons, which are stabilized by coordination with Dy ions (Fig. 2c,d). The C_s(10528)–C₇₂ cage was found before in Sc₂S@C₇₂,⁸ Sc₂C₂@C₇₂,⁹ and presumably Dy₂S@C₇₂.^3g The C₂(13333)–C₇₄ cage was predicted theoretically as a plausible isomer of Sc₂C₂@C₇₄ ¹⁰ and was found recently in Ho₂O@C₇₄.^5c

Fig. 2 (a and b) Thermal ellipsoids of Dy₂O@C_2n·Ni^II(OEP) crystals with 30% probability showing only the major dysprosium sites: (a) Dy₂O@C_s(10528)–C₇₂, (b) Dy₂O@C₂(13333)–C₇₄; the solvent molecules and minor Dy sites are omitted, Dy is green, O is red, Ni is purple, N is blue, C is grey, adjacent-pentagon pairs are highlighted in orange. (c and d) Interaction of the major Dy₂O site with the closest cage motif in (c) Dy₂O@C₇₂, and (d) Dy₂O@C₇₄. (e and f) Endohedral Dy₂O units with disordered Dy sites and selected structural parameters in (e) Dy₂O@C₇₂, and (f) Dy₂O@C₇₄.

Carbon cages and oxygen atoms are fully ordered in the crystals, but Dy atoms are less ordered with 2–3 sites for each metal atom (Fig. 2e and f). In Dy₂O@C₇₂ the Dy–O–Dy angle is 138.2(4)° for the major Dy sites (occupancy 0.60 and 0.75), whereas in Dy₂O@C₇₄ the Dy₂O cluster is close to the linear shape, similar to the Ho₂O cluster in Ho₂O@C₂(13333)–C₇₄.^5c Dy–O bonds are very short, 1.980(5)–2.059(5) Å, but the disorder in Dy positions does not allow detailed analysis. Optimization of the molecular structures performed at the PBE-D level (PAW 4f-in-core potentials, VASP 5.0 code¹¹) resulted in the Dy–O distances and Dy–O–Dy angles of 2.025 Å/138° in Dy₂O@C₇₂ and 2.038 Å/180° in Dy₂O@C₇₄.

Apparently, the variation of the Dy₂O cluster shape from bent to linear is dictated by the Dy⋯Dy distance, which changes from 3.743(1) Å in Dy₂O@C₇₂ to 4.030(2) Å in Dy₂O@C₇₄ (DFT values are 3.784 and 4.076 Å, respectively). In due turn, the location of Dy atoms inside the fullerene is determined by the arrangement of pentalene units. The distance between centroids of pentalenes in X-ray structures is 8.151 Å in Dy₂O@C₇₂ and 8.597 Å in Dy₂O@C₇₄. Thus, the shape of the Dy₂O cluster is imposed by the fullerene cage form-factor.^1c Note that in Dy₂O@C₈₂ isomers the cluster is bent despite the larger cage size than in Dy₂O@C₇₄: in fact, Dy⋯Dy distances in all Dy₂O@C₈₂ structures are shorter than in Dy₂O@C₇₄.⁷ At the same time, the linear shape of the cluster was found in Ho₂O@D_2d(23)–C₈₄, which also has an elongated shape of the fullerene cage.^5d

DFT-based molecular dynamics simulations showed that at 300 K on a timescale of 100 ps metal atoms oscillate near their optimized positions (Fig. S4†). IR spectra calculated from the Fourier transform of the time-dependent dipole moment resemble those computed for the static model and agree well with the experimental spectra (Fig. S5†). Antisymmetric Dy–O stretching vibration is found at 680 cm⁻¹ in Dy₂O@C₇₂ and 700 cm⁻¹ in Dy₂O@C₇₄.

Electronic absorption spectra of both compounds extend to the near-IR region with the lowest-energy bands at 1137 nm (1.09 eV) in Dy₂O@C₇₂ and 1240 nm (1.00 eV) in Dy₂O@C₇₄ (Fig. 1c). The spectrum of Dy₂O@C₇₂ exhibits a close similarity to the spectra of Sc₂S@C₇₂,⁸ Sc₂C₂@C₇₂,⁹ and Dy₂S@C₇₂^3g featuring the same C_s(10528)–C₇₂ cage. However, the cluster composition has a noticeable impact on the lowest energy excitation, found at 1076 nm (Sc₂S), 1082 nm (Sc₂C₂), or 1115 nm (Dy₂S). The absorption spectrum of Dy₂O@C₇₄ is virtually identical to that of Ho₂O@C₂(13333)–C₇₄.^5c

Further insight into the electronic structure of Dy₂O clusterfullerenes is obtained from the electrochemical studies. Cyclic voltammograms are shown in Fig. S6† and redox potentials are listed in Table 1. Both Dy₂O@C₇₂ and Dy₂O@C₇₄ exhibit one reversible oxidation and three reversible reduction steps as well as poorly reversible fourth reduction and second oxidation steps. DFT calculations show that frontier molecular orbitals of both molecules are localized on their fullerene cages (Fig. S7†), which suggests that the redox processes do not affect the Dy₂O cluster. Comparison of the redox potentials of Dy₂O@C₇₂ to Sc₂S@C₇₂ ⁸ and Sc₂C₂@C₇₂ ⁹ (Table S2†) reveals that the variation of the optical gap discussed above is also reflected in their electrochemical gaps despite the predominant localization of the frontier MOs on the carbon cage in all molecules.

Table 1 Redox potentials^a of Dy₂O@C_s(10528)–C₇₂ and Dy₂O@C₂(13333)–C₇₄

EMF	O–II	O–I	R–I	R–II	R–III	R–IV	GapEC
a Measured in TBAPF6/o-dichlorobenzene and referred versus Fe(Cp)2^+/0.
Dy2O@C72	0.87	0.33	−1.09	−1.56	−2.18	−2.55	1.42
Dy2O@C74	1.18	0.52	−0.81	−1.17	−2.16	−2.56	1.33

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Magnetic properties

SQUID magnetometry. Magnetic properties of Dy₂O@C₇₂ and Dy₂O@C₇₄ powders were studied by SQUID magnetometry. Fig. 3a and b show low-temperature magnetization curves measured for two clusterfullerenes with a sweep rate of 2.9 mT s⁻¹. Dy₂O@C₇₂ shows open hysteresis up to 7 K (Fig. S8†). In the isostructural Dy₂S@C₇₂, the hysteresis is much narrower and is closing already at 3 K.^3g Thus, once again Dy₂O cluster is found to exhibit more robust single molecule magnetism than the Dy₂S analog with the same fullerene cage. Earlier, superior SMM properties were found for Dy₂O@C₈₂ when compared to the isostructural Dy₂S@C₈₂.⁷ At the same time, the use of softer sulfur co-ligands was found to be beneficial to increase the axiality of other types of single-molecule magnets.¹²

Fig. 3 Magnetic hysteresis curves of Dy₂O@C₇₂ (a) and Dy₂O@C₇₄ (b) measured with the sweep rate 2.9 mT s⁻¹. Insets show comparison of χ_FC and χ_ZFC curves (μ₀H = 0.2 T, temperature sweep rate 5 K min⁻¹); dashed blue lines denote T_irrev as the temperature, at which χ_FC and χ_ZFC curves bifurcate.

Magnetic hysteresis in Dy₂O@C₇₄ closes above 14 K (Fig. S8†). The shape of the hysteresis with the abrupt decay of magnetization near 0 T is different from that of Dy₂O@C₇₂ and other dinuclear Dy-clusterfullerenes. Such a “waist-restricted” or “butterfly” hysteresis is characteristic for the quantum tunnelling of magnetization (QTM) in zero field and is common for single-ion magnets, but is not typical for dinuclear SMMs.

Blocking of magnetization is analysed by comparing magnetic susceptibility measured after cooling the sample in zero field (χ_ZFC) and during cooling the sample in field (χ_FC).¹³ Blocking temperature T_B defined as the peak temperature of χ_ZFC is near 4 K in Dy₂O@C₇₂ and 6.7 K in Dy₂O@C₇₄ (Fig. 3) when measured in a field of 0.2 T with the temperature sweep rate of 5 K min⁻¹. However, both fullerenes show bifurcation of χ_ZFC and χ_FC curves (defined as T_irrev¹³) until noticeably higher temperatures, reaching 8 K for Dy₂O@C₇₂ and 14 K for Dy₂O@C₇₄. The uncommon shape of χ_ZFC, being higher than χ_FC, is caused in Dy₂O@C₇₄ by the fast QTM at small fields. It should be also noted that T_B and T_irrev values are kinetic parameters and depend on the magnetic field, sweep rate, and some technical aspects of the measurements (see Fig. S9–S11† for further measurements and ref. 4d for a more detailed discussion). More universal parameter is the temperature T_B100, at which relaxation time of magnetization is 100 s. As determined from the relaxation time measurements described below, T_B100 of Dy₂O@C₇₂ is 3.4 K in zero field and 2.6 K at 0.2 T. For Dy₂O@C₇₄, the T_B100 value in zero field is not defined because QTM limits the relaxation time, whereas in the field of 0.2 T the value is 5.0 K.

Relaxation times of magnetization τ_M below T_irrev (Tables S3–S8 and Fig. S12†) were determined by the stretched exponential fitting of magnetization decay curves recorded after the fast sweep of the magnetic field from 5 T to a required value. Unfortunately, the isolable amounts of the clusterfullerenes are insufficient for the measurement of relaxation times at higher temperatures by AC magnetometry.

Fig. 4a shows the magnetic field dependence of τ_M measured at a constant temperature. For Dy₂O@C₇₂, τ_M decays fast with the field from 523 s at zero field to 95 s at 0.4 T and then tends to level off. This τ_M(H) dependence is a clear manifestation of the direct relaxation mechanism. For Dy₂O@C₇₄, relaxation rate in 0 T could not be measured because of the fast QTM, and the lowest field studied is 0.05 T. Field dependence at 2.5 K first shows a fast increase of τ_M from 152 s at 0.05 T to 750–780 s at 0.2–0.35 T. This τ_M(H) dependence corresponds to the gradual quenching of the QTM by increasing Zeeman splitting. With the further field increase beyond 0.35 T, relaxation accelerates due to the contribution of the direct mechanism as in Dy₂O@C₇₂.

Fig. 4 Magnetization relaxation times of Dy₂O@C₇₂ and Dy₂O@C₇₄: (a) Field dependence of τ_M at 1.8 K (Dy₂O@C₇₂) and 2.5 K (Dy₂O@C₇₄). Note that the smallest field for Dy₂O@C₇₄ is 0.05 T since the measurements in zero field are not possible because of the fast QTM process; (b) Temperature dependence of τ_M in two different fields, dots are measured values, solid lines are fits by a combination of Raman and direct mechanisms (eqn (1)), dashed lines are contributions of Raman process for Dy₂O@C₇₂ (blue) and Dy₂O@₇₄ (red).

The temperature dependence of τ_M shown in Fig. 4b cannot be described by a direct process alone. Equally good fits are obtained either for a combination of the direct and Raman mechanisms:

τ_M⁻¹(T) = C_d,HT^n_d + C_RT^n_R (1)

or by a combination of the direct and Arrhenius process:

τ_M⁻¹(T) = C_d,HT^n_d + τ₀⁻¹exp(−U^eff/T) (2)

For the direct mechanism n_d should be 1, and C_d,H is field-dependent. For each fullerene, temperature dependence was measured in two different fields (Fig. 4b), and then two datasets were fitted jointly either with eqn (1) or with eqn (2) keeping field-independent parameters identical (Fig. S13 and S14†). Eqn (1) gives more physically sound interpretation of the relaxation of magnetization and will be discussed further. For Dy₂O@C₇₂, the procedure returned n_d = 1.43 ± 0.13, C_{d,0 T} = (5.2 ± 0.1)·10⁻⁴ s⁻¹ K^−1.4, C_{d,0.2 T} = (1.8 ± 0.2)·10⁻³ s⁻¹ K^−1.4, n_R = 3.7 ± 0.4, and C_R = (8 ± 4)·10⁻⁵ s⁻¹ K^−3.7. The n_d value is higher than 1, which may be due to the phonon bottleneck effect (which increases n_d to 2). For Dy₂O@C₇₄, we obtained n_d = 1.23 ± 0.14, C_{d,0.2 T} = (1.3 ± 0.3)·10⁻⁴ s⁻¹ K^−1.25, C_{d,0.8 T} = (4.9 ± 0.5)·10⁻⁴ s⁻¹ K^−1.25, n_R = 3.28 ± 0.14, and C_R = (4 ± 1)·10⁻⁵ s⁻¹ K^−3.3. For both molecules the direct mechanism dominates at lower temperature and at higher fields, whereas the field-independent Raman process with n_R of 3–4 takes over at higher temperatures. These relatively small n_R values indicate that optical phonons contribute strongly to the relaxation of magnetization.¹⁴ Contribution of the Raman mechanism is shown for each molecule in Fig. 4b. When eqn (2) was used for the fit, Arrhenius processes with effective barriers of 14–16 K were obtained (see ESI†). These values are much smaller than the ligand-field splitting of Dy³⁺ ions in Dy₂O@C_2n molecules, but are considerably larger than the energy difference between the states with ferromagnetic and antiferromagnetic coupling of magnetic moments (see below), and thus cannot match the spin energy levels of the system.

Dy⋯Dy interactions and magnetic anisotropy. To get better understanding of the Dy⋯Dy coupling, magnetization curves of Dy₂O@C_72,74 molecules were simulated and fitted using powder averaging in the PHI code¹⁵ with the following spin Hamiltonian:where Ĥ_LFi are single-ion ligand-field Hamiltonians of Dy³⁺ with ab initio computed parameters, j₁₂ is the coupling constant between dysprosium moments, and Ĥ_ZEE is Zeeman term. Dy³⁺ moments are treated in the |J,m_J〉 basis sets (⁶H_15/2 multiplet). In this notation, the energy difference between FM and AFM-coupled states of the molecule is ΔE_AFM–FM = 225j₁₂ cos(α), where α is the angle between quantization axes of two Dy³⁺ ions.

The ligand-field parameters for Dy ions were computed ab initio at the CASSCF(9,7)/SO-RASSI¹⁶ level for DyYO@C₇₂ and DyYO@C₇₄ molecules. Given the considerable disorder of experimental structures, DFT-optimized atomic coordinates were used. Ab initio calculations showed that Dy³⁺ ions in both clusterfullerenes have easy-axis magnetic anisotropy with the quantization axes aligned along the corresponding Dy–O bonds with a deviation of 2°. In |J,m_J〉 representation, the four lowest energy Kramers doublets (KDs) are almost pure states with m_J of 15/2 (ground state), 13/2 (near 340 cm⁻¹), 11/2 (near 720 cm⁻¹), and 9/2 (near 1030 cm⁻¹) (Table S9†). Transition probabilities between these states are thus very low (Fig. 5). Further KDs have a more mixed nature, and at higher temperatures the relaxation of magnetization following the Orbach mechanism is expected to proceed via the fifth KD at 1180–1200 cm⁻¹, which resembles the situation found experimentally in Dy₂ScN@C₈₀ with the Orbach barrier of 1206 ± 15 cm⁻¹.^3e The overall LF splitting is 1337 cm⁻¹ in Dy₂O@C₇₂ and 1329 cm⁻¹ in Dy₂O@C₇₄. In brief, both clusterfullerenes have strongly axial magnetic anisotropy with similar energies and state compositions, and their low-temperature magnetic properties are determined exclusively by the ground state KDs.

Fig. 5 (a) DFT-optimized molecular structures of Dy₂O@C₇₂ and Dy₂O@C₇₄ showing alignment of magnetic moment in ferromagnetically-coupled ground-state doublet (Dy is green, O is red, adjacent pentagon pairs are rose; magnetic moments of Dy ions are shown as green arrows). (b) CASSCF/RASSI-computed ligand-field splitting for Dy³⁺ ions in Dy₂O@C₇₂ and Dy₂O@C₇₄; the thickness of light blue lines corresponds to transition probability. The insets in (b) show Dy-coordinated fragments of the fullerene cage and quantization axis of Dy (green line); Dy–C distances shorter than 2.6 Å are shown as bonds. The scale on the left shows the energy in cm⁻¹, on the right in K.

The coupling constants j₁₂ defining the scale of Dy⋯Dy interactions are determined by fitting of the experimental magnetization curves to eqn (3) (Fig. S15 and S16†). For Dy₂O@C₇₂, the best fit is obtained for the ferromagnetic coupling with j₁₂ = 0.009 ± 0.002 cm⁻¹, which gives ΔE_AFM–FM = 1.5 ± 0.3 cm⁻¹. For Dy₂O@C₇₄, the optimal j₁₂ is less than 0.001 cm⁻¹ with the uncertainty of ±0.002 cm⁻¹. The FM and AFM states in Dy₂O@C₇₄ are thus degenerate within |ΔE_AFM–FM| < 0.4 cm⁻¹, which means that Dy³⁺ moments are essentially decoupled.

If a magnetic moment of one of the centers in a dinuclear FM-coupled system is flipped, the system arrives in the AFM-coupled state (and vice versa). As long as the FM and AFM states have a different energy, the ΔE_AFM–FMdifference acts as a barrier preventing the QTM, because the latter requires degeneracy of the energy levels. In all dinuclear Dy-clusterfullerenes studied to date the ΔE_AFM–FM difference, either positive (FM) or negative (AFM), was large enough to prevent the QTM in zero field. Dy₂O@C₇₄ is the first dinuclear Dy-clusterfullerene exhibiting efficient zero-field QTM, which can be explained by the vanishing Dy⋯Dy coupling. In Dy₂O@C₇₂, Dy⋯Dy coupling is also weak but still sufficient to quench the QTM in zero field.

The Dy⋯Dy coupling in the Dy₂O clusters found in this work – weak FM in Dy₂O@C₇₂ and negligible in Dy₂O@C₇₄ – is in a sharp contrast with the situation in Dy₂O@C₈₂ isomers, featuring strong AFM coupling with ΔE_AFM–FM of (5.4–12.9) cm⁻¹.⁷ Such a large variation of the strength of magnetic interactions in seemingly very similar molecules requires a closer look. The overall Dy⋯Dy coupling in dinuclear spin systems can be divided into exchange and dipolar contributions, ΔE^exch_AFM–FM and ΔE^dip_AFM–FM. The latter can be computed exactly when molecular structures and orientations of magnetic moments are known. Using DFT-optimized structures and orientation of quantization axes from ab initio calculations, we obtain ΔE^dip_AFM–FM of 2.99 cm⁻¹ in Dy₂O@C₇₂ and 2.56 cm⁻¹ in Dy₂O@C₇₄. Thus, dipolar interactions favour FM arrangement of Dy³⁺ moments and are of the same size as in Dy₂O@C₈₂ isomers with ΔE^dip_AFM–FM of 2.5–3.0 cm⁻¹. Apparently, small overall coupling in Dy₂O@C_72,74 results from the cancellation of dipolar coupling by exchange interactions, which are therefore antiferromagnetic. To yield the experimentally determined ΔE_AFM–FM energies, ΔE^exch_AFM–FM values should be −1.5 ± 0.3 cm⁻¹ in Dy₂O@C₇₂ and −2.5 ± 0.4 cm⁻¹ in Dy₂O@C₇₄. Thus, it is the antiferromagnetic exchange coupling in the Dy₂O cluster that is changing strongly from one fullerene to another and is therefore responsible for the considerable variation of the magnetic properties in the series of Dy₂O clusterfullerenes. The factors determining exchange interactions in Dy-clusterfullerenes are not very clear yet, and further studies of Dy-oxide clusterfullerenes with different fullerene cages are needed to establish structure–property correlations.

Conclusions

In this work we isolated Dy₂O@C_s(10528)–C₇₂ and Dy₂O@C₂(13333)–C₇₄ clusterfullerenes violating the isolated pentagon rule and characterized their structural, electronic, and magnetic properties. The shape of the Dy₂O cluster is determined by the form-factor of the carbon cage and location of pentalene units, leading to the bent cluster in Dy₂O@C₇₂ but linear one in Dy₂O@C₇₄. Both fullerenes are single molecule magnets showing hysteresis up to 7 K in Dy₂O@C₇₂ and 14 K in Dy₂O@C₇₄, the latter being the highest temperature among Dy-clusterfullerenes. The magnetic Dy⋯Dy interactions in Dy₂O clusters are characterized by the ferromagnetic dipolar coupling counterbalanced by the antiferromagnetic exchange. In Dy₂O@C₇₄, these two contributions cancel each other completely leading to decoupled Dy moments and zero-field quantum tunnelling of magnetization. As a result, magnetization of Dy₂O@C₇₄ relaxes fast in zero field, but the relaxation becomes slow once the tunnelling is quenched in a finite magnetic field. This study demonstrates that the shape flexibility of the Dy₂O cluster and its conformity with the fullerene form-factor is reflected in the substantial variation of the Dy₂O-clusterfullerene magnetic properties with the carbon cage. This cage-adaptive SMM behaviour suggests that even stronger variability of magnetic properties may be achieved in future exploration of lanthanide-oxide clusterfullerenes with different carbon cages.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

G. Velkos and W. Yang contributed equally to this work. The authors acknowledge funding from the European Union's Horizon 2020 research and innovation programme, European Research Council (grant agreement No. 648295 to A. A. P.), and Marie Skłodowska-Curie action (grant agreement No. 748635 to S. M. A.), and the Deutsche Forschungsgemeinschaft (grants PO 1602/4-1 and 1602/5-1 to A. A. P.). C. N. thanks the National Science Foundation China (NSFC 91961109, 51302178), the NSF of Jiangsu Province (BK20171211), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the project of scientific and technologic infrastructure of Suzhou (SZS201708). Computational resources were provided by the Center for High Performance Computing at the TU Dresden. We appreciate the help from Dr Anja U. B. Wolter and Sebastian Gaβ in magnetic measurements and technical support with computational resources in IFW Dresden by Ulrike Nitzsche.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 1974305 and 1974314. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0sc00624f

‡ Equal contribution.

§ Crystals were grown by layering the benzene solution of nickel octaethylporphyrin (Ni(OEP)) onto the CS₂ solution of the Dy₂O@C_2n (2n = 72, 74) isomers. The as-prepared crystals Dy₂O@C_s(10528)–C₇₂·Ni(OEP)·2(C₆H₆) and Dy₂O@C₂(13333)–C₇₄·Ni(OEP)·C₆H₆·CS₂ were measured with Bruker APEX II at 120 and 173 K, respectively. The structures were solved using direct methods¹⁷ and refined on F2 using full-matrix least-squares using the SHELXL2015 crystallographic software package.¹⁸ Hydrogen atoms were inserted at calculated positions and constrained with isotropic thermal parameters. The crystal data are presented in Table S1.† The data can be obtained free of charge from the Cambridge Crystallographic Data Centre with CCDC no. 1974305 and 1974314,† respectively.

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