Rotation of fullerene molecules in the crystal lattice of fullerene/porphyrin: C₆₀ and Sc₃N@C₈₀

Abstract

The dynamics of molecules in the solid-state is important to understand their physicochemical properties. The temperature-dependent dynamics of Sc₃N@C₈₀ and C₆₀ in the crystal lattice containing nickel octaethylporphyrin (NiOEP) was studied with variable temperature X-ray diffraction (VT-XRD). The results indicate that the fullerene cages (both C₆₀ and C₈₀) in the crystal lattice present stronger libration than the co-crystallized NiOEP in the temperature range of 100–280 K. In contrast to the fullerene cage, the Sc₃N cluster shows pronounced rotation roughly perpendicular to the plane of the co-crystallized NiOEP molecule driven by temperature. The obtained temperature dependent dynamic behavior of the Sc₃N cluster is different from that of Ho₂LuN and Lu₃N, regardless of their rather similar structure, indicating the effect of the mass and size of the metal ions.

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The empty inner space of fullerenes offers the possibility to use them as nano-containers to store atoms/clusters, leading to compounds known as endohedral fullerenes.¹ The encapsulation of trimetallic nitride clusters is well known and the resulting nitride clusterfullerenes show great potential in biomedicine and molecular magnetism.^2–8 The dynamics of fullerene molecules in the crystal lattice are fundamentally interesting. Recently, we unraveled the temperature driven molecular spinning top movement of Ho₂LuN/Lu₃N nitride clusters inside librating I_h(7)-C₈₀ fullerene cages (hereafter the symmetry notation I_h(7) will be omitted for clarity) and circular movement of Dy₂ inside benzyl functionalized C₈₀.^9–11 The fullerene/metal octaethylporphyrin (MOEP) co-crystallization method is widely applied to elucidate fullerene structures. However, the temperature dependent fullerene dynamics are rarely touched, considering the large number of such co-crystals reported. This is likely owing to the challenges presented by the need for high-quality crystals and technical effort required to undertake variable temperature X-ray diffraction measurements (VT-XRD). Phase transitions in crystals containing fullerenes were observed for several cases, such as, Li@C₆₀/NiOEP,¹² C₆₀/S₈,¹³ C₆₀/CS₂,¹⁴ C₆₀/C₆H₆,¹⁵ C₆₀/C₂Cl₄.¹⁶ C₆₀/CoOEP was reported more than two decades ago.¹⁷ Recently, the effect of the metal ion in the porphyrin and the role of the solvent in the crystallization of C₆₀/MOEP (M = Co, Ni, Cu, and Zn) was thoroughly investigated.^18,19 However, the temperature driven fullerene molecule dynamics in the crystal lattice of co-crystals with MOEP are still unclear for this type of fullerene. The structure of the nitride clusterfullerene Sc₃N@C₈₀ was reported more than two decades ago.² Later on, several efforts to elucidate the ordered crystal structure of Sc₃N@C₈₀ based on other co-crystallization routes were reported, a reasonably ordered structure was achieved with o-xylene,²⁰ disordered structures were obtained with cyclic Zn bis-porphyrin,²¹ and Sc₃N@C₈₀ anions with counterions,^22,23 meanwhile, structures with well-ordered fullerene cages and disordered Sc₃N clusters have been observed with o-dichlorobenzene,²² and decapyrrylcorannulene.²⁴ In strong contrast with the enthusiasm to get an ordered Sc₃N@C₈₀ structure with alternative co-crystallization strategies, the temperature dependent dynamics of Sc₃N@C₈₀ in the crystal lattice was never touched. Herein, we report on the temperature dependent dynamics of Sc₃N@C₈₀ in the crystal lattice, revealing the metallic atom effect on the dynamics of M₃N@C₈₀. Additionally, as a comparison, dynamics of C₆₀ in a co-crystal of fullerene/NiOEP were unraveled to shed light on the understanding of the temperature dependent dynamics of fullerene/NiOEP crystals.

High-quality single crystals were obtained by co-crystallization of fullerenes (obtained by DC arc discharge synthesis as described previously²⁵) with NiOEP.¹⁷ X-ray diffraction data collection was carried out at the MX14.2 beamline of the BESSY storage ring (Berlin-Adlershof, Germany).²⁶ XDSAPP2.0 suite was employed for data processing.^27,28 The structure was solved by direct methods and refined by SHELXL-2018.²⁹ Hydrogen atoms were added geometrically, and refined with a riding model. The crystal data are presented in Tables S1 and S2 in the ESI.†Fig. 1 shows the structures measured at variable temperatures from 100 to 280 K, the structures are shown from one specific direction to intuitively compare the temperature effect. The C₆₀ molecule is nestled by four ethyl groups of the NiOEP, while the Sc₃N@C₈₀ molecule is embraced by all eight ethyl groups, which is archetypical for fullerene/NiOEP co-crystals. Overall, the temperature changing from 100 to 280 K does not enable rotation while the libration of the fullerene cage increased with the temperature as shown by the increasing size of the thermal ellipsoids as shown in Fig. 1 and 2. However, when comparing the increasing size of the thermal ellipsoids between fullerene molecule and the co-crystallized NiOEP, it does show that the fullerene cage librates stronger than its companion NiOEP. The temperature dependent dynamic behaviours of C₆₀ and C₈₀ in the crystal lattice are not significantly different in spite of the different relationships to the co-crystallized NiOEP molecule. The thermal ellipsoids of fullerene cage carbons are characteristic with small radial increments along with large lateral increments with temperature (shown in the highlighted part of Fig. 1 and Fig. S1†), indicating the increasing librational amplitudes experienced by the fullerene cages. The rather isotropic feature of the lateral ellipsoids is a sign of the isotropic libration of fullerene cages on the NiOEP. At 100 K, the shortest cage carbon to Ni distances are 2.766(3) Å and 3.006(2) Å for Sc₃N@C₈₀ and C₆₀, respectively. More details on higher temperatures are presented in Table S3 in the ESI.† The large difference between the cage carbon to Ni distance mirrors the rather different relationships between C₆₀ or C₈₀ and the NiOEP. C₆₀ prefers to interact with half of the NiOEP molecule while Sc₃N@C₈₀ prefers the whole NiOEP, the plausible reason is the different size of the C₆₀ and C₈₀ in spite of the same icosahedral symmetry. The structure of Sc₃N@C₈₀ is consistent with the reported structures from co-crystals of pristine Sc₃N@C₈₀ as shown in Fig. S2–S10 in the ESI.†

Fig. 1 Molecular structures of C₆₀/NiOEP and Sc₃N@C₈₀/NiOEP measured at variable temperatures. The solvent molecules are omitted for clarity. The displacement parameters are shown at the 30% probability level. Color code: grey for carbon, blue for nitrogen, white for hydrogen, red for nickel, and pink for scandium. To compare the ellipsoid changes upon temperature between cage carbon and NiOEP carbon, ellipsoids of C1P of NiOEP and C61 of C₈₀ fullerene cage (both atoms are highlighted with a light blue circle in the related structures) at 100 and 280 K are highlighted at 80% probability level, the N1 of the Sc₃N cluster is drawn to show the orientation of the C61 as well as its ellipsoid changes on temperature.

Fig. 2 Comparison of equivalent atomic displacement parameters of Sc₃N@C₈₀·NiOEP·2(C₆H₆), C₆₀/NiOEP/C₇H₈/C₆H₆ and Ho₂LuN@C₈₀·NiOEP·2(C₆H₆)⁹ as a function of temperature between 100 and 280 K. The encapsulated N and co-crystallized NiOEP were highlighted with grey rectangles, while the encapsulated metals were highlighted with cyan rectangles.

Fig. 3 presents the distribution of the Sc₃N cluster at variable temperatures from 100 to 280 K with comparison of the analogous Ho₂LuN cluster, comparison between all measured temperatures is shown in Fig. S11.† The size of the metal ions represents the site occupancies, which are presented in detail in Table S4 in the ESI.† The N atom presents as ordered in the whole temperature ranging from 100 to 280 K, while the Sc atoms show increasing number of sites with increasing temperature, a sign of increasing movement at higher temperature. This is similar to the case of recently reported M₃N@C₈₀ (M₃ = Ho₂Lu, Lu₃),⁹ because the encapsulated M₃N cluster rotates with the N as rotation center. However, there is a clear difference when comparing the Sc₃N@C₈₀ to its analogue Ho₂LuN@C₈₀. Ho₂LuN shows temperature dependent dynamics of mimicking the motion of spinning top, while the Sc₃N shows roughly free rotation on the plane nearly perpendicular to the NiOEP plane. This is further shown in Fig. 4 with the observed electron density maps at 100 and 280 K. This is significant, because considering the very similar structure of Ho₂LuN@C₈₀ and Sc₃N@C₈₀ from the perspectives of fullerene cage (the same), the M₃N cluster (both planar cluster with similar M–N bond lengths and M–N–M bond angles), and the M-cage carbon distances, the temperature dependent dynamics would be expected to be similar. However, it is different. The plausible explanation is that the mass and size of the M³⁺ ions matter. When the encapsulated cluster is Ho₂LuN or Lu₃N, the temperature dependent dynamics of the cluster are the same because the mass and size differences between Ho(M = 165 g mol⁻¹, r³⁺ = 0.901 Å (ref. 30)) and Lu(M = 175 g mol⁻¹, r³⁺ = 0.861 Å (ref. 30)) are not significant when comparing them to the much lighter Sc(M = 45 g mol⁻¹, r³⁺ = 0.745 Å (ref. 30)). It is worth to mention that Balch et al. highlighted the interesting phenomenon in M₃N@C₈₀ cocrystals with NiOEP, which shows a strong tendency for M₃N to be situated over one of the N–Ni–N set of bonds within the NiOEP.^31,32 The temperature driven rotation of Sc₃N and Ho₂LuN clusters in C₈₀ cage relative to the co-crystallized NiOEP viewed from the direction perpendicular to porphyrin plane are presented in Fig. S12.†

Fig. 3 Molecular structure of Sc₃N@C₈₀·NiOEP·2(C₆H₆) measured with single-crystal X-ray diffraction at variable temperatures from 100 to 280 K. The metal sites are shown as spheres whose radii are scaling proportional to the site occupancy (the bigger the sphere, the higher the occupancy). Color code: grey for carbon, pink for Sc, and blue for N. As comparison, the molecular structure of Ho₂LuN@C₈₀·NiOEP·2(C₆H₆) measured with single-crystal X-ray diffraction at variable temperatures from 100 to 280 K was shown.⁹ The metal sites are shown as spheres whose radii are scaling proportional to the site occupancy (the bigger the sphere, the higher the occupancy). Color code: grey for carbon, brown for Lu, cyan for Ho, and blue for N.

Fig. 4 Observed electron density maps of the Sc₃N cluster. (a) Plane map passing through the encapsulated Sc₃N (main site composed of Sc2, Sc3, and Sc9) at 100 K. (b) Plane map roughly perpendicular to the NiOEP plane passing through the encapsulated Sc₃N (relatively main site composed of Sc2, Sc9, and Sc12) at 280 K. (c) Plane map roughly parallel to the NiOEP plane passing through the encapsulated N atom at 280 K.

Conclusion

The temperature-dependent dynamics of Sc₃N@C₈₀ and C₆₀ in the crystal lattice containing the widely used co-crystallization reagent, nickel octaethylporphyrin (NiOEP), was studied with variable temperature X-ray diffraction. The results indicate that the rotation of fullerene cages (both C₆₀ and C₈₀) in the crystal lattice is suppressed in the temperature range of 100–280 K, while libration of the fullerene cages comparing with the co-crystallized NiOEP is clearly promoted with increasing temperature. The most striking result is that the temperature dependent dynamic behavior of the encapsulated Sc₃N cluster is different from that of its analogues, Ho₂LuN/Lu₃N, regardless of the rather similar structure, which can be explained by the difference in mass and size of the M³⁺ ions. The results shed light on how the dynamics of the widely surveyed M₃N@C₈₀ system are influenced by metal ion mass and size.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors appreciate the great support of and fruitful discussion with Dr Alexey A. Popov. Diffraction data have been collected on BL14.2 at the BESSY II electron storage ring operated by the Helmholtz-Zentrum Berlin; we would particularly like to acknowledge the help and support of Dr Manfred Weiss and his group members during the experiments at BESSY II. Y. H. thanks the China Scholarship Council (CSC) for financial support.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2027144–2027154. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0qi01101k

‡ These authors contributed equally to this work.

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