Fullerene rotation dictated by benzene–fullerene interactions

Abstract

The movement of metal ions inside the fullerene cage signifies the unique structural character of metallofullerenes; however, concrete details on this movement are extremely difficult to observe. In this work, we elucidated the structure of Dy₂ScN@C₈₀ using variable temperature single crystal X-ray diffraction. Dynamic disorders with the Dy₂ScN and C₈₀ rotations driven by temperature were precisely presented in detail. The position of the solvent molecule benzene in the crystal lattice proved to have a powerful impact on hindering fullerene rotation. These results present a deep understanding of the unique structural character of metallofullerenes, facilitating their high-accuracy structure–property relationship investigation.

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The fullerene cage provides a robust confining arena for metal clusters that are otherwise unstable, allowing for the investigation of their peculiar structures/interactions and interesting properties. Versatile metal clusters have been discovered to be hosted by fullerene cages, and these fullerenes show application potential in biomedicine and materials science.^1–4 Elucidating the structures of these metallofullerenes with single-crystal X-ray diffraction proved powerful in understanding these novel molecules.^5–14 Determining the structure with high accuracy is fundamentally important to understand their properties such as bonding interactions between the entities of metallofullerenes and the crystal field around the metal ions.^15–23 The ubiquitous disorders (more than 95%) in metallofullerene crystals originate from the round shape of fullerene cages, which tend to rotate in the crystal lattice, and the shallow potential energy surface supplied by the fullerene cage to hold the encapsulated metal ions.^24–26 Co-crystallization with metal octaethylporphyrin or decapyrrylcorannulene significantly constrained the rotation of fullerene cages.^27,28 However, the rotation of metal clusters in the fullerene cages persists as its intrinsic character.^{24–26,29,30} Trimetallic nitride clusterfullerenes have attracted broad interest since their discovery.^6,31 A diversified combination of the metal ions from transition and rare-earth metals produces molecules with fascinating single-molecule magnetism represented by DyM₂N@C₈₀ and Dy₂MN@C₈₀ (M = Sc,^32–36 Y,^37,38 La,³⁸ ErSc,³⁹ Lu,⁴⁰ or V ⁴¹).⁴² The combination of cluster rotation with mixed metals presents a great challenge in their structural elucidation by single crystal X-ray diffraction because the overlapping between different metal ions precludes their accurate assignment.^33,38,39,43 Recently, we observed the snapshots of M₃N rotation inside the C₈₀ cage with spinning top character for heavy metal ions (Ho and Lu),²⁶ while strong rotation for light metal ions (Sc).²⁴ Interestingly, the C₈₀ cage rotation in these crystals was blocked up to 280 K. On the other hand, it is notable that Sc₃N@C₇₀ shows fullerene cage rotation starting from 160 K.²⁵ Since the symmetry of C₇₀ (C_2v) is lower than that of C₈₀ (I_h), it is expected that C₈₀ has a lower energy barrier for rotation, thus requiring a lower temperature to drive its rotation in the same environment. The driving force for fullerene cage rotation in the crystal lattice is fascinating and still unclear. Herein, we precisely determined the structure of Dy₂ScN@C₈₀ using variable temperature single crystal X-ray diffraction. The C₈₀ cage and Dy₂ScN cluster rotations driven by temperature were precisely recorded with great details. Surprising effects of solvent benzene–fullerene interactions on the fullerene rotations were disclosed, revealing the subtle driving force for fullerene cage rotation in the crystal lattice.

Single crystals were obtained by co-crystallization of Dy₂ScN@C₈₀ in benzene with nickel octaethylporphyrin (NiOEP) in benzene through solvent diffusion/evaporation. X-ray diffraction data collection was carried out at the BESSY storage ring (BL14.2, Berlin-Adlershof, Germany).⁴⁴ The XDSAPP2.0 suite was employed for data processing.^45,46 The structures were solved using direct methods and refined using SHELXL-2018.⁴⁷ Hydrogen atoms were added geometrically and refined with a riding model. The crystal data are presented in Table S1 in the ESI.† The asymmetric unit contains one intact NiOEP, one intact fullerene, one intact benzene, and two halves of benzene. The fullerene cage (fully ordered I_h(7)-C₈₀ at 100 K, (symmetry notation I_h(7) will be omitted hereafter)) sits onto the NiOEP as shown in Fig. 1. The packing characteristics show the typical packing character of fullerene–NiOEP co-crystals, in which one fullerene molecule is supported by one NiOEP, as shown in the DySc₂N@C₈₀-NiOEP co-crystals.³²

Fig. 1 The structure of Dy₂ScN@C₈₀·NiOEP·2C₆H₆ with omitted benzene molecules (a) and the structure of Dy₂ScN with coordinated fullerene cage fragments (b) observed at 100 K. The site occupancies with atom labels and bond lengths as well as the distance between metal ions and the coordinated hexagons/pentagons (in Å) are shown in the figure. Colour code: grey for C, white for H, blue for N, red for Ni, pink for Sc, and green for Dy. The thermal ellipsoids are set at 30% probability.

The encapsulated Dy₂ScN cluster presents a slight disorder in the structure obtained at 100 K as shown in Fig. 1 and Fig. S1 in the ESI.† Three sites were refined with a site occupancy ratio of 0.67 : 0.29 : 0.04 for Dy₂Sc. The significant size differences of Dy and Sc in the round C₈₀ cage push the N out of the C₈₀ centroid (vide infra); a similar disorder of N with three positions accordingly should be present along with the disordered Dy₂Sc cluster. However, because of the rather short distances between the possible positions and the large differences of the site occupancies, it is difficult to refine all these positions accurately. Two N sites were refined with a site occupancy ratio of 0.81 : 0.19. The structure of Dy₂ScN is evaluated with the main position Dy₂ScN, which shows M–N bond lengths of 1.95(2), 2.07(2), and 2.079(5) Å, and bond angles of ∠Dy1N1ADy2 = 120.8(5)°, ∠Dy1N1ASc1 = 122.9(6)°, and ∠Dy2N1ASc1 = 116.3(3)°, indicating a planar Dy₂ScN cluster. The N was pushed out of the centroid of C₈₀ by the asymmetrical Dy₂Sc cluster by 0.19(1) Å. The Dy ion far away from NiOEP is located under a hexagon/pentagon with a Dy–hexagon distance of 2.0095(6) Å (Dy–C bond lengths range from 2.345(4) to 2.599(4) Å), while the other Dy ion is located under a hexagon with a Dy–hexagon distance of 1.9670(5) Å (Dy–C bond lengths range from 2.433(4) to 2.464(4) Å). The Sc ion is located under a hexagon/pentagon with a Sc–hexagon distance of 1.851(8) Å (Sc–C bond lengths range from 2.238(7) to 2.492(7) Å). For comparison, the Dy–hexagon distance of DySc₂N@C₈₀ is 2.0241(8) Å, while the Dy–N and Sc–N bond lengths are 2.096(6), 1.965(6), and 1.978(6) Å,³² showing noticeable compression of Dy₂ScN by the C₈₀ cage.

Variable temperature single crystal X-ray diffraction was used to observe the detailed metal movement in the fullerene cage. In striking contrast to the previous reports on analogous M₃N@C₈₀ (M₃ = Ho₂Lu, Lu₃, Sc₃) without fullerene cage rotation until 280 K,^24,26 the fullerene cage here presents rotation at 180 and 260 K as shown in Fig. 2, with 50% of them rotating to a new orientation, which shows the same interaction with the co-crystallized NiOEP. Both orientations are related by a mirror symmetry passing through NiOEP, as shown in Fig. S2 in the ESI.† The surprisingly different dynamic behaviours of C₈₀ cages in a similar crystal lattice originate from the effects of benzene–fullerene interactions (π–π and C–H⋯π), as shown in Fig. 3, and Fig. S3 and Fig. S4 in the ESI.† The contribution of C–H⋯π is small compared to the π–π interactions,⁴⁸ while the positions of two benzene molecules interacting with the fullerene through π–π interactions show a surprisingly powerful effect on the fullerene rotation, i.e., the fullerene starts rotation from 180 K when the two benzene molecules sit parallel, while the fullerene rotation is blocked until 280 K when the two benzene molecules sit with a magic angle of 125.1(8)° determined at 100 K.

Fig. 2 The structures of Dy₂ScN@C₈₀·NiOEP·2C₆H₆ viewed perpendicular to the NiOEP plane, observed at variable temperatures of (a) 100, (b) 180 and (c) 260 K. The structures with omitted benzene for clarity are shown in the upper panel, while the structures of the Dy₂ScN cluster are highlighted in the middle panel, and the lower panel shows the Dy₂ScN structure with a metal volume equal to the site occupancy, and the three main metal positions are labelled (I labels the position far away from NiOEP; II and III label the positions near the NiOEP, with the equivalent Dy occupancies noted in the figure) with their electron density gauged with equivalent Dy occupancies. Colour code: grey for C, white for H, blue for N, red for Ni, pink for Sc, and green for Dy. The thermal ellipsoids are set at 30% probability except for C₈₀. C₈₀ is highlighted with red/cyan to show the orientations.

Fig. 3 The structures of Dy₂ScN@C₈₀·NiOEP·2C₆H₆ in the C2/c space group (a, c and e) and Lu₃N@C₈₀·NiOEP·2C₆H₆ in the P1̄ space group (b, d, f, data from ref. 26) viewed perpendicular to the NiOEP plane. The ball-stick model is shown in the upper panel, while the space-filling structures are shown in the middle/lower panels. Colour code: red/cyan for the fullerene cage, pink for NiOEP, purple for benzene interacting with the fullerene cage through π–π interactions, and cyan for benzene interacting with the fullerene cage through C–H⋯π interactions.

The Dy₂ScN cluster shows heavy disorder at 180 and 260 K, which hampers the assignment of Dy and Sc; thus, we applied heavier Dy with free occupancies to refine the positions of Dy/Sc.³⁸ The positions of metal ions at 180 and 260 K show a tendency of “spinning top” rotation as observed for M₃N@C₈₀ (M₃ = Ho₂Lu, Lu₃).²⁶ The possibility of two metal ions near the NiOEP (labelled as II and III in Fig. 2) rotating to the far away position (labelled as I) is impossible to clarify in the case of M₃N@C₈₀ (M₃ = Ho₂Lu, Lu₃) because of similar electron densities at the three positions (I, II, and III). Dy₂ScN@C₈₀ is ideal to clarify such a possibility with substantially different Dy and Sc electron densities (1Sc ≈ 0.25Dy in the refinement considering the effects of scattering power). The electron density imbalance of 0.50 : 0.75 (normalized with Dy) between II and III at 100 K is averaged off to 0.60 : 0.59 at 180 K, while the imbalance between I and II/III shows a tendency of averaging off with increasing temperature as shown in Fig. 2. Contrary to the disordered N at 100 K, the N is fully ordered at 180 and 260 K and is located at the centroid of C₈₀. Because the N is pushed out of the centroid of C₈₀ by the inequivalent Dy/Sc ions (vide supra), it does not occupy the crystallographically ordered N position. The flattened ellipsoids of N at 180 and 260 K hint at the rotation of N around the C₈₀ centroid (see Fig. S5 in the ESI† for details). Thus, the assumption that the main position represents the real structure in single crystal X-ray diffraction involving disorder should be applied with caution. It is worth noting that the structures obtained at 180 and 260 K result in misrepresented Dy₂SCN structures as shown in Table S2 in the ESI.† The Dy–N bond lengths of 2.07(2) and 2.079(5) Å determined at 100 K change to 2.052(5) and 1.949(6) Å at 180 K and to 2.019(9) and 1.935(11) Å at 260 K, showing remarkable discrepancies.

For most M₃N@C₈₀ co-crystallized with NiOEP, two metal atoms of M₃N prefer to sit above the N–Ni–N bonds.^{24,26,49–51} The recently reported M₂@C₈₀-CF₃ (M = Tb, Nd) co-crystallized with NiOEP also shows such tendency.^5,52 The Dy₂ScN cluster shown in Fig. S6 in the ESI† reveals that the encapsulated Dy₂ScN sits above the N–Ni–N bonds at 100 K, while exhibiting rotation at 180 and 260 K, showing the subtle interaction between metal ions and the NiOEP molecule.

In conclusion, the structure of Dy₂ScN@C₈₀ was determined by variable temperature single crystal X-ray diffraction. The temperature-driven dynamics of Dy₂ScN@C₈₀ in the lattice was precisely recorded in great detail. The angle between the two benzenes interacting with the fullerene cage through π–π interaction shows a surprising ability to control the fullerene cage rotation. These results present a deep understanding of the metallofullerene structures, facilitating their high-accuracy structure–property relationship investigation.

Author contributions

Y. W. synthesized fullerenes, Y. W. and F. L. prepared the crystals and conducted the single crystal X-ray diffraction measurement and analysis. F. L. wrote the manuscript with a contribution from Y. W.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This research was financially supported by the Deutsche Forschungsgemeinschaft (LI 3055/3-1) and the Jiangsu Specially Appointed Professorship. 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. The authors appreciate the help of Dr Manfred Weiss and his team during the experiments at BESSY II.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2330927–2330929. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi00831f

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