Rotation of fullerene molecules in the crystal lattice of fullerene/porphyrin: C 60 and Sc 3 N@C 80 †

The dynamics of molecules in the solid-state is important to understand their physicochemical properties. The temperature-dependent dynamics of Sc 3 N@C 80 and C 60 in the crystal lattice containing nickel octaethylporphyrin (NiOEP) was studied with variable temperature X-ray di ﬀ raction (VT-XRD). The results indicate that the fullerene cages (both C 60 and C 80 ) 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 Sc 3 N 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 Sc 3 N cluster is di ﬀ erent from that of Ho 2 LuN and Lu 3 N, regardless of their rather similar structure, indicating the e ﬀ ect of the mass and size of the metal ions. The temperature-dependent dynamics of Sc 3 N@C 80 and C 60 in the crystal lattice containing the widely used co-crystallization reagent, nickel octaethylporphyrin (NiOEP), was studied with variable temperature X-ray di ﬀ raction. The results indicate that the rotation of fullerene cages (both C 60 and C 80 ) 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 Sc 3 N cluster is di ﬀ erent from that of its analogues, Ho 2 LuN/Lu 3 N, regardless of the rather similar structure, which can be explained by the di ﬀ erence in mass and size of the M 3+ ions. The results shed light on how the dynamics of the widely surveyed M 3 N@C 80 system are influenced by metal ion mass and size.

Recently, the effect of the metal ion in the porphyrin and the role of the solvent in the crystallization of C 60 /MOEP (M = Co, Ni, Cu, and Zn) was thoroughly investigated. 18,19However, 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 Sc 3 N@C 80 was reported more than two decades ago. 2 Later on, several efforts to elucidate the ordered crystal structure of Sc 3 N@C 80 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 Sc 3 N@C 80 anions with counterions, 22,23 meanwhile, structures with well-ordered fullerene cages and disordered Sc 3 N clusters have been observed with o-dichlorobenzene, 22 and decapyrrylcorannulene. 24In strong contrast with the enthusiasm to get an ordered Sc 3 N@C 80 structure with alternative co-crystallization strategies, the temperature dependent dynamics of Sc 3 N@C 80 in the crystal lattice was never touched.Herein, we report on the temperature dependent dynamics of Sc 3 N@C 80 in the crystal lattice, revealing the metallic atom effect on the dynamics of M 3 N@C 80 .Additionally, as a comparison, dynamics of C 60 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 previously 25 ) with NiOEP. 17X-ray diffraction data collection was carried out at the MX14.2 beamline of the BESSY storage ring (Berlin-Adlershof, Germany). 26XDSAPP2.0 suite was employed for data processing. 27,28The structure was solved by direct methods and refined by SHELXL-2018. 29ydrogen atoms were added geometrically, and refined with a † 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.a Leibniz Institute for Solid State and Materials Research (IFW Dresden), Helmholtzstraße 20, 01069 Dresden, Germany.E-mail: f.liu@ifw-dresden.deriding 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 C 60 molecule is nestled by four ethyl groups of the NiOEP, while the Sc 3 N@C 80 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 C 60 and C 80 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 Sc 3 N@C 80 and C 60 , 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 C 60 or C 80 and the NiOEP.C 60 prefers to interact with half of the NiOEP molecule while Sc 3 N@C 80 prefers the whole NiOEP, the plausible reason is the different size of the C 60 and C 80 in spite of the same icosahedral symmetry.The structure of Sc 3 N@C 80 is consistent with the reported structures from co-crystals of pristine Sc 3 N@C 80 as shown in Fig. S2-S10 in the ESI.† Fig. 3 presents the distribution of the Sc 3 N cluster at variable temperatures from 100 to 280 K with comparison of the analogous Ho 2 LuN 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 M 3 N@C 80 (M 3 = Ho 2 Lu, Lu 3 ), 9 because the encapsulated M 3 N cluster rotates with the N as rotation center.However, there is a clear difference when comparing the Sc 3 N@C 80 to its analogue Ho 2 LuN@C 80 .Ho 2 LuN shows temperature dependent dynamics of mimicking the motion of spinning top, while the Sc 3 N shows roughly free rotation on the plane nearly perpen- 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 C 80 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 Sc 3 N cluster is drawn to show the orientation of the C61 as well as its ellipsoid changes on temperature.dicular 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 Ho 2 LuN@C 80 and Sc 3 N@C 80 from the perspectives of fullerene cage (the same), the M 3 N 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 M 3+ ions matter.When the encapsulated cluster is Ho 2 LuN or Lu 3 N, the temperature dependent dynamics of the cluster are the same because the mass and size differences between Ho(M = 165 g mol −1 , r 3+ = 0.901 Å (ref.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 Ho 2 LuN@C 80 •NiOEP•2(C 6 H 6 ) measured with single-crystal X-ray diffraction at variable temperatures from 100 to 280 K was shown. 9he 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.

Conclusion
The temperature-dependent dynamics of Sc 3 N@C 80 and C 60 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 C 60 and C 80 ) in the crystal lattice is suppressed in the temperature range of 100-280 K, while libration of the fullerene cages comparing with the cocrystallized NiOEP is clearly promoted with increasing temperature.The most striking result is that the temperature dependent dynamic behavior of the encapsulated Sc 3 N cluster is different from that of its analogues, Ho 2 LuN/Lu 3 N, regardless of the rather similar structure, which can be explained by the difference in mass and size of the M 3+ ions.The results shed light on how the dynamics of the widely surveyed M 3 N@C 80 system are influenced by metal ion mass and size.

Fig. 1
Fig. 1 Molecular structures of C 60 /NiOEP and Sc 3 N@C 80 /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 C 80 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 Sc 3 N cluster is drawn to show the orientation of the C61 as well as its ellipsoid changes on temperature.

Fig. 3
Fig.3Molecular structure of Sc 3 N@C 80 •NiOEP•2(C 6 H 6 ) 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 Ho 2 LuN@C 80 •NiOEP•2(C 6 H 6 ) measured with single-crystal X-ray diffraction at variable temperatures from 100 to 280 K was shown.9The 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
Fig. 4 Observed electron density maps of the Sc 3 N cluster.(a) Plane map passing through the encapsulated Sc 3 N (main site composed of Sc2, Sc3, and Sc9) at 100 K. (b) Plane map roughly perpendicular to the NiOEP plane passing through the encapsulated Sc 3 N (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.