Flexible decapyrrylcorannulene hosts

The assembly of spherical fullerenes, or buckyballs, into single crystals for crystallographic identification often suffers from disordered arrangement. Here we show a chiral configuration of decapyrrylcorannulene that has a concave ‘palm’ of corannulene and ten flexible electron-rich pyrryl group ‘fingers’ to mimic the smart molecular ‘hands’ for self-adaptably cradling various buckyballs in a (+)hand-ball-hand(−) mode. As exemplified by crystallographic identification of 15 buckyball structures representing pristine, exohedral, endohedral, dimeric and hetero-derivatization, the pyrryl groups twist with varying dihedral angles to adjust the interaction between decapyrrylcorannulene and fullerene. The self-adaptable electron-rich pyrryl groups, susceptible to methylation, are theoretically revealed to contribute more than the bowl-shaped palm of the corannulene in holding buckyball structures. The generality of the present decapyrrylcorannulene host with flexible pyrryl groups facilitates the visualization of numerous unknown/unsolved fullerenes by crystallography and the assembly of the otherwise close-packed spherical fullerenes into two-dimensional layered structures by intercalation.

F ullerenes, also known as cage-like 'buckyballs', represent one class of the most remarkable molecules discovered in the last three decades 1 . However, a number of examples have been retracted due to problematical structural determinations that are largely attributed to the lack of reliable analytical approaches [2][3][4] . Crystallography is regarded as the most reliable approach towards the unambiguous identification of molecular structures, but the presence of severe disorder defects in the single crystals of spherical fullerenes frequently leads to ambiguous geometric identification. Exohedral derivatization has been employed to reduce the disorder defects, as exemplified by determination of the C 60 cage structure through the crystallographic study of its derivative obtained from a cycloaddition reaction involving OsO 4 and 4-tert-butylpyridine 5 . The exohedral derivatization method is even adequate to crystallographically identify some of the wellknown fullerene derivatives, such as α-PC 71 BM 6 , a dominant isomer of the most prevalent electron acceptor [6,6]-phenyl-C 71 butyric acid methyl ester. Alternatively, the supramolecular assembly of buckyballs into co-crystals with a selected 'buckycatcher' host is also a straightforward approach for reducing the disorder defects and identifying the pristine structures of various fullerenes [7][8][9][10][11][12][13][14] . In buckycatcher/buckyball (host/guest) systems, that are generally applied in a stoichiometric ratio of 1:1, the most prevalent buckycatchers are porphyrin compounds such as M II (OEP) (OEP=octaethylporphyrin) [15][16][17] . Tens of fullerenes, especially endohedral ones 18 , have been identified in the form of supramolecular M II (OEP)/fullerene co-crystals since the groundbreaking work by Balch and co-workers in 1999 15 .
In addition to M II (OEP), concave corannulene 19 is an ideal host for fullerenes with convex surfaces. Scott and co-workers first reported the assembly of C 60 with a penta-tert-butyl corannulene derivative 20 in the solid state and a corannulene polysulfide 21 in solution, illustrating that the attachment of adaptable substituents to the periphery of corannulene is an effective way to tune the electron distribution of corannulene and consequently reinforce the anticipated attraction between corannulene and fullerenes. Similarly, the supramolecular assemblies formed between our recently synthesized hexathiolated trithiasumanenes 22 and fullerenes demonstrate the importance of functionalization with electron-rich substituents. Instead of pristine corannulene, doping heteroatoms in corannulene can also enhance its electron-donating and polarizing abilities, leading to stronger supramolecular interactions between heteroatom-doped corannulene 12 and fullerenes. On the other hand, an increase in surface contact between the host and guest molecules also favours the formation of assemblies. Recent computations revealed that the binding energy of a bowl-shaped polyarene and buckyball was proportional to the surface contact area 23 . Indeed, both dibenzocorannulene 24 and double concave biscorannulenes 7 have enhanced associations with fullerenes due to their expanded concave surfaces, but the resultant rigid cavities limit the compatibility of these derivatives to self-adaptly interact with fullerenes upon changing shapes/types of buckyballs.
Herein, we design and synthesize a unique decapyrrylcorannulene (abbreviated as DPC) that mimics a molecular 'hand', where ten pyrryl groups resemble flexible 'fingers' on the periphery of a bowl-shaped corannulene 'palm'. Benefiting from the matched shape, electron-rich property, expanded surface contact and flexible pyrrole-corannulene dihedral angles, the DPC host is capable of holding almost all commonly known types of fullerene such as pristine, exohedral, endohedral, dimeric and heteroderivatized structure as well as fulleroid and pentagon-fused fullerene.   Fig. 2a, c, the 10 pyrryl groups all twist in the same direction but with varying dihedral angles (Supplementary Fig. 9 and Supplementary  Table 3), which gives 2a and 2b molecules chiral configurations in the crystals. This chirality derives from P vs M twisting of the 10 pyrryl groups in the crystal, rather than the concave vs convex geometry of the corannulene core 29 . With the attachment of pyrryl groups to the periphery of corannulene, the bowl depth was reduced from the value of 0.87 Å reported for parent corannulene 29 to 0.61 Å for 2a and 0.60 Å for 2b, indicating a decrease in the corannulene curvature, which is possibly due to the repulsion of the 10 pyrryl groups (Fig. 2a, c, Supplementary  Fig. 18 and Supplementary Table 11), similar to the results shown for decaphenylcorannulene 30 . However, based on the distance from the mean plane of the centre corannulene core pentagon to the α-carbon of the pyrrole ring, the concave surfaces of 2a and 2b actually expanded relative to that of parent corannulene, and, in some senses, the concave depth increased to 1.97 Å for 2a and 1.96 Å for 2b, as shown in Fig. 2a, c. The inversion barrier energy of 2a was calculated to be 3.11 kcal mol −1 by density functional theory (DFT) calculations, a value much lower than those of pristine corannulene (10.46 kcal mol −1 ) and sumanene (19.29 kcal mol −1 ) calculated at the same level of theory (see Supplementary Table 12 and 15).

Results and discussion
Usually, buckybowls tend to stack into a columnar supramolecular structure in a concave-to-convex orientation 31,32 , and few cases of structures that differ from columnar packing have been reported 33 . Unexpectedly, 2a exhibits an alternate convex-toconvex plus concave-to-concave columnar stacking structure along the a axis, as shown in Fig. 2b. The stacking distance is 3.34 Å between neighbouring convex-to-convex faces and 7.36 Å between concave-to-concave faces, which form untethered clamlike cavities, implying that fullerene C 60 with a diameter of~7 Å may be allowed to intercalate into the untethered clam-like cavity. In addition to the unique concave-to-concave packing orientation, a layer-by-layer structure is observed in the crystal, as shown in the a-c plane of the crystal depicted in Fig. 2b. Such an exceptional packing pattern might be mainly due to the influence of the flexible pyrryl groups, which afford multiple selfadaptable C-H•••π interactions (edge to face) in both the convexto-convex and concave-to-concave packing orientations. Note that the generation of these significant C-H•••π interactions mainly depends on the ability of the flexible pyrryl groups to adopt self-adaptive and suitable dihedral angles (ranging from 62.27°to 66.80°, as seen in Supplementary Table 3). Methylsubstituted DPC (2b) adopts a completely different packing mode than 2a, as shown in Fig. 2d. With the introduction of methyl groups, the previous convex-to-convex stacking orientation of 2a still exists in the packing structure of 2b, but the unique concaveto-concave stacking mode is transformed into a more compact cross-orthogonal packing pattern (see Supplementary Fig. 19 and Supplementary Data 1), with implication about 2b unsuitable for forming supramolecular assemblies with fullerenes in the crystal state.
Supramolecular behaviours of the DPC host/buckyball guest systems. The structure of 2a featuring an expanded concave depth (1.97 Å) matches well with the convex structure of the fullerene surface, implying that the formation of a supramolecular assembly between 2a and fullerenes is possible. DFT calculations predicted a higher binding energy for the assembly of 2a with C 60 than for the assembly of pristine corannulene with C 60 (31.39-31.44 vs 15.46 kcal mol −1 ) (see Supplementary Table 13). Indeed, 2a undergoes strong supramolecular assembly with C 60 and C 70 to form 2a/C 60 and 2a/C 70 supramolecules in a stoichiometric ratio of 1:1 in solution on the basis of the 1 H NMR complexation-induced chemical shifts 21 Supplementary Fig. 16). Such strong supramolecular interactions are also attributed to the introduction of electron-rich pyrryl groups on the periphery of corannulene. By contrast, the previously reported corannulene derivatives, pentakis(tert-butyl)corannulene 20 and dibenzo-[a,g] corannulene 24 were unable to form supramolecular assemblies with C 60 in solution.
The supramolecular assembly behaviour of 2a with fullerenes in the solid state can be characterized by X-ray crystallography (see Supplementary Tables 4-8). With the introduction of C 60 , as shown in Fig. 3a, the clam-like cavities formed by the concave-toconcave stacking orientation of 2a are occupied by C 60 molecules in the co-crystal of 2a/C 60 . The shell of the clam-like cavity cooperatively opens into a V shape with an angle of 58.02° (  Fig. 3a). Upon the involvement of C 60 , the dihedral angles of pyrryl group and corannulene rim in 2a/C 60 co-crystal vary from 57.07°to 68.61°(see Supplementary Table 9), the variation range is larger than the span of 62.27°−66.80°observed in the crystal of pristine 2a (see Supplementary Table 3), reflecting the pyrryl groups are adaptable and capable of adjusting the interactions between 2a and C 60 . The asymmetric unit consists of one pair of fully ordered 2a molecules and one molecule of C 60 , which is disordered with two orientations. Interestingly, in each pair of 2a molecules the 10 pyrryl rings in one 2a molecule all twist in the opposite direction relative to those in another molecule with varying dihedral angles (Supplementary Table 9), which gives the 2a molecules a pair of chiral hand-like configuration. Each pair of chiral 2a molecules [marked as (+) and (−)] cradle a C 60 cage in a structure strongly resembling a ball held by two hands as shown in Fig. 3a. The notation 2DPC{C 60 } is adopted to specify this unique supramolecular (+)hand-ball-hand(−) assembly in which the buckyball C 60 is cradled by both chiral DPC hosts. Based on the interesting concept reported in Balch's article 34 , our DPC hosts provides a valuable opportunity to sort chiral fullerene enantiomers in the future.
Within the unit cell of 2DPC{C 60 }, fullerene is positioned asymmetrically between the pair of 2a molecules. Measured from the plane of the five hub carbons of 2a to the centroid of C 60 , the penetration depths of C 60 into (+) and (−) 2a are 6.64 Å and 6.69 Å (Fig. 3a, Supplementary Table 10), which are shorter than those observed in the supramolecular corannulene/C 60 (6.94 Å) 20 and azabuckybowl/C 60 (6.82 Å) 12 . Such a short contact suggests the occurrence of strong π-π interactions between 2a and C 60 .
In the crystal packing structure of 2DPC{C 60 }, the C 60 molecules are completely surrounded by the DPC hosts of 2a and aggregate in a zigzag fashion along the b axis, as shown in Fig. 3b. Along the c axis, the DPC hosts and buckyballs stack head-to-tail, forming a one-dimensional badminton-shaped packing structure, and the adjacent columns pack with the opposite badminton shape. The π-π or C-H•••π intermolecular contacts that exist between the DPC host and buckyball and between two DPC host molecules are tuneable through the dihedral angles of the pyrryl groups. In particular, the weak interactions present in the structure are quite sensitive for the formation of 2DPC{C 60 } because another DPC derivative, decakis (3,4-dimethylpyrryl)corannulene 2b, in which all (β)H atoms of the pyrryl groups are replaced by methyl groups, does not easily assemble with any fullerenes, as suggested in the crystal structure of 2b (see Supplementary Fig. 19 and Supplementary Data 1). This difference in behaviour highlights the importance of the cooperative and adaptable weak intermolecular interactions, especially the appropriate C-H•••π interactions, within the cocrystal of 2DPC{C 60 }.
The merit of 2a is embodied by the generality for holding any fullerene, regardless of the shape/type of fullerene derivative. As shown in Fig. 4, all the common types of fullerenes, including pristine (C 60 , C 70 , C 90 ), exohedral (six methanofullerene 35 derivatives, three fullerene hydride derivatives 36 , and one fulleroid 37 derivative), endohedral (Sc 3 N@C 80 ) 16 , and dimeric/ hetero-[(C 59 N) 2 ] 38 fullerenes, have been co-crystallized in the forms of 2DPC{fullerene} suitable for crystallographic identification. Specifically, six methanofullerene derivatives with various functional groups such as phenyl, pyridyl and ester groups are compatible with 2a for co-crystallization (Fig. 4d-i). Although a large number of literatures have discussed the synthesis of fullerene hydride derivatives, the involved structures have rarely been identified by crystallography. Here the DPC host 2a help to visualize the fullerene hydride derivatives, including an IPRdefying (IPR=Isolated Pentagon Rule 39 ) C 65 H 6 and two unsolved C 60 HCH 3 and C 60 HPh (Fig. 4j-l). As shown in Fig. 4d, e, h, i, and m, in addition, isomeric PC 71 BM (α-and β 1 -PC 71 BM) and C 71 H 2 (isomers I-III, see Supplementary Note 4) can be successfully held by the DPC host 2a no matter where the derivative groups are located in the skeleton of fullerene isomers. In particular, the fulleroid of C 2v -C 71 H 2 -III was synthesized in 2010 40 but whose crystal structure has not been solved heretofore. In the present work, we succeeded in obtaining a high-quality single co-crystal of 2DPC{C 2v -C 71 H 2 -III}, in addition to the newly isolated isomers  (Fig. 4m) reveals that the methylene group indeed stays on the equator of C 70 , the same with the theoretical prediction 40 . Remarkably, the exact crystal structure of another long-known pendent fullerene, (C 59 N) 2 dimer, which was first reported by Wudl and co-workers in 1995 38 as the first heterofullerene dimer, has remained a mystery until now. Owing to the adaptability of DPC host 2a, we succeeded in determining the molecular structure of this fullerene dimer via cocrystallization with 2a for the first time ( Fig. 4o and   a b  Supplementary Tables 7-10). Our present result strongly supports the theoretical prediction by Lee et al. 42 that the gauche conformer is the lowest-energy structure, and the dihedral angle of deflection for two nitrogen atoms is approximately 65.57°in the co-crystal, slightly larger than that in the theoretical calculation 42 . The merit of 2a is also reflected by the flexibility in cradling various buckyballs. Taking the pristine C 70 , exohedral α-PC 71 BM, endohedral Sc 3 N@C 80 , fulleroid C 2v -C 71 H 2 -III, and dimeric/ hetero-(C 59 N) 2 Table 9), the flexibility of the 2DPC{fullerene} assembly conformations is attributed to ten pyrryl groups of 2a that are able to self-adjust the dihedral angles for matching different shape/type of fullerene. As shown, for most of supramolecular 2DPC{fullerene}, V-shaped conformations are found (Fig. 4a-h,  j-m). The angles of the two 2a molecules vary between 56.31°and 68.55°for the representative examples (Supplementary Table 10). Interestingly, a racemate of α-PC 71 BM is found disordered with the occupancy of 0.5:0.5, while the V-shaped DPC hosts are observed to be fully ordered (Supplementary Fig. 10). For isomeric C s -C 71 H 2 -I (Fig. 4h) and C 2v -C 71 H 2 -III (Fig. 4m) 2a takes a V-shape conformation, but for C s -C 71 H 2 -II ( Fig. 4i) 2a adopts an almost parallel sandwich-like conformation in the cocrystals. Additionally, for Sc 3 N@C 80 , the conformation of the 2DPC{ Sc 3 N@C 80 } changes into sandwich-like structure (Fig. 4n). Interestingly, a sandwich-like conformation is found for 2DPC {(C 59 N) 2 } as well, in which the two 2a molecules are dramatically dislocated so that each DPC host molecule can tightly bind one of the fullerene cages in (C 59 N) 2 (Fig. 4o), further demonstrating the flexibility of the present DPC host 2a.

as representative examples (Supplementary
Although the goal of sorting chiral fullerene enantiomers was not achieved in the example of 2DPC{α-PC 71 BM}, it is worth trying more experiments in the future. Sorting chiral fullerene enantiomers might be possible since DPC can become chiral upon crystallization though it is intrinsically achiral in solution, as mentioned in Balch's article 34 . This important characteristic provides valuable opportunities and new thoughts for sorting fullerene enantiomers by DPC. The merit of 2a is even further shown in the assembly of various fullerenes into two-dimensional (2D) layered structures. Benefiting from the flexible pyrryl groups, 2a itself adopts a unique layer-by-layer packing mode that constructs potential untethered cavities for fullerene intercalation (Fig. 2b). Interestingly, in the supramolecular assembly of DPC host 2a with any buckyball, the fullerene molecules could intercalate into the cavities to form multilayered matrix structures composed of alternating fullerene mono layers and DPC double layers as outlined in Fig. 5 for C 60 and Supplementary Fig. 11 for the representative fullerene molecules (including C 70 , Sc 3 N@C 80 , α-PC 71 BM and C 2v -C 71 H 2 -III). In the multi-layered structure, the DPC layer adopts a slightly corrugated arrangement, but the fullerene layers form an interesting 2D molecular assembly with C 60 centroid-to-centroid separations of 14.23 Å and 14.12 Å along the a axis and the c axis, respectively (Fig. 5b), and an interlayer spacing of 16.93 Å (Fig. 5a). The 2D arrangement of metallofullerenes was reported as one special assembly example by Shinohara and co-workers 43 . Note that controlled fabrications of ordered 2D fullerene molecular nanostructures are challenging and mainly accomplished by means of metal surface-supported self-assembly approaches previously [44][45][46][47] .
Theoretical understanding of the assembly of DPC host/ buckyball guest. To understand the assembly between fullerenes and 2a, DFT theoretical calculations were performed on DPC (2a) and the representative 2DPC{fullerene} co-crystals (Supplementary Note 8). Similar to the parent corannulene molecule, the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of 2a are both doubly degenerate 48 . However, the degenerate HOMO of 2a is mainly localized on the pyrryl groups, while the LUMO is distributed on the corannulene core, and the HOMO-LUMO gap (2.80 eV) is lower than that of pristine corannulene (4.39 eV) at the B3LYP-D3BJ/6-31 G(d,p) level 49 (see Supplementary Fig. 17). The total interaction energies of 2DPC{fullerene} range from −62.60 to −76.54 kcal mol −1 , as presented in Fig. 6 and Table S13. Taking C 60 as an example, the total interaction energy, namely, ΔE int F/2DPC , is −62.60 kcal mol −1 , which is much larger than that of the double concave buckycatcher reported by Sygula and co-workers 7 (−35.60 kcal mol −1 at the same calculation level, see Supplementary Data 1).
In addition, according to fragmentation methods 50 , 2a can be divided into pyrryl fragments (PFs) and corannulene fragment (CF) by capping the dangling bonds with H atoms. As a result, the total interaction energy of 2DPC{fullerene}, namely, ΔE int F/2DPC , can be divided into three parts, including the interaction energy between fullerene and twenty PFs (ΔE int F/P ), the interaction energy between fullerene and two CFs (ΔE int F/C ) and the two-body interaction correlation energy between PFs and CFs (ΔE corr ). That is, ΔE int F/2DPC = ΔE int F/P + ΔE int F/C + ΔE corr .  5 Layered structure of the co-crystal formed between DPC (2a) and C 60 . a, b Edge-on view (a) and top view (b) of the space-filling structure of C 60 and the stick structure of DPC. FL is a monolayer of C 60 , and DL is a slightly corrugated double DPC layer. C atom is shown in gold, N atom in navy blue, and C 60 in purple. All hydrogens and solvent molecules (CS 2 ) in the co-crystal have been omitted for clarity As outlined in Fig. 6 and Supplementary Table 14, the partial interaction energies of PFs and fullerenes are generally 1.5-1.9 times larger than those of CF and fullerenes in the representative supramolecules, revealing that the pyrryl fragments make a larger contribution to the total interaction energy than the corannulene fragments (ΔE int F/P > ΔE int F/C ). In addition, the ΔE int F/2DPC (black columns) increases with the ΔE int F/P (red columns), whereas the ΔE int F/C (blue columns) changes slightly, suggesting that the variation in the total interaction energies mainly depends on changes in the partial interaction energies of PFs and fullerene.
Accordingly, we have demonstrated that the DPC host molecule with a concave corannulene and flexible groups is able to adaptably hold various fullerenes to form (+)hand-ball-hand (−) supramolecular 2DPC{fullerene} assemblies, as exemplified by 15 typical fullerenes including pristine, endohedral, exohedral, dimeric and hetero-fullerene as well as fulleroid and non-IPR fullerene. Such a particular interaction between DPC and fullerene can be attributed to the electron-rich property, expanded and matched surface contact, and especially the flexible and self-adaptable pyrryl groups. DFT theoretical calculations confirm that the crucial contribution of the pyrryl groups is actually more than that of corannulene core in the supramolecular assemblies of 2DPC{fullerene}. Besides, benefitting from the flexible pyrryl groups and the concave corannulene, the DPC hosts themselves assemble into a layer-by-layer crystal, suitable for intercalation of various fullerenes as two-dimensional structures. In the future, expectedly, more fullerenes may be crystallographically identified based on the supramolecular (+) hand-ball-hand(−) 2DPC{fullerene} co-crystals, and the DPC host-decorated materials may be capable of separating buckyballs in more complex matrixes.

Methods
Synthesis and separation of compound 2. To a DMF (15 ml) solution of NaH (60% oil dispersion, 80.7 mg, 2.00 mmol for 2a or 136.0 mg, 3.40 mmol for 2b) was added 135.1 mg (2.01 mmol) of pyrrol for 2a or 323.0 mg (3.40 mmol) of 3,4dimethylpyrrol for 2b at 0°C. After the evolution of H 2 gas ceased, the reaction mixture was stirred for an additional 30 min at the same temperature, and then, decachlorocorannulene 1 (100.0 mg, 0.17 mmol) was added. The reaction mixture was stirred for an additional 12 h at 25°C before it was poured into ice water (20 ml). Then, the mixture was added to dichloromethane (20 ml). The organic layers were separated, and the aqueous layer was thoroughly extracted with dichloromethane. The combined organic layers were washed with water and brine and dried over magnesium sulfate. After filtration and evaporation of the solvent, product 2a was separated by an alkaline aluminium oxide column, and product 2b was separated by silica gel column chromatography using dichloromethane/hexane (1:1) as the eluent. Finally, 45.3 mg (0.05 mmol) of 2a or 40.1 mg (0.034 mmol) of 2b was obtained. The isolated yield is approximately 30% for 2a or 20 % for 2b.
X-ray diffraction analysis. Slow evaporation from a solution of 2a and fullerenes in toluene or a mixture of toluene and dichloromethane or a mixture of disulfide and dichloromethane provided co-crystals of 2DPC{fullerene} suitable for X-ray crystal structure analysis (see Supplementary Note 5 and Supplementary Table 2 for detail). In each case, a suitable crystal was mounted with mineral oil on a glass fibre and transferred to an Agilent SuperNova diffractometer with a Cu Kα (λ = 1.54184 Å) microfocus X-ray source. The data were processed using CrysAlis Pro . The structure was solved and refined using full-matrix least-squares based on F 2 with the programs SHELXT and SHELXL-2015 51 within OLEX2 52 . The intensities were corrected for Lorentz and polarization effects. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed using AFIX instructions. Program SQUEEZE, a part of the PLATON package 53 of crystallographic software, was used to calculate the solvent disorder area and remove its contribution from the intensity data if needed.