Corannulene and its penta-tert-butyl derivative co-crystallize 1 : 1 with pristine C₆₀-fullerene

Abstract

The first X-ray structural determinations of pristine fullerene C₆₀, cocrystallized 1 : 1 with corannulene and with its pentaalkyl-substituted derivative, 1,3,5,7,9-penta-tert-butyl-corannulene, have now been achieved.

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The chemistry and properties of corannulene (1) have been of long-standing interest to many research groups, but particularly so in recent years due to its structural resemblance to segments of the C₆₀-fullerene. Corannulene represents the smallest C₆₀-fullerene fragment which is built around a five-membered ring and is also the first “geodesic” polyarene to have been synthesized.¹ Barth and Lawton first reported its synthesis via a 16-step procedure in 1966.² Since then more practical syntheses of corannulene have been developed, some being variants³ of flash vacuum pyrolysis approaches,⁴ and more recently, variants of the solution-phase methods originally developed by the Siegel group.⁵ As a result of these advances, controlled substitution at corannulene's rim has been possible to achieve in synthetically-useful amounts which has resulted in diverse derivatives whose properties have been studied.⁶ Recently, Petrukhina's group showed that the “hub” of corannulene could also be functionalized via Friedel–Crafts alkylation to form stable salts which could be further functionalized at the adjacent carbon on the rim to produce neutral bifunctionalized derivatives.⁷

The curved concave surface of corannulene has long been postulated as being ideally suited for supramolecular binding to the electron-deficient convex C₆₀. There are many computational studies which have shown that, at least in the gas phase, complexation from the concave face of corannulene to a C₆₀ molecule should be energetically favourable;⁸ however, direct experimental proof has not been demonstrated until now.

Corannulene has indeed been shown to form a complex with the cation of C₆₀ in the gas phase.⁹ More recently, Fasel and coworkers reported a Cu(110)-surface supported corannulene : C₆₀ host–guest system.¹⁰ Both of these findings are consistent with the hypothesis of a preferred concave-corannulene convex-C₆₀ complementary π–π interaction. We have previously shown¹¹ that corannulenes 2 and 3, which are more electron-rich than corannulene itself, exhibit modest binding constant values of 280–475 M⁻¹ as measured by ¹H-NMR studies in toluene-d₈. Later, a “Venus fly-trap”-resembling corannulene derivative,¹²pentakis(1,4-benzodithiino)corannulene (4), proved to be a better host for C₆₀, having a binding constant of 1420 M⁻¹. In probably the best previously-demonstrated evidence of a corannulene concave-face binding to C₆₀, Sygula et al. reported a double concave “buckycatcher” containing two opposing concave corannulene subunits linked via a central motif. In addition to showing a much higher binding constant of ∼8600 M⁻¹ by ¹H-NMR studies in toluene-d₈, these authors obtained an X-ray crystal structure of a 1 : 1 C₆₀ : buckycatcher complex.¹³

Despite the studies described above, no solution-phase or solid-phase complexation of “pristine” or un-modified, corannulene with “pristine” C₆₀¹⁴ has been reported to date. We now report that we have succeeded in obtaining the first X-ray structures of both a stable pristine 1 : 1 corannulene : C₆₀ co-crystal (Fig. 1) and also of a C₆₀ : penta-tert-butylated corannulene, 5 (Fig. 2). Compound 5 was the first alkyl-substituted corannulene derivative for which an X-ray structure was previously determined¹⁵ and it, like corannulene itself, shows no evidence for complexation with C₆₀ in solution studies.

Fig. 1 50% probability ellipsoid representation of the asymmetric unit of C₆₀ : 1, with the minor disorder component of C₆₀ omitted for clarity.

Fig. 2 30% probability ellipsoid representation of C₆₀ : 5, with the minor disorder component of C₆₀ omitted for clarity.

A solution of corannulene containing an approximately equimolar amount of C₆₀ in toluene was allowed to slowly evaporate over a period of several weeks during which time red prisms formed from which a single crystal suitable for X-ray diffraction measurements was obtained. In the case of C₆₀ : 5 suitable crystals for X-ray diffraction measurements could only be obtained from the slow evaporation of a solution containing equimolar amounts of 5 and C₆₀ in 1,2-dichlorobenzene.

For C₆₀ : 1, the C₆₀ molecule was disordered and was modeled with four different orientations. The major component was located completely from difference maps, and subsequent atom positions for smaller components were determined and refined by first locating the highest peaks from difference maps for the next major disorder component, using Monte Carlo methods to calculate the remaining atom positions and then refining these positions with a central dummy atom that acted as the pivot. This was repeated for all orientations. Refinement values are still high,¹⁶ and reflect the fact that the C₆₀ crystallized with many different orientations. An investigation into modeling this as a spherical shell of diffuse electron density is ongoing and will be reported elsewhere. For C₆₀ : 5, the C₆₀ molecule was disordered and modeled with two different orientations that were refined.¹⁷ Two disordered lattice solvent 1,2-dichlorobenzene molecules were present in the asymmetric unit.

The C₆₀ molecules in the structure of C₆₀ : 1 pack in a zigzag manner (coincident with the two-fold screw axes), with centroid–centroid separations of 10.04 Å (close to the van der Waals limit of fullerene, 10.0 Ź⁸) and an angle for three successive C₆₀ molecule centroids of 135.9° (Fig. 3). This packing motif is common for close contacts between C₆₀ molecules in host–guest crystals,^18,19 and, in particular, the 1 : 1 C₆₀:calix[5]arene complex exhibits this packing motif.²⁰ The depth of penetration of the C₆₀ into the corannulene is 6.94 Å, measured from the centroid of the corannulene five-membered ring to the centroid of C₆₀, and 3.75 Å, measured from the shortest distance from the concave surface of 1 to a C₆₀ surface (“A” in Fig. 4). Other short contacts of 6.64 Å and 3.21 Å can be seen between the centroid of a six-membered ring of the convex face of a second corannulene to the centroid of C₆₀, and to the C₆₀ surface, respectively (see “B” in Fig. 4). Each C₆₀ is in close contact (less than 10.3 Ź⁸) with six other C₆₀ molecules. For uncomplexed 1 the average bowl depth at −100 °C is 0.875 Å,¹⁵ compared to 0.89 Å in the current study. This small difference does not indicate any significant change in geometry upon co-crystallization with C₆₀. By way of contrast, an average depth of 0.72 Å was reported for uncomplexed 5¹⁵ whereas 0.59 Å was found for its C₆₀ complex in this study.

Fig. 3 Capped stick representation of the extended packed unit cell (0 ≤ x ≤ 1, −0.5 ≤ y ≤ 1.5, 0 ≤ z ≤ 1) for C₆₀ : 1. Minor disorder components of C₆₀ and H-atoms omitted for clarity.

Fig. 4 30% probability ellipsoid representation of C₆₀ : 1 (minor disorder components of C₆₀ omitted for clarity). Dashed lines indicate: A: concave surface of 1 to C₆₀ short contact 3.75 Å; B: convex surface of 1 (symmetry operator 1.5 − x, −½ + y, ½ − z) to C₆₀ short contact of 3.21 Å; and C: C₆₀ centroid–centroid (symmetry operator ½ − x, ½ + y, ½ − z) contact of 10.04 Å.

The packing motif of C₆₀ : 5 is similar to that of C₆₀ : 1; the C₆₀ molecules pack in a zigzag manner (coincident with the two-fold screw axes), with centroid–centroid separations of 10.16 Å, and an angle of 97.4° for three successive C₆₀ molecule centroids (Fig. 5). By analogy with the close contacts seen in 1 : C₆₀, the depth of penetration of the C₆₀ into 5 is 6.78 Å and 3.30 Å respectively. Multiple methyl C–H (5)–C₆₀ interactions are present, with contacts ranging from 2.78–2.90 Å, while lattice solvent dichlorobenzene molecules also interact with C₆₀via π(solvent)–π(C₆₀) (3.35 Å) and C–Cl(solvent)–C₆₀ (3.02 Å) associations. Finally, unlike the C₆₀ : 1 complex, significant host–host interactions are also present; the host molecules are staggered with respect to their five-membered rings, with centroid–centroid distances of 3.30 Å (Fig. 2). The relatively greater ordering of the “guest” C₆₀ in this structure is consistent with observations made by us previously in which the tetra-tert-butyl-alkylated calix[4]naphthalene²¹ proved to be a better host for C₆₀ than the corresponding unsubstituted calix[4]naphthalene, presumably due to the extra methyl–π interactions that can take place between the host and guest.^22a–c

Fig. 5 Capped stick representation of the extended packed unit cell (0 ≤ x ≤ 1, −0.5 ≤ y ≤ 1.5, 0 ≤ z ≤ 1) for C₆₀ : 5, showing the zigzag motif with respect to C₆₀. Minor disorder components of C₆₀, H-atoms and lattice solvent C₆H₄Cl₂ omitted for clarity.

In general, it would be expected that strong host–guest interactions should lead to reduced disorder for C₆₀. In the structures reported here, C₆₀ was disordered for both C₆₀ : 1 and C₆₀ : 5. This is particularly problematic for C₆₀ : 1, where the fullerene showed many different orientations and would be better described as a spherical shell of electron density. While there is shape complementarity between C₆₀ and corannulene, the disorder we have found in the C₆₀ : 1 structure points to the absence of a strong host–guest interaction for any particular orientation of C₆₀ with respect to the corannulene.

The authors thank NSERC, Memorial University of Newfoundland and the U.S. Dept. of Energy for financial support. The following individuals are acknowledged: Dr Khalil Abboud (U. of Florida) for helpful discussions on the refinement of C₆₀ : 1; Dr Hilary Jenkins (McMaster Analytical X-ray Diffraction Facility) for the data collection for C₆₀ : 5; Dr Christine Beavers (Advanced Light Source, LBL. Berkeley) and Dr Lee Daniels (Rigaku Americas Corporation) for helpful discussions on the solution and refinement of C₆₀ : 5.

Notes and references

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16. Crystal data for C₆₀:1: (C₆₀)·C₂₀H₁₀, M = 970.96, monoclinic, a = 13.0695(10) Å, b = 18.6034(15) Å, c = 17.3068(13) Å, α = 90.00°, β = 110.768(8)°, γ = 90.00°, V = 3934.5(5) ų, T = 163(2) K, space group P2₁/n, Z = 4, μ(MoKα) = 0.094 mm⁻¹, 35 719 reflections measured, 7297 independent reflections (R_int = 0.0537). Final R₁ = 0.1551 (I > 2σ(I)) and wR(F²) = 0.4859 (all data). The goodness of fit on F² was 2.112. C₆₀ was disordered and modeled with four orientations that were refined using a SHELXL SUMP command with total occupancy = 1. SIMU restraints were applied to all C–C bonds in C₆₀. CCDC 864755.
17. Crystal data for C₆₀:5: C₆₀·C₄₀H₅₀·1.5(C₆H₄Cl₂), M = 1472.00, monoclinic, a = 21.975(6) Å, b = 15.264(4) Å, c = 23.000(7) Å, α = 90.00°, β = 117.783(5)°, γ = 90.00°, V = 6825(3) ų, T = 100(2) K, space group P2₁/n, Z = 4, μ(CuKα) = 1.674 mm⁻¹, 35 649 reflections measured, 14 175 independent reflections (R_merge = 0.0989). Final R₁ = 0.1184 (I > 2σ(I)) and wR(F²) = 0.3355 (all data). The goodness of fit on F² was 1.050. The selected crystal was a non-merohedral twin (hklf5). C₆₀ was disordered with two occupancy-refined orientations. SHELXL SADI and SIMU restraints were applied to all C–C bonds in C₆₀. CCDC 864756.
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22. For some other examples see (a) S. Mizyed, M. Ashram, D. O. Miller and P. E. Georghiou, J. Chem. Soc., Perkin Trans. 2, 2001, 1916–1919; (b) A. Hosseini, S. Taylor, G. Accorsi, N. Armaroli, C. A. Reed and P. D. W. Boyd, J. Am. Chem. Soc., 2006, 128, 15903–15913; (c) W. Van Rossom, O. Kundrat, T. H. Ngo, P. Lhotak and W. Dehaen, Tetrahedron Lett., 2010, 51, 2423–2426.

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Footnote

† Electronic supplementary information (ESI) available: Additional crystallographic details and figures. CCDC 864755 (1) and 864756 (5). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2cc30652b

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