Letter pubs.acs.org/OrgLett
Entropy-Driven Ball-in-Bowl Assembly of Fullerene and Geodesic Phenylene Bowl Koki Ikemoto,†,§ Ryo Kobayashi,‡ Sota Sato,†,§ and Hiroyuki Isobe*,†,‡,§ †
Department of Chemistry, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan Advanced Institute for Materials Research, Tohoku University, Aoba-ku, Sendai 980-8577, Japan § JST, ERATO Isobe Degenerate π-Integration Project, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan ‡
S Supporting Information *
ABSTRACT: Complexation of C60 at a conical region of a nanometer-sized geodesic phenylene bowl has been demonstrated. Proton NMR spectroscopy showed formation of a 1:1 complex that was driven by entropy gains for the assembly. Crystallographic analyses revealed its unique ball-in-bowl structure, and the presence of smoothly curved surfaces was unveiled at their interfaces.
S
upramolecular assembly of carbonaceous entities has attracted much interest in the growing field of nanocarbons. Combinations of nanocarbon entities with fullerenes are of particular interest because of their concave−convex molecular recognition as well as their unique carbon-rich architectures.1 The concave shape at conical regions of curved nanocarbons accommodates fullerene molecules, which has ubiquitously been observed by transmission electron microscopy.2−4 The conical regions are also chemically reactive sites for creation of openings5 but, despite such potentially defective structures, are favorable sites for fullerene encapsulation (Figure 1). However, supramolecular chemistry at the conical yet defective graphitic carbon is still in its infancy because appropriate molecular entities that can serve as their segmental models are scarcely available. We recently designed and synthesized a nanometer-sized bowl (1) comprising 20 phenylene units.6 Its sp2-carbon networks are not fully completed for a perfect graphitic sheet, but the geodesic constraints from a pentagonal phenylene array at the center renders the molecule in its bowl shape. We envisioned that the molecule might serve as an ideal model of defective, hydrogenterminated nanocarbons with conical shapes to investigate its supramolecular assembly with fullerenes. In this study, we disclosed association behaviors of the bowl-shaped molecule 1 with a spherical [60]fullerene (C60) molecule.7 The solutionphase association was investigated by NMR analyses to reveal its fundamental thermodynamics. The association was predominantly driven by entropy gains for the assembly, which might give insight toward understanding the nanocarbon assembly. Molecular structures of the complex were investigated by crystallographic analyses to show interesting structural features of the ball-in-bowl assembly. We first noted changes in proton resonances of 1 in NMR spectra upon addition of C60 and systematically investigated the changes in the chemical shifts. Thus, we prepared a mixture of 1 © 2017 American Chemical Society
Figure 1. Fullerene accommodations at defective conical nanocarbons. (a) A bowl-shaped, conical region of nanocarbons accommodates fullerene molecules.2 (b) Ball-in-bowl assembly between fullerene and a geodesic phenylene bowl (1).
and C60 at a total concentration of 1.0 × 10−4 M in chloroformd and measured 14 proton NMR spectra by varying ratios of each component.8 As shown in Figure 2a for the measurement at 298 K, all the resonances shifted gradually toward an upfield region upon increasing the ratio of C60. The gradual change showed that the exchange processes were fast on the NMR time scale, and consequently, time-averaged resonances of free and bound species were observed. The chemical-shift changes Received: March 27, 2017 Published: April 14, 2017 2362
DOI: 10.1021/acs.orglett.7b00899 Org. Lett. 2017, 19, 2362−2365
Letter
Organic Letters
Figure 2. Spectroscopic analyses for the association of 1⊃C60 in chloroform-d at a total concentration of 1.0 × 10−4 M. (a) Proton NMR spectra from a mixture of different ratios of C60 and 1. (b) Representative binding isotherm at 298 K.9 (c) A 1/T−ln Ka plot for the van’t Hoff analysis.
of seven proton resonances were then averaged to obtain Δδ values for each spectrum to elucidate the thermodynamics for the association. Analysis of Δδ values in a form of the Job plot revealed the stoichiometry of the complex and confirmed the formation of a 1:1 complex, 1⊃C60 (Figure S1). The binding isotherm was then analyzed by a curve-fitting method with a quadratic equation7,9 to afford the association constant (Ka) from triplicate experiments as (3.4 ± 0.1) × 104 M−1 at 298 K (Figure 2b). This association constant was >10-fold larger than those of small bowl-shaped molecules such as corannulene derivatives10 and was comparable to those of divalent bowlshaped receptors such as buckycatcher.7,11 We then analyzed temperature-dependence of the Ka values to elucidate further details of the association thermodynamics. The association behaviors for six data points in a temperature range of 283−323 K were respectively investigated (Figures S2−S6), and the 1/T−ln Ka plot was obtained (Figure 2c). The van’t Hoff analysis of the plot revealed the thermodynamics for the association as ΔG = −6.1 ± 0.2 kcal/mol, ΔH = −2.0 ± 0.1 kcal/mol, and − TΔS = −4.1 ± 0.1 kcal/mol (298 K; ΔS = +13.9 ± 0.4 cal/mol·K). The result shows that the association of the ball-in-bowl assembly of 1⊃C60 was predominantly driven by the entropy gain. To the best of our knowledge, the entropy-driven nature of ball-in-bowl assembly of carbon-rich entities has not been disclosed,7 and further experimental and theoretical investigations on related systems may deepen our understanding of dispersion forces at the curved interfaces.12,13 Entropy-driven associations of other nanometer-sized concaveconvex assembly such as tube-sphere14 and bowl-in-bowl6 combinations indicated that the present entropy gains might originate from desolvation of crystalline solvates of the curved carbon-rich components.15 A similar entropy-driven association was found with bowl-in-bowl dimerization to form 12, but its − TΔS value (−12.0 kcal/mol) was larger than the present one (−4.1 kcal/mol), which might be ascribed to larger contacting
areas for the dimerization. Solvent effects on the association thermodynamics are of interest12,14 and will be investigated in the future. Finally, we obtained a single crystal of 1⊃C60, and diffraction studies with synchrotron X-ray revealed precise structures of the complex. As shown in Figure 3a, the fullerene molecule was expectedly located at the concave area of the bowl-shaped molecule. Two different orientations of C60 were observed, and these orientations possessed their center of gravity around an identical position. This observation may indicate the presence of rotational freedom at the complex state, which should be beneficial to reduce the entropy cost for the association.12,14 Analyses of the crystal structures with the Hirshfeld surfaces16−18 revealed key features of the contacting surfaces of each component.19 The Hirshfeld surface of 1 showed that the concave area accommodated the fullerene molecule (Figure 3b). Its de color mapping clarifying the distance from the surface to the external atoms showed the close contacts (green dots) at the center of the bowl. The contacts were predominantly present at the pentagonal ring of the bowl, and the outer hexagonal rings possessed fewer contacts. Curvedness color mappings of 1 showed the absence of inflection points (yellow-green areas) except at the hydrogenterminated opening sites of the macrocycles. The curvedness mapping of C60 for a bottom view shows the corresponding contact surface to support the absence of inflection lines and, consequently, confirmed the presence of smoothly curved surfaces at the interface.20 In contrast, curvedness color mappings on the C60 surface for a top view showed the presence of inflection lines, which were created by three adjacent bowl-shaped molecules above this molecule (see Figure S7 for the crystal packing). In summary, ball-in-bowl assembly of C60 and a geodesic phenylene bowl (1) was studied. Solution-phase analyses revealed the 1:1 complex formation as well as the presence of 2363
DOI: 10.1021/acs.orglett.7b00899 Org. Lett. 2017, 19, 2362−2365
Organic Letters
■
Letter
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b00899. Experimental details, variable temperature NMR analyses, and single-X-ray crystallographic data (PDF) X-ray data for 1⊃C60 (CIF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Sota Sato: 0000-0002-7395-2112 Hiroyuki Isobe: 0000-0001-8907-0694 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This study was partly supported by JST ERATO (JPMJER1301) KAKENHI (24241036, 25102007, 16K04864, 17K05772). We thank Dr. M. Oinuma, Dr. S. Takahashi, and Ms. C. Yang (ERATO) for their synthetic work and KEK PF (Research 2015G097) for the use of the X-ray diffraction instruments.
■
REFERENCES
(1) (a) Iijima, S. Phys. B 2002, 323, 1−5. (b) Monthioux, M. Carbon 2002, 40, 1809−1823. (c) Krive, I. V.; Shekhter, R. I.; Jonson, M. Low Temp. Phys. 2006, 32, 887−905. (2) For instance, see Figure 3 of ref 3a. (3) (a) Ajima, K.; Yudasaka, M.; Suenaga, K.; Kasuya, D.; Azami, T.; Iijima, S. Adv. Mater. 2004, 16, 397−401. (b) Yuge, R.; Yudasaka, M.; Miyawaki, J.; Kubo, Y.; Ichihashi, T.; Imai, H.; Nakamura, E.; Isobe, H.; Yorimitsu, H.; Iijima, S. J. Phys. Chem. B 2005, 109, 17861−17867. (c) Yuge, R.; Yudasaka, M.; Miyawaki, J.; Kubo, Y.; Isobe, H.; Yorimitsu, H.; Nakamura, E.; Iijima, S. J. Phys. Chem. C 2007, 111, 7348−7351. (4) (a) Zhu, S.; Xu, G. Nanoscale 2010, 2, 2538−2549. (b) Karousis, N.; Suarez-Martinez, I.; Ewels, C. P.; Tagmatarchis, N. Chem. Rev. 2016, 116, 4850−4883. (5) (a) Bekyarova, E.; Kaneko, K.; Yudasaka, M.; Kasuya, D.; Iijima, S.; Huidobro, A.; Rodriguez-Reinoso, F. J. Phys. Chem. B 2003, 107, 4479−4484. (b) Ajima, K.; Yudasaka, M.; Maigné, A.; Miyawaki, J.; Iijima, S. J. Phys. Chem. B 2006, 110, 5773−5778. (6) Ikemoto, K.; Kobayashi, R.; Sato, S.; Isobe, H. Angew. Chem., Int. Ed. 2017, DOI: 10.1002/anie.201702063. (7) (a) Matsuno, T.; Sato, S.; Isobe, H. Curved π-Receptors. In Comprehensive Supramolecular Chemistry II; Atwood, J., Ed.; Elsevier, 2017, in press. (b) Yu, C.; Jin, Y.; Zhang, W. Chem. Rec. 2015, 15, 97− 106. (c) Canevet, D.; Pérez, E. M.; Martin, N. Angew. Chem., Int. Ed. 2011, 50, 9248−9259. (8) The bowl forms a dimer in solution, but the dimeric species was absent under highly diluted conditions around 10−4 M due to its small association constant (K ∼ 5.5 × 10 M−1; ref 6). (9) Thordarson, P. Chem. Soc. Rev. 2011, 40, 1305−1323. (10) (a) Mizyed, S.; Georghiou, P. E.; Bancu, M.; Cuadra, B.; Rai, A. K.; Cheng, P.; Scott, L. T. J. Am. Chem. Soc. 2001, 123, 12770−12774. (b) Georghiou, P. E.; Tran, A. H.; Mizyed, S.; Bancu, M.; Scott, L. T. J. Org. Chem. 2005, 70, 6158−6163. (11) (a) Sygula, A.; Fronczek, F. R.; Sygula, R.; Rabideau, P. W.; Olmstead, M. M. J. Am. Chem. Soc. 2007, 129, 3842−3843. (b) Yanney,
Figure 3. Crystallographic analyses of 1⊃C60. (a) Crystal structures. (b) Hirshfeld surfaces.
tight association with the association constant on the order of 104 M−1. The results show that defective yet curved graphitic sheets may tightly capture fullerene guests through concave− convex molecular recognition despite the presence of defects. Interestingly, the association was mainly driven by the entropy gains, which may be a common characteristic feature for supramolecular assembly with curved carbon-rich interfaces.6,14 For such unique supramolecular assembly, its large contacting area as well as its shape matching at the concave-convex interface should play a crucial role to enable the tight association. Examination of other fullerene guests is of interest to deepen our understanding in the shape recognition with the geodestic bowl7,14 and will be investigated in due course. Crystallographic analyses revealed the molecular structures with smoothly curved surfaces at the interface. Dynamic motions of this ball-in-bowl assembly are thus of particular interest and will be studied in due course.14,19 2364
DOI: 10.1021/acs.orglett.7b00899 Org. Lett. 2017, 19, 2362−2365
Letter
Organic Letters M.; Fronczek, F. R.; Sygula, A. Angew. Chem., Int. Ed. 2015, 54, 11153−11156. (12) (a) Isobe, H.; Nakamura, K.; Hitosugi, S.; Sato, S.; Tokoyama, H.; Yamakado, H.; Ohno, K.; Kono, H. Chem. Sci. 2015, 6, 2746− 2753. (b) Isobe, H.; Nakamura, K.; Hitosugi, S.; Sato, S.; Tokoyama, H.; Yamakado, H.; Ohno, K.; Kono, H. Chem. Sci. 2016, 7, 2929− 2932. (13) (a) Grimme, S. Chem. - Eur. J. 2012, 18, 9955−9964. (b) Ambrosetti, A.; Alfé, D.; DiStasio, R. A., Jr.; Tkatchenko, A. J. Phys. Chem. Lett. 2014, 5, 849−855. (c) Antony, J.; Sure, R.; Grimme, S. Chem. Commun. 2015, 51, 1764−1774. (14) (a) Isobe, H.; Hitosugi, S.; Yamasaki, T.; Iizuka, R. Chem. Sci. 2013, 4, 1293−1297. (b) Hitosugi, S.; Iizuka, R.; Yamasaki, T.; Zhang, R.; Murata, Y.; Isobe, H. Org. Lett. 2013, 15, 3199−3201. (c) Matsuno, T.; Sato, S.; Iizuka, R.; Isobe, H. Chem. Sci. 2015, 6, 909−916. (d) Matsuno, T.; Sato, S.; Yokoyama, A.; Kamata, S.; Isobe, H. Angew. Chem., Int. Ed. 2016, 55, 15339−15343. (15) Marcus, Y.; Smith, A. L.; Korobov, M. V.; Mirakyan, A. L.; Avramenko, N. V.; Stukalin, E. B. J. Phys. Chem. B 2001, 105, 2499− 2506. (16) (a) Wolff, S. K.; Grimwood, D. J.; McKinnon, J. J.; Turner, M. J.; Jayatilaka, D.; Spackman, M. A. CrystalExplorer, ver. 3.1; University of Western Australia, 2012. (b) McKinnon, J. J.; Spackman, M. A.; Mitchell, A. S. Acta Crystallogr., Sect. B: Struct. Sci. 2004, 60, 627−668. (c) Spackman, M. A.; Jayatilaka, D. CrystEngComm 2009, 11, 19−32. (d) Makha, M.; McKinnon, J. J.; Sobolev, A. N.; Spackman, M. A.; Raston, C. L. Chem. - Eur. J. 2007, 13, 3907−3912. (17) Residual electron densities from disordered solvent molecules were removed from the crystal structure via the SQUEEZE method. As a result, the Hirshfeld surfaces were deformed at edge areas having contacts with these molecules (see also ref 19). (18) van der Sluis, P.; Spek, A. L. Acta Crystallogr., Sect. A: Found. Crystallogr. 1990, 46, 194−201. (19) Sato, S.; Yamasaki, T.; Isobe, H. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 8374−8379. (20) Inflection lines were observed in cyclo-para-phenylene with a polygonal shape at its biaryl linkages between phenylene units (refs 14 and 19).
2365
DOI: 10.1021/acs.orglett.7b00899 Org. Lett. 2017, 19, 2362−2365
