From Endohedral Complexes to Endohedral Fullerene Covalent

Article pubs.acs.org/JPCC

From Endohedral Complexes to Endohedral Fullerene Covalent Derivatives: A Density Functional Theory Prognosis of Chemical Transformation of Water Endofullerene H2O@C60 upon Its Compression Denis Sh. Sabirov* Institute of Petrochemistry and Catalysis, Russian Academy of Sciences, 450075 Ufa, Russia W Web-Enhanced Feature * S Supporting Information *

ABSTRACT: Currently, there are no examples of transformation of endohedral C60 complexes, which result in a chemical reaction between the encapsulated molecule and the carbon cage. Such reactions open new opportunities for producing the poorly studied endohedral covalent derivatives of C60, which may be of great interest for fullerene chemistry and material science. In the present work, we have performed accurate theoretical study on the compression of the recently synthesized H2O@C60 endofullerene. Calculations show that inner-cavity chemical reaction C60 + H2O can take place if H2O@C60 is compressed in the direction of two opposite pentagons. In the case of hexagon-to-hexagon compression, there is no chemical reaction, and relaxation of the compressed structure leads to the original H2O@C60. Though the calculated heat effect of the mentioned chemical transformation is endothermic, the product of the reaction is a minimum on potential energy surface.

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INTRODUCTION An attractive idea to obtain covalent endoderivatives of C60 fullerene occurs in the fullerene chemistry from time to time. The endohedral location of some addends has been discussed in the works devoted to highly hydrogenated fullerene derivatives1 and C60 fullerene secondary ozonides;2 the inner reactivity of carbon nanotubes toward diverse reagents (e.g., F• 3 and S34) has been also investigated. Theoretically, if the fullerene skeleton is highly distorted by multiple addends attached, the endohedral location of small atoms (e.g., H or F) may become favorable. The attention to this problem is caused not only by the opportunity to synthesize compounds with unusual structure and properties but also in the aspects of the use of C60 nanocapsules for energy and gases storage. In contrast to covalent fullerene endoderivatives, endohedral complexes of fullerenes (endofullerenes) are well-known. These are unique topological compounds, in which atoms, ions, or clusters are encapsulated into a fullerene skeleton.5 Until recently, endofullerenes were obtained by laser evaporation of graphite in the presence of a gas (or a salt) whose atoms have to be trapped. So, the atoms get inside during the assembly of the cage (examples of introducing small atoms inside C60 under high pressure are scarce). The situation changed with the advent of a synthetic strategy, called molecular surgery.6−9 It consists of chemically making an orifice in C60, filling the formed open-cage fullerene derivative by desired molecules and final chemical restoration of the open carbon skeleton to its pristine state. Obtained in 2011,9 endofullerene H2O@C60 is a masterpiece of molecular surgery. At the moment, it is the only endohedral complex of C60 produced by this strategy and © 2012 American Chemical Society

containing a triatomic molecule inside. It attracts the attention by the size of the encapsulated guest. The water molecule has been isolated from the environment by the C60 cage, which is very important for understanding of H2O properties when it is not connected by hydrogen bonds with its neighbors. In addition, the complex can be considered as an interesting nanoobject, which combines incompatibly: here, the smallest hydrophobic cavity encages a single polar molecule. Endofullerene H2O@C60 is a young compound. Its properties have been studied in several experimental and theoretical works. For example, the interconversion of ortho- and parawater spin isomers in H2O@C60 has been tracked in real time.10 The influence of paramagnetic moieties, added to the C60 cage, on the proton relaxation in fullerene derivatives with encapsulated water allows manipulating the ortho- and paraisomers.11 This complex became an object of two theoretical studies: DFT-based description of its structure, IR spectrum and dipole moment9 and molecular simulation of orientational relaxation of water molecule inside C60.12 Also we should mention a theoretical search13 of the global energy minima of endohedral (H2O)n@C60 and (H2O)n@C180, and exohedral (H2O)n···C180 water−fullerene clusters with n ≤ 20 using basinhopping global optimization. The authors13 have found that the small C60 cavity is able to encapsulate exothermically only one water molecule, whereas for the larger C180 cavity this number grows up to 17 molecules. Received: October 29, 2012 Revised: December 6, 2012 Published: December 10, 2012 1178

dx.doi.org/10.1021/jp310673j | J. Phys. Chem. C 2013, 117, 1178−1182

The Journal of Physical Chemistry C

Article 12

The previously discovered endofullerenes have been the subject of numerous theoretical studies.14 Though it is very difficult to classify works on theoretical chemistry of endofullerenes at the moment, we can identify the main branches of research in this area: e.g., aspects of endofullerenes stability,15,16 the impact of endoatoms on exohedral reactivity,17 calculations of physical characteristics (bulk modulus,18−20 dipole polarizability,21,22 photoionization,23 and nonlinear optical response properties,24 etc.). In addition, a few theoretical works deals with the question whether a chemical bond with carbon cage is possible inside a fullerene. For example, calculations of different motifs of the binding of a hydrogen atom to heterofullerenes C59X (X = B, N, P) by the B3LYP/6-31G* method have shown a preferential formation of the exobonds compared to their endocounterparts.25,26 In the present study, the covalent binding of water to the carbon skeleton upon H2O@C60 endofullerene compression has been investigated theoretically. Molecular simulation using the modern DFT method with Perdew−Burke−Ernzerhof (PBE) functional has shown that the pressure-induced inner reactivity of such complexes may vary depending on the direction of the carbon cage compression.

V=

20

i=1 j=1

4

i=1 j=1

(2)

where vij denotes the volumes of triangular pyramids, which are calculated by coordinates of the four constituent atoms:

vij =

x1 − xO y1 − yO z1 − zO 1 x −x y −y z −z 2 O 2 O 2 O 6 x3 − xO y3 − yO z 3 − zO

(3)

Calculated as described above, the volume of intact C60 fullerene (163.0 Å3) is in good agreement with the previous estimation38 (160.5 Å3). Compression Simulations. Compressions have been simulated by combination of two potential energy surfaces (PES). PES-1 corresponds to the approaching of the selected polygons to each other (two pentagons or two hexagons). When curvature indices of atoms of these polygons become zero (i.e., they lie in the same plane with the neighboring atoms), the further compression involves also the neighboring atoms (5 for PP- and 6 for HH-compression) and is described by PES-2. The displacement ΔL has been chosen as a reaction coordinate for the process:

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COMPUTATIONAL DETAILS DFT Method. The potential energy surface scanning and all optimizations have been performed by the density functional theory method PBE/3ζ27,28 and Broyden−Fletcher−Goldfarb− Shanno optimization algorithm implemented in the Priroda program.29 The 3ζ basis set describes electronic configurations of molecular systems by the orbital basis sets of contracted Gaussian-type functions (5s,1p)/[3s,1p] for H, (11s,6p,2d)/ [6s,3p,2d] for C and O, which have been used in combination with the density-fitting basis sets of uncontracted Gaussian-type functions (5s,2p) for H, and (10s,3p,3d,1f) for C and O atoms. The PBE/3ζ method reproduces structures and physicochemical characteristics of fullerenes and their derivatives with high accuracy,30−35 especially the measured IR spectra of C6030,32 and C70.32 Calculation of Auxiliary Geometric Parameters. Cartesian coordinates of the structures have been used for calculation of auxiliary geometric parameters − curvature index (k) and nuclear volume (V). Curvature index for each atom has been found as k = 2 sin θP/a

3

∑ ∑ vij(P) + ∑ ∑ vij(H)

ΔL = L0 − L

(4)

where L denotes the distance between the approaching polygons at each step of the PES scanning, L0 is a respective initial distance (6.715 Å for pentagon−pentagon and 6.520 Å for hexagon−hexagon). The heat effect of the reaction 7 has been calculated as the difference between the total energies E of the final C60OH2 and the starting H2O@C60 with inclusion of zero-point vibrational energy corrections εZPV and the temperature corrections Hcorr (T = 298 K): ΔHr° = (Etot + εZPV + Hcorr)C60OH2 − (Etot + εZPV + Hcorr)H2O@C60

(5)

Calculation of Polarizability. Components of polarizability tensors have been calculated in terms of the finite field approach as the second-order derivatives of the total energy E with respect to the homogeneous external electric field. They have been calculated in the arbitrary coordinate system and then diagonalized. Eigenvalues of polarizability tensor αxx, αyy, and αzz have been used for the calculation of the mean polarizabilities of the molecules: 1 α = (αxx + αyy + αzz) (6) 3

(1)

where a is the average distance between an atom and its three neighbors and θP is a pyramidality angle. Our algorithm for calculation of curvature indices has been tested and expounded earlier.36,37 Then, for each ion, we find O is the center of mass (it is always inside the cage) and divide the polyhedron (initial or compressed C60) on 32 disjoint simplexes by drawing a vector from O in the direction of each vertex. The nuclear framework of C60 consists of 12 pentagons and 20 hexagons. For convenience, we consider the simplexes, constructed on the pentagons (P) and hexagons (H), separately (in general, they are nonplanar). Then we triangulate the geometrical bodies, obtained by the first partition: each P and H is divided by three and four triangular pyramids, respectively. The general formula for calculation of volume is:

The PBE/3ζ-based finite field approach allows reproducing the measured mean polarizabilitites of C60 and C70 fullerenes.30,32,35

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RESULTS AND DISCUSSION Two variants of H2O@C60 compression have been considered: in the direction of opposite pentagons (PP-compression) and in the direction of opposite hexagons (HH-compression). Energy profiles demonstrate that the total energy (E) increases with the selected polygons approaching each other in both cases (Figure 1). The analogous plots have been obtained for X@C60 (X = Ne, Ar, Kr) compression.19,20 Those works have estimated maximal energies the structure being able to support 1179



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Figure 2. Evolution of H2O@C60 structure under HH (clockwise) and PP compression (counterclockwise). Black arrows indicate compression; red ones correspond to the relaxation of the compressed structures. Animations of the relaxed optimizations of the most deformed D structure and G structure are available.

the original, its relaxed optimization results in the pristine water endofullerene (marked as A in Figures 1 and 2). Another situation is observed for the highly PP-compressed structure D. When the distance between the displaced pentagons equals 2.80 Å, the crushed water molecule binds to the carbon cage another way: the oxygen atom forms an epoxide cycle with a 5.6 bond (the carbon−carbon bond is not cleaved), and hydrogen atoms bind to the carbon atoms of the opposite part of the cage (Figure 2). Surprisingly, in contrast to the previous case, the relaxed optimization of this structure leads to the endohedral fullerene derivative (Figure 2, Drelaxed structure), in which O and H atoms remain attached to the carbon cage (though upon the optimization, one of the H atoms migrates from its initial position). The described C60OH2 structure is quite unusual but nevertheless, according to the vibration modes solving, it is a minimum on the potential energy surface (its Hessian contains no imaginary frequencies; the lowest frequency is 222.8 cm−1). So, the pressure-induced transformation of H2O@C60 takes place:

Figure 1. Dependences of nuclear volume and total energy on the degree of H2O@C60 compression: (a) for PP compression; (b) for HH compression.

by B3PW91/6-31G method. These energies for Ar@C60 and Kr@C60 (5205.8 and 4784.9 kJ mol−1, respectively20) are comparable with critical energies for PP and HH compression of H2O@C60, which equal 5729.2 and 5209.2 kJ mol−1. Figure 2 demonstrates several structures of the compressed H2O@C60 endofullerene: B, C, D for PP compression and E, F, G for HH compression. Upon the selected polygons approaching, the nuclear volume of C60 cage decreases from 163.2 Å3 for intact H2O@C60 to ∼130.0 Å3 for D and G (Figure 1). The relaxed optimizations of B, C, E, and F structures, as expected, result in the initial endofullerene (A). The highly deformed D and G structures do not remain the topology of the pristine C60 framework. In the case of the HHcompressed G structure, the distance between the displaced hexagons is 2.83 Å, and the crushed water molecule binds to carbon surface inside. The oxygen atom breaks a 5.6 carbon− carbon bond, forming the oxidoannulene structure with a −O− bridge, while hydrogen atoms add to different parts of the fullerene cage: one is attached in the hexagon with the oxidoannulene moiety and another is attached to the opposite part of the cage. Despite the fact that this structure is far from

H 2O@C60 → C60OH 2

(7)

As expected, reaction 7 is very endothermic: its heat effect equals to +926.1 kJ mol−1. In terms of transition state theory, the path from the initial endohedral complex to covalent endoderivative via reaction 7 may occur through several transition states, which are needed for estimation of activation energy of the process. However, the effective activation energy of the direct and inverse processes can be estimated roughly as eff Eact = Etot,D − Etot,H2O@C60

(8)

eff Eact,reverse = Etot,D − Etot ,C60OH2

(9)

which are the differences in total energies of the PP-compressed initial (D) and relaxed structures (C60OH2) and original endofullerene structure (A) (here, we did not take into account zero-point vibration energies). According to eqs 8 and 9, the activation energy of reaction 7 is extremely high and equals 1180



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5729.2 kJ mol−1; the activation energy of the reverse process is 4776.39 kJ mol−1; i.e., two local minima, corresponding to H2O@C60 (A) and C60OH2 (Drelaxed), are separated by a very high barrier. It may be interpreted as follows: transformation (7), occurred once and will not happen in the reverse direction due to the comparably high barrier. Thus, the transformation of endohedral complex to endohedral fullerene covalent derivative seems surreal. However, we should note that the present calculations correspond to the compounds in ground states. As shown previously,39 high pressures can induce the transition of C60 fullerene to its excited states. Another way to perform thermodynamically unfavorable transformations is the use of high temperatures and external electric fields. The possible ways to covalent fullerene endoderivatives need a specific study, and here we report a principal possibility of transformation H2O@C60 → C60OH2, induced by compression. To elucidate the ways of monitoring such chemical reactions, we have studied some properties of the C60OH2 compound. The designations of the addition patterns according to IUPAC nomenclature for fullerene derivatives40 leads to the name 1(endo),2(endo)-epoxy-43(endo),60(endo)-dihydro(C 60-I h)[5.6]fullerene. Its structure has a C1 symmetry and is distorted in comparison to C60 and H2O@C60. It is clearly demonstrated with local curvature indices k of the carbon atoms. In the fullerene and its water endohedral complex, all the carbon atoms have k = 0.28 Å−1. The product of reaction 7 is characterized with more spread k values (Figure 3): the endohedrally functionalized atoms have almost zero curvature, whereas k indices of their neighbors are larger than the average curvature of carbon surface (0.29−0.39 Å−1 against the average 0.28 Å−1). In addition, the volume of the endoderivative is slightly higher than C60 and H2O@C60 volumes (Table 1). The calculated IR spectrum of C60OH2 (Figure 3) significantly differs from that of H2O@C60: it is stronger in the 500−1600 cm−1 region and contains new bands, e.g., symmetric and asymmetric stretching modes of the C−H bond (2230 and 2406 cm−1) (Figure 4). We point particular attention to polar characteristics of C60OH2 because earlier9 the different polarities of C60 and H2O@C60 allowed their efficient separation by high performance liquid chromatography. According to PBE/3ζ calculations, the dipole moments of H2O@C60 and C60OH2 do not differ significantly, but mean polarizabilities are not the same (Table 1). The complex H2O@C60 has a mean polarizability close to that of α(C60); i.e., the depression of polarizability takes place. Estimated as

Figure 3. Structure of 1(endo),2(endo)-epoxy-43(endo),60(endo)dihydro(C60-Ih)[5.6]fullerene and local curvatures of endohedrally functionalized carbon atoms and their nearest neighbors.

Table 1. Calculated Nuclear Volumes (V), Dipole Moments (μ), and Mean Polarizabilities (α) of the Molecules under Studya) molecule C60 H2O H2O@C60 C60OH2

μ, D 41

0.0 (0.0 ) 2.0 (1.86, gas phase42) 0.45 0.39

α, Å3

V, Å3

82.7 (76.4 ± 8.0 ) 1.08 (1.4643) 82.8 87.9 41

163.0 163.2 164.3

a

The corresponding experimental values are given in parentheses if available.

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Δα(H 2O@C60) = α(H 2O@C60) − α(C60) − α(H 2O) (9)

CONCLUSION In this work, using DFT theory, we have studied two variants of H2O@C60 endofullerene compression to find out the principal possibility of the chemical reaction between the trapped polar molecule and the nonpolar cage. As calculations show, such a process is possible only in the case of compression in the direction of two opposite pentagons and leads to an endohedral covalent fullerene derivative. The activation energies of both direct and inverse processes are extremely high. On the one hand, it means that transformation “endohedral comlex → endohedral derivative” is difficult for the implementation. On the other hand, if it has been implemented, endoderivative C60OH2 does not turn back to the initial substance. Obviously,

it equals to 1 Å3. So, Δα(H2O@C60) ≈ α(H2O), which can be interpreted as a sheltering water molecule by the carbon cage from external electric fields. The calculated H2O@C60 polarizability is in agreement with the previous consideration of C60 fullerene as a perfect Faraday cage.44 The same depression of polarizability Δα is observed for endofullerenes with noble gas atoms.21,22 When H2O@C60 is transformed to C60OH2, the mean polarizability of a molecular system increases to 87.9 Å3. Thus, IR spectra can be informative to monitor reaction 7, whereas the different electric field responses, caused by unequal polarizabilities of these compounds, can be useful for their separation. 1181



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R.; Lei, X.; Levitt, M. H.; Li, Y.; Mamone, S.; Murata, Y.; Nagel, U.; Nishida, T.; Ollivier, J.; Rols, S.; Rõõm, T.; Sarkar, R.; Turro, N. J.; Yang, Y. Proc. Natl. Acad. Sci. 2012, 109, 12894−12898. (11) Li, Y.; Chen, J. Y.-C.; Lei, X.; Lawler, R. G.; Murata, Y.; Komatsu, K.; Turro, N. J. J. Phys. Chem. Lett. 2012, 3, 1165−1168. (12) Bucher, D. Chem. Phys. Lett. 2012, 534, 38−42. (13) Hernández-Rojas, J.; Monteseguro, V.; Bretón, J.; Gomez Llorente, J. M. Chem. Phys. 2012, 399, 240−244. (14) Osuna, S.; Swart, M.; Solà, M. Phys. Chem. Chem. Phys. 2011, 13, 3585−3603. (15) Cioslowski, J.; Fleischmann, E. D. J. Chem. Phys. 1991, 94, 3730−3734. (16) Levin, A. A.; Breslavskaya, N. N. Russ. Chem. Bull., Int. Ed. 2005, 54, 1999−2002. (17) Osuna, S.; Swart, M.; Solà, M. Chem.Eur. J. 2009, 15, 13111− 12123. (18) Kang, J. W.; Hwang, H. J. Nanotechnology 2004, 15, 614−621. (19) Wang, Z. Y.; Su, K. H.; Fan, H. Q.; Li, Y. L.; Wen, Z. Y. Mol. Phys. 2008, 106, 703−716. (20) Wang, Z. Y.; Su, K. H.; Yao, X.-P.; Li, Y.-L.; Wang, F. Mater. Chem. Phys. 2010, 119, 406−417. (21) Yan, H.; Yu, Sh.; Wang, X.; He, Y.; Huang, W.; Yang, M. Chem. Phys. Lett. 2008, 456, 223−226. (22) Sabirov, D. Sh.; Bulgakov, R. G. JETP Lett. 2010, 92, 662−665. (23) Ludlow, J. A.; Lee, T.-G.; Pindzola, M. S. J. Phys. B: At. Mol. Opt. Phys. 2010, 43, 235202. (24) Ma, F.; Li, Z.-R.; Zhou, Z.-J.; Wu, D.; Li, Y.; Wang, Y.-F.; Li, Z.Sh. J. Phys. Chem C 2010, 114, 11242−11247. (25) Buchachenko, A. L.; Breslavskaya, N. N. Russ. Chem. Bull., Int. Ed. 2005, 54, 5154. (26) Buchachenko, A. L.; Breslavskaya, N. N. Russ. Chem. Bull., Int. Ed. 2007, 56, 1283−1288. (27) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868. (28) Laikov D. N. Ph.D. Dissertation, Moscow State University, 2000. (29) Laikov, D. N.; Ustynyuk, Yu. A. Russ. Chem. Bull., Int. Ed. 2005, 54, 820−826. (30) Sabirov, D. Sh.; Khursan, S. L.; Bulgakov, R. G. J. Mol. Graph. Model. 2008, 27, 124−130. (31) Shestakov, A. F. Russ. J. Gen. Chem. 2008, 78, 811−821. (32) Sabirov, D. Sh.; Bulgakov, R. G. Chem. Phys. Lett. 2011, 506, 52−56. (33) Tulyabaev, A. R.; Khalilov, L. M. Comput. Theor. Chem. 2011, 976, 12−18. (34) Pankratyev, E. Yu.; Tulyabaev, A. R.; Khalilov, L. M. J. Comput. Chem. 2011, 32, 1993−1997. (35) Sabirov, D. Sh.; Garipova, R. R.; Bulgakov, R. G. Chem. Phys. Lett. 2012, 523, 92−97. (36) Sabirov, D. Sh.; Khursan, S. L.; Bulgakov, R. G. Russ. Chem. Bull., Int. Ed. 2008, 57, 2520−2525. (37) Sabirov, D. Sh.; Bulgakov, R. G. Comput. Theor. Chem. 2011, 963, 185−190. (38) Adams, G. B.; O’Keeffe, M.; Ruoff, R. S. J. Phys. Chem. 1994, 98, 9465−9469. (39) Khavryutchenko, A. V.; Khavryutchenko, V. D. Z. Naturforsch. 2005, 60a, 41−46. (40) Nomenclature and Terminology of Fullerenes: A Preliminary Survey. Pure Appl. Chem. 1997, 69, 1411−1434. (41) Compagnon, I.; Antoine, R.; Broyer, M.; Dugourd, Ph.; Lermé, J.; Rayane, D. Phys. Rev. A 2001, 64, 025201. (42) Badyal, Y. S.; Saboungi, M.-L.; Price, D. L.; Shastri, S. D.; Haeffner, D. R.; Soper, A. K. J. Chem. Phys. 2000, 112, 9206−9208. (43) Vereshchagin, A. N. Polarizability of Molecules; Nauka: USSR, 1980 (in Russian). (44) Delaney, P.; Greer, J. C. Appl. Phys. Lett. 2004, 84, 431−433.

Figure 4. Calculated IR spectra for H2O@C60 endofullerene (spectrum shifted along the ordinate) and the product of its pressure-induced reaction 7.

the analogous situation takes place in the case of the endofullerenes with another reactive filling inside. To prove this proposition, we will perform the further studies.

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ASSOCIATED CONTENT

S Supporting Information *

Cartesian coordinates of A−G and Drelaxed structures; curvature indices for H2O@C60 and C60OH2; output file with Hessian calculation of C60OH2. This material is available free of charge via the Internet at http://pubs.acs.org. W Web-Enhanced Feature *

Animation of the relaxed optimizations of the most deformed D and G structures.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author is grateful to Dmitry Rudakov (Bashkir State University) for his assistance at all stages of the work. The work was supported by the Presidium of Russian Academy of Sciences (Program No. 24 “Foundations of Basic Research of Nanotechnologies and Nanomaterials”).

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REFERENCES

(1) Goldshleger, N. F.; Moravsky, A. P. Russ. Chem. Rev. 1997, 66, 323−342. (2) Bulgakov, R. G.; Nevyadovsky, E. Yu.; Ponomareva, Yu. G.; Sabirov, D. Sh.; Razumovsky, S. D. Russ. Chem. Bull., Int. Ed. 2006, 55, 1372−1379. (3) Chen, Zh.; Thiel, W.; Hirsch, A. Chem. Phys. Chem. 2003, 4, 93− 97. (4) Castillo, Á .; Lee, L.; Greer, A. J. Phys. Org. Chem. 2012, 25, 42− 49. (5) Yang, S.; Dunsch, L. Endohedral Fullerenes; John Wiley & Sons, Ltd.: New York, 2006. (6) Murata, M.; Murata, Y.; Komatsu, K. Chem. Commun. 2008, 6083−6094. (7) Whitener, K. E.; Frunzi, M.; Iwamatsu, S.; Murata, S.; Cross, R. J.; Saunders, M. J. Am. Chem. Soc. 2008, 130, 13996−13999. (8) Stanisky, C. M.; Cross, R. J.; Saunders, M. J. Am. Chem. Soc. 2009, 131, 3392−3395. (9) Kurotobi, K.; Murata, Y. Science 2011, 333, 613−616. (10) Beduz, C.; Carravetta, M.; Chen, J. Y.-C.; Concistrè, M.; Denning, M.; Frunzi, M.; Horsewill, A. J.; Johannessen, O. G.; Lawler, 1182


From Endohedral Complexes to Endohedral Fullerene Covalent

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