Communication pubs.acs.org/IC
Unconventional Electronic Structure and Chlorination/ Dechlorination Mechanisms of #1911C64 Fullerene Jing-Shuang Dang,† Wei-Wei Wang,‡,† Xiang Zhao,*,† and Shigeru Nagase‡ †
Institute for Chemical Physics & Department of Chemistry, School of Science, Xi’an Jiaotong University, Xi’an 710049, China Fukui Institute for Fundamental Chemistry, Kyoto University, Kyoto 606-8103, Japan
‡
S Supporting Information *
ABSTRACT: We report a computational work on the electronic structure and derivatization of #1911C64. By means of computations based on density functional theory, we find that #1911C64 in states of closed-shell singlet (CS), open-shell singlet (OS), and triplet are iso-energetic with an energy difference less than 0.1 kcal mol−1. The regioselective chlorine additions on CS, OS, and triplet C64 are studied, and the formation of experimentally observed C64Cl4 and C64Cl8 have been successfully elucidated for the first time. In addition, the dechlorination processes of formed chlorofullerenes are also explored. In contrast to the radical Cl addition, the reverse reaction is a themolysis process, and the decomposition sequence is proved to be simply determined by the C−Cl bond length.
Figure 1. Schlegel diagram of #1911C64.
T
he isolated pentagon rule (IPR) has thus far served as a fundamental criterion in predicting the stability of fullerene molecules.1 In a stable cage-like structure (such as C60 or C70),1,2 the 12 five-membered rings are separated by hexagons to avoid the local strain caused by adjacent pentagons. Instead, the pristine fullerene cages with pentagon pairs (so-called as non-IPR fullerenes) were proved to be unstable in theory and difficult to capture in experiments. However, since the synthesis of non-IPR C50Cl10 by Xie in 2004,3 exohedral chlorination was introduced as an effective approach to obtain the non-IPR fullerene isomers. Molecular stabilization of non-IPR fullerenes can be achieved by sp3 hybridization of the carbons on the fused pentagonal fragments at elevated temperatures. Using this method, various fullerene chlorides in different sizes have been yielded.4−17 Among those synthesized fullerene chlorides, derivatization of C64 is a special case. In 2008 and 2012, Xie et al. captured two different C64 derivatives C64Cl4 and C64Cl8, respectively.6,14 These two chlorides originate from the same parental #1911C64 (labeled by spiral code18), but the distribution and amount of Cl are distinct. As shown in Figure 1 and Figure S1 in the Supporting Information (SI), three fused pentagons on the surface of #1911C64 form a highly reactive triple-directly fusedpentagon (TDFP) subunit in which the center carbon is shared by three pentagonal rings (5/5/5 site, C1) and three neighbored carbons are the common atoms of pentagon−pentagon− hexagon junctions (5/5/6 sites, C2, C3, and C4).19 In C64Cl4, all four carbons in the TDFP fragment are saturated by chlorines (Figure 2).6 However, for C64Cl8, one of the reactive 5/5/6 junctional carbons (e.g., C4 in Figure 2) in the TDFP region does not bond with Cl but keeps the sp2 state after chlorination.14 Why © XXXX American Chemical Society
Figure 2. Structures of C64Cl4 and C64Cl8.
can different chlorination products be generated from the same parental C64 and why does the reactive carbon in TDFP not bond with Cl? As far as we know, to date there has been no perfect explanation on the formation of two distinct chlorofullerenes. In this Letter, based on density functional theory (DFT) calculations of the electronic structures of #1911C64, the regioselective Cl addition and the reverse decomposition processes have been explored. The formation mechanisms of experimentally observed chlorofullerenes as well as the partially dechlorined products were elucidated for the first time. To verify the electronic ground state of #1911C64 (abbreviated as C64), optimizations at different spin states were carried out at the level of (U)B3lyp/6-311G(d,p). Interestingly, as listed in Table 1, C64 in triplet, open-shell singlet (OS), and closed-shell singlet (CS) states exhibits almost the same potential energy, with a difference of less than 0.1 kcal mol−1. Moreover, the free energies (ΔG) of C64 at 298.15 and 1000 K (temperature for fullerene chlorination15) were computed, and the calculated splitting energies between OS and triplet are still as small as 0.4 and 0.2 kcal mol−1, respectively. Structurally, the triplet C64 is C3v symmetric, and the lengths of three 5−5 bonds are 1.423 Å equivalently. On the other hand, the CS and OS C64 are degenerated to Cs symmetry, and the 5−5 bonds are divided into Received: March 14, 2016
A
DOI: 10.1021/acs.inorgchem.6b00642 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry Table 1. Relative Energy (ΔE, kcal mol−1), Free Energies at 298.15 and 1000 K (ΔG, kcal mol−1), and ⟨S2⟩ Value of #1911 C64 at Different States state
⟨S2⟩
ΔE
ΔG (rt)
ΔG (1000 K)
closed-shell singlet (CS) open-shell singlet (OS) triplet quintet
0.00 0.56 2.03 6.03
0.1 0.0 0.0 36.8
1.3 0.0 0.4
2.2 0.0 0.2
electronic effect is more important than the structural effect for chlorination site selectivity. The spin distribution of formed C64Cl is shown in Table S5. Since the unsaturated 5/5/6 junctional C3 (or equivalent C4) displays the largest spin density of 0.27, the second Cl radical is preferred to bind with C3 to generate the bis-adduct shown in Figure 4b with the Er of −56.6 kcal mol−1. Subsequently, according to the HOMO distribution on C64Cl2, the central carbon C1 provides the largest coefficient of 16.4% on the HOMO, whereas the distributions from other carbons are less than 6% (Table S6), suggesting that the third step can feasibly take place on the center to generate a trichloride (labeled as C64Cl3), with a calculated Er of −53.5 kcal mol−1. Furthermore, as shown in Figure 4c and Table S7 in the SI, the spin of open-shell C64Cl3 is mainly localized on two patterns: (a) unsaturated C4 (spin density of 0.42), which is in the highly reactive TDFP region, and (b) two equivalent atoms, C5 and C6, with a spin density of 0.24. Therefore, the two possibilities of further addition on C64Cl3 were taken into account. When the foreign chlorine radical adds on C4, as shown in Figure 4d, the product (labeled as C64Cl4-a) is the same as the C64Cl4 detected in experiments.6 On the other hand, another tetra-adduct (labeled as C64Cl4-b) can also be generated if chlorine addition takes place on C5 (Figure 4e). Energetically, the Er’s of C64Cl4-a and C64Cl4-b are −58.0 and −43.8 kcal mol−1, respectively, implying that C64Cl4-a should be the thermodynamically favorable product. On the basis of C64Cl4-b, we further studied the formation of experimentally observed C64Cl8. The multistep Cl additions are shown in Figure 5. For each step, the carbon with the largest
two patterns with lengths of 1.390 (1.393) Å and 1.442 (1.440) Å in the case of CS (OS) species. The HOMO diagram of the CS state and spin density distributions of OS and triplet states are shown in Figure 3. It is evident that the HOMO (or spin) of
Figure 3. HOMO (isosurface = 0.05 au) and spin distributions (density = 0.01 au) of #1911C64 at different states.
singlet C64 is distributed on the sides of a symmetric surface, whereas the spin density of triplet C64 is mainly located on the three equivalent 5/5/6 sites. To the best of our knowledge, this is the first report describing the unconventional electronic structure of C64, which may be important for understanding the nature of open-shell fullerenes and corresponding derivatization processes. Now, we focus on the chlorination of C64. According to the well-accepted radical addition mechanism proposed by Rogers and Fowler,20 chlorination on the fullerene surface is preferred to occur on the carbon with highest HOMO contribution or largest spin density. Therefore, based on the calculated HOMO and spin distribution of C64 in Figure 3 and Tables S1−S3 in the SI, the first Cl radical is predicted to add on the 5/5/6 junctional carbon (i.e., C2), no matter whether the parental C64 is singlet or triplet (Figure 4a). In order to verify the preference, geometrical
Figure 5. #1911C64 chlorination process (II).
HOMO or spin density is selected as the chlorination site (Tables S8−S12), and finally we successfully obtained the octachlorided C64Cl8 (Figure 5e), which was captured in 2012.14 Energetically, the calculated Er from step 5 to 8 is −27.9, −45.8, −30.0, and −52.0 kcal mol−1, respectively. It should be noted that, in the sixth step, the spin density of reactant C64Cl5 is delocalized on four different sites (Figure 5b and Table S9). Hence, all the four hexa-adducts were optimized, and the results (Table S10) show that the addition on C11 with the largest spin density of 0.29 is more favored in energy than other addition patterns. Therefore, C11 is considered as the feasible addition site. Overall, the formation of two C64 derivatives was successfully elucidated using DFT calculations. C64Cl4 is predicted as the major product, whereas C64Cl8 should be the minor one, thermodynamically. By employing the step-by-step radical addition mechanism, we found the chlorination processes on CS, OS, and triplet #1911C64 are identical, although their electronic structures are distinct. Beside chlorination, it is of interest to consider the reverse process. Different from the radical addition, the dechlorination in
Figure 4. #1911C64 chlorination process (I).
optimizations on all possible C64Cl structures were carried out. The results listed in Table S4 clearly show that the most stable monochloride is the isomer in which the Cl is located on C2, with the reaction energy (Er = E(C64Cln) − E(C64Cln−1) − E(•Cl)) of −56.8 (−56.7) kcal mol−1 from CS (OS or triplet) C64. It is worth noting that although the centered 5/5/5 carbon C1 exhibits the largest pyramidalization angle (π-orbital axis vector, POAV),14,21,22 the corresponding C64Cl is less stable because of the smaller HOMO contribution, which suggests that the B
DOI: 10.1021/acs.inorgchem.6b00642 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
■
experiments was proved as a thermolysis process and the decomposition on two or more chemically equivalent sites took place simultaneously.14 Therefore, we predict that the site selectivity is simply dependent on the C−Cl bond length (or bond strength). For example, in the case of C64Cl4 (Figure S2), the three 5/5/6 carbons are equivalent and the C−Cl bonds (1.824 Å) are longer than the center one (1.770 Å). The calculated dissociation energies of the two types of C−Cl bonds are 58.0 (5/5/6 site) and 71.1 (5/5/5 site) kcal mol−1, respectively. So the first step of dechlorination takes place on the three 5/5/6 sites to obtain an unexpected monochloride in which the Cl is located on the central carbon C1. Such a monochloride has been observed by using mass spectrometry.6,14 Interestingly, as mentioned above, this species cannot be obtained by radical addition because of the small HOMO density on C1 of pristine C64. However, by chlorination and partial dechlorination, the intermediates are found to be yielded, which provides a novel way to obtain some special fullerene derivatives. As for C64Cl8, similar analysis based on the variation of C−Cl bond lengths was carried out, and we found the decomposition process should be like that (Figure S3): C64Cl8 → C64Cl7 → C64Cl5 → C64Cl3 → C64Cl. This sequence is entirely consistent with the results of multistage mass spectra, and therefore we believe our predication on chlorofullerene decomposition is reasonable and the C−Cl bond lengths are convenient and good indicators.14 In summary, the unconventional electronic structure and the chlorination and reverse dechlorination mechanisms of #1911C64 were proposed for the first time. Although the current calculations suggested identical radical addition processes among CS, OS, and triplet #1911C64, it is still of interest to consider the role of initial state of parental fullerene on chemical derivatizations, i.e. the metal-cage charge transfer nature of endohedral metallofullerenes.
■
REFERENCES
(1) Kroto, H. W. Nature 1987, 329, 529−531. (2) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162−163. (3) Xie, S. Y.; Gao, F.; Lu, X.; Huang, R. B.; Wang, C. R.; Zhang, X.; Liu, M. L.; Deng, S. L.; Zheng, L. S. Science 2004, 304, 699−699. (4) Tan, Y. Z.; Han, X.; Wu, X.; Meng, Y. Y.; Zhu, F.; Qian, Z. Z.; Liao, Z. J.; Chen, M. H.; Lu, X.; Xie, S. Y.; Huang, R. B.; Zheng, L. S. J. Am. Chem. Soc. 2008, 130, 15240−15241. (5) Tan, Y. Z.; Liao, Z. J.; Qian, Z. Z.; Chen, R. T.; Wu, X.; Liang, H.; Han, X.; Zhu, F.; Zhou, S. J.; Zheng, Z.; Lu, X.; Xie, S. X.; Huang, R. B.; Zheng, L. S. Nat. Mater. 2008, 7, 790−794. (6) Han, X.; Zhou, S. J.; Tan, Y. Z.; Wu, X.; Gao, F.; Liao, Z. J.; Huang, R. B.; Feng, Y. Q.; Lu, X.; Xie, S. Y.; Zheng, L. S. Angew. Chem., Int. Ed. 2008, 47, 5340−5343. (7) Tan, Y. Z.; Xie, S. Y.; Huang, R. B.; Zheng, L. S. Nat. Chem. 2009, 1, 450−460. (8) Tan, Y. Z.; Li, J.; Zhou, T.; Feng, Y. Q.; Lin, S. C.; Lu, X.; Zhan, Z. P.; Xie, S. Y.; Huang, R. B.; Zheng, L. S. J. Am. Chem. Soc. 2010, 132, 12648−12652. (9) Ioffe, I. N.; Chen, C.; Yang, S.; Sidorov, L. N.; Kemnitz, E.; Troyanov, S. I. Angew. Chem., Int. Ed. 2010, 49, 4784−4787. (10) Tan, Y. Z.; Li, J.; Zhu, F.; Han, X.; Jiang, W. S.; Huang, R. B.; Zheng, Z.; Qian, Z. Z.; Chen, R. T.; Liao, Z. J.; Xie, S. Y.; Lu, X.; Zheng, L. S. Nat. Chem. 2010, 2, 269−273. (11) Zhou, T.; Tan, Y. Z.; Shan, G. J.; Zou, X. M.; Gao, C. L.; Li, X.; Li, K.; Deng, L. L.; Huang, R. B.; Zheng, L. S.; Xie, S. Y. Chem. - Eur. J. 2011, 17, 8529−8532. (12) Amsharov, K. Y.; Ziegler, K.; Mueller, A.; Jansen, M. Chem. - Eur. J. 2012, 18, 9289−9293. (13) Mueller, A.; Ziegler, K.; Amsharov, K. Y.; Jansen, M. Eur. J. Inorg. Chem. 2011, 2011, 268−272. (14) Shan, G. J.; Tan, Y. Z.; Zhou, T.; Zou, X. M.; Li, B. W.; Xue, C.; Chu, C. X.; Xie, S. Y.; Huang, R. B.; Zhen, L. S. Chem. - Asian J. 2012, 7, 2036−2039. (15) Tan, Y. Z.; Chen, R. T.; Liao, Z. J.; Li, J.; Zhu, F.; Lu, X.; Xie, S. Y.; Li, J.; Huang, R. B.; Zheng, L. S. Nat. Commun. 2011, 2, 420. (16) Gao, C. L.; Li, X.; Tan, Y. Z.; Wu, X. Z.; Zhang, Q.; Xie, S. Y.; Huang, R. B. Angew. Chem., Int. Ed. 2014, 53, 7853−7855. (17) Tan, Y. Z.; Zhou, T.; Bao, J.; Shan, G. J.; Xie, S. Y.; Huang, R. B.; Zheng, L. S. J. Am. Chem. Soc. 2010, 132, 17102−17104. (18) Fowler, P. W.; Manolopoulos, D. E. An Altas of Fullerenes; Oxford University Press: Oxford, 1995. (19) Wang, C. R.; Shi, Z. Q.; Wan, L. J.; Lu, X.; Dunsch, L.; Shu, C. Y.; Tang, Y. L.; Shinohara, H. J. Am. Chem. Soc. 2006, 128, 6605−6610. (20) Rogers, K. M.; Fowler, P. W. Chem. Commun. 1999, 2357−2358. (21) Haddon, R. C. Acc. Chem. Res. 1988, 21, 243−249. (22) Haddon, R. C. Science 1993, 261, 1545−1550.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00642. Computational methods, Schlegel diagram of #1911C64, relative energies, spin densities, HOMO distributions, Cartesian coordinates of fullerene and derivatives, and dechlorination processes of #1911C64Cl4 and #1911C64Cl8 (PDF)
■
Communication
AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-29-82665671. Fax: +86-29-82668559. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work has been financially supported by the National Natural Science Foundation of China (21503157, 21573172), the China Postdoctoral Science Foundation (2014M562402), the Shannxi Province Postdoctoral Science Foundation, and the Specially Promoted Research Grant (22000009) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. C
DOI: 10.1021/acs.inorgchem.6b00642 Inorg. Chem. XXXX, XXX, XXX−XXX
