Experimental and Theoretical Approach to Variable Chlorination

Communication Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX

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Experimental and Theoretical Approach to Variable ChlorinationPromoted Skeletal Transformations in Fullerenes: The Case of C102 Olga N. Mazaleva,† Ilya N. Ioffe,† Fei Jin,‡ Shangfeng Yang,*,‡ Erhard Kemnitz,§ and Sergey I. Troyanov*,† †

Chemistry Department, Moscow State University, Leninskie Gory, 119991 Moscow, Russia Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Materials for Energy Conversion & Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China § Institute of Chemistry, Humboldt University of Berlin, Brook-Taylor-Strasse 2, 12489 Berlin, Germany ‡

S Supporting Information *

ever, the usual lack of isolated intermediates hampers experimental verification of the general pathways and mechanistic details of the transformations. While the starting IPR chlorides are easier to capture by, e.g., keeping the chlorination temperature below the values that trigger cage transformations, the intermediate rearrangement products rarely form isolable crystalline phases.2a,d This lack of experimental information prompts topological analysis of the likely pathways between the starting and final cages2a,d,j,k and quantum-chemical modeling of the SWR2c,i and C2L2e,g processes. Herein we report an interesting case of skeletal transformations where the same C102 cage can be transformed by three different pathways. Fullerene C102 has 616 IPR isomers.3 The most stable of them, C102(603), was recently isolated and structurally characterized as the C102(603)Cl18/20 chloride.4 Also, a non-IPR fullerene chloride #283794C102Cl20 was shown to form from another IPR isomer, C102(19) (also known as #341061C102), via two SWR steps.2h Now we present the new data on #283794 C102Cl20 cocrystallized with its precursor #258508C102Cl20 and on the two novel products of skeletal transformations in C102(19), nonclassical C98(NC2)Cl26 (the numeral after NC indicates the number of heptagons in the cage) and non-IPR C96Cl28, that form respectively via two or three C2L acts. The fullerene soot was synthesized by a Krätschmer−Huffman direct-current arc discharge method with nondoped graphite rods under a helium pressure of 400 mbar. The extracted fullerene mixture was first separated by high-performance liquid chromatography (HPLC) in toluene using a preparative 5PYE column. The target fraction with a retention time of 43.5−47.1 min was then subjected to recycling HPLC with a semipreparative Buckyprep column, and its main subfraction underwent an additional recycling HPLC step with a semipreparative Buckyprep-M column. According to mass spectrometric (MS) analyses, that afforded a fraction of C102 admixed with C100 [see the Supporting Information (SI) for more details]. In a series of chlorination experiments, around 0.02−0.03 mg samples of the C102 fraction were placed in glass ampules together with ca. 0.4 mL of VCl4 and a drop of SbCl5. The evacuated and sealed ampules were heated at 350−360 °C (Caution! The pressure of VCl4 reaches ca. 30 bar under the

ABSTRACT: The first example of three alternative chlorination-promoted skeletal transformation pathways in the same fullerene cage is presented. Isolated-pentagonrule (IPR) C102(19) undergoes both Stone−Wales rotations to give non-IPR #283794C102Cl20 and C2 losses to form nonclassical C98 and non-IPR C96. X-ray structural characterization of the transformation products and a theoretical study of their formation pathways are reported.

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ince the discovery of chlorination-promoted skeletal transformations in higher fullerenes in 2009 (Stone−Wales rearrangements, or SWRs)1a and 2010 (C2 losses, or C2Ls),1b the library of otherwise inaccessible transformed fullerenes has grown tremendously.2 A series of non-isolated-pentagon-rule (IPR) isomers with adjacent pentagons and nonclassical (NC) structures with heptagonal rings have been obtained from various conventional IPR precursors that range from C76 to C102. Remarkably, many of the transformations found to date are multistep and, in some cases, combine SWR and C2L steps.1,2 Also, the transformations can be branched. Thus, C100(18) (IPR isomer numbers in parentheses follow the spiral code ordering3), whose chlorination ultimately gives nonclassical C96(NC3)Cl20 and C94(NC1)Cl22 as products of competing SWR and C2L steps.2g The skeletal processes are also highly diverse in terms of the resulting topological changes in the carbon cages. Initial findings suggested that the SWR processes usually introduce fused pentagons with chlorinated junctions, while the C2L eliminations create heptagons, but later studies have demonstrated that the formation of heptagons via SWR processes and their destruction via C2Ls are also possible. A brief topological survey of the previously observed skeletal transformations was presented in one of our earlier studies.2g The growing range of experimentally captured skeletal transformations suggests that they are quite ubiquitous, yet different skeletal isomers strongly differ in proneness to those processes. Thus, chlorination of C86(17) is not accompanied by skeletal changes,2b while C86(16) can undergo consecutive C2L acts.1b,2a Clearly, such differences in reactivity are due to the carbon cage topology itself and also due to the effects of the topology-induced equilibrium chlorination patterns.1a,2c,g,i How© XXXX American Chemical Society

Received: October 5, 2017

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DOI: 10.1021/acs.inorgchem.7b02554 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry experimental conditions.). After 8−10 days, the formation of small crystals was observed, and the ampules were cooled and opened. Washing with HCl and water to remove SbCl5 and VCl4 gave the final product as small orange crystals. X-ray diffraction studies5 revealed the formation of two cocrystallized non-IPR chlorides: #283794 C102Cl20 (reported previously2h) and #258508C102Cl20. Unexpectedly, longer than 3−6 months of heating afforded further products: nonclassical C98(NC2)Cl26 and non-IPR C96Cl28 (Figure 1).5 The following density functional theory (DFT) analysis of the products utilizes PBE/TZ2p calculations done with use of the PRIRODA software.6

form via C2Ls. Their much slower formation is indicative of higher activation barriers compared to the SWR acts that yield #283794 C102Cl20. The topological analysis shows that both structures can form from the starting C102(19) via only the C2L acts without any accompanying SWR processes. The general scheme of transformations to give C98(NC2)Cl26 from a likely precursor C102(19)Cl20 is shown in Figure 3. Here, the two

Figure 1. Views of three molecules, non-IPR #283794C102Cl20, nonclassical C98(NC2)Cl26, and non-IPR C96Cl28, obtained by different transformation routes from IPR C102(19). Fused cage pentagons are shown in red, whereas cage heptagons are highlighted with blue.

Figure 3. Schlegel diagram presentation of the transformation of C102(19) to the nonclassical C98(NC2)Cl26 via two C2L steps. Black circles denote the positions of Cl attachments. C−C bonds to be removed in the next C2L step are indicated with small ovals.

The shortest transformation pathways of C102(19) (no. 341061 in the full list of isomers) to #283794C102Cl20 include two SWR steps. The two alternative sequences are shown in Figure 2. Previously,2h we already suggested the SWR-then-

C2L stages are separated by a step of additional chlorination. Also involved is “chlorine dance”, equilibrium rearrangement of chlorination patterns that proceeds much faster that the cage transformations.2i,7 The designations C2L1 and C2L3 that follow ref 2g refer to abstraction of the pentagon−hexagon edge where the hexagon has one or three adjacent pentagons, respectively. Remarkably, the SWR processes to give #283794C102Cl20 and the C2Ls occur in the same region of the carbon cage. Probably, that is due to the chlorination pattern in the initial chloride of the parent C102(19) (the likely candidate is #341061C102Cl20 in Figure 2) that features a continuous chain of adjacent chlorine addends exactly in the region of interest. Although the suggested mechanisms of the cage rearrangements do not strictly require the presence of such chlorinated chains, most of the documented C2L and SWR processes occur within them, thus pointing to the favorable effects of the chainlike chlorination patterns on the energetics of skeletal transformations.1,2 The general mechanism of C2 elimination from chlorinated fullerenes was suggested elsewhere.2e It is based on consecutive cleavage of the C−C bonds that connect the C2 fragment to the rest of the cage, assisted by the formation of new C−C bonds and by the quenching of dangling bonds with labile chlorine addends. There are two alternative variants of the process: (a) consecutive cleavage of all four bonds around the C2 fragment or (b) cleavage of only two bonds on the opposite ends to give a −C2Cl2− bridge eliminated as C2Cl4 after additional chlorination. Our calculations for both the C2L3 and C2L1 steps in Figure 3 identified option a as more energetically preferable (see the SI). Its scheme is presented in Figure 4. Calculations have demonstrated that the limiting stage in both the C2L3 and C2L1 steps of Figure 3 is the opening of the >C CCl2 bridge into a singly bonded C2Cl3 moiety, transition state TS11. In the C2L3 step, comparably limiting is the preceding formation of the >CCCl2 bridge itself (TS7; see the SI for more details). The limiting activation barrier for the C2L3 step,

Figure 2. Schlegel diagram presentation of the possible pathways of SWR transformations of the IPR #341061C102Cl20 into the non-IPR #283794 C102Cl20. Cage pentagons are filled with red (with fused pentagons highlighted). The C−C bonds to be rotated are encircled with ovals. The relative energy values are given for chlorinated and, in parentheses, pristine cages.

SWR′ pathway with a more exothermic first stage to be more likely. Unfortunately, the energy values in ref 2h were partly incorrect, but the updated data shown in Figure 2 favor the SWRthen-SWR′ pathway even more strongly. The discovery of the admixture of precursory #258508C102Cl20 corroborates the theoretical suggestions. The two other products of skeletal transformations in C102(19), nonclassical C98(NC2)Cl26, and non-IPR C96Cl28 B

DOI: 10.1021/acs.inorgchem.7b02554 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

heptagons. The calculated activation barrier, again due to the TS11 transition state, was found to be ca. 300 kJ mol−1 (see more details in the SI). Presently, we lack data that concludes whether such an increased activation energy is characteristic of those C2L processes where no heptagons are implicated. Like a similar value for a more common C2L1 process in the formation pathway of C98(NC2)Cl26, it may result from the local geometric features of the carbon cage. It also appears possible that both computed activation barriers are overestimated. Anyway, the qualitative trends that show comparable activation energy toward C98(NC2)Cl26 and non-IPR C96Cl28 agree well with the concurrent formation of the two compounds under the same conditions. In summary, we have observed an interesting example of branching of the skeletal transformation pathways in higher fullerenes, where the time factor turned out to be highly important. While the temperature conditions of the experiments are limited by increased pressures that develop in the reaction ampules, the increased reaction times can enable the formation of new, previously unavailable, non-IPR and nonclassical fullerene derivatives. It may be worth revisiting even the previously studied higher fullerenes in order to investigate the possibility of deeper carbon cage shrinkage as a result of further C2L acts.

Figure 4. General scheme of the C2 abstraction process. TS# designations refer to the transition states.

ca. 250 kJ mol−1 relative to the starting compound, is in agreement with the reaction conditions and our previous computational estimates.2g In the C2L1 step, however, the limiting barrier reaches 300 kJ mol−1, a value somewhat too high for the synthetic temperatures of 350−400 °C, although it qualitatively reflects the trend of increased reaction time. Presently, we do not see viable alternatives to the proposed mechanism, and one can hypothesize that the DFT values are overestimated. Formation of the non-IPR C96Cl28 (or #185115C96Cl28) via three C2L steps has several particularly interesting aspects. There are two alternatives for the order of the initial steps, one of them being the same as that in the case of C98(NC2)Cl26. In both cases, the second step eliminates one of the common pentagon− pentagon edges, thus destroying the heptagon formed in the preceding step. One of the said alternative pathways is shown in Figure 5. The DFT data demonstrate that the limiting activation barriers are mostly the same (ca. 250 kJ mol−1); i.e., the order of C2Ls can be arbitrary. The third step is a novel kind of C2L process (C2L5) that removes a common pentagon−pentagon edge situated between the two hexagons and thus neither creates nor destroys any

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02554. Data on HPLC separation, MS spectra, and DFT results (PDF) Accession Codes

CCDC 1578277−1578279 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +007 495 9395396. Fax: +007 495 9391240. *E-mail: [email protected]. Fax/Tel: +86 551 63601750. ORCID

Shangfeng Yang: 0000-0002-6931-9613 Erhard Kemnitz: 0000-0002-5300-3905 Sergey I. Troyanov: 0000-0003-1663-0341 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research (Grants 15-03-04464, 15-03-05083, and 16-53-53012), the National Natural Science Foundation of China (Grants 21132007, 21371164, and 2151101074), and the Deutsche Forschungsgemeinschaft (Grant Ke-489/39-2). We are thankful to the supercomputer center of the Moscow State University for computational support.8

Figure 5. Schlegel diagram presentation of the transformation of C102(19) to the nonclassical C96Cl28 via three C2L steps. C−C bonds to be removed in the next C2L step are indicated with small ovals. C

DOI: 10.1021/acs.inorgchem.7b02554 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

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(7) (a) Troshin, P. A.; Lyubovskaya, R. N.; Ioffe, I. N.; Shustova, N. B.; Kemnitz, E.; Troyanov, S. I. Synthesis and Structure of the Highly Chlorinated [60]Fullerene C60Cl30 with Drum-Shaped Carbon Cage. Angew. Chem., Int. Ed. 2005, 44, 234−237. (b) Troshin, P. A.; Łapiński, A.; Bogucki, A.; Połomska, M.; Lyubovskaya, R. N. Preparation and spectroscopic properties of chlorofullerenes C60Cl24, C60Cl28, and C60Cl30. Carbon 2006, 44, 2770−2777. (8) Sadovnichy, V.; Tikhonravov, A.; Voevodin, V.; Opanasenko, A. “Lomonosov”: Supercomputing at Moscow State University. Contemporary High Performance Computing; CRC Press: Boca Raton, FL, 2013; pp 283−307.

REFERENCES

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DOI: 10.1021/acs.inorgchem.7b02554 Inorg. Chem. XXXX, XXX, XXX−XXX