Stable C92(26) and C92(38) as Well as Unstable C92(50) and C92(23

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Stable C92(26) and C92(38) as Well as Unstable C92(50) and C92(23) Isolated-Pentagon-Rule Isomers As Revealed by Chlorination of C92 Fullerene Runnan Guan,† Fei Jin,† Shangfeng Yang,*,† Nadezhda B. Tamm,‡ and Sergey I. Troyanov*,‡

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Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, China ‡ Chemistry Department, Moscow State University, Leninskie Gory, Moscow 119991, Russia S Supporting Information *

CF3 derivatives, C92(38)(CF3)14/16, and a chloro derivative with an averaged composition of C92Cl20.84, which contained C92(38)Cl20 and C92(38)Cl22 overlapping in the same crystallographic site.5b Herein we report the isolation and structural characterization of chloro derivatives of IPR C92 isomers C92(38) and C92(26). The presence of isomers C92(50) and C92(23) in the fullerene soot was confirmed through the isolation of heptagoncontaining nonclassical C90(NC)Cl22 and non-IPR C90Cl26, respectively, which were obtained via C2 losses from C92 cages. The chlorination patterns are discussed in terms of the formation of isolated CC double bonds and aromatic substructures on carbon cages. In a series of chlorination experiments, high-performance liquid chromatography (HPLC) C92 fractions were chlorinated with excess VCl4 or a VCl4/SbCl5 mixture in thick-walled glass ampules at 350−360 °C for a time period of several weeks to several months. (Caution! At 350−360 °C, the pressure of VCl4 is 20−25 bar.) After removal of excess VCl4/SbCl5 by treatment with HCl/H2O, small crystals were studied by X-ray diffraction with the use of synchrotron radiation (Table S1). The experiments carried out for the shorter reaction time revealed the molecular structures of C92(38)Cl18, C92(38)Cl22, and C92(26)Cl24. In the prolonged chlorination experiments, the crystals of non-IPR C90Cl26 and nonclassical C90(NC)Cl22 were isolated. Whereas isomers C92(38) and C92(82)5a elute first, isomers C92(26), C92(23), and C92(50) elute as the last subfractions in recycling HPLC (Figure S1). It should be noted that the separation of a C92 fraction into subfractions does not result in the isolation of individual C92 isomers because the elution times of the first and last subfractions are very close. It was also found that chloro derivatives of the IPR isomers of C92 can be isolated after days or 1−2 weeks, while the formation of chlorinated derivatives of nonclassical or non-IPR C90 requires much longer reaction times. These observations suggest slower kinetics of cage transformations, as reported earlier for the cases of other higher fullerenes.6 The C92(38)Cl18 and C92(26)Cl24 molecules with an IPR carbon cage as well as nonclassical C90(NC)Cl22 and non-IPR C90Cl26 are shown in Figure 1. While the two IPR carbon cages

ABSTRACT: High-temperature chlorination of C92 fractions, followed by single-crystal X-ray diffraction, resulted in the structure determination of C92(38)Cl18, C92(38)Cl22, C92(26)Cl24, nonclassical C90(NC)Cl22, and non-isolated-pentagon-rule C90Cl26. Two latter chloro derivatives were obtained by chlorination-promoted cage transformations via a single C2 loss from C92(50) and a combination of a C2 loss from C92(23) and three Stone− Wales rearrangements. The chlorination patterns are stabilized by the formation of isolated CC bonds and aromatic substructures on carbon cages. The presence of C92 isomer numbers 23, 26, and 50 in the arc-discharge fullerene soot has been confirmed for the first time.

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he progress in the investigation and even identification of higher fullerenes is hampered because of their low abundance and the existence of numerous isomers. Structural studies of some empty higher fullerenes such as C86, C90, and C96 were possible through their cocrystallization with metal porphyrins,1 which restricts the rotational/librational mobility of fullerenes in crystals. Derivatization of higher fullerenes also benefits the formation of compounds with ordered crystal structures. Chlorination or trifluoromethylation of fullerenes followed by the structural characterization of individual derivatives was proven to be an effective tool for the identification of many pristine fullerene isomers from C84 up to C108.2 Fullerene C92 has 86 topologically possible isolated-pentagonrule (IPR) isomers.3 Theoretical studies of the formation energies pointed out several C92 isomersD2-C92(82), D2C92(81), D2-C92(84), C1-C92(38), and D3-C92(28)as the most stable ones (the numerals in parentheses indicate the isomer numbers according to the spiral algorithm3), however, with different orders of stability depending on the calculation method.4a For example, the density-functional tight-binding method gave a stability row with the leading position of isomer 28: D3-C92(28), C2-C92(26), D2-C92(84), D2-C92(82), and C1C92(38).4b The first crystallographic study was performed for C92(CF3)16 and revealed the presence of C92(82) and/or C92(81) cages.5a High-temperature trifluoromethylation and chlorination allowed the isolation and structural characterization of several © XXXX American Chemical Society

Received: January 15, 2019

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DOI: 10.1021/acs.inorgchem.9b00144 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

isolation of two benzenoid rings, thus contributing to the stability of the whole molecule. The structure of C92(26)Cl24 is a chloro derivative of a new IPR isomer of C92, C2-C92(26), which has not been detected before in the fullerene soot. According to some theoretical calculations, C2-C92(26) is one of the most stable IPR isomers of C92 fullerene.4 Our DFT calculations confirmed the second place in the stability row with 21 kJ mol−1 above the most stable isomer C92(28). Its carbon cage contains two closely arranged groups of four pentagons (Figure 2). Noticeably, the chlorination pattern of 24 Cl attachments is strongly asymmetric so that a long chain of eight adjacent Cl attachments is present in one group, whereas the second group contains 10 Cl attachments, resulting in the formation of four isolated CC bonds in this cage region. It is worth noting that the isolation of two CC bonds occurs because of additions in THJs, shown by green arrows in Figure 2. The structure of C90(NC)Cl22 contains two C atoms less than the starting C92 fullerene. It is characterized by the presence of two pairs of fused pentagons and a heptagon in a nonclassical carbon cage (Figure 3). This structure is experimental evidence

Figure 1. Projections of C92(38)Cl18 (a), C92(26)Cl24 (b), C90(NC)Cl22 (c and d), and #86239C90Cl26 (e and f).

retain roughly their ellipsoidal or spherical shapes also in chloro derivatives, non-IPR and nonclassical cages are significantly distorted by flattening or the presence of concave regions, respectively. The C92(38)Cl18 molecule represents a new chloro derivative of C92(38), which is a rather stable isomer of C92 with a formation energy only 27.9 kJ mol−1 higher than that for C92(28) according to our density functional theory (DFT) calculations.7 In the carbon cage of C92(38)Cl18, three isolated CC double bonds and three nearly isolated benzenoid rings are present, as can be seen in the Schlegel diagram of Figure 2. As

Figure 3. Schlegel diagram representation of the cage transformation from C92(50)Cl24 to the experimental C90(NC)Cl22 (left and middle). Schlegel diagram of the isolated non-IPR #86239C90Cl26 (right). Cage pentagons are highlighted in red, whereas a heptagon is shown in blue. Black circles denote the positions of the attached Cl atoms. Isolated CC bonds and isolated benzenoid rings are indicated. A small oval in the Schlegel diagram of C92(50)Cl24 denotes a C−C bond to be removed.

Figure 2. Schlegel diagrams of C92(38)Cl18, C92(38)Cl22, and C92(26)Cl24. Cage pentagons are highlighted in red. Black circles denote the positions of the attached Cl atoms. Isolated CC bonds and benzenoid rings are indicated. Green arrows indicate the additions in the THJ positions.

of the existence of rather unstable isomer C92(50) [63.6 kJ mol−1 relative to C92(28)] in the arc-discharge fullerene soot. The reconstruction of a cage transformation revealed unambiguously that the starting cage is C92(50) and a single act of a C2 loss is necessary to form the cage of C90(NC). This transformation is demonstrated schematically by the Schlegel diagrams in Figure 3. Similar one-step C2 losses of the type C2L39 were reported for chlorination-promoted cage transformations from C96(114) to C94(NC),10a from C90(28) to C88(NC),10b from C88(33) to C86(NC),10c and from C86(16) to C84(NC).8a In some cases, both the starting and final structures were confirmed by singlecrystal X-ray diffraction. The chlorination pattern of C90(NC)Cl22 is characterized by the attachments to all sites of pentagon−pentagon fusions. The higher energy of such attachments is due to the more favorable pyramidalization angles at the sites of pentagon−pentagon− hexagon junctions compared to the sites of pentagon− hexagon−hexagon junctions. Two isolated CC bonds and three isolated or nearly isolated benzenoid rings contribute to further stabilization of the molecule. Another isolated chloride of C90, non-IPR C90Cl26, must be regarded as the chlorination product of C92 because no C90 fullerenes were detected by mass spectrometry (MS) analysis of

a comparatively rare case for fullerene derivatives with more than 12 addends, one of the cage pentagons has no Cl attachment. Several analogous molecular structures of fullerene chlorides with unoccupied pentagons are reported in the literature, for example, in C86(16)Cl28,8a C88(17)Cl16,8b and C88(7)Cl24.8c The chlorination pattern of C92(38)Cl22 isolated in this work is the same as that reported earlier in a disordered structure where C92(38)Cl22 and C92(38)Cl20 overlap in the same crystallographic site.5b Nevertheless, the packing motifs of C92(38)Cl 22 and C92(38)Cl20/22 are rather different. A comparison of the chlorination patterns of C92(38)Cl18 and C92(38)Cl22 demonstrates their close similarity and some differences (Figure 2). Their chlorination patterns have 14 attachment positions in common. The chlorination pattern of C92(38)Cl22 is characterized by the presence of four isolated CC bonds and three isolated or nearly isolated benzenoid rings. There is one Cl attachment in the position of a triplehexagon junction (THJ), which is indicated with an arrow. Such types of attachments to fullerene cages are generally not favored, but in this case, the attachment in the THJ completes the B

DOI: 10.1021/acs.inorgchem.9b00144 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry the starting materials. The Schlegel diagram of C2-#86239C90Cl26 demonstrates the presence of two pairs of fused pentagons in the carbon cage (Figure 3). In the chlorination pattern, both common pentagon−pentagon edges are chlorinated, as expected. Most chlorinated sites are arranged in extended chains of adjacent Cl attachments, which is typical for chloro derivatives of fullerenes with a high degree of chlorination.11 A possible pathway of chlorination-promoted cage transformation from C92 to #86239C90 can be tentatively suggested based on a reconstructive topological analysis. It is obvious that the formation of both pairs of fused pentagons occurs via two Stone−Wales rotations of partially chlorinated C−C bonds denoted by arrows in the Schlegel diagram in Figure 3. These types of cage rearrangements in pyracylene fragments of fullerene carbon cages are known as SWR1.9 Two other cage transformation acts, the loss of a C2 fragment (C2L) from C92(23) and a Stone−Wales rotation of the type SWR2 (formation or elimination of a cage heptagon), should occur in another cage region. Altogether, four transformation steps are necessary to transform the cage of C92(23) into #86239C90, which is illustrated in Figure S3 in more detail. The presence of the C92(23) isomer in the arc-discharge fullerene soot was not expected because of its very high formation energy [115 kJ mol−1 relative to C92(28)]. It is important to note that C92(23) is a “single” isomer; i.e., it cannot be transformed to an IPR C92 by SWRs. Therefore, annealing during high-temperature fullerene synthesis under the formation of more stable C92 cages should be hampered significantly. Indeed, several “single” fullerene isomers with very high relative formation energies in the range of 63−125 kJ mol−1 have been unambiguously confirmed by structural studies.1b,c,12 It is instructive to compare the average chlorination energies of the C92Cln and C90Cln compounds isolated in this work. It is known from previous studies of fullerene chlorides that the average C−Cl energy per one Cl atom changes insignificantly, slightly decreasing with the number of attached Cl atoms regardless of the cage size.8b,13 The presence of stabilizing local aromatic substructures and isolated CC bonds may slightly increase the C−Cl bond energy. Our DFT calculations7 of the average chlorination enthalpy per one Cl atom in the IPR C92(38)Cl18/20/22 gave values of 5.7, 4.9, and 3.1 kJ mol−1 higher than the chlorination enthalpy of D3d-C60Cl30 taken as a standard.14 The average C−Cl energy in C92(26)Cl24, which has destabilizing attachments in THJs, was found to be virtually the same as that in D3d-C60Cl30. In contrast, the fullerene chlorides containing two pairs of fused pentagons show much higher average C−Cl bond energies, 13.3 and 19.0 kJ mol−1 in C90(NC)Cl22 and #86239C90Cl26, respectively. The large chlorination energy wins comprise the main driving forces of the skeletal transformations in C92(50) and C92(23). In summary, high-temperature chlorination of C92 fractions resulted in the isolation and structural characterization of several chloro derivatives of the IPR C92 and non-IPR and nonclassical C90. The structures of C92(38)Cl18/20/22 reveal the factors defining the chlorination patterns such as the formation of isolated CC double bonds and aromatic substructures on a carbon cage. A new, relatively stable isomer, C92(26), was isolated as C92(26)Cl24, and its chlorination pattern shows an interesting combination of several isolated CC bonds and ortho chains of Cl attachments. The isolation and structural characterization of nonclassical C90(NC)Cl22 and non-IPR #86239 C90Cl26 allowed us to suggest respectively the presence of rather unstable IPR isomers C92(50) and C92(23) in the

fullerene soot. Topological reconstruction revealed that a C2 loss and a four-step transformation, a combination of a C2 loss and Stone−Wales rearrangements, are necessary to produce respectively C90(NC)Cl22 and #86239C90Cl26 from the corresponding C92 cages. DFT calculations of the average C−Cl bond energy confirmed the existence of large driving forces of skeletal transformation under the formation of fused pentagon pairs on carbon cages.

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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.9b00144. Compound isolation, MS and UV/vis data of C92 fractions, and selected crystallographic data (PDF) Accession Codes

CCDC 1890016−1890020 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]. Fax/Tel: +86 551 3601750. *E-mail: [email protected]. Tel: +007 495 9395396. Fax: +007 495 9391240. ORCID

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the National Natural Science Foundation of China (Grant 51572254) and the Russian Foundation for Basic Research (Grant 19-03-00733).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.9b00144 Inorg. Chem. XXXX, XXX, XXX−XXX