New Isolated-Pentagon-Rule Isomers of Fullerene C98 Captured as

Apr 17, 2017 - High-temperature chlorination of C98-containing mixtures afforded crystals of several chloro derivatives, and their structure elucidati...
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New Isolated-Pentagon-Rule Isomers of Fullerene C98 Captured as Chloro Derivatives Fei Jin,† Shangfeng Yang,*,† and Sergey I. Troyanov*,‡ †

Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Materials for Energy Conversion, and Department of Materials Science and Engineering, University of Science and Technology of China (USTC), Hefei 230026, China ‡ Chemistry Department, Moscow State University, Leninskie Gory, 119991 Moscow, Russia S Supporting Information *

There are several theoretical studies on IPR isomers of C98, which report the highest stability of isomer C2-C98(248) but differ in the stability orders of other isomers.9 Here, we report on the isolation and structural characterization of new isomers of C98 nos. 107, 109, and 120 as chloro derivatives. High-temperature chlorination of high-performance liquid chromatography (HPLC) C98-containing fractions allowed the structure determination of C98(107,109)Cl20/22, C98(120)Cl18, and C98(120)Cl22. In addition, the known isomer C98(248) was isolated as chlorides with a high chlorination degree, C98(248)Cl24/26. Significant features of the chlorination pattern are discussed in terms of the formation of stable substructures on fullerene cages. Fullerene soot was synthesized by a Krätschmer−Huffman direct-current arc-discharging method with an undoped graphite rod under 400 mbar of helium. The as-produced soot was Soxhlet-extracted by CS2 for 24 h. The fullerene extract was subjected to multistep HPLC separation in toluene. The fraction eluted between 42.5 and 46.5 min in the first step was further separated by HPLC in a recycling mode, affording two fractions with a moderate relative abundance of C98, whereas both C96 and C100 fullerenes were also present in approximately equal amounts according to mass spectrometric analyses [see the Supporting Information (SI) for more details]. These two fractions as well as the third one isolated previously (mainly C98 admixed with C96; see ref 10) were used for chlorination experiments. C98-containing subfractions in amounts of ca. 0.02−0.03 mg each were placed in thick-walled glass ampules, and an excess of VCl4 (ca. 0.4 mL) and a drop of SbCl5 were added. The ampules were evacuated and heated at 350 °C for 4−12 weeks. (Caution! The pressure of VCl4 reaches ca. 30 bar under the experimental conditions.) After the ampules were cooled and opened, the reaction products were washed with HCl and water to remove excess SbCl5 and VCl4, leaving tiny orange-to-red-colored crystals. The crystals obtained were studied by an X-ray diffraction (XRD) using synchrotron radiation, resulting in five molecular structures (see the SI for more crystallographic details). The crystals of a chloride obtained from the previously isolated C98 fraction were identified as C98(248)Cl24/26. The crystals isolated in two different experiments upon chlorination of the C98containing fraction eluted first in the present work gave two slightly different structures of C98(107,109)Cl20/22. Finally, the

ABSTRACT: Fullerene C98 possesses 259 isomers obeying the isolated pentagon rule (IPR), from which two, nos. 116 and 248, have been confirmed earlier as chloro derivatives. High-temperature chlorination of C98containing mixtures afforded crystals of several chloro derivatives, and their structure elucidation by X-ray crystallography revealed the presence of new isomers, nos. 107, 109, and 120, in the fullerene soot. Evidence for an isomer of no. 111 is also presented. In addition, a new chloride of the known isomer 248 has been isolated and structurally studied. The chlorination patterns of the chlorides are discussed in terms of the formation of isolated CC bonds and aromatic substructures on the fullerene cages.

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he fullerene soot obtained by an arc-discharge method contains, besides the commonly known C60 and C70, a number of pristine higher fullerenes up to C100 and even higher, however in progressively smaller amounts.1 Higher fullerenes may exist as numerous isomers, which additionally complicates their investigation and even structural identification. 13C NMR spectroscopy is proven to be an effective identification tool for higher fullerenes up to C88.2 Isomers of even higher pristine fullerenes could be unambiguously identified by crystallographic studies of either cocrystals with metal porphyrins (e.g., C90 and C96)3 or exohedral derivatives.4 The method of high-temperature chlorination with an excess of inorganic chlorides (SbCl5, VCl4, ICl, etc.) turned out to be very successful for the identification of higher fullerenes. Using the chlorination method, many isomers of higher fullerenes have been identified for the first time as in the cases of C90−C965 and C100−C108.6 Higher fullerene C98 can exist in the form of 259 isomers obeying the isolated pentagon rule (IPR),7 and it was confidently detected in the fullerene soot by mass spectrometry.1 However, because of the low abundance of C98 in the fullerene soot and the difficulties in the isolation of compositionally pure fractions, no experimental study of specific isomers of C98 was reported until recently, whereas several isomers of still higher fullerenes up to C108 have already been identified.6 Using the chlorination method, the first two IPR isomers of C98 fullerene nos. 248 and 116 were confirmed very recently as chloro derivatives, C98(248)Cl22 and C98(116)Cl20, respectively (the IPR isomer numbering in parentheses is according to the spiral algorithm7).8 © 2017 American Chemical Society

Received: March 3, 2017 Published: April 17, 2017 4780

DOI: 10.1021/acs.inorgchem.7b00568 Inorg. Chem. 2017, 56, 4780−4783

Communication

Inorganic Chemistry

partially isolated benzenoid ring is converted into the completely isolated one owing to the attachment of two additional Cl atoms so that the molecular structure is stabilized by four isolated CC bonds and four isolated and one partially isolated benzenoid rings. The addition of two Cl atoms in C1-C98(248)Cl24 occurs in a symmetrical way, thus restoring the two-fold symmetry of the whole molecule, in which all five benzenoid rings become completely isolated. Similarly, two-fold-symmetry-assisted successive additions of pairs of CF3 groups were reported for the C88(33)(CF3)16/18/20 molecules.11 A remarkable feature of the chlorination patterns of C98(248)Cl22/24/26 is the attachment of two Cl atoms in the positions of triple-hexagon junctions (THJs), which are generally less suitable for the attachment to fullerene cages.12 It can be seen on the Schlegel diagrams in Figure 2 that each attachment in a THJ contributes to the complete isolation of three benzenoid rings, thus increasing the stability of the whole molecules. In the structure of composition C98Cl21.46, 20 attached Cl atoms possess full occupancies while two Cl atoms have partial occupancies of 0.728(5), thus indicating the presence of two chlorides with 20 and 22 attached Cl atoms (Figure 3). The

C98-containing fraction eluted later in the present work resulted in two chloro derivatives, C98(120)Cl18 and C98(120)Cl22, isolated respectively after ampule heating for 4 and 12 weeks. The isolation of C98(248)Cl24/26 in the present study is not surprising because the previous study of the chlorination products of the same HPLC fraction gave C98(248)Cl22,8 with the reaction time being 10 and 4 weeks, respectively. In fact, both C1-C98(248)Cl24 (Figure 1a) and C2-C98(248)Cl26 crystallo-

Figure 1. Views of the C98(248)Cl24 (a), C98(107)Cl20 (b), C98(120)Cl18 (c), and C98(120)Cl18 (d) molecules. The view in part a is along the two-fold molecular axis, whereas views in parts b−d are parallel to the molecular mirror planes.

Figure 3. Schlegel diagrams of C1-C98(109)Cl22 and Cs-C98(107)Cl20. Cage pentagons are shown with a red color. Black circles denote the positions of the attached Cl atoms. The occupied THJ position is shown by an arrow. Isolated CC bonds and benzenoid rings are indicated.

graphically independent molecules are found in the same crystal in a 1:1 ratio. Our density functional theory (DFT) calculations confirmed that isomer C98(248) belongs to the most stable IPR isomers of C98 (relative formation energy of 1.5 kJ mol−1), being practically isoenergetic with two other most stable isomers nos. 148 (0.0 kJ mol−1) and 253 (1.2 kJ mol−1).10 It is worth noting that the chlorination pattern of C1C98(248)Cl24 inherits that of C2-C98(248)Cl22 determined earlier,8 with two additional Cl atoms being attached asymmetrically on the cage (Figure 2). In C1-C98(248)Cl24, one

arrangement of 20 fully occupied Cl atoms is mirror symmetric, whereas two partially populated Cl atoms are attached on the one side of the cage. In addition, one C−C bond of the carbon cage appears to be disordered between two positions, which are rotated relative to one another by 90° (a so-called SW cross). This implies the presence of two isomeric cages of C98, C1C98(109) and Cs-C98(107), with refined populations of 0.62(2) and 0.38(2), respectively. The appearance of SW crosses was reported for some other structures of fullerene derivatives in which the arrangement of exohedral addends was the same but the carbon cages differed by the orientation of one or two C−C bonds. The most striking examples among chlorinated higher fullerenes are known for C78(2,3)Cl18,13a C90(34,46)Cl32,13b and C104(811,812)Cl24.13c Although the occupancies of two additional Cl atoms and two cages are somewhat different, it can be assumed that the less populated Cs-C98(107) presumably adopts a mirror-symmetric chlorination pattern with 20 attached Cl atoms (Figure 1b) and the asymmetric C1-C98(109) cage bears mainly 22 Cl atoms. The second C98(107,109)Cl20/22 structure obtained in the other chlorination experiment has a composition of C98Cl21.25 with somewhat lower populations of two additional Cl atoms [0.628(5)]. An important difference from the C98Cl21.46 structure is the presence of the second SW cross, which is arranged mirrorsymmetrically on the cage. The relative populations of formally disordered C−C bonds in two SW crosses are different, 0.47/

Figure 2. Schlegel diagrams of C1-C98(248)Cl24 and C2-C98(248)Cl26. Cage pentagons are shown with red color. Black circles denote the positions of the Cl atoms in C2-C98(248)Cl228 and C2-C98(248)Cl26, whereas two gray circles designate additional Cl atoms in C98(248)Cl24. The occupied positions of THJs are indicated with arrows. Also shown are isolated CC bonds and benzenoid rings. 4781

DOI: 10.1021/acs.inorgchem.7b00568 Inorg. Chem. 2017, 56, 4780−4783

Communication

Inorganic Chemistry

have calculated relative chlorination enthalpies of 4.2, 6.0, and 5.3 kJ mol−1, whereas C98(120)Cl22 and C98(120)Cl18 gave values of 3.3 and 4.5 kJ mol−1, respectively. The latter value seems to be too small for a chloride with only 18 attached Cl atoms, which can result from its unusual chlorination pattern with two unoccupied cage pentagons. In summary, the high-temperature chlorination of C98containing HPLC fractions followed by an XRD study of crystalline chlorides resulted in the discovery of three new IPR isomers of C98 fullerene nos. 107, 109, and 120 and the isolation of chlorides of the known C98(248) with 24 and 26 attached Cl atoms. The relative stabilities of the five experimentally confirmed C98 isomers lie in a wide range from the most stable C98(248) and C98(120) to the rather unstable C98(107). The formation of favorable but sometimes unusual chlorination patterns correlates well with the presence of stabilizing substructures on the fullerene cage such as benzenoid rings and isolated CC bonds.

0.53(2) and 0.76/0.24(3), which can be interpreted as the presence of chlorides (with 20 or 22 Cl atoms) of three isomeric C98 cages, C1-C98(109) (51%), Cs-C98(107) (36%), and CsC98(111) (13%) (see the SI for details). Our DFT calculations revealed that C1-C98(109) and CsC98(111) belong to isomers of moderate stability with formation energies of 19.0 and 26.0 kJ mol−1 relative to C98(148), whereas the isomer C98(107) is significantly less stable (42.8 kJ mol−1). In spite of these energetic relationships, which do not contradict the presence of isomer Cs-C98(111), its existence needs an independent experimental confirmation in future studies. Chloro derivatives of isomer Cs-C98(120), which belongs to the most stable IPR isomers of C98 [1.5 kJ mol−1 relative to C98(148)], were obtained as two chlorides, Cs-C98(120)Cl18 and Cs-C98(120)Cl22, both possessing mirror-symmetrical chlorination patterns (Figure 1c,d). In an unusual chlorination pattern of Cs-C98(120)Cl18, 18 attached Cl atoms are distributed very unevenly on the C98(120) fullerene cage, thus occupying only one cage hemisphere (Figure 4) and leaving two cage pentagons

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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.7b00568. Data on HPLC separation, MS spectra of C98-containing fractions, details of DFT calculations, and selected data on X-ray crystallography (PDF) Crystallographic data in CIF format (CIF) Crystallographic data in CIF format (CIF) Crystallographic data in CIF format (CIF) Crystallographic data in CIF format (CIF) Crystallographic data in CIF format (CIF)

Figure 4. Schlegel diagrams of Cs-C98(120)Cl18 and Cs-C98(120)Cl22. Cage pentagons are shown with a red color. Black circles denote the positions of the attached Cl atoms. The occupied positions of THJs are shown by arrows. Isolated CC bonds and benzenoid rings are indicated.

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AUTHOR INFORMATION

Corresponding Authors

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

unoccupied. Two attachments in THJs contribute to the formation of three isolated benzenoid rings, and the whole arrangement is characterized by the presence of four such rings and three isolated CC bonds. An incomplete occupation of 12 pentagons by 12 or more addends has been found in some other chlorinated fullerenes, such as C88(7)Cl12, C88(7)Cl24, C88(33)Cl12/14,14a and C88(17)Cl16,14b with the latter possessing rather similar addition pattern with three isolated benzenoid rings, three isolated CC bonds, and two unoccupied pentagons. The structure of Cs-C98(120)Cl22 contains only 14 attached Cl atoms located in the same positions as those of Cs-C98(120)Cl18 and, in the absence of the addition in THJs, possesses only one isolated and two nearly isolated benzenoid rings, whereas the number of isolated CC bonds is increased to four. Furthermore, all pentagons are occupied with one or two Cl atoms, which makes the addition pattern more usual among chlorinated fullerenes.15 In order to estimate the stability of C98 chloro derivatives, the average enthalpy of chlorination per one Cl atom was calculated and compared with the standard value for D3d-C60Cl30.16a The previously known tendency of decreasing the energy of C−Cl bonds with increasing number of Cl atoms attached to the fullerene cage regardless of its size was confirmed.16b For chlorides of C98(248) with 22, 24, and 26 attached Cl atoms, the average relative chlorination enthalpies are 3.3, 2.0, and 0.9 kJ mol−1, respectively. The most probable chloro derivatives of new C98 isomers, C98(109)Cl22, C98(107)Cl20, and C98(111)Cl20,

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 supported by the National Natural Science Foundation of China (Grants 21132007, 21371164, and 2151101074) and the Russian Foundation for Basic Research (Grants 15-03-04464 and 16-53-53012).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b00568 Inorg. Chem. 2017, 56, 4780−4783