Rebuilding C60: Chlorination-Promoted Transformations of the

Jun 29, 2018 - Department of Chemistry, Moscow State University, 119991 Moscow, Leninskie gory , Russia. ‡ Institute of Chemistry, Humboldt Universi...
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Rebuilding C60: Chlorination-Promoted Transformations of the Buckminsterfullerene into Pentagon-Fused C60 Derivatives Victor A. Brotsman,† Nadezhda B. Tamm,† Vitaliy Yu. Markov,† Ilya N. Ioffe,† Alexey A. Goryunkov,† Erhard Kemnitz,*,‡ and Sergey I. Troyanov*,† †

Department of Chemistry, Moscow State University, 119991 Moscow, Leninskie gory, Russia Institute of Chemistry, Humboldt University of Berlin, Brook-Taylor.-Str.2, 12489 Berlin, Germany

‡

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S Supporting Information *

ABSTRACT: In recent years, many higher fullerenes that obey the isolated pentagon rule (IPR) were found capable of rearranging into molecules with adjacent pentagons and even with heptagons via chlorination-promoted skeletal transformations. However, the key fullerene, buckminsterfullerene Ih-C60, long seemed insusceptible to such rearrangements. Now we demonstrate that buckminsterfullerene yet can be transformed by chlorination with SbCl5 at 420−440 °C and report X-ray structures for the thus-obtained library of non-IPR derivatives. The most remarkable of them are non-IPR C60Cl24 and C60Cl20 with fundamentally rearranged carbon skeletons featuring, respectively, four and five fused pentagon pairs (FPPs). Further high-temperature trifluoromethylation of the chlorinated mixture afforded additional non-IPR derivatives C60(CF3)10 and C60(CF3)14, both with two FPPs, and a nonclassical C60(CF3)15F with a heptagon, two FPPs, and a fully fused pentagon triple. We discuss the general features of the addition patterns in the new non-IPR compounds and probable pathways of their formation via successive Stone−Wales rearrangements.

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INTRODUCTION Dopant-free syntheses of fullerene soot give only the structures that obey the isolated pentagon rule (IPR),1 that is, do not feature edge-sharing pentagons.1,2 Furthermore, the chemistry of pristine fullerenes, including C60, C70, and higher fullerenes C74−C108, is almost exclusively limited to exohedral derivatization without any changes in the cage connectivity pattern.3−5 However, theoretical studies of non-IPR systems pointed out that the derivatized non-IPR cages can be thermodynamically advantageous over the respective IPR compounds.6 The first experimental observation of a nondestructive IPR-to-non-IPR cage transformation was reported in 2009.7 Non-IPR 18917 C76Cl24 with as many as five fused pairs of pentagons (FPPs) (cage numbering in superscript is according to the spiral algorithm as applied to the full list of the pentagonhexagon cages2) was isolated from the products of 350 °C chlorination of IPR D2-C76(1) with SbCl5 (IPR isomer numbers in parentheses are according to their ordering by the spiral algorithm,2 alternative designation is 19150C76). Theoretical analysis has revealed7,8 that formation of 18917 C76Cl24 requires seven successive Stone−Wales rearrangements (SWRs),9 that is, rotations of 6:6 C−C bonds within pyracylene subunits by 90°. Later studies produced more examples of chlorination-promoted SWRs in higher fullerenes in the range of C76−C102, revealed their considerable exothermicity brought about by formation of chlorinated pentagon−pentagon junctions, and shed more light on their detailed mechanisms.10−18 SWR rotations were found to take © XXXX American Chemical Society

place not only in pyracylene patches consisting of two pentagons and two hexagons (subtype designated as SWR1) but also in the patches of one pentagon and three hexagons, thus creating nonclassical cages with heptagonal rings (SWR2).10−12,15,16 Together with the SWRs, another kind of cage transformation has been found to occur under hightemperature chlorination: partly mechanistically similar C2 abstractions.12,14−16,19,20 Earlier, C2 abstraction from the buckminsterfullerene was also observed under harsh solidphase fluorination conditions.21 In the IPR cages, C2 losses involve 5:6 bonds and thus constitute a different pathway to nonclassical cages with heptagons. In some of the presently known cases of complex skeletal rearrangements, SWR and C2 loss steps coparticipate in consecutive and/or branched multistep pathways.12,14−16,20 Apart from synthesizing endohedral metallo- or clusterfullerenes, an alternative approach to the non-IPR and/or nonclassical isomers consists in arc-discharge or combustion syntheses from graphite with addition of dopants (CCl4, Cl2, CH4) to the reactor atmosphere. Chlorine or hydrogen atoms stabilize the sterically unfavorable pentagon−pentagon junctions thus enabling survival of the lower non-IPR fullerenes C50, C54, C56 as well as non-IPR C60, C64, C70, and C72 in the chlorinated or hydrogenated form.22−34 It has been hypothesized that the Cl or H atoms are captured by the unconventional carbon cages already formed in the carbon Received: April 12, 2018

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DOI: 10.1021/acs.inorgchem.8b00976 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry plasma rather than participate in the process of their assembly.31 The thus-obtained non-IPR chlorides of C60 and C70 were found, similarly to the SWR products, to possess much higher energetic stability than their IPR counterparts.29,31,32 On the one hand, an advantage of the dopantmodified syntheses from graphite is accessibility of the compounds without any conceivable IPR precursors, such as the trailblazing C50Cl10 reported in 2004.22 On the other hand, transformations of the presynthesized IPR fullerenes can provide a narrower, better-defined range of products based on a specifically selected precursor. In addition, a predefined precursor can facilitate understanding of the transformation mechanisms. Until now, the transformational approach worked rather well with higher fullerenes but seemed inapplicable to the two most abundant IPR fullerenes C60 (1812C60) and C70 (8149C70). Their chlorination with SbCl5 did not bring about any skeletal changes and only afforded various IPR chlorides up to D3dC60Cl30 and C70Cl28 (a mixture of isomers), respectively.35,36 Yet there remained a hope that the potential of the time and temperature factors had not been fully exploited. Indeed, there was an example of C78(2) that initially produced only chlorides with unchanged carbon skeleton up to C78(2)Cl30,37 yet sixweek heating eventually transformed it into a nonclassical (NC) and non-IPR chloride C78(NC2)Cl24.12 The C78 case prompted us to try harsher chlorination conditions on C60, and that approach finally effected diverse and deep transformations of the buckminsterfullerene cage.

Figure 1. Schlegel diagrams of the non-IPR C60 derivatives 1−6 obtained in the present work. Cage pentagons are shown with red filling, and the cage heptagon is shown with blue filling. The positions of attached (black ▲) CF3, (black ●) Cl, and (green ●) F addends are indicated. Also shown are isolated double bonds and benzenoid and naphtalenoid substructures.

reported 1809C60Cl8, 1804C60Cl12,29 and 1809C60H8,30 obtainable by arc-discharge or combustion syntheses from graphite, the present set of products shows greater diversity of structures and provides examples of deeper transformations. One can see that the pentagon−pentagon junctions in all compounds are functionalized (with one exception in compound 6), which provides energetic stabilization thereof, even despite the sterically unfavorable adjacent addition of bulky CF3 groups. Obviously, formation of the non-IPR carbon cages in 1−6 from the IPR Ih-C60 cage results from chlorination-promoted SWRs similar to those previously observed in higher fullerenes C76−C102.7−13,17,18 The structural data for the compounds 1−6 will be presented below in the order of increasing length of the required SWR sequence. The molecules of the two CF3-only derivatives 4a and 5 with the same non-IPR cage, namely, 1809C60(CF3)10O and 1809 C60(CF3)14, are shown in Figure 2a,b. Epoxide formation in 1809C60(CF3)10 occurs during recrystallization, and the mass spectrum of the initial HPLC-isolated fraction reveals compound 4 without oxygen (see Supporting Information, Figure S8). The C2v symmetric non-IPR carbon cage of 1809C60 contains two FPPs (see Figure 1 for the Schlegel diagram). It can be created by an SWR of any of the equivalent 6:6 C−C bonds of the parent buckminsterfullerene. Hence, any multistep SWR transformation of the IPR C60 includes 1809 C60 as the first intermediate. In contrast, formation of 1809 C60Cl8 in the arc-discharge synthesis with chlorinecontaining additives was hypothesized to occur via stabilization of the independently formed 1809C60 cage by Cl addends.29,31 Trifluoromethylation patterns of 1809C60(CF3)10 (4) and 1809 C60(CF3)14 (5) (Figure 1) are both mirror symmetric, preserving either of the two mirror planes of the C2v-1809C60 cage (see the Schlegel diagram of 4a in Figure S9 in Supporting Information). A characteristic feature of the CF3 patterns in 4 and 5 is that all common edges of FPPs are stabilized by the CF3 addends, whereas in the IPR C60(CF3)2n compounds (2n = 2−12) adjacent CF3 attachment is generally less favorable.40 Other CF3 groups show para-attachment with respect to each other, except for one more adjacent pair of addends in 1809C60(CF3)14. It is worth noting that the addition patterns of both 1809C60(CF3)10 and 1809C60(CF3)14 include the

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RESULTS AND DISCUSSION Previously, chlorination of the IPR C60 with an excess of SbCl5 at 340−360 °C was known to produce IPR D3d-C60Cl30 as the final product.35 In the present work, chlorination was performed at higher temperatures of 420−440 °C and resulted in the formation of non-IPR chlorides. X-ray diffraction study of the crystals obtained from chromatographically separated fractions revealed three non-IPR chlorides: 1810C60Cl24 (1), 1805 C60Cl24 (2), and 1794C60Cl20 (3). Notwithstanding relative complexity of the reaction mixtures, high-performance liquid chromatography (HPLC) separation of the products was not complicated by the light sensitivity issues reported in the literature for lower IPR chlorides C60Cl2−10.38 Compounds 1 and 2 were found to dominate in the chlorination products, each with ca. 15% yield related to the starting buckminsterfullerene, and compound 3 was recovered with a smaller 3% yield. To recover more non-IPR products, we employed the trick of high-temperature trifluormethylation of the chlorination products followed by HPLC separation. One can assume that trifluoromethylation leaves the skeleton connectivity intact, since it gives a plentitude of conventional CF3 derivatives with the IPR fullerenes, more than 50 for C60 alone,39 but not a single transformed one. Recently, the chlorination-trifluoromethylation approach was successfully applied to the IPR D2-C76 to recover several derivatives of a new non-IPR C76 cage.11 In the present case of C60, it enabled X-ray structural elucidation of the non-IPR 1809C60(CF3)10 (4), 1809 C60(CF3)14 (5), and nonclassical C60(NC)(CF3)15F (6). A general overview of compounds 1−6 presented as Schlegel diagrams is given in Figure 1. As one can see, they feature two to five fused pentagon pairs (FPPs). Particularly interesting is nonclassical C60(NC)(CF3)15F (6) with two FPPs, a fully fused pentagon triple, and a heptagon. Compared to the previously B

DOI: 10.1021/acs.inorgchem.8b00976 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Top and side views of two isomers of non-IPR C60Cl24, C2v-1810C60Cl24 (a, b) and D2d-1805C60Cl24 (c, d). The two compounds differ by the position of only one C−C bond situated in the centers of (a, c). The FPPs are highlighted with red.

Figure 2. Views of trifluoromethylated non-IPR C1-1809C60(CF3)10O (a), Cs-1809C60(CF3)14 (b), and nonclassical C1-C60(NC)(CF3)15F (c, d). Fused pentagons are shown with red and the cage heptagon in (c, d) is shown with blue. The F atom connected directly to the carbon cage is highlighted with brighter green.

be obtained from the buckminsterfullerene via two SWR1 rotations of the pair of opposite 6:6 C−C bonds, the two overlaying ones in the center of Figure 3b. The D2h symmetry of the 1810C60 cage is reduced to C2v due to less symmetric chlorination pattern. All four FPP junctions are stabilized with the Cl atoms, and further stabilization likely comes from breakdown of the π-system into four isolated benzenoid rings and six isolated double CC bonds. Mostly similar to C2v-1810C60Cl24 is D2d-1805C60Cl24 (2), also with four FPPs (Figures 1 and 3c,d). Compound 2 features the same chlorination motif and conjugated substructures as 1 and is formally coupled to 1 via an SWR1 rotation of the 6:6 bond shown in the center of Figure 3a. In structures 1 and 2, the four chlorinated pentagon− pentagon edges constitute the longest C−C bonds with average lengths of 1.594 Å (1) and 1.596 Å (2). Isolated CC bonds are the shortest ones in both structures with averaged lengths of 1.348 Å (1) and 1.337 Å (2). The average length of 24 C−Cl bonds is 1.793 Å in 1 and 1.788 Å in 2, but the chlorines attached to the pentagon−pentagon junctions are bound stronger (av lengths of 1.774 (1) and 1.770 (2)) than the ones at the other FPP sites (1.790 Å (1) and 1.785 Å (2)) and even more so than the ones in the isolated pentagons (1.814 Å (1) and 1.809 Å (2)). The latter values fall in the common range for the IPR fullerene chlorides.5,37 The most remarkable of the reported compounds is D5-1794C60Cl20 (3) with its lenslike carbon cage featuring five FPPs (Figures 1 and 4). The closed equatorial loop of chlorine

subpattern of the previously reported 1809C60Cl8.29 As expected, the longest C−C bonds in the carbon cages are those of the sp3−sp3 type with average (av) values of 1.61 Å (4a) and 1.59 Å (5). The isolated CC bond in the structure of 5 is 1.33 Å long. The molecular structure of C60(NC)(CF3)15F (6) is remarkable due to both its nonclassical, non-IPR carbon cage and its unusual addition pattern (Figures 1 and 2c,d). The C60(NC) carbon cage comprises a heptagon, two pairs of fused pentagons, and a fully fused triple of pentagons. It can be obtained from the common 1809C60 intermediate via an SWR2 process that occurs in a patch consisting of three hexagons and one pentagon and thus creates a heptagon. Previously, similar SWR2 processes have been observed in several higher fullerenes: C76(1),11 C78(2),12 and C100(18).15,16 An uncommon feature of the functionalization pattern of 6 is a fluorine addend at the center of the fused pentagon triple surrounded by three adjacent CF3 addends (Figure 2c,d). Fully fused pentagon triples with all four junction sites likewise functionalized have been previously observed in the non-IPR 1911 C64Cl423 and the nonclassical C84(NC2)Cl26.14 Of the sites in the common pentagon−pentagon edges, however, one remains free, which is likely due to the steric issues. Those edges provide the longest C−C bonds with an average length of 1.566 Å. The C(cage)−F bond has a length of 1.384(5) Å, typical of the fluorinated C60 compounds.41 It presently remains unclear, how a fluorine addend could have been cleaved from a CF3 group. Such cases have not been known in the chemistry of IPR cages, but likewise mixed CF3 + F addition patterns have recently been observed in a transformed non-IPR C76.11 Furthermore, mass spectrometry revealed many more C60(CF3)nFm compounds among the products of the present synthesis (see the mass spectrum in Figure S4 in the Supporting Information), and one can expect them to have non-IPR C60 cages as well. Turning to the isolated non-IPR chlorides, the carbon cage of C2v-1810C60Cl24 (1) with four FPPs (Figures 1 and 3a,b) can

Figure 4. Top and side views of D5-1794C60Cl20. The five fused pentagon pairs are shown with red. C

DOI: 10.1021/acs.inorgchem.8b00976 Inorg. Chem. XXXX, XXX, XXX−XXX

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and 57 kJ mol−1 more stable than the most stable IPR isomers of C60(CF3)10 and C60(CF3)14. Those values could have been even higher if not for the steric effects caused by the adjacently attached bulky addends at the pentagon−pentagon junctions. With less bulky chlorine addends and more FPPs, the thermodynamic driving force is, expectedly, much higher: 1810 C60Cl24 (1) and 1805C60Cl24 (2) with four FPPs are, respectively, 267 and 336 kJ mol−1 more stable than the IPR compound Td-C60Cl24.45 For structure 3 with five FPPs, no direct IPR counterpart is presently known, but compared to a hypothetical IPR D5d-C60Cl20, an analogue of saturnene C60F20 with the same closed loop addition pattern,42 D5-1794C60Cl20 is more favorable by remarkable 599 kJ mol−1. Another instructive characteristic is the change in the average C−Cl bond energy. Relative to the IPR D3d-C60Cl30,46 1810C60Cl24, 1805 C60Cl24, and 1794C60Cl20 were found to show stronger chlorine binding by, respectively, 24, 28, and 53 kJ mol−1 per Cl atom. Much higher value in 1794C60Cl20 reflects the fact that half of its chlorines are attached to the pentagon−pentagon junctions, compared to only one-third in the other two compounds. The increase in chlorine binding energy is in line with the shortening of the average C−Cl bond length from 1.793 Å (1) and 1.788 (2) to 1.781 (3). The need for higher temperatures to trigger skeletal rearrangements in buckminsterfullerene gives rise to a question, whether the Ih-C60 cage is harder to transform in itself, or other effects are possibly implicated. The most obvious is the effect of the chlorination motif. It depends on the arrangement of chlorines which of the C−C bonds can be rotated: at least one site of the rotated bond must be chlorinated, preferably both, and yet more preferably the bond must be a part of a longer chain of chlorinated sites.8,10 At the same time, the chlorination pattern determines the energetic effect of an SWR act: bond rotation is usually exothermic if the emerging pentagon−pentagon junction(s) is chlorinated, and some less straightforward steric aspects can play a role as well. Since SWRs proceed much slower than chlorine migration,8 an equilibrium distribution of chlorination patterns is established prior to each SWR step, including the equilibrium between different numbers of chlorine addends due to the exchange with SbCl5/SbCl3. Hence, if only a particular subset of isomeric chlorides is prone to skeletal transformations, the equilibrium content of the said isomers enters as a factor into the apparent transformation rate. Low abundance of the relevant chlorination patterns can thus considerably impede the rearrangements. In the highly symmetric terminal product of chlorination of the buckminsterfullerene, IPR D3d-C60Cl30,35 all possible SWR rotations are equivalent and give 1809C60Cl30. Our calculations demonstrate them to have a gross endothermic effect of 106 kJ mol−1 and a relatively high activation barrier of 291 kJ mol−1 compared to the ca. 240 kJ mol−1 barrier calculated for the limiting SWR stage in the IPR C76(1).8 That would translate into some 120 °C increase in the reaction temperature with respect to the C76(1) case (340−350 °C), quite in agreement with our present observations. However, formation of 1809 C60Cl30 is calculated to be highly endothermic, and stabilization of 1809C60 is possible only via partial dechlorination and chlorine migration to give a complex mixture of isomers of 1809C60Cl28 and possibly of other compositions. A comprehensive computational investigation of the whole range of possible products is presently unfeasible, but we considered

atoms cuts its π-system into the two corannulene caps. Previously, a double corannulene structure has been observed in the IPR C60F20 (“saturnene”)42 and the first discovered nonIPR chloride 271C50Cl10.22 Also to be mentioned in this regard are 18917C76Cl24, a considerably flattened molecule with two vinylcoronene substructures,7,8 and the recent nonclassical C78(NC2)Cl24, also flattened and with a single closed loop of chlorines.11 All 5:5, 5:6, and 6:6 C−C bonds within the chlorinated loop are roughly equally elongated with average length of 1.584 Å. The average length of C−Cl bonds is 1.781 Å. In Figure 5, we give a scheme of the SWR bond rotations through which the presently obtained cages are related to the

Figure 5. Shortest Stone−Wales pathways from the buckminsterfullerene (1812C60, top left) to the non-IPR C60 cages identified in the present study. The bonds to be rotated are indicated by colored ovals, the color code being in correspondence with the color of the arrows. SWR1 are classical Stone−Wales rotations in pyracylene fragments; SWR2 is a nonclassical rotation within a patch of three hexagons and one pentagon.

buckminsterfullerene. The carbon cage obtained after the first rotation, C2v-1809C60 with two FPPs, which is observed in the trifluoromethylated compounds 4 and 5, offers a number of inequivalent subsequent SWR options. Thus, one further SWR2 bond rotation gives the nonclassical C60(NC) cage of compound 6, while one of the possible SWR1 acts provides the D2h-1810C60 cage of compound 1 with four FPPs. Another cage with similarly arranged four FPPs, D2d-1805C60 of compound 2, forms from the buckminsterfullerene in altogether three SWR steps, either by one SWR1 act from 1810C60 or via an alternative pathway that involves Cs-1804C60 as the second intermediate. Cs-1804C60 has not been observed in the present work, but it had been captured previously as an arc-discharge product 1804 C60Cl12.29 Finally, the D5d-1794C60 cage of compound 3 with five FPPs, the most symmetric of the presently reported cages, requires five SWR steps to form. In view of that, it is not surprising that the yield of 3 is lower than those of 1 and 2. On the contrary, it is quite remarkable that the observed difference in abundance is not as dramatic as the difference in the SWR pathways length. Formation of the non-IPR and nonclassical C60 derivatives reported herein was found to be highly thermodynamically favored. According to our DFT calculations performed with the use of the PRIRODA software43 at the PBE/TZ2P level,44 1809 C60(CF3)10 (4) and 1809C60(CF3)14 (5) are, respectively, 65 D

DOI: 10.1021/acs.inorgchem.8b00976 Inorg. Chem. XXXX, XXX, XXX−XXX

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mol−1 is not much higher than that of the second stage of the D5d-1794C60Cl20 formation pathway. Once formed, 1810C60 will be rapidly stabilized as 1810C60Cl24 (1) via additional chlorination and rearrangement of the chlorination motif. Thus, the computational data support feasibility of our hypothesis that IPR D5d-C60Cl20 can be implicated in the transformations and that harsher reaction conditions are required to compensate for its minor abundance in the reaction mixture. Of course, other possible IPR precursors can be involved too, and the wealth of various intermediate compounds may actually be quite considerable.

several most stable structures of 1809C60Cl28 and found that they cannot offer any further energetically favorable SWR steps. In view of the above, it is quite likely that the starting buckminsterfullerene chloride involved in the transformations is different from IPR D3d-C60Cl30, at least when we consider any transformations beyond 1809C60. One can hypothesize that some other buckminsterfullerene chloride(s) with fewer chlorine addends can form in the reaction mixture as a result of, for example, partial dechlorination at elevated temperatures. Lower abundance of such chlorides would slow the skeletal transformations as discussed above. One obvious candidate for a starting compound is the previously mentioned saturnene-like IPR D5d-C60Cl20. Our computational search identifies it as the most stable IPR isomer of C60Cl20. With average C−Cl bond energy only 3 kJ mol−1 lower than that of the IPR D3d-C60Cl30, IPR D5d-C60Cl20 is sufficiently stable to be present in a hypothetic partially dechlorinated mixture. Furthermore, it features the same chlorination pattern as the final D5-1794C60Cl20 and thus does not require any chlorine migration. The respective five-step Stone−Wales pathway is sketched in Figure 6. The least

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CONCLUSIONS Thus, the buckminsterfullerene finally appeared to be as prone to skeletal transformations as higher fullerenes. Even though C60 requires considerably higher reaction temperatures, its major abundance and lower price signify more important advantages, as they enable large-scale synthesis of the non-IPR and nonclassical structures. On the one hand, the presently achievable integral transformation yield of above 33% is already rather high, and one can hope to refine it further and maybe also to achieve higher selectivity toward particular products by means of gentle tuning of the reaction conditions. On the other hand, the versatility of the presently reported transformed compounds provides new instructive insights into the diversity of accessible skeletal rearrangement pathways in the seemingly rigidly built fullerene frameworks. Previously, the attempts to recover a pristine non-IPR C60 cage from its chloride formed by dopant-modified arcdischarge syntheses were of little success.47 Potentially higher yields in the present transformational approach suggest better chances to obtain pristine non-IPR and nonclassical cages and to study their chemistry. After almost 10 years of just accumulating the library of skeletal rearrangements in fullerenes, we now see a possibility to make a step forward.

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EXPERIMENTAL SECTION

Synthesis and Isolation. In a typical experiment, C60 (50 mg) was placed into a thick-walled glass ampule, and an excess of SbCl5 was added. The ampule was evacuated and heated at 420−440 °C for 2−4 d. Caution! The pressure of SbCl5 reaches ca. 30 bar under the experimental conditions. After cooling and opening the ampule, the reaction product was heated at 100−150 °C under vacuum to remove the remaining SbCl5 and SbCl3. The black residue thus obtained contained fullerene chlorides and some hydrolyzed antimony compounds. The chlorinated product was dissolved in toluene, filtered to remove the antimony compounds, and subjected to HPLC separation in toluene using a semipreparative Buckyprep column (for more detail see the Supporting Information). The isolated fractions were characterized by matrix-assisted laser desorption ionization time-offlight (MALDI TOF) mass spectroscopy that revealed the presence of C60Cln compounds with n ranging from 6 to 24 (selected data are presented in the Supporting Information). After slow removal of the solvent, several chromatographic fractions gave small crystals. X-ray diffraction study with the use of synchrotron radiation afforded crystal structures of the crystal solvates of the non-IPR C60Cl24 (two isomers, 1 and 2) and C60Cl20 (3) (for selected crystallographic data and CCDC deposition numbers see Supporting Information, Table S1). The yields of the two most abundant C60Cl24 compounds (1 and 2) found in the first fractions reached ca. 15% for each of them, with further ca. 3% of the less abundant C60Cl20 (3). Some of the chlorinated mixtures were subjected to trifluoromethylation with gaseous CF3I in ampules at 450 °C for 2−4 h, similarly to the previously reported treatment of higher fullerenes48 and C76

Figure 6. Schlegel diagram presentation of a possible five-step cage transformation of D 5d - 1812 C 60 Cl 20 (“chlorosaturnene”) into D5-1794C60Cl20 (3). The C−C bonds to be rotated in the SWR1 step to follow are indicated with small ovals.

exothermic step is the first one (42 kJ mol−1) that creates 1809 C60Cl20 with two half-chlorinated pentagon−pentagon junctions. Formation of one fully chlorinated junction in each of the steps 2−4 makes them considerably more exothermic: 135, 122, and 111 kJ mol−1, respectively. Finally, the last, fifth step has the largest exothermic effect of 189 kJ mol−1 due to formation of two fully chlorinated junctions instead of half-chlorinated ones. Obviously, one would anticipate the first step to be the limiting one, and its activation energy is calculated to be 227 kJ mol−1, decreasing to 199 kJ mol−1 for the subsequent step. The calculated value is expectedly below the barrier of the endothermic SWR rotation in IPR D3d-C60Cl30 and is almost on par with the exothermic first SWR step in the IPR 19150 C76Cl28 that undergoes transformations at much lower reaction temperatures.10 IPR D5d-C60Cl20 can also account for the formation of at least 1810C60Cl24 (1). Indeed, although the heat of transformation of 1809C60Cl20 into the respective 1810 C60Cl20 is only 45 kJ mol−1, the activation barrier of 219 kJ E

DOI: 10.1021/acs.inorgchem.8b00976 Inorg. Chem. XXXX, XXX, XXX−XXX

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chlorination products11 Caution! The pressure of CF3I reaches ca. 6−7 bar under the experimental conditions. The raw trifluoromethylated mixtures analyzed by MALDI TOF mass spectroscopy revealed the presence of the C60(CF3)n and C60(CF3)nFm compounds with n ranging from 10 to 18 and m between 0 and 13 (a typical MALDI mass spectrum is given in Supporting Information, Figure S4). Trifluoromethylated products were dissolved in n-hexane and subjected to HPLC separation using preparative or semipreparative Buckyprep columns (see Supporting Information for more detail). After slow evaporation and recrystallization from toluene, some chromatographic fractions gave small crystals of the non-IPR compounds C60(CF3)10O (4a) and C60(CF3)14 (5) as well as of the nonclassical C60(CF3)15F (6), as was revealed by the X-ray diffraction studies. In the case of C60(CF3)10 (4) investigated before recrystallization, the MALDI MS data revealed only trace amounts of oxygen, which suggested that the oxidation occurred only upon subsequent recrystallization (see the MS spectra in Supporting Information, Figure S9). The yields of the isolated trifluoromethylated compounds were rather low (less than 1%), partly due to the two-step isolation process and broad distribution of the products over numerous HPLC fractions. X-ray Crystallography. Synchrotron X-ray data for crystals of 1− 6 were collected at 100 K on BL14.3 and BL14.2 at the BESSY storage ring (Berlin, Germany) using a MAR225 detector (λ = 0.8950 Å) or a pixel detector Pilatus2M (λ = 0.8266 Å). All structures were solved and anisotropically refined using the SHELX package. Selected crystallographic data and CCDC deposition numbers are given in Supporting Information. Theoretical Calculations. The quantum-chemical calculation of formation energies, chlorination enthalpies, and activation barriers were performed at the DFT level of theory with the use of the PRIRODA software43 with efficient implementation of density fitting (resolution-of-identity) for the pure exchange-correlation functionals. The PBE exchange-correlation functional44 and a built-in TZ2P basis set were used.

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ACKNOWLEDGMENTS This work was partially supported by the Deutsche Forschungsgemeinschaft (Ke-489/39-2). We would like to acknowledge the assistance of Dr. S. Feiler during the synchrotron diffraction experiments. The theoretical calculations were performed using the equipment of the shared research facilities of HPC computing resources at the Lomonosov Moscow State Univ.

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00976. Synthesis, isolation, and characterization of chlorination and trifluoromethylation products. Selected crystallographic data and pathway details of cage transformations (PDF) Accession Codes

CCDC 1828797−1828802 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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Corresponding Authors

*E-mail: [email protected]. (E.K.) *E-mail: [email protected]. (S.I.T.) ORCID

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

The authors declare no competing financial interest. F

DOI: 10.1021/acs.inorgchem.8b00976 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00976 Inorg. Chem. XXXX, XXX, XXX−XXX