Article pubs.acs.org/IC
Capturing the Fused-Pentagon C74 by Stepwise Chlorination Cong-li Gao,‡,† Laura Abella,‡,§,† Yuan-Zhi Tan,† Xin-Zhou Wu,† Antonio Rodríguez-Fortea,*,§ Josep M. Poblet,*,§ Su-Yuan Xie,*,† Rong-Bin Huang,† and Lan-Sun Zheng† †
State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China § Departament de Química Física i Inorgànica, Universitat Rovira i Virgili, c/Marcel·lí Domingo 1, 43007 Tarragona, Spain S Supporting Information *
ABSTRACT: As a bridge to connect medium-sized fullerenes, fusedpentagon C74 is still missing heretofore. Of 14 246 possible isomers, the first fused-pentagon C74 with the Fowler−Manolopoulos code of 14 049 was stabilized as C74Cl10 in the chlorine-involving carbon arc. The structure of C74Cl10 was identified by X-ray crystallography. The stabilization of pristine fused-pentagon C74 by stepwise chlorination was clarified in both theoretical simulation with density functional theory calculations and experimental fragmentation with multistage mass spectrometry.
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involving arc-discharge of graphite,14 and definitely characterized as C1(14 049)-C74Cl10 by mass spectra and single-crystal X-ray diffraction.
INTRODUCTION So far, all the clearly characterized structures of C74 derivatives are based on the same isomer of C74 with a D3h symmetry and a Fowler−Manolopoulos code of 14 246,1 namely, the sole isolated pentagon rule (IPR)-satisfying isomer among 14 246 C74 isomers.2 Although a number of non-IPR fullerenes have been stabilized by endohedral or exohedral derivatization in previous reports, the non-IPR structure in the family of C74 was unclear in experiments. Because of a small highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO; HL) energy gap, which confers kinetic instability,3 the IPR C74 is subject to polymerization resulting in insolubility and difficult extraction from raw soot using organic solvents.4,5 However, the polymerized solids can be reduced into C74 anion and separated from the carbon soot by electrochemical method.4 They also can sublime out of the soot in high temperature5 and then, respectively, react with K2PtF66 and gaseous CF3I7,8 into C74F38 and C74(CF3)12. A series of endohedral metallofullerenes M@C74 containing divalent or trivalent metal are synthesized in experiments, such as Ca@C74,9,10 Ba@C74,11 Y@C74(CF3)(I), and Y@C74(CF3) (II).12 The insoluble endohedral metallofullerene, La@C74, was solubilized and isolated as dichlorophenyl derivative, La@C74(C6H3Cl2), and characterized by Xray crystallography.13 Heretofore no direct evidence from 13C NMR and singlecrystal X-ray diffraction confirms the occurrence of reactive non-IPR C74 in carbon arc. Now we report a hollow non-IPR C74 isomer with a Fowler−Manolopoulos code of 14 049,1 which was captured by exohedral chlorination, extracted from toluene solution of carbon soot produced in a chlorine© XXXX American Chemical Society
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RESULTS AND DISCUSSION Single crystals of C1(14 049)-C74Cl10 suitable for X-ray diffraction analysis were grown in toluene solution, and the structure of this chlorofullerene derivative of C1(14 049)-C74 was unambiguously revealed (Figure 1). In contrast to the nonIPR endofullerenes of C74 and the IPR exofullerenes of C74, the pristine cage of C1(14 049)-C74Cl10, which has C1 symmetry, represents the first hollow pentagon-fused isomer in the C74 family. The unfavorable local strain at the vertexes of the pentagon fusion in C1(14 049)-C74 can be released through
Figure 1. Top (left) and side (right) views of C1(14 049)-C74Cl10 as ORTEP drawings with thermal ellipsoids at the 50% probability level. The fused pentagons in the structure are highlighted in blue. Received: December 4, 2015
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DOI: 10.1021/acs.inorgchem.5b02824 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 2. Multistage mass spectra (MSn, n = 1−9) of C1(14 049)-C74Cl10. The mother ions C74Clx− (x = 10, 9, 8, 7, 6, 5, 4, 3, 1) selected for the next dechlorination are marked by rhombus.
Figure 3. Representation of the HOMO (for C74, C74Cl4, C74Cl6, C74Cl8, and C74Cl10) or the spin density (for C74Cl3, C74Cl5, C74Cl7, and C74Cl9) distributions, along with the corresponding Schlegel diagrams, for different steps of the chlorination pathway. The green dots in the Schlegel diagrams indicate the positions of the chlorine atoms. The black arrows in the three-dimensional structure and the black dots in the Schlegel diagrams show the position where the next chlorine atom is added.
Purified C1(14 049)-C74Cl10 has a vivid brick-red color in toluene solution and shows a similar UV absorption as #8064 C70Cl1020 over a wide range of wavelength (Figure S2 of the Supporting Information). In addition to the initial absorption at 690 nm, a series of strong absorptions are located at 358, 414, 458, 494, 539, and 581 nm. Such a wide range of absorption may be useful for technological applications in some fields such as fullerene-based solar cells.
bonding to two chlorine atoms. In addition to the two chlorine atoms associated with the pentagon fusion, eight chlorine atoms are bonded at eight additional pentagon−hexagon− hexagon vertexes in a chain of sp3-hybridized carbon atoms, resulting in an aromatic biphenyl and a C42 fragment. This addition pattern is analogous to previously reported #11188 C72Cl415,16 and #4348C66Cl10,17,18 and in good accord with the local aromaticity principle.19 B
DOI: 10.1021/acs.inorgchem.5b02824 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
and Figure S4 in the Supporting Information). In addition, these two atoms show the highest atomic orbital contributions to the HOMO of C74 (see Figure 3a). Once the first chlorine was added in any of the two C atoms of the pentalene bond, the two possible C74Cl radicals that could be generated show important values of the spin density on the other C atom of the pentalene. Therefore, both strain and electronic factors point to sequential addition to the pentalene bond for the first two chlorination steps. On the basis of the highest contribution to the HOMO of C74Cl2 the next position to be chlorinated is one [5, 6, 6] C atom contiguous to the pentalene bond, indicated by an arrow in C74Cl2 or as a black dot in the Schlegel diagram in Figure 3b. For the C74Cl3 radical, which shows several active sites with rather high spin densities, the most stable regioisomer was found to be that with the four chlorine atoms placed in the fused pentagons in a zigzag manner (see Figure 3c). Therefore, analogous to #11188C72Cl4 and other non-IPR chlorofullerenes, there are four Cl atoms placed at the fused pentagons through one [5, 5] bond in a zigzag manner.15,16 Further chlorination on C74Cl4 was investigated. The fifth addition takes place again in the carbon atom with the highest HOMO contribution (see Figure 3d), and the sixth addition is to the position with the highest spin density in C74Cl5 (Figure 3e). Chlorination up to C74Cl10 follows the same patterns: the lowest-energy regioisomers are those having highest HOMO (for systems with an even number of Cl atoms) or highest spin density contributions (for systems with an odd number of Cl atoms). Other positions that present high HOMO contributions or high spin densities were also analyzed, but in all cases the selected positions are the ones that led to the regioisomers with the lowest energy. Thus, stepwise chlorination from C1(14 049)-C74 to C1(14 049)-C74Cl10 might take place following two simple rules; that is, (i) addition on structures with an even number of chlorines is directed by the highest atomic orbital contributions to the HOMO and (ii) addition on structures with an odd number of chlorines is governed by the topology of the spin densities. It is worth remarking here that these results are in good agreement with the chlorination patterns established so far17−20 and with the experimental results as exemplified by dechlorination in the multistage mass spectra in negative ion mode (Figure 2). The introduction of chlorine atoms effectively reduces reactive sites of the bare cage, preventing from polymerization and improving the solubility. We observed that the H-L gaps show a gradual increase upon chlorination up to C1(14 049)-C74Cl10 (see Table 1). In addition, bond energies between carbon and chlorine atoms in C1(14 049)-C74 were found to be similar as in other chlorinated fullerenes detected so far (ca. −50 kcal mol−1). The bond energies for the first and second chlorines, which attach to the pentalene bond, are ca. −60 kcal mol−1. We checked that the bond energies for dichlorination at other positions of the cage are much lower. The C−Cl bond energies for radicals C1(14 049)-C74Clx (x = 3, 5, 7, and 9) are significantly smaller than those for closed-shell species (x = 4, 6, 8, and 10). The bond energies for x = 6 and x = 8 are rather high (−58 kcal mol−1), but the energy for the addition of the 10th chlorine is by far the largest, which is related to the high stability of the formed product C1(14 049)-C74Cl10. Despite the above analysis, it is unnecessary to rule out that the C1(14 049)-C74Cl10 formation occurs through chlorination of the isomeric D3h(14 246)-C74, which is existent in the fullerene soot.4−8 Topologically it is possible to undergo a single Stone−Wales rotation23 from the IPR D3h(14246)-C74 to
The molecular composition of C74Cl10 was confirmed by mass spectrometry (MS) using an atmosphere pressure chemical ionization (APCI) source (Figure 2). The APCI furnace temperature was 350 °C. As the inset shown in Figure 2, the dominant isotopic pattern of the strong peak around m/z 1243.7 matches well with the simulated C74Cl10. Compared with C72Cl4,15,16 C74Cl10 has a good thermal stability at high temperature area, and the bare carbon cage C74 can be observed by fragmentizing of mother C74Cl10 molecules in collision with He atoms in the multistage mass spectrometry21 (see the Supporting Information). The strong peak at 887.9 m/z confirms that the non-IPR C74 fullerene is produced in the graphite arc-discharge and existed in the gas phase. In principle, the kinetic stability of a molecule can be related with its H-L gap. As discussed above, the IPR D3h(14246)-C74 isomer is elusive in the soot of graphite arc-discharge due to its small H-L gap.3,4 In addition, this D3h(14 246)-C74 isomer is found to have an open-shell electronic structure according to computation at density functional theory level,22 which confers it a radical-like character prone to polymerize. When we compare the H-L gaps of the non-IPR C1(14 049)-C74 and C1(14 049)-C74Cl10 [The HOMO and LUMO of C1(14 049)C74 and C1(14 049)-C74Cl10 are shown in Figure S3 of the Supporting Information], we observe that chlorination of C1(14 049)-C74 induces a change in the H-L gap from 0.344 eV in the pristine structure to 1.727 eV in the chloride (at BP86/ TZP level). The latter is even larger than that in C60 (1.658 eV), implying that the external chlorine atoms clearly increase the kinetic stability of the cage and stabilize the fullerene. Even though the C1(14 049)-C74 is viable in the gas phase as inferred from the multistage mass spectra (vide supra), the nonIPR C1(14 049)-C74 cage is assumed much more reactive to chlorine than the IPR D3h(14246)-C74 cage because of the enhanced reactivity of the pentalene [5,5] bond compared to the [5,6] and [6,6] bonds of the IPR structure. The IPR cage shows significantly lower energy than the non-IPR C1(14 049)C74 one (21 kcal mol−1 at BP86/TZP level), but the relative stability is already changed for the dichlorinated system. The energy of the non-IPR C1(14 049)-C74Cl2 isomer is 11 kcal mol−1 lower than that of D3h(14246)-C74Cl2 isomer (see Figure S5 in the Supporting Information). Therefore, we analyze below, using computational tools, the most favorable positions for the sequential chlorination steps of C1(14 049)-C74, which seems to be a plausible mechanism, among other possibilities, for the formation of C1(14 049)-C74Cl10. Sometimes, the reactivity of a given fullerene can be rationalized easily from the simple inspection of energies and shapes of its molecular orbitals. In particular, the radical addition to a non-IPR fullerene can be understood from the atomic orbital contributions to the HOMO and the spin density distribution for cages with an odd number of Cl atoms. The shapes of the HOMOs for the closed-shell systems C 1 (14 049)-C 74 , C 1 (14 049)-C 74 Cl 2 , C 1 (14 049)-C 74 Cl 4 , C 1 (14 049)-C 74 Cl 6 , C 1 (14 049)-C 74 Cl 8 , and C 1 (14 049)C74Cl10 together with the spin densities for the radicals C1(14 049)-C74Cl3, C1(14 049)-C74Cl5, C1(14 049)-C74Cl7, and C1(14 049)-C74Cl9, are drawn in Figure 3 along with the stepwise chlorination pathway. The first two chlorine atoms are added sequentially to the pentalene bond of the fused pentagons so that strain is maximally released.18 The C atoms in these pentalene bonds are by far the two most pyramidalized atoms in the structure (pyramidalization angles of 16.25° and 14.89°, see Table S1 C
DOI: 10.1021/acs.inorgchem.5b02824 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry *E-mail:
[email protected]. (X.Y.S.)
Table 1. C−Cl Bond Energies and HOMO−LUMO Gaps (eV) for Chlorinated Systems at Different Stages of Chlorination, C1(14 049)-C74Clx (x = 0−10)a,b system
C−Cl BE
C74 C74Cl1 C74Cl2 C74Cl3 C74Cl4 C74Cl5 C74Cl6 C74Cl7 C74Cl8 C74Cl9 C74Cl10
−59.5 −60.3 −49.4 −51.5 −45.8 −58.6 −43.2 −58.2 −44.4 −64.9
Author Contributions ‡
These authors contributed equally. All authors have given approval to the final version of the manuscript.
H-L gap
Notes
0.344
The authors declare no competing financial interest.
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0.763
ACKNOWLEDGMENTS This work was supported by the 973 Project (2014CB845601) and the National Science Foundation of China (U1205111, 21390390, 51572231). This work was also supported by the Spanish Ministerio de Ciencia e Innovación (Project No. CTQ2014-52774-P) and by the Generalitat de Catalunya (2014SGR-199 and XRQTC). L.A. thanks the Generalitat de Catalunya for a predoctoral fellowship (FI-DGR 2014) and also the Chinese Government for a scholarship to study in China (Bilateral Program 2015-1016).
0.813 1.275 1.297 1.727
a
BE are in kilocalories per mole, and H-L gaps are in electronvolts. b The BE are computed as BE = E(C 1 (14 049)-C 74 Cl x ) − E(C1(14 049)-C74Clx−1) − E(Cl). H-L gaps only for closed-shell systems.
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the non-IPR C1(14 049)-C74, as stated in the caption of Figure S5. The examples of isomeric transformations in chlorinated higher fullerenes as well as in inorganic systems have been observed under solvothermal conditions in seated vessel,24−28 but the pressure of chlorine in the arc-discharge reaction is obviously lower than those in the solvothermal reactions. The detailed discussion on this topic is apparently beyond the scope of the present work.
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CONCLUSION Overall, the reactive small band gap fullerene C1(14 049)-C74, representing the first non-IPR isomer of C74 characterized by Xray single-crystal diffraction, was successfully stabilized by regioselective chlorination in the graphite arc-discharge. The stepwise chlorination from C1(14 049)-C74 to the experimental product C1(14 049)-C74Cl10 was rationalized by means of density functional theory computations, which is helpful for clarifying the mechanism responsible for stabilization of nonIPR fullerenes in the chlorine-involving graphite arc-discharge process. Multistage mass spectrometry supports the existence of pristine C1(14 049)-C74 in the gas phase, which may provide new experimental evidence for understanding the formation mechanism responsible for medium-sized fullerenes that is still a puzzle to chemists.
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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.5b02824. Synthesis of C1(14 049)-C74Cl10 in the chlorine-involving arc-discharge, HPLC isolation of C1(14 049)-C74Cl10, Xray diffraction analysis, UV−vis spectrum of C1(14 049)C74Cl10, computational details (PDF) Crystallographic data for C 1 (14 049)-C 74 Cl 10 ·C 7 H 8 (CIF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. (A.R.F.) *E-mail:
[email protected]. (J.M.P.) D
DOI: 10.1021/acs.inorgchem.5b02824 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.5b02824 Inorg. Chem. XXXX, XXX, XXX−XXX
