Fusing Pentagons in a Fullerene Cage by Chlorination: IPRD2-C76Rearranges into non-IPR C76Cl24

Nonclassical C86 and C88 Chlorofullerenes via Complex Chlorination-Promoted Skeletal Transformations of IPR Isomers of C88 and C96

Upon high-temperature (340-360 °C) chlorination with VCl4, an isolated-pentagon-rule (IPR) C2-C88(33) fullerene is just chlorinated to C88(33)Cl22 and C88(33)Cl28, but a VCl4/SbCl5 mixture promotes a five-step cage transformation to nonclassical C86(NC2)Cl30 with two heptagons and five pairs of fused pentagons. Another chlorination-promoted seven-step transformation of an IPR fullerene removes as many as four C2 fragments from C96 to give a nonclassical C88(NC1)Cl30 with cage heptagon and six fused pairs of pentagons. We discuss the driving forces behind the observed transformations and probable detailed pathways thereof.

Chlorination-Promoted Skeletal Transformations in Isolated-Pentagon Rule (IPR) Isomers of Fullerene C86 to Non-IPR Chloro- and Trifluoromethyl Derivatives

Fullerene C86 contains two isomers obeying the Isolated-Pentagon Rule (IPR), CS-C86(16) and C2-C86(17). Both isomers undergo unprecedented skeletal transformations at high-temperature (400 °C) chlorination with SbCl5. One-step Stone-Wales rearrangement (SWR) in C86(17) results in the pentagon-fused #63614C86 cage found in the structure of #63614C86Cl24. CF3 derivatives with the same cage, two isomers of #63614C86(CF3)18 and #63614C86(CF3)18O2, were obtained by high-temperature trifluoromethylation of the chlorination products with CF3I, followed by HPLC separation. The skeletal transformation of C86(16) proceeds via two SWRs under the formation of a #63624C86 cage with one fused-pentagon pair found in the structure of #63624C86(CF3)18. The addition patterns in skeletally transformed molecules are discussed in detail, disclosing the influence of the pentagon fusions, isolated C=C bonds, and benzenoid rings on the stability of the molecules with non-IPR C86 cages. The chlorination-promoted SWRs in C86 isomers have been observed for the first time, which contribute a lot to the understanding of skeletal transformations in fullerenes.

Dimeric and 1D polymeric low-chlorinated C60 fullerenes, (C60Cl5)2 and (C60Cl4)∞

Low-chlorinated fullerenes, dimeric (C₆₀Cl₅)₂ and one-dimensional, polymeric (C₆₀Cl₄)_∞, were obtained by high-temperature (270 °C) chlorination of C₆₀ with a SbCl₅/SbCl₃ mixture, as revealed by X-ray crystallography. The compounds were characterized by IR and Raman spectroscopy and theoretical calculations. This is the first observation of a fullerene polymer with single C-C bonding and neutral building blocks.

Crippling the C70 fullerene: non-classical C68Cl26(OH)2 and C68Cl25(OH)3 with three heptagons and only fused pentagons via chlorination-promoted skeletal transformations

High-temperature (440 °C) chlorination of C₇₀ with SbCl₅ promotes Stone-Wales transformations and loss of the C₂ fragment, which results in a non-classical C₆₈Cl₂₈ partially hydrolyzed to C₆₈Cl₂₆(OH)₂ and C₆₈Cl₂₅(OH)₃. X-ray diffraction reveals an unprecedented C₆₈ cage with three heptagons and 15 pentagons arranged in fused pairs and triples. The shortest possible transformation pathways include one C₂ loss step and four Stone-Wales transformation steps.

Fused-Pentagon C70Cl6 and C70Cl8 Obtained via Chlorination-Promoted Skeletal Transformation of IPR C70

The isolated-pentagon-rule (IPR) D_5h-C₇₀ fullerene is least susceptible to skeletal transformations in comparison with higher fullerenes and even C₆₀. A cage transformation in IPR C₇₀ via a one-step Stone-Wales rearrangement was accomplished by high-temperature (440 °C) ampule chlorination with SbCl₅. Subsequent dechlorination at 450 °C, followed by high-performance liquid chromatography separation, allowed the isolation of non-IPR C₇₀Cl₆ and C₇₀Cl₈. X-ray diffraction study revealed the presence of an unprecedented C₇₀ carbon cage, possessing two pairs of fused pentagons and the chlorination patterns located on one cage hemisphere. A high energetic and thermal stability of both non-IPR chlorides was also confirmed by theoretical calculations of formation energies. Pathways of skeletal transformations of IPR C₇₀ in comparison with those in C₆₀ are discussed.

Fused-Pentagon Isomers of C60 Fullerene Isolated as Chloro and Trifluoromethyl Derivatives

The carbon cage of buckminsterfullerene I_h -C₆₀ , which obeys the Isolated-Pentagon Rule (IPR), can be transformed to non-IPR cages in the course of high-temperature chlorination of C₆₀ or C₆₀ Cl₃₀ with SbCl₅ . The non-IPR chloro derivatives were isolated chromatographically (HPLC) and characterized crystallographically as ¹⁸⁰⁹ C₆₀ Cl₁₆ , ¹⁸¹⁰ C₆₀ Cl₂₄ , and ¹⁸⁰⁵ C₆₀ Cl₂₄ , which contain, respectively two, four, and four pairs of fused pentagons in the carbon cage. High-temperature trifluoromethylation of the chlorination products with CF₃ I afforded a non-IPR CF₃ derivative, ¹⁸⁰⁷ C₆₀ (CF₃ )₁₂ , which contains four pairs of fused pentagons in the carbon cage. Addition patterns of non-IPR chloro and CF₃ derivatives were compared and discussed in terms of the formation of stabilizing local substructures on fullerene cages. A detailed scheme of the experimentally confirmed non-IPR C₆₀ isomers obtained by Stone-Wales cage transformations is presented.

Fused-pentagon C70Cl26 obtained via chlorination-promoted Stone-Wales cage transformations of C70

High-temperature (360 °C) chlorination of C₇₀ with VCl₄ or SbCl₅ yields only IPR C₇₀Cl_26/28. Chlorination with SbCl₅ at 440 °C resulted in a skeletal transformation via a two-step Stone-Wales rearrangement and the formation of non-IPR ⁸⁰⁰⁵C₇₀Cl₂₆ with two fused pentagon pairs in the carbon cage which was established by single crystal X-ray diffraction.

Strain Release of Fused Pentagons in Fullerene Cages by Chemical Functionalization

According to the isolated pentagon rule (IPR), for stable fullerenes, the 12 pentagons should be isolated from one another by hexagons, otherwise the fused pentagons will result in an increase in the local steric strain of the fullerene cage. However, the successful isolation of more than 100 endohedral and exohedral fullerenes containing fused pentagons over the past 20 years has shown that strain release of fused pentagons in fullerene cages is feasible. Herein, we present a general overview on fused-pentagon-containing (i.e. non-IPR) fullerenes through an exhaustive review of all the types of fused-pentagon-containing fullerenes reported to date. We clarify how the strain of fused pentagons can be released in different manners, and provide an in-depth understanding of the role of fused pentagons in the stability, electronic properties, and chemical reactivity of fullerene cages.

Chlorination-Promoted Skeletal Transformations of Fullerenes

Classical fullerenes are built of pentagonal and hexagonal rings, and the conventional syntheses produce only those isomers that obey the isolated-pentagon rule (IPR), where all pentagonal rings are separated from each other by hexagonal rings. Upon exohedral derivatization, the IPR fullerene cages normally retain their connectivity pattern. However, it has been discovered that high-temperature chlorination of fullerenes with SbCl₅ or VCl₄ can induce skeletal transformations that alter the carbon cage topology, as directly evidenced by single crystal X-ray diffraction studies of the chlorinated products of a series of fullerenes in the broad range of C₆₀ to C₁₀₂. Two general types of transformations have been identified: (i) the Stone-Wales rearrangement (SWR) that consists of a rotation of a C-C bond by 90°, and (ii) the removal of a C-C bond, i.e., C₂ loss (C2L). Single- or multistep SWR and/or C2L transformations afford either classical non-IPR fullerenes bearing fused pentagons (highlighted in red in the TOC picture) or nonclassical (NCx) fullerenes with x = 1-3 heptagonal rings (highlighted in blue in the TOC picture) often flanked by fused pentagons. Several subtypes of the SWR and C2L processes can be further discerned depending on the local topology of the transformed region of the cage. Under the chlorination conditions, the non-IPR and NC carbon cages that would be energetically unfavorable and mostly labile in their pristine state are instantaneously stabilized by chlorination of the pentagon-pentagon junctions and by delimitation of the original spherical π-system into smaller favorable aromatic fragments. The significance of the chlorination-promoted skeletal transformations within the realm of fullerene chemistry is demonstrated by the growing body of examples. To date, these include single- and multistep SWRs in the buckminsterfullerene C₆₀ and in the higher fullerenes C₇₆(1), C₇₈(2), C₈₂(3), and C102(19), single and multistep C2Ls (i.e., cage shrinkage) in C₈₆(16), C₈₈(33), C₉₀(28), C₉₂(50), C₉₆(80), C₉₆(114), and C102(19), and multistep combinations of SWRs and C2Ls in C₈₈(3), C₈₈(33), and C₁₀₀(18), (IPR isomer numbering in parentheses is according to the spiral algorithm). Remarkably, an IPR precursor can give rise to versatile transformed chlorinated fullerene cages formed via branched pathways. The products can be recovered either in their initial chlorinated form or as more soluble CF₃/F derivatives obtained by an additional trifluoromethylation workup. Reconstruction of the skeletal transformation pathways is often complicated due to the lack of the isolable intermediate products in the multistep cases. Therefore, it is usually based on the principle of selecting the shortest pathways between the starting and the final cage. The quantum-chemical calculations illustrate the detailed mechanisms of the SWR and C2L transformations and the thermodynamic driving forces behind them. A particularly important aspect is the interplay between the chlorination patterns and the regiochemistry of the skeletal transformations.

Chlorination-Promoted Cage Transformation of IPR C92 Discovered via Trifluoromethylation under Formation of Non-classical C92 (NC)(CF3 )22

High-temperature trifluoromethylation of isolated-pentagon-rule (IPR) fullerene C₉₂ chlorination products followed by HPLC separation of C₉₂ (CF₃ )_n derivatives resulted in the isolation and X-ray structural characterization of IPR C₉₂ (38)(CF₃ )₁₈ and non-classical C₉₂ (NC)(CF₃ )₂₂ . The formation of C₉₂ (38)(CF₃ )₁₈ as the highest CF₃ derivative of the known isomer C₉₂ (38) can be expected. The formation of C₉₂ (NC)(CF₃ )₂₂ was interpreted as chlorination-promoted cage transformation of C₉₂ (38) followed by trifluoromethylation of non-classical C₉₂ (NC) chloride. Noticeably, C₉₂ (NC)(CF₃ )₂₂ shows the highest degree of trifluoromethylation among all known CF₃ derivatives of fullerenes. The addition patterns of C₉₂ (38)(CF₃ )₁₈ and C₉₂ (NC)(CF₃ )₂₂ are discussed and compared to the chlorination patterns of C₉₂ (38)Cl_n compounds.

Salvaging Reactive Fullerenes from Soot by Exohedral Derivatization

The awesome allotropy of carbon yields innumerable topologically possible cage structures of molecular carbon. This field is also related to endohedral metallofullerenes constructed by metal-atom encapsulation. Stable and soluble empty fullerenes and endohedral metallofullerenes are available in pure form in macroscopic amounts from carbon arc production or other physical processes followed by extraction and subsequent chromatographic separation. However, many other unidentified fullerene species, which must be reactive and insoluble in their pristine forms, remain in soot. These "missing" species must have extremely small HOMO-LUMO gaps and may have unconventional cage structures. Recent progress in this field has demonstrated that reactive fullerenes can be salvaged by exohedral derivatization, which can stabilize the reactive carbon cages. This concept provides a means of preparing macroscopic amounts of unconventional fullerenes as their derivatives.

Synthesis, Isolation and Structures of Trifluoromethylated Fullerenes D2 -C76 , C76 (1)(CF3 )10-18

High-temperature trifluoromethylation of fullerene C76 chlorination products followed by HPLC separation of C76 (CF3 )n derivatives resulted in the isolation and X-ray structural characterization of thirteen C76 (1)(CF3 )n compounds including nine new isomers such as one isomer of C76 (1)(CF3 )10 , two C76 (1)(CF3 )12 , three C76 (1)(CF3 )14 , one C76 (1)(CF3 )16 , and two isomers of C76 (1)(CF3 )18 . Depending on their addition patterns, C76 (1)(CF3 )n isomers are divided into three subgroups and discussed in terms of trifluoromethylation pathways and relative formation energies.

Formation of C2v-C72(11188)Cl4: A Particularly Stable Non-IPR Fullerene

Halogenation has been one of the most used strategies to explore the reactivity of empty carbon cages. In particular, the higher reactivity of non-IPR fullerenes, i.e., those fullerenes that do not satisfy the isolated pentagon rule (IPR), has been used to functionalize and capture these less stable fullerenes. Here, we have explored the stability of the non-IPR isomer C₇₂(11188) with C_2v symmetry, which is topologically linked to the only IPR isomer of C₇₀, as well as its reactivity to chlorination. DFT calculations and Car-Parrinello molecular dynamics simulations suggest that chlorination takes places initially in nonspecific sites, once carbon cages are formed. When the temperature in the arc reactor decreases sufficiently, Cl atoms are trapped on the fullerene surface, migrating from not-so-favored positions to reach the most favored sites in the pentalene. We have also discussed why cage C_2v-C₇₂(11188) is found to take four chlorines, whereas cage C₁-C₇₄(14049) is observed to capture 10 of them, even though these two fullerenes are closely related by a simple C₂ insertion.

Chlorination-promoted skeletal transformation of IPR C76 discovered via trifluoromethylation under the formation of non-IPR C76(CF3)nFm

High-temperature chlorination of IPR D₂-C₇₆ followed by trifluoromethylation resulted in X-ray structures of non-classical, non-IPR C₇₆(CF₃)₁₄, C₇₆(CF₃)₁₄F₂, and C₇₆(CF₃)₁₆F₆.

Versatility of chlorination-promoted skeletal transformation pathways in C76 fullerene

Chlorination-promoted cage transformations in C₇₆ produce a new non-IPR C₇₆Cl₃₀ molecule revealing the considerable versatility of concurrently accessible skeletal transformation pathways.

Experimental and Theoretical Approach to Variable Chlorination-Promoted Skeletal Transformations in Fullerenes: The Case of C102

The first example of three alternative chlorination-promoted skeletal transformation pathways in the same fullerene cage is presented. Isolated-pentagon-rule (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.

Skeletal Transformation of a Classical Fullerene C88 into a Nonclassical Fullerene Chloride C84Cl30 Bearing Quaternary Sequentially Fused Pentagons

A classical fullerene is composed of hexagons and pentagons only, and its stability is generally determined by the Isolated-Pentagon-Rule (IPR). Herein, high-temperature chlorination of a mixture containing a classical IPR-obeying fullerene C₈₈ resulted in isolation and X-ray crystallographic characterization of non-IPR, nonclassical (NC) fullerene chloride C₈₄(NC2)Cl₃₀ (1) containing two heptagons. The carbon cage in C₈₄(NC2)Cl₃₀ contains 14 pentagons, 12 of which form two pairs of fused pentagons and two groups of quaternary sequentially fused pentagons, which have never been observed in reported carbon cages. All 30 Cl atoms form an unprecedented single chain of ortho attachments on the C₈₄ cage. A reconstruction of the pathway of the chlorination-promoted skeletal transformation revealed that the previously unknown IPR isomer C₈₈(3) is converted into 1 by two losses of C₂ fragments followed by two Stone-Wales rearrangements, resulting in the formation of very stable chloride with rather short C-Cl bonds.

C100 is Converted into C94 Cl22 by Three Chlorination-Promoted C2 Losses under Formation and Elimination of Cage Heptagons

Chlorination of the C100 (18) fullerene with a mixture of VCl4 and SbCl5 gives rise to branched skeletal transformations affording non-classical (NC) C94 (NC1)Cl22 with one heptagon in the carbon cage together with the previously reported C96 (NC3)Cl20 with three heptagons. The three-step pathway to C94 (NC1)Cl22 starts with two successive C2 losses of 5:6 CC bonds to give two cage heptagons, whereas the third C2 loss of the 5:5 CC bond from a pentalene fragment eliminates one of the heptagons. Quantum-chemical calculations demonstrate that the two unusual skeletal transformations-creation of a heptagon in C96 (NC3)Cl20 through a Stone-Wales rearrangement and the presently reported elimination of a heptagon through C2 loss-are both characterized by relatively low activation energy.

Structures of chlorinated fullerenes, IPR C₉₆Cl₂₀ and non-classical C₉₄Cl₂₈ and C₉₂Cl₃₂: evidence of the existence of three new isomers of C₉₆

Chlorination of various HPLC fractions of C96 with a mixture of VCl4 and SbCl5 at 340-360 °C and single-crystal X-ray diffraction study of the products led to the identification of three new IPR isomers of C96. The C96(175) isomer forms a stable chloride, C96(175)Cl20, while chlorides of two other new isomers, C96(114) and C96(80), undergo cage shrinkage yielding C94(NC1)Cl28 and C96(NC2)Cl32 with non-classical (NC) cages. These two NC chlorides contain, respectively, one and two heptagons flanked by pairs of fused pentagons and are stabilized by chlorine attachment to the emerging pentagon-pentagon junctions. Thus, the number of the experimentally confirmed C96 isomers has reached nine, which corroborates the empirical rule that the C(6n) fullerenes exhibit particularly rich isomerism.

The first structural confirmation of a C102 fullerene as C102Cl20 containing a non-IPR carbon cage

The chlorination of a pristine C102 fullerene separated by HPLC from fullerene soot afforded crystals of C102Cl20 with a non-IPR (IPR = isolated pentagon rule) cage containing two pairs of fused pentagons; structural reconstruction of a two-step Stone-Wales rearrangement revealed the starting IPR isomer (no. 19) of C102.

Regioselective chlorine-addition reaction toward C54Cl8 and role of chlorine atoms in Stone-Wales rearrangement

By means of density functional theory, detailed studies of regioselective chlorine-addition reactions of two C(54)Cl(8) isomers disclose a highly competitive advantage of (#540)C(54)Cl(8) in the chlorofullerene formation process. The regioselectivity of the addition pattern in (#540)C(54)Cl(8) is found to be dependent on both local and general factors. Special structural relationships reveal that the pristine cage of (#540)C(54)Cl(8) can transform to that of (#864)C(56)Cl(10) and (#913)C(56)Cl(12) through both C(2) addition and Stone-Wales rearrangement. It is found that Stone-Wales rearrangement, which is believed to be a high energy barrier reaction, can be facilitated remarkably well if chlorine atoms participate in the rearrangement process. Furthermore, investigation into the electronic properties of C(54) exohedral fullerenes reveal the different impacts of halogen and hydrogen atoms.

X-ray structures of Sc2C2@C2n (n = 40-42): in-depth understanding of the core-shell interplay in carbide cluster metallofullerenes

X-ray analyses of the cocrystals of a series of carbide cluster metallofullerenes Sc(2)C(2)@C(2n) (n = 40-42) with cobalt(II) octaethylporphyrin present new insights into the molecular structures and cluster-cage interactions of these less-explored species. Along with the unambiguous identification of the cage structures for the three isomers of Sc(2)C(2)@C(2v)(5)-C(80), Sc(2)C(2)@C(3v)(8)-C(82), and Sc(2)C(2)@D(2d)(23)-C(84), a clear correlation between the cluster strain and cage size is observed in this series: Sc-Sc distances and dihedral angles of the bent cluster increase along with cage expansion, indicating that the bending strain within the cluster makes it pursue a planar structure to the greatest degree possible. However, the C-C distances within Sc(2)C(2) remain unchanged when the cage expands, perhaps because of the unusual bent structure of the cluster, preventing contact between the cage and the C(2) unit. Moreover, analyses revealed that larger cages provide more space for the cluster to rotate. The preferential formation of cluster endohedral metallofullerenes for scandium might be associated with its small ionic radius and the strong coordination ability as well.

Capturing the most-stable C56 fullerene cage by in situ chlorination

The most-stable (#916)C(56) carbon cage has been captured by in situ chlorination during the radio frequency furnace process. The resulting exohedral (#916)C(56)Cl(12) was separated and unambiguously characterized by single crystal X-ray structure determination. The discovery of (#916)C(56) provides evidence for a thermodynamically controlled mechanism of fullerene formation, and on the other hand shows that the in situ chlorination does not remarkably influence the fullerene formation itself but just results in the capture of preformed cages. A detailed analysis of the chlorination pattern of (#916)C(56)Cl(12) reveals the main factors controlling the reactivity of non-IPR fullerenes. A high degree of aromatization was observed in the remaining π-system by considering geometric criteria and nucleus-independent chemical-shift analysis (NICS). Along with the well-known stabilization of pentagon-pentagon junctions during chlorination, the formation of aromatic islands plays an important role in the stabilization of the fullerene cage and also in the determination of the chlorination pattern. Based on these empirical rules, the preferable addition patterns for non-IPR fullerene cages can be easily predicted.