Rebuilding C60: Chlorination-Promoted Transformations of the Buckminsterfullerene into Pentagon-Fused C60 Derivatives

Oxidative cage opening in the C70 fullerene facilitated by preceding trifluoromethylation

Two novel derivatives of the C70 fullerene with 9- and 10-membered cage openings were obtained by means of oxidation and decarbonylation of C70(CF3)8. The major product, C70(O)(CF3)8O2, features a cleaved C-C bond transformed into two carbonyl functions plus an ether bridge. The second product, C69O(CF3)8O, has one of the carbonyls replaced with another ether bridge. We provide a DFT analysis of the possible formation pathways to give the oxidized compounds under the action of pyridine N-oxide.

Non-classical (NC), heptagon-containing fullerenes obtained via chlorination-promoted cage transformations: C76(NC2a)Cl24 and C76(NC2b)Cl28

High-temperature chlorination of C76 fullerene with SbCl5 proceeds via five Stone-Wales rearrangements, resulting in non-classical (NC) C76(NC1a)Cl24 with two heptagons and 14 pentagons partically fused in pairs and triples. C76(NC2b)Cl28 with isomeric carbon cage was obtained by chlorination-promoted cage shrinkage of C80via two C2 losses. The pathways of skeletal cage trasformations, the chlorination patterns, and formation energies are discussed in detail.

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.

Structural Chemistry of Pentagon-Fused C82 Fullerene Derivatives #39173C82(CF3)14,16,18 and #39173C82Cl28

High-temperature chlorination of the most stable Isolated-Pentagon-Rule (IPR) isomer of fullerene C₈₂, C₂-C₈₂(3), invariably produces non-IPR ^#39173C₈₂Cl₂₈, containing one pentagon-pentagon fusion in the carbon cage. High-temperature trifluoromethylation of ^#39173C₈₂Cl₂₈ followed by HPLC separation resulted in the isolation and structure elucidation of eight ^#39173C₈₂(CF₃)_n (n = 14, 16, 18) compounds. Structural chemistry of ^#39173C₈₂(CF₃)_14,16,18 and ^#39173C₈₂Cl₂₈ is characterized by the variation of the addition patterns in the region of a pentagon-pentagon fusion. The regiochemistry of CF₃ addition in the remaining cage region is similar to that of the known IPR C₈₂(3)(CF₃)_n compounds. Theoretical calculations revealed that ^#39173C₈₂(CF₃)_n possess lower thermodynamic stability than isomeric IPR derivatives.

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.

Chloro- and Trifluoromethyl Derivatives of a Pentagon-Fused C60: 1810C60Cl24, 1810C60Cl20, and 1810C60(CF3)14

The carbon cage of I_h-C₆₀, obeying the isolated-pentagon rule (IPR), can be transformed to the non-IPR D_2h-¹⁸¹⁰C₆₀ cage via two successive Stone-Wales rearrangements in the course of high-temperature chlorination of C₆₀ with SbCl₅. Two chloro derivatives, C_2v-¹⁸¹⁰C₆₀Cl₂₄ and C_2v-¹⁸¹⁰C₆₀Cl₂₀, have been isolated by high-performance liquid chromatography (HPLC). High-temperature trifluoromethylation of the chlorination products with CF₃I, followed by HPLC separation, afforded a non-IPR CF₃ derivative, C_s-¹⁸¹⁰C₆₀(CF₃)₁₄. Structural elucidation of the isolated compounds revealed that all eight sites of pentagon-pentagon fusions on the carbon cage are preferentially occupied by Cl atoms or CF₃ groups. According to density functional theory calculations, chloro and CF₃ derivatives of ¹⁸¹⁰C₆₀ are more stable than the isomeric derivatives of ¹⁸⁰⁹C₆₀ or IPR ¹⁸¹²C₆₀, possessing respectively four or no sites of pentagon fusion in their carbon cages.

Trifluoromethyl derivatives of pentagon-fused C60: 1809C60(CF3)n (n = 10, 12, 14, 16)

The carbon cage of buckminsterfullerene I_h-C₆₀, obeying the Isolated-Pentagon Rule (IPR), can be transformed to the non-IPR C_2v-¹⁸⁰⁹C₆₀ cage by a single Stone-Wales rearrangement (SWR) in the course of high-temperature chlorination of C₆₀ with SbCl₅. The following high-temperature trifluoromethylation of the chlorination products with CF₃I afforded non-IPR CF₃ derivatives, ¹⁸⁰⁹C₆₀(CF₃)_n. X-ray diffraction studies of ¹⁸⁰⁹C₆₀(CF₃)_n (n = 10, 12, 14, 16) revealed that the sites of pentagon-pentagon fusions on the carbon cage are preferentially occupied by CF₃ groups. The addition patterns of ¹⁸⁰⁹C₆₀(CF₃)_n and related ¹⁸⁰⁹C₆₀Cl_n are compared, demonstrating a prevailing role of pentagon-pentagon fusions in the stability and structural chemistry of these compounds. Further SWR skeletal transformations of ¹⁸⁰⁹C₆₀ are discussed and compared with the experimental data available.

Three Isolated-Pentagon-Rule Isomers of C96 Fullerene Isolated as Trifluoromethyl Derivatives

The family of experimentally confirmed isolated-pentagon-rule (IPR) isomers of C₉₆ fullerene is extended by trifluoromethylation of a C₉₆ fraction of the fullerene soot, high-performance liquid chromatography separation of CF₃ derivatives, and a single-crystal X-ray diffraction study of C₉₆(CF₃)_n compounds with the use of synchrotron radiation. New cage isomers were revealed in C₉₆(94)(CF₃)_18/20 and C₉₆(182)(CF₃)₁₈ compounds, whereas isomer C₉₆(181), previously known in the adduct with nickel porphyrinate, was confirmed in C₉₆(181)(CF₃)_18/20 derivatives. Common and special features of the addition patterns of CF₃ groups on C₉₆ carbon cages are discussed in more detail. The investigated isomers belong to the most stable C₂-C₉₆(181) and slightly less stable C₁-C₉₆(94) and C₂-C₉₆(182) among the altogether 15 experimentally confirmed IPR isomers of C₉₆ fullerene.

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.

Generalized network theory of physical two-dimensional systems

The properties of a wide range of two-dimensional network materials are investigated by developing a generalized network theory. The methods developed are shown to be applicable to a wide range of systems generated from both computation and experiment; incorporating atomistic materials, foams, fullerenes, colloidal monolayers, and geopolitical regions. The ring structure in physical networks is described in terms of the node degree distribution and the assortativity. These quantities are linked to previous empirical measures such as Lemaître's law and the Aboav-Weaire law. The effect on these network properties is explored by systematically changing the coordination environments, topologies, and underlying potential model of the physical system.

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.