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

Capturing nonclassical C70 with double heptagons in low-pressure combustion

A double-heptagon-containing C₇₀H₆ (dihept-C₇₀H₆) was isolated and unambiguously characterized in the soot of low-pressure combustion, which shares the identical heptagonal cage as dihept-C₇₀Cl₆ previously identified in the products of carbon arc, and thus represents the first nonclassical fullerene isolable in both carbon arc and combustion.

How to Stabilize a Heptagon-Containing C80 Cage by Endohedral Derivation

Nonclassical fullerene is a new member of the fullerene family. In the present work, a systematic investigation on La_xSc_3-xN@C₈₀ (x = 0-3) covering both classical and nonclassical C₈₀ cages was performed utilizing density functional theory combined with statistical mechanics. At absolute zero, LaSc₂N@Hept(6)-C_s(2)-C₈₀ with a heptagon-containing nonclassical carbon is the second most stable isomer, whereas at the temperature range of endohedral metallofullerene (EMF) formation, due to the large vibrational frequencies, LaSc₂N@Hept(6)-C_s(2)-C₈₀ is the third most abundant isomer, and its mole fraction is very low, accounting for the low experimental yield of LaSc₂N@Hept(6)-C_s(2)-C₈₀; La₂ScN@Hept(6)-C_s(2)-C₈₀, and La₃N@Hept(6)-C_s(2)-C₈₀ are the overwhelming isomers of the corresponding series, but compared with the cases of Sc₃N@C₈₀ and LaSc₂N@C₈₀, La₂ScN and La₃N clusters suffer much larger constraints from the C₈₀ cages, perhaps preventing the synthesis of La₂ScN@C₈₀ and La₃N@C₈₀ species. Because of the large mole fractions and large electron donation and back-donation of La₂ScN@Hept(6)-C_s(2)-C₈₀ and La₃N@Hept(6)-C_s(2)-C₈₀, it can be inferred that La₂ScN and La₃N clusters may be used to stabilize some other larger nonclassical fullerene cages. This work will provide useful insights into the origins of stabilization of nonclassical fullerene cages by endohedral derivation and guidelines for synthesis EMF with nonclassical cages.

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.

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.

Double Negatively Curved C70 Growth through a Heptagon-Involving Pathway

All previously reported C₇₀ isomers have positive curvature and contain 12 pentagons in addition to hexagons. Herein, we report a new C₇₀ species with two negatively curved heptagon moieties and 14 pentagons. This unconventional heptafullerene[70] containing two symmetric heptagons, referred to as dihept-C₇₀ , grows in the carbon arc by a theoretically supported pathway in which the carbon cluster of a previously reported C₆₆ species undergoes successive C₂ insertion via a known heptafullerene[68] intermediate with low energy barriers. As identified by X-ray crystallography, the occurrence of heptagons facilitates a reduction in the angle of the π-orbital axis vector in the fused pentagons to stabilize dihept-C₇₀ . Chlorination at the intersection of a heptagon and two adjacent pentagons can greatly enlarge the HOMO-LUMO gap, which makes dihept-C₇₀ Cl₆ isolable by chromatography. The synthesis of dihept-C₇₀ Cl₆ offers precious clues with respect to the fullerene formation mechanism in the carbon-clustering process.

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.

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.

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.

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.