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.

M@C78 (M = U, Th): Inherent Topological Connectivity Existed in Thermodynamically Stable Isomers and the Possibility of an Endohedral Fullerene Containing One Heptagon Ring

The density functional theory combined with statistical thermodynamic analyses of M@C₇₈ (M = U and Th) demonstrated that four isomers, M@D_3h(24109)-C₇₈, M@C_2v(24107)-C₇₈, M@C₁(22595)-C₇₈, and M@C₁(23349)-C₇₈, and a nonclassical isomer, M@C₁(id7)-C₇₈, containing one heptagon ring possess outstanding thermodynamic stabilities in the two M@C₇₈ series. Especially, the M@C₁(id7)-C₇₈ isomer is the first nonclassical C₇₈ fullerene that can exist stably. Importantly, these five fullerene cages are found to be related in the form of Stone-Wales (SW) transformations. Geometric analyses disclosed that, unlike lanthanide metals, actinide metals are more likely to bond with sumanene-type hexagonal rings when they are encapsulated in IPR C₇₈ cages. Frontier molecular orbital analysis showed that both U and Th atoms donate four electrons to the C₇₈ carbon cages.

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.

Significant Roles of a Particularly Stable Two-Center Two-Electron Lu-Lu σ Bond in Lu2@C86: Electronic Structure of Lu and Radius of Lu2

There is still dispute over the stability of endohedral metallofullerenes (EMFs) M₂C_2n, and recently, multiform lutetium-based dimetallofullerenes have been dropped in experiments. The thermodynamic stabilities of Lu₂C₈₆ EMFs are revealed by density functional theory (DFT) in conjunction with statistical thermodynamic analyses. Inevitably, besides the experimentally reported Lu₂@C_2v(63751)-C₈₆, Lu₂@C_s(63750)-C₈₆, and Lu₂@C_s(63757)-C₈₆, other three metal carbide clusterfullerenes, Lu₂C₂@D_2d(51591)-C₈₄, Lu₂C₂@C₁(51383)-C₈₄, and Lu₂C₂@C_s(id207430)-C₈₄, rather than Lu₂@C₈₆ are first characterized as thermodynamically stable isomers of Lu₂C₈₆. Specially, the C_s(id207430)-C₈₄ is a newly non-classical fullerene containing one heptagon, which is stabilized via encaging Lu₂C₂. Another interesting phenomenon is that the outer fullerene cages of thermodynamically stable Lu₂C_82-88 molecules are geometrically connected through C₂ addition/loss and Stone-Wales (SW) transformation, suggesting a special relationship between thermodynamic stabilities and geometries of Lu₂C_82-88 EMFs. Furthermore, the electronic configurations of (Lu₂)⁴⁺@C₈₆^4- and (Lu₂C₂)⁴⁺@C₈₄^4- were confirmed. A significantly stable two-center two-electron (2c-2e) Lu-Lu σ single bond is formed in Lu₂@C₈₆. By comparing M-M bonds in M₂@C_2v(63751)-C₈₆ (M = Sc, Y, La, and Lu), two significant factors, the valence atomic orbital (ns) of metal atoms and radius of M²⁺, are found to determine the stability of the M-M bond in the C_2v(63751)-C₈₆. Additionally, the simulated UV-vis-NIR spectra of thermodynamically stable Lu₂C₈₆ isomers were simulated, which further disclose their electronic features.

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.

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.

Structural interconnections and the role of heptagonal rings in endohedral trimetallic nitride template fullerenes

Recent experiments indicate that fullerene isomers outside the classical definition can also encapsulate metallic atoms or clusters to form endohedral metallofullerenes. Our systematic study using DFT calculations, suggests that many heptagon-including nonclassical trimetallic nitride template fullerenes are similar in stability to their classical counterparts, and that conversion between low-energy nonclassical and classical parent cages via Endo-Kroto insertion/extrusion of C2 units and Stone-Wales isomerization may facilitate the formation of endohedral trimetallic nitride fullerenes. Close structural connections are found between favored isomers of trimetallic nitride template fullerenes from C78 to C82 . It appears that the lower symmetry and local deformations associated with introduction of a heptagonal ring favor encapsulation of intrinsically less symmetrical mixed metal nitride clusters. © 2016 Wiley Periodicals, Inc.