Cage Shrinkage of Fullerene via a C2 Loss: from IPR C90(28)Cl24 to Nonclassical, Heptagon-Containing C88Cl22/24

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.

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.

Gas phase ion chemistry of titanium-oxofullerene with ligated solvents

We investigated the gas phase fragmentation events of highly symmetric fullerene-like (FN-like) titanium oxo-cluster anions, [H₁₂Ti₄₂O₆₀(OCH₃)₄₂(HOCH₃)₁₀(H₂O)₂]^2- (1) and [H₇Ti₄₂O₆₀(OCH₃)₄₂(HOCH₃)₁₀(H₂O)₃]^1- (2). These oxo-clusters contain a closed cage Ti₄₂O₆₀ core, protected by a specific number of methoxy, methanol, and water molecules acting as ligands. These dianionic and monoanionic species were generated in the gas phase by electrospray ionization of the H₆[Ti₄₂(μ³-O)₆₀(OⁱPr)₄₂(OH)₁₂] (TOF) cluster in methanol. Collision induced dissociation studies of 1 revealed that upon increasing the collision energy, the protecting ligands were stripped off first, and [Ti₄₁O₅₈]^2- was formed as the first fragment from the Ti₄₂O₆₀ core. Thereafter, systematic TiO₂ losses were observed giving rise to subsequent fragments like [Ti₄₀O₅₆]^2-, [Ti₃₉O₅₄]^2-, [Ti₃₈O₅₂]^2-, etc. Similar fragments were also observed for monoanionic species 2 as well. Systematic 23 TiO₂ losses were observed, which were followed by complete shattering of the cage. We also carried out computational studies using density functional theory (DFT) to investigate the structures and fragmentation mechanism. The fragmentation of TOF was comparable to the fragmentation of C₆₀ ions, where systematic C₂ losses were observed. We believe that this is a consequence of topological similarity. The present study provides valuable insights into the structural constitution of TOF clusters and stability of the parent as well as the resulting cage-fragments in the gas phase.

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.

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.

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.

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.

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.

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.

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.

Chlorination of IPR C100 fullerene affords unconventional C96 Cl20 with a nonclassical cage containing three heptagons

Chlorination of C100 fullerene with a mixture of VCl4 and SbCl5 afforded C96Cl20 with a strongly unconventional structure. In contrast to the classical fullerenes containing only hexagonal and pentagonal rings, the C96 cage contains three heptagonal rings and, therefore, should be classified as a fullerene with a nonclassical cage (NCC). There are several types of pentagon fusions in the C96 cage including pentagon pairs and pentagon triples. The three-step pathway from isolated-pentagon-rule (IPR) C100 to C96(NCC-3hp) includes two C2 losses, which create two cage heptagons, and one Stone-Wales rotation under formation of the third heptagon. Structural reconstruction established C100 isomer no. 18 from 450 topologically possible IPR isomers as the starting C100 fullerene. Until now, no pristine C100 isomers have been confirmed based on the experimental results.