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.     

New Isolated‐Pentagon‐Rule and Skeletally Transformed Isomers of C100 Fullerene Identified by Structure Elucidation of their Chloro Derivatives

High‐temperature chlorination of C₁₀₀ fullerene followed by X‐ray structure determination of the chloro derivatives enabled the identification of three isomers of C₁₀₀ from the fullerene soot, specifically numbers 18, 425, and 417, which obey the isolated pentagon rule (IPR). Among them, isomers C₁‐C₁₀₀(425) and C₂‐C₁₀₀(18) afforded C₁‐C₁₀₀(425)Cl₂₂ and C₂‐C₁₀₀(18)Cl_28/30 compounds, respectively, which retain their IPR cage connectivities. In contrast, isomer C_2v‐C₁₀₀(417) gives C_s‐C₁₀₀(417)Cl₂₈ which undergoes a skeletal transformation by the loss of a C₂ fragment, resulting in the formation of a nonclassical (NC) C₁‐C₉₈(NC)Cl₂₆ with a heptagon in the carbon cage. Most probably, two nonclassical C₁‐C₁₀₀(NC)Cl_18/22 chloro derivatives originate from the IPR isomer C₁‐C₁₀₀(382), although both C₁‐C₁₀₀(344) and even nonclassical C₁‐C₁₀₀(NC) can be also considered as the starting isomers.     

Quantum Chemical Insight of the Dimetallic Sulfide Endohedral Fullerene Sc2S@C70: Does It Possess the Conventional D5h Cage?

Like C₆₀, C₇₀ is one of the most representative fullerenes in fullerene science. Even though there are 8149 C₇₀ isomers, only two of them have been found before: the conventional D_5h and an isolated pentagon rule (IPR)‐violating C_2v(7854). Through the use of quantum chemical methods, we report a new unconventional C₇₀ isomer, C₂(7892), which survives in the form of dimetallic sulfide endohedral fullerene Sc₂S@C₇₀. Compared with the IPR‐obeying C₇₀ and the C_2v(7854) fullerene with three pairs of pentagon adjacencies, the C₂(7892) cage violates the isolated pentagon rule and has two pairs of pentagon adjacencies. In Sc₂S@C₂(7892)‐C₇₀, two scandium atoms coordinate with two pentalene motifs, respectively, presenting two equivalent Sc? S bonds. The strong coordination interaction, along with the electron transfer from the Sc₂S cluster to the fullerene cage, results in the stabilization of the non‐IPR endohedral fullerene. The electronic structure of Sc₂S@C₇₀ can be formally described as [Sc₂S]⁴⁺@[C₇₀]^4?; however, a substantial overlap between the metallic orbitals and cage orbitals has also been found. Electrochemical properties and electronic absorption, infrared, and ¹³C NMR spectra of Sc₂S@C₇₀ have been calculated theoretically.     

Capturing the Most‐Stable C56 Fullerene Cage by In Situ Chlorination

The most‐stable ^#916C₅₆ carbon cage has been captured by in situ chlorination during the radio frequency furnace process. The resulting exohedral ^#916C₅₆Cl₁₂ was separated and unambiguously characterized by single crystal X‐ray structure determination. The discovery of ^#916C₅₆ 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 ^#916C₅₆Cl₁₂ 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.     

The First Experimentally Confirmed Isolated Pentagon Rule (IPR) Isomers of Higher Fullerene C98 Captured as Chlorides,C98(248)Cl22 and C98(116)Cl20

High‐temperature chlorination of pristine C₉₈ fullerene isomers separated by HPLC from the fullerene soot afforded crystals of C₉₈Cl₂₂ and C₉₈Cl₂₀. An X‐ray structure elucidation revealed, respectively, the presence of carbon cages of the most stable C₂‐C₉₈(248) and rather unstable C₁‐C₉₈(116), which represent the first isolated pentagon rule (IPR) isomers of fullerene C₉₈ confirmed experimentally. The chlorination patterns of the chlorides are discussed in terms of the formation of isolated C=C bonds and aromatic substructures on the fullerene cages.     

La2@Cs(17 490)‐C76: A New Non‐IPR Dimetallic Metallofullerene Featuring Unexpectedly Weak Metal–Pentalene Interactions

Although all the pure‐carbon fullerene isomers above C₆₀ reported to date comply with the isolated pentagon rule (IPR), non‐IPR structures, which are expected to have different properties from those of IPR species, are obtainable either by exohedral modification or by endohedral atom doping. This report describes the isolation and characterization of a new endohedral metallofullerene (EMF), La₂@C₇₆, which has a non‐IPR fullerene cage. The X‐ray crystallographic result for the La₂@C₇₆/[Ni^II(OEP)] (OEP=octaethylporphyrin) cocrystal unambiguously elucidated the C_s(17 490)‐C₇₆ cage structure, which contains two adjacent pentagon pairs. Surprisingly, multiple metal sites were distinguished from the X‐ray data, which implies dynamic behavior for the two La³⁺ cations inside the cage. This dynamic behavior was also corroborated by variable‐temperature ¹³⁹ La NMR spectroscopy. This phenomenon conflicts with the widely accepted idea that the metal cations in non‐IPR EMFs invariably coordinate strongly with the negatively charged fused‐pentagon carbons, thereby providing new insights into modern coordination chemistry. Furthermore, our electrochemical and computational studies reveal that La₂@C_s(17 490)‐C₇₆ has a larger HOMO–LUMO gap than other dilanthanum‐EMFs with IPR cage structures, such as La₂@D_3h(5)‐C₇₈ and La₂@I_h(7)‐C₈₀, which implies that IPR is no longer a strict rule for EMFs.     

Stable Non‐IPR C60 and C70 Fullerenes Containing a Uniform Distribution of Pyrenes and Adjacent Pentagons

The most abundant fullerenes, C₆₀ and C₇₀, and all the pure carbon fullerenes larger than C₇₀, follow the isolated‐pentagon rule (IPR). Non‐IPR fullerenes containing adjacent pentagons (APs) have been stabilized experimentally in cases where, according to Euler’s theorem, it is topologically impossible to isolate all the pentagons from each other. Surprisingly, recent experiments have shown that a few endohedral fullerenes, for which IPR structures are possible, are stabilized in non‐IPR cages. We show that, apart from strain, the physical property that governs the relative stabilities of fullerenes is the charge distribution in the cage. This charge distribution is controlled by the number and location of APs and pyrene motifs. We show that, when these motifs are uniformly distributed in the cage and well‐separated from one other, stabilization of non‐IPR endohedral and exohedral derivatives, as well as pure carbon fullerene anions and cations, is the rule, rather than the exception. This suggests that non‐IPR derivatives might be even more common than IPR ones.     

Chlorination‐promoted Transformation of Isolated Pentagon Rule C78 into Fused‐pentagons‐ and Heptagons‐containing Fullerenes

Cage transformations in fullerenes are rare phenomena which are still not fully understood. We report the first skeletal transformation of an Isolated‐Pentagon‐Rule (IPR) isomer of C₇₈ fullerene upon high‐temperature chlorination which proceeds by six‐step Stone–Wales rearrangements affording non‐IPR, non‐classical (NC ) C₇₈(NC 2)Cl₂₄ with two cage heptagons, six pairs of fused pentagons, and an unprecedented loop‐like chlorination pattern. The following loss of a C₂ unit results in C₇₆(NC 3)Cl₂₄ containing three cage heptagons.     

Isolated pentagon rule violating endohedral metallofullerenes explained using the Hückel rule: A statistical mechanical study of the C84 Isomeric Set

Fullerenes and their structure and stability have been a major topic of discussion and research since their discovery nearly 30 years ago. The isolated pentagon rule (IPR) has long served as a guideline for predicting the most stable fullerene cages. More recently, endohedral metallofullerenes have been discovered that violate the IPR. This article presents a systematic, temperature dependent, statistical thermodynamic study of the 24 possible IPR isomers of C₈₄ as well as two of the experimentally known non‐IPR isomers (51365 and 51383), at several different charges (0, ?2, ?4, and ?6). From the results of this study, we conclude that the Hückel rule is a valid simpler explanation for the stability of fused pentagons in endohedral metallofullerenes. © 2014 Wiley Periodicals, Inc.     

C100 is Converted into C94Cl22 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.     

First Isomers of Pristine C104 Fullerene Structurally Confirmed as Chlorides,C104(258)Cl16 and C104(812)Cl24

Isolation and characterization of very large fullerenes is hampered by a drastic decrease of their content in fullerene soot with increasing fullerene size and a simultaneous increase of the number of possible IPR (Isolated Pentagon Rule) isomers. In the present work, fractions containing mixtures of C₁₀₂ and C₁₀₄ were isolated in very small quantities (several dozens of micrograms) by multi‐step recycling HPLC from an arc‐discharge fullerene soot. Two such fractions were used for chlorination with a VCl₄/SbCl₅ mixture in glass ampoules at 350–360 °C. The resulting chlorides were investigated by single‐crystal X‐ray diffraction using synchrotron radiation. By this means, two IPR isomers of C₁₀₄, numbers 258 and 812 (of 823 topologically possible isomers), have been confirmed for the first time as chlorides, C₁‐C₁₀₄(258)Cl₁₆ and D₂‐C₁₀₄(812)Cl₂₄, respectively, while an admixture of C₂‐C₁₀₄(811)Cl₂₄ was assumed to be present in the latter chloride. DFT calculations showed that pristine C₁₀₄(812) belongs to rather stable C₁₀₄ cages, whereas C₁₀₄(258) is much less stable.     

Unusual Chlorination Patterns of Three IPR Isomers of C88 Fullerene in C88(7)Cl12/24, C88(17)Cl22, and C88(33)Cl12/14

High‐temperature chlorination of three IPR isomers of fullerene C₈₈, C₂‐C₈₈(7), C_s‐C₈₈(17), and C₂‐C₈₈(33), resulted in the isolation and X‐ray structural characterization of C₈₈(7)Cl₁₂, C₈₈(7)Cl₂₄, C₈₈(17)Cl₂₂, and C₈₈(33)Cl_12/14. Chlorination patterns of C₈₈(7) and C₈₈(33) isomers are unusual in that one or more pentagons remain free from chlorination while some other pentagons are occupied by two or three Cl atoms. The addition patterns of the isolated chlorides are discussed in terms of the distribution of twelve pentagons on the carbon cages and the formation of stabilizing isolated C=C bonds and benzenoid rings.     

Regioselective Cage Opening of La2@D2(10611)‐C72 with 5,6‐Diphenyl‐3‐(2‐pyridyl)‐1,2,4‐triazine

The thermal reaction of the endohedral metallofullerene La₂@D₂(10611)‐C₇₂, which contains two pentalene units at opposite ends of the cage, with 5,6‐diphenyl‐3‐(2‐pyridyl)‐1,2,4‐triazine proceeded selectively to afford only two bisfulleroid isomers. The molecular structure of one isomer was determined using single‐crystal X‐ray crystallography. The results suggest that the [4+2] cycloaddition was initiated in a highly regioselective manner at the C? C bond connecting two pentagon rings of C₇₂. Subsequent intramolecular electrocyclization followed by cycloreversion resulted in the formation of an open‐cage derivative having three seven‐membered ring orifices on the cage and a significantly elongated cage geometry. The reduction potentials of the open‐cage derivatives were similar to those of La₂@D₂‐C₇₂ whereas the oxidation potentials were shifted more negative than those of La₂@D₂‐C₇₂. These results point out that further oxidation could occur easily in the derivatives.     

The Most Stable Isomers of Giant Fullerenes C102 and C104 Captured as Chlorides,C102(603)Cl18/20 and C104(234)Cl16/18/20/22

The chlorination of HPLC fractions with pristine giant fullerenes, C₁₀₂ and C₁₀₄, followed by X‐ray crystallographic study of chlorides, C₁₀₂(603)Cl_18/20 and C₁₀₄(234)Cl_16–22, confirmed the presence of the most stable IPR (IPR=Isolated Pentagon Rule) isomers, C₁₀₂(603) and C₁₀₄(234), in the fullerene soot. The discussion concerns the chlorination patterns of polychlorides and relative stability of pristine isomers of C₁₀₂ and C₁₀₄ fullerenes.     

Chlorination‐Promoted Skeletal‐Cage Transformations of C88 Fullerene by C2 Losses and a CC Bond Rotation

High‐temperature chlorination of fullerene C₈₈ (isomer 33) with VCl₄ gives rise to skeletal transformations affording several nonclassical (NC) fullerene chlorides, C₈₆(NC1)Cl_24/26 and C₈₄(NC2)Cl₂₆, with one and two heptagons, respectively, in the carbon cages. The branched skeletal transformation including C₂ losses as well as a Stone–Wales rearrangement has been comprehensively characterized by the structure determination of two intermediates and three final chlorination products. Quantum‐chemical calculations demonstrate that the average energy of the C?Cl bond is significantly increased in chlorides of nonclassical fullerenes with a large number of chlorinated sites of pentagon–pentagon adjacency.     

Bingel–Hirsch Addition on Endohedral Metallofullerenes: Kinetic Versus Thermodynamic Control

An extensive theoretical study of the Bingel–Hirsch addition of bromomalonate on scandium nitride endohedral fullerenes has been carried out. The prototypical and highly symmetrical Sc₃N@I_h‐C₈₀, with a structure that satisfies the isolated pentagon rule (IPR), and the non‐IPR Sc₃N@D₃(6140)‐C₆₈ fullerene show analogous reaction paths despite the distinct topology of the carbon networks and different rotation freedom of the internal nitride cluster. For the two metallofullerenes, our results predict that the reaction takes place under kinetic control yielding open‐cage fulleroids on [6,6] bonds, which is in good agreement with experimental data. The theoretical studies also show that predicting the reactivity of endohedral metallofullerenes is not straightforward and often an accurate analysis of the potential energy surface is required.     

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.     

C68 Fullerene Isomers,Anions, and their Metallofullerenes: Charge‐Stabilizing Different Isomers

The complete set of 6332 classical isomers of the fullerene C₆₈ as well as several non‐classical isomers is investigated by PM3, and the data for some of the more stable isomers are refined by the DFT‐based methods HCTH and B3LYP. C₂:0112 possesses the lowest energy of all the neutral isomers and it prevails in a wide range of temperatures. Among the fullerene ions modeled, C₆₈^2?, C₆₈^4? and C₆₈^6?, the isomers C₆₈^2?(C_s:0064), C₆₈^4?(C_2v:0008), and C₆₈^6?(D₃:0009) respectively, are predicted to be the most stable. This reveals that the pentagon adjacency penalty rule (PAPR) does not necessarily apply to the charged fullerene cages. The vertical electron affinities of the neutral C_s:0064, C_2v:0008, and D₃:0009 isomers are 3.41, 3.29, and 3.10 eV, respectively, suggesting that they are good electron acceptors. The predicted complexation energy, that is, the adiabatic binding energy between the cage and encapsulated cluster, of Sc₂C₂@C₆₈(C_2v:0008) is ?6.95 eV, thus greatly releasing the strain of its parent fullerene (C_2v:0008). Essentially, C₆₈ fullerene isomers are charge‐stabilized. Thus, inducing charge facilitates the isolation of the different isomers. Further investigations show that the steric effect of the encaged cluster should also be an important factor to stabilize the C₆₈ fullerenes effectively.     

Further test of the isolated pentagon rule: Thermodynamic and kinetic stabilities of C84 fullerene isomers

Thermodynamic and kinetic stabilities of 73 C₈₄ fullerene isomers were estimated from the MM3 heats of formation and the recently defined bond resonance energies (BREs), respectively. The BRE represents the contribution of a given π bond in a molecule to the topological resonance energy (TRE). All π bonds shared by two pentagons turned out to be highly reactive without exceptions. C₈₄ fullerene isomers with such π bonds must be incapable of survival during harsh synthetic processes. Thus, the isolated pentagon rule (IPR) proved to be applicable to such large fullerene cages. For sufficiently large fullerenes like C₈₄, some isolated-pentagon isomers are also predicted to be very unstable with highly antiaromatic π bonds. © 1996 by John Wiley & Sons, Inc.     

Dimetallic sulfide endohedral metallofullerene Sc2S@C76: Density functional theory characterization

In terms of density functional theory combined with statistic mechanics computations, we investigated a dimetallic sulfide endohedral fullerene Sc₂S@C₇₆ which has been synthesized without any characterization in experiments. Our theoretical study reveals that Sc₂S@T_d(19151)‐C₇₆ which satisfies the isolated‐pentagon rule (IPR) possesses the lowest energy, followed by three non‐IPR structures (Sc₂S@C_2v(19138)‐C₇₆, Sc₂S@C_s (17490)‐C₇₆, and Sc₂S@C₁(17459)‐C₇₆). To clarify the relative stabilities of those isomers at high temperatures, enthalpy–entropy interplay has been taken into consideration. Calculation results indicate that three species Sc₂S@T_d(19151)‐C₇₆, Sc₂S@C_2v(19138)‐C₇₆, and Sc₂S@C₁(17459)‐C₇₆ have noticeable molar fractions at the fullerene‐formation temperature region (500–3000K), and the Sc₂S@C₁(17459)‐C₇₆ with one pentagon pair becomes the most predominant isomer above 1800 K, suggesting that the unexpected non‐IPR structure is thermodynamically favorable at elevated temperatures. In addition, the structural characteristics, electron features, UV‐vis‐NIR adsorptions, and ¹³C NMR spectra of those three stable structures are introduced to assist experimental identification and characterization in future. © 2014 Wiley Periodicals, Inc.