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.     

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.     

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.     

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.     

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.     

Blending Non‐Group‐3 Transition Metal and Rare‐Earth Metal into a C80 Fullerene Cage with D5h Symmetry

Rare‐earth metals have been mostly entrapped into fullerene cages to form endohedral clusterfullerenes, whereas non‐Group‐3 transition metals that can form clusterfullerenes are limited to titanium (Ti) and vanadium (V), and both are exclusively entrapped within an I_h‐C₈₀ cage. Non‐Group‐3 transition‐metal‐containing endohedral fullerenes based on a C₈₀ cage with D_5h symmetry, V_xSc_3?xN@D_5h‐C₈₀ (x=1, 2), have now been synthesized, which exhibit two variable cluster compositions. The molecular structure of VSc₂N@D_5h‐C₈₀ was unambiguously determined by X‐ray crystallography. According to a comparative study with the reported Ti‐ and V‐containing clusterfullerenes based on a I_h‐C₈₀ cage and the analogous D_5h‐C₈₀‐based metal nitride clusterfullerenes containing rare‐earth metals only, the decisive role of the non‐Group‐3 transition metal on the formation of the corresponding D_5h‐C₈₀‐based clusterfullerenes is unraveled.     

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.     

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.     

Synthesis of Long‐Sought C66 with Exohedral Stabilization

Previously reported fused‐pentagon fullerenes stabilized by exohedral derivatization do not share the same cage with those stabilized by endohedral encapsulation. Herein we report the crystallographic identification of ^#4348C₆₆Cl₁₀, which has the same cage as that of previously reported Sc₂@C₆₆. According to the geometrical data of ^#4348C₆₆Cl₁₀, both strain relief (at the fused pentagons) and local aromaticity (on the remaining sp²‐hybrided carbon framework) contribute to the exohedral stabilization of this long‐sought 66 carbon atom cage.     

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.     

The saturated hydrides of C<Subscript>60</Subscript>:F<Subscript>5</Subscript>F<Subscript>6</Subscript> PK F<Subscript>5</Subscript>F<Subscript>7</Subscript> isomers

The carbon cages composed of pentagons and heptagons (F₅F₇ isomers) are the analogs of fullerenes composed of pentagons and hexagons (F₅F₆ isomers). To provide insight into the structures and stability of the hydrides of F₅F₆ and F₅F₇ isomers, systematical density functional theory calculations are performed on all the 1,812 F₅F₆–C₆₀H₆₀ and 56 F₅F₇–C₆₀H₆₀. The calculated results demonstrate that the isomer with lowest/highest energy has most/fewest fused pentagons for both F₅F₆ and F₅F₇ hydrides and the stability of these hydrides increase with the number of fused pentagons roughly. The lowest energy F₅F₆–C₆₀H₆₀ and F₅F₇–C₆₀H₆₀ are 237.1 and 152.5 kcal/mol lower in energy than the isolated pentagon rule (IPR) C₆₀H₆₀, respectively; however, these two parent cages are 529.6 and 660.0 kcal/mol higher in energy than the IPR C₆₀. The calculations suggest that heptagon-containing cages, not only those violating the IPR can be the candidate cages for fullerene derivatives and the possible repulsion between the added atoms may play an important role in determining the structures and stability of the hydrides of carbon cages.     

Tb3N@C84: an improbable, egg-shaped endohedral fullerene that violates the isolated pentagon rule

The structure of isomer 2 of Tb3N@C84 has been determined through single-crystal X-ray diffraction on Tb3N@C84.NiII(OEP).2(C6H6). The carbon cage has a distinct egg shape due to the presence of a single pair of fused pentagons at one apex of the molecule. Thus, although 24 IPR structures are available to the C84 cage, Nature utilizes one of the 51 568 isomeric structures that do not conform to the IPR for this unusual molecule. The Tb3N portion of isomer 2 of Tb3N@C84 is strictly planar. One Tb atom is nestled within the fold of the fused pentagons, while the other Tb atoms are disordered over four pairs of sites.     

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.     

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.     

Structure, stability, and cluster-cage interactions in nitride clusterfullerenes M3N@C2n (M = Sc, Y; 2n = 68-98): a density functional theory study

Extensive semiempirical calculations of the hexaanions of IPR (isolated pentagon rule) and non-IPR isomers of C(68)-C(88) and IPR isomers of C(90)-C(98) followed by DFT calculations of the lowest energy structures were performed to find the carbon cages that can provide the most stable isomers of M(3)N@C(2n) clusterfullerenes (M = Sc, Y) with Y as a model for rare earth ions. DFT calculations of isomers of M(3)N@C(2n) (M = Sc, Y; 2n = 68-98) based on the most stable C(2n)(6-) cages were also performed. The lowest energy isomers found by this methodology for Sc(3)N@C(68), Sc(3)N@C(78), Sc(3)N@C(80), Y(3)N@C(78), Y(3)N@C(80), Y(3)N@C(84), Y(3)N@C(86), and Y(3)N@C(88) are those that have been shown to exist by single-crystal X-ray studies as Sc(3)N@C(2n) (2n = 68, 78, 80), Dy(3)N@C(80), and Tb(3)N@C(2n) (2n = 80, 84, 86, 88) clusterfullerenes. Reassignment of the carbon cage of Sc(2)@C(76) to the non-IPR Cs: 17490 isomer is also proposed. The stability of nitride clusterfullerenes was found to correlate well with the stability of the empty 6-fold charged cages. However, the dimensions of the cage in terms of its ability to encapsulate M(3)N clusters were also found to be an important factor, especially for the medium size cages and the large Y(3)N cluster. In some cases the most stable structures are based on the different cage isomers for Sc(3)N and Y(3)N clusters. Up to the cage size of C(84), non-IPR isomers of C(2n)(6-) and M(3)N@C(2n) were found to compete with or to be even more stable than IPR isomers. However, the number of adjacent pentagon pairs in the most stable non-IPR isomers decreases as cage size increases: the most stable M(3)N@C(2n) isomers have three such pairs for 2n = 68-72, two pairs for n = 74-80, and only one pair for n = 82, 84. For C(86) and C(88) the lowest energy IPR isomers are much more stable than any non-IPR isomer. The trends in the stability of the fullerene isomers and the cluster-cage binding energies are discussed, and general rules for stability of clusterfullerenes are established. Finally, the high yield of M(3)N@C(80) (Ih) clusterfullerenes for any metal is explained by the exceptional stability of the C(80)(6-) (Ih: 31924) cage, rationalized by the optimum distribution of the pentagons leading to the minimization of the steric strain, and structural similarities of C(80) (Ih: 31924) with the lowest energy non-IPR isomers of C(760(6-), C(78)(6-), C(82)(6-), and C(84)(6-) pointed out.     

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‐C₇₀Cl₆ isolable by chromatography. The synthesis of dihept‐C₇₀Cl₆ offers precious clues with respect to the fullerene formation mechanism in the carbon‐clustering process.     

Theoretical study on monometallic cyanide cluster fullerenes YCN@C72

The geometries, stabilities, and electronic properties of new endohedral fullerene YCN@C₇₂ have been investigated by the B3LYP and PBE1PBE density functional (DFT) methods. The C_2v(11188)‐C₇₂ cage, which violates the isolated pentagon rule (IPR) with a pair of fused pentagons, is predicted to be the lowest energy isomer for both empty and YCN@C₇₂. The relatively large HOMO‐LUMO gap (B3LYP: 1.48 eV, PBE1PBE: 1.68 eV) for YCN@C_2v(11188)‐C₇₂ reveals this structure kinetic stability. Significantly, the encased YCN cluster adopts a triangular structure inside the C_2v(11188)‐C₇₂ cage, similar to the results reported on YCN@C_s(6)‐C₈₂ and TbCN@C₂(5)‐C₈₂. Furthermore, the vertical ionization potential and electron affinity, UV‐vis‐NIR and IR spectra of YCN@C_2v(11188)‐C₇₂ have been predicted to facilitate future experimental characterization. © 2015 Wiley Periodicals, Inc.     

Synthesis and Structure of LaSc2N@Cs(hept)‐C80 with One Heptagon and Thirteen Pentagons

The synthesis and single‐crystal X‐ray structural characterization of the first endohedral metallofullerene to contain a heptagon in the carbon cage are reported. The carbon framework surrounding the planar LaSc₂N unit in LaSc₂N@C_s(hept)‐C₈₀ consists of one heptagon, 13 pentagons, and 28 hexagons. This cage is related to the most abundant I_h‐C₈₀ isomer by one Stone–Wales‐like, heptagon/pentagon to hexagon/hexagon realignment. DFT computations predict that LaSc₂N@C_s(hept)‐C₈₀ is more stable than LaSc₂N@D_5h‐C₈₀, and suggests that the low yield of the heptagon‐containing endohedral fullerene may be caused by kinetic factors.     

The fused heterocycle 2‐benzyl‐3‐methyl‐1‐phenyl‐4‐phenyl­sulfonyl‐2H,4H‐pyrrolo­[3,4‐b]­indole

Crystal structure determination of the title mol­ecule, C₃₀H₂₄N₂O₂S, reveals that the pyrrole ring in this fused heterocycle, although presumably strained and reactive in cyclo­addition reactions, does not differ appreciably from N‐methyl­pyrrole except for a shorter C—C single bond [1.409 (4) Å] in the pyrrole ring.