Capturing the Antiaromatic #6094C68 Carbon Cage in the Radio‐Frequency Furnace

Although all fullerenes do not satisfy the classical aromaticity condition, as a result of their nonplanar nature, they experience effective stabilization due to extensive cyclic π‐electron delocalization and exhibit pronounced “spherical aromaticity”. This feature has raised the question of the opposite phenomenon, that is, the existence of antiaromatic carbon cages. Here the first experimental evidence of the existence of antiaromatic fullerenes is reported. The elusive ^#6094C₆₈ was effectively captured as C₆₈Cl₈ by in situ chlorination in the gas phase during radio‐frequency synthesis. The chlorinated cage was separated by means of multistage HPLC, and its connectivity unambiguously determined by single‐crystal X‐ray analysis. Halogen‐stripped pristine ^#6094C₆₈ was monitored by mass spectrometry of the chlorinated C₆₈Cl₈ cage. Quantum chemical calculations reveal the highly antiaromatic character of ^#6094C₆₈, in accordance with all geometric, energetic, and magnetic criteria of aromaticity. Chlorine addition leads to substantial stabilization of the cage owing to aromatization in the resulting C₆₈Cl₈, which explains its high abundance in the primary fullerene soot. This work provides new insights into the process of fullerene formation and better understanding of aromaticity phenomena in general.     

Synthesis and structural characterization of highly chlorinated C70, C70Cl28

Chlorination of [70]fullerene with SbCl(5), VCl(4) or PCl(5) yielded C(70)Cl(28) comprising three isomers, all containing four isolated benzenoid rings in the fullerene cage. This demonstrates, for the first time for C(70) derivatives, a stabilization effect due to planar aromaticity.     

M4 @Si28 (M=Al,Ga): metal-encapsulated tetrahedral silicon fullerene

It is known that silicon fullerenes cannot maintain perfect cage structures like carbon fullerenes. Previous density-functional theory calculations have shown that even with encapsulated species, nearly all endohedral silicon fullerenes exhibit highly puckered cage structures in comparison with their carbon counterparts. In this work, we present theoretical evidences that the tetrahedral fullerene cage Si(28) can be fully stabilized by encapsulating a tetrahedral metallic cluster (Al(4) or Ga(4)). To our knowledge, this is the first predicted endohedral silicon fullerene that can retain perfectly the same cage structure (without puckering) as the carbon fullerene counterpart (T(d)-C(28) fullerene). Density-functional theory calculations also suggest that the two endohedral metallosilicon fullerenes T(d)-M(4)@Si(28) (M=Al and Ga) can be chemically stable because both clusters have a large highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap ( approximately 0.9 eV), strong spherical aromaticity (nucleus-independent chemical shift value of -36 and -44), and large binding and embedding energies.     

An entrant of smaller fullerene: C56 captured by chlorines and aligned in linear chains

A smaller fullerene C56 (#913) is stabilized, isolated, and crystallographically characterized as C56Cl10. The geometric parameters of C56Cl10 imply the otherwise unstable cage of C56 can be stabilized by chlorination through releasing its surface strains and maintaining fragmental aromaticity. An unexpected C Cl...ClC short contact, as well as the linear alignment with pearl-necklace-shaped, is revealed in C56Cl10 crystal.     

Aromaticity of Heterofullerenes C18BxNy (x+y=2) and Their Molecular Ions

The aromaticity of all possible substituted fullerene isomers of C18N2, C18B2, C18BN, and their molecularions which originate from the C20 (Ih) cage were studied by the topological resonance energy (TRE) and the percentage topological resonance energy methods. The relationship between the aromaticity of C18BxNy isomers and the sites where the heteroatoms dope at the C20 (Ih) cage is discussed. Calculation results show that at the neutral and cationic states all the isomers are predicted to be antiaromatic with negative TREs, but their polyvalent anions are predicted to be aromatic with positive TREs. The most stable isomer is formed by heteroatom doping at the 1,11-sites in C18N2, C18B2, and C18BN. Heterofullerenes are more aromatic than C20. The stability order in the neutral states is C18N2>C18BN>C18B2>C20. The stability order in closed-shell is C18B2 8->C20 6->C18BN6->C18N2 4-. This predicts theoretically that their polyvalent anions have high aromaticity.     

Four isomers of c(96) fullerene structurally proven as c(96) cl(22) and c(96) cl(24)

Direct proof of the cage connectivities of four isomers of C(96) , the highest isolable empty fullerene, has been achieved. C(96) fractions, which were isolated from fullerene soot by recycling HPLC, were chlorinated and the resulting single crystals of C(96) Cl(22) and C(96) Cl(24) were studied by X-ray diffraction using synchrotron radiation. D(2) -C(96) (183)Cl(24) (see structure; gray C, green Cl) was obtained in two crystalline modifications.     

Studies on the encapsulation of various anions in different fullerenes using density functional theory calculations and Born-Oppenheimer molecular dynamics simulation

The density functional theory (DFT)-based Becke's three parameter hybrid exchange functional and Lee-Yang-Parr correlation functional (B3LYP) calculations and Born-Oppenheimer molecular dynamics (BOMD) simulations have been performed to understand the stability of different anions inside fullerenes of various sizes. As expected, the stability of anion inside the fullerene depends on its size as well as on the size of the fullerene. Results show that the encapsulation of anions in larger fullerenes (smaller fullerene) is energetically favorable (not favorable). The minimum size of the fullerene required to encapsulate F(-) is equal to C(32). It is found from the results that C(60) can accommodate F(-), Cl(-), Br(-), OH(-), and CN(-). The electron density topology analysis using atoms in molecule (AIM) approach vividly delineates the interaction between fullerene and anion. Although F(-)@C(30) is energetically not favorable, the BOMD results reveal that the anion fluctuates around the center of the cage. The anion does not exhibit any tendency to escape from the cage.     

Expanding the number of stable isomeric structures of the C80 cage: a new fullerene Dy3N@C80

The production, isolation, and spectroscopic characterization of a new Dy3N@C80 cluster fullerene that exhibits three isomers (1-3) is reported for the first time. In addition, the third isomer (3) forms a completely new C80 cage structure that has not been reported in any endohedral fullerenes so far. The isomeric structures of the Dy3N@C80 cluster fullerene were analyzed by studying HPLC retention behavior, laser desorption time-of-flight (LD-TOF) mass spectrometry, and UV-Vis-NIR and FTIR spectroscopy. The three isomers of Dy3N@C80 were all large band-gap (1.51, 1.33, and 1.31 eV for 1-3, respectively) materials, and could be classified as very stable fullerenes. According to results of FTIR spectroscopy, the Dy3N@C80 (I) (1) was assigned to the fullerene cage C80:7 (I(h)), whereas Dy3N@C80 (II) (2) had the cage structure of C80:6 (D(5h)). The most probable cage structure of Dy3N@C80 (III) (3) was proposed to be C80:1 (D(5d)). The significant differences between Dy3N@C80 and other reported M3N@C80 (M = Sc, Y, Gd, Tb, Ho, Er, Tm) cluster fullerenes are discussed in detail, and the strong influence of the metal on the nitride cluster fullerene formation is concluded.     

Application of sulfur as a matrix for laser desorption/ionization in the characterization of halogenated fullerenes.

The application of sulfur as a matrix for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) analysis of highly chlorinated and fluorinated fullerenes is reported. Control over fluorofullerene fragmentation which resulted in the domination of the molecular peak C(60)F(36)(-) was achieved, with the optimal matrix-to-analyte ratio found to be 1000:1. We suggest the possible mechanism of the molecular ion formation according to the charge transfer between the sulfur anions and C(60)F(36). For the first time the LDI and MALDI mass spectra of the highly chlorinated fullerene C(60)Cl(x)(x(max) approximately 32) are presented. The formation of odd chlorine ions (positive and negative) is observed. We conclude that use of sulfur as a matrix leads to a significant decrease in fragmentation of the halogenated fullerenes.     

DFT study on the stabilities of the heterofullerenes Sc3N@C67B, Sc3N@C67N, and Sc3N@C66BN

On the basis of calculations using density functional theory, we investigated the relative stabilities of all isomers of Sc3N@C67B and Sc3N@C67N as well as those of stable isomers of Sc3N@C66BN. As a result, we predict that Sc3N@C68 can be doped substitutionally with a boron atom much better than C60. This effect can be ascribed to the favorable electrostatic attraction between the encased Sc3N cluster and the polar C-B bonds of the fullerene cage, which show the important role played by the encapsulated atoms in stabilizing the fullerene. A difference in the interaction also determines the regiospecificity of Sc3N@C67B. On the contrary, N-doping of the fullerenes forming Sc3N@C67N is much less favorable than that in C60 or C70. A judicious choice of stable isomers of Sc3N@C66BN among a vast number of possible isomers indicates that Sc3N@C68 can also be doped with a pair of B and N atoms better than C60 under the simultaneous existence of B and N sources. Relative stabilities of various isomers of the BN-substituted fullerenes can be understood in terms of the combined electrostatic effects in the B- and N-substitutions of Sc3N@C68 complemented by a specific local preference in the N-substitution and the formation of a B-N bond.     

The smallest stable fullerene, M@C28 (m = Ti, Zr, U): stabilization and growth from carbon vapor

The smallest fullerene to form in condensing carbon vapor has received considerable interest since the discovery of Buckminsterfullerene, C(60). Smaller fullerenes remain a largely unexplored class of all-carbon molecules that are predicted to exhibit fascinating properties due to the large degree of curvature and resulting highly pyramidalized carbon atoms in their structures. However, that curvature also renders the smallest fullerenes highly reactive, making them difficult to detect experimentally. Gas-phase attempts to investigate the smallest fullerene by stabilization through cage encapsulation of a metal have been hindered by the complexity of mass spectra that result from vaporization experiments which include non-fullerene clusters, empty cages, and metallofullerenes. We use high-resolution FT-ICR mass spectrometry to overcome that problem and investigate formation of the smallest fullerene by use of a pulsed laser vaporization cluster source. Here, we report that the C(28) fullerene stabilized by encapsulation with an appropriate metal forms directly from carbon vapor as the smallest fullerene under our conditions. Its stabilization is investigated, and we show that M@C(28) is formed by a bottom-up growth mechanism and is a precursor to larger metallofullerenes. In fact, it appears that the encapsulating metal species may catalyze or nucleate endohedral fullerene formation.     

Most stable structure of fullerene[20] and its novel activity toward addition of alkene: a theoretical study

Structures and stabilities of fullerene C20 and C20- have been investigated by the density functional theory and CCSD(T) calculations. In consideration of the Jahn-Teller distortion of Ih-symmetric C20, possible subgroup symmetries have been used in the full geometry optimization. On the basis of relative energetics, vibrational analyses, and electron affinities, fullerenes C20 and C20- have most stable D2h and Ci structures, respectively. The controversy on the relative stability of fullerene[20] arises from the use of different subgroups in calculation and the basis set dependence in vibrational analysis. Predicted nucleus-independent chemical shift values show that the most stable fullerene C20 and its derivatives C20(C2H2)n and C20(C2H4)n (n=1-3) exhibit remarkable aromaticity, while C20(C2H2)4 and C20(C2H4)4 have no spherical aromaticity. The C20 (D2h) cage has remarkable activity toward the addition of olefin, and such feasibility of the addition reaction is ascribed to strong bonding interactions among frontier molecular orbitals from C20 and olefin. Calculations indicate that both C20(C2H2)n and C20(C2H4)n have similar features in electronic spectra.     

The way of stabilizing non-IPR fullerenes and structural elucidation of C(54)Cl(8)

Recently, a new non-IPR chlorofullerene C(54)Cl(8) was isolated experimentally (Science 2004, 304, 699). To explore the ways to stabilize non-IPR fullerenes, the authors studied all of the possible isomers of C(54) fullerene and some of the C(54)Cl(8) isomers at PM3, B3LYP/3-21G, and B3LYP/6-31G* levels. Combined with analysis of pentagon distributions, bond resonance energies, and steric strains, C(54):540 with the least number of 5/5 bonds was determined to be the thermodynamically best isomer for the C(54)Cl(8). Based on C(54):540, the most probable structure of the experimental C(54)Cl(8) was elucidated. The results suggested one of the necessary conditions of stabilizing non-IPR fullerenes: chemical derivatizations of either endohedral complexation or exohedral addition need to sufficiently stabilize all of the kinetically unstable 5/5 bonds of the cages.     

Comparative investigation on non-IPR C68 and IPR C78 fullerenes encaging Sc3N molecules

A computational study on the experimentally detected Sc(3)N@C(68) cluster is reported, involving quantum chemical analysis at the B3LYP/6-31G level. Extensive computations were carried out on the pure C(68) cage which does not conform with the isolated pentagon rule (IPR). The two maximally stable C(68) isomers were selected as initial Sc(3)N@C(68) cage structures. Full geometry optimization leads to a confirmation of an earlier assessment of the Sc(3)N@C(68) equilibrium geometry (Nature 2000, 408, 427), namely an eclipsed arrangement of Sc(3)N in the C(68) 6140 frame, where each Sc atom interacts with one pentagon pair. From a variety of theoretical procedures, a D(3h) structure is proposed for the free Sc(3)N molecule. Encapsulated into the C(68) enclosure, this unit is strongly stabilized with respect to rotation within the cage. The complexation energy of Sc(3)N@C(68) cage is found to be in the order of that determined for Sc(3)N@C(80) and exceeding the complexation energy of Sc(3)N@C(78). The cage-core interaction is investigated in terms of electron transfer from the encapsulated trimetallic cluster to the fullerene as well as hybridization between these two subsystems. The stabilization mechanism of Sc(3)N@C(68) is seen to be analogous to that operative in Sc(3)N@C(78). For both cages, C(68) and C(78), inclusion of Sc(3)N induces aromaticity of the cluster as a whole.     

The role of an asymmetric nitride cluster on a fullerene cage: the non-IPR endohedral DySc2N@C76

The first non-IPR C(76) cage based on a mixed metal nitride cluster, DySc(2)N@C(76), was successfully synthesized and isolated. DySc(2)N@C(76) is a stable fullerene with a small band gap of 0.96 eV. According to the FTIR spectroscopic study in combination with extensive DFT calculations, the cage structure of DySc(2)N@C(76) has been assigned to the non-IPR C(s): 17490-I isomer having two pairs of the adjacent pentagons. DySc(2)N@C(76)provides the first example of stabilization of the non-IPR C(76) cage by encapsulation of an asymmetric DySc(2)N mixed cluster, revealing the role of the cluster structure on the stability of the fullerene cage. As the asymmetric DySc(2)N cluster has a more suitable geometry for the inner space of the C(76) cage compared to that of the homogeneous clusters like Sc(3)N or Dy(3)N, the highest yield for C(76)-based cluster fullerenes with the Dy(x)Sc(3)-(x)N mixed nitride cluster is achieved for the DySc(2)N@C(76)     

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.     

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.     

Can Substituted Cyclopentadiene Become Aromatic or Antiaromatic?

Cyclopentadiene derivatives with electronegative (F, Cl) or electropositive (H(3)Si, Me(3)Si) bis-5,5-substituents were studied at the B3LYP/6-311G* level of theory. It was found that there is no special stabilization or destabilization for any of the derivatives; the energetic effects that were previously attributed to aromatic stabilization or antiaromatic destabilization are the result of interactions in the reference systems. A nucleus-independent chemical shift (NICS) scan study at the HF-GIAO/6-311+G* theoretical level of these and similar derivatives suggest that they all show different magnitudes of diamagnetic ring current. None of the derivatives shows a paramagnetic ring current. Thus, cyclopentadienes are neither aromatic nor antiaromatic. It is also concluded that a diamagnetic ring current is perhaps necessary but certainly not a sufficient condition for aromaticity. The NICS scan procedure describes the type of ring current in the system, whereas a single isotropic NICS value (i.e., NICS(1)) may wrongly assign the type of ring current. It is shown that neither NICS(1) nor the NICS scan procedure can be used as a single aromaticity criterion.     

Theoretical studies of the substitution patterns of boron-nitrogen (BN) fullerenes: from C50 up to C20B15N15 CBN ball

Study on the patterns of successive BN pair substitution in C50 fullerene and the chemical and electronic properties of these substitutionally doped heterofullerenes has been carried out with semiempirical (AM1 and MNDO) and density functional (B3LYP/3-21G) theories. The BN units prefer to stay together following "single bond", "hexagon filling", and "continuity and equatorial belt" rules. The driving force governing the stabilities of these BN-doped fullerenes is the strain of the cage. Compared with C50, the redox activity of C50-2x(BN)x (x = 1-15) isomers decreases and becomes weaker by increasing the number of BN units, while the aromaticity of the C50-2x(BN)x derivatives decreases and is independent of the number of BN units but related to the substitution positions. The main infrared absorptions are similar for all the C50-2x(BN)x isomers and the infrared spectrum becomes simpler and stronger with increasing the number of BN groups.     

Synthesis and antiviral activity of highly water-soluble polycarboxylic derivatives of [70]fullerene

We report here a facile preparation of highly water-soluble derivatives C(70)[p-C(6)H(4)(CH(2))(n)COOH](8) (n = 2, 3) starting from readily available chlorinated [70]fullerene precursors C(70)Cl(8) and C(70)Cl(10). The synthesized fullerene derivatives showed pronounced antiviral activity in vitro, particularly against human immunodeficiency virus (HIV) and influenza A virus (subtypes H1N1 and H3N2).