Structural Aspects of Fullerene ChemistryA Journey through Fullerene Chirality

Manipulating the Structural Transformation of Fullerene Microtubes to Fullerene Microhorns Having Microscopic Recognition Properties

We report the production of fullerene microtubes (FMTs), having solid cores bisecting their tubular cavities, from solutions of mixtures of fullerene C₆₀ and C₇₀ and have demonstrated the structural transformation of FMTs to fullerene microhorns (FMHs) upon their exposure to alcohol/mesitylene mixtures at 25 °C. The conically shaped microhorns have hollow interiors and exhibit preferential recognition of silica particles over fullerene C₇₀, polystyrene (PS) latex, PS hydroxylate, or PS carboxylate particles of similar dimensions due to strong electrostatic interactions between negatively charged FMHs and positively charged silica particles.

Palladium-catalyzed decarboxylative heterocyclizations of [60]fullerene: preparation of novel vinyl-substituted [60]fullerene-fused tetrahydrofurans/pyrans/quinolines

A general and practical methodology for the preparation of novel vinyl-substituted [60]fullerene-fused tetrahydrofurans/pyrans/quinolines through palladium-catalyzed decarboxylative heterocyclizations of [60]fullerene with vinylethylene/2-alkylidenetrimethylene carbonates and vinyl carbamates was developed. Without additives or ligands, the Pd(PPh3)4-catalyzed transformations undergo decarboxylative O- and N-heteroannulation processes to efficiently furnish structurally diverse [60]fullerene-fused heterocyclic derivatives.

Molecular Structure and Magnetic and Optical Properties of Endometallonitridofullerene Sc3 N@Ih -C80 in Neutral, Radical Anion, and Dimeric Anionic Forms

A series of compounds with Sc₃ N@I_h -C₈₀ in the neutral, monomeric, and dimeric anion states have been prepared in the crystalline form and their molecular structures and optical and magnetic properties have been studied. The neutral Sc₃ N@I_h -C₈₀ ⋅3 C₆ H₄ Cl₂ (1) and (Sc₃ N@I_h -C₈₀ )₃ (TPC)₂ ⋅5 C₆ H₄ Cl₂ (2, TPC=triptycene) compounds both crystallized in a high-symmetry trigonal structure. The reduction of Sc₃ N@I_h -C₈₀ to the radical anion resulted in dimerization to form diamagnetic singly bonded (Sc₃ N@I_h -C₈₀ ^- )₂ dimers. In contrast to {[2.2.2]cryptand(Na⁺ )}₂ (Sc₃ N@I_h -C₈₀ ^- )₂ ⋅2.5 C₆ H₄ Cl₂ (3) with strongly disordered components, we synthesized new dimeric phases {[2.2.2]cryptand- (K⁺ )}₂ (Sc₃ N@I_h -C₈₀ ^- )₂ ⋅2 C₆ H₄ Cl₂ (4) and {[2.2.2]cryptand- (Cs⁺ )}₂ (Sc₃ N@I_h -C₈₀ ^- )₂ ⋅2 C₆ H₄ Cl₂ (5) in which only one major dimer orientation was found. The thermal stability of the (Sc₃ N@I_h -C₈₀ ^- )₂ dimers was studied by EPR analysis of 3 to show their dissociation in the 400-460 K range producing monomeric Sc₃ N@I_h -C₈₀ ^.- radical anions. This species shows an EPR signal with a hyperfine splitting of 5.8 mT. The energy of the intercage C-C bond was estimated to be 234±7 kJ mol^-1 , the highest value among negatively charged fullerene dimers. The EPR spectra of crystalline (Bu₃ MeP⁺ )₃ (Sc₃ N@I_h -C₈₀ ^.- )₃ ⋅C₆ H₄ Cl₂ (6) are presented for the first time. The salt shows an asymmetric EPR signal, which could be fitted by three lines. Two lines were attributed to Sc₃ N@I_h -C₈₀ ^.- . Hyperfine splitting is manifested above 180 K due to the hyperfine interaction of the electron spin with the three scandium atoms (a total of 22 lines with an average splitting of 5.32 mT are observed at 220 K). Furthermore, each of the 22 lines is additionally split into six lines with an average separation of 0.82 mT. The large splitting indicates intrinsic charge and spin density transfer from the fullerene cage to the Sc₃ N cluster. Both the monomeric and dimeric Sc₃ N@I_h -C₈₀ ^- anions show an intrinsic shift of the IR bands attributed to the Sc₃ N cluster and new bands corresponding to these species appear in the NIR range of their UV/Vis/NIR spectra, which allows these anions to be distinguished from neutral species.

General One-step Synthesis of Symmetrical or Unsymmetrical 1,4-Di(organo)fullerenes from Organo(hydro)fullerenes through Direct Oxidative Arylation

A general one-step synthesis of symmetrical or unsymmetrical 1,4-di(organo)fullerenes from organo(hydro)fullerenes (RC₆₀H) is realized by direct oxidative arylation. The new combination of catalytic trifluoromethanesulfonic acid (TfOH) and stoichiometric o-chloranil is the first to be used to directly generate an R-C₆₀⁺ intermediate from common RC₆₀H. Unexpectedly, the in situ generated R-C₆₀⁺ intermediate is shown to be quite stable in whole ¹³C NMR spectroscopy characterization in the absence of cation quenching reagents. Because the direct oxidation of common RC₆₀H to form the corresponding R-C₆₀⁺ has never been realized, the present combination of TfOH and o-chloranil solves the challenges associated with the formation of stable RC₆₀⁺ cations from common RC₆₀H without any coordination of an R group.

Bowl Inversion in an Exo-type Supramolecule in the Solid State

Bowl inversion is a unique property of buckybowls. The polarity and assembly configuration of buckybowls are reversed after bowl inversion. So far, this unique phenomenon has been studied in solution and on surface, but not in solid state due to spatial constraint. Now a series of exo-type supramolecular assemblies of trithiasumanene and nanographene are investigated. Tuning the electron density of the nanogaphene component was found to directly affect the binding constant of the complex. Reversible bowl inversion in the solid state was then successfully achieved by subjecting the trithiasumanene-nanographene assembly with the weakest binding strength to repeated heating-cooling cycles, which was unambiguously observed by single crystal X-ray diffraction.

Chiral Molecular Carbon Nanostructures

Chirality is a fascinating property present in naturally occurring and artificial molecules and materials, observable as chiroptical behavior. The emerging area of carbon nanostructures has undergone tremendous development, with a wide variety of carbon nanoforms reported over the last two decades. However, despite interest in merging chirality and nanocarbons, this has been successfully achieved only in empty fullerenes, whereas in other kinds of fullerenes or carbon nanostructures such as carbon nanotubes, graphene, and graphene quantum dots (GQDs), to name the most popular systems, it is almost unknown. Therefore, controlling chirality in carbon nanostructures currently represents a major challenge for the chemical community. In this Account, we show our progress in the synthesis of chiral molecular carbon nanostructures, namely, metallofullerenes, endohedral fullerenes, GQDs, and curved molecular nanographenes, by using asymmetric catalysis and both top-down and bottom-up chemical approaches. Furthermore, we bring in a new family of lesser-known molecular chiral bilayer nanographenes, where chirality is introduced from the starting helicene moiety and a single enantiomer of the nanographene is synthesized. Some important landmarks in the development of chiral molecular carbon nanostructures shown in this Account are the application of synthesis-tailored, enantiomerically pure metallofullerenes as catalysts for hydrogen transfer reactions and the use of endohedral fullerenes to determine the effect of the incarcerated molecule in the carbon cage on the cis-trans stereoisomerization of optically active pendent moieties. Furthermore, the first top-down synthesis of chiral GQDs by functionalization with chiral alcohols is also presented. An emerging alternative to GQDs, when the desire for purity and atomistic control outweighs the cost of multistep synthesis, is the bottom-up approach, in which molecular nanographenes are formed in precise sizes and shapes and enantiomeric control is feasible. In this regard, a singular and amazing example is given by our synthesis of a single enantiomer of the first chiral bilayer nanographene, which formally represents a new family of molecular nanographenes with chirality controlled and maintained throughout their syntheses. The aforementioned synthetic chiral nanostructures represent groundbreaking nanocarbon systems where chirality is a further dimension of structural control, paving the way to a new scenario for carbon nanoforms in which chirality selection determines the properties of these novel carbon-based materials. Fine-tuning of such properties is envisioned to impact biomedical and materials science applications.

Metal-Free Synthesis of N-Alkyl-2,5-Unsubstituted/Monosubstituted Fulleropyrrolidines: Reaction of [60]Fullerene with Paraformaldehyde and Amines

A series of scarce N-alkyl-2,5-unsubstituted/monosubstituted fulleropyrrolidines were synthesized in moderate to excellent yields by the simple one-step thermal reaction of [60]fullerene with primary/secondary amines in the presence of paraformaldehyde without the addition of valuable metal salts. Intriguingly, the reaction with primary amines unexpectedly afforded N-alkyl-2,5-unsubstituted fulleropyrrolidines instead of the anticipated 2,5-monosubstituted fulleropyrrolidines. A plausible reaction pathway is proposed to elucidate the above-mentioned reaction process based on the experimental results.

Spectroscopic characterisation of centropolyindanes

A highly promising class of three-dimensional polyaromatic hydrocarbons comprises the centropolyindanes. The characteristic feature of these compounds is the mutual fusion of several molecules of indane along the saturated C-C bonds of their cyclopentane rings. Among the polycyclic aromatic hydrocarbons, the centropolyindanes are special because of the saturated core of sp3-hybridised carbon atoms embedded in a three-dimensional environment of aromatic building blocks. While the centropolyindanes and their numerous derivatives have been studied in detail by NMR spectroscopy, mass spectrometry and X-ray diffraction, investigation of their vibrational features, and especially those of the neopentane core present in most cases, have not been performed so far. In the present paper, we report the first systematic study of a set of centropolyindanes by vibrational spectroscopy, using inelastic neutron scattering (INS), infrared and Raman spectroscopies.

A retro Baeyer-Villiger reaction: electrochemical reduction of [60]fullerene-fused lactones to [60]fullerene-fused ketones

An unprecedented retro Baeyer–Villiger reaction has been achieved by the electrochemical reduction of [60]fullerene-fused lactones in the presence of acetic acid at room temperature, affording [60]fullerene-fused ketones in excellent yields within a short time.

A highly efficient electrochemical reduction of [60]fullerene-fused lactones to [60]fullerene-fused ketones, a formal process of retro Baeyer–Villiger reaction, has been achieved for the first time. The electrochemically generated dianionic [60]fullerene-fused lactones can be transformed into [60]fullerene-fused ketones in the presence of acetic acid in 85–91% yields. Control experiments have been performed to elucidate the reaction mechanism. The products have been characterized with spectroscopic data and single-crystal X-ray analysis. Moreover, the electrochemical properties have also been investigated.

Electron transport and ultrafast spectroscopic studies of new methanofullerenes bearing a heteroatom in the exohedral chain

Heteroatom-bearing methanofullerenes with exciting photophysical and electron transport properties are shown to be an addition to the variety of fullerene derivatives for use as n-type materials in organic electronics.

Negatively charged singly-bonded dimers of C1-[C70(CF3)10] and bare C70 fullerene

Singly-bonded {C₇₀(CF₃)₁₀⁻}₂ and (C₇₀⁻)₂ dimers are obtained as crystalline salts allowing their molecular structure, and optical and magnetic properties to be studied.

Reaction of the C60 radical anion with alkyl halides

A new reductive alkylation of C₆₀ with α-bromo-1,3-dicarbonyl compounds, benzyl halide, and 1,2-bis(dihalomethyl)benzene has been reported.

Acylation of Homoatomic Ge9 Cages and Subsequent Decarbonylation

The direct acylation of Ge₉ Zintl clusters by the reaction of K[Ge₉ {Si(SiMe₃ )₃ }₃ ] with acyl chlorides in hexane or toluene solutions is presented, leading to the neutral, carbonyl-derivatized products [{Si(SiMe₃ )₃ }₃ Ge₉ (CO)R'] (R'=Me, iPr, tBu, Ph, Bz, cyclopropylmethyl, phenethyl, 4-vinylphenyl). This reaction is applicable to a wide range of acyl chlorides and allows for diverse functionalization of Ge₉ Zintl clusters. [{Si(SiMe₃ )₃ }₃ Ge₉ (CO)tBu] readily releases CO at ambient conditions under formation of [{Si(SiMe₃ )₃ }₃ Ge₉ tBu]. This temperature-dependent decarbonylation most likely proceeds via a radical Norrish-type I α-bond cleavage. Except for R'=tBu and Bz all obtained acyl-derivatized Ge₉ cluster compounds do not release CO even at elevated temperatures. All compounds were characterized by NMR spectroscopy. [{Si(SiMe₃ )₃ }₃ Ge₉ (CO)R'] (R'=tBu, Ph, Me, iPr) as well as [{Si(SiMe₃ )₃ }₃ Ge₉ tBu] were further structurally characterized by single crystal X-ray diffraction.

Facile Separation, Spectroscopic Identification, and Electrochemical Properties of Higher Trifluoromethylated Derivatives of [70]Fullerene

We survey the structure and electronic properties of the family of higher trifluoromethylated C70 (CF3 )n molecules with n=14, 16, 18, and 20. Twenty-two available compounds, of which thirteen are newly obtained and characterized, demonstrate the broad diversity of π-system topologies, which enabled us to study the interplay between the CF3 addition pattern and the electronic properties. UV/Vis spectroscopic and cyclic voltammetric studies demonstrate the importance of the exact addition pattern rather than the plain number of addends. Of particular interest is the skew pentagonal pyramid (SPP) addition pattern, which enables formation of closed-shell cyclopentadienyl anions C70 (CF3 )n-1- through CF3 detachment upon electron transfer. A detailed study of the process is presented for a SPP-C70 (CF3 )16 where potentiostatic electrolysis at the second reduction potential gives C70 (CF3 )15- oxidizable to a persistent C70 (CF3 )15. radical. Together with the literature data for the lower C70 (CF3 )n compounds with n=2-12, the present results show good correlation between the experimental boundary level positions and the DFT predictions. The compounds turn out to be electron acceptor molecular semiconductors with experimental LUMO energies and HOMO-LUMO gaps within the ranges of -4.3 to -3.7 eV and 1.6 to 3.3 eV, respectively, depending on the shape of the conjugated fragments. The HOMO levels fall within the range of -5.6 to -6.9 eV and show linear correlation with the number of addends.

KO tBu-Promoted C4 Selective Coupling Reaction of Phenols and [60]Fullerene: One-Pot Synthesis of 4-[60]Fullerephenols under Transition-Metal-Free Conditions

A KO ^tBu-promoted direct coupling reaction of phenols and [60]fullerene was disclosed. The reaction occurs exclusively at the C4-position of phenols with high regioselectivity and provides an efficient and inexpensive manner to various 4-[60]fullerephenols in good yields. The electrochemical properties of the products render the method attractive and valuable.

Enantioselective Synthesis of Chiral-at-Cage o-Carboranes via Pd-Catalyzed Asymmetric B-H Substitution

Carborane cage chirality is an outstanding issue of great interest as the icosahedral carboranes have wide applications in medicinal and materials chemistry. The synthesis of optically active carborane derivatives, whose chirality is associated with the substitution patterns on the polyhedron, will open new avenues to carborane chemistry. We report herein an efficient method to achieve chiral-at-cage arylation of o-carboranes with high regio- and enantioselectivities by a strategy of palladium-catalyzed asymmetric intramolecular B-H arylation and cyclization. This represents the first example of the enantioselective reaction on carboranes, providing an efficient way for the construction of chiral-at-cage compounds with new skeletons.

When metal clusters meet carbon cages: endohedral clusterfullerenes

Fullerenes have the characteristic of a hollow interior, and this unique feature triggers intuitive inspiration to entrap atoms, ions or clusters inside the carbon cage in the form of endohedral fullerenes. In particular, upon entrapping an otherwise unstable metal cluster into a carbon cage, the so-called endohedral clusterfullerenes fulfil the mutual stabilization of the inner metal cluster and the outer fullerene cage with a specific isomeric structure which is often unstable as an empty fullerene. A variety of metal clusters have been reported to form endohedral clusterfullerenes, including metal nitrides, carbides, oxides, sulfides, cyanides and so on, making endohedral clusterfullerenes the most variable and intriguing branch of endohedral fullerenes. In this review article, we present an exhaustive review on all types of endohedral clusterfullerenes reported to date, including their discoveries, syntheses, separations, molecular structures and properties as well as their potential applications in versatile fields such as biomedicine, energy conversion, and so on. At the end, we present an outlook on the prospect of endohedral clusterfullerenes.

TEMPO-Mediated Synthesis of Tetrahydropyridinofullerenes: Reaction of [60]Fullerene with α-Methyl-Substituted Arylmethanamines and Aldehydes in the Presence of 4-Dimethylaminopyridine

A series of scarce tetrahydropyridinofullerenes were synthesized by the metal-free-mediated reaction of [60]fullerene with cheap and easily available α-methyl-substituted arylmethanamines and aldehydes in the presence of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-dimethylaminopyridine (DMAP) in moderate to good yields comparable to the previously reported data for most monoadducts. The in situ generation of azadienes played a crucial role in the successful synthesis of tetrahydropyridinofullerenes. A plausible reaction mechanism was proposed to elucidate the reaction process.

Stereoselective synthesis of cyclopentafullerenes: the reaction of [60]fullerene with aldehydes and triethylamine promoted by magnesium perchlorate

The magnesium perchlorate-mediated reaction of [60]fullerene with aldehydes and triethylamine generated a series of rare amino-substituted cyclopentafullerenes with high stereoselectivity.

Metal-free synthesis of fulleropyrrolidin-2-ols: a novel reaction of [60]fullerene with amines and 2,2-disubstituted acetaldehydes

The reaction of [60]fullerene with amines and 2,2-disubstituted acetaldehydes in the absence of metal salts afforded a series of novel fulleropyrrolidin-2-ols in moderate yields.

Synthesis and self-sensitized photo-oxidation of 2-fulleropyrrolines by palladium(ii)-catalyzed heteroannulation of [60]fullerene with benzoyl hydrazone esters

A Pd(OAc)₂-catalyzed N-heteroannulation reaction was exploited to synthesize N-unsubstituted 2-fulleropyrrolines and the photo-oxidation of 2-fulleropyrrolines was first investigated.