Empty Fullerenes Research Articles

(Invited) Organic Functionalization of Lithium-Ion-Endohedral Fullerenes

Lithium-ion-containing fullerene Li+@C60 has very high electron affinity and low-lying LUMO level at -4.3 eV due to encapsulated positive charged lithium ion in the fullerene cage. Because of this property, reactivity of Li+@C60 on organic functionalization is very different from empty fullerene. In this presentation we will show the synthesis of lithium-ion-containing PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) (Li+@PCBM). The [5,6]- and [6,6]-isomers of [Li+@PCBM]PF6 – were successfully prepared, and the structure of [6,6]-[Li+@PCBM]PF6 – was elucidated by X-ray crystallography. In addition, we will discuss reactivity of the Diels-Alder reaction on Li+@C60. The Diels–Alder (DA) reaction of fullerene is one of the most important reaction in the fullerene chemistry. We studied the reaction of Li+@C60 with cyclopentadiene, which were chosen as an diene, and found that this reaction was highly efficient, with an equilibrium constant that was more than 1000-fold that for the reaction with empty C60. The DA reaction of [Li+@C60](PF6 –) with 1,3-cyclohexadiene was also kinetically and computationally investigated to precisely determine the kinetic parameters. Compared with empty C60, Li+@C60 reacted 2400-fold faster at 303 K. The result was well explained by DFT calculation at the M06-2X/6-31G(d) level of theory considering the reactant complex with dispersion corrections as well as by the frontier molecular orbital theory. Figure 1

Key Structural Motifs To Predict the Cage Topology in Endohedral Metallofullerenes

We show that the relative isomer stability of fullerene anions is essentially governed by a few simple structural motifs, requiring only the connectivity information between atoms. Relative energies of a large number of isomers of fullerene anions, C(2n)(q) (2n = 68-104; q = -2, -4, -6), can be satisfactorily reproduced by merely counting the numbers of seven kinds of hexagon-based motifs. The dependence of stability on these motifs varies with the charge state, which reflects the fact that the isomeric form of the carbon cage in endohedral metallofullerenes (EMFs) often differs from that in neutral empty fullerenes. The chemical origin of the stabilization differences between motifs is discussed on the basis of electronic and strain effects as well as aromaticity. On the basis of this simple model, the extraordinary abundance of the icosahedral C80 cage in EMFs can be easily understood. We also provide an explanation for why the well-known isolated pentagon rule is often violated in smaller EMFs. Finally, simple topological indices are proposed for quantitatively predicting the relative stability of fullerene anions, allowing a rapid determination of suitable hosting cages in EMFs by just counting three simple structural motifs.

2-Aminoethanol Extraction as a Method for Purifying Sc3N@C80 and for Differentiating Classes of Endohedral Fullerenes on the Basis of Reactivity.

Extraction with 2-aminoethanol is an inexpensive method for removing empty cage fullerenes from the soluble extract from electric-arc-generated fullerene soot that contains endohedral metallofullerenes of the type Sc3N@C2n (n = 34, 39, 40). Our method of separation exploits the fact that C60, C70, and other larger, empty cage fullerenes are more susceptible to nucleophilic attack than endohedral fullerenes and that these adducts can be readily extracted into 2-aminoethanol. This methodology has also been employed to examine the reactivity of the mixture of soluble endohedral fullerenes that result from doping graphite rods used in the Krätschmer-Huffman electric-arc generator with the oxides of Y, Lu, Dy, Tb, and Gd. For example, with Y2O3, we were able to detect by mass spectrometry several new families of endohedral fullerenes, namely Y3C108 to Y3C126, Y3C107 to Y3C125, Y4C128 to Y4C146, that resisted reactivity with 2-aminoethanol more than the empty cage fullerenes and the mono- and dimetallo fullerenes. The discovery of the family Y3C107 to Y3C125 with odd numbers of carbon atoms is remarkable, since fullerene cages must involve even numbers of carbon atoms. The newly discovered families of endohedral fullerenes with the composition M4C2n (M = Y, Lu, Dy, Tb, and Gd) are unusually resistant to reaction with 2-aminoethanol. Additionally, the individual endohedrals, Y3C112 and M3C102 (M = Lu, Dy, Tb and Gd), were remarkably less reactive toward 2-aminoethanol.

Isolation and Crystallographic Characterization of Gd3N@D2(35)-C88 through Non-Chromatographic Methods.

While several nonchromatographic methods are available for the isolation and purification of endohedral fullerenes of the type M3N@Ih-C80, little work has been done that would allow other members of the M3N@C2n family to be isolated with minimal chromatography. Here, we report that Gd3N@D2(35)-C88 can be isolated from the multitude of endohedral and empty cage fullerenes present in carbon soot obtained by electric-arc synthesis using Gd2O3-doped graphite rods. The procedure developed utilizes successive precipitation with the Lewis acids CaCl2 and ZnCl2 followed by treatment with amino-functionalized silica gel. The structure of the product was identified by single-crystal X-ray diffraction.

Theoretical study of electron transfer process between fullerenes and neurotransmitters; acetylcholine, dopamine, serotonin and epinephrine in nanostructures [neurotransmitters].C n complexes.

Neurotransmitters are the compounds which allow the transmission of signals from one neuron to the next across synapses. They are the brain chemicals that communicate information throughout brain and body. Fullerenes are a family of carbonallotropes, molecules composed entirely of carbon, that take the forms of spheres, ellipsoids, and cylinders. Various empty carbon fullerenes (Cn) with different carbon atoms have been obtained and investigated. Topological indices have been successfully used to construct effective and useful mathematical methods to establish clear relationships between structural data and the physical properties of these materials. In this study, the number of carbon atoms in the fullerenes was used as an index to establish a relationship between the structures of neurotransmitters (NTs) acetylcholine (AC) 1, dopamine (DP) 2, serotonin (SE) 3, and epinephrine (EP) 4 as the well-known redox systems and fullerenes C n (n = 60, 70, 76, 82, and 86) which create [NT].Cn; A-1 to A-5 up to D-1 to D-5. The relationship between the number of carbon atoms and the free energy of electron transfer (ΔG et(n); n = 1-4) is assessed using the Rehm-Weller equation for A-1 to A-5 up to D-1 to D-5 supramolecular [NT].Cn complexes. The calculations are presented for the four reduction potentials ( (Red.) E 1 to (Red.) E 4 ) of fullerenes C n . The results were used to calculate the four free energy values of electron transfer (ΔG et(1) to ΔG et(4)) of the supramolecular complexes A-1 to A-8 up to D-1 to D-8 for fullerenes C60 to C120. The first to fourth free activation energy values of electron transfer and the maximum wavelength of the electron transfers, ΔG (#) et(n) and λ et (n = 1-4), respectively, were also calculated in this study for A-1 to A-8 up to D-1 to D-8 in accordance with the Marcus theory.

Mechanically modulated electronic properties of water-filled fullerenes

Abstract

Unprecedented Chemical Reactivities and Potential Utilities of Paramagnetic Endohedral Metallofullerenes

The inside metals of endohedral metallofullerenes (EMFs) lead remarkable features, such as unique molecular structures, and magnetic and electronic properties.[1] Among EMFs, paramagnetic EMFs are of particular interest because interplay between π-electron spins on the fullerene cage and inside metal atoms is expected to produce unconventional magnetic features. Unique properties of paramagnetic EMFs may lead to unique chemical reactivities. In this regard, despite the former efforts, much work is still needed for a better control on the chemical reactivity of the increasing variety of EMFs. For an easier accessibility to EMFs based materials, a wider availability of chemical functionalization methods on EMFs is required. Meanwhile, La@C82 has extensively been investigated as a prototype paramagnetic fullerene because it is the first EMF which was reported the successful isolation for the first time.[2] Moreover, it was demonstrated that a family of lanthanum containing fullerenes were produced and that extraction with toluene yielded abundantly La@C82. La@C82 has an electronic structure which is best described as [La]3+[C82]3- with an open-shell electronic character that is a consequence of transferring three electrons from lanthanum to C82(Scheme). The resulting electron spin imposes unique chemical reactivity, radical character, and magnetic properties. In this talk, I will present remarkable chemical reactivities and molecular properties of paramagnetic EMFs; La@C 2v -C82,[3] La@Cs -C82 [4] and Ce@C 2v -C82.[5] Their unique characteristics cannot be expected in ordinary chemical compounds and empty fullerenes, and are promising for creating novel electronic and magnetic materials with utilizing their unprecedented electronic properties.

Electrochemical Activation of the Least Reactive Endohedral Fullerene: Sc3N@I h-C80

The low reactivity of endohedral fullerenes compared with empty fullerenes makes their functionalization and further applications very challenging. To overcome their low reactivity, part of our research is focused on exploring alternative synthetic pathways for their functionalization. Some examples include the exohedral functionalization through photochemical reactions and electrosynthesis. Electrochemical methods in particular provide control over the state of oxidation/reduction of the endohedral compounds. Extensive research has been done with anionic empty fullerenes species but until recently, only some anionic endohedral fullerenes used as nucleophilic reagents have been explored. However, cationic radical species of endohedral compounds as electrophilic reagents for exohedral functionalizations have never been reported. Furthermore, regioselective and enantioselective addition reactions are also not well studied. Using electrochemical methods of endohedral fullerenes we can tune the HOMO-LUMO energy levels of the species in play to perform reactions that are unsuccessful by other methods. Herein, we report the first asymmetric 1,3-dipolar cycloaddition using the cation radical of Sc3N@I h-C80 through both electrochemical and chemical pathways using controlled potential electrolysis and chemical oxidation, respectively.

Isoxazoline-Ring Fused Derivatives of Sc3n@C80 and C60: The Internal Effect on the Exohedral Configuration

Fullerenes are unique among all carbon allotropes as their molecular structures can be fully confirmed by single-crystal X-ray diffraction. With metal atoms or metal complexes encapsulated, endohedral metallofullerenes (EMFs) redistribute electrons between the internal unit and the cage, and thus afford different electronic structures and different chemical properties compared to empty fullerenes. However, because of the relatively low yield of endohedral metallofullerenes, only a limited number of chemical modifications have been reported for them. In this work, an isoxazoline ring was added to Sc3N@C80 for the first time to form only one mono-adduct with high regio-selectivity (2b). An analogous product was obtained for empty cage C60 (2a). X-ray crystallography has revealed their structures, as shown in Fig. 1. The addition site of exohedral moieties of C60 was the [6,6]-bond while it is a [5,6]-bond for Sc3N@C80. Inside the Sc3N@C80, disorder of the Sc atoms represents the dynamic motion of the cluster, and the sum of the three Sc–N–Sc angles (342°) suggests that the trimetallic nitride cluster slightly derives from a coplanar structure, confirming the motion control effect of the exohedral modification. Moreover, it is very interesting to find that the configuration of exohedral moieties differed in 2a and 2b.In detail, the methoxy indicated by the arrow shown in Fig. 1 occurs at opposite sides of the benzene plane. These results reveal the significant internal effect on the exohedral configuration. Figure 1. Drawings of 2a and 2b showing thermal ellipsoids at the 30% probability level. Arrows indicate the special methoxy group that changes conformations when added to different fullerenes. Solvent molecules are omitted for clarity. Figure 1

ChemInform Abstract: Emergence of Highly Elaborated π‐Space and Extending Its Functionality Based on Nanocarbons: New Vistas in the Fullerene World

AbstractReview: [151 refs.

Structural relationships and theoretical study of the free energies of electron transfer, electrochemical properties, and electron transfer kinetic of cephalosporin antibiotics derivatives with fullerenes in nanostructure of [R]·C n (R = cefadroxil, cefepime, cephalexin, cefotaxime, cefoperazone and ceftriaxone) supramolecular complexes

Various empty carbon fullerenes (C n ) with different carbon atoms have been obtained and investigated. Cephalosporin antibiotics and its derivatives have important medicinal properties. The β-lactam class of antibiotics has a broad spectrum of antimicrobial properties. Their antibacterial and pharmacokinetic properties have wide therapeutic applications. Topological indices have been successfully used to construct effective and useful mathematical methods to establish clear relationships between structural data and the physical properties of these materials. In this study, the number of carbon atoms in fullerenes was used as an index to establish a relationship between the structures of cefadroxil, cefepime, cephalexin, cefotaxime, cefoperazone and ceftriaxone (β-lactam antibiotics) and fullerenes (C n , n = 60, 70, 76, 82 and 86), which create [cefadroxil]·C n , [cefepime]·C n , [cephalexin]·C n , [cefotaxime]·C n , [cefoperazone]·C n and [ceftriaxone]·C n . The relationship between the number of fullerene carbon atoms and the free energies of electron transfer (ΔG et(1)–ΔG et(4)) are assessed using the Rehm-Weller equation for A-1 to A-5, B1 to B-5, C-1 to C-5, D-1 to D-5, E-1 to E-5 and F-1 to F-5 of the supramolecular complexes [R]·C n (where R = cefadroxil, cefepime, cephalexin, cefotaxime, cefoperazone and ceftriaxone) complexes. The calculations are presented for the four reduction potentials (Red. E 1–Red. E 4 ) of fullerenes C n . The results were used to calculate the four free energies of electron transfer (ΔG et(1)–ΔG et(4)) of the cephalosporin-fullerene supramolecular complexes A-1 to A-5, B1 to B-5, C-1 to C-5, D-1 to D-5, E-1 to E-5 and F-1 to F-5 for fullerenes C60–C120. The free energies of activation for electron transfer, Δ $$ G_{{_{{{\text{et}}\left( n \right)}} }}^{{^{\# } }} $$ (n = 1–4) were also calculated for these complexes in accordance with the Marcus theory. In this study, was presented the calculated wavelengths (λ(n); n = 1–4; in nm) of the photoelectron transfer process as well in the nanostructure complexes.

In-depth understanding of the chemical properties of rarely explored carbide cluster metallofullerenes: a case study of Sc2 C2 @C3v (8)-C82 that reveals a general rule.

The chemical properties of carbide-cluster metallofullerenes (CCMFs) remain largely unexplored, although several new members of CCMFs have been discovered recently. Herein, we report the reaction between Sc2 C2 @C3v (8)-C82 , which is viewed as a prototypical CCMF because of its high abundance, and 3-triphenylmethyl-5-oxazolidinone (1) to afford the corresponding pyrrolidino derivative Sc2 C2 @C3v (8)-C82 (CH2 )2 NTrt (2; Trt=triphenylmethyl). Single-crystal X-ray crystallography studies of 2 revealed that the reaction takes place at a [6,6]-bond junction, which is directly over the encapsulated C2 unit and is far from either of the two scandium atoms. On the basis of theoretical calculations and by considering previously reports, we have found that a hexagonal carbon ring on the cage of Sc2 C2 @C3v (8)-C82 is highly reactive toward different reagents due to the overlap of high p-orbital axis vector (POAV) angles and large LUMO coefficients. We propose that this highly concentrated area of reactivity is generated by the encapsulation of the Sc2 C2 cluster because this region is absent from the empty fullerene C3v (8)-C82 . Moreover, the absorption and electrochemical results confirm that derivative 2 is more stable than pristine Sc2 C2 @C3v (8)-C82 , thus illuminating its potential applications.

ChemInform Abstract: Recent Progress in the Chemistry of Endohedral Metallofullerenes

AbstractReview: 81 refs.

Emergence of Highly Elaborated π-Space and Extending Its Functionality Based on Nanocarbons: New Vistas in the Fullerene World

Abstract How the electronic properties and chemical reactivity of empty fullerenes can be changed upon endohedral metal-doping? How the dynamic behavior and position of encapsulated metal atoms can be controlled? How its functionality can be extended by chemical treatments? We have addressed all aspects of endohedral metallofullerenes, including the synthesis, isolation, structure, chemical properties, and theoretical considerations and their applications in the related fields to answer these questions.

Thermal Reactions of Fullerenes with Cyclic Organosilicon and Organogermanium Compounds: Silylation and Germylation of C60 and C70

ABSTRACTThermal reactions of empty fullerenes with disilirane (1) and digermirane (2) afforded the corresponding bis‐silylated and bis‐germylated derivatives. When a bicyclic fused silirane (3a) was heated in the presence of C70, two silylene adducts were obtained through thermal extrusion of silylenes. These results provide a complementary method to the photochemical addition of reactive silicon and germanium compounds, which have been long employed for derivatization of fullerenes. Electrochemical analyses revealed the redox properties of the silylated and germylated C70, indicating the electronic perturbation caused by silyl and germyl groups introduced on the fullerene cages. Theoretical calculations were also conducted for understanding of the electronic property of the fullerene derivatives.

Recent progress in the chemistry of endohedral metallofullerenes.

Putting metal atoms or metallic clusters into fullerenes has generated a new class of hybrid molecules, defined as endohedral metallofullerenes (EMFs), possessing novel structures and fascinating properties which are different from those of empty fullerenes. In particular, it has been revealed that the chemical properties of the cage carbons of EMFs depend strongly on the nature of the internal metallic species, such as their electronic configuration, location and even motion. Since the first report describing the successful derivatization of La@C82 in 1995, great efforts have been devoted to the chemical modification of EMFs during the last two decades. Although earlier studies mainly focused on readily available species such as M@C82, M2@C80 and M3N@C80 and the related results have been systematically summarized in our previous review paper (Chem. Commun., 2011, 47, 5942-5957), recent concerns about some relatively rare EMFs have developed rapidly. Moreover, taking advantage of single crystal X-ray crystallography, we can now clearly demonstrate the mutual influences between the internal metallic species and the chemical behaviours of the surrounding cage carbons, and the addends as well. In this article, we present recent achievements in the chemical functionalization of EMFs, which were mainly published during the last four years. For consistency, we will still pay special attention to the role that the metals play in controlling the properties of the whole EMF molecules. In this review, however, we will not only focus on concrete experimental results such as X-ray crystallographic and NMR spectroscopic data but will also include computational studies which have indeed enhanced our understanding of the chemical properties of EMFs. Applicable materials based on EMFs are also mentioned but are not discussed in detail.

Endohedral Metallofullerenes – Filled Fullerene Derivatives Towards Multifunctional Reaction Center Mimics

In recent years, endohedral metallofullerenes have attracted tremendous interest not only in physics and chemistry, but also in interdisciplinary areas such as materials and biological sciences. In this concept article we highlight recent results on different endohedral metallofullerenes based on lanthanides and their derivatives. The chemical and excited state reactivities of endohedral metallofullerenes are discussed for various endohedral clusters. Most important is the part that covers spectroscopic and kinetic assays of reductive and oxidative charge transfer evolving from photoexcited electron donors and electron acceptors, respectively, in a variety of electron donor-acceptor conjugates. Towards the end, we refer to the applications of endohedral metallofullerenes in photovoltaic devices that feature greater efficiency than devices fabricated with empty fullerenes. Herein, we focus mainly on results obtained in the groups of Akasaka, Echegoyen, and Guldi.

Synthesis, extraction, and chromatographic purification of higher empty fullerenes and endohedral gadolinium metallofullerenes

Higher empty and endohedral gadolinium metallofullerenes were prepared by electric arc synthesis, isolated, and separated chromatographically to high degree of purity (99 wt %). The samples obtained were characterized by chromatography, optical spectroscopy, and mass spectrometry. According to small-angle neutron scattering data, the endohedral metallofullerenes in o-xylene form a molecular solution in which the molecules interact to form molecular pairs.

Vibrational spectra of lanthanide endofullerenes with different symmetries of molecular structure

Quantum-chemical calculations of the geometric structure and vibrational spectra of lanthanide endofullerenes have been carried out. The vibrational frequencies of lanthanide atoms depend substantially on the symmetry of the molecular structure of the endofullerene and on the distance between the metal atom and the carbon cage. The infrared spectra of the endofullerenes M@C60 contain vibrations that are forbidden by symmetry for the empty fullerene C60. A change in the vibrational spectra due to the encapsulation of a metal atom in fullerenes with a C60 cage is considerably more pronounced than that of the higher fullerenes. In the vibrational spectra, there are lines not characteristic of C60, which indicates the presence of M@C60 endofullerenes in a mixture with C60 fullerenes.

Rates and energetics of intramolecular electron transfer processes in conjugated metallofullerenes

In this paper, we report on the design, redox potentials, excited state energies and radical ion pair state energies in electron donor-acceptor conjugates comprising the electron-donating π-extended tetrathiafulvalene and several electron-accepting fullerenes. To this end, we contrast an empty fullerene, that is, C₆₀, with two endohedral metallofullerenes, that is, open-shell La@C₈₂ and closed-shell La₂@C₈₀, in terms of charge separation and charge recombination dynamics.