Solubility of fullerenes in n-alkanoic acids C2–C9

The solubility of fullerene C₆₀ and a fullerene mixture [C₆₀ (75%), C₇₀ (24%), C_76–80 (1%)] in linear alkanoic acids (C₂–C₉) was determined at 20°C. The solubilities of C₆₀ and a fullerene mixture in carboxylic acids were examined in relation to the number of carbon atoms in the carboxylic acid.     

Electrochemical formation and properties of two-component films of transition metal complexes and C60 or C70

The electrochemically active polymers have been formed during electro-reduction carried out in solution containing fullerenes, C₆₀ or C₇₀, and transition metal complexes of Pd(II), Pt(II), Rh(III), and Ir(I). In these films, fullerene moieties are covalently bounded to transition metal atoms (Pd and Pt) or their complexes (Rh and Ir) to form a polymeric network. All films exhibit electrochemical activity at negative potentials due to the fullerene cages reduction process. For all studied metal complexes, yields of formation of films containing C₇₀ are higher than yields of electrodeposition of their C₆₀ analogs. C₇₀ /M films also exhibit higher porosity in comparison to C₆₀/M layers. The differences in film morphology and efficiency of polymer formation are responsible for differences in electrochemical responses of these films in acetonitrile containing supporting electrolyte only. C₇₀/M films shows more reversible voltammeric behavior in negative potential range. They also show higher potential range of electrochemical stability. Processes of film formation and electrochemical properties of polymers depend on the transition metal ions or atoms bonding fullerene cages into polymeric network. The highest efficiency of polymerization was observed for fullerene/Pd and fullerene/Rh films. In the case of fullerene/Pd films, the charge transfer processes related to the fullerene moieties reduction in negative potential range exhibit the best reversibility among all of the studied systems. Capacitance performances of C₆₀/Pd and C₇₀/Pd films deposited on the porous Au/quartz electrode were also compared. Capacitance properties of both films are significantly affected by the conditions of electropolymerization. Only a fraction of the film having a direct contact with solution contributes to pseudocapacitance. Capacitance properties of these films also depend on the size of cations of supporting electrolyte. The C₇₀/Pd film exhibits much better capacitance performance comparison to C₆₀/Pd polymer.     

Observation of radioactive hetero-fullerenes using radiochemical techniques

Fullerenes, C₆₀ and C₇₀, were irradiated by 8 and 10 MeV deuterons. The irradiated samples were dissolved in CS₂ and filtered to remove insoluble by-products. Finally, radioactive fullerenes and products, such as fullerene dimers, trimers labeled with¹³N, were isolated and detected in the liquid phase by radiochromatography. This suggests that the energy rich¹³N atoms successfully incorporated into the fullerene cages and that the disrupted cages reacted with neighboring cages.     

Solution ozonolysis of C<Subscript>70</Subscript>: stable products and chemiluminescence

For the first time the total and relative contents of the stable ozonolysis products of fullerene C₇₀ solutions were identified by IR spectroscopy and elemental and chemical analyses. At the 100% conversion of C₇₀ a mixture of products corresponding to the empirical formula C₇₀O_14.3H_0.21 (epoxides: polyketones: polyesters: secondary fullerene ozonides (SFOZ): acids = 1.07: 6: 6: 0.21: 1.02) is formed. The content of polyketones, polyesters, acids, and SFOZ increases during the whole ozonolysis time (1 h). The number of oxygen atoms in epoxides C₇₀O _n (n = 1–4) is lower than that in epoxides C₆₀O _n (n = 1–6) formed by the ozonolysis of fullerene C₆₀. The kinetic curve of accumulation of epoxides C₇₀O _n (n = 1–4) passes through a maximum, which is observed 0.5 min after the beginning of ozonolysis. No epoxides were identified among the products 3.5 min after the ozonolysis. The photoluminescence (PL) (λ_max = 645 and 685 nm) of fullerene polyketones in glassy EtO₂/EtOH solutions frozen at 77 K was observed. This PL is much brighter, than that of polyketones formed upon the ozonolysis of fullerenes C₆₀. For the first time the chemiluminescence (CL) was detected and studied upon the ozonolysis of C₇₀ solutions at 300 K. The CL emitters are excited states of fullerene polyketones. The CL spectrum is partially overlapped with the known CL spectrum appeared upon the ozonolysis of C₆₀ (λ_max = 685 nm) but contains the greater number of maxima (λ_max = 645 and 685 nm), which is related to a lower symmetry of the C₇₀ oxidation products.     

Theoretical Investigation of Mono and Multiply Oxygenated C70 Fullerenes

We have applied DFT calculations to investigate atomic arrangements of fullerene oxides, C₇₀O _n with n = 1, 5, 15, 20, and 25. The oxidation reaction energies are generally exothermic. In the case of C₇₀O, the most stable configuration is the one in which the oxygen atom is chemisorbed on the C–C bond at the equatorial belt of fullerene with oxidoannulene like structure. In highly oxygenated fullerenes, the addition of oxygen atoms on the [6, 6] bonds near the pole area of the C₇₀ leads to lower values of reaction energies. Among these, C₇₀O₂₀ with three oxygen atoms adsorbed on a benzenoid ring yields the most energetically favorable configuration. Different positions of the oxygen atoms on the surface of the mono oxygenated fullerenes lead to significant differences in their ¹⁷O NQR parameters. Meanwhile, the ¹⁷O NQR parameters of the highly oxygenated fullerenes divide their oxygen nuclei into a few groups. For example, the electric field gradient tensor for oxygen atoms chemisorbed on the [6, 5] bonds with larger η values becomes considerably asymmetric in comparison to those chemisorbed on the [6, 6] bonds. These are also reflected in the calculated natural charges of oxygen atoms.     

A theoretical investigation on the structures and stabilities of C<Subscript>60</Subscript>X<Subscript>18</Subscript> and C<Subscript>70</Subscript>X<Subscript>10</Subscript> (X = H,F, Cl,and Br)

A density functional theory study was performed on fullerene derivatives C₆₀X₁₈ and C₇₀X₁₀ (X = H, F, Cl, and Br). The calculated results show that the lowest energy isomers are IPR-satisfying for C₆₀X₁₈ (X = H, F, Cl, and Br). It is found that the addition patterns of X (X = Cl and Br) are different from those of X (X = H and F) for C₆₀, demonstrating that the stability of fullerene derivatives is partly attributed to the steric repulsion and electronegativity of added atoms. However, the lowest energy isomers are IPR-violating for C₇₀X₁₀ (X = H, F, and Cl), suggesting that many more fullerene derivatives may violate the isolated pentagon rule.     

Transition metal coated fullerenes

Compound clusters of fullerene molecules and transition metal atoms having the composition C₆₀M_x and C₇₀M_x with x = 0..150 and M ∈ {Ti, Zr, V, Y, Ta, Nb} were produced using laser vaporisation in a low-pressure inert gas aggregation cell. Intensity anomalies in the mass spectra correlate with the atomic radii of the different metals indicating the formation of complete metal layers around the central fullerene molecule. Using high laser intensities the metal-fullerene clusters can be transformed into metcars and metal-carbides. Photofragmentation spectra of preselected C₆₀Ta_x indicate that the fullerene cage is destroyed for x ≥ 3.     

Methyl methacrylate polymerization in the presence of C60 (C70) and molecular characteristics of fullerene-containing poly(methyl methacrylate)

Kinetics was studied of bulk polymerization of methyl methacrylate initiated by dicyclohexyl peroxydicarbonate in the presence of tri-n-butylborane and fullerene C₆₀ (or C₇₀) at variable ratio initiator: fullerene. The deceleration of the polymerization in the first stage of the reaction (below 10% conversion) was established by dilatometric method that depended on the fullerene concentration and the mode of its addition to the monomer. It was shown that at similar ratios initiator: fullerene the C₆₀ inhibited the polymerization process considerably stronger than C₇₀. The gel-permeation chromatography revealed the widening of the molecular weight distribution of the poly(methyl methacrylates) containing C₆₀ or C₇₀ compared to its analog synthesized under the same conditions without fullerene. It was established that in the fullerene-containing poly(methyl methacrylates) all the framework nanospecies are linked by covalent bonds and are mostly accumulated in the low-molecular fractions. The effect of the covalently bound fullerene on the molecular characteristics of polymers were investigated by translational isothermal diffusion, high-speed sedimentation, and viscometry     

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.     

Reductive Benzylation of Dimetallo Hexaaryl[70]Fullerenes on the Equatorial Region

Dianions of dimetallic hexa(organo)[70]fullerene [(C₅R₅)₂Ru₂C₇₀Ar₆]^2? (R=H, Me; Ar=Ph, 4‐MeC₆H₄, 4‐tBuC₆H₄) react with benzylic bromide to yield the dibenzylated product dimetallic octa(organo)[70]fullerene (C₅R₅)₂Ru₂C₇₀Ar₆(CH₂Ar′)₂ (Ar′=Ph, 4‐MeO₂CC₆H₄), where the benzylic groups are attached to the equatorial belt region of [70]fullerene; this region is generally considered to be rather unreactive. This unusual structure was unambiguously determined by X‐ray crystallography. Theoretical studies on the electronic properties of the monoanionic intermediate indicated that the highest spin density resides on the two carbon atoms in the belt region; one of them then couples with a benzylic radical to yield the octa(organo)fullerene product after ionic substitution of the fullerene anion with a benzylic bromide. Electrochemical analysis of the hexa(organo) and octa(organo) ruthenium complexes suggests that the modification of the belt region does not affect the electronic communication between the two metal centers.     

Distribution of C60 and C70 fullerenes in the extraction system (C60 + C70)-α-pinene-ethanol-H2O

The distribution of C₆₀ and C₇₀ fullerenes in the extraction system (C₆₀ + C₇₀)-α-pinene-ethanol-H₂O was studied at constant C₆₀ to C₇₀ ratio and variable total fullerene concentration at 25°C. The relationship between the C₆₀ and C₇₀ content in ethanol (I) and α-pinene (II) phases is nonlinear over the entire fullerene concentration range.     

Effects of Precursors and Plasma Parameters on Fullerene Synthesis in RF Thermal Plasma Reactor

Synthesis of fullerenes from graphite powders of different grade was studied in a radiofrequency (RF) plasma reactor. Dependence of fullerene yield on the properties and feed rate of precursors and on the helium content of plasma gas was studied in details. The fullerene yield was influenced by the mean size and the thermal conductivity of graphite particles on the one hand, and the helium content of the gas phase on the other. Soot containing fullerene mixture of 5.9% was produced in best conditions found in this work. The main component of the fullerene mixture was C₆₀. In addition, it contained about 30% of C₇₀ (corresponding to a C₆₀/C₇₀ mass ratio of 2.64). Higher fullerenes such as C₈₄ were also detected by mass spectroscopy (MS) and high performance liquid chromatography (HPLC).     

New method for synthesis of fullerenes and fullerene hydrides from benzene

New and simple method for synthesis of C₆₀ fullerene and fullerene (C₆₀, C₇₀, and C₈₂) hydrides was developed. The substances synthesized were characterized by laser mass spectrometry, IR spectroscopy, temperature-programmed desorption mass spectrometry, and X-ray phase and chemical analyses.     

“Chemical portrait” of fullerene molecules

Chemical activity of fullerene molecules is associated with a partially radical character of these molecules caused by the presence of effectively unpaired electrons (EUE). Values of the total number of N _D and partial distribution N _DA of EUE over the atoms in a singlet state of molecules C₆₀ and C₇₀ have been calculated. “Chemical portraits” of molecules are presented, and N _DA values are proposed to be used as indicators for chemical activity of atoms and to predict thereby the position of favorable atom-atom contacts in addition reactions with participation of these molecules. Bases of “computational synthesis” procedures for fullerene C₆₀ derivatives are exemplified by initial phases of its fluorination.     

Production of endohedral 133Xe-fullerene by ion implantation

Ion implantation was applied to the production of endohedral ¹³³Xe-fullerene. Using an isotope separator, ¹³³Xe ions were implanted into a fullerene target of C₆₀ and C₇₀ produced by vacuum evaporation on a Ni backing. An HPLC analysis following dissolution of the fullerene targets in o-dichlorobenzene corroborated the formation of ¹³³Xe@C₆₀ and ¹³³Xe@C₇₀, showing a strong correlation between C₆₀/C₇₀ and ¹³³Xe. The observed tailing following ¹³³Xe peaks in the elution curves suggests a possibility of the isolation of endohedral ¹³³Xe-fullerene from empty fullerene.     

Reductive Hydrogenation of Cs‐C70(CF3)8 and C1‐C70(CF3)10

CF₃‐derivatized fullerenes prove once again to be promising scaffolds for regioselective fullerene functionalization: now with the smallest possible addends—hydrogen atoms. Hydrogenation of C_s‐C₇₀(CF₃)₈ and C₁‐C₇₀(CF₃)₁₀ by means of reduction with Zn/Cu couple in the presence of water proceeds regioselectively, yielding only one major isomer of C₇₀(CF₃)₈H₂ and only two for C₇₀(CF₃)₁₀H₂, whose addition patterns are combined in the only abundant isomer of C₇₀(CF₃)₁₀H₄. The observed selectivity is governed by the electronic structure of trifluoromethylated substrates. Interestingly, we discovered that Clar's theory can be utilized to predict the regiochemistry of functionalization, and we look forward to testing it on forthcoming cases of derivatization of pre‐functionalized fullerene building blocks.     

Pressure effect on heterogeneous trifluoromethylation of fullerenes and its application

The first systematic study of heterogeneous fullerene trifluoromethylation using an innovative gradient-temperature gas-solid reactor revealed a significant effect of CF₃I pressure on the conversion of C₆₀ and C₇₀ into trifluoromethylated products and on the range of fullerene(CF₃)_n compositions that were obtained. The design of the reactor allowed us to lower the residence times of fullerene(CF₃)_n species in the hot zone which resulted in the significant differences in relative isomeric distributions as compared to the earlier methods. For the first time, gram quantities of trifluoromethylated fullerenes were prepared using the new reactor, and the selective synthesis of a single-isomer C₆₀(CF₃)₂ was developed. The relative reactivity of C₇₀ as a CF₃ radical scavenger was found to be much lower than that of C₆₀, especially at an early radical addition stage, which led to the cost-efficient synthesis of C₆₀(CF₃)₂ from a fullerene extract.     

[6,6]‐Open and [6,6]‐Closed Isomers of C70(CF2): Synthesis,Electrochemical and Quantum Chemical Investigation

Novel difluoromethylenated [70]fullerene derivatives, C₇₀(CF₂)_n (n=1–3), were obtained by the reaction of C₇₀ with sodium difluorochloroacetate. Two major products, isomeric C₇₀(CF₂) mono‐adducts with [6,6]‐open and [6,6]‐closed configurations, were isolated and their homofullerene and methanofullerene structures were reliably determined by a variety of methods that included X‐ray analysis and high‐level spectroscopic techniques. The [6,6]‐open isomer of C₇₀(CF₂) constitutes the first homofullerene example of a non‐hetero [70]fullerene derivative in which functionalisation involves the most reactive bond in the polar region of the cage. Voltammetric estimation of the electron affinity of the C₇₀(CF₂) isomers showed that it is substantially higher for the [6,6]‐open isomer (the 70‐electron π‐conjugated system is retained) than the [6,6]‐closed form, the latter being similar to the electron affinity of pristine C₇₀. In situ ESR spectroelectrochemical investigation of the C₇₀(CF₂) radical anions and DFT calculations of the hyperfine coupling constants provide evidence for the first example of an inter‐conversion between the [6,6]‐closed and [6,6]‐open forms of a cage‐modified fullerene driven by an electrochemical one‐electron transfer. Thus, [6,6]‐closed C₇₀(CF₂) constitutes an interesting example of a redox‐switchable fullerene derivative.     

The X-Ray Crystal Structures of a 1,9- and a 7,8-Diels-Alder monoadduct of C70

Accurate low-temperature X-ray crystal structures of the isomeric Diels-Alder monoadducts of C₇₀ 1a , bridged at the [6,6] bond between C(1) and C(9), and 1b , bridged at the [6,6] bond between C(7) and C(8), have been determined. The latter structure is the first one solved for a 7,8-C₇₀ monoadduct. The C₁-symmetrical 1a and C_s-symmetrical 1b co-crystallize with CS₂ molecules in the space groups P2₁/c and Pnma, respectively, and the distances between bridgehead atoms are 1.603(3) and 1.584(3) Å. The degree of pyramidalization of atoms involved in [6,6] bonds near the ‘unsubstituted’ pole and near the equator of the fullerene was estimated; it is shown for the unsubstituted pole region that atoms corresponding to C(1) and C(9) are slightly more pyramidal than those corresponding to C(7) and C(8), in agreement with ab initio calculations obtained for the C₇₀ spheroid. Some aspects of the crystal packings are discussed.     

Preparation and study of hydrides of fullerenes C60 and C70

Fullerene hydrides were prepared by hydrogenation of fullerences C₆₀ and C₇₀ using proton transfer from 9,10-dihydroanthracene to fullerene and were studied by mass spectrometry (electron impact, field desorption), IR, UV, and¹H and¹³C NMR spectroscopy. The main product of the hydrogenation of C₆₀ is C₆₀H₃₆, which is sufficiently stable. Hydrogenation of fullerene C₇₀ gives a series of polyhydrides C₇₀H _n (n=36–46), and the main product is C₇₀H₃₆. The dehydrogenation of C₆₀H₃₆ by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone is not quantitative and results in the formation of fullerene derivatives along with C₆₀. The comparison of the IR and¹H and¹³C NMR spectral data for solid C₆₀H₃₆ with the theoretical calculations suggests that the fullerene hydride has aT-symmetric structure and contains four isolated benzenoid rings located at tetrahedral positions on the surface of the closed skeleton of the molecule. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya. No. 4, pp. 671–678, April, 1997.