V. P. Bubnov

Endohedral metallofullerenes M@C82 (M=La, Y): synthesis and transport properties

Endohedral metallofullerenes M@C2n (M=La, Y) were synthesized by the arc-discharge method using optimum electric arc parameters. It was also shown that an organic solvent (N,N-dimethylformamide) is successfully used in selective extraction of M@C2n. We identified the resulting products by mass spectrometry, electron spectrophotometry, EPR and X-ray fluorescence spectroscopy. The resulting N,N-dimethylformamide extracts of La@C2n and Y@C2n are shown to be free of empty fullerenes and appear as a mixture of endometallofullerenes M@C2n whose main ingredient is M@C82 (∼80 wt%). Transport properties of the extracts of La@C82 and Y@C82 were studied using pressed pellets. It was found that oxygen and heat treatment affect conductivity of endometallofullerenes. Heat treatment results in a three-order increase of conductivity from 10−5 upto 10−2 Ohm−1 cm−1

Spin-transfer complexes of endohedral metallofullerenes: ENDOR and NMR evidences

The aim of this research was to answer the question whether the area of localization of unpaired electron in a paramagnetic endohedral metallofullerene is restricted by the fullerene shell or a “spin-leakage” beyond the fullerene cage is possible. Herein, we report an ENDOR investigation of La@C82 embedded into the polycarbonate films. The intensive 1H-ENDOR signal has been revealed. Since the La-EMF does not contain hydrogen atoms, this result testifies to the contact hyperfine interaction of the unpaired electron of La-EMF with the matrix hydrogen atoms, i.e. electron spin density on the polymer protons. We also report a NMR investigation of a liquid solution of the same La-EMF in hexamethylphosphoramide (HMPA), molecules of which contain the NMR active nucleus, phosphorus-31. The paramagnetic shift of the 31P NMR signal of bulk HMPA molecules in the presence of La-EMF has been revealed. Thus, much as the charge-transfer complexes, the paramagnetic EMF molecules can form the spin-transfer complexes in which the electron spin density partially localizes beyond the fullerene cage on atoms of the matrix in which the EMF molecules are embedded.

EPR Study of Reactivity of Endohedral Metallofullerenes La@C82 and Y@C82

Abstract Reactivity of La@C82 in the mixture with C60 and C70 vs L2pto, phosphoryl radical ∗P(O)(OR)2, and CF3COOH has been studied using EPR spectroscopy. Whereas two former reagents did not change EPR spectrum, treatment with CF3COOH in toluene caused the disappearance of minor octet and a new signal was observed. Similar behaviour was found upon treatment of Y@C82 with CF3COOH. It is interpreted as selective protonation of one isomer of endohedral metallofullerene.

Harnessing Electron Transfer from the Perchlorotriphenylmethide Anion to Y@C82(C2v) to Engineer an Endometallofullerene‐Based Salt

We show that electron transfer from the perchlorotriphenylmethide anion (PTM(-)) to Y@C82(C2v) is an instantaneous process, suggesting potential applications for using PTM(-) to perform redox titrations of numerous endohedral metallofullerenes. The first representative of a Y@C82-based salt containing the complex cation was prepared by treating Y@C82(C2v) with the [K(+)([18]crown-6)]PTM(-) salt. The synthesis developed involves the use of the [K(+)([18]crown-6)]PTM(-) salt as a provider of both a complex cation and an electron-donating anion that is able to reduce Y@C82 C2v). For the first time, the molar absorption coefficients for neutral and anionic forms of the pure isomer of Y@C82(C2v) were determined in organic solvents with significantly different polarities.

Novel proficient method for isolation of endometallofullerenes from fullerene-containing soots by two-stepo-xylene–N, N-dimethylformamide extraction

We have developed a novel proficient method of extraction of endohedral metallofullerenes from fullerene-containing soots. The method consists of two extraction steps. In the first step,o-xylene is used to extract empty fullerenes from the soot. In the second step, the same soot is extracted by polar solvent N,N-dimethylformamide. This method has been applied for isolation of endohedral La- and Y-fullerenes from the soots obtained in the electric-arc reactor. The resultant DMF extracts contain virtually no C60 , C70 or other empty fullerenes. The yield of the DMF extracts reaches 1% of the primary soot mass. The simple extraction method developed here enables production of large amounts of concentrated M@C2n.

EPR spectrum of the Y@C82 metallofullerene isolated in solid argon matrix: hyperfine structure from EPR spectroscopy and relativistic DFT calculations

The EPR spectrum of the Y@C(82) molecules isolated in solid argon matrix was recorded for the first time at a temperature of 5 K. The isotropic hyperfine coupling constant (hfcc) A(iso) = 0.12 +/- 0.02 mT on the nucleus (89)Y as derived from the EPR spectrum is found in more than two times greater than that obtained in previous EPR measurements in liquid solutions. Comparison of the measured hfcc on a metal atom with that predicted by density-functional theory calculations (PBE/L22) indicate that relativistic method provides good agreement between experiment in solid argon and theory. Analysis of the DFT calculated dipole-dipole hf-interaction tensor and electron spin distribution in the endometallofullerenes with encaged group 3 metal atoms Sc, Y and La has been performed. It shows that spin density on the scandium atom represents the Sc d(yz) orbital lying in the symmetry plane of the C(2v) fullerene isomer and interacting with two carbon atoms located in the para-position on the fullerene hexagon. In contrast, the configuration of electron spin density on the heavier atoms, Y and La, is associated with the hybridized orbital formed by interaction of the metal d(yz) and p(y) electronic orbitals.

Production of carbon soot with a high content of C60 and C70 fullerenes by electric arc

The effect of the parameters of the electric arc (helium pressure, current and voltage, clearance between electrodes) and of the peculiarities of the setup design (arrangement of electrodes, distance between the arc and the cooling surface, temperature of the soot condensation surface) on the yield of fullerenes has been studied. Conditions for producing soot with a C60 and C70 content up to 43% (toluene extract) have been found.

Electric-arc synthesis of soot with a high content of higher fullerenes

The influence of potassium carbonate additions from 1 to 12 wt % and the helium pressure on the content of higher fullerenes C76, C78, C80, C82, C84, C86, etc., in soot is studied. The optimum conditions of the evaporation of composite graphite electrodes are found: they provide the formation of soot with a high content of higher fullerenes (up to 11.4 wt %) in an extract.

Endohedral fullerene Gd2C2@C82 as a strongly correlated electron system

Isomerically pure endohedral fullerene with a gadolinium carbide cluster Gd2C2@C82(Cs) has been synthesized by the electric-arc method, extracted from soot by o-dichlorobenzene, separated chromatographically, and characterized by mass spectroscopic and spectrophotometric methods. The energy and optical absorption spectra of the synthesized compound have been calculated for the first time within the Hubbard model. Good agreement of the experimental and calculated optical absorption curves has been observed.

Stability and mobility of the endohedral metallofullerene of La@C82 in polycarbonate polymer films

An La@C82-doped polymer (bisphenol-A polycarbonate) was prepared. EPR spectra of La@C82 in a solid polymer film and in a polycarbonate solution in o-dichlorobenzene were studied. The EPR spectrum shape of La@C82 was shown to be sensitive to the phase state of the polymer. La@C82 in the film was found to have a high chemical stability below the glass formation temperature, thus permitting one to use the polymer for the storage and study of endohedral metallofullerenes.