Quantum chemical calculations and NMR spectroscopic data suggest that the two isomers of Y@C82(CF3)5 prepared by trifluoromethylation of the endohedral metallofullerene (EMF) contain chains of four 1,4‐C6(CF3)2 edge‐sharing hexagons (see picture). In striking contrast to the empty fullerenes, which form complex mixtures with up to 22 CF3 groups, EMF Y@C82 only forms products with one, three, and five CF3 groups.

We have examined various parameters of the electric arc synthesis of endohedral metallofullerenes (EMFs or M@C2 n ). The influence of some experimental parameters (M/C ratio, helium vapor pressure, and values of the direct current arc) on yields of EMFs has been studied. The optimization of these parameters allows us to bring the EMF yield to a record level of 6−8% of the primary soot mass. Furthermore, a selective extraction sequence [o-xylene (five cycles) − dimethylformamide (seven cycles)] has been studied in detail in order to apply this technique to the estimation of the EMF yield. It has also been shown that polar organic solvents, among them dimethylformamide (DMF), dimethylacetamide (DMA), and dimethylsulfoxide (DMSO), are the most favorable ones for a selective extraction of EMF.

An efficient method for the synthesis of trifluoromethyl derivatives of endohedral gadolinium containing metallofullerenes was proposed. High purity (98-99%) trifluoromethyl derivatives Gd@C 82 (CF 3 ) 5 (two isomers) and Gd 2 @C 80 (CF 3 ) have been synthesized for the first time. They were isolated and characterized by HPLC, MALDI TOF mass spectrometry, and UV-Vis spectroscopy.Endohedral metallofullerenes (EMFs) are unique nanocarbon compounds containing one or several metal atoms inside the fullerene molecule. A wide list of ele ments from the D. I. Mendeleev Table, whose atoms are incorporated into a fullerene molecule, is presently known. 1,2 The EMFs containing atoms from the second (Ca, Sr, Ba) and third (Sc, Y, La) Groups, as well as lanthanides, are most actively studied. The most part of the EMFs have such an electronic structure that all or some valence electrons of the metal atom, which is situat ed inside the fullerene cage, transfer to the fullerene molecule. As a result, the EMF molecule gains pro perties that differ from those of "empty" fullerenes. The presence of additional weakly bound electrons at the external shell of the EMF molecules imparts proper ties of a strong reducing agent to these compounds and causes their high reactivity and a possibility to participate in various chemical reactions leading to the addition of diverse atoms, molecules, or radicals to the EMFs. Suc cessful functionalization of "empty" fullerenes and the synthesis of new materials 3 from these fullerenes sug gest that one should search for routes of practical use of the EMFs in the synthesis of their derivatives with higher solubility and unique electric, magnetic, optical, chemi cal, and biological properties.At present, the chemistry of the EMFs, unlike the properties of the well known exohedral modifica tions of "empty" fullerenes, is poorly studied. Such an important question as an effect of different metal atoms incapsulated into the fullerene shell on the reactivity of the EMFs remains unanswered. This situation is related to the restricted accessibility of the EMFs caused by difficulties of isolation of pure EMFs, which requires the labor consuming and expensive multistage separa tion using HPLC. 1,2,4 As a result, the chromatographi cally pure EMFs are available only in milligram amounts, which impedes the development of the EMF chemistry. Presently, the state of investigation in this area can be compared with that of the studies of fullerenes before the discovery of the efficient electric arc method for syn thesis of fullerenes C 60 , 5 which was proposed in 1990. Due to this method, fullerenes became accessible to a wide range of synthetic chemists, and this was the start of the vigorous development of the C 60 chemistry. Un like fullerenes, the problems of synthesis of the EMFs in preparative amounts has not been solved yet, explaining a small number of studies on EMF functionalization.The evolution of investigations in the field of the EMFs can be seen from several studies. The first EMF derivatives were ...

Graphical abstract

Ribbon pattern: An isomer of C84(CF3)12 fullerene with the C84‐C2(11) cage (see molecular structure determined by X‐ray crystallography) demonstrates a new principle of hollow higher fullerene reactivity: For 1,4‐additions of bulky groups that produce ribbons or loops of edge‐sharing p‐C6X2 hexagons, the “most reactive” double bonds remain intact.

Ribbon pattern: An isomer of C84(CF3)12 fullerene with the C84‐C2(11) cage (see molecular structure determined by X‐ray crystallography) demonstrates a new principle of hollow higher fullerene reactivity: For 1,4‐additions of bulky groups that produce ribbons or loops of edge‐sharing p‐C6X2 hexagons, the “most reactive” double bonds remain intact.

Quantenchemische Rechnungen und NMR‐Daten sprechen dafür, dass die beiden Y@C82(CF3)5‐Isomere, die durch Trifluormethylierung des endohedralen Metallofullerens (EMF) erhalten wurden, Ketten aus vier kantenverknüpften 1,4‐(CF3)2C6‐Sechsecken enthalten (siehe Bild). Während die leeren Fullerene komplexe Mischungen mit bis zu 22 CF3‐Gruppen bilden, entstehen aus dem EMF Y@C82 nur Produkte mit einer, drei und fünf CF3‐Gruppen.

Endohedral metallofullerenes Y 2 @C 84 , Ce 2 @C 78 , and M@C 82 (M = Y, Ce) were synthe sized by the electric arc method and isolated from the soot using extraction with o dichlo robenzene. Pure (98%) endohedral dimetallofullerenes Y 2 @C 84 and Ce 2 @C 78 were isolated for the first time from o dichlorobenzene extracts using HPLC and characterized by mass spec trometry and spectrophotometry.Unique structure of endohedral metallofullerenes (EMF) and diversity of their properties, depending on the nature of encapsulated metal and fullerene, excite great interest from the viewpoint of studying their chemical and physicochemical properties. It can be expected that suc cess in the area of synthesis and investigation of the physi cochemical properties of EMF would form a basis for practical use and creation of novel materials with unique electrical, magnetic, optical, chemical, and biological properties, for instance, organic ferromagnetics, laser and Seignette electric materials, and pharmaceutical and ra diopharmaceutical preparations. 1-4However, EMF are poorly studied up to present. Pub lished data on their chemical properties are virtually lack ing. The main reason for this situation is restricted acces sibility of EMF because of problems in their synthesis and isolation in preparative amounts.The known methods for the synthesis of EMF (laser and electric arc vaporization of composite graphite elec trodes) and traditional methods of their isolation from soot (extraction with toluene, carbon disulfide, o dichlo robenzene (DCB)) make it possible to obtain extracts in low yield (0.5-3%) and EMF content (0.1-1%). 1,5,6 Multistep HPLC is used to isolate pure EMF from the extracts. However, the chromatographic process is very labor consuming because of the low EMF content in the extracts. Pure EMF (96-99%) have been prepared so far only in milligram quantities and are almost inaccessible for a wide range of researchers. 1It should be mentioned that imperfect methods for EMF isolation prevent the optimization of conditions for their synthesis. As a rule, only final results are presented in the literature, while no conditions for the preparation of rare earth metal containing graphite electrodes are dis cussed and no analysis of the influence of the electric arc parameters and constructive features of electric arc setups on an increase in the yield of EMF in the soot is given. In this work, we synthesized for the first time and character ized endohedral dimetallofullerenes Y 2 @C 84 and Ce 2 @C 78 by the optimization of all steps of the EMF synthesis. ExperimentalThe process of synthesis of EMF with yttrium and cerium includes four steps, each of which plays a substantial role in the synthesis: (1) preparation of the composite graphite electrodes containing rare earth metals Y and Ce; (2) electric arc vaporiza tion of the composite graphite electrodes and preparation of the EMF containing soot; (3) isolation of fullerenes and EMF from the soot by extraction; (4) isolation of individual EMF by two step HPLC.The composite...