Study On Soluble Endohedral Metallofullerenes With Large Cages

Currently, carbon nanomaterials have been considered as the most promising materials due to the unique properties and applications in many fields. As the only zero-dimensional crystalline form of carbon allotrope with accurate molecular structures, the fullerene interior can host metal atoms or metal clusters. Such species with encapsulated metals, namely metallofullerenes, are particularly attractive because the metal atoms interact strongly with the carbon cage. From this perspective, on account of the existence of numerous isomers, the structures, properties and the interaction between metal and cage of metallofullerenes with large carbon cages are still a terra incognita and it no doubt represents a seemingly endless frontier as a new member of fullerene family. Consequently, the research in this dissertation mainly focused on the synthesis, extraction, separation, characterization and chemical modification of a family of lanthanum-containing endohedral fullerenes with large cages. The main results and new findings in this work are summarized as follows:1. A wide array of soluble dilanthanum endohedral fullerenes extends from La2Cg4to La2C106have been successfully synthesized by the direct current (dc) arc discharge method.29isomers were isolated for the first time, namely, La2@C94(five isomers), La2@C96(five isomers), La2@C98(five isomers), La2@C100(five isomers), La2@C102(four isomers) La2@C104(two isomers) and La2@C106(three isomers). The purity, cage symmetry and stability of all of the isomers were characterized by LDI-TOF mass spectrometry, HPLC and Vis/NIR spectroscopy. Based on the absorption characterizations, La2C94(I), La2C106(Ⅰ) and La2C100(V) are supposed to be La2C2@D3(85)-C92, La2C2@D3d(822)-C104and La2@D5(450)-C100, respectively. In addition, in the consideration of previously theoretical calculations, the other four La2C94isomers are supposed to be La2C2@C2(64)-C92, La2C2@C1(67)-C92, La2@C2(126)-C94and La2@C2(121)-C94, respectively. Moreover, the electrochemical results reveal that isomeric influence on the electrochemical properties of higher metallofullerenes is apparent. The endohedrals with cage larger than Cioo present smaller bandgaps around1.0eV.2. The three most abundant isomers, La2C2n (2n=102,104,106), have been structurally characterized as carbide cluster metallofullerenes La2C2@D5(450)-C100, La2C2@Cs(574)-C102and La2C2@C2(816)-C104by single-crystal X-ray diffraction. Being different from other midsized carbide cluster metallofullerenes, in which carbide clusters exhibit butterfly configurations, the La2C2clusters in these three larger cages tend to adopt slightly distorted linear and nearly planar structures. Both D5(45O)-C100and Cs(574)-C102exhibit ideal nanotubular shape, whereas C2(816)-C104is considered as a defective tubular structure due to the insertion of a pyracylene unit into the cylindrical segment of the cage. On the basis of the single-crystal X-ray diffraction (XRD) crystallographic results, we found that the fullerene cage shrinks unexpectedly when the large La2C2cluster is encapsulated due to the strong bonding interactions of the two lanthanum atoms with the C2-unit and the cage carbon atoms as well. Moreover, the defective tubular structure of C2(816)-C104can rearrange to the other three ideal tubular fullerene cages by elimination of the pyracylene motif. In the consideration of the similarities between the structural features and physical properties of nanotubular fullerenes and those of end-capped CNTs, the structural deformation behavior and formation mechanism of end-capped CNTs have been studied at the atomic level by single-crystal X-ray diffraction analysis for the first time. It is proposed that the axial strain mainly occurs in the side-wall segment of the corresponding finite-length capped zigzag (10,0) nanotube under the internal Coulombic force (1.2%) is about two to six times higher than those in the caps. Besides, the defective CNTs are supposed to be a parent structure of ideal carbon nanotubes in the arc discharge process.3. The1,3-dipolar reaction of a typical carbide cluster metallofullerene (Sc2C2@C3V(8)-C82) and two large-sized carbide cluster metallofullerenes (La2C2@D5(450)-C100and La2C2@Cs(574)-C102) with3-triphenylmethyl-5-oxazolidinone are studied for the first time, respectively. It is found that the reactivity of carbide cluster metallofullerenes and the number of substituted groups of their derivatives are obviously influenced by the cage size and structure. Multiple adducts formed in same conditions precluding thus the definitive structural characterization of the products, while the production of multi-adducts can’t be inhibited even by decreasing temperature. In addition, single-crystal X-ray crystallography studies of the corresponding pyrrolidino derivative Sc2C2@C3v(8)-C82(CH2)2NTrt (2; Trt=triphenylmethy1) revealed that the reaction takes place at a [6,6]-bond junction, which is directly over the encapsulated C2unit 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 Sc2C2@C3v(8)-C82is 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 Sc2C2 cluster because this region is absent from the empty fullerene C3v(8)-C82.