Uranium Based Endohedral Actinide Fullerenes:Studies On Molecular Structure And Actinide Chemical Bonds

Endohedral metallofullerenes(EMFs),due to its diversity of both the entrapped units and structure of carbon cage,unique electronic structure and special physical and chemical properties,has shown great potential on the applications in field of biomedical,organic photoelectric etc.To date,many rare-earth metals and neighboring elements(group II,III,IV)have been reported to form endohedral fullerene.And the molecular structures of endohedral fullerenes have extended from monometallic endohedral fullerenes to dimetallic endohedral fullerenes,trimetalic endohedral fullerenes and endohedral clusters fullerenes.The structures,properties and applications of the above-described endohedral fullerenes are mainly based on lanthanide metals and lanthanide-based clusters fullerenes,which have been systematically studied.In contrast,the study of actinide endohedral fullerenes has been limited to theoretical model calculations and a few spectroscopic charaterizations.A systematic study of experimental and theoretical study of these EMFs are yet to be established.Recently,on one hand,theoretical actinides chemistry studies have paid much attention to the nature of actinides metal-metal bonds and actinides metal-nonmetal multiple bonds.Fullerene carbon cage,as a special host,on the other hand,can stabilize the metal ions and embedded clusters,in which unqiue metal-metal bonds and metal-nonmetal bonds might be formed.Thus,external carbon cage may provide a unique chemical environment for the study of actinides metal-metal bonds and actinide metal-nonmetal multiple bonds.Therefore,the experimental research of endohedral actinide fullerenes is of great scientific significance in the perspectives of both fullerene and actinides chemistry.In this dissertation,we reported the synthesis,isolation and characterization of uranium based endohedral actinide fullerenes for the first time and carried out the following works:(1)We have reported the X-ray structures of two uranium mono-EMFs,U@C2(5)-C82 and U@C2v(9)-C82,and provided theoretical evidence for cage isomer dependent charge transfer states for U.Results from density functional theory(DFT)calculations combined with experiments show that U@C2(5)-C82 has a tetravalent electronic configurations corresponding to U4+@C2(5)-C824-.The isomeric U@C2v(9)-C82,however,has a trivalent electronic configuration corresponding to U3+@C2v(9)-C823-.These are the first X-Ray crystallographic structures of uranium EMFs and this it is also the first observation of metal oxidation state dependence on carbon cage isomerism for mono-EMFs.(2)We have reported the synthesis and isolation of a dimetallic actinide EMF,U2@C80.This compound was fully characterized by mass spectrometry,single crystal X-ray crystallography,UV-vis-NIR,cyclic voltammetry and X-ray absorption spectroscopy(XAS).The single crystal X-ray crystallographic analysis unambiguously assigned the molecular structure to U2@Ih(7)-C80.In particular,the crystallographic data revealed that the U-U distance is within the range of 3.46-3.79 ?,which is shorter than the 3.9 ? previously predicted for an elongated weak U-U bond inside the C80 cage.The XAS results reveal that the formal charge of the U atoms trapped inside the fullerene cage is +3,which agrees with the computational and crystallographic studies that assign a hexaanionic carbon cage,(IhC80)6-.Theoretical studies confirm the presence of a U-U bonding interaction and suggest that the weak U-U bond in U2@Ih(7)-C80 is strengthened upon reduction and weakened upon oxidation.The comprehensive characterization of U2@Ih(7)-C80 and the overall agreement between the experimental data and theoretical investigations provide experimental proof and deeper understanding for actinide metal-metal bonding interaction inside a fullerene cage.(3)We have reported that,utilizing Ih(7)-C80 fullerenes as nanocontainers,an unprecedented diuranium carbide cluster,UV=C=UV,has been encapsulated and stabilized in the form of UCU@Ih(7)-C80.The UCU@Ih(7)-C80 endohedral fullerene was prepared utilizing the Kr?tschmer-Huffman arc discharge in He atmosphere using U3O8 as the metal source and was then co-crystallized with nickel(II)octaethylporphyrin(NiII-OEP)to produce UCU@Ih(7)-C80·[NiII-OEP] as single crystals.X-ray diffraction analysis reveals a cage- stabilized,carbide-bridged,bent UCU cluster with unexpectedly short uranium-carbon distances(2.03 ?)indicative of standard covalent U=C double-bond character.The structural features of UCU@Ih(7)-C80 and the covalent nature of the U(f1)=C double bonds were further affirmed through various spectroscopic and theoretical analysis.