Synthesis,Molecular Structure And Bonding Motif Study Of Actinide Endohedral Metallofullerenes

Fullerene cage,which act as a special molecular nanocontainer,is capable of entrapping a diversity of metal atoms or clusters with different structures,leading to the formation of endohedral metallofullerenes(EMFs).The distinctive electronic and physicochemical properties of EMFs,derived from the unique interaction between the embedded species and the carbon cage,shed light on great potentials for the applications of EMFs in various fields such as biomedicine,organic photovoltaic devices,thermoelectric materials and molecular devices.Actinide endohedral metallofullerenes represent a novel type of fullerenes which encapsulate actinide atoms and clusters.On one hand,the study of actinide endohedral metallofullerenes enriches conventional fullerene research fields and has significant importance on probing the structural features and formation mechanism of EMFs.On the other hand,understanding the properties of actinide clusters and bonding motifs is significant for the understanding of the electronic structure and reactivity of 5f elements.Over the years,the possible actinide metal-metal and metal-nonmetal ligands bonds in actinide EMFs have been proposed and discussed by theoreticians.Previous studies have also shown that the inner cavity of fullerene cage can provide a unique environment to stabilize unexpected metallic clusters which are not accessible by conventional synthetic methods.Thus,actinide EMFs could be an ideal candidate for studying the structure and properties of actinide bonding motifs.In this context,the comprehensive characterization and in-depth theoretical analysis of novel actinide EMFs have great significance for fundamental actinide chemistry as well as for fullerene research.Herein,in this thesis,we report the synthesis and characterization of novel actinide metallofullerenes as well as the molecular and electronic structure and actinide bonding properties.The contents of this thesis include the following three aspects(1)We employed modified DC arc discharge method for the synthesis of three novel non-isolated-pentagon-rule(IPR)actinide EMFs and single crystal X-ray crystallography unambiguously assigned the molecular structures as U@C1(17418)-C76,U@C1(28324)-C80,and Th@C1(28324)-C80.Theoretical calculations show that the stabilization of these unique non-IPR fullerenes originates from the four-electron transfer and the resulting strong host-guest interactions between the actinide ions and the fullerene cages.This work demonstrates that the metal-cage interactions in actinide EMFs show remarkable differences from those previously known for their lanthanide analogues and provides new insights and deeper understanding into the formation and stabilization mechanism of endohedral metallofullerenes.(2)We have reported the successful synthesis of two novel diuranium carbide cluster EMFs,U2C2@Ih(7)-C80 and U2C2@D3h(5)-C78,both of which were characterized by single crystal X-ray crystallography,nuclear magnetic resonance spectroscopy(NMR),X-ray absorption spectroscopy(XAS),UV-vis-NIR as well as quantum-chemical calculations.The experimental and theoretical analyses confirm that U takes a different oxidation state in the novel uranium clusters,U4+,than in the previously reported U2C@Ih(7)-C80(U5+).U2C2@Ih(7)-C80 and U2C2@D3h(5)-C78 are the first examples of structurally characterized diuranium(?)carbides,forming a butterfly shape in which the two bridged C atoms are linked by a C?C triple bond.The U-C bonds in the U2C2@Ih(7)-C80 and U2C2@D3h(5)-C78 are predominantly ionic,while U2C inside Ih(7)-C80 contains two covalent U=C double bonds.In addition,different from the inherently bent structure of U=C=U,the observed UC2U"butterfly"structure is dependent on the steric hindrance imposed by the fullerene cages.This work indicates that,due to the variable oxidation states of actinide elements,the electronic structure and the bonding motif of endohedral actinide cluster fullerenes can be substantially different from those of their lanthanide analogues.This study also shows that fullerene cages can act as effective nanocontainers to stabilize and study unexpected actinide clusters.(3)We have synthesized and characterized a novel dimetallic endohedral fullerene,Th2@Ih(7)-C80.The single crystal structure clearly shows that the two Th atoms are separated by 3.816 A,which is in great agreement with the calculated value 3.817 A.In particular,quantum chemistry results proves that Th2@Ih(7)-C80 contains an unprecedented strong covalent thorium-thorium bond between two rarely accessible Th3+ions,giving the effective bond order(EBO)of 0.99.The theoretical calculation also suggests that the overlap between 7s/6d hybrid thorium orbitals is so large that the bond still exists at Th-Th separations larger than 6 A.This study demonstrates the authenticity of covalent actinide metal-metal bonds in a stable compound and deepens our understanding of f element metal bonds,paving the way for future experimental studies of these fundamentally important yet far from fully understood bonding motifs.