Synthesis,Isolation And Characterization Of Novel Uranium-Based Endohedral Cluster Fullerenes

Endohedral metallofullerenes(EMFs)are unique molecules in which metal or mixed metal-nonmetal clusters were encapsulated inside the empty fullerene cages.Since the first EMF La@C82 was reported in 1985,the EMF family has expanded rapidly,and many kinds of different EMFs have been reported,including various single metallic EMFs,dimetallic and even mixed metal-nonmetal cluster fullerenes,etc.To date,researches on EMFs have been mainly focused on some metals of group ?,? and ? such as Sc,Ti,V,Y and most lanthanide metals.The knowledge about the structures and properties of these fullerenes have been systematically summarized.In contrast,the research on actinide-based EMFs has long been limited to computational prediction and preliminary mass signals and spectroscopic observations.Thus,this work is devoted to the synthesis,separation and characterization of novel uranium-based cluster fullerenes,which is briefly introduced as follows:(1)The metal oxides,U3O8 and Sc2O3,were adopted as the metal source and the novel actinide-lanthanide mixed metallic EMF,Sc2UC@C80,was successfully synthesized by the Kratschmer-Huffman arc discharge method.This compound is fully characterized by single crystal X-ray diffraction analysis,UV-vis-NIR spectroscopy,Raman spectroscopy,IR spectroscopy and cyclic voltammetry.Based on the single crystal X-ray diffraction analysis,the carbon cage of Sc2UC@C50 was assigned to the Ih(7)-C50 cage structure.Spectroscopic analyses further confirmed this assignment.The analysis of cluster configuration revealed that the encapsulated cluster contains a U=C bond of 2.01 A.which,to the best of our knowledge,is the shortest U=C bond confirmed by crystallography so far.The oxidation state of the U is+4,as suggested by the crystallographic results.It marked the first successful combination of actinide and lanthanide in the single molecular scale.Besides,the vibrational characteristics and the redox behavior of Sc2U@Ih(7)-C80 show significant differences from those of the U2C@Ih(7)-C80,indicating a major influence of the replacement of uranium by two scandium atoms on the physiochemical properties of the endohedral fullerene(2)The metal oxide,U3O8,was adopted as the metal source and the novel diuranium nitride cluster fullerene,U2N@C80,was successfully synthesized by the Kratschmer-Huffman arc discharge method.It was fully characterized by single crystal X-ray diffraction analysis,Raman and IR spectroscopy and cyclic voltammetry.The fullerene cage was unambiguously assigned to Ih(7)-C80 by single crystal X-ray diffraction analysis.The two U atoms are asymmetrically bonded with the central N with notably different bond lengths of 2.058 A and 1.943 A,respectively,which is indicative of the different intensity of the two U-N bonds as well as the different valence states of the two U anions.The bond angle of U-N-U is 150°,which is larger than that of the UCU cluster of the previously reported U2C@Ih(7)-C80(142.8°).The novel UNU cluster and the unique bonding nature breaks the generally accepted trimetallic nitride template in the field of EMF research,which has been unexceptionally followed by all the previously reported nitride cluster fullerenes.The results suggest that the unique multiple valence states of uranium can facilitate the stabilization of novel clusters inside fullerene cages.