Actinide Metallic Endohedral Fullerenes: Synthesis of Th@C2n and U@C2n(n=36-50) and the Electrochemical Properties of Th@C82 and U@C82

Abstract

A series of actinide metallic endohedral fullerenes, Th@C2n (n=36-50) and U@C2n (n=36-50), were synthesized by arc-discharging method with graphite rods packed with graphite powder and metal oxide (ThO2 , U3O8 , respectively) under an atmosphere of helium. Among these novel endohedral fullerenes, Th@C82 and U@C82 were found to be most abundant species and one isomer for each endohedral fullerene has been isolated and purified by multistage HPLC methods respectively. The purified Th@C82 and U@C82 were characterized by mass spectrometry, UV-Vis-NIR absorption spectroscopy and for the first time, their electrochemical properties were investigated by cyclic voltammetry (CV). The UV-Vis-NIR absorption and electrochemical properties of U@C82 show large resemblance to those of previously reported lanthanide monometallic fullerenes, such as La@C82 ,Y@C82 ,Gd@C82 etc., which suggests that the oxidation state of the uranium atom in U@C82 is likely to be +3. Th@C82 , compared to those of the U@C82 as well as the lanthanide monometallic fullerenes, however, demonstrated significant differences both in the retention time on the Buckyprep and Buckyprep-D columns and the absorption spectra. In particular, Th@C82 demonstrated unique electrochemical properties. In the cathodic range, Th@C82 shows four irreversible reductive steps in which the first reduction potential was found to be -1.04V, far more negative than those of all lanthanides monometallic fullerenes. In the anodic range, the oxidative behavior of Th@C82 is similar to that of U@C82 , displaying a reversible first oxidation step at 0.49 V, and irreversible second oxidation step at 0.90 V. Thus the electrochemical potential gap of Th@C82 was determined as 1.53V. This value is much higher than those of the M@C82 of trivalent lanthanides and is the largest for all the mono-metal endohedral fullerenes discovered to date. This work demonstrated unique physical and electrochemical properties of actinide metallic endohedral fullerenes and their potential of application in material sciences.