(Invited) Synthesis of Ion-Endohedral Fullerenes by Plasma Implantation

Abstract

Endohedral metallofullerenes (EMFs), which are the hybrid materials consisting of fullerene cage and encapsulated metal(s), are one of the well-studied nanocarbon materials during the past 3 decades.[1] As a result of significant progress in the development of synthetic procedures and purification processes, a variety of different types of EMFs, such as mono-metallofullerenes, di-metallofullerenes and metal-cluster-fullerenes with various carbon-cage sizes, are currently available. We previously reported development of an efficient method to access EMFs, so-called plasma implantation.[2] This approach allowed us to achieve scalable synthesis of lithium-ion-encapsulated [60]fullerene Li+@C60 X−, a stable ionic form of lithium encapsulated fullerene. The compound can be regarded as “ion-endohedral fullerene” consisting of encapsulated ion, neutral fullerene cage and external counter anion X−, and it should not be categorized into general metallofullerenes because of its ion-pair structure. Despite the unique properties arising from its ionic natures, no other ionic metallofullerenes have been well studied so far.[3] For systematic and deeper studies on ion-endohedral fullerenes, we focused on Li+-endohedral [70]fullerene (Li+@C70) because C70 is the second abundant and therefore widely studied fullerene next to C60. The Li+@C70 was synthesized as PF6 − and TFSI− (TFSI=bis(trifluoromethanesulfonyl)imide) salts using our improved plasma implantation method followed by oxidative purification process. 7Li NMR spectrum showed a sharp singlet signal at −32.81 ppm (vs LiCl/D2O standard) indicating that the Li+ was encapsulated in the C70 cage. 13C NMR spectrum showed 5 aromatic peaks, which evidenced the defectless C70 structure. In addition to the NMR signals originated from the Li+@C70 core, 19F and 31P NMR spectra showed clear signals assigned to each external counter anion. The structure of Li+@C70 was unambiguously confirmed by X-ray crystal structure analysis of the TFSI− salt. The crystal structure showed encapsulation of Li+ by C70 cage as well as the 1:1 ion-paired structure of the cationic Li+@C70 core and external TFSI− anion.Moreover, we also performed collision experiments using sodium plasma to synthesize sodium-ion-endohedral [60]fullerene. Laser desorption ionization (LDI) mass spectrum of the crude products showed signals at m/z= 720 and 743 assignable to the molecular ion of empty C60 and the Na+@C60, respectively. The target compound was isolated as TFSI− salt by oxidative purification process followed by electrolyte-added HPLC. There was only a mass peak at m/z= 743 after the purification, which evidenced the complete removal of empty C60. 23Na NMR spectrum showed a sharp singlet signal at −59.6 ppm (vs NaCl/D2O standard), evidenced the formation of the target Na+@C60.