Thermodynamically stable [4 + 2] cycloadducts of lanthanum-encapsulated endohedral metallofullerenes.

Yuta Takano,Takeshi Akasaka,M Ángeles Herranz,Nazario Martín,Yuki Nagashima

doi:10.3762/bjoc.10.65

Abstract

The [4 + 2] cycloaddition of o-quinodimethanes, generated in situ from the sultine 4,5-benzo-3,6-dihydro-1,2-oxathiin 2-oxide and its derivative, to La metal-encapsulated fullerenes, La2@C80 or La@C82, afforded the novel derivatives of endohedral metallofullerenes (3a,b, 4a,b and 5b). Molecular structures of the resulting compounds were elucidated using spectroscopic methods such as MALDI–TOF mass, optical absorption, and NMR spectroscopy. The [4 + 2] adducts of La2@C80 (3a,b, and 4a,b) and La@C82 (5b), respectively, retain diamagnetic and paramagnetic properties, as confirmed by EPR spectroscopy. Dynamic NMR measurements of 4a at various temperatures demonstrated the boat-to-boat inversions of the addend. In addition, 5b revealed remarkable thermal stability in comparison with the reported [4 + 2] cycloadduct of pentamethylcyclopentadiene and La@C82 (6). These findings demonstrate the utility of sultines to afford thermodynamically stable endohedral metallofullerene derivatives for the use in material science.

Highlights

Endohedral metallofullerenes (EMFs) are a family of nanocarbons, which encapsulated one or more metal atoms inside a hollow carbon cage [1,2,3,4]
The encapsulation results in the electron transfer from metal atoms to the fullerene cage, which leads to unique electronic, magnetic, and chemical properties for EMFs that cannot be expected for empty fullerenes
Synthesis and characterization of La2@C80 cycloadducts o-Quinodimethanes 2a and 2b were generated in situ by thermolysis of the corresponding sultines 1a and 1b in toluene at 80 °C (Scheme 1)

Summary

Introduction

Endohedral metallofullerenes (EMFs) are a family of nanocarbons, which encapsulated one or more metal atoms inside a hollow carbon cage [1,2,3,4]. The encapsulation results in the electron transfer from metal atoms to the fullerene cage, which leads to unique electronic, magnetic, and chemical properties for EMFs that cannot be expected for empty fullerenes. The spectrum of 3b shows the specific absorption band around 700 nm, which strongly suggests the electric nature of a [6,6]-closed adduct of the La2@C80 derivatives [15], because the absorption spectra of fullerenes and their derivatives are mainly attributable to π–π* transitions, which reflects the distinctive fingerprints of the π-electron system topology of the fullerene cage.

Results

Conclusion