Synthesis and EPR Studies of Radicals and Biradical Anions of C60 Nitroxide Derivatives

A series of C60 derivatives substituted with the nitroxide group 2,2,6,6-tetramethylpiperidine-1-oxyl have been synthesized. A detailed EPR and ENDOR study of the neutral radicals and of their reduction products is reported. All the neutral species give EPR spectra consisting of a main triplet of lines with 15 G splitting by the 14N nucleus, typical of nitroxide radicals. Some of them show additional well-resolved splittings by methyl and methylene protons, which are discussed in relation to the conformation of the nitroxide ring. When the compounds are progressively reduced under vacuum by contact with an alkali metal mirror, new EPR lines are observed; a 1:1:1 triplet with a splitting constant half that of a nitroxide radical and g ) 2.0030 is attributed to biradical anions where one electron is located in the fullerene moiety and experiences a strong exchange coupling with the nitroxide unpaired electron. Frozen solution spectra of these species allow the determination of the electron-electron dipolar interaction parameters which are compared with theoretical MO calculations. The experimental values agree with a spin distribution of one unpaired electron mostly in the equatorial plane of the fullerene. Another single line which appears in the spectrum as the reduction proceeds further on (g ) 1.9999) is attributed to a new species: possibly a substituted fullerene radical anion in which the nitroxide group is irreversibly reduced.