Fullerides of Pyrrolidine-Functionalized C60

Abstract

The spectral characteristics of the fulleride(1−) anions of pyrrolidine-functionalized C60 derivatives have been investigated in order to assess the degree to which the parent C60 moiety retains its distinctive redox and chromophoric properties upon functionalization. Such comparison data are necessary if C60 is to fulfill its anticipated role as a versatile electron-accepting chromophore and a multielectron reservoir. Three systems have been investigated: a simple, monomeric, N-methylpyrrolidine derivative, 1, a pyrrolidine-linked tetraphenylporphyrin/C60 dyad, 2, and a pair of bifullerenes, 3 and 4, with adjacent and remote dispositions of the C60 moieties, respectively. Near infrared spectra of the fulleride(1−) ions of these derivatives are notably similar to C601- with only small energy shifts of the band envelope. 1H and 13C NMR data on 11- are consistent with unpaired spin density confined largely to the C60 moiety rather than delocalized onto the pyrrolidine functionality. The EPR spectra of all of the pyrrolidine-functionalized fulleride(1−) species are characterized by sharp doublets (ΔH = 1−2 G) and g values less than the free electron value. Unlike the EPR spectrum of C601-, there is little temperature dependence of the line width. EPR evidence for ball-to-ball spin−spin interactions are observed in the fulleride of 3 but not in 4 or in the CuII/C601- metalated porphyrin dyad. The X-ray crystal structure of the porphyrin-C60 dyad 2 has been determined. It is the first X-ray structural characterization of a pyrrolidine-functionalized C60 species and confirms that the azamethine ylide addition has occurred across a 6:6 ring juncture. The crystal packing of the dyad reveals an intermolecular interaction of the C60 ball nestled in remarkably close approach to the porphyrin plane. The closest approach of a fullerene carbon atom to the mean plane of the porphyrin is ca. 2.75 Å. This interaction is notably similar to that envisaged for porphyrin-functionalized chromatographic supports used to separate fullerenes. A novel donor/acceptor relationship is proposed to account for this close interaction.