Reactions of C_(70) ~(2-) with organic halides revisited: Unusual magnetic equivalence for the diastereotopic methylene protons in 2,5-(PhCH_2)_2C_(70)

摘要

The reactions of C_(70) ~(2-) with organic halides result in several isomeric products. However, the structures of the isomers have not been identified unambiguously and the reactions still remain elusive even though the first report of the reaction appeared more than ten years ago. Herein, the reactions of C_(70) ~(2-) with ArCH_2Br (Ar = Ph, o-, m-, and p-BrC_6H_4) are revisited. Two types of isomeric para-adducts of 2,5- and 7,23-(ArCH_2)_2C_(70) are obtained and characterized with the X-ray single-crystal diffractions, HRMS, ~1H and ~(13)C NMR, and UV-vis spectroscopies. An unusual singlet resonance instead of the typical AB quartet resonance is shown for the C2 diastereotopic methylene protons in 1H NMR spectrum of 2,5-(PhCH_2)_2C_(70) recorded at ambient temperature. The unexpected magnetic equivalence is studied with the variable-temperature (VT) NMR and density functional theory (DFT) calculations. The results show that the greater local curvature in the C70 polar region is likely responsible for the unusual magnetic equivalence exhibited by the C2 diastereotopic methylene protons of 2,5-(PhCH_2) _2C_(70), indicating that under certain cases, it is improbable to elucidate the spectral data of fullerene derivatives with a less symmetrical carbon cage in analogy with those of the C_(60) derivatives.