Reaction of C₆₀²⁻ with Organic Halides Revisited in DMF: Proton Transfer from Water to RC₆₀⁻ and Unexpected Formation of 1,2-Dihydro[60]fullerenes

The reactions of dianionic C60 with organic halides, which have been studied extensively in benzonitrile (PhCN), are revisited in a different solvent medium, N,N-dimethylformamide (DMF), by using ArCH2Br (Ar = Ph, C6H4CH3, C6H4Br). Interestingly, instead of the 1,4-R2C60 adducts, which are the typical products when the reactions are carried out in PhCN, the 1,2-dihydro[60]fullerenes (1,2-HRC60) have been obtained as the major products in DMF, except for the case of o-CH3C6H4CH2Br probably due to the steric effect. The obtained 1,2-dihydro[60]fullerenes have been characterized with single-crystal diffraction, 1H and 13C NMR, high-resolution mass spectrometry (HRMS), and UV−vis. Further examination with deuterated reagents including PhCD2Br, DMF-d7, and D2O has revealed that the fullerenyl hydrogen of 1,2-dihydrofullerenes originates from traces of water residue in DMF. When the reaction is re-examined in PhCN with the addition of an excessive amount of water, the yield of 1,2-dihydrofullerene increases significantly, but is still lower compared with that obtained in DMF, demonstrating a considerable solvent effect on the reactivity of C602−. A possible mechanism accounting for such a difference is proposed. The work has presented an alternative protocol for effective preparation of 1,2-dihydro[60]fullerenes, and may also provide clues toward a better understanding of the proton transfer process for anionic C60-mediated reducing reactions involving H2O.