Isolation and characterisation of C60F4, C60F6, C60F8, C60F7CF3 and C60F2O, the smallest oxahomofullerene; the mechanism of fluorine addition to fullerenes

Abstract

From the reaction of [60]fullerene with K₂PtF₆ at 470 °C, we have isolated C₆₀F₄, C₆₀F₆ (mixed with C₆₀F₇CF₃), C₆₀F₈ and C₆₀F₂O. The ¹⁹F NMR spectrum of C₆₀F₂O comprises a single line at −69.3 ppm, consistent with it being the simplest oxahomofullerene. The AA′BB′ spectrum for C₆₀F₄ (double doublets at −141.82 and −142.78 ppm) shows it to be isostructural with C₆₀H₄, the addends in each case being in a 1,2,3,4-arrangement. The spectrum for C₆₀F₆ consists of doublets at −124.39 and −142.41 and a triplet at −139.78 ppm (coupling confirmed by a 2D spectrum), showing the fluorines to be in a previously-conjectured S-shaped motif, giving the molecule overall C₂ symmetry. The C₆₀F₈ spectrum comprises five peaks in a 1 : 2 : 2 : 2 : 1 ratio, so that the molecule has C_s symmetry based on a T-shaped motif, also conjectured previously. The peak couplings and symmetry indicate that the structure is created by addition of three pairs of fluorines across contiguous double bonds, followed by 1,8-addition of the final fluorine pair; this unique latter step is attributed to the ability of a pentagon containing three sp³ carbons to accommodate a double bond, due to the reduction in strain. The C₆₀F₈ structure is part of the motif of C₆₀F₁₆ and C₆₀F₁₈, indicating it to be an intermediate on the pathway to formation of these compounds. The spectrum for C₆₀F₇CF₃ shows the presence of one major and one minor isomer, the probable structures of which are deduced.