Unusual addition patterns in trifluoromethylation of [60]fullerene

Abstract

From pyrolytic trifluoromethylation of [60]fullerene with CF₃CO₂Ag at 300 °C we have isolated ca. sixty C₆₀(CF₃)_n isomers (numbers in parentheses) as follows: n = 2 (1), 4 (8), 6 (13), 8 (21) 10 (11), 12 (5), 14 (4), twenty-one of which have been characterised by ¹⁹F NMR. Compounds with addition levels up to n = 20 have also been identified. With increasing value of n, yields decrease and the separation of compounds of similar HPLC retention time but different addend levels becomes more difficult. Many of the ¹⁹F NMR spectra show combinations of quartets and septets (the latter tending to be more downfield) due to ‘linear’ addend arrays. The spectra are consistent with addition across both 6:6- and 5:6-ring junctions [double (1,2) and single (1,6) bonds, respectively], giving corresponding coupling constants for adjacent addends of ca. 14.5 and 12.0 Hz respectively, the differences being attributable to the different 1,2- and 1,6-bond lengths. The ¹³C NMR spectrum of C₆₀(CF₃)₂ shows the CF₃ groups are in either a 1,4- or 1,6-relationship; the UV-vis band appears at 442 nm. Other unsymmetrical tetra-adducts are comprised of isolated pairs of CF₃ groups. The exceptionally large number of derivatives and isomers, (much greater than in any other fullerene reaction), no dominant product, and unusual addition pattern indicates that thermodynamic stability is not of primary importance in governing product formation. EI mass spectrometry of trifluoromethylfullerenes is characterised by loss of CF₃ groups, the more highly addended compounds also showing fragmentation by CF₂ loss, attributable to steric compression. The CF₃ group shows strong IR bands at ca. 1260 and 1190 cm⁻¹. The compounds are stable to aq. acetone, which contrasts to the behaviour of fluorofullerenes. Trifluoromethylation by the Scherer radical (C₉F₁₉˙) gave addition of up to eight CF₃ groups, together with hydrogen in some products. During EI mass spectrometry of some of these, loss of HF attributable to CF₃ and H adjacency can occur, giving CF₂-containing derivatives.