Lower trifluoromethyl[70]fullerene derivatives: novel structural data and an survey of electronic properties

We revisit a series of trifluoromethylated C70(CF3)n compounds with n = 2-12 to provide a comprehensive comparative characterization thereof by means of a combination of physico-chemical methods. X-ray diffraction studies, including first structural determinations of the major isomers of C70(CF3)2, C70(CF3)4, and the third most abundant isomer of C70(CF3)8 as co-crystals with octaethylporphyrin Ni(II), produce accurate CF3 group rotation angles and intergroup F⋯F distances that enable more reliable 19F NMR signal assignment based on the through-space JFF spin-spin coupling relation to the F⋯F distances. Cyclic voltammetry measurements were performed in a potential range that covers oxidation on the one end and reduction to tri- or tetraanionic states on the other. It was demonstrated that the CF3 addition patterns have marked effects on the redox potentials. A consistent set of the experimental HOMO energy values of the C70(CF3)n molecules is reported for the first time in complement to the LUMO data, and the both sets of results were found to demonstrate good correlation with quantum-chemical DFT predictions. Most of the compounds exhibit electrochemically reversible one-electron reductions up to trianionic (sometimes even tetraanionic) state, a notable exception being C70(CF3)10 with irreversibility of its very first reduction. Bulk electrolysis of C70(CF3)12 at its 4-th reduction potential identifies CF3 detachment as the most likely cause of the observed irreversible processes yielding C70(CF3)11−anions as was identified by means of MALDI MS.