Cyanation of Fullerene(CF3)n Does Not Always Increase Electron Affinity: Experimental and Theoretical Study

Trifluoromethylfullerenes have highly tunable electronic properties and can be used as adaptable building blocks for advanced organic electronics. Previously, our group has shown that the frontier orbitals of a series of C₆₀(CF₃)_n are greatly influenced by the addition pattern of CF₃ groups, which in turn, has a profound influence on electron affinities.¹ Judicious additions of strong electron-withdrawing groups, such as CN or CF₃, to fullerene cages are typically expected (and theoretically predicted) to increase electron affinity.

Indeed, regioselective synthesis of a new SPP derivative with three unique addends, C₆₀(CF₃)₄(CN)H, made possible due to the use of a new isomer of C₆₀(CF₃)₄ as a substrate, resulted in the experimental and theoretical identification of a family of superhalogen fullerene radicals, species with the gas-phase electron affinity higher than that of the most electronegative halogens, F and Cl.² The addition of one CN group to C₆₀(CF₃)₄ increases the measured gas-phase electron affinity by more than 1 eV to 4.28(1) eV; such a large increase is likely due to a significant stabilization of the skew pentagonal pyramid pattern (SPP), and electron-withdrawing effect of CN group. In contrast, the measured electron affinity of another SPP open shell molecular species, C₆₀(CF₃)₄H, is considerably lower, 3.96(1) eV.

Regioselective cyanation of several C₇₀(CF₃)_n  compounds has also been carried out using recently described technique.³ In the case of C₁-C₇₀(CF₃)₁₀, the addition of two CN groups results in an expected increase in the measured gas-phase electron affinity by 0.22 eV. Surprisingly, the addition of two electron-withdrawing CN groups to C_s-C₇₀(CF₃)₈ results in a decrease in the measured gas-phase electron affinity! This decrease is observed for two different CN addition patterns. The underlying reasons and implications for the design of electrophilic fullerene derivatives are rationalized using DFT calculations.

References

 1           Popov, A. A.; Kareev, I. E.; Shustova, N. B.; Stukalin, E. B.; Lebedkin, S. F.; Seppelt, K.; Strauss, S. H.; Boltalina, O. V.; Dunsch, L. J. Am. Chem. Soc. 2007, 129, 11551.

    2           Clikeman, T. T.; Deng, S. H. M.; Avdoshenko, S.; Wang, X.-B.; Popov , A. A.; Strauss, S. H.; Boltalina, O. V. Chem. Eur. J. 2013, 19, 15404.

    3           Clikeman, T. T.; Kuvychko, I. V.; Shustova, N. B.; Chen, Y.-S.; Popov, A. A.; Boltalina, O. V.; Strauss, S. H. Chem. Eur. J. 2013, 19, 5070.