Exohedrally stabilized C70 isomer with adjacent pentagons characterized by crystallography

Abstract

All C₇₀ isomers other than the well-known D_5h-symmetric C₇₀ (#8149) are highly elusive due to their defiance of the isolated pentagon rule (IPR), and, in turn, have long been underrated for years. Here we show a non-IPR hollow C₇₀ (#8064) with C₂-symmetry stabilized by exohedral chlorination as ^#8064C₇₀Cl₁₀. Its connectivity with two pairs of fused pentagons was determined unambiguously by single crystal X-ray diffraction analysis. Based on the geometric criterion in terms of the pyramidalization angle as well as theoretical computations on the Kekulé structure and the negative nucleus independent chemical shift, the exohedral stabilization of the labile ^#8064C₇₀ is rationalized by releasing the local strain relevant to the fused pentagons and achieving the aromaticity of the remaining sp²-hybridized carbon framework in the exohedral derivative of ^#8064C₇₀. This new entrant in the C₇₀ family, in addition to two previously reported non-IPR isomers (by Yang, Dunsch et al. for Sc₃N@^#7854C₇₀ and by Echegoyen et al. for Sc₂S@^#7892C₇₀), provides valuable opportunities for experimental and theoretical research involving non-IPR isomers of C₇₀ beyond the common D_5h-C₇₀.