Double Negatively Curved C₇₀ Growth through a Heptagon-Involving Pathway

All previously reported C₇₀ isomers have positive curvature and contain 12 pentagons in addition to hexagons. Herein, we report a new C₇₀ species with two negatively curved heptagon moieties and 14 pentagons. This unconventional heptafullerene[70] containing two symmetric heptagons, referred to as dihept-C₇₀ , grows in the carbon arc by a theoretically supported pathway in which the carbon cluster of a previously reported C₆₆ species undergoes successive C₂ insertion via a known heptafullerene[68] intermediate with low energy barriers. As identified by X-ray crystallography, the occurrence of heptagons facilitates a reduction in the angle of the π-orbital axis vector in the fused pentagons to stabilize dihept-C₇₀ . Chlorination at the intersection of a heptagon and two adjacent pentagons can greatly enlarge the HOMO-LUMO gap, which makes dihept-C₇₀ Cl₆ isolable by chromatography. The synthesis of dihept-C₇₀ Cl₆ offers precious clues with respect to the fullerene formation mechanism in the carbon-clustering process.