An Unconventional Hydrofullerene C₆₆H₄ with Symmetric Heptagons Retrieved in Low-Pressure Combustion

The combustion has long been applied for industrial synthesis of carbon materials such as fullerenes as well as carbon particles (known as carbon black), but the components and structures of the carbon soot are far from being clarified. Herein, we retrieve an unprecedented hydrofullerene C₆₆H₄ from a soot of a low-pressure combustion of benzene-acetylene-oxygen. Unambiguously characterized by single-crystal X-ray diffraction, the C₆₆H₄ renders a nonclassical geometry incorporating two heptagons and two pairs of fused pentagons in a C_{2 v} symmetry. The common vertexes of the fused pentagons are bonded with four hydrogen atoms to convert the hydrogen-linking carbon atoms from sp² to sp³ hybridization, which together with the adjacent heptagons essentially releases the sp²-bond strains on the abutting-pentagon sites of the diheptagonal fused pentagon C₆₆ (dihept-C₆₆). DFT computations suggest the possibility for an in situ hydrogenation process leading to stabilization of the dihept-C₆₆. In addition, the experiments have been carried out to study heptagon-dependent properties of dihept-C₆₆H₄, indicating the key responsibility of the heptagon for changing hydrocarbon activity and electronic properties. The present work with the unprecedented double-heptagon-containing hydrofullerene successfully isolated and identified as one of the low-pressure combustion products shows that the heptagon is a new building block for constructing fullerene products in addition to pentagons and hexagons in low-pressure combustion systems.