Chemistry through cocrystals: pressure-induced polymerization of C2H2·C6H6 to an extended crystalline hydrocarbon

Abstract

The 1 : 1 acetylene–benzene cocrystal, C₂H₂·C₆H₆, was synthesized under pressure in a diamond anvil cell (DAC) and its evolution under pressure was studied with single-crystal X-ray diffraction and Raman spectroscopy. C₂H₂·C₆H₆ is stable up to 30 GPa, nearly 10× the observed polymerization pressure for molecular acetylene to polyacetylene. Upon mild heating at 30 GPa, the cocrystal was observed to undergo an irreversible transition to a mixture of amorphous hydrocarbon and a crystalline phase with similar diffraction to i-carbon, a nanodiamond polymorph currently lacking a definitive structure. Characterization of this i-carbon-like phase suggests that it remains hydrogenated and may help explain previous observations of nanodiamond polymorphs. Potential reaction pathways in C₂H₂·C₆H₆ are discussed and compared with other theoretical extended hydrocarbons that may be obtained through crystal engineering. The cocrystallization of benzene with other more inert gases may provide a novel pathway to selectively control the rich chemistry of these materials.