Chemistry through cocrystats: pressure-induced potymerization of C2H2•C6H6 to an extended crystalline hydrocarbont

The 1:1 acetylene-benzene cocrystal, C2H2 center dot 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 center dot C6H6 is stable up to 30 GPa, nearly 10x 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 tacking 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 center dot 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.