Insight into the high-pressure stability of hydrazobenzene and pressure-induced polymerization of azobenzene from crystal structure analysis

While hydrazobenzene is stable up to at least 28 GPa, azobenzene undergoes a polymerization reaction at much lower pressure of ca. 18 GPa, however, preserving the azo groups. Although a polymerization mechanism was proposed by us in a previous study, the high-pressure structure of azobenzene close to the reaction remains unknown. We propose here to solve the high-pressure structures of azobenzene and hydrazobenzene at pressures up to the polymerization reaction of azobenzene, and up to the stability limit of hydrazobenzene at >28 GPa. This will give us most accurate information on the possible reaction path of the polymerization of azobenzene and the influence of N–H...C hydrogen bonding on the superior high-pressure stability of hydrazobenzene if compared to azobenzene. Overall, the results of the proposed study will contribute to the understanding of the relation between the molecular packing and the mechanisms of pressure-induced polymerization reactions in aromatic compounds.