The Role of π-Stacking in the Composition of Phloroglucinol and Phenazine Cocrystals

Cocrystallization of phloroglucinol (PHG) and phenazine (Phen) afforded cocrystals of 1:1.5, 1:1.75, and 1:2 PHG·Phen stoichiometry upon grinding the two components in different starting ratios and then using suitable solvents for single crystal growth. The phenol···pyridine O−H···N synthon directs 1:1.5 bimolecular organization of PHG and Phen molecules in a π-stacked motif (cocrystal 1). Additional phenazine molecules insert in the π-stacks to give a higher proportion of the aromatic species in cocrystals 2 and 3 (1:1.75 and 1:2). A hydrate cocrystal of 1:2:1 composition (4) was also obtained. The π-stack dimer motif of phenazine templated by the 1,3-(OH)2 moiety of phloroglucinol at about van der Waals distance in these crystal structures is postulated to promote the crystallization of the β-phenazine polymorph having a sandwich herringbone motif. These results suggest a role for cocrystal formers as hydrogen bond additives that favor and stabilize specific motifs for the crystallization of new polymorphs.