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 center dot Phen stoichiometry upon grinding the two components in different starting ratios and then using suitable solvents for single crystal growth. The phenol center dot center dot center dot pyridine O-H center dot center dot center dot N synthon directs 1: 1.5 bimolecular organization of PHG and Phen molecules in pi-stacked motif (cocrystal 1). Additional phenazine molecules insert in the pi-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 pi-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 beta-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.