1-(Pyridin-4-yl)-4-thiopyridine (PTP) in the crystalline state - pure PTP and a cocrystal and salt

The first in situ preparation and single-crystal structure identification of pure 1-(pyridin-4-yl)-4-thiopyridine (PTP), C₁₀H₈N₂S, a simple and basic derivative of mercaptopyridine, from a crystallization mixture is described. The same PTP was found in two multicomponent crystal forms with 3,5-dinitrobenzoic acid as a classic two-component cocrystal, namely, 1-(pyridin-4-yl)-4-thiopyridine-3,5-dinitrobenzoic acid (1/1), C₇H₄N₂O₆·C₁₀H₈N₂S, and with 2-hydroxy-3,5-dinitrobenzoic acid as a salt formed via proton transfer from the hydroxy group of the acid to the pyridyl N atom of PTP, namely, 4-(4-sulfanylidene-1,4-dihydropyridin-1-yl)pyridin-1-ium 1-carboxy-3,5-dinitrophenolate, C₁₀H₉N₂S⁺·C₇H₃N₂O₇^-. The protonation energy of PTP is 944.64 kJ mol^-1, indicating slightly greater N-basicity compared to pyridine, a well characterized and very basic chemical reference. A variety of molecular interactions can be observed in the three new crystal structures of PTP, which are all discussed in detail. Our findings confirm those of previous studies, indicating that PTP and 4-mercaptopyridine may, under suitable conditions, be chemically converted to one another, and that this process can be stimulated by light (UV-Vis).