Ionic cocrystals of dithiobispyridines: the role of I...I halogen bonds in the building of iodine frameworks and the stabilization of crystal structures.

It has been confirmed that mercaptopyridines undergo spontaneous condensation in redox reaction with iodine-forming dithiopyridines. In the solid state, these compounds are protonated at the N atoms and cocrystallize with iodine forming salt structures, namely, 2-[(pyridin-2-yl)disulfanyl]pyridinium triiodide sesquiiodine, C10H9N2S2+·I3-·1.5I2, and 4,4'-(disulfanediyl)dipyridinium pentaiodide triiodide, C10H10N2S22+·I5-·I3-. Dithiopyridine cations are packed among three-dimensional frameworks built from iodide anions and neutral iodine molecules, and are linked by hydrogen, halogen and chalcogen interactions. Quantum chemical computations indicated that dithiopyridines exhibit anomalously high nitrogen basicity which qualify them as potential proton sponges.