Pressure Effects on Crystallization, Polymorphism, and Solvation of 4,4′-Bipyridinium Perchlorate

Pressure efficiently changes the crystallization preferences of three polymorphs and three solvates of 4,4'-bipyridinium perchlorate, [C10H9N2]+ClO4-. At ambient pressure a triclinic dihydrate [C10H9N2]+ClO4-center dot 2H(2)O is formed of the solutions in water and in the water/methanol mixture, while the methanol solution in open vials yields concomitant monoclinic quarterhydrate 4([C10H9N2]+ClO4-)center dot H2O and triclinic anhydrate [C10H9N2]+ClO4-. At 0.30 GPa an orthorhombic anhydrate is formed, stable to 0.60 GPa, when it collapses into a monoclinic phase. The transition changes the conformation of cations from planar to twisted, differently than the analogue phase transition in [C10H9N2]Br+center dot H2O. Recrystallizations from the methanol solution above 0.55 GPa leads to methanol solvate [C10H9N2]+ClO4-center dot CH3OH. In the solvates, chains of the cations are aggregated through either NH+center dot center dot center dot N bonds (like in the neat polymorphs) or NH+center dot center dot center dot OH center dot center dot center dot N bonds, or both these bonds in one crystal. The pyridinium protons are disordered in all these structures, which implies disproportionation between neutral molecules, C10H8N2, monocations [C10H9N2](+) and dications [C10H10N2](2+). In quarterhydrate 4([C10H9N2]+ClO4-)center dot H2O the disorder of protons propagates through the hydrogen bonds mediated by disordered water molecules, also involved in weak H-bonds to perchlorate anions.