Energetically Favorable Conformation of Malonic Acid Achieved through Pressure-Induced Phase Transitions in Its Cocrystal with 4,4’-Vinylenedipyridine

High pressure is a stimulus known to affect the structure and, hence, properties of the solid state. However, its effects on multicomponent organic crystals remain understudied compared to those on monocomponent crystals or MOFs. Meanwhile, the exposure of cocrystals and solvates to high pressure was successfully employed to modify their chemical nature by inducing a proton transfer reaction, which resulted in their transformation into organic salts. Nevertheless, further studies on Brønsted–Lowry acid–base systems are necessary to expand the available data and achieve a thorough understanding of how different multicomponent crystals behave under extreme conditions. In this work, a 1:1 cocrystal of 4,4′-vinylenedipyridine and malonic acid (VIN·MAL) was investigated under high pressure by X-ray diffraction on isothermal compression up to approximately 2 GPa. Two solid-to-solid phase transitions to Phases II and III were observed. The first is associated with lowering the symmetry of the crystal from monoclinic to triclinic, leading to the formation of a two-domain twin from the initial single crystal. The symmetry of the crystal is preserved during the second phase transition. Malonic acid molecules were discovered to adopt a more energetically favorable conformation in high-pressure phases, with the one observed in Phase III being the most stable of all three phases of VIN·MAL. Furthermore, the pressure-induced proton transfer reaction is discussed in the context of the critical donor···acceptor distance and spectroscopic data measured at atmospheric pressure, revealing that the first proton transfer takes place under room conditions, with protons continuously migrating between MAL and VIN molecules.