The curious case of proton migration under pressure in the malonic acid and 4,4′-bi­pyridine cocrystal

Pressure was successfully used to induce single and double proton-transfer reactions in malonic acid and the 4,4′-bi­pyridine cocrystal. After contrasting with similar literature examples, an extended correlation between the ΔpK _a values of coformers and the pressure necessary to initiate proton-transfer reactions is unveiled.

In the search for new active pharmaceutical ingredients, the precise control of the chemistry of cocrystals becomes essential. One crucial step within this chemistry is proton migration between cocrystal coformers to form a salt, usually anticipated by the empirical ΔpK _a rule. Due to the effective role it plays in modifying intermolecular distances and interactions, pressure adds a new dimension to the ΔpK _a rule. Still, this variable has been scarcely applied to induce proton-transfer reactions within these systems. In our study, high-pressure X-ray diffraction and Raman spectroscopy experiments, supported by DFT calculations, reveal modifications to the protonation states of the 4,4′-bi­pyridine (BIPY) and malonic acid (MA) cocrystal (BIPYMA) that allow the conversion of the cocrystal phase into ionic salt polymorphs. On compression, neutral BIPYMA and monoprotonated (BIPYH⁺MA⁻) species coexist up to 3.1 GPa, where a phase transition to a structure of P2₁/c symmetry occurs, induced by a double proton-transfer reaction forming BIPYH₂ ²⁺MA²⁻. The low-pressure C2/c phase is recovered at 2.4 GPa on decompression, leading to a 0.7 GPa hysteresis pressure range. This is one of a few studies on proton transfer in multicomponent crystals that shows how susceptible the interconversion between differently charged species is to even slight pressure changes, and how the proton transfer can be a triggering factor leading to changes in the crystal symmetry. These new data, coupled with information from previous reports on proton-transfer reactions between coformers, extend the applicability of the ΔpK _a rule incorporating the pressure required to induce salt formation.