The salt–cocrystal spectrum in salicylic acid–adenine: the influence of crystal structure on proton‐transfer balance

At one extreme of the proton‐transfer spectrum in cocrystals, proton transfer is absent, whilst at the opposite extreme, in salts, the proton‐transfer process is complete. However, for acid–base pairs with a small ΔpK_a (pK_a of base ? pK_a of acid), prediction of the extent of proton transfer is not possible as there is a continuum between the salt and cocrystal ends. In this context, we attempt to illustrate that in these systems, in addition to ΔpK_a, the crystalline environment could change the extent of proton transfer. To this end, two compounds of salicylic acid (SaH) and adenine (Ad) have been prepared. Despite the same small ΔpK_a value (≈1.2), different ionization states are found. Both crystals, namely adeninium salicylate monohydrate, C₅H₆N₅⁺·C₇H₅O₃^?·H₂O, I , and adeninium salicylate–adenine–salicylic acid–water (1/2/1/2), C₅H₆N₅⁺·C₇H₅O₃^?·2C₅H₅N₅·C₇H₆O₃·2H₂O, II , have been characterized by single‐crystal X‐ray diffraction, IR spectroscopy and elemental analysis (C, H and N) techniques. In addition, the intermolecular hydrogen‐bonding interactions of compounds I and II have been investigated and quantified in detail on the basis of Hirshfeld surface analysis and fingerprint plots. Throughout the study, we use crystal engineering, which is based on modifications of the intermolecular interactions, thus offering a more comprehensive screening of the salt–cocrystal continuum in comparison with pure pK_a analysis.