Dicarboxylic Acid-Based Co-Crystals of Pyridine Derivatives Involving Structure Guiding Unconventional Synthons: Experimental and Theoretical Studies

Four co-crystals involving dicarboxylic acids and pyridine derivatives, viz. (ox)0.5(2-CNpy) (1), (adp)(4-CNpy)2 (2), (tp)(4-CNpy)2 (3) and (adp)(3-CNpy)2 (4) (ox = oxalic acid, tp = terephthalic acid, adp = adipic acid, CNpy = cyanopyridine), have been synthesized at room temperature in water medium. Crystal-structure analysis of co-crystal 1 reveals the presence of unconventional O···π(oxalic acid)-hole interaction with the C-C bond of ox moiety, along with parallel nitrile–nitrile interactions. The structural topologies of co-crystals 2–4 unfold the presence of antiparallel nitrile–nitrile interactions involving the CNpy moieties. The molecular associations involving the H-bonds and other unconventional contacts among the co-formers of the multicomponent co-crystals are analyzed using density functional theory (DFT) calculations combined with molecular electrostatic potential (MEP) surface, quantum theory of atoms-in-molecules (QTAIM) and noncovalent interaction (NCI) plot computational tools. The computational studies revealed the presence of unconventional O···π-hole interaction in 1 and the H-bonded synthons with π-stacked nitrile contacts involving CNpy moieties in co-crystals 2–4. The energetic features of the noncovalent contacts reveal the crucial roles of the H-bonding synthons and π-stacking interactions in the multicomponent compounds