Structural Studies on Solvates of Cyclic Imide Tethered Carboxylic Acids with Pyridine and Quinoline

Structures of eight solvates of cyclic imide tethered carboxylic acids and aromatic tetra carboxylic acids with pyridine (solvate I−VI) and quinoline (VII−VIII) are determined. Different types of hydrogen bond motifs (discrete or cyclic) in these solvates are identified. Solvates I and II possess discrete O−H···N interactions, solvates III and VIII possess combinations of cyclic interactions arising from O−H···N and C−H···O interactions, solvates IV, V, and VII have both the above namely discrete and cyclic types of interactions, whereas, solvate VI is an exception which possesses discrete O−H···N as well as R²₂(8) types of interactions and provides a model system for incomplete cleavage of dimeric assembly of carboxylic acid moiety. On the basis of the results of various hydrogen bond motifs, density functional theory calculations (DFT) on similar motifs generated from formic acid and its interaction with pyridine and quinoline are carried out. In the case of a pyridine formic acid system, DFT calculations show that the energy difference between the cyclic R²₂(7) motif and the discrete motif is ∼0.6 kcal/mol. Such a small difference accounts for the formation of both types of structural patterns in solvates I−V depending on the steric requirements. The observed motif of VI is established by comparison of theoretical energies between a dimeric carboxylic acid moiety generated from two formic acids interactions and a trimeric moiety that exhibits two formic acids and pyridine interactions. The energy of different types of hydrogen bond motifs formed by the interactions between quinoline and formic acid is also calculated. Calculations based on DFT show that the salt formation between formic acid and pyridine is not a favorable process, but it may occur in the case of quinoline.