Positional Isomerism and Conformational Flexibility Directed Structural Variations in the Molecular Complexes of Dihydroxybenzoic Acids

Abstract

Mutual disposition and conformational preferences of functional groups can induce variations in the nature and types of interactions and hence the molecular arrangements in the rigid crystal environment. We comprehensively analyzed this effect in a series of 13 (of which 9 are novel) (Khan et al. Cryst. Growth Des. 2009, 9, 2354−2362; Varughese et al. Chem.Eur. J. 2006, 12, 1597−1609) molecular complexes of positional isomers of dihydroxybenzoic acid with trans-1,2-bis(4-pyridyl)ethene and 1,2-bis(4- pyridyl)ethane. Seven of the complexes exist as salts, with an observed carboxyl to pyridine heteroatom proton transfer, which can be explained on the basis of ΔpKa analysis. In all the complexes, carboxyl/carboxylate functionalities interact consistently with pyridine/ pyridinium moieties. The −OH groups, in contrast, are more versatile with the formation of diverse interaction types: −OH···carboxyl (O−H···O), −OH···carboxylate (O−H···O−), and −OH···pyridine (O−H···N) hydrogen bonds. Hirshfeld surface analysis and computed interaction energy values were utilized to determine the hierarchical ordering of the interactions and further to highlight the significance of weak interactions such as π···π and C−H···π in structure stabilization. In ionic complexes, these secondary interactions become more expressed, with an enhanced contribution from electrostatic elements. The energetic bias toward the complex formation is evident from the calculated cohesive energies of the complexes visà- vis their parent components.