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

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 vis-à-vis
their parent components.