Unconventional hydrogen bonding and [pi]-stacking in two substituted pyridine carboxamides

The crystal structures of twopara-substituted aryl derivatives of pyridine-2-carboxamide, namelyN-(4-fluorophenyl)pyridine-2-carboxamide, C12H9FN2O, (I), andN-(4-nitrophenyl)pyridine-2-carboxamide, C12H9N3O3, (II), have been studied. Compound (I) exhibits unconventional aryl-carbonyl C--H...O and pyridine-fluorine C--H...F hydrogen bonding in two dimensions and well defined π-stacking involving pyridine rings in the third dimension. The conformation of (II) is more nearly planar than that of (I) and the intermolecular interactions comprise one-dimensional aryl-carbonyl C--H...O hydrogen bonds leading to a stepped or staircase-like progression of loosely π-stacked molecules. The close-packed layers of planar π-stacked molecules are related by inversion symmetry. Two alternating interplanar separations of 3.439(1) and 3.476(1)Å are observed in the crystal lattice and are consistent with a repetitive packing sequence,ABA'B'AB..., for the π-stacked inversion pairs of (II). [PUBLICATION ABSTRACT]