A structural systematic study of four isomers of difluoro-N-(3-pyridyl)benzamide.

The four isomers 2,4-, (I), 2,5-, (II), 3,4-, (III), and 3,5-difluoro-N-(3-pyridyl)benzamide, (IV), all with formula C(12)H(8)F(2)N(2)O, display molecular similarity, with interplanar angles between the C(6)/C(5)N rings ranging from 2.94 (11) degrees in (IV) to 4.48 (18) degrees in (I), although the amide group is twisted from either plane by 18.0 (2)-27.3 (3) degrees. Compounds (I) and (II) are isostructural but are not isomorphous. Intermolecular N-H...O=C interactions form one-dimensional C(4) chains along [010]. The only other significant interaction is C-H...F. The pyridyl (py) N atom does not participate in hydrogen bonding; the closest H...N(py) contact is 2.71 A in (I) and 2.69 A in (II). Packing of pairs of one-dimensional chains in a herring-bone fashion occurs via pi-stacking interactions. Compounds (III) and (IV) are essentially isomorphous (their a and b unit-cell lengths differ by 9%, due mainly to 3,4-F(2) and 3,5-F(2) substitution patterns in the arene ring) and are quasi-isostructural. In (III), benzene rotational disorder is present, with the meta F atom occupying both 3- and 5-F positions with site occupancies of 0.809 (4) and 0.191 (4), respectively. The N-H...N(py) intermolecular interactions dominate as C(5) chains in tandem with C-H...N(py) interactions. C-H...O=C interactions form R(2)(2)(8) rings about inversion centres, and there are pi-pi stacks about inversion centres, all combining to form a three-dimensional network. By contrast, (IV) has no strong hydrogen bonds; the N-H...N(py) interaction is 0.3 A longer than in (III). The carbonyl O atom participates only in weak interactions and is surrounded in a square-pyramidal contact geometry with two intramolecular and three intermolecular C-H...O=C interactions. Compounds (III) and (IV) are interesting examples of two isomers with similar unit-cell parameters and gross packing but which display quite different intermolecular interactions at the primary level due to subtle packing differences at the atom/group/ring level arising from differences in the peripheral ring-substitution patterns.