An isomer grid of nine fluoro-N-(pyridyl)benzamides (Fxx) (x = para-/meta-/ortho-) has been examined to correlate structural relationships between the experimental crystal structure and ab initio calculations, based on the effect of fluorine (Fx) and pyridine N-atom (x) substitution patterns on molecular conformation. Eight isomers form N-H⋅⋅⋅N hydrogen bonds, and only one (Fom) aggregates via intermolecular N-H⋅⋅⋅O=C interactions exclusively. The Fpm and Fom isomers both crystallize as two polymorphs with Fpm_O (N-H⋅⋅⋅O=C chains, P-syn) and Fpm_N (N-H⋅⋅⋅N chains, P-anti) both in P2(1)/n (Z' = 1) differing by their meta-N atom locations (P-syn, P-anti; N(pyridine) referenced to N-H), whereas the disordered Fom_O is mostly P-syn (Z' = 6) compared with Fom_F (P-anti) (Z' = 1). In the Fxo triad twisted dimers form cyclic R(2)(2)(8) rings via N-H⋅⋅⋅N interactions. Computational modelling and conformational preferences of the isomer grid demonstrate that the solid-state conformations generally conform with the most stable calculated conformations except for the Fxm triad, while calculations of the Fox triad predict the intramolecular N-H⋅⋅⋅F interaction established by spectroscopic and crystallographic data. Comparisons of Fxx with related isomer grids reveal a high degree of similarity in solid-state aggregation and physicochemical properties, while correlation of the melting point behaviour indicates the significance of the substituent position on melting point behaviour rather than the nature of the substituent.
The three title isomers, 4‐, (I), 3‐, (II), and 2‐fluoro‐N′‐(4‐pyridyl)benzamide, (III), all C12H9FN2O, crystallize in the P21/c space group (No. 14) with similar unit‐cell parameters and are isomorphous and isostructural at the primary hydrogen‐bonding level. An intramolecular C—H...O=C interaction is present in all three isomers [C...O = 2.8681 (17)–2.884 (2) Å and C—H...O117–118°], with an additional N—H...F [N...F = 2.7544 (15) Å] interaction in (III). Intermolecular amide–pyridine N—H...N hydrogen bonds link molecules into one‐dimensional zigzag chains [graph set C(6)] along the [010] direction as the primary hydrogen bond [N...N = 3.022 (2), 3.049 (2) and 3.0213 (17) Å]. These are augmented in (I) by C—H...π(arene) and cyclic C—F...π(arene) contacts about inversion centres, in (II) by C—F...F—C interactions [C...F = 3.037 (2) Å] and weaker C—H...π(arene)/C—H...F contacts, and in (III) by C—H...π(arene) and C=O...O=C interactions, linking the alternating chains into two‐dimensional sheets. Typical amide N—H...O=C hydrogen bonds [as C(4) chains] are not present [N...O = 3.438 (2) Å in (I), 3.562 (2) Å in (II) and 3.7854 (16) Å in (III)]; the C=O group is effectively shielded and only participates in weaker interactions/contacts. This series is unusual as the three isomers are isomorphous (having similar unit‐cell parameters, packing and alignment), but they differ in their interactions and contacts at the secondary level.
The title compound, C19H12F2N2O2, a 2:1 product of the reaction of 2-fluorobenzoyl chloride and 2-aminopyridine, crystallizes with a disordered 2-fluorobenzene ring adopting two conformations [ratio of occupancies = 0.930 (4):0.070 (4)] in one of the two independent molecules (differing slightly in conformation) comprising the asymmetric unit. In the crystal structure, C—H⋯O and C—H⋯π(arene) interactions are present.
The title compound, C19H12F2N2O2, a 2:1 product of the reaction of 3-fluorobenzoylchloride and 2-aminopyridine crystallizes with a disordered 3-fluorobenzene ring adopting two conformations [ratio of occupancies 0.959 (4):0.041 (4)]. In the crystal structure, there are no classical hydrogen bonds and interactions comprise C—H⋯O in the form 2(C—H)⋯O=C [with motif R 2 1(5)]; C—H⋯π(arene) interactions are also present.
Halogen atoms are typically located at the periphery of organic molecules and are thus ideally positioned to be involved in intermolecular interactions. Halogen bonding (XB) describes any interaction where halogen atoms function as electrophilic species. XB can be described by the general scheme D•••X-Y where X is the electrophilic halogen atom (Lewis acid, XB donor), D is a donor of electron density (Lewis base, XB acceptor), and Y is carbon, nitrogen, halogen, etc. (Scheme 1)[1]. The main features of the interaction will be given and the close similarity with hydrogen bonding will become apparent. Some heuristic principles will be presented in order to develop a rational crystal engineering based on XB. The potential of the interaction will be shown by useful applications in different fields spanning synthetic chemistry, material science, and bioorganic chemistry.
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