Formation of isostructural solid solutions in 2,6-disubstitutedN-phenylformamides andN-phenylthioamides

Abstract

In order to investigate possible isostructural solid solutions of disubstituted N-phenylformamides and thioamides, we have studied the re-crystallization of pairs of compounds selected from 2,6-difluoro-N-phenylformamide (I), 2,6-dichloro-N-phenylformamide (II), 2,6-dimethyl-N-phenylformamide (III), 2,6-dichloro-N-phenylthioamide (IV), 2,6-dimethyl-N-phenylthioamide (V), 2,6-diisopropyl-N-phenylformamide (VI) and 2,6-diisopropyl-N-phenylthioamide (VII). For single-component 2,6-disubstituted-N-phenylformamides only the trans form occurs in the pure crystal, while for thioamides the cis form occurs, with only one exception. By forming solid solutions of pairs of these molecules the resulting structures all adopt similar N-H...O/S chains in the crystals. Solid solutions (1), (2) and (3), resulting from the mixing of (I) and (II), (II) and (III), and (IV) and (V), respectively, are all isostructural with each other (space group Pbca). Only co-crystal (1) is isostructural to both starting materials, while (2) is isostructural to only one of the starting pair, (II). Solid solution (3), which adopts the same Pbca structure as (1) and (2), is different to the monoclinic structures of both the reactants. Solid solution (4) is monoclinic, with similar hydrogen-bonded chains, and isostructural to the two components, resulting from the composition from the mixing of (VI) and (VII). Isostructural indices were used to quantify crystal-packing similarities and differences. Occupancy factors of the reactants in each co-crystal differ widely.

N-(2,3,5,6-Tetrafluoropyridin-4-yl)formamide

“…The formamide group and pyridine ring in (I) are somewhat twisted with a dihedral angle of 13.21 (5) . This is similar to N-phenyl-data reports formamide (Omondi et al, 2014) (dihedral angle between the benzene ring and formamide groups = 10.5 ) but in contrast to N-(2,6-difluorophenyl)formamide (Omondi et al, 2009b) and N-(2,6-dibromophenyl)formamide (Omondi et al, 2009a) where the equivalent dihedral angles are 58.4 and 83.2 , respectively. The latter structures indicate that the steric effects likely increase from H to F to Br; however, this large deviation from planarity is not observed in (I).…”

“…1). The amide bond lengths for (I) are 1.218 (3) A ˚and 1.366 (3) A ˚for the C O and C-N bonds, respectively, which are in good agreement with the corresponding bonds in the related compounds N-phenylformamide (Omondi et al, 2014), N-(2,6-difluorophenyl)formamide (Omondi et al, 2009b), and N-(2,6-dibromophenyl)formamide (Omondi et al, 2009a). As observed in other structures, the amide bond lengths for (I) are consistent with little to no N C/C-O bond resonance contribution.…”

“…The latter structures indicate that the steric effects likely increase from H to F to Br; however, this large deviation from planarity is not observed in (I). Furthermore, the torsion angle of 179.0 (2) for C3-N2-C6-O1 in (I) indicates a near anti-conformation, but in structures with a benzene ring, the carbonyl-benzene conformation is syn regardless of aromatic substituents (Omondi et al, 2009a(Omondi et al, ,b, 2014. Taken together, these indicate that the pyridine ring is playing a role in the structure beyond the sterics of the aromatic ring substituents.…”

Seeking Rules Governing Mixed Molecular Crystallization

“…Current understanding of principles governing the mixed crystallization of organic and inorganic compounds is based largely on work carried out decades ago by Kitaigorodsky and co-workers, which has not been subjected to extensive reexamination. , These pioneering studies suggested that a series of mixed crystals with continuously variable compositions in all proportions can only be obtained when the components are similar enough to crystallize isostructurally . However, more recent work has challenged this notion and shown that mixed crystals with a wide range of compositions can be formed from pairs that are not known to have a close crystallographic relationship. − These recent advances, which include the discovery that mixed crystals can be effective seeds for inducing crystallization of the individual components, have made mixed crystallization an exciting area of research.…”

Solid Solutions in the Xanthone–Thioxanthone Binary System: How Well Are Similar Molecules Discriminated in the Solid State?

“…For example, as we have shown in our recent work−solid solutions may form when a similarly shaped molecule that does not disrupt the hydrogen bonding network in the crystal structure or can participate in this bonding equally well is introduced in the system. 10 Solid solutions have been discovered and studied for multicomponent systems of the following: enantiomers; 11−17 diastereomers; 13,18−21 molecules differing by single/double bonds between two carbon atoms; 22,23 molecules differing by halogen (and halogen/methyl) substituents; 24−28 molecules differing by S/O atoms; 29,30 molecules differing by F/H atoms; 31 aromatic systems differing by presence/absence of a heteroatom; 32−34 molecules differing by methyl/ethyl substituents. 35 Nevertheless, up to date, only somewhat rough and empirical guidelines could be established to define the criteria for solid solution formation, and there are cases when even very similar molecules do not mix in the solid state.…”