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Isomorphism in Two (E)-1-(4-Halophenyl)-N-[1-(4-Methylphenyl)-1H-Imidazol-4-yl]Methanimines (Halide = Cl, Br)

The crystal structures of two imidazole-4-imines, (E)-1-(4-chlorophenyl)-N-[1-(4-methylphenyl)-1H-imidazol-4-yl]methanimine (1, C17H14ClN3), and (E)-1-(4-bromophenyl)-N-[1-(4-methylphenyl)-1H-imidazol-4-yl]methanimine, (2, C17H14BrN3), are isomorphous, the isostructurality index is 99.4 %. Both compounds crystallize in the triclinic space group P-1 with unit cell parameters at 100(1) K as follows: for (1), a = 7.9767(5) Å, b = 10.9517(7) Å, c = 16.6753(12) Å, α = 80.522(6)°, β = 87.046(6)°, γ = 89.207(5)°, and for (2), a = 8.0720(7) Å, b = 10.9334(10) Å, c = 16.8433(13) Å, α = 81.161(7)°, β = 86.605(7)°, γ = 89.505(7)°. The structures contain two symmetry—independent but conformationally similar molecules in the asymmetric unit (Z’ = 2). In both compounds the overall twist of the molecule, defined as the dihedral angle between the terminal phenyl ring planes is significant, around 56°. The crystal packing is determined mainly be weak specific intermolecular interactions: the C–H···N hydrogen bonds connect molecules into infinite chains, and the chains are linked via C–H···X hydrogen bonds and by π–π interactions. This study illustrates the significant role of the weak interactions, which—in spite of their weakness—can robustly repeat in the crystal structures of similar compounds. Two imidazole-4-imines, (E)-1-(4-chlorophenyl)-N-[1-(4-methylphenyl)-1H-imidazol-4-yl]methanimine, and (E)-1-(4-bromophenyl)-N-[1-(4-methylphenyl)-1H-imidazol-4-yl]methanimine, are isomorphous, the isostructurality index is 99.4 %.


Introduction
Nucleophilic substitution of the nitroimidazole system proved to be a versatile tool for modification of the imidazole moiety [1,2]. The presence of the nitro group, essential for the reaction mentioned, becomes not only unnecessary but it can interfere with the next potential synthetic transformation. The reduction of the nitro group, relatively easy in benzene derivatives, has proven challenging in azole systems due to metastability of the resulting amines [3,4]. To this end the reduction and further protection of the nitro functionality appears to be an important synthetic problem.

Results and Discussion
Compounds (1) and (2) are isostructural. It is possible to estimate the degree of isostructuralism using the descriptors introduced by Kálmán et al. [8] in his work on socalled ''main-part'' isostructuralism, or a more specific index defined by Kubicki and Szafrański [9]. The unit-cell similarity index is 0.01, very close to the ideal value of 0. Also the values of both isostructurality indices are close to the ideal values: according to [8] 99.3 % (ideal value 100 %), and according to [9] 0.985 (ideal value 1.0). The isostructuralism can be seen in the almost identical packing diagrams (Fig. 3).
Both compounds crystallize with two symmetry-independent molecules A and B in the asymmetric part of the unit cell. The results of the normal probability plot analysis [10,11] for bond lengths and angles show that there are no   Due to the isostructurality, the discussion can be made on one of the compound only (1), the geometrical values for the second one (2) will be given in square brackets.
The bond lengths and angles pattern is typical; the short N41-C42 bonds prove the double-bond character. The intraannular bond angles within the phenyl rings reflect the nature of the substituents, generally in accordance with the findings of Domenicano and Murray-Rust [12] and Exner and Böhm [13].
The conformation of the molecule can be described by the dihedral angles between the subsequent planar fragments: methylphenyl group (A), imidazole ring (B), CÀN CÀC bridge (C) and halophenyl group (D). All these groups are essentially planar, maximum deviation from the leastsquares plane is as small as 0.006 (2) Table 1. It can be found that the 'sense' of the twists between the subsequent planes is the same, and the overall twist is roughly equal to the sum of the individual angles.
The crystal packing is determined mainly by the specific, directional weak interactions. The main motif, an *ABAB* chain along [010] direction, is created by relatively short and linear C5-HÁÁÁN3 hydrogen bonds ( Fig. 4; Table 2). In the high resolution structure of 1-phenyl-4nitroimidazole [14] the topological analysis of the electron density distribution showed the critical points connected to the identical C5-HÁÁÁN3 hydrogen bonds. The neighboring chains are connected along [101], the long axis of the molecule, by very weak C-HÁÁÁX and C-HÁÁÁp hydrogen bonds (Table 2; Fig. 5).

Experimental
Preparation of Compounds 1-(4-Methylphenyl)-4-nitroimidazole was reduced in a current of hydrogen under atmospheric pressure at room  temperature in methanol as a solvent. Freshly prepared Raney nickel was used as a catalyst. The TLC analysis showed that the starting material was consumed within an hour giving an undefined intermediate. The reaction was carried out until the entire intermediate compound was reacted out-usually 6 h. After filtering off the metal catalyst, because of its reactivity [2,3], the resulting amine, was not worked out but immediately treated with aromatic  aldehydes. The addition of a catalytic amount of acetic acid caused the precipitation of the corresponding Schiff's bases in good yields.   Crystallography Diffraction data were collected at 100(1) K by the x-scan technique on an Agilent Xcalibur CCD-detector diffractometer with Sapphire detector and using graphite-monochromatized Mo K a radiation (k = 0.71073 Å ). The data were corrected for Lorentz-polarization as well as for absorption effects [15]. Accurate unit-cell parameters were determined by a least-squares fit of 3,746 (1) and 5,533 (2) reflections of highest intensity, chosen from the whole experiment. The structures were solved with SIR92 [16] and refined with the full-matrix least-squares procedure on F 2 by SHELXL97 [17]. Scattering factors incorporated in SHELXL97 were used. The function Rw(jF o j 2 -jF c j 2 ) 2 was minimized, with w -1 =[r 2 (F o ) 2 ? (AÁP) 2 ? BÁP] (P = (max (F o 2 , 0) ? 2F c 2 )/3). The final values of A and B are listed in Table 1. Anisotropic atomic displacement parameters were refined for all non-hydrogen atoms. The hydrogen atoms were placed in geometrically idealized positions, and refined as rigid groups with their U iso 's as 1.2 or 1.5 (methyl) times U eq of the appropriate carrier atom. Programs XP [17] and Mercury [18] were used for graphic representations, used in Figures. Relevant crystal data are listed in Table 3, together with refinement details.
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