Crystal Structures of Potentially Mesomorphic Alkyloxy-Substituted 4-Nitrobenzene Derivatives (Alk–O–C₆H₄–NO₂, Alk = С₄Н₉, С₁₀Н₂₁, С₁₂Н₂₅)

Abstract

Three 4-alkyloxy-substituted nitrobenzene derivatives with Alk = С₄Н₉ (I), С₁₀Н₂₁ (II), and С₁₂Н₂₅ (III) were studied by X-ray diffraction and differential scanning calorimetry (DSC). For compound II, the X-ray diffraction data sets were collected at different temperatures (150, 303, 308, and 310 K). According to the DSC data, all three compounds exhibit no mesomorphism, either enantiotropic or monotropic. The crystal packing of I is not typical of mesomorphic compounds. In the crystal structures of II and III, there are no weak directional interactions between functional groups of the molecules, which could ensure the structuring of the melt.

INTRODUCTION

This work is a continuation of systematic research on potentially mesomorphic dichotomous organic compounds aimed to reveal the factors responsible for the occurrence or the absence of different types of mesomorphism, either enantiotropic or monotropic, in these compounds. Dichotomous compounds are composed of at least two chemically and structurally contrasting moieties, e.g., include a flexible aliphatic chain and a rigid aromatic moiety.

According to the previous studies [1–16], crystals of mesomorphic compounds are characterized by two main distinguishing features. First, their molecular packings consist of alternating loosely packed aliphatic layers and closely packed aromatic regions. Second, the closely packed layers are stabilized by weak directional interactions (secondary bonds) between functional groups of aromatic moieties. These interactions include hydrogen bonds, weak С–Н⋅⋅⋅O/N and C–H⋅⋅⋅π hydrogen bonds, stacking interactions, etc. These interactions, along with dispersion interactions, are responsible for the close packing of the layers, while the loose structure of aliphatic regions is due to high conformational flexibility of aliphatic chains, resulting in an increased in their effective size.

It was shown [6, 16] that atoms of aliphatic chains vibrate with higher amplitudes compared to the atoms of the rigid core, and this difference increases with increasing temperature. This suggests that the melting of crystals should be anomalous. An increase in the temperature first leads to the violation of the ordering of the aliphatic chains with the retention of structuring of the closely packed aromatic regions followed by the appearance of a viscous structured liquid (a mesophase). The structuring of the mesophase is associated with the retention of secondary bonds in the crystal.

According to the observations, the occurrence of mesomorphic features in compounds of a homologous series (for example, in alkyloxy-substituted phenyl benzoates) depends on the melting point. The lower the melting point, the more probable the occurrence of mesomorphism in the compound. The mesophase is stable up to the temperature, at which secondary bonds that are responsible for the mesophase formation are broken. If the melting of the crystal occurs at rather high temperatures, weak interactions can be broken at temperatures below the melting point. In this case, the mesophase formation is impossible because of the absence of structuring elements in the melt.

Alkyloxy derivatives of benzene are usually low-melting compounds. Hence, these compounds are attractive candidates to study the mesomorphism and also for the refinement of the mesophase model.

Here we report and discuss the results of investigation of 4-alkyloxy-substituted nitrobenzene derivatives with Alk = С₄Н₉ (I), С₁₀Н₂₁ (II), and С₁₂Н₂₅ (III) by single-crystal X-ray diffraction (XRD) (for II, the X-ray diffraction data sets were collected at different temperatures (150, 303, 308, and 310 K)) and differential scanning calorimetry (DSC). The molecular structures of two derivatives (II and III) were studied in [17, 18] at 291 K. We repeated the study of the structures of these compounds in order to obtain the data not only on the geometry but also on the character of thermal motion of the atoms of the aliphatic and aromatic moieties and their dependence on the temperature.

EXPERIMENTAL

DSC Study

Compounds I, II, and III were purchased from Sigma-Aldrich. A sample comprising of several single crystals of compounds I, II, and III, which were grown from a benzene−acetonitrile mixture, was placed on a Linkam DSC600 optical DSC system equipped with an Olympus BX43 polarized light microscope. Thermograms were recorded at a scanning rate of 5 deg/min. According to the DSC analysis, all these compounds do not form a mesophase upon melting or cooling of the isotropic melt. The melting point (enthalpy of melting) of I, II, and III is 33.9°С (21 kJ/mol), 44.3°С (51 kJ/mol), and 50.4°С (57 kJ/mol), respectively. The cooling of the isotropic melt is accompanied by the crystallization at a temperature (enthalpy) of 20.4°С (20 kJ/mol), 25.7°С (48 kJ/mol), and 38.8°С (58 kJ/mol) for I, II, and III, respectively, i.e., below the melting point.

X-Ray Diffraction Study

Mesomorphic compounds often exhibit polymorphism. Therefore, in order to ensure the identity of the samples of compounds I, II, and III studied by two different methods (XRD and DSC), the X-ray diffraction data were collected from the single crystals, which were preliminary studied by DSC.

A single crystal was mounted on a Bruker SMART APEX-II diffractometer, and the unit cell parameters and X-ray diffraction intensities were measured (MoK_α radiation, graphite monochromator, ω-scanning technique). The structure of compound I was studied at 210 K; of compound III, at room temperature (296 K). The X-ray diffraction study of compound II was performed at 150, 303, 308, and 310 K.

The X-ray diffraction data were processed using the SAINT software [19]. The crystal structures were solved by direct methods and refined by the least-squares method based on F  ² with anisotropic displacement parameters for all non-hydrogen atoms. Although it was possible to locate hydrogen atoms in difference Fourier maps, they were positioned geometrically and included in the refinement using a riding model. Principal crystallographic data and the X-ray diffraction data collection and refinement parameters are given in Table 1.

Table 1.   Crystallographic data and the X-ray diffraction data collection and refinement statistics for structures I, II, and III

All calculations were performed using the Olex-2 [20], ShelXT [21], and XH [22] program packages. The structural data were deposited with the Cambridge Crystallographic Data Centre (CCDC 2019429 (I), 2019430 (II 150 K), 2019431 (II 303 К), and 2109432 (III)) and are available at deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk/data_request/cif.

RESULTS AND DISCUSSION

DSC Study

According to the DSC analysis, all the three compounds do not form a mesophase both upon heating of the crystals or cooling of the isotropic melt. These compounds are characterized by the formation of supercooled melts, i.e., their melting points are much higher than the crystallization temperatures. The ratio of the melting point to crystallization temperature is 33.9/20.4, 44.3/25.7, and 50.4/38.8°С for compounds I, II, and III, respectively. This can apparently be accounted for as follows. The thermal motion of atoms of the aliphatic group increases to a greater extent compared to the atoms of the aromatic moiety, which significantly affects the overall shape of the molecules. An increase in the temperature can lead to significant differences in the vibration amplitudes of the aromatic and aliphatic moieties. Hence, the effective shape of the molecules at the temperature of crystallization from the solution and the melt may be greatly different. Apparently, the cooling of the melt can lead even to the formation of another crystal modification. Since the size of the molecule increases with increasing temperature, the crystal obtained from the melt will have a lower density and, consequently, a lower melting point. This is the most reasonable explanation for the difference in the melting points and crystallization temperatures.

X-Ray Diffraction Study Molecular structure

The crystals of all three compounds contain two crystallographically independent molecules with very similar geometric parameters. The molecular structure of I and the atomic displacement ellipsoids determined from the experimental data at 210 K are presented in Fig. 1. A slight increase in the thermal motion of the terminal atoms of the alkyl chain compared to the other atoms is observed.

Fig. 1.

Two crystallographically independent molecules in crystal I: (а) the molecular structures with displacement ellipsoids drawn at the 50% probability level; (b) the superimposition of two molecules.

The crystallographically independent molecules adopt different conformations (Fig. 1b). In both molecules, the С2–С1–О1–С7 (С2А–С1А–О1А–С7А) torsion angles have almost identical values (4.47° and 4.64°) due to the conjugation of one electron pair of the oxygen atom and the π system of the benzene ring. The butyl moiety in both molecules is nearly planar, and this plane is inclined to the plane of the benzene ring. The dihedral angle between these planes in one molecule is 5.18° (the twist and bend components of this angle are 4.8° and 7.8°, respectively); in another molecule, 109.5° (109.6° and 40.7°, respectively).

The structures of the crystallographically independent molecules of II and the atomic displacement ellipsoids determined from the experimental data at 303, 308, and 310 K, which are slightly lower than the melting point (317.3 K), are presented in Fig. 2. Table 2 summarizes the equivalent isotropic displacement parameters of three terminal atoms of the aliphatic chain (С14, С15, and С16) and, for the sake of comparison, of the first atom of this chain (С7) bound to the oxygen atom at 303, 308, and 310 К. As can be seen in Table 2, an increase in the temperature has only a slight effect on the character of thermal motion of the atoms in the molecule, as opposed to mesomorphic 4-(decyloxybenzylidene)-4'-toluidine studied previously at 120, 295, 320, and 333 K [6]. For the latter compound, not only the thermal motion of the terminal moieties (propyl and then butyl) of the aliphatic chain increases but also their disorder enhances with increasing temperature, which confirms the assumption that the melting of mesomorphic compounds starts in the aliphatic chains of the molecules.

Fig. 2.

Molecular structure of II at 303, 308, and 310 K with displacement ellipsoids drawn at the 70% probability level.

Table 2.   Equivalent isotropic displacement parameters (Ų) of selected atoms of the aliphatic chain in two crystallographically independent molecules of II at 303, 308, and 310 K

An increase in the thermal vibrations of the terminal moieties of the aliphatic chain is pronounced, although it is weaker compared to that observed for 4-(decyloxybenzylidene)-4'-toluidine. The latter compound, unlike II, is mesomorphic. An increase in the thermal vibrations of aliphatic atoms with increasing distance from the initial С7 atom is also observed in compound III, the independent molecules of which (Fig. 3) are structurally similar to molecules of II.

Fig. 3.

Molecular structure of III with the atom-numbering scheme; displacement ellipsoids are drawn at the 50% probability level.

Crystal packing

The crystal packing of I is not typical of mesomorphic crystals (Fig. 4). In the crystal of I, the aromatic and aliphatic regions are not separated. The crystal packings of isostructural compounds II and III are qualitatively similar. A fragment of the crystal packing of II is shown in Fig. 5. In this packing, the aliphatic layers alternate with aromatic layers, which is typical of mesomorphic crystals. The shortest intermolecular distances between atoms of adjacent aliphatic chains in II are larger than the sums of the van-der-Waals radii of the corresponding atoms (Fig. 6). A similar pattern is observed in the crystals of III, which differs only in the corresponding distances; however, these distances are also larger than the standard distances for the С⋅⋅⋅С, Н⋅⋅⋅Н, and С⋅⋅⋅Н contacts (~3.6, 2.4, and 3.2 Å, respectively). Therefore, one condition of mesomorphism is satisfied in these crystal structures.

Fig. 4.

Fragment of the crystal packing of I.

Fig. 5.

Fragment of the crystal packing of II.

Fig. 6.

Shortest distances (Å) between atoms of adjacent aliphatic moieties (determined from the experimental data at 120 K).

The situation is different with the fulfillment of the second condition that requires the existence of weak directional interactions between functional groups of aromatic moieties. All three compounds exhibit a tendency to form stacking dimers. In structure I the distance between the centroids of the benzene rings in the dimer (Fig. 7) is 3.87 Å in both independent molecules, which may correspond to a weak stacking interaction. However, since the first characteristic feature of the crystal packing of mesomorphic compounds is absent, this compound is of interest only because it provides evidence for the possible formation of weak directional interactions between functional groups of the aromatic moiety in homologs of this series. Besides, a weak С–Н⋅⋅⋅О hydrogen bond (Н⋅⋅⋅О, 2.60 and 2.72 Å) can exist in this compound; the corresponding H atoms are acidic because they are located in the benzene ring in the ortho position to the electron-withdrawing AlkO substituent.

Fig. 7.

Fragment of the crystal packing of I; the distances are given in Å.

The crystal packing of III (Fig. 8) exhibits a tendency to form a stacking dimer, with the benzene rings being arranged in an antiparallel centrosymmetric fashion, although the distances between their π systems (4.12 and 4.19 Å) are too large for this interaction to have an effect on the thermal behavior of the compound. Unlike structure I, weak С–Н⋅⋅⋅О hydrogen bonds are absent in structure III.

Fig. 8.

Fragment of the crystal packing of compound III with possible π-stacking interactions; the distances are given in Å.

The crystal packing of II contains similar structural units (Fig. 9). Although the distances between the π systems of the benzene rings in this structure are shorter than those in III, they are too long to maintain the structuring of the melt. Besides, the DSC thermograms of compounds II and III do not show thermal effects in the crystalline phase. This indicates that the observed intermolecular contacts are not specific interactions. Hence, the absence of mesomorphism in these compounds is attributed to the fact that the condition of the existence of weak directional interactions is not fulfilled in this crystal. These interactions are absent due apparently to insufficient conformational flexibility of the molecules. Actually, as is typical of alkyloxy-substituted benzenes, the plane of the aliphatic moiety in all molecules of II and III is coplanar with the plane of the benzene ring due to a conjugation of the oxygen lone pair with the π system of the ring. As opposed to alkyloxy-substituted benzene derivatives, alkyl-substituted benzenes are non-planar. In the latter derivatives, the orientation of the alkyl substituent with the same planar structure of the carbon skeleton is rigidly fixed with respect to the plane of the benzene ring, i.e., this substituent is more prone to adjustment of the molecules to form stacking dimers.

Fig. 9.

Fragment of the crystal packing of II; the distances are given in Å.

CONCLUSIONS

The investigation of AlkO–C₆H₄–NO₂, where Alk = C₄H₉ (I), C₁₀H₂₁ (II), and C₁₂H₂₅ (III), by X-ray diffraction and DSC demonstrated that these compounds do not exhibit mesomorphic properties. In the crystals of II and III, there are no weak directional interactions capable of providing the structuring of the melt. The crystal packing of I differs from that typical of mesomorphic compounds.