Crystal structure of 4-[(3-methylbut-3-enoyl)oxy]phenyl 4-n-hexyloxybenzoate

The molecule is non-planar and the dihedral angle between the phenyl rings is 50.72 (4)°. Only a weak directional interaction of the C—H⋯O type combines molecules in infinite chains running along the a axis.


Chemical context
Phenylbenzoates bearing a rather long aliphatic substituent at the benzene ring are potentially mesogenic compounds. On melting, these compounds often form smectic or nematic phases. Cases where these compounds exhibit a monotropic mesomorphism, i.e. do not form the mesophase on melting but instead form it on cooling the isotropic melt, are also known. The structural studies of these compounds are of great interest as these investigations make it possible to clarify the structure of the mesophase and propose a mechanism of phase transitions in a crystal-mesophase-isotropic system.
In this work we performed an X-ray structural determination and DSC study of the title compound. According to DSC the compound is non-mesomorphic, exhibiting three solidstate modifications: Cr III 367.7 K Iso 350.6 K Cr II 349.9 K Cr I .

Structural commentary
The unit cell contains one independent molecule whose structure is shown in Fig. 1. The molecule is non-planar. Five planar fragments can be selected in it, viz. benzene rings C8-C13 (plane I) and C2-C7 (plane II), ester groups C2/C1/O1/ O2 (plane III) and O4/O5/C20/C21 (plane IV) and the hexyloxy group O3/C14-C19 (plane V). The dihedral angles between the planes I/II, II/III, II/V, I/III and I/IV are 50.72 (4), 4.84 (5), 7.05 (3), 52.82 (4) and 55.50 (5) , respectively. According to the CSD Groom et al., 2016), the dihedral angle between the planes of the benzene rings in phenylbenzoates varies over a rather wide range (30-90 ) having a normal distribution with the maximum at $60 . The obtained values of the dihedral angles in the structure provide evidence that the ester group C2/C1/O1/O2 is in a -conjugation with the benzene ring C2-C7 bonded to the ester group through a C-C bond and is out of -conjugation with the benzene ring C8-C13 bonded with it through a C-O bond. The same feature is characteristic of the second ester group bounded with the benzene ring C8-C13 through a C-O bond. This group is also strongly rotated from the plane of the indicated benzene ring and does not participate in conjugation with it. As is usual for liquid crystal compounds with a rather long alkyloxy chain O-C n H 2n+1 (n > 4), this substituent has an extended structure and its plane is nearly coplanar with the plane of the corresponding benzene ring.

Supramolecular features
It is known that crystal packing of mesogenic compounds is characterized by certain features, one of which is the separation of the packing into alternating aromatic and aliphatic areas, as shown in Fig. 2. Another feature is that the aromatic areas are closely packed, whereas the aliphatic areas have a very loose crystal packing. The close packing is formed as a result of many non-directional van der Waals and weak directional interactions. The most typical directional interactions are weak hydrogen bonds C-HÁ Á ÁO/N,stacking and C-HÁ Á Á interactions (Nangia, 2002;Janiak, 2000;Chen et al., 2009), as well as usual hydrogen bonds. The loose aliphatic areas involve only a few van der Waals contacts. These peculiarities bring about specific melting of the mesogenic compounds. Upon a rise in temperature, melting starts from the loose aliphatic areas, whereas the aromatic areas retain their ordering over a certain time, resulting in mesophase formation. All these peculiarities have been observed in the crystal packing of alkyl-and alkyloxycyanobiphenyls (Kuz'mina & Kucherepa, 2011;Kuz'mina et al., 2012), alkyloxybenzoic acids (Kuz'mina et al., 2009), n-(alkyloxybenzilidene)-n 0 -tolyidines (Kuz'mina et al., 2016) and phenylbenzoates (Konstantinov et al., 2013;Kuz'mina et al., 2014), which represent a precursor of the mesophase.
The crystal packing of the title compound is shown in Fig. 3. Both aforementioned features of mesogenic compound crystal packing are lacking in the compound. An analysis of the intermolecular distances of the aliphatic chain atoms indicates that there are no loosely packed areas, which explains lacking the mesomorphism for this compound.
In the crystal, only C9-H9Á Á ÁO1 contacts between translationally (along the a axis) related molecules may be considered to be weak hydrogen bonds (Table 1, Fig. 4). The H9Á Á ÁO1 distances are equal to 2.47 Å , which corresponds to common values. The H9 atom is rather acidic to participate in a weak hydrogen bond since it is situated at the ortho position to the accepting ester group. A detailed analysis of the crystal Two variants of crystal packing for mesogenic compounds; rectangles denote aromatic fragments and zigzags denote aliphatic side chains; d 2 > d 1 .

Figure 1
The molecular structure of the title compound. Displacement ellipsoids are shown at the 50% probability level. The H atoms are presented as a small spheres of arbitrary radius.
packing did not reveal contacts that could be considered to be weak directional interactions of other types. Interestingly, on cooling the isotropic melt of the compound, the formed crystal modifications Cr II and Cr I differ from that found in the crystal modification grown from solution at room temperature. Nevertheless, these modifications are also non-mesomorphous. The lack of mesomorphism of the compound in all crystal modifications may be explained by the occurrence of the branched metacryl group at the benzene ring C8-C13 that efficiently fills the adjacent areas in the crystal packing, thus restricting the displacement of the aliphatic chains.

Synthesis and crystallization
The compound was prepared by the reaction of 4-n-hexyloxybenzoic acid with 4-methacryloyloxyphenol using N,N-dicyclohexylcarbodiimide in dichloromethane solution according to the procedure described by Hassner & Alexanian (1978). The product was purified by column chromatography and then recrystallized from acetone. Its purity was checked by thin-layer chromatography.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were located from a difference Fourier synthesis and refined isotropically without constrains and restrains.  Computer programs: SMART and SAINT (Bruker, 2009), SHELXS97 and SHELXL97 (Sheldrick, 2008) and OLEX2 (Dolomanov et al., 2009).

Figure 4
Translation related (along the a axis) molecules.

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.