An exploration of O—H⋯O and C—H⋯π interactions in a long-chain-ester-substituted phenylphenol: methyl 10-[4-(4-hydroxyphenyl)phenoxy]decanoate

The superstructure of 4-(9-methyloxycarbonylnonyloxy)phenylphenol is dominated by O—H⋯O and C—H⋯O hydrogen-bonding and C—H⋯π interactions. Hirshfeld surface, fingerprint plot, interaction energy and energy framework analyses were used to explore the nature and strength of the intermolecular interactions.


Chemical context
In a gel, the scaffold molecules (the gelator) assemble into a network of fibers, which trap large numbers of solvent molecules by way of non-covalent interactions (Weiss, 2014). Organogels, which are obtained by dissolving a small amount of a low-molecular-mass organic gelator in an organic solvent, have myriad uses, including drug delivery and biomedical diagnostics (Wu & Wang, 2016;Tibbitt et al., 2016), medical implants (Liow et al., 2016;Yasmeen et al., 2014), and tissue engineering (Xavier et al., 2015;Yan et al., 2015).
For a gel, self-assembly of a three-dimensional arrangement of molecules incorporating a large number of solvent molecules results in a thermodynamically stable state, whereas self-assembly followed by crystallization gives a solid. The factors resulting in gelation rather than crystallization are subtle and, as a result, there are few examples of single-crystal structure determinations of organogelators (Adhikari et al., 2016;Rojek et al., 2015;Cui et al., 2010;Martin et al., 2016;Geiger, Zick et al., 2017;.
Traditional hydrogen bonding, van der Waals forces, andand C-HÁ Á Á interactions play important roles in determining the stability of organogels and crystalline lattices. The combination of solid-state structural data obtained via X-ray diffraction analysis and interaction energies determined using computational techniques affords a powerful means of ISSN 2056-9890

Structural commentary
MBO10Me was isolated as a side product during the synthesis of the corresponding bis(ester-substituted)biphenyl, 4,4 0 -bis(9methyloxycarbonylnonyloxy)biphenyl, BBO10Me (see Scheme below). Although BBO10Me readily forms stable gels in a variety of solvents, MBO10Me does not behave as an organogelator in any of the solvents examined. The solid-state structures of BBO6Me and BBO6Et have been reported . BBO6Me behaves as an organogelator, but BBO6Et does not. The two compounds are isostructural and a comparative energy framework analysis (Turner et al., 2015) showed that the ethyl ester exhibits weaker intercolumnar interactions. The structural characterization of MBO10Me was undertaken in an effort to better understand the subtle differences in the strengths of the intermolecular interactions that control gelation. Fig. 1 shows the molecular structure of MBO10Me with the atom-labeling scheme. The dihedral angle between the two phenyl rings is 6.6 (2) and the C6-C1-C7-C12 torsion angle is À6.3 (4) . The ester chain adopts a straight-chain conformation, as is found in similar structures (Geiger, Zick et al., 2017;, which maximizes the intermolecular van der Waals interactions. The ester chain is, however, tilted out of the plane of the phenyl ring to which it is attached, with a C13-O2-C4-C3 torsion angle of 173.2 (3) .

Supramolecular features
As seen in Table 1 and Fig. 2, O-HÁ Á ÁO hydrogen bonds, in which the phenol group is the donor and the ester carbonyl group is the acceptor, and C-HÁ Á ÁO hydrogen bonds, in which the methyl group is the donor and the phenol is the acceptor, result in sheets parallel to the ac plane that are composed of interlinked R 4 4 (52) rings. The structure is extended into the third dimension via C-HÁ Á Á interactions involving phenyl ring hydrogen atoms and the systems of both phenyl rings (see Fig. 3 and Table 1). The result is a columnar structure similar to that observed in BBO6Me and BBO6Et  with an important difference: the columns are joined by an O-HÁ Á ÁO hydrogenbonding network in which the phenol is the donor and the ester carbonyl is the acceptor (Table 1 and Fig. 2).

Database survey
A search of the Cambridge Structural Database (CSD, V5.38, last update May 2017; Groom et al., 2016) for 4,4 0 -biphenols yielded 21 structures, excluding those in which the biphenol was coordinated to a metal. There are 15 examples of structures with biphenol molecules in which the dihedral angle between phenyl rings is 2 or less. [The calculated rotational barrier in the gas phase for 4,4 0 -biphenyl is ca 8 kJ mol À1 (Johansson & Olsen, 2008).] In the title compound, MBO10Me, the dihedral angle between the two phenyl rings is 6.6 (2) .  Table 1 Hydrogen-bond geometry (Å , ).

Figure 1
View of the molecular structure of MBO10Me, showing the atom-labeling scheme. Displacement ellipsoids are drawn at the 50% probability level.

Hirshfeld surface analysis, interaction energies
Using CrystalExplorer17 (Turner et al., 2017), the Hirshfeld surface and fingerprint plots were calculated (see Section 9 for details). As seen in Fig. 4, the closest intermolecular contacts involve the phenol group. Each of the types of hydrogenbonding interactions are clearly discernible in the fingerprint plot. The presence of C-HÁ Á Á bonding is also apparent. The HÁ Á ÁO and HÁ Á ÁC surface-contact coverages are 17.6% and 22.9%, respectively. No significantinteractions are are observed [the closest ring centroid-to-ring centroid distance is 4.921 (2) Å ]. Table 2 shows the results of the interaction energy calculations (see Section 9 for details). The results are represented graphically in Fig. 5 as framework energy diagrams (Turner et al., 2015). In an energy framework, the cylinder size correlates to the strength of the interaction. The framework is reminiscent of that observed in the bis(substituted) compounds with interactions that are primarily dispersive in nature between the six nearest intracolumnar neighbors. However, the intercolumnar interactions, which possess the O-HÁ Á ÁO hydrogen bonding, have greater electrostatic components. These findings show that the van der Waals and C-HÁ Á Á interactions result in significantly favorable intermolecular attractive forces, surpassing the strength of the intercolumnar O-HÁ Á ÁO interaction.
Based on the three structures reported to date, a columnar supramolecular structure appears to be a common feature of long-chain ester compounds with a biphenyl core. The findings reported herein support the rationale posited for the difference in gelation ability exhibited by BBO6Me and BBO6Et

Figure 3
Partial crystal packing diagram of MBO10Me, emphasizing the C-HÁ Á Á interactions. Only H atoms involved in these interactions are shown. Table 2 Interaction energies.
N refers to the number of molecules with an R molecular centroid-to-centroid distance (Å ). Energies are in kJ mol À1 .    Energy framework diagram for separate electrostatic (top, red) and dispersion (middle, green) components of MBO10Me and the total interaction energy (bottom, blue). The energy factor scale is 120 and the cut-off is 5.00 kJ mol À1 . , i.e., the strength of the intercolumnar interactions. The O-HÁ Á ÁO hydrogen bonds between columns in MBO10Me are about twice the strength of the intercolumnar interactions found in BBO6Me (À15.5 kJ mol À1 ) and three times that found in BBO6Et (À10.1 kJ mol À1 ). A possible explanation for the lack of gelation ability of MBO10Me is that the stronger intercolumnar interactions favor formation of the crystal lattice rather than incorporation of a large number of solvent molecules giving a gel.

Gelation studies
The gelation behavior of MBO10Me was examined in noctanol, n-hexanol, n-butanol and ethanol. Gelation attempts were carried out using a 2.0% (wt/wt) of the compound and solvent in a screw-capped vial. The mixture was heated until all the solid dissolved and was then allowed to cool to room temperature. Formation of a gel is indicated when inversion of the vial yields no movement of the solvent.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All H atoms were located in difference-Fourier maps. H atoms were refined using a riding model, with C-H = 0.95 Å and U iso (H) = 1.2U eq (C) for the aromatic positions, C-H = 0.99 Å and U iso (H) = 1.2U eq (C) for the methylene groups, and C-H = 0.98 Å and U iso (H) = 1.5U eq (C) for the methyl group. The phenolic H atom was refined freely, including the isotropic displacement parameter. A meaningless Flack parameter and corresponding standard deviation were observed.

Hirshfeld surface, fingerprint plots, interaction energy calculations
Hirshfeld surfaces, fingerprint plots, interaction energies and energy frameworks (Turner et al., 2015) were calculated using CrystalExplorer17 (Turner et al., 2017). Interaction energies were calculated employing the CE-B3LYP/6-31G(d,p) functional/basis set combination and are corrected for basis set superposition energy using the counterpoise method. The interaction energy is broken down as  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.12 e Å −3 Δρ min = −0.18 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.