Crystal structure of butane-1,4-diyl bis(furan-2-carboxylate)

The asymmetric unit of the title compound consists of one half-molecule, the whole all-trans molecule being generated by an inversion centre. In the crystal, the molecules interconnected by C—H⋯O and C—H⋯π interactions.


Chemical context
To suppress global warming, materials derived from fossil fuels have been attempted to be replaced with plant-based products. For example, plant-derived furan-2,5-dicarboxylic acid is expected to be substituted for terephthalic acid, raw materials of aromatic polyesters such as poly(ethylene terephthalate) and poly(butylene terephthalate) (abbreviated herein as PBT) (Gandini et al., 2016); therefore, in the future, the substitute for PBT will possibly be poly(butylene 2,5furandicarboxylate) (PBF), the alternate copolymer of furan-2,5-dicarboxylic acid and butane-1,4-diol.
The ultimate mechanical stiffness of polymers mostly corresponds to the crystalline modulus in the chain-axis direction at 0 K and depends largely on the chain conformation (Kurita et al., 2018). Therefore, it is of significance to determine conformations of polymers in crystal and to relate such structural information to their mechanical properties. PBT is known to exhibit two crystal structures of and forms (Yokouchi et al., 1976;Desborough & Hall, 1977). The form adopts gauche + (g + ), gauche + (g + ), trans (t), gauche À (g À ) and gauche À (g À ) conformations in the O-CH 2 -CH 2 -CH 2 -CH 2 -O unit (referred hereafter to as the spacer), while the form has a near all-trans spacer. It is known that mechanical stresses induce the -totransformation, which will absorb impact and avoid fracture. Owing to such remarkable structural characteristics, PBT has been used for engineering plastics superior in impact resistance.
Single crystal X-ray structure analysis of butane-1,4-diyl dibenzoate (BT), a model compound of PBT, showed that its spacer lies in a tgttt conformation different from that of PBT (Palmer et al., 1985). A powder X-ray diffraction study on PBF (Zhu et al., 2013) has estimated dihedral angles of its spacer to be 180 (trans), 66 (+synclinal), 99 (+anticlinal), 124 (+anticlinal) and 148 (+anticlinal) and hence quite different from those of PBT and BT. In this study, we have conducted a single-crystal X-ray diffraction experiment on a model ISSN 2056-9890 compound of PBF, butane-1,4-diyl bis(furan-2-carboxylate) (BF), to investigate its spacer conformation and intermolecular interactions and compare them with those of PBF, BT and PBT.

Structural commentary
The BF spacer of the title compound adopts an all-trans conformation ( Fig. 1), which is different from those of PBF as well as PBT and BT. The unit cell includes four molecules, each of which is located on an inversion centre, and hence one half-molecule corresponds to the asymmetric unit. The furan O1/C1-C4 ring is planar, while the carboxy O2/C5/O3 plane is slightly twisted form the furan ring, with a dihedral angle of 4.00 (15) .

Supramolecular features
In the crystal, the BF molecules are interconnected by C-HÁ Á ÁO interactions (Table 1) to form a molecular sheet parallel to (102) (Fig. 2). The sheets are further linked via a C-HÁ Á Á interaction (Table 1 and Fig. 3), forming a threedimensional network. In the BT crystal (Palmer et al., 1985), the benzene rings face to each other to form intermolecularinteractions with centroid-centroid distances of 4.169 (2) and 3.910 (2) Å . In addition, the benzene rings act as donors in C-HÁ Á Á interactions. As stated above, BF seems to prefer the C-HÁ Á ÁO interactions and adopt a spacer conformation so as to fulfill the C-HÁ Á ÁO interactions efficiently, whereas BT and PBT (Yokouchi et al., 1976;Desborough & Hall, 1977) tend to adapt a spacer conformation to forminteractions.

Figure 2
A packing diagram of the title compound, showing the molecular sheet formed by C-HÁ Á ÁO interactions (blue lines).

Figure 3
A packing diagram of the title compound, showing the intermolecular C-HÁ Á Á interactions (blue dotted lines) between the molecular sheets.

Figure 1
The molecular structure of the title compound, showing the atomlabelling scheme. Atoms with suffix a are generated by the symmetry operation (Àx + 3 2 , Ày + 1 2 , Àz). Displacement ellipsoids are drawn at the 50% probability level. H atoms are represented by spheres of arbitrary size.

Synthesis and crystallization
Furan-2-carbonyl chloride (2.2 ml, 22 mmol) was added dropwise under a nitrogen atmosphere to butane-1,4-diol (0.89 ml, 10 mmol) and pyridine (6.0 ml) put in a three-necked flask dipped in ice-water and stirred at room temperature for 28 h. The reaction mixture was extracted with chloroform (10 ml) and water (10 ml), and the organic layer was washed thrice with aqueous sodium bicarbonate (10%), dried over anhydrous sodium sulfate overnight and filtrated. The filtrate was condensed on a rotary evaporator, and the residue was dried in vacuo and identified by 1 H and 13 C NMR as BF (yield 73%).
A small amount of BF was dissolved in benzene in a small phial, which was put in a larger phial including a small volume of n-hexane. The outer vessel was capped and stood still. After a few weeks, single crystals were found to precipitate at the bottom of the inner phial.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were geometrically positioned with C-H = 0.95 and 0.99 Å for the aromatic and methylene groups, respectively, and were refined as riding with U iso (H) = 1.2U eq (C).

Special details
Experimental. SADABS (Sheldrick, 1996) 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.