Crystal structures of 2-(benzenecarbothioyloxy)ethyl benzenecarbothioate and 2-(benzenecarbothioyloxy)ethyl benzoate

The title compounds are monomeric models for a polythionoester and a poly(ester-co-thionoester). The molecules adopt all-trans structures with intermolecular C⋯S close contacts and C—H⋯π interactions. Both crystals have almost the same molecular packing in space group P21/c.


Chemical context
Compounds expressed as C 6 H 5 -C( X)-Y-CH 2 -CH 2 -Y-C( X)-C 6 H 5 (X, Y = O or S) can be considered to be monomeric models for polymers, [-C( X)-C 6 H 4 -C( X)-Y-CH 2 -CH 2 -Y-] x , namely, X = Y = O, poly(ethylene terephthalate) (designated herein as polymer A); X = O and Y = S, poly(ethylene dithioterephthalate) (polymer B); X = Y = S, poly(ethylene tetrathioterephthalate) (polymer C); X = S and Y = O, poly(ethylene dithionoterephthalate) (polymer D). It is well established that the solution, mechanical and thermal properties of such aromatic polymers are essentially determined by the conformational characteristics of the Y-CH 2 -CH 2 -Y unit (referred hereafter to as the spacer) and intermolecular interactions between the benzene rings (Sasanuma, 2009;Sasanuma et al., 2013). In expectation that replacement of oxygen by sulfur at the X or Y site would affect the spacer conformation, andand C-HÁ Á Á interactions of the benzene rings, and thus lead to variations in the physical properties, we synthesized polymers B and C, and characterized them by X-ray diffraction, NMR spectroscopy, thermal analyses, molecular orbital calculations and statistical mechanics of the chain molecules . Herein, the monomeric models for polymers A-D are termed models A-D, respectively.
By molecular orbital calculations at the second-order Møller-Plesset perturbation (MP2) level with moderate-size basis sets, we have determined the most stable conformations of the Y-CH 2 -CH 2 -Y parts of the models and evaluated their free energies relative to that of the all-trans form as follows: model A, tgt and À1.1 kcal mol À1 (Sasanuma, 2009); model B, g AE tg Ç and À3.1 kcal mol À1 ; ISSN 2056-9890 model C, g AE tg Ç and À2.1 kcal mol À1 ; model D, tgt and À1.7 kcal mol À1 (this study). We have also predicted that an asymmetric model compound, C 6 H 5 -C( O)-O-CH 2 -CH 2 -O-C( S)-C 6 H 5 (model E), would be most stabilized in the tg AE g Ç conformation with a free energy of À1.8 kcal mol À1 (this study). However, not all the models and polymers crystallize in the lowest-energy conformations: model (polymer) A, ttt (ttt) (Pé rez & Brisse, 1976;Daubeny et al., 1954); model (polymer) B, g AE tg Ç (g AE tg Ç ) (Deguire & Brisse, 1988;; model (polymer) C, g AE tg Ç (amorphous) (Abe et al., 2011;. In the crystals, the molecules adopt conformations so as to form intermolecular interactions effectively and minimize the total of intramolecular and intermolecular interaction energies. Interestingly, however, models A-C crystallize in the same spacer conformation as those of the corresponding polymers; therefore, the crystal structure of the model suggests the polymer conformation. This study has aimed to determine crystal structures of the title compounds (models D and E) to predict the crystal conformations of polymers D and E on the above hypothesis.

Structural commentary
The molecule of model D lies on an inversion centre and the asymmetric unit contains one half-molecule. The central O-CH 2 -CH 2 -O unit adopts an all-trans conformation (Fig. 1). The molecule of model E is also located on an inversion centre and the O-CH 2 -CH 2 -O bond sequence is in an all-trans conformation. Since the molecule has carbonyl and thiocarbonyl groups, the atoms S and O (S1 and O2) are each assumed to be disordered over two equivalent sites about the inversion centre with equal occupancies (Fig. 2). Consequently, it was proved that all the models (A, D and E) with the O-CH 2 -CH 2 -O spacer crystallize with all-trans structures, although models A, D and E in the free state are most stabilized in tgt, tgt, and tg AE g Ç conformations, respectively.

Supramolecular features
The compounds of models D and E are isotypic and crystallize in the space group P2 1 /c. There are no classical hydrogen bonds but intermolecular close contacts between atoms C and S [C1-S1 i = 3.391 (3) and 3.308 (3) Å for models D and E, respectively; symmetry code: (i) x, -y + 1 2 , z -1 2 ]. Both compounds also have C-HÁ Á Á interactions (Tables 1 and 2  The molecular structure of model E, showing atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. The unlabelled atoms are related to the labelled atoms by inversion symmetry (symmetry code: 2 À x, 2 À y, 2 À z,). H atoms are represented by spheres of arbitrary size.

Figure 1
The molecular structure of model D, showing atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. The unlabelled atoms are related to the labelled atoms by inversion symmetry (symmetry code: 2 À x, 2 À y, 2 À z,). H atoms are represented by spheres of arbitrary size. Table 1 C-HÁ Á Á interaction geometry (Å , ) for model D.
Cg1 is the centroid of the C2-C7 ring.
Cg1 is the centroid of the C2-C7 ring.

Synthesis and crystallization
Benzoyl chloride (10.0 ml, 87 mmol) was added dropwise under a nitrogen atmosphere to ethylene glycol (2.4 ml, 43 mmol) and pyridine (7.0 ml, 87 mmol) placed in a fournecked flask connected to a drying tube filled with calcium chloride, and the mixture was stirred at room temperature overnight. Water was added to the reaction mixture to yield a precipitate, which was collected by filtration, dissolved in chloroform, washed thrice with 5% aqueous solution of sodium bicarbonate, and dried over anhydrous magnesium sulfate overnight. The liquid phase was separated by filtration and condensed on a rotary evaporator, and the residue was recrystallized from ethanol (15 ml). The white crystallites thus obtained were dried under reduced pressure at room temperature overnight to yield ethane-1,2-diyl dibenzoate (8.3 g, 71%). The synthesized ethane-1,2-diyl dibenzoate (0.10 g, 0.37 mmol) was ground in a mortar and mixed thoroughly with Lawesson's reagent (0.24 g, 0.59 mmol), and the powder mixture was moved to a 15 ml vial container and placed in a Yuasa PRE-7017R microwave oven. The powder was heated under the following microwave irradiation at 500 W: on for 2.0 min -off for several seconds -on for 1.0 min. The above handling was repeated ten times to obtain the product sufficiently.
The crude product was extracted with chloroform and condensed under reduced pressure. The residue was dissolved in a mixed solvent of ethyl acetate and n-hexane (1:9 v/v) and subjected to column chromatography. The yellowish fraction (R f = 0.5) was collected and condensed, and the residue underwent column chromatography again with a mixed solvent of toluene and n-hexane (1:5 v/v). Two yellow fractions [(1) R f = 0.1 and (2) 0.3 À 0.5] were stratified and collected separately. The layer (1) was condensed and recrystallized from ethanol to yield a yellow solid, which was identified as 2-(benzenecarbothioyloxy)ethyl benzoate (model E, yield 23%) by 1 H and 13 C NMR, and the layer (2) was condensed and dried at room temperature overnight to yield a red solid, which was identified as 2-(benzenecarbothioyloxy)ethyl benzenecarbothioate (model D, yield 0.9%). A small quantity of model D was dissolved in chloroform in a thin vial container. The vessel was placed in a larger vial containing a small amount of n-hexane, and the outer container was capped. After a week, single crystals were found to be formed in the inner vessel. Single crystals of model E were prepared similarly.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. 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) = 1.2 U eq (C).

Funding information
This study was partially supported by the Grants-in-Aid for Scientific Research (C) (16 K05906) from the Japan Society for the Promotion of Science.

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.

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.