The crystal structures, Hirshfeld surface analyses and energy frameworks of two hexathiapyrazinophane regioisomers; 2,5,8,11,14,17-hexathia-[9.9](2,6,3,5)-pyrazinophane and 2,5,8,11,14,17-hexathia-[9.9](2,5,3,6)-pyrazinophane

The title hexathiapyrazinophanes are regioisomers, having a central tetra-2,3,5,6-methylenepyrazine unit with two –S—CH2—CH2—S—CH2—CH2—S– chains linking the methylene C atoms at positions 2 and 6 and 3 and 5 in the m-bis regioisomer, but linking the methylene C atoms at positions 2 and 5 and 3 and 6 in the p-bis regioisomer.


Chemical context
Ligands with mixed hard and soft binding characters, such as O, N and S donor atoms, are known to display diverse coordination modes by binding selectively to metal centres giving rise to unusual coordination geometries (Kim et al., 2018;Klinga et al., 1994;Lockhart et al., 1992). Three regioisomers, o, m and p, of a bis-dioxadithia-benzenophane (L, O 4 S 4 ) have been reported on by the group of Shim Sung Lee (Kim et al., 2018). The structures of a number of metal complexes have also been described; for example, both o-bis L and m-bis L form one-dimensional coordination polymers with AgPF 6 (Siewe et al., 2014), while with lead(II) perchlorate a binuclear complex was obtained with o-bis L and a one-dimensional coordination polymer with m-bis L (Kim et al., 2018). In all four complexes the metal atoms coordinate to both the O and S atoms.
The coordination chemistry of the title compound m-bis L1 (I), an N 2 S 6 thiapyrazinophane, has also been studied and shown to form a binuclear complex with CuBr 2 and a twodimensional coordination polymer with CuI (Assoumatine & Stoeckli-Evans, 2020e). In both cases, the ligand coordinates to both the N and S atoms. Herein, we report on and compare the crystal structures, the Hirshfeld surfaces and the energy frameworks of the regioisomers m-bis L1 (I) and p-bis L1 (II).

Figure 1
A view of the molecular structure of compound I, the regioisomer m-bis L1, with atom labelling for the asymmetric unit [symmetry code: (i) Àx, Ày, Àz + 1]. Displacement ellipsoids are drawn at the 50% probability level. For clarity, the minor components of the disordered atoms in the chains have been omitted.

Supramolecular features
In the crystal of I, molecules pack in layers that lie parallel to the (101) plane, as shown in Fig. 3. In the crystal of II, molecules are linked by C-HÁ Á ÁS hydrogen bonds, forming corrugated layers that lie parallel to the ac plane (Table 2 and 4. Hirshfeld surface analyses, two-dimensional fingerprint plots and energy frameworks for I (m-bis L1) and II (p-bis L1).
The Hirshfeld surface analysis (Spackman & Jayatilaka, 2009), the associated two-dimensional fingerprint plots and the calculation of the energy frameworks (McKinnon et al., 2007;Turner et al., 2015) were performed with CrystalExplorer17.5 (Turner et al., 2017), following the protocol of Tiekink and collaborators (Tan et al., 2019). The Hirshfeld surface is colour-mapped with the normalized contact distance, d norm , from red (distances shorter than the sum of the van der Waals radii) through white to blue (distances longer than the sum of the van der Waals radii). The energy frameworks are represented by cylinders joining the centroids of molecular pairs using red, green and blue colour codes for the E elect (electrostatic potential forces), E disp (dispersion forces) and E total (total energy) energy components, respectively. The radius of the cylinder is proportional to the magnitude of the interaction energy.
A summary of the short interatomic contacts in I (m-bis L1) and II (p-bis L1) is given in Table 3. The Hirshfeld surfaces of I Table 1 Hydrogen-bond geometry (Å , ) for I.

Figure 4
A view along the b axis of the crystal packing of II, with the C-HÁ Á ÁS hydrogen bonds (Table 2) shown as dashed lines.

Figure 3
A view along the b axis of the crystal packing of I. For clarity, the minor components of the disordered atoms in the chains and the H atoms have been omitted. and II mapped over d norm , are given in Fig. 5a and b, respectively. The faint red spots indicate that short contacts are significant in the crystal packing of both compounds. The Hirshfeld surfaces mapped over the calculated electrostatic potential for I and II, given in Fig. 6a and b, respectively, are very similar. The red and blue regions represent negative and positive electrostatic potentials, respectively. The red regions around the sulfur atoms indicate their participation in the C-HÁ Á ÁS contacts (see Table 3).
For both I and II the interatomic contacts are dominated by dispersion forces, as can be seen when comparing the elec-trostatic potential (E elect ) and dispersion (E disp ) energy frameworks in Fig. 8a and b, respectively. The energy frameworks ( Fig. 8) were adjusted to the same scale factor of 80 with a cut-off value of 5 kJ mol À1 within a radius of 6 Å about a central molecule, and were obtained using the wave function calculated at the HF/3-21G level theory.

Synthesis and crystallization
Synthesis of 2,5,8,11,14,17-hexathia-[9.9](2,6,3,5)-pyrazinophane (I): A 500 ml three-necked flask was equipped with a reflux condenser, a 50 ml addition funnel, and a magnetic stirring bar.    (a) The energy frameworks for I viewed down the b-axis direction, (b) the energy frameworks for II viewed down the c-axis direction: comprising, E elect (electrostatic potential forces), E disp (dispersion forces) and E total (total energy) for a cluster about a reference molecule.
(0.62 g, 11 mmol) was dissolved in a solution of MeOH/ CH 2 Cl 2 (250 ml, 1/1 v/v) in the flask. To this well-stirred mixture was added slowly and dropwise through the addition funnel, a solution of 1 g (2.21 mmol) of 2,3,5,6-tetrakis-(bromomethyl)pyrazine (Ferigo et al., 1994;Assoumatine & Stoeckli-Evans, 2014) and bis-(2-mercaptoethyl)sulfide (0.6 ml, 4.42 mmol, 95%) dissolved in CH 2 Cl 2 (25 ml), at a rate of ca 10 ml h À1 . The mixture was stirred for a further 20 h. The reaction mixture was taken to dryness on a rotary evaporator. The residue was extracted into CH 2 Cl 2 (300 ml), washed with water (3 Â 30 ml), dried over anhydrous MgSO 4 , filtered and then evaporated to dryness. The resultant yellowish solid was chromatographed over deactivated silica gel using CH 2 Cl 2 as eluent. The main eluted fraction was evaporated to give a white solid, which was dried under vacuum to obtain 0.42 g (43% yield) of pure L1 (m.p. 581-584 K, with decomposition). Slow evaporation of a CHCl 3 solution of L1 gave colourless rod-like crystals of I, the m-bis L1 regioisomer, after ca one month Synthesis of 2,5,8,11,14,17-hexathia-[9.9](2,5,3,6)-pyrazinophane (II): Pale-yellow block-like crystals of compound II were obtained unexpectedly during a complexation reaction of L1 with ZnI 2 (Assoumatine, 1999). It is difficult to imagine that the complexation reaction resulted in the transformation of m-bis L1 (I) into p-bis L1 (II). We believe it is more likely that the latter was obtained in small quantities during the various syntheses of L1 and was present in the main eluted fraction used subsequently for the complexation reaction. There are no analytical or spectroscopic data available for this compound.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The C-bound H atoms were included in calculated positions and treated as riding on their parent C atom: C-H = 0.98 Å with U iso (H) = 1.2U eq (C). In I atoms C4 and C5 of the -CH 2 -S-CH 2 -CH 2 -S-CH 2 -CH 2 -S-CH 2 -chain are disordered over two positions. They were refined with a fixed occupancy ratio (C4A:C4B and C5A:C5B) of 0.85:0.15.
Intensity data were measured using a STOE IPDS-1 onecircle diffractometer. For the monoclinic system often only 93% of the Ewald sphere is accessible, which explains why the B alert diffrn_reflns_laue_measured_fraction_full value low at 0.957 for compound I is given. This involves 76 random reflections out of the expected 1765 for the IUCr cutoff limit of sin / = 0.60 for I.   program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015); molecular graphics: Mercury (Macrae et al., 2020); software used to prepare material for publication: SHELXL2018/3 (Sheldrick, 2015), PLATON (Spek, 2020) and publCIF (Westrip, 2010). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.80 e Å −3 Δρ min = −0.35 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.