Silver(I) nitrate two-dimensional coordination polymers of two new pyrazinethiophane ligands: 5,7-dihydro-1H,3H-dithieno[3,4-b:3′,4′-e]pyrazine and 3,4,8,10,11,13-hexahydro-1H,6H-bis([1,4]dithiocino)[6,7-b:6′,7′-e]pyrazine

On reaction with silver nitrate the new pyrazinethiophanes, 5,7-dihydro-1H,3H-dithieno[3,4-b:3′,4′-e]pyrazine and 3,4,8,10,11,13-hexahydro-1H,6H-bis([1,4]dithiocino)[6,7-b:6′,7′-e]pyrazine, both form two-dimensional coordination polymers.


Chemical context
Ligands with mixed hard and soft binding characters, such as N and S donor atoms, are known to display diverse coordination properties, either by binding selectively to metal centers or by coordination to a wide range of metal cations giving rise to unusual coordination geometries. The title compounds 5,7-dihydro-1H,3H-dithieno[3,4-b:3 0 ,4 0 -e]pyrazine (L1), and 3,4,8,10,11,13-hexahydro-1H,6H-bis ([1,4]dithiocino)[6,7-b:-6 0 ,7 0 -e]pyrazine (L2), are new N 2 S x (x = 2 in L1 and = 4 in L2) ligands designed for the formation of coordination polymers (Assoumatine, 1999). In L1, both the nitrogen and sulfur ISSN 2056-9890 potential coordination sites are orientated exo to their respective rings. Because of this and the rigidity of the entire molecule, the potential chelating ability appears compromised, as stated by Shimizu and colleagues, who prepared a number of Ag I polymer networks with the benzene analogue of L1, 5,7-dihydro-1H,3H-benzo[1,2-c:4,5-c 0 ]dithiophene (Shimizu et al., 1998;1999;Melcer et al., 2001). A search of the Cambridge Structural Database (Groom et al., 2016) revealed that L2 is unique and no benzene analogue or complexes of this analogue have been described. Using the nomenclature of the group of Shim Sung Lee (Siewe et al., 2014;Kim et al., 2016Kim et al., , 2018, L2 can be described as the bis-ortho-l regioisomer. Although, in view of the small size of the macrocycles, it is unlikely that either a meta-or a para-bis-l regioisomer could be formed.

Structural commentary
The molecular structure of ligand L1 is illustrated in Fig. 1. The molecule possesses inversion symmetry and consists of two sulfur atoms linked by a rigid tetra-2,3,5,6-methylenepyrazine unit. The molecule is planar (r.m.s. deviation = 0.008 Å ) with the pyrazine ring being located about a center of symmetry. Both the nitrogen and sulfur potential coordination sites are orientated exo to their respective rings. The molecular structure of ligand L2 is illustrated in Fig. 2. The molecule also possesses inversion symmetry with the pyrazine ring being located about a center of symmetry. It consists of two S-CH 2 -CH 2 -S chains linked by the central rigid tetra-2,3,5,6-methylenepyrazine unit, forming eightmembered rings. The configuration of these rings fits best to the definition for a twist-boat-chair (Evans & Boeyens, 1988;Spek, 2020), with a pseudo twofold rotation axis bisecting the C1-C2 and C4-C5 bonds and their symmetry equivalents. The molecule is step-shaped with six potential sites for coordination.

Figure 2
The molecular structure of L2, with atom labelling; symmetry code: (i) Àx + 3 2 , Ày + 1 nitrato anion bridging two equivalent silver atoms (Fig. 3). Selected bond lengths and bond angles are given in Table 1. The central pyrazine ring is situated about an inversion center and the silver atom Ag1 and atoms N2 and O2 of the nitrato anion lie on a twofold rotation axis. Atom Ag1 has a fourfold AgO 2 S 2 coordination sphere with a distorted shape. The fourfold geometry index 4 has a value of 0.74 ( 4 = 1 for a perfect tetrahedral geometry, 0 for a perfect square-planar geometry and 0.85 for perfect trigonal-pyramidal geometry; Yang et al., 2007). The intermediate value of 0.74 tends towards a see-saw arrangement. This seems reasonable in view of the fact that atom Ag1 is located on a twofold rotation axis. The reaction of L2 with silver nitrate also leads to the formation of a two-dimensional coordination polymer (II, Fig. 4). Selected bond lengths and bond angles are given in Table 2. While the ligand has a step-shape in the solid state with one eight-membered ring directed above the pyrazine ring and the other below ( Fig. 2), in the complex it has a boat shape with both eight-membered rings directed to the same side of the pyrazine ring (Fig. 4). The configuration of these rings again fits best to the definition for a twist-boat-chair (Evans & Boeyens, 1988;Spek, 2020), with a pseudo twofold rotation axis bisecting bonds C1-C2 and C7-C8 and bonds C3-C4 and C10-C11. The nitrato anion coordinates to the silver atom in a monodentate manner via atom O11 (Fig. 4, Table 2). The silver atom Ag1 has a fourfold AgOS 3 coordination sphere with a distorted shape. The fourfold geometry index 4 has a value of 0.75, which again tends towards a seesaw arrangement.
The pyrazine N atoms are not involved in coordination to the silver atom in either I or II; the silver atom prefers coordination to the S atoms in both complexes. The role of the nitrato anion in I is essential in forming the two-dimensional network, bridging two equivalent silver atoms, while in II the nitrato anion coordinates to atom Ag1 in a monodentate manner. There is a significant difference in the Ag-S bond lengths and the Ag-O bond lengths in compounds I and II (cf. Tables 1 and 2), which are discussed in x5. Database survey.

Supramolecular features
In the crystals of both L1 and L2, there are no significant intermolecular interactions present (Figs. 5 and 6, respectively).

Figure 5
Crystal packing of L1 viewed along the c axis. The molecules stack in columns up the a axis.
In the crystal of II, the coordination networks lie parallel to the ab plane ( Fig. 9). They are linked by C-HÁ Á ÁO and C-HÁ Á ÁS hydrogen bonds, forming a supramolecular framework ( Fig. 10 and Table 4).  Table 3 Hydrogen-bond geometry (Å , ) for I.

Figure 8
A view along the a axis of the crystal packing of complex I. The hydrogen bonds are shown as dashed lines (Table 3).

Figure 9
A view along the c axis of the crystal packing of complex II, illustrating the formation of the metal-organic network. The silver atoms are shown as grey balls. For clarity, the H atoms have been omitted.

Figure 10
A view along the a axis of the crystal packing of complex II. The hydrogen bonds are shown as dashed lines (Table 4). For clarity, only the H atoms involved in these interactions have been included.

Hirshfeld surface analysis and two-dimensional fingerprint plots
The Hirshfeld surface (HS) analyses (Spackman & Jayatilaka, 2009) and the associated two-dimensional fingerprint plots (McKinnon et al., 2007) were performed with Crystal-Explorer17 (Turner et al., 2017) following the protocol of Tiekink and collaborators (Tan et al., 2019). A summary of the short interatomic contacts in L1 and L2 is given in Table 5. The Hirshfeld surfaces of L1 and L2 mapped over d norm are given in Fig. 11a and b, respectively. They show that there are no short significant interatomic contacts present in the crystal of L1, while the red spots indicate that short contacts are significant in the crystal of L2.

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.41, last update November 2019; Groom et al., 2016) for the benzene analogue of L1, i.e. 5,7-dihydro-1H,3Hbenzo[1,2-c:4,5-c 0 ]dithiophene, gave ten hits. Five compounds concern silver(I) coordination complexes involving various  Melcer et al., 2001). The latter are two reports of the same compound, cf. unit-cell parameters and space group. The compound MIZHAE is a three-dimensional coordination polymer with a fourfold geometry index 4 value of 0.80 (close to a trigonal-pyramidal geometry) for the silver atom, which has an AgN 2 S 2 coordination sphere. NUTBUZ is a twodimensional coordination polymer. Here, the silver atom has a fivefold AgN 2 S 3 coordination sphere with a distorted shape; the fivefold geometry index 5 is 0.77 ( 5 = 1 for perfect trigonal-pyramidal geometry and = 0 for perfect squarepyramidal geometry; Addison et al., 1984). NUTCAG is a twodimensional coordination polymer with a 4 value of 0.73 for the silver atom, which has an AgNS 3 coordination sphere. QACYUO (and QACYUO0) is a two-dimensional coordination polymer, with the silver atom having a fourfold AgOS 3 coordination sphere with a trigonal-pyramidal geometry, the fourfold geometry index 4 being 0.83. The Ag-S bond lengths involving the fourfold coordinated silver atoms vary from 2.4708 (13) Å in NUTCAG to 2.6077 (7) Å in QACYUO/01. The values of the various Ag-S bond lengths in I and II fall within these limits (see Tables 1 and 2). While in the ligand L1 the five-membered thiophene rings are planar, in the above mentioned structures and in complex I they have envelope configurations with the S atom as the flap.
A search of the CSD for the benzene analogue of L2, or complexes of this analogue, gave zero hits.

Synthesis and crystallization
The reagent tetra-2,3,5,6-bromomethyl-pyrazine (TBr) was first synthesized by Ferigo et al. (1994), and its crystal structure has been reported (CSD refcode: TOJXUN; Assoumatine & Stoeckli-Evans, 2014). The IR spectra for ligands L1 and L2, and for complexes I and II, are given in Fig. S1 in the supporting information. Synthesis Ligand L1 was first prepared by the reaction of TBr with Na 2 SÁ9H 2 O, using the procedure of Shimizu et al. (1998). This gave a crude brown solid, which was chromatographed on deactivated silica gel with CH 2 Cl 2 as eluent to yield 35% of a white solid.
The yield could be increased by as much as 11% using a method similar to that described by Boekelheide et al. (1973). Well-ground Na 2 SÁ9H 2 O (1.06 g, 4.42 mmol, Aldrich 99%) was dissolved in a solution of MeOH/CH 2 Cl 2 (100 ml, 1/1 v/v) in a three-necked flask (500 ml) equipped with a reflux condenser topped by a CaCl 2 drying tube, an addition funnel (50 ml) and a magnetic stirring bar. To this mixture was added slowly through the addition funnel a solution of TBr (1 g, 2.21 mmol) in CH 2 Cl 2 (25 ml). The reaction mixture was stirred vigorously for 3 h. Removal of the solvent resulted in a brown residue that was extracted into CH 2 Cl 2 (200 ml), washed with water (3 Â 30 ml), dried over anhydrous MgSO4 and then, after filtration, evaporated to dryness. The resultant residue was chromatographed over deactivated silica gel using CH 2 Cl 2 as eluent. The main eluted fraction was evaporated to give a white solid that was dried under vacuum yielding pure L1 (m.p. 518-521 K, with decomposition). Colourless rod-like crystals were formed from a concentrated solution of pure L1 in CH 2 Cl 2 , after standing for one week at 278 K. Synthesis of 3,4,8,10,11,13-hexahydro-1H,6H-bis([1,4]dithiocino)[6,7-b:6 0 0 0 ,7 0 0 0 -e]pyrazine (L2): A 500 ml three-necked flask was equipped with a reflux condenser, a 50 ml addition funnel, and a magnetic stirring bar. The entire system was purged and kept under a nitrogen atmosphere using vacuum line techniques. Then well-ground Cs 2 CO 3 (3.52 g, 10.80 mmol, Fluka 99%) was suspended in DMF (250 ml) in the flask. To this well-stirred suspension was added dropwise through the addition funnel a solution of TBr (1 g, 2.21 mmol) and 1,2-ethanedithiol (0.4 ml, 4.76 mmol, 98%) dissolved in DMF (50 ml), at a rate of about 10 ml h À1 . The mixture was stirred for a further 20 h and then filtered.
The orange filtrate was evaporated under reduced pressure. The residue was extracted into CH 2 Cl 2 (300 ml) then washed with water (3 Â 30 ml), dried over anhydrous MgSO 4 and then, after filtration, evaporated to dryness. The resultant residue was chromatographed over deactivated silica gel using CH 2 Cl 2 as eluent. The main eluted fraction was evaporated to give a white solid that was dried under vacuum to obtain 0.35 g (50% yield) of pure L2 (m.p. 541-544 K, with decomposition). Slow evaporation at room temperature of a solution of L2 in CHCl 3 in a 5 mm diameter glass tube gave colourless platelike crystals. Synthesis of complex I: A solution of L1 (15 mg, 0.08 mmol) in THF (5 ml) was introduced into a 16 mm diameter glass tube and layered with MeCN (2 ml) as a buffer zone. Then a solution of AgNO 3 (14 mg, 0.08 mmol) in MeCN (5 ml) was added very gently to avoid possible mixing. The glass tube was sealed and left in the dark at room temperature for at least two weeks, whereupon colourless needle-like crystals of complex I were isolated in the buffer zone. Synthesis of complex II: A solution of L2 (20 mg, 0.06 mmol) in CH 2 Cl 2 (10 ml) was introduced into a 16 mm diameter glass tube and layered with MeCN (2 ml) as a buffer zone. Then a solution of AgNO 3 (10 mg, 0.06 mmol) in MeCN (5 ml) was added very gently to avoid possible mixing. The glass tube was sealed and left in the dark at room temperature for at least three weeks, whereupon thin, colourless plate-like crystals of complex II were isolated at the interface between the two solutions. No analytical data are available for this complex.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 6. The C-bound H atoms were included in calculated positions and treated as riding on the parent atoms: C-H = 0.97-0.98 Å with U iso (H) = 1.2U eq (C). For L1, the rather high R int value of 0.159 is due to the poor quality, viz. large mosaic spread, of the crystal.