Silver(I) nitrate complexes of three tetrakis-thioether-substituted pyrazine ligands: metal–organic chain, network and framework structures

The reaction of silver nitrate with the ligands 2,3,5,6-tetrakis[(methylsulfanyl)methyl]pyrazine, 2,3,5,6-tetrakis[(phenylsulfanyl)methyl]pyrazine and 2,3,5,6-tetrakis[(pyridin-2-ylsulfanyl)methyl]pyrazine, three tetrakis-thioether-substituted pyrazine ligands, lead, respectively, to the formation of compounds with a metal–organic chain, a metal–organic network and a metal–organic framework structure.

The reaction of the ligand 2,3,5,6-tetrakis[(methylsulfanyl)methyl]pyrazine (L1) with silver(I) nitrate led to {[Ag (C 12 H 20 N 2 S 4 )](NO 3 )} n , (I), catena-poly[[silver(I)--2,3,5,6-tetrakis[(methylsulfanyl)methyl]pyrazine] nitrate], a compound with a metal-organic chain structure. The asymmetric unit is composed of two half ligands, located about inversion centres, with one ligand coordinating to the silver atoms in a bis-tridentate manner and the other in a bis-bidentate manner. The charge on the metal atom is compensated for by a free nitrate anion. Hence, the silver atom has a fivefold S 3 N 2 coordination sphere. The reaction of the ligand 2,3,5,6-tetrakis[(phenylsulfanyl)methyl]pyrazine (L2) with silver(I) nitrate, led to [Ag 2 (NO 3 ) 2 (C 32 H 28 N 2 S 4 )] n , (II), poly[di--nitrato-bis{-2,3,5,6tetrakis[(phenylsulfanyl)methyl]pyrazine}disilver], a compound with a metalorganic network structure. The asymmetric unit is composed of half a ligand, located about an inversion centre, that coordinates to the silver atoms in a bistridentate manner. The nitrate anion coordinates to the silver atom in a bidentate/monodentate manner, bridging the silver atoms, which therefore have a sixfold S 2 NO 3 coordination sphere. The reaction of the ligand 2,3,5,6tetrakis[(pyridin-2-ylsulfanyl)methyl]pyrazine (L3) with silver(I) nitrate led to [Ag 3 (NO 3 ) 3 (C 28 H 24 N 6 S 4 )] n , (III), poly[trinitrato{ 6 -2,3,5,6-tetrakis[(pyridin-2ylsulfanyl)methyl]pyrazine}trisilver(I)], a compound with a metal-organic framework structure. The asymmetric unit is composed of half a ligand, located about an inversion centre, that coordinates to the silver atoms in a bis-tridentate manner. One pyridine N atom bridges the monomeric units, so forming a chain structure. Two nitrate O atoms also coordinate to this silver atom, hence it has a sixfold S 2 N 2 O 2 coordination sphere. The chains are linked via a second silver atom, located on a twofold rotation axis, coordinated by the second pyridine N atom. A second nitrate anion, also lying about the twofold rotation axis, coordinates to this silver atom via an Ag-O bond, hence this second silver atom has a threefold N 2 O coordination sphere. In the crystal of (I), the nitrate anion plays an essential role in forming C-HÁ Á ÁO hydrogen bonds that link the metal-organic chains to form a three-dimensional supramolecular structure. In the crystal of (II), the metal-organic networks (lying parallel to the bc plane) stack up the a-axis direction but there are no significant intermolecular interactions present between the layers. In the crystal of (III), there are a number of C-HÁ Á ÁO hydrogen bonds present within the metal-organic framework. The role of the nitrate anion in the formation of the coordination polymers is also examined.

Structural commentary
The reaction of the ligand 2,3,5,6-tetrakis[(methylsulfanyl)methyl]pyrazine (L1) with silver(I) nitrate, led to the formation of a metal-organic chain (MOC) structure, (I) (Fig. 1). Selected bond lengths and angles involving the Ag1 atom are given in Table 1. The asymmetric unit is composed of two half ligands, located about inversion centres, with one ligand coordinating to the silver atom in a bis-tridentate manner and the other in a bis-bidentate manner. Their pyrazine rings are almost normal to one another, making a dihedral angle of 88.6 (2) . The charge on the metal atom is compensated for by a free nitrate anion. The silver atom, Ag1, has a fivefold S 3 N 2 coordination sphere with a highly distorted shape and a 5 value of 0.63 ( 5 = 0 for an ideal square-pyramidal coordination sphere, and = 1 for an ideal trigonal-pyramidal coordination sphere; Addison et al., 1984). Within the MOC structure, there are significant C-HÁ Á ÁS interactions present, involving the thioether substituent that does not coordinate to the silver atom, viz. atom S3 (Table 4 and Fig. 1).
formation of a metal-organic network (MON) structure, (II) (Fig. 2). Selected bond lengths and angles involving atom Ag1 are given in Table 2. The asymmetric unit is composed of half a ligand, located about an inversion centre, a silver atom and a nitrate anion. The ligand coordinates to the silver atoms in a bis-tridentate manner. The nitrate anion coordinates to the silver atom in a bidentate/monodentate manner, bridging the silver atoms, which therefore have a sixfold S 2 NO 3 coordination sphere, best described as a highly distorted octahedron ( Table 2).
The reaction of the ligand 2,3,5,6-tetrakis[(pyridin-2-ylsulfanyl)methyl]pyrazine (L3) with silver(I) nitrate, led to the formation of a metal-organic framework (MOF) structure, (III) (Fig. 3). Selected bond lengths and angles involving atoms Ag1 and Ag2 are given in Table 3. The asymmetric unit is composed of half a ligand, located about an inversion centre, a silver atom and a nitrate anion, plus half a second AgNO 3 unit located about a twofold rotation axis. The organic ligand coordinates to the silver atoms (Ag1), in a bis-tridentate manner. One pyridine N atom, N2, bridges the monomeric units, so forming a chain structure along the b-axis direction. The nitrate O atoms, O11 and O13, coordinate to silver atom Ag1, hence it has a highly distorted octahedral S 2 N 2 O 2 coordination sphere (Table 3). The chains are linked via a second silver atom, Ag2, located on a twofold rotation axis, coordinated by the second pyridine N atom, N3. A second nitrate anion, also lying about the twofold rotation axis, The molecular entities of compound (II), with atom labelling for the asymmetric unit. For the ligand, unlabelled atoms are related to the labelled atoms by symmetry operation (i) Àx + 2, Ày + 2, Àz + 1; other symmetry codes are (ii) x, Ày + 3 2 , z + 1 2 ; (iii) Àx + 2, y + 1 2 , Àz + 1 2 . Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: (i) Àx þ 2; Ày þ 2; Àz þ 1; (ii) x; Ày þ 3 2 ; z þ 1 2 .

Table 3
Selected geometric parameters (Å , ) for (III). coordinates to this silver atom via an Ag2-O21 bond, hence silver atom Ag2 has a T-shaped N 2 O coordination sphere. It can be seen from Tables 1-3 that the Ag-N(pyrazine) and Ag-S bond lengths differ considerably for the three compounds. In compound (I), the Ag1-N2 bond length, involving the ligand that coordinates in a bis-bidentate manner, is considerably shorter at 2.436 (5) Å , compared to the Ag1-N1 bond length of 2.714 (4) Å , involving the ligand that coordinates in a bis-tridentate manner. These Ag-N(pyrazine) bond lengths contrast with those for compounds (II) and (III), where both ligands coordinate in a bis-tridentate manner, with values of 2.527 (4) and 2.578 (3) Å , respectively. The Ag1-S bond lengths in compound (I) are almost the same, varying from 2.5895 (15) to 2.5987 (16) Å . These distances are shorter than those in (II), which are 2.6560 (15) and 2.6790 (14) Å , but similar to bond length Ag1-S2 ii = 2.6010 (11) Å in (III). The longest Ag-S distance [2.7943 (13) Å ] is found for bond Ag1-S1 in (III). Finally, in compound (III), the two Ag-N(pyridine) bond lengths also differ; Ag1-N2 i is 2.267 (3) Å , while bond length Ag2-N3 is shorter at 2.208 (3) Å (see Table 3). Despite the large variation in the Ag-N(pyrazine), Ag-S or Ag-N(pyridine) bond lengths, which perhaps indicates how flexible the ligands are, the values are within the limits observed for similar silver coordinating pyrazine, thioether or pyridine ligands, when compared to the values observed for such structures present in the Cambridge Structural Database (Groom et al., 2016). The various histograms of the bond lengths have skewed-right distributions and the values vary from 2.10 to 2.75 Å for Ag-research communications Acta Cryst. (2017). E73, 434-440 Assoumatine and Stoeckli-Evans [Ag(C 12 H 20 N 2 S 4 ](NO 3 ) and two related compounds 437 Table 5 Hydrogen-bond geometry (Å , ) for (III).

Figure 4
A partial view, normal to plane (110), of the metal-organic chain structure of compound (I). The H atoms have been omitted for clarity   Table 4), and only those H atoms involved in intermolecular C-HÁ Á ÁO hydrogen bonds have been included.

Figure 6
A view along the a axis of compound (II), illustrating the role of the NO 3 À anion in forming the network structure. H atoms have been omitted for clarity N(pyrazine), from 2.48 to 2.79 Å for Ag-S, and 1.90 to 2.99 Å for Ag-N(pyridine).

Supramolecular features
In the crystal of (I), the metal-organic chains ( Fig. 4) propagate along [101]. They are linked via a number of C-HÁ Á ÁO hydrogen bonds (Table 4), forming a three-dimensional supramolecular structure, as illustrated in Fig. 5.
In the crystal of (II), the metal-organic networks extend parallel to the bc plane and stack up the a axis ( Fig. 6), but there are no significant intermolecular interactions present between the layers.
In the crystal of (III), the metal-organic framework ( Fig. 7) is reinforced by a number of C-HÁ Á ÁO hydrogen bonds ( Table 5). The voids in this three-dimensional structure, occupied by disordered solvent molecules, amount to only ca 3.7% of the total volume of the unit cell.
The role of the anion in coordination chemistry is often essential for the formation of multi-dimensional structures. The nitrate anion can be present as an isolated anion, coordinating to the metal atom or even bridging metal atoms. A search of the CSD for silver nitrate complexes yielded 2192 hits, among which it was noted that the nitrate anion can coordinate in at least 10 different manners. In the present study, three different situations are observed. In (I), the nitrate anion is present as an isolated anion. Its role here is to form C-HÁ Á ÁO hydrogen bonds, resulting in the formation of a three-dimensional supramolecular structure ( Fig. 5 and Table 4). In (II), the nitrate anion is essential in forming the network structure. The -Ag-L2-Ag-L2-chains, which propagate along [010], are linked by the nitrate anion in the [001] direction, so forming the metal-organic network ( Fig. 6 and Table 2). Finally, there are two independent nitrate anions present in (III). They coordinate to the metal atoms in different manners, but they do not appear to be the essential elements in forming the three-dimensional framework ( Fig. 7 and Table 3). Here, it is the presence of the pyridine rings, which twist about the S-C ar bonds, that enables the metal atoms to cross-link, so forming the metal-organic framework.

Compound (I):
A solution of L1 (50 mg, 0.16 mmol; Assoumatine & Stoeckli-Evans, 2014a) in CH 2 Cl 2 (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 (27 mg, 0.16 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 yellow platelike crystals of complex (I) were isolated at the interface between the two solutions. IR (KBr disc, cm À1 ): = 2985 w, 2912 w, 1406 bm, 1341 bs, 1141 w, 1115 w, 982 w, 828 w, 777 w, 701 vw, 478 vw.

Compound (II):
A solution of L2 (50 mg, 0.09 mmol; Assoumatine et al., 2007) 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 (15 mg, 0.09 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 yellow block-like crystals of complex (II) were isolated from the bottom of the tube. IR

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 6. Complexes (I) and (II) were measured at 293 K on a four-circle diffractometer, while complex (III) was measured at 223 K on a one-circle imageplate diffractometer. In complex (I), the nitrate ion is positionally disordered and atoms O12A/O12B and O13A/O13B were refined with a fixed occupancy ratio of 0.5:0.5. No absorption correction was applied for complex (II) owing to the irregular shape of the crystal, and as there were no suitable reflections for scans. For complex (III), a region of disordered electron density (25 electrons for a solvent-accessible volume of 130 Å 3 ) was corrected for using the SQUEEZE routine in PLATON (Spek, 2015). Their formula mass and unit-cell characteristics were not taken into account for the final model. For complexes (I) and (II), only one equivalent of data were measured, hence R int = 0. In all three complexes, the H atoms were included in calculated positions and refined as riding: C-H = 0.96-0.97 Å for (I), 0.93-0.97 Å for (II) and 0.94-0.98 Å for (III), with U iso (H) = 1.5U eq (C-methyl) and 1.2U eq (C) for other H atoms.

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.

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.