Crystal structure of a pyrazine-2,3-dicarboxamide ligand and of its silver(I) nitrate complex, a three-dimensional coordination polymer

The reaction of the ligand N 2,N 3-bis(pyridin-4-ylmethyl)pyrazine-2,3-dicarboxamide with silver(I) nitrate led to the formation of a three-dimensional coordination polymer.


Chemical context
The title ligand, N 2 ,N 3 -bis(pyridin-4-ylmethyl)pyrazine-2,3dicarboxamide (L1), is one of a series of ligands synthesized in order to study the superexchange in supramolecular complexes formed using pyrazine carboxamide derivatives and first row transition metal ions (Cati, 2002;Cati et al., 2004). To the best of our knowledge, neither the synthesis nor the crystal structure of (L1) have been described previously. It is very similar to the ligand N 2 ,N 3 -bis(pyridin-2-ylmethyl)pyrazine-2,3-dicarboxamide (L2), for which a number of transition metal complexes have been described, including some interesting tetranuclear 2Â2 grid-like and square complexes (Hausmann et al., 2003;Klingele et al., 2007) (Cati et al., 2004), exhibit anion encapsulation, and magnetic susceptibility measurements indicate that they are weakly anti-ferromagnetic, with J values of À5.87 and À2.64 cm À1 , respectively.

Structural commentary
The title ligand (L1) crystallized as a dihydrate, and its molecular structure is illustrated in Fig. 1. The molecule is U-shaped with the carboxamide groups (C6/N3/C5/O1) being cis to one another, making a dihedral angle of 81.6 (5) . The terminal pyridine rings (N4/C7-C11) are inclined to one another by 58.5 (4) . There is an intramolecular N-HÁ Á ÁN hydrogen bond present, forming an S(5) ring motif ( Fig. 1 and Table 1).
The reaction of the ligand with silver(I) nitrate led to the formation of a three-dimensional coordination polymer (I). The coordination of the ligand to the silver ions is illustrated in Fig. 2. Selected bond lengths and angles in (I) are given in Table 2. The asymmetric unit is composed of a silver ion, located on a twofold rotation axis, half a ligand molecule and half a nitrate anion. The full molecule of the ligand is generated by twofold rotational symmetry, with this twofold axis bisecting the C4-C4 i bonds of the pyrazine ring and the Ag1-Ag1 i bond ( Table 2). The carboxamide groups (C6/N3/ C5/O1) are now trans to one another, making a dihedral angle of 65.8 (4) . The terminal pyridine rings (N4/C7-C11) are inclined to one another by 6.6 (3) . Two ligands effectively wrap around a Ag-Ag bond of 3.1638 (11) Å , forming a figure-of-eight-shaped molecule, with each silver ion being coordinated by two pyridine N atoms. The silver ions are each further coordinated by the carboxamide O atom, O1, of neighbouring molecules, hence forming a three-dimensional framework, illustrated in Fig. 3. If one considers that the silver ion, Ag1, is fivefold coordinate (N 2 O 2 Ag i ) then its coordination sphere can be described as distorted trigonal-bipyramidal, with a 5 value of 0.8 ( 5 = 1 for perfect trigonalpyramidal geometry and 0 for perfect square-pyramidal geometry; Addison et al., 1984). However, if one considers the Ag1 ion to be fourfold coordinate, N 2 O 2 , with a 4 value of 0.55, its coordination sphere can be described as intermediate between trigonal-pyramidal and seesaw ( 4 = 1 for a perfect tetrahedral geometry and 0 for a perfect square-planar geometry. For intermediate structures, including trigonalpyramidal and seesaw, 4 falls within the range of 0 to 1; Yang et al., 2007). The nitrate anion that does not coordinate to the silver(I) ion is positionally disordered, and also located about a twofold rotation axis.

Supramolecular features
In the crystal of ligand (L1), molecules are linked by N-HÁ Á ÁO(water) hydrogen bonds forming chains propagating along the c-axis direction (Table 1 and Fig. 4). Parallel to this chain of molecules is a chain of hydrogen-bonded water molecules (Table 1 and Fig. 4), which is linked to the chain of (L1) molecules by O-HÁ Á ÁN hydrogen bonds, forming columns propagating along the c axis. The columns are linked by C-HÁ Á ÁO and C-HÁ Á ÁN hydrogen bonds, forming a three-dimensional supramolecular structure (Table 1 and Fig. 5).
In (I), the nitrate anion is situated in the cavities of the three-dimensional framework and is linked to the framework by N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds (Table 3  A view of the molecular structure of ligand (L1), with atom labelling. Displacement ellipsoids are drawn at the 50% probability level. The intramolecular N-HÁ Á ÁN contact is shown as a dashed line (see Table 2).

Table 2
Selected geometric parameters (Å , ) for (I).  6). The nitrate anion in (I) is not essential for forming the three-dimensional structure, although it may act as a template for the formation of the framework (Batten et al., 2009). This (Shu et al., 2006), in which there are nitrate anions coordinating the silver ions in a 3 fashion and present also in the framework cavities. There are, of course, other examples reported in the Cambridge Structural Database (Groom et al., 2016).
In describing compound (I) as a three-dimensional coordination polymer, we make here the distinction between a coordination polymer and a metal-organic framework. Both have a three-dimensional framework but there are no cavities, even small ones, in the structure of (I). Hence, it should be classed as a three-dimensional coordination polymer according to the IUPAC recommendations on the 'Terminology of metal-organic frameworks and coordination polymers' (Batten et al., 2013).

Database survey
A search of the Cambridge Structural Database (Version 5.38, update February 2017; Groom et al., 2016) for Ag-Ag complexes, excluding silver ion clusters of any kind, gave 321 hits. Limiting the search to Ag-Ag complexes with each silver ion coordinated by two pyridine N atoms, gave 95 hits. The Ag-Ag distances vary between ca 2.6-3.6 Å . One compound, bis[ 2 -2,7-di-tert-butyl-9,9-dimethyl-N,N 0 -bis[(3-pyridyl)methyl]xanthene-4,5-dicarboxamide]disilver bis(trifluoromethanesulfonate) chloroform solvate (HIFKUD; Yue et al., 2007), is particularly interesting because it too involves a  A view along the c axis of the three-dimensional framework of complex (I), showing the AgÁ Á ÁO bonds as dashed lines (see Table 2). The nitrate anions and the C-bound H atoms have been omitted for clarity.

Figure 5
A view along the a axis of the crystal packing of ligand (L1). The columns of (L1) molecules, linked by hydrogen bonds involving the water molecules, are indicated by blue circles. The hydrogen bonds are shown as dashed lines (see Table 1), and for clarity, only the H atoms involved in hydrogen bonding have been included.

Synthesis and crystallization
Ligand (L1) was prepared using the same procedure as for ligand (L2) (Cati et al., 2004). Dimethyl pyrazine-2,3-dicarboxylate (1.96 g, 10 mmol; Alvarez-Ibarra et al., 1994) and an excess of 4-(aminomethyl)pyridine (3.24 g, 30 mmol) in 35 ml of methanol were heated to reflux and heating was continued for 72 h in a two-necked flask (100 ml). The brown solution that formed was concentrated and 15 ml of water were added, which precipitated quantitatively ligand (L1 A solution of (L1) (46 mg; 0.126 mmol) in 6 ml CHCl 3 was introduced into a 13 mm diameter glass tube. It was layered with methanol (ca 2 ml) used as a buffer zone. A solution of AgNO 3 (21 mg, 0.126 mmol) in MeOH (6 ml) was then added gently to avoid possible mixing. The glass tube was sealed with a perforated parafilm and left at room temperature. Colourless block-like crystals were obtained after a few days (yield 60 mg, 92%

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. For the ligand (L1), the NH and water H atoms were located in difference-Fourier maps and refined with distance restraints: O-H = 0.85 (2) Å , N-H = 0.88 (2) Å with U iso (H) = 1.5U eq (O) and 1.2U eq (N). In the final cycles of refinement, the water H atoms were treated as riding atoms. For complex (I), the NH H atoms were included in calculated positions and treated as riding: N-H = 0.88 Å with U iso (H) = 1.2U eq (N). For both compounds, the C-bound H atoms were included in calculated positions and refined as riding: C-H = 0.95-0.99 Å with U iso (H) = 1.2U eq (C). The nitrate anion is positionally disordered about a twofold rotation axis and was refined with fixed occupancies (N10A and N10B = 0.5, O11 and O13 = 0.5, O12 and O14 = 0.25), and all their ADP's were made equal to that of atom O11. Using a one-circle image-plate diffraction system it is not possible to measure 100% of the Ewald sphere, particularly for triclinic or monoclinic systems. This is the case for ligand (L1), which crystallized in the monoclinic space group Pc and for which only 94.7% of the Ewald sphere was accessible.

Funding information
Funding for this research was provided by: Swiss National Science Foundation; University of Neuchâ tel..   (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015), PLATON (Spek, 2009) and publCIF (Westrip, 2010).  (6) 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.