A two-dimensional CdII coordination polymer: poly[diaqua[μ3-5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylato-κ5 O 2:O 3:O 3,N 4,N 5]cadmium]

The reaction of cadmium dichloride with the ligand 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylic acid leads to the formation of a two-dimensional coordination polymer.


Chemical context
The crystal structure of the ligand 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylic acid (H 2 L) and the chloride, perchlorate and hexafluorophosphate salts, have been reported on previously (Alfonso et al., 2001). Interestingly, the ligand crystallizes as a zwitterion in all four compounds. The reaction of H 2 L with CuBr 2 (ratio 1:2) leads to the formation of a onedimensional coordination polymer. On exposure to air, the compound loses the solvent of crystallization and four water molecules, transforming into a two-dimensional coordination polymer (Neels et al., 2003). In both cases, there are two crystallographically independent fivefold-coordinated copper atoms present and they all have almost perfect square-pyramidal geometry. Recently, we have reported on the crystal structures of the dimethyl and diethyl ester of the H 2 L ligand (Alfonso & Stoeckli-Evans, 2016a). The reaction of the dimethyl ester of H 2 L with CdCl 2 and HgCl 2 leads to the formation of isotypic one-dimensional coordination polymers (Alfonso & Stoeckli-Evans, 2016b). There the ligand coordinates to the metal atom via the pyridine N atoms, and they have MN 2 Cl 2 fourfold bisphenoidal coordination geometry.

Structural commentary
The reaction of 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylic acid with cadmium dichloride leads to the formation of the title two-dimensional coordination polymer (Fig. 1). Here the metal atom is sevenfold coordinated by one pyrazine N atom (N1), one pyridine N atom (N3) and two water O atoms (O1W and O2W), and by two carboxylate O atoms (O1 and O3). Atom O1 bridges two cadmium atoms to form a Cd 2 O 2 unit situated about a centre of inversion; the Cd1Á Á ÁCd1 ii distance is 3.8753 (8) Å , while the Cd-O1 and Cd-O1 ii bonds are, respectively, 2.371 (4) and 2.427 (4) Å , and the Cd1-O1Á Á ÁCd1 ii and O1-CdÁ Á ÁO1 ii bond angles are 107.74 (13) and 72.26 (13) , respectively. As can be seen in Fig. 1, the ligand coordinates to the cadmium atom in a tridentate (N,N,O) and a monodentate manner (O). It can be seen from the carboxylate C-O bond lengths [C15-O1 and C15-O2 are 1.255 (6) and 1.253 (6) Å , respectively, while C16-O3 and C16-O4 are 1.258 (6) and 1.227 (6) Å , respectively] that the negative charge is distributed over the O-C-O group for the first, but located on atom O3 for the second.
Selected bond lengths and angles involving atom Cd1 are given in Table 1 (4) and 2.377 (4) Å ], while the bridging Cd1Á Á ÁO1 ii distance is the longest at 2.427 (4) Å . The geometry of the sevenfold-coordinated cadmium atom can best be described as a distorted pentagonal bipyramid, with atoms O1,N1,N3,O2W,O1 ii in the basal plane and atoms O1W,O3 i in the apical positions with an O1W-Cd1-O3 i bond angle of 157.41 (15) ( Table 1).

Supramolecular features
In the crystal, the two-dimensional polymer networks lie parallel to the bc plane, as illustrated in Figs. 2 and 3. The networks are aligned back-to-back along the a axis, with the non-coordinating pyridine rings directed into the space between the networks (Fig. 4). Within the networks there are a number of O-HÁ Á ÁO hydrogen bonds present, involving the water molecules and the carboxylate O atoms (Table 2 and Fig. 5). There are also C-HÁ Á ÁO and C-HÁ Á ÁN hydrogen bonds present within the network (Table 2). A view along the a axis of the title two-dimensional coordination polymer. The C-bound H atoms have been omitted for clarity.

Figure 3
A view along the c axis of the title two-dimensional coordination polymer. The C-bound H atoms have been omitted for clarity.

Figure 5
A view normal to plane (110) of the O-HÁ Á ÁO hydrogen bonds (dashed lines; see Table 2) within the polymer network, involving the carboxylate O atoms (red balls) and the coordinating water molecules. The C atoms and C-bound H atoms of the ligand have been omitted for clarity.

Figure 4
A view in projection down the c axis of the crystal packing of the title twodimensional coordination polymer. The C-bound H atoms have been omitted for clarity.
3.8753 (8) Å , and 107.74 (13) and 72.26 (13) , respectively, in the title compound. However, such an arrangement is extremely common for cadmium(II) complexes (over 600 hits in the CSD) and the bond lengths and angles vary enormously; for example the CdÁ Á ÁCd distance varies from ca 3.0 to 4.3 Å , the Cd-OÁ Á ÁCd angle varies from ca 82 to 119 and the O-CdÁ Á ÁO angle from ca 60 to 90 .

Synthesis and crystallization
The synthesis of the ligand 5,6-bis(pyridin-2-yl)pyrazine-2,3dicarboxylic acid (H 2 L) has been reported previously (Alfonso et al., 2001). Synthesis of the title coordination polymer: H 2 L (32 mg, 0.10 mmol) was added to an aqueous solution (25 ml) of CdCl 2 Á2H 2 O (22 mg, 0.10 mmol). The colourless solution immediately obtained was stirred for 1 h at room temperature. The reaction mixture was then filtered and the filtrate allowed to evaporate slowly at room temperature. After two weeks, small colourless plate-like crystals of the title compound were obtained, separated by filtration and dried in air (

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The water H atoms were located in a difference Fourier map and refined with distance restraints: O-H = 0.84 (2) and HÁ Á ÁH = 1.35 (2) Å , with U iso (H) = 1.5U eq (O). The C-bound H atoms were included in calculated positions and treated as riding atoms: C-H = 0.94 Å with U iso (H) = 1.2U eq (C). The best crystal available was extremely thin (0.01 mm) and as the shape of the crystal was irregular it was not possible to carry out a numerical absorption correction. The displacement ellipsoids for two carboxylate O atoms (O2 and O4) and a water O atom (OW1) are large but attempts to split these atoms were not successful.  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).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq  (7)