Isotypic one-dimensional coordination polymers: catena-poly[[dichloridocadmium]-μ-5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylato-κ2 N 5:N 6] and catena-poly[[dichloridomercury(II)]-μ-5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylato-κ2 N 5:N 6]

The title complexes are isotypic one-dimensional coordination polymers. The metal ions are bridged by binding to the N atoms of the two pyridine rings, and have an MN2Cl2 bisphenoidal coordination geometry. In the crystals of both compounds, the polymer chains are linked via pairs of C—H⋯Cl hydrogen bonds, forming corrugated slabs parallel to the ac plane.


Chemical context
The crystal structures of the dimethyl and diethyl esters of 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylic acid (L1H 2 ; Alfonso et al., 2001) have been reported on recently (Alfonso & Stoeckli-Evans, 2016). They were originally synthesized to study the hydrolysis of these esters with first row transition metals (Alfonso, 1999). Subsequent studies of their reaction with d 10 or post-transition metals lead to the formation of the title compounds, and we report herein on the syntheses and crystal structures of the title isotypic cadmium(II) and mercury(II) coordination polymers. ISSN 2056-9890

Structural commentary
In compounds (I) and (II), the metal atom is located on a twofold rotation axis and a second such axis bisects the C ar -C ar bonds of the pyrazine ring; as illustrated in Fig. 1 for the cadmium complex (I), and in Fig. 2 for the mercury complex (II). Details of the bond lengths and bond angles involving the metal atoms are given in Table 1 for (I), and in Table 2 for (II). The metal atoms are bridged by binding to the N atoms of the two pyridine rings, N2 and N2 i ; Cd1-N2 = 2.3862 (17) Å in (I) and Hg1-N2 = 2.590 (5) Å in (II  (5) Å . The difference in the metal-N pyrazine and metal-N pyridine bond lengths is 0.389 (2) Å for the Cd-N bonds but only 0.286 (5) Å for the Hg-N bonds (see Tables 1 and 2).
The fourfold coordination geometry of the metal atoms differ slightly, as illustrated in Fig. 3 a structural overlap of the two compounds. In (I) atom Cd1 has a 4 parameter of 0.53, while for the Hg1 atom in (II) the 4 parameter = 0.30 (extreme values: 4 = 0 for square-planar, 1 for tetrahedral and 0.85 for trigonal-pyramidal geometry; Yang et al., 2007). When also considering the values of the Cl-M-Cl and N-M-N bond angles in both compounds (see Tables 1 and 2) A view of the molecular structure of compound (I), showing the atom labelling [symmetry codes: (a) Àx + 1 2 , y, Àz + 3 2 ; (b) Àx + 3 2 , y, Àz + 3 2 ]. Displacement ellipsoids are drawn at the 50% probability level

Figure 3
A view of the structural overlap of the cadmium complex (I) in blue and the mercury complex (II) in red; also illustrating the slight difference in the bisphenoidal coordination geometry of the two metal atoms (MN 2 Cl 2 ).
It can be seen from Fig. 4, a structural overlap of the ligand itself (Alfonso & Stoeckli-Evans, 2016) with the coordinating ligand in compound (I), that both the pyridine ring involving atom N4, and the carboxylate group, involving atoms O1 and O2, have been rotated by ca 100 and 160 , respectively, on coordination to the metal atom. While the pyrazine ring is ideally planar in the ligand (r.m.s. deviation = 0.032 Å ), on coordination it is less planar with r.m.s. deviations of 0.096 and 0.092 Å for (I) and (II), respectively.

Supramolecular features
In the crystals of both compounds, the polymer chains are linked via a pair of C-HÁ Á ÁCl hydrogen bonds, forming corrugated slabs parallel to the ac plane, as illustrated in Fig. 5. Within the slabs, the hydrogen bonding forms R 2 2 (16) and R 2 2 (18) type loops, as shown in Fig. 6. Details of the hydrogen bonding are given in Table 3 for compound (I) and Table 4 for compound (II). There are no other significant intermolecular interactions present for either structure.  Table 3 Hydrogen-bond geometry (Å , ) for (I).

Figure 4
A view of the structural overlap of the ligand (Me 2 L, green; Alfonso & Stoeckli-Evans, 2016) and the coordinating ligand (blue) in compound (I).

Figure 5
A view along the a axis of the crystal packing of compound (I). The hydrogen bonds are shown as dashed lines (see Table 3; only H atom H9A has been included).
Synthesis of compound (I): CdCl 2 Á2H 2 O (22 mg, 0.1 mmol) in 25 ml of dry MeOH was slowly added to a solution of Me 2 L in 10 ml of dry MeOH. The colourless solution that formed was stirred at room temperature for 1 h, then filtered to remove any impurities. The filtrate was allowed to stand over several days until colourless square rod-like crystals were obtained (yield: 40 mg, 75%). Elemental analysis for C 18  Synthesis of compound (II): Me 2 L (35 mg, 0.1 mmol) was added in solid form to a solution of HgCl 2 Á2H 2 O (30 mg, 0.1 mmol) in 25 ml of dry MeOH. The colourless solution immediately obtained was stirred at room temperature for 2 h, filtered to remove any impurity, and the filtrate allowed to evaporate slowly. After two days colourless needle-like crystals were obtained (yield: 47mg, 76%). Elemental analysis for C 18 Table 4; only H atom H9A has been included).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. For both compounds the C-bound H atoms were included in calculated positions and treated as riding atoms: C-H = 0.93-0.97 Å with U iso (H) = 1.5U eq (Cmethyl) and 1.2U eq (C) for other H-atoms. For the mercury complex (II), R int = 0.000 as only one equivalent was measured. 1184-1188. Groom