Isotypic MnII and FeII binuclear complexes of the ligand 5,6-bis(pyridin-2-yl)-pyrazine-2,3-dicarboxylic acid

The reaction of manganese dichloride and iron dichloride with the ligand 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylic acid leads to the formation of isotypic binuclear complexes which have a cage-like structure.


Chemical context
The syntheses and crystal structures of the ligand 5,6-bis-(pyridin-2-yl)pyrazine-2,3-dicarboxylic acid (H 2 L) and three different salts, have been described by Alfonso et al. (2001), and it was noted that the ligand crystallizes as a zwitterion in all four compounds. The reaction of H 2 L with CuBr 2 (ratio 1:2) led to the formation of a one-dimensional coordination polymer. On exposure to air, this compound loses the solvent of crystallization and four water molecules, transforming into a polymeric two-dimensional network structure (Neels et al., 2003). In both cases, there are two crystallographically independent fivefold-coordinated copper atoms present, each having an almost perfect square-pyramidal geometry. Recently, we have reported on the crystal structure of the cadmium dichloride complex of ligand H 2 L, which is a twodimensional coordination polymer (Alfonso & Stoeckli-Evans, 2016). Herein, we describe the syntheses and crystal structures of the title isotypic binuclear complexes, (I) and (II), formed by the reaction of H 2 L with, respectively, MnCl 2 and FeCl 2 .

Structural commentary
The complete molecules of complexes (I) and (II) are generated by inversion symmetry, as shown in Figs. 1 and 2, respectively. The metal atoms are sixfold coordinated by one pyrazine N atom (N1), one pyridine N atom (N3), two water O atoms (O1W and O2W), and by two carboxylate O atoms, O1 and O3 i [symmetry code: (i) Àx + 2, Ày + 2, Àz + 2]. Hence, the ligand coordinates to the metal atoms in a tridentate (N,N,O) and a monodentate (O) manner. Atom O3 is bridg-ing, so leading to the formation of a cage-like complex situated about a centre of inversion; illustrated in Fig. 3 for the Fe II complex, (II). The metal-metal distances are Mn1Á Á ÁMn1 i ca 6.58 Å , while the Fe1Á Á ÁFe1 i distance is ca 6.50 Å . Selected bond lengths and angles for compounds (I) and (II), are given in Tables 1 and 2, respectively.

Figure 1
A view of the molecular structure of compound (I), with atom labelling. Unlabelled atoms are related to the labelled atoms by inversion symmetry (Àx + 2, Ày + 2, Àz + 2). Displacement ellipsoids are drawn at the 50% probability level. The solvate water molecules have been omitted for clarity.

Figure 2
A view of the molecular structure of compound (II), with atom labelling. Unlabelled atoms are related to the labelled atoms by inversion symmetry (Àx + 2, Ày + 2, Àz + 2). Displacement ellipsoids are drawn at the 50% probability level. The solvate water molecules have been omitted for clarity.

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.37, last update May 2016; Groom et al., 2016) for the ligand H 2 L, and its dimethyl ester, gave eight hits. Some of these structures have been mentioned in the Chemical context above. In the case of (I) and (II), the ligand coordinates to the metal atom in a tridentate (N,N,O) and monodentate (O) manner. This coordination mode of H 2 L is the same as that observed in the Cd II two-dimensional coordination polymer (Alfonso & Stoeckli-Evans, 2016). The pyridine rings and the carboxylate groups are orientated with respect to the pyrazine ring in a very similar manner for all three compounds.

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 compound (I): H 2 L (64 mg, 0.20 mmol) was added in solid form to an aqueous solution (15 ml) of MnCl 2 Á4H 2 O (45 mg, 0.20 mmol). The yellow solution immediately obtained was stirred for 10 min at room temperature, filtered and the filtrate allowed to slowly evaporate. After two weeks orange-yellow rod-like crystals were obtained. They were separated by filtration and dried in air (yield: 54 mg, 54.5%). Selected IR bands (KBr pellet, cm À1 ): 3226(br, s), Synthesis of compound (II): A degassed aqueous solution (20 ml) of H 2 L (32 mg, 0.10 mmol) was treated with FeCl 2 Á4H 2 O (20 mg, 0.10 mmol). The violet solution immediately obtained was stirred under N 2 at 343 K for 1 h, filtered and the filtrate allowed to slowly evaporate. After two months deep-violet block-like crystals were obtained. They were separated by filtration and air dried (yield: 20 mg, 44.6%). Precipitation of small amounts of iron(III) hydroxide accompanied the formation of the crystals. Selected IR bands (KBr pellet, cm A view along the a axis of the crystal packing of compound (II), showing the hydrogen bonds as dashed lines (see Table 4). The offsetinteractions are shown as dark-blue dashed lines and for clarity only the C-bound H atoms, H7 and H8, have been included.

Figure 4
A view along the c axis of the chain of complexes propagating along the a axis direction. The hydrogen bonds are shown as dashed lines (see Table 3).

Figure 5
A view along the normal to the bc plane of the crystal packing of compound (I). The hydrogen bonds are shown as dashed lines (see Table 3), and the C-bound H atoms have been omitted for clarity.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. For both (I) and (II), the water H atoms were located in difference Fourier maps and refined with distance restraints: O-H = 0.84 (2) Å . The C-bound H atoms were included in calculated positions and treated as riding atoms: C-H = 0.93 Å for (I) and 0.94 Å for (II), with U iso (H) = 1.2U eq (C). Intensity data for (I) were collected at 293 K on a four-circle diffractometer. Only one equivalent of data was measured, hence R int = 0, and as no suitable -scans could be measured no absorption correction was applied. For compound (II), the data were collected at 223 K using a onecircle image-plate diffractometer with which it is not possible to measure 100% of the Ewald sphere, particularly for the triclinic system, hence a small cusp of data was inaccessible.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Mn1 0.70864 (6)  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.59 e Å −3 Δρ min = −0.69 e Å −3 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.

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