Poly[triaquabis(μ2-3-carboxypyrazine-2-carboxylato)dilithium(I)]

In the title compound, [Li2(C6H3N2O4)2(H2O)3]n, the coordination number for both independent Li+ cations is five. One of the Li+ ions has a distorted trigonal–bipyramidal geometry, coordinated by one of the carboxyl O atoms of a 3-carboxypyrazine-2-carboxylate ligand, two O atoms from two water molecules, and an N and a carboxylate O atom of a second 3-carboxypyrazine-2-carboxylate ligand. The other Li+ ion also has a distorted trigonal–bipyramidal geometry, coordinated by one water molecule and two 3-carboxypyrazine-2-carboxylate ligands through an N and a carboxylate O atom from each. One of the carboxyl groups of the two ligands takes part in an intramolecular O—H⋯O hydrogen bond. The stabilization of the crystal structure is further assisted by O—H⋯O, O—H⋯N and C—H⋯O hydrogen-bonding interactions involving the water molecules and carboxylate O atoms.

In the title compound, [Li 2 (C 6 H 3 N 2 O 4 ) 2 (H 2 O) 3 ] n , the coordination number for both independent Li + cations is five. One of the Li + ions has a distorted trigonal-bipyramidal geometry, coordinated by one of the carboxyl O atoms of a 3carboxypyrazine-2-carboxylate ligand, two O atoms from two water molecules, and an N and a carboxylate O atom of a second 3-carboxypyrazine-2-carboxylate ligand. The other Li + ion also has a distorted trigonal-bipyramidal geometry, coordinated by one water molecule and two 3-carboxypyrazine-2-carboxylate ligands through an N and a carboxylate O atom from each. One of the carboxyl groups of the two ligands takes part in an intramolecular O-HÁ Á ÁO hydrogen bond. The stabilization of the crystal structure is further assisted by O-HÁ Á ÁO, O-HÁ Á ÁN and C-HÁ Á ÁO hydrogenbonding interactions involving the water molecules and carboxylate O atoms.

Comment
The systematic design of metal-organic frameworks has became the most fascinating and challenging area of research particularly during the last decade (Lehn, 1995;Haiduc & Edelmann, 1999). Hence, the synthesis of novel coordination polymers has advanced rapidly because of their applications in many areas such as, hydrogen storage (Kitagawa et al., 2004;Mueller et al., 2006), ion-exchange resins (Pancholi & Patel, 1996) and catalysis (Janiak, 2003). Multidendate carboxylic acids are found to be excellent ligands for the synthesis of coordination polymers giving the structures with a diverse range of topologies and conformations, due to the carboxylate groups being able to coordinate to a metal centre as a mono-, bi-, or multidentate ligand (Erxleben, 2003;Ye et al., 2005;. Although most of the studies conducted in this area is primarily focused on coordination polymers containing transition metals as connectors, such as Zn, Ni and Co (Sreenivasulu & Vittal, 2004;Fei, Ang et al., 2006), there is little attention on the Group I metal (López Garzón et al., 2003;Gao et al., 2005;Chen et al., 2007).
As shown in Fig. 1, compound (I) is a polymeric dinuclear complex with two kinds of Li atoms, two kinds of pyrazine-2,3-dicarboxylate ligands and three water molecules in the asymmetric unit. The geometries of the two independent Li atoms are distorted trigonal-bipyramidal, while the coordination modes of the pyrazine-2,3-dicarboxylate ligands are chelation. The Li1 ion has a five-coordinate geometry and achieves the coordination number by bonding to one of the carboxylate O atom of pyrazine-2,3-dicarboxylate ligand, two O atoms from two water molecules and a chelation pyrazine-2,3-dicarboxylate ligand (through the interactions by utilizing both N and O atoms) of the adjacent molecule. The Li2 ion has also distorted trigonal-bipyramidal geometry, with one water molecule, one chelation ligand molecule (through the interactions by utilizing both N and O atoms of the same ligand) and symmetry related chelation pyrazine-2-3-dicarboxylate ligand. There is no metal to-metal interaction; the Li-Li distance is 7.221 (2) Å. The Li-O distances are in the range 1.980 (5) Å to 2.074 (4) Å (for Li1) and 1.901 (5) Å to 1.974 (4) Å (for Li2), in accordance with the corresponding values reported for other lithium complexes (Chen et al. 2007;Kim et al. 2007). Li-N bond lengths also lie within the normal ranges found for similar bonds in the literature (Grossie et al. 2006). The C-O distances are comparable with structurally similar compounds (Chen et al. 2007). There are appreciable differences between the two carboxyl groups of the each ligand molecule. The C-O distances at C6 and C12 are (1.228 (3) Å, 1.275 and 1.216 (3) Å, 1.283 (3) Å respectively), and these are fairly typical for supplementary materials sup-2 a carboxylic acid group (Speakman, 1972). On the other hand, those at C5 and C11 are (1.236 (3) Å, 1.268 (3) Å and 1.247 (3) Å, 1.258 (3) Å respectively), giving a strong indication of a carboxylate ion. As is typically the case, the mean value of the four C-O distances in the different carboxyl/carboxylate groups is almost the same, at 1.254 (3) (Table 2).

Experimental
Li 2 CO 3 (220 mg, 3 mmol) was carefully added to an aqueous solution (20 ml) of pyrazine 2,3-dicarboxylic acid (1008 mg, 6 mmol), until no further bubbles formed. The reaction mixture gave a colourless and clear solution which was stirred at 323 K for 10 h, until it solidified. The solid product was then redissolved in water (5 ml) and allowed to stand for a day at ambient temperature, after which transparent fine crystals were harvested.

Refinement
All H atoms were repositioned geometrically. They were initially refined with soft restraints on the bond lengths and angles to regularize their geometry (C-H = 0.93 Å, O-H in the range 0.86 − 0.94 Å) and U iso (H) (in the range 1.2-1.5 times U eq of the parent atom), after which the positions were refined with riding constraints.