catena-Poly[[aqualithium(I)]-μ-3-carboxy-5,6-dimethylpyrazine-2-carboxylato-κ4 O 2,N 1:O 3,N 4]

The asymmetric unit of the title compound, [Li(C8H6N2O4)(H2O)]n, comprises three Li cations, two of which are located on a twofold rotation axis, two carboxylate anions and three water molecules, of which two are situated on the twofold rotation axis being aqua ligands. Both carboxylate anions are in μ2-bridging mode. All Li ions show a trigonal–bipyramidal coordination mode; the two located in special positions are bridged through N,O-bonding sites generating a polymeric ribbon along the c-axis direction. The Li cation in a general position creates an independent polymeric ribbon through N,O-bonding sites of the two symmetry-related ligands; the trigonal–bipyramidal coordination is completed by an aqua ligand. In both carboxylate anions, a carboxylate and a carboxylic acid group form an intramolecular hydrogen bond. The polymeric ribbons running along [001] are interconnected by hydrogen bonds in which the water molecules act as donors and carboxylate O atoms act as acceptors, giving rise to a three-dimensional architecture.

The asymmetric unit of the title compound, [Li(C 8 H 6 N 2 O 4 )-(H 2 O)] n , comprises three Li cations, two of which are located on a twofold rotation axis, two carboxylate anions and three water molecules, of which two are situated on the twofold rotation axis being aqua ligands. Both carboxylate anions are in 2 -bridging mode. All Li ions show a trigonal-bipyramidal coordination mode; the two located in special positions are bridged through N,O-bonding sites generating a polymeric ribbon along the c-axis direction. The Li cation in a general position creates an independent polymeric ribbon through N,O-bonding sites of the two symmetry-related ligands; the trigonal-bipyramidal coordination is completed by an aqua ligand. In both carboxylate anions, a carboxylate and a carboxylic acid group form an intramolecular hydrogen bond. The polymeric ribbons running along [001] are interconnected by hydrogen bonds in which the water molecules act as donors and carboxylate O atoms act as acceptors, giving rise to a three-dimensional architecture.
Data collection: CrysAlis PRO (Oxford Diffraction, 2010); cell refinement: CrysAlis PRO; data reduction: CrysAlis PRO; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL. The asymmetric unit of the title compound contains three Li cations (two of them located on the rotation twofold axes), two ligand and three water molecules (two of them located on the rotation twofold axes). Two water-coordinated Li ions bridged by a ligand via both N,O bonding sites form the type 1 molecular ribbon; the type 2 ribbon is built of units composed of a water-coordinated Li cation and a ligand which also uses its N,O bonding sites ( Fig.1, Table 1). Both ligands act in µ 2 bridging mode. All three Li cations show slightly distorted trigonal bipyramidal coordination geometry.
The Li11 cation is situated in the equatorial plane composed of O11, O11 ii and O15 atoms; N11 and N11 ii atoms are in the apical positions. The equatorial plane of Li12 coordination is formed by O13, O13 iii and O16 atoms and N14and N14 iii atoms are at the apices; Li12 is coplanar with the equatorial ligand plane. However, Li21 cation is 0.0142 (2) Å out of the equatorial plane of O21, O21 i and O25 atoms; N21 and N21 i are at the apices. The Li-O and Li-N bond distances (Table 2), fall in the range observed in the structures of other Li complexes with diazine carboxylate ligands. Methyl carbon and pyrazine ring atoms of both ligands are coplanar with r.m.s of 0.0062 (1) Å in the ligand 1 and 0.0193 (2) Å in the ligand 2. The carboxylic groups C11/O11/O12 and C18/O13/O14 form with the ligand 1 ring dihedral angles of 6.1 (1)° and 10.9 (1)°, respectively. The dihedral angles between ligand 2 ring and carboxyl groups C27/O21/O22 and C28/O23/O24 are 1.2 (1)° and 9.0 (1)°, respectively. In both ligands the second carboxyl O atoms remain protonated and act as donors in the short intramolecular hydrogen bonds. Bond distances and bond angles within the ligand molecules do not differ from those reported in the structure of the parent acid (Vishwershwar et al., 2001). Two ribbons of the same type form pairs which propagate in the [001] direction. The planes of ribbon 1 and ribbon 2 pairs are inclined 91.9 (3)° each to the other (Fig. 2). They are held together by a system of hydrogen bonds in which water molecules act as donors and carboxyl O atoms as acceptors (Table 3).

Experimental
To 50 mL of a solution of 5,6-dimethylpyrazine-2,3-dicarboxylic acid dihydrate in doubly distilled water an 1 N solution of LiOH was added by drops until pH reached 5.5. Then, the solution was boiled under reflux with stirring for 5 h. After cooling to room temperature, the solution was left to crystallize. The material which was found after evaporation to dryness was recrystallized from cold water. The obtained single-crystal blocks were washed with cold ethanol and dried in air.

Refinement
Hydrogen atoms belonging to water molecules and the carboxylic group were located in a difference map and refined isotropically while twelve methyl H atoms were located at calculated positions and treated as riding on the parent C atoms with C-H=0.96 Å.

catena-Poly[[aqualithium(I)]-µ-3-carboxy-5,6-dimethylpyrazine-2-carboxylato-κ 4 O 2 ,N 1 :O 3 ,N 4 ]
Crystal data [Li(C 8 (Clark & Reid, 1995). Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.