catena-Poly[[lithium-μ2-(dihydrogen pyrazine-2,3,5,6-tetracarboxylato)-κ6 O 2,N 1,O 6;O 3,N 4,O 5-lithium-di-μ-aqua-κ4 O:O] 2.5-hydrate]

The title coordination polymer, {[Li2(C8H2N2O8)(H2O)2]·2.5H2O}n, is built up from molecular ribbons propagating in the c-axis direction of the orthorhombic unit cell; the ligand bridges two Li+ ions using both its N,O,O′-bonding sites and adjacent Li+ ions are bridged by pairs of water molecules. The coordination geometry of the metal ion is distorted trigonal bipyramidal, with the ligand O atoms in the axial sites. Two of the carboxylate groups of the ligand remain protonated and form short symmetric O—H⋯O hydrogen bonds. In the crystal, the ribbons interact via a network of O—H⋯O hydrogen bonds in which coordinating water molecules act as donors and carboxylate O atoms within adjacent ribbons act as acceptors, giving rise to a three-dimensional framework. O—H⋯N interactions are also observed. The asymmetric unit contains quarter of the ligand and the complete ligand has 2/m symmetry; the Li+ ion lies on a special position with m.. site symmetry. Both bridging water molecules have m2m site symmetry and both lattice water molecules have m.. site symmetry; one of the latter was modelled with a site occupancy of 0.25.

The title coordination polymer, {[Li 2 (C 8 H 2 N 2 O 8 )(H 2 O) 2 ]Á-2.5H 2 O} n , is built up from molecular ribbons propagating in the c-axis direction of the orthorhombic unit cell; the ligand bridges two Li + ions using both its N,O,O 0 -bonding sites and adjacent Li + ions are bridged by pairs of water molecules. The coordination geometry of the metal ion is distorted trigonal bipyramidal, with the ligand O atoms in the axial sites. Two of the carboxylate groups of the ligand remain protonated and form short symmetric O-HÁ Á ÁO hydrogen bonds. In the crystal, the ribbons interact via a network of O-HÁ Á ÁO hydrogen bonds in which coordinating water molecules act as donors and carboxylate O atoms within adjacent ribbons act as acceptors, giving rise to a three-dimensional framework. O-HÁ Á ÁN interactions are also observed. The asymmetric unit contains quarter of the ligand and the complete ligand has 2/m symmetry; the Li + ion lies on a special position with m.. site symmetry. Both bridging water molecules have m2m site symmetry and both lattice water molecules have m.. site symmetry; one of the latter was modelled with a site occupancy of 0.25.
Data collection: CrysAlis PRO (Agilent, 2011); 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.  (Table 2). Within a ligand two carboxylate groups remain protonated to maintain charge balance and form short, intramolecular symmetric hydrogen bonds of 2.409 (1) Å (Table 3). The ligand ring is almost planar (r.m.s. is 0.0003 (1) Å; the carboxylate C17/O1/O12 group makes with it a dihedral angle of 2.6 (1)°. Bond distances and bond angles within the hetero-ring do not differ from those reported in the structures of two other Li complexes with the title ligand (Starosta & Leciejewicz, 2010). The Fourier map shows two solvation water molecules (O5 and O6), both at special positions. The refinemt reveals a disorder of the O6 aqua molecule with 0.25 positional occupancy i.e. two molecules at random in a unit cell. This molecule locates coplanarly with the N1, O3, O4 and Li1 atoms at a distance of 2.538 (2) Å from the latter. The ribbons are held together by a system of hydrogen bonds in which coordinated and solvation water molecules act as donors and carboxylate O atoms as acceptors giving rise to a three-dimensional framework. The structures of two other Li complexes with the title ligand have been recently reported. The structural unit of the title polymer is closely related to that one observed in the structure of the first compound which cosists of discrete dimeric molecules built of two aqua-coordinated Li ions bridged by the ligand molecule via both its N,O,O bonding sites (Starosta & Leciejewicz, 2014). The structure of the second complex is built of anions each consisting of an aqua coordinated Li ion cheleted to a doubly deprotonated ligand molecule and of aqua coordinated Li cations (Starosta & Leciejewicz, 2010).

Experimental
An aqueous solution containing 2 mmol of lithium nitrate and a small excess over 1 mmol of pyrazine-2,3,5,6-tetracarboxylic acid dihydrate was heated under reflux with stirring at ca 330 K for 10 h. After cooling to room temperature the solution was left to evaporate. Three days later well formed colorless blocks of the title compound were found, which were washed with cold methanol and dried in the air.

Refinement
Water and carcoxylate H atoms were found in the Fourier map and refined isotropically.  A fragment of a [001] chain in the title compound with 50% probability displacement ellipsoids. Symmetry code: i x, y, -z + 1; ii -x + 1, y, z; iii x, -y, -z + 1.

Figure 2
The alignment of the ribbons viewed along the crystal a direction.

catena-Poly[[lithium-µ 2 -(dihydrogen pyrazine-2,3,5,6-tetracarboxylato)-κ 6 O 2 ,N 1 ,O 6 ;O 3 ,N 4 ,O 5 -lithium-di-µ-aquaκ 4 O:O] 2.5-hydrate]
Crystal data [Li(C 8  neighbouring sites H atoms treated by a mixture of independent and constrained refinement w = 1/[σ 2 (F o 2 ) + (0.0592P) 2 + 0.9014P] where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.25 e Å −3 Δρ min = −0.29 e Å −3 Special details 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ.  (17)