Poly[diaqua(μ2-3-carboxypyrazine-2-carboxylato)(μ2-pyrazine-2,3-dicarboxylic acid)potassium(I)]

The structural unit of the title compound, [K(C6H3N2O4)(C6H4N2O4)(H2O)2]n, consists of one potassium cation, one hydrogen pyrazine-2,3-dicarboxylate anion, one pyrazine-2,3-dicarboxylic acid molecule and two water molecules; this is twice the asymmetric unit, since the potassium cation lies on an inversion centre. Each anion or acid molecule is linked to two potassium cations, while the potassium cation has contacts to four symmetry-equivalent organic ligands, with two different coordination modes towards this cation. In addition, each potassium cation is coordinated by two water O atoms, raising the coordination number to eight. One of the carboxyl groups of the acid retains its H atom, which forms a hydrogen bond to a coordinated water molecule. The other carboxyl group is deprotonated in half of the ligands and protonated in the other half, taking part in a strong O—H⋯O hydrogen bond disordered over an inversion centre. The stabilization of the crystal structure is further assisted by O—H⋯O and O—H⋯N hydrogen bonds in which water acts as the donor.

The structural unit of the title compound, [K(C 6 H 3 N 2 O 4 )-(C 6 H 4 N 2 O 4 )(H 2 O) 2 ] n , consists of one potassium cation, one hydrogen pyrazine-2,3-dicarboxylate anion, one pyrazine-2,3dicarboxylic acid molecule and two water molecules; this is twice the asymmetric unit, since the potassium cation lies on an inversion centre. Each anion or acid molecule is linked to two potassium cations, while the potassium cation has contacts to four symmetry-equivalent organic ligands, with two different coordination modes towards this cation. In addition, each potassium cation is coordinated by two water O atoms, raising the coordination number to eight. One of the carboxyl groups of the acid retains its H atom, which forms a hydrogen bond to a coordinated water molecule. The other carboxyl group is deprotonated in half of the ligands and protonated in the other half, taking part in a strong O-HÁ Á ÁO hydrogen bond disordered over an inversion centre. The stabilization of the crystal structure is further assisted by O-HÁ Á ÁO and O-HÁ Á ÁN hydrogen bonds in which water acts as the donor.

S1. Comment
Pyrazine-2,3-dicarboxylic acid (Takusagawa & Shimada, 1973) and its dianion (Richard et al., 1973;Nepveu et al., 1993) have been reported to be well suited for the construction of multidimentional frameworks (nD, n = 1-3), owing to the presence of two adjacent carboxylate groups (O donor atoms) as substituents on the N-heterocyclic pyrazine ring (N donor atoms). In recent years, a variety of metal-organic compound of pyrazine-2,3-dicarboxylic acid have been characterized crystallographically due to growing interest in supramolecular chemistry. These include the calcium 1997b), sodium (Tombul et al., 2006) and caesium (Tombul et al., 2007) complexes. We present here the synthesis and crystal structure of the hydrated potassium complex, (I), formed with pyrazine-2,3-dicarboxylic acid.
The structural unit of the title compound, (I), contains one potassium cation, one hydrogen pyrazine-2,3-dicarboxylate anion, one pyrazine-2,3-dicarboxylic acid molecule and two water molecules; this is twice the asymmetric unit, as the potassium ion lies on an inversion centre. Pyrazine-2,3-dicarboxylic acid is, on average, only half deprotonated at one of the carboxylate groups (O1) and together with the symmetry-related oxygen atom (O1 v ) which is also half deprotonated, completes the charge balance of the cation. In the crystal structure, the anion or acid molecule is linked to two potassium cations, while the K + cation is surrounded by four organic ligands, two of which are coordinated by utilizing both N and O atoms and the other two are coordinated solely by O atoms. In addition, each potassium cation is coordinated by two water molecules, achieving a coordination number of eight. The primary coordination comprises six oxygen atoms, together with two nitrogen atoms. The planes of the carboxylic/carboxylate groups (O4/C5/O1) and (O2/C6/O5) form dihedral angles with the ring plane of 54.33 (14) and 53.75 (14)°, respectively. The K-O distances are in the range 2.877 (2) Å to 3.089 (2) Å, in accordance with the corresponding values reported for other potassium complexes (Clegg & Liddle, 2004;Cuesta et al., 2003).
In the crystal structure, an asymmetric strong hydrogen bond occurs, linking carboxylate O atoms (Table 2). Atom H1 is involved in this bond and maintains the charge balance within the structure. The ordered carboxyl group forms a hydrogen bond in which water serves as acceptor. The water molecules are involved in normal, slightly bent, hydrogen bonds with hydrogen pyrazine-2,3-dicarboxylate (Table 2); the acceptors are carboxylate O atoms and N atoms of the aromatic ring.

S2. Experimental
K 2 CO 3 (346 mg, 2.5 mmol) was carefully added to an aqueous solution (20 ml) of pyrazine-2,3-dicarboxylic acid (1680 mg, 10 mmol), until no further bubbles formed. The reaction mixture gave a colourless and clear solution which was stirred at 333 K for 2.5 h, until it solidified. The solid product was redissolved in water (10 ml) and allowed to stand for a week at room temperature, after which transparent fine crystals were harvested.

S3. Refinement
The H atoms were all located in a difference map, but those attached to carbon atoms were repositioned geometrically and treated as riding, with C-H in the range 0.93-0.98 Å and U iso (H) = 1.2U eq (C). O-bound H atoms were refined freely.

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.