Di-μ-aqua-bis[aqua(5-carboxylato-1H-1,2,3-triazole-4-carboxylic acid-κ2 N 3,O 4)lithium]

The crystal structure of the title compound, [Li2(C4H2N3O4)2(H2O)4], contains centrosymmetric dinuclear molecules in which two LiI ions are bridged by two water O atoms. The metal ion is coordinated by one N,O-bidentate ligand and three water O atoms (one of them is symmetry generated), with one of the bridging water O atoms in the apical position of a distorted square pyramid. The carboxylate group that participates in coordination to the metal ion remains protonated; the other is deprotonated and coordination inactive. An intramolecular O—H⋯O hydrogen bond between carboxylate groups is observed. In the crystal, dimers are linked by O—H⋯O, O—H⋯N and N—H⋯O hydrogen bonds, generating a three-dimensional network.


Table 2
Hydrogen-bond geometry (Å , ). indicates clearly that the O2 atom is protonated and acts as a donor in a fairly short intra-molecular hydrogen bond of 2.538 (2) Å to the O3 atom as an acceptor. The C7/O3/O4 carboxylic group remains deprotonated and coordination inactive. The bond distances and bond angles within the triazole ring do not differ from those reported in the structures of other complexes, for example, with Co and Ni (Tong et al., 2011). The dimers form molecular sheets ( Fig. 2) in which they interact via an extensive hydrogen bond network; coordinated water molecules are as donors a hetero-ring N atom and carboxylate O atoms as acceptors (Table 3).

Experimental
1 mmol of 1,2,3-triazole-4,5-dicarboxylic acid and ca2 mmol s of lithium hydroxide dissolved in 50 ml of hot, doubly distilled water were boiled under reflux with stirring for ten hours and then left to crystallize at room temperature.
Colourless blocks deposited after a week among polycrystalline material. After extraction, the crystals were washed with cold ethanol and dried in the air.

Refinement
H atoms belonging to water molecules, the carboxylate group and hetero-ring N atom were located in a difference map and refined isotropically.

Figure 2
The packing of the dimers with hydrogen bonds shown as dashed lines.

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 O1 0.21219 (