Poly[diammonium [(μ4-butane-1,2,3,4-tetracarboxylato)zincate] tetrahydrate]

In the title compound, {(NH4)2[Zn(C8H6O8)]·4H2O}n, the asymmetric unit contains one ammonium cation, half of a butane-1,2,3,4-tetracarboxylate anion, one Zn2+ cation and two water molecules. The butane-1,2,3,4-tetracarboxylate ligand is located about an inversion centre at the mid-point of the central C—C bond. The Zn2+ cation is situated on a twofold rotation axis and is surrounded by four O atoms from four symmetry-related butane-1,2,3,4-tetracarboxylate anions in a distorted tetrahedral environment. In turn, each anion coordinates to four Zn2+ cations. The bridging mode of the anions leads to a three-dimensional framework structure with channels extending along [110] and [010] in which the ammonium cations and the water molecules are located. N—H⋯O and O—H⋯O hydrogen bonding between the cations and water molecules and the uncoordinating O atoms of the carboxylate groups consolidates the crystal packing.

In the title compound, {(NH 4 ) 2 [Zn(C 8 H 6 O 8 )]Á4H 2 O} n , the asymmetric unit contains one ammonium cation, half of a butane-1,2,3,4-tetracarboxylate anion, one Zn 2+ cation and two water molecules. The butane-1,2,3,4-tetracarboxylate ligand is located about an inversion centre at the mid-point of the central C-C bond. The Zn 2+ cation is situated on a twofold rotation axis and is surrounded by four O atoms from four symmetry-related butane-1,2,3,4-tetracarboxylate anions in a distorted tetrahedral environment. In turn, each anion coordinates to four Zn 2+ cations. The bridging mode of the anions leads to a three-dimensional framework structure with channels extending along [110] and [010] in which the ammonium cations and the water molecules are located. N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonding between the cations and water molecules and the uncoordinating O atoms of the carboxylate groups consolidates the crystal packing.
The asymmetric unit of compound (I) contains half of the butane-1,2,3,4-tetracarboxylate anion, one ammonium cation, two water molecules and one Zn 2+ cation. The butane-1,2,3,4-tetracarboxylate anion has an inversion centre located at the mid point of the central 3-C, and 4-C bond (symmetry code -x, -y, -z). The Zn 2+ ion lies on a twofold rotation axis. It is surrounded by four O atoms from four symmetry-related butane-1,2,3,4-tetracarboxylate anions, forming a tetrahedral coordination geometry. Of the four coordinating O atoms, two come from the carboxylate groups in 1-position, while the other two come from the carboxylate groups in 3-position. The Zn-O bond lengths are almost equal.
They are hydrogen bonded to each other through O-H···O and N-H···O interactions and also hydrogen-bonded to the uncoordinating O atoms of the carboxylate groups of the anion.
Single crystals of the title compound were obtained by reacting zinc(II) acetate dihydrate with butane-1,2,3,4-tetracarboxylic acid in basic solution in the presence of 3,5-dimethyl pyrazole. However, 3,5-dimethyl pyrazole does not appear in the title compound. It should be noted that single crystals could not be obtained by evaporating an appropriate solution of the title compound in water or organic solvents. We found that it can be dissolved in a concentrated solution of ammonia; thus its single crystals were grown by slow evaporating its ammonia solution.

Experimental
Butane-1,2,3,4-tetracarboxylic acid (22.3.1 mg, 0.10 mmol) was dissolved in 10 ml of methanol, to this solution zinc acetate dihydrate (42.6 mg, 0.2 mmol), and 3,5-dimethylpyrazole (19.2 mg, 0.2 mmol) was added. The solution was stirred for about 2 h at room temperature, and a large amount of precipitate formed. To the suspensition concentrated ammonia solution was added until the precipitate dissolved completely. The solution was filtered into a test tube and was left standing at room temperature. Several days later colorless block crystals could be obtained.

Figure 1
The molecular components of the structure of (I), showing the atom-numbering scheme and displacement ellipsoids at the 30% probability level.  The packing of the structure of (I) showing the channel formation. Hydrogen bonds are displayed with 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.