A second monoclinic polymorph of ethylenediammonium bis(hydrogen squarate) monohydrate

The title compound, C2H10N2 2+·2HC4O4 −·H2O, a new polymorph of ethylenediammonium bis(hydrogen squarate) monohydrate, was synthesized by slow evaporation of an acid solution. The asymetric unit contains two hydrogen squarate anions, two half-molecules of protonated ethylenediamine arranged around a twofold axis and one water molecule. In the crystal, N—H⋯O and O—H⋯O hydrogen bonds between the hydrogen squarate anions, protonated N atoms from the amine group and water molecules lead to a three-dimensional framework. In particular, the cohesion between the squarate groups is ensured by very short intermolecular hydrogen bonds bonds. The title compound crystallized together with the previously reported polymorph [Mathew et al. (2002 ▶). J. Mol. Struct. 641, 263–279].


Comment
In the course of a study on mixed squarate of amines and metals, the role of the amine group has been investigated in the topology of the organic-inorganic framework. The preparation did not lead to a mixed compound but to a new hydrogen squarate of ethylenediammonium.
The compound is a polymorph of the compound previously reported by Mathew et al., 2002, whose molecular framework is also stabilized by hydrogen bonds (Fig. 1, Table 1). In the title compound hydrogen bonds connect the hydrogen squarate units along the a axis in the form of zigzag chains, which are connected to each other along the c axis through hydrogen bonds implying the water molecules, then forming a layer. Amine groups are situated in between neighbour layers and connected to them along the b axis through hydrogen bonds leading to a molecular three-dimensional framework (Table 1, Fig. 2).
The main differences between the structures of the two polymorphs reside in the orientation of the amine groups related to that of the mean planes of the squarate groups. Indeed, in the title structure, the ethylenediammonium cations are perpendicular to the squarate groups, while the mean planes between these two molecules in the already reported polymorph deviate to 56.2 (2)°.
The slow evaporation at room temperature leads after some hours to the formation of both polymorphs. A metal salt seems to be necessary to the synthesis of the title compound even if its role has not been clearly established.

Refinement
All H atoms were found from Fourier difference maps but those attached to C and N atoms were fixed geometrically and treated as riding with C-H = 0.98 Å and N-H = 0.87 Å with U iso (H) = 1.2U eq (C) or U iso (H) = 1.5U eq (N). The H attached to the water molecule and those of the hydroxyl groups were refined using restraints: O-H= 0.92 (1)Å and H···H= 1.42 (2)Å) for the water and O-H = 1.05 (2)Å for the hydroxyl H with U iso (H) = 1.5U eq (O). In the last cycles of refinement, they were treated as riding on their parent O atoms.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2σ(F 2 ) is used only for calculating Rfactors(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.