Crystal structure of propane-1,3-diaminium squarate dihydrate

The crystal structure of propane-1,3-diaminium squarate dihydrate (space group P4bm, Z = 2) features a triperiodic N—H⋯O and O—H⋯O hydrogen-bond network with a polar c axis.

Propane-1,3-diaminium squarate dihydrate, C 3 H 12 N 2 2+ •C 4 O 4 2À •2H 2 O, results from the proton-transfer reaction of propane-1,3-diamine with squaric acid and subsequent crystallization from aqueous medium.The title compound crystallizes in the tetragonal crystal system (space group P4bm) with Z = 2.The squarate dianion belongs to the point group D 4h and contains a crystallographic fourfold axis.The propane-1,3-diaminium dication exhibits a C 2v -symmetric allanti conformation and resides on a special position with mm2 site symmetry.The orientation of the propane-1,3-diaminium ions makes the crystal structure polar in the c-axis direction.The solid-state supramolecular structure features a triperiodic network of strong hydrogen bonds of the N-H� � �O and O-H� � �O types.

Chemical context
Squaric acid (H 2 C 4 O 4 ; systematic name: 3,4-dihydroxycyclobut-3-ene-1,2-dione) and its derivatives have been widely studied in organic chemistry and materials science (Grus et al., 2021;Laramie et al., 2017;Wurm & Klok, 2013;Xia & Wang, 2017).Squaric acid analogues have also attracted attention in medicinal chemistry (Chasa ´k et al., 2021;Ruseva et al., 2022).In structural chemistry, the interest in squaric acid and its mono-and dianions arises mainly from their planar, symmetrical and strained molecular structures and their diverse hydrogen-bonding patterns in the solid state (Allen et al., 2013;Gilli et al., 2001).As a strong diprotic organic acid with pK a1 = 0.59 � 0.09 and pK a2 = 3.48 � 0.023 at 298 K (as determined by potentiometric titrations; Schwartz & Howard, 1970), squaric acid readily forms proton-transfer compounds with nitrogen bases and a wide variety of structurally characterized examples can be found in the Cambridge Structural Database (CSD; Groom et al., 2016).In the present contribution, we describe the crystal structure of the dihydrate of the proton-transfer compound propane-1,3-diaminium squarate.

Supramolecular features
Apart from ion-ion interactions between propane-1,3diaminium dications and squarate dianions, hydrogen bonding dominates the solid-state structure of the title compound.The protonated amino group joins two squarate ions via N-H� � �O hydrogen bonds.The remaining hydrogen-bond donor site of the aminium group forms an N-H� � �O hydrogen bond to a water molecule (Fig. 2).The water mol-ecule in turn acts as hydrogen-bond acceptor towards two squarate ions, which results in a triperiodic hydrogen-bond network (Fig. 3).Table 1 lists the corresponding hydrogenbond parameters, which are characteristic of strong hydrogen bonds (Thakuria et al., 2017).The centroid-centroid distance between the squarate ions in the [001] direction corresponds to the c lattice parameter.A packing index of 67.8% (Kitajgorodskij, 1973), as calculated with PLATON (Spek, 2020), indicates a relatively open structure.This lends support to the view that strong hydrogen bonding governs the structure rather than van der Waals close packing.

Figure 3
The tetragonal unit cell of the title compound, projected along the c-axis direction.Dashed lines illustrate hydrogen bonds.Colour scheme: C, grey; H, white; N, blue; O, red.

Synthesis and crystallization
Starting materials were obtained from commercial sources and used as received.A solution of propane-1,3-diamine (148 mg, 2 mmol) in 25 mL of ethanol was mixed with a solution of squaric acid (228 mg, 2 mmol) in 40 mL of distilled water.
After stirring at 333 K for 4 h, the mixture was left at ambient conditions.After three weeks, colourless crystalline material was collected by filtration and air-dried.Colourless crystals of the title compound suitable for single-crystal X-ray diffraction were grown from methanol/water (1:1) by the slow evaporation method.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Hydrogen-atom positions were refined freely, and U iso (H) values were set 1.2U eq (C, N, O) to improve the data/parameter ratio.The direction of the polar axis was chosen to give a Flack x parameter, as calculated by Parsons' quotient method (Parsons et al., 2013), close to zero.
In the absence of significant anomalous scattering, however, the polar axis direction could not be determined reliably in view of the high standard uncertainty of the Flack x parameter (Flack & Bernardinelli, 1999).For this reason, the presence of inversion twinning also cannot be excluded.(Sheldrick, 2008), SHELXL2019/3 (Sheldrick, 2015), DIAMOND (Brandenburg, 2018) and publCIF (Westrip, 2010).

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. Fractional

Fig. 1
Fig. 1 shows the molecular structures of the components of the title compound in the crystal.The squarate dianion exhibits D 4h point-group symmetry and contains a crystallographic fourfold rotation axis with the direction [001].The propane-

Table 2
Experimental details.