Piperazinium bis(dihydrogenarsenate)

The (H2AsO4) anion in (I) shows its normal tetrahedral geometry about As, with the usual distinction (Table 1) between protonated and unprotonated As—O bond lengths (Wilkinson & Harrison, 2004). The piperazinium dication lies on a centre of inversion and adopts a typical chair conformation. As well as coulombic forces, the component species in (I) interact by way of a network of N—H O and O—H O hydrogen bonds (Table 2). The (H2AsO4) units are linked into infinite sheets (Fig. 2) by the O—H O hydrogen bonds. The O3—H1 O2 interaction (see Table 2 for symmetry codes) results in centrosymmetric dimeric pairs of (H2AsO4) tetrahedra linked by pairs of O—H O hydrogen bonds. The O4—H2 O1 hydrogen bond links these dimers into an

The (H 2 AsO 4 ) À anion in (I) shows its normal tetrahedral geometry about As, with the usual distinction (Table 1) between protonated and unprotonated As-O bond lengths (Wilkinson & Harrison, 2004). The piperazinium dication lies on a centre of inversion and adopts a typical chair conformation.
As well as coulombic forces, the component species in (I) interact by way of a network of N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds ( Table 2). The (H 2 AsO 4 ) À units are linked into infinite sheets (Fig. 2) by the O-HÁ Á ÁO hydrogen bonds. The O3-H1Á Á ÁO2 i interaction (see Table 2 for symmetry codes) results in centrosymmetric dimeric pairs of (H 2 AsO 4 ) À tetrahedra linked by pairs of O-HÁ Á ÁO hydrogen bonds. The O4-H2Á Á ÁO1 ii hydrogen bond links these dimers into an The molecular structure of (I) (50% displacement ellipsoids and H atoms are drawn as spheres of arbitrary radius). The hydrogen bond is indicated by a dashed line. [Symmetry code: (i) Àx, Ày, 1 À z.] infinite sheet (Fig. 3) lying parallel to (100). The AsÁ Á ÁAs i and AsÁ Á ÁAs ii separations are 4.0148 (3) and 5.0190 (3) Å , respectively. The topological connectivity of the As atoms defines a 6 3 sheet (O'Keeffe & Hyde, 1996), i.e. every As node participates in three polyhedral six-ring loops.
The anionic sheets are bridged by piperazinium cations, each of which participates in two N-HÁ Á ÁO interactions from each of its NH 2 groups to nearby dihydrogenarsenate tetrahedra. This results (Fig. 3) in organic and inorganic layers that alternate along the a axis. A similar layered structure has been reported for guanidinium dihydrogenarsenate, CH 6 N 3 Á-H 2 AsO 4 (Wilkinson & Harrison, 2005), despite the different cation:anion ratios in the two compounds. Other ammonium hydrogenarsenate salts contain isolated pairs of tetrahedra (Todd & Harrison, 2005) or polymeric chains of anions (Wilkinson & Harrison, 2004).

Experimental
A 0.5 M aqueous piperazine solution (10 ml) was added to a 0.5 M aqueous H 3 AsO 4 solution (10 ml) to give a clear solution. Crystals of (I) were obtained as the water evaporated over the course of a few days.  Table 1 Selected geometric parameters (Å , ).

Figure 3
The packing in (I), showing the (100)  H atoms bound to O atoms were found in difference Fourier maps and refined as riding on their carrier O atoms in their as-found relative positions. H atoms bound to N and C atoms were placed in idealized positions (C-H = 0.97 Å and N-H = 0.90 Å ) and refined as riding. The constraint U iso (H) = 1.2U eq (carrier) was applied in all cases.
Data collection: SMART (Bruker, 1999); cell refinement: SAINT (Bruker, 1999); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 (Farrugia, 1997) and ATOMS (Shape Software, 2004); software used to prepare material for publication: SHELXL97. 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