Propane-1,2-diaminium hydrogenarsenate

The title compound, (C3H12N2)[AsHO4], is a mol­ecular salt containing a network of propane-1,2-diaminium cations and hydrogenarsenate anions [mean As—O 1.686 (2) A]. The crystal packing involves cation-to-anion N—H⋯O and anion-to-anion O—H⋯O hydrogen bonds, the latter resulting in dimeric associations of two adjacent hydrogenarsenate anions.

The [HAsO 4 ] 2À hydrogenarsenate group in (I) shows its normal tetrahedral geometry [mean As-O 1.686 (2) Å ], with the protonated As1-O4 vertex showing its usual lengthening relative to the unprotonated As-O bonds ( Table 1). The propane-1,2-diaminium cation is disordered over two overlapped positions (Fig. 1). This positional disorder manifests itself as a terminal methyl group (atoms C3 or C4) being attached to either C1 or C2, with 50% occupancy in each case. The N atoms and atoms C1 and C2 of the two orientations of the cation are not resolved. Allowing for the disorder, this ion is chiral, but crystal symmetry generates a 50:50 mix of enantiomers, which is consistent with the racemic starting material. Atoms N1 and N2 are close to being trans with respect to the C1-C2 backbone of the molecule (Table 1).
As well as electrostatic attractions, the component species in (I) interact by means of a network of O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds ( Table 2). The (HAsO 4 ) 2À units are linked into inversion-generated dimeric pairs by way of the O4-H1Á Á ÁO2 i bond (see Table 2 for symmetry code), with a resulting As1Á Á ÁAs1 i separation of 4.3963 (4) Å . This situation is distinct from that observed in related materials, where chains (Lee & Harrison, 2003) and sheets (Wilkinson & Harrison, 2005) of (di)hydrogenarsenate ions linked by O-HÁ Á ÁO bonds are seen.

Experimental
Aqueous propane-1,2-diamine solution (0.5 M, 10 ml) was added to aqueous H 3 AsO 4 solution (0.5 M, 10 ml) to result in a clear mixture. Aqueous ammonia was added to this solution to raise the pH to about 12, which is beyond the second end-point for H 3 AsO 4 (i.e. the predominant solution species is HAsO 4 2À ). Crystals of (I) grew as the water evaporated over the course of a few days.
Crystal data (C 3 Table 1 Selected geometric parameters (Å , ). Table 2 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 1; Ày; Àz; (ii) x; y þ 1; z; (iii) Àx þ 1 2 ; y þ 1 2 ; Àz þ 1 2 ; (iv) Àx þ 1; Ày þ 1; Àz; The organic cation is orientationally disordered, such that the two positions of atoms N1, N2, C1, and C2 overlap and cannot be resolved. The site-occupation factors of atoms C3 and C4 refined to 50% within experimental error and were both fixed at 0.50 for the final cycles of refinement. The O-bound H atom was found in a difference map and refined as riding in its as-found relative position. The H atoms bonded to C and N were located in idealized positions, with N-H = 0.91 Å and C-H = 0.98-0.99 Å , and refined as riding, allowing for free rotation of the -NH 3 groups. The constraint U iso (H) = 1.2U eq (carrier) or U iso (H) = 1.5U eq (methyl carrier) was applied.
We thank the EPSRC National Crystallography Service (University of Southampton) for the data collection. The packing for (I), with all C-bound H atoms omitted for clarity. Hydrogen bonds are indicated by dashed lines.
The N atoms and atoms C1 and C2 of the two orientations of the molecule are not resolved. Allowing for the disorder, this molecular ion is chiral, but crystal symmetry generates a 50:50 mix of enantiomers, which is consistent with the racemic starting material. Atoms N1 and N2 are close to being trans with respect to the C1-C2 backbone of the molecule ( Table 1).
As well as electrostatic attractions, the component species in (I) interact by means of a network of O-H···O and N-H···O hydrogen bonds ( Table 2). The (HAsO 4 ) 2− units are linked into inversion-symmetry generated dimeric pairs by way of the O4-H1···O2 i bond (see Table 2 for symmetry code), with a resulting As1···As1 i separation of 4.3963 (4) Å. This situation is distinct from that observed in related materials, where chains (Lee & Harrison, 2003) and sheets (Wilkinson & Harrison, 2005) of (di)hydrogenarsenate moieties linked by O-H···O bonds are seen.

S2. Experimental
Aqueous propane 1,2-diamine solution (0.5 M, 10 ml) was added to aqueous H 3 AsO 4 solution (0.5 M, 10 ml) to result in a clear mixture. Aqueous ammonia was added to this solution to raise the pH to about 12, which is beyond the second endpoint for H 3 AsO 4 (i.e. the predominant solution species is HAsO 4 2− ). Plate-like [Shard below?] crystals of (I) grew as the water evaporated over the course of a few days.

S3. Refinement
The organic cation is orientationally disordered, such that the two positions of atoms N1, N2, C1, and C2 overlap and cannot be resolved. The site-occupation factors of atoms C3 and C4 refined to 50% within experimental error and were both fixed at 0.50 for the final cycles of refinement. The O-bound H atom was found in a difference map and refined as riding in its as-found relative position. The H atoms bonded to C and N were located in idealized positions, with N-H = 0.91 Å and C-H = 0.98-0.99 Å, and refined as riding, allowing for free rotation of the -NH 3 groups. The constraint U iso (H) = 1.2U eq (carrier) or U iso (H) = 1.5U eq (methyl carrier) was applied.

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.