Propane-1,3-diaminium bis(dihydrogenarsenate)

# 2005 International Union of Crystallography Printed in Great Britain – all rights reserved The title compound, (C3H12N2)[H2AsO4]2, contains a network of propane-1,3-diaminium cations and dihydrogenarsenate anions [mean As—O = 1.682 (2) Å]. The crystal packing involves anion-to-anion O—H O hydrogen bonds, resulting in double chains of dihydrogenarsenate tetrahedra. Cation-toanion N—H O hydrogen bonds generate a three-dimensional overall structure. One C atom occupies a special position with twofold symmetry.

The [H 2 AsO 4 ] À dihydrogenarsenate group in (I) has normal tetrahedral geometry [mean As-O = 1.682 (2) Å ], with the protonated As1-O1 and As1-O2 vertices showing their expected lengthening relative to the unprotonated As1-O3 and As1-O4 bonds, which have formal partial double-bond character ( Table 1). The propane-1,3-diaminium cation, which is generated by twofold symmetry from the atoms of the asymmetric unit (C2 occupies a special position with site symmetry 2), shows no unusual geometrical features.
As well as electrostatic attractions, the component species in (I) interact by means of a network of cation-to-anion N-HÁ Á ÁO and anion-to-anion O-HÁ Á ÁO hydrogen bonds ( Table 2). The [H 2 AsO 4 ] À units are linked into polymeric double chains (Fig. 2) propagating along [010] by way of inversion-symmetry-generated pairs of O2-H2Á Á ÁO4 iii and O1-H1Á Á ÁO4 ii bonds (see Table 2 for symmetry codes). The first of these bonds results in 'dimers' of dihydrogenarsenate tetrahedra, which in turn are linked into double chains by the second hydrogen bond. In graph-set notation (Bernstein et al., 1995), these bonding patterns correspond to R 2 2 (8) and R 4 4 (12) loops, respectively. This scheme results in every [H 2 AsO 4 ] À group in the chain forming two hydrogen bonds to its neighbours and accepting two hydrogen bonds from its neighbours. The AsÁ Á ÁAs iii (via O2-H2Á Á ÁO4 iii ) and AsÁ Á ÁAs ii (via O1-H1Á Á ÁO4 ii ) separations are 4.5325 (4) and 4.6549 (4) Å , respectively (symmetry codes as in Table 2).
The organic species interacts with the dihydrogenarsenate anions by way of three N-HÁ Á ÁO hydrogen bonds [mean HÁ Á ÁO = 2.00 Å , mean N-HÁ Á ÁO = 158 and mean NÁ Á ÁO = 2.892 (3) Å ], such that the [010] dihydrogenarsenate double chains are crosslinked in the a and c directions to result in a three-dimensional network (Fig. 3). A PLATON (Spek, 2003) analysis of (I) indicated the presence of two short C-HÁ Á ÁO contacts (Table 2) although their structural significance is not clear.
A mass of plate-and slab-like crystals of (I) grew as the water evaporated over the course of a few days.
The I-centred unit cell was chosen in preference to the C-centred setting (space group C2/c) to avoid a very obtuse angle of 127 (Mighell, 2003). The O-bound H atoms were found in difference maps and refined as riding on their carrier O atoms in their as-found relative positions. H atoms bonded to C and N atoms were placed in idealized positions (C-H = 0.97 Å and N-H = 0.89 Å ) and refined as riding, allowing for free rotation of the -NH 3 group. The constraint U iso (H) = 1.2U eq (carrier) was applied in all cases. The highest difference peak is 0.95 Å from O3 and the deepestdifference hole is 1.20 Å from As1.
HSW thanks the Carnegie Trust for the Universities of Scotland for an undergraduate vacation studentship. 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.