Comment

The title compound, (C₃H₁₂N₂)[HAsO₄]·H₂O, (I) (Fig. 1), was prepared as part of our ongoing structural studies of hydrogen-bonding interactions in protonated-amine (di)hydrogen arsenates (Lee & Harrison, 2003a; Wilkinson & Harrison, 2004; Todd & Harrison, 2005). In particular, (I) complements propane-1,3-diaminium bis(dihydrogenarsenate), (C₃H₁₂N₂)[H₂AsO₄]₂ (Wilkinson & Harrison, 2005), prepared under different pH conditions.

The [HAsO₄]^2- hydrogenarsenate group in (I) has normal tetrahedral geometry [mean As-O = 1.687 (2) Å], with the protonated As1-O4 vertex showing its usual lengthening relative to the unprotonated As-O bonds (Table 1). The propane-1,3-diaminium cation shows no unusual geometrical features.

As well as electrostatic attractions, the component species in (I) interact by means of a network of O-HO and N-HO hydrogen bonds (Table 2). The [HAsO₄]^2- units and water molecules are linked into polymeric chains (Fig. 2) propagating along [010] by way of anion-to-anion O4-H1O2ⁱ and water-to-anion O5-H14O1 and O5-H15O2ⁱⁱ bonds (Table 2). This arrangement results in an unusual R₃³(10) graph-set (Bernstein et al., 1995) motif. The As1As1ⁱ separation is 4.7991 (3) Å.

The organic species interacts with the hydrogenarsenate/water chains by way of six N-HO hydrogen bonds [mean HO = 1.89 Å, mean N-HO = 171° and mean NO = 2.793 (2) Å]. One of the acceptor O atoms is part of a water molecule, and the other five are parts of hydrogenarsenate groups. This hydrogen-bonding scheme results in a three-dimensional network (Fig. 3).

The hydrogen-bonded hydrogenarsenate/water chains in (I) are different from the motifs seen in related structures. In bis(cycloheptylaminium) hydrogenarsenate monohydrate (Todd & Harrison, 2005) and bis(benzylammonium) hydrogenarsenate monohydrate (Lee & Harrison, 2003c), hydrogen-bonded dimers of [HAsO₄]^2- units occur, with the dimers bridged into double chains by intervening water molecules. In the unhydrated piperidinium dihydrogenarsenate (Lee & Harrison, 2003b) and t-butylammonium dihydrogenarsenate (Wilkinson & Harrison, 2004), single chains of [H₂AsO₄]^- anions occur with each adjacent dihydrogenarsenate pair linked by a pair of hydrogen bonds. In propane-1,3-diaminium bis(dihydrogenarsenate) (Wilkinson & Harrison, 2005), the same organic cation as found in (I) is combined with dihydrogenarsenate [H₂AsO₄]^- groups, with the latter forming double chains.

Figure 1 A view of (I) with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen bonds are indicated by dashed lines.

Figure 2 Detail of a hydrogen-bonded (dashed lines) hydrogenarsenate/water chain in (I).

Figure 3 The crystal packing of (I). Dashed lines indicate hydrogen bonds.

Experimental

0.5 M aqueous propane-1,3-diamine solution (10 ml) was added to 0.5 M aqueous H₃AsO₄ solution (10 ml) to result in a clear solution. Aqueous ammonia was added to this solution to raise the pH to about 12, which is beyond the second end-point for H₃AsO₄ (i.e. the predominant species is [HAsO₄]^2-). Platy crystals of (I) grew as the water evaporated over the course of a few days.

Crystal data

(C3H12N2)[HAsO4]·H2O Mr = 234.09 Monoclinic, P 21 /c a = 7.1327 (2) Å b = 16.8046 (6) Å c = 7.9402 (2) Å = 113.253 (2)° V = 874.42 (5) Å3 Z = 4 Dx = 1.778 Mg m-3 Mo K radiation Cell parameters from 2043 reflections = 2.9-27.5° = 3.87 mm-1 T = 120 (2) K Plate, colourless 0.32 × 0.24 × 0.03 mm

Data collection

Nonius KappaCCD diffractometer and scans Absorption correction: multi-scan(SADABS; Bruker, 1999)Tmin = 0.370, Tmax = 0.892 11562 measured reflections 2002 independent reflections 1804 reflections with I > 2(I) Rint = 0.036 max = 27.5° h = -9 8 k = -20 21 l = -10 10

Refinement

Refinement on F2 R[F2 > 2(F2)] = 0.022 wR(F2) = 0.055 S = 1.05 2002 reflections 103 parameters H-atom parameters constrained w = 1/[2(Fo2) + (0.0218P)2 + 0.7899P] where P = (Fo2 + 2Fc2)/3 (/)max = 0.001 max = 0.49 e Å-3 min = -0.53 e Å-3 Extinction correction: SHELXL97 Extinction coefficient: 0.0032 (6)

Table 1 Selected geometric parameters (Å, °) As1-O1 1.6612 (14) As1-O2 1.6746 (13) As1-O3 1.6814 (14) As1-O4 1.7302 (13) N1-C1-C2-C3 175.33 (16) C1-C2-C3-N2 175.49 (16)

Table 2 Hydrogen-bond geometry (Å, °) D-HA D-H HA DA D-HA O4-H1O2i 0.93 1.71 2.6207 (19) 166 O5-H14O1 0.92 1.79 2.709 (2) 177 O5-H15O2ii 0.89 1.98 2.858 (2) 169 N1-H2O1iii 0.91 1.81 2.711 (2) 173 N1-H3O3i 0.91 1.96 2.855 (2) 166 N1-H4O5iv 0.91 1.90 2.798 (2) 168 N2-H11O3ii 0.91 1.90 2.802 (2) 170 N2-H12O2v 0.91 1.95 2.851 (2) 172 N2-H13O3 0.91 1.84 2.743 (2) 175 Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) .

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 = 0.99 Å and N-H = 0.91 Å) and refined as riding, allowing for free rotation of the -NH₃ groups. The constraint U_iso(H) = 1.2U_eq(carrier) was applied in all cases.

Data collection: COLLECT (Nonius, 1998); cell refinement: SCALEPACK (Otwinowski & Minor, 1997); data reduction: SCALEPACK, DENZO (Otwinowski & Minor, 1997) and SORTAV (Blessing, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: SHELXL97.