supplementary materials

The title compound, (I), was prepared as part of our ongoing studies of hydrogen bonding interactions in the molecular salts of arsenic oxo-anions (Wilkinson & Harrison, 2007a,b). Its asymmetric unit contains two organic cations, two anions and three-and-a-half water molecules (Fig. 1).

The tetrahedral (HAs1O₄)²⁻ anion in (I) shows the expected distinction (Table 1) between its protonated and unprotonated As—O bond lengths. The situation for the (HAs2O₄)²⁻ group is less clear cut, and seems to be correlated with disorder of its H atoms (see below). Both C₂H₁₀N₂²⁺ dications are in their extended conformations with all their backbone torsion angles close to 180°. Three fully occupied (O9, O10, O11) water molecules and one half occupied (O12) water molecule complete the structure. O12 cannot be more than half occupied due to a close O12···O12^xx (xx = −x, 1 − y, −z) contact of 2.21 (1) Å.

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 (HAsO₄)²⁻ units are linked by way of O—H···O bonds into distinctive, isolated, tetramers (Fig. 2). The complete assembly is generated by inversion, thus the central O5—H2···O5ⁱ (see Table 2 for symmetry code) link must be disordered (i.e. O5—H2···O5ⁱ + O5ⁱ—H2ⁱ···O5 bonds) or possibly a symmetric O5···H2···O5ⁱ bond (i.e. the H atom occupies the inversion centre (Wilson, 2001). These possibilities could not be distinguished in the present experiment and a disordered model was assumed. Either of these possibilities necessitates disorder of the H atoms of the As2-hydrogenarsenate group, with one of the (HAsO₄)²⁻ moieties directing its H atom elsewhere. The extended As2—O6 bond length suggests that O6 bears the H atom and if so, it makes an O—H···O link to the disordered O12 water molecule O atom (Fig. 2).

The cations interact with the anion tetramers by way of a large number of N—H···O bonds (Table 2), with each NH₃⁺ group making three such links, as is typically seen in these systems (Wilkinson & Harrison, 2007a). Unfortunately the H atoms of the water molecules in (I) could not be located in the present study. The disorder of the As2 H atoms and O12 seems to require that at least some of the water H atoms are also disordered, and the situation is too uncertain to allow their geometrical placement with any confidence.

The packing for (I) results in a distinctive structure (Fig. 3) in which hydrogen-bonded (001) sheets of tetrahedral tetramers are bridged by the organic molecules to result in a framework encapsulating [100] channels, which are occupied by the water molecules.

The structure of 1,6-diamminiumhexane bis(dihydrogenarsenate), C₆H₁₈N₂·(H₂AsO₄)₂, (II), (Wilkinson & Harrison, 2007a), in which the arsenic-containing anion bears two protons, is entirely different to that of (I). In (II), infinite sheets of H-bonded tetrahedra arise and the centrosymmetric cation adopts a gauche conformation. Other types of supramolecular networks based on hydrogen bonded organic cations and arsenate tetrahedra are descibed by Wilkinson & Harrison (2007b).