Received 19 February 2007
The title compound, C5H14N22+·2H2AsO4-, contains a network of centrosymmetric doubly protonated 2-methylpiperazinium cations, showing disorder of the methyl group, accompanied by dihydogenarsenate anions. The component species interact by way of cation-to-anion N-HO and anion-to-anion O-HO hydrogen bonds, the latter leading to infinite sheets of the H2AsO4- anions containing R66(24) supramolecular loops.
The tetrahedral H2AsO4- anion in (I) [mean As-O = 1.677 (2) Å], shows the usual distinction (Table 1) between protonated and unprotonated As-O bond lengths (Wilkinson & Harrison, 2007). The complete 2-methylpiperazinium dication is generated by inversion. This must result in disorder, as each dication is chiral at C2. Thus, the two enantiomers are superimposed in the long-range structure of the crystal, with all the atoms of the ring overlapped. A typical chair conformation for the six-membered ring arises, and atom C3 of the methyl group is equatorial to the ring in both disorder components.
As well as Coulombic forces, the component species in (I) interact by way of a network of anion-to-anion O-HO and cation-to-anion N-HO hydrogen bonds (Table 2). The hydrogen-bonding scheme and overall structure in (I) are very similar to those in piperazinium bis(dihydrogenarsenate), (II) (Wilkinson & Harrison, 2007). In both (I) and (II), the H2AsO4- units are linked into infinite (100) layers by the O-HO bonds. A distinctive feature of the sheets are supramolecular R66(24) rings (Bernstein et al., 1995) built up from six tetrahedra, the rings being stabilized by N-HO bonds from the organic cations (Fig. 2). For the two inter-tetrahedral O-HO interactions, the AsAsi and AsAsii (see Table 2 for symmetry codes) separations for (I) are 4.3061 (3) and 4.7599 (3) Å, respectively, which are distinctly different from the values of 4.0148 (3) and 5.0190 (3) Å for the topologically equivalent network in (II).
| || Figure 1 |
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 double-dashed line. Only one disorder component of the cation is shown. [Symmetry code: (iv) -x, -y, 1 - z.]
| || Figure 2 |
Detail of a six-membered ring of H2AsO4- groups in (I), in polyhedral representation, with attached organic cations. Only one disorder component of each cation is shown and the C-bound H atoms are omitted for clarity. The HO parts of the O-HO hydrogen bonds are coloured yellow and the HO parts of the N-HO hydrogen bonds are light blue. The As1* and As1# tetrahedra are generated by the symmetry operations (1 - x, 1 - y, 1 - z) and (1 - x, + y, - z), respectively.
To an aqueous racemic 2-methylpiperazine solution (10 ml, 0.5 M) was added an aqueous H3AsO4 solution (10 ml, 0.5 M), resulting in a clear solution. Chunks and blocks of (I) grew as the water evaporated over the course of a few days; these were harvested by vacuum filtration and rinsed with acetone.
The C3 methyl group is disordered over two positions in the molecule. Crystal symmetry dictates equal occupancy for both components. The O-bound H atoms were found in a difference map and refined as riding in their as-found relative positions, with Uiso(H) = 1.2Ueq(O) (see Table 2 for distances). The C- and N-bonded H atoms were placed in idealized positions (C-H = 0.96-0.97 Å and N-H = 0.90 Å) and refined as riding, with Uiso(H) = 1.2Ueq(C,N) or 1.5Ueq(methyl C). The methyl group was allowed to rotate, but not to tip, to best fit the electron density.
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.
HSW thanks the Carnegie Trust for the Universities of Scotland for an undergraduate vacation studentship.
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Shape Software (2004). ATOMS. Shape Software, Kingsport, Tennessee, USA.
Sheldrick, G. M. (1997). SHELXS97 and SHELXL97. University of Göttingen, Germany.
Wilkinson, H. S. & Harrison, W. T. A. (2007). Acta Cryst. E63, m26-m28.