2-Methyl­piperazinium bis­(dihydrogenarsenate)

Wilkinson, H.S.; Harrison, W.T.A.

doi:10.1107/S1600536807008392

Volume 63
Part 3
Pages m900-m901
March 2007

Received 19 February 2007
Accepted 19 February 2007
Online 28 February 2007

Key indicators
Single-crystal X-ray study
T = 293 K
Mean (C-C) = 0.004 Å
Disorder in main residue
R = 0.029
wR = 0.074
Data-to-parameter ratio = 29.0
Details

2-Methylpiperazinium bis(dihydrogenarsenate)

Hazel S. Wilkinson ^a and William T. A. Harrison ^a ^*

^aDepartment of Chemistry, University of Aberdeen, Meston Walk, Aberdeen AB24 3UE, Scotland
Correspondence e-mail: w.harrison@abdn.ac.uk

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.

Comment

The title compound, (I) (Fig. 1), was prepared as part of our ongoing studies of hydrogen-bonding interactions in the molecular salts of oxo-anions (Wilkinson & Harrison, 2007).

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 H

O parts of the O-H

O hydrogen bonds are coloured yellow and the H

O parts of the N-H

O 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, $[{1\over 2}]$ + y, $[{3\over 2}]$ - z), respectively.

Experimental

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.

Crystal data

C₅H₁₄N₂²⁺·2AsH₂O₄^-
M_r = 384.06
Monoclinic, P 2₁ /c
a = 6.7537 (3) Å
b = 8.1753 (4) Å
c = 12.7105 (5) Å
= 107.501 (2)°
V = 669.31 (5) Å³
Z = 2
Mo K radiation
= 5.02 mm^-1
T = 293 (2) K
0.37 × 0.12 × 0.03 mm

Data collection

Bruker SMART 1000 CCD diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 1999) T_min = 0.258, T_max = 0.864
6913 measured reflections
2409 independent reflections
1929 reflections with I > 2(I)
R_int = 0.026

Refinement

R[F² > 2(F²)] = 0.029
wR(F²) = 0.074
S = 1.06
2409 reflections
83 parameters
H-atom parameters constrained
_max = 0.65 e Å^-3
_min = -0.67 e Å^-3

Table 1
Selected bond lengths (Å)

As1-O1	1.6477 (16)
As1-O2	1.6531 (16)
As1-O3	1.6974 (16)
As1-O4	1.7082 (17)

Table 2
Hydrogen-bond geometry (Å, °)

D-HA	D-H	HA	DA	D-HA
O4-H2O1ⁱ	0.84	1.76	2.599 (3)	174
O3-H1O2ⁱⁱ	0.86	1.70	2.548 (2)	169
N1-H4O2	0.90	1.79	2.689 (2)	175
N1-H3O1ⁱⁱⁱ	0.90	1.80	2.685 (2)	168
Symmetry codes: (i) -x+1, -y+1, -z+1; (ii) $[-x+1, y+{\script{1\over 2}}, -z+{\script{3\over 2}}]$ ; (iii) -x, -y+1, -z+1.

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 U_iso(H) = 1.2U_eq(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 U_iso(H) = 1.2U_eq(C,N) or 1.5U_eq(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.

Acknowledgements

HSW thanks the Carnegie Trust for the Universities of Scotland for an undergraduate vacation studentship.

References

Bernstein, J., Davis, R. E., Shimoni, L. & Chang, N.-L. (1995). Angew. Chem. Int. Ed. Engl. 34, 1555-1573.
Bruker (1999). SMART (Version 5.624), SAINT (Version 6.02A) and SADABS (Version 2.03). Bruker AXS Inc., Madison, Wisconsin, USA.
Farrugia, L. J. (1997). J. Appl. Cryst. 30, 565.
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.

metal-organic compounds