Comment

The crystal structures of (protonated) amine hydrogen phosphites containing [HPO₃]^2- or [H₂PO₃]^- oxo-anions are of crystallochemical interest in terms of the interplay between the hydrogen bonds linking the cations, anions, and, if applicable, water molecules together (Averbuch-Pouchot, 1993a,b; Harrison, 2003a,b).

The asymmetric unit of the title compound, (I), consists of two half-molecule {C₂H₆N} fragments of (C₄H₁₂N₂)²⁺ piperizinium cations, an [HPO₃]^2- hydrogen phosphite group and a water molecule. Inversion symmetry (Fig. 1) generates the two complete piperizinium cations, and the water O atom is disordered over two adjacent sites (see Experimental). The hydrogen phosphite group shows its usual (Harrison, 2003a) pseudo-pyramidal geometry [mean d(P-O) = 1.521 (2) Å; mean (O-P-O) = 112.48 (9)°] and the organic species adopt typical chair conformations.

As well as electrostatic forces, the component species in (I) interact by means of O-HO and N-HO hydrogen bonds (Table 2), and possibly a C-HO interaction (see below). Infinite chains of alternating [HPO₃]^2- and H₂O moieties are formed (Fig. 2) along [010] as a result of the water-to-phosphite O-HO hydrogen bonds, with the repeating units generated by translation symmetry. The resulting P1P1ⁱⁱ (Fig. 2; see Table 2 for symmetry code) separation of 6.5706 (7) Å is naturally much larger than the typical PP separations (4.7-4.9 Å) seen when [H₂PO₃]^- dihydrogen phosphite units link together by way of P-O-HO-P interactions without an intervening water molecule (Averbuch-Pouchot, 1993a, Harrison, 2003a).

The piperizinium cations crosslink the [010] [HPO₃]^2--H₂O chains by way of the N-HO hydrogen bonds (Table 2), with all four bonds close to linear [mean (N-HO) = 168°]. A short C1-H5O4a^iv (Table 2) interaction was identified in a PLATON (Spek, 2003) analysis of (I). If it is not merely a packing artefact, it may provide some additional coherence between the piperizinium cations and the water component of the [HPO₃]^2--H₂O [010] chains, although its role, if any, in the disordering of the water molecule O4 atom is not obvious.

Figure 1 View of (I) (50% displacement ellipsoids; H atoms are drawn as small spheres of arbitrary radius). The disordered O4b species is omitted. Symmetry codes: (i) -x, 1 - y, -z; (ii) 1 - x, 1 - y, -z.

Figure 2 Detail of a [010] hydrogen phosphite-water chain with the HO components of the hydrogen bonds indicated by dashed lines (atom O4b not shown). Symmetry codes: (i) x, y + 1, z; (ii) x, y - 1, z.

Figure 3 Unit-cell packing in (I) projected onto (010). The HO components of the hydrogen bonds are indicated by dashed lines. O4b and all C-H H atoms are omitted for clarity.

Experimental

H₃PO₃ (0.82 g; 1 mmol) and piperizine hexahydrate (1.92 g; 0.01 mmol) were dissolved in 10 ml deionized water, resulting in a clear solution. Block-shaped crystals of (I) grew as the water evaporated over several days.

Crystal data

C6H12N22+·HPO32-·H2O Mr = 186.15 Monoclinic, P21/c a = 12.2476 (8) Å b = 6.5706 (4) Å c = 10.6592 (8) Å = 92.744 (1)° V = 856.8 (1) Å3 Z = 4 Dx = 1.443 Mg m-3 Mo K radiation Cell parameters from 2470 reflections = 3.3-29.8° = 0.30 mm-1 T = 293 (2) K Block, colourless 0.27 × 0.23 × 0.19 mm

Data collection

Bruker SMART1000 CCD diffractometer scans Absorption correction: multi-scan (SADABS; Bruker, 1999) Tmin = 0.925, Tmax = 0.949 6211 measured reflections 2468 independent reflections 1930 reflections with I > 2(I) Rint = 0.022 max = 30.0° h = -17 16 k = -8 9 l = -14 12

Refinement

Refinement on F2 R[F2 > 2(F2)] = 0.042 wR(F2) = 0.131 S = 1.02 2468 reflections 100 parameters H-atom parameters constrained w = 1/[2(Fo2) + (0.0845P)2] where P = (Fo2 + 2Fc2)/3 (/)max < 0.001 max = 0.78 e Å-3 min = -0.44 e Å-3

Table 1 Selected bond lengths (Å) for (I) P1-O3 1.5151 (13) P1-O2 1.5230 (12) P1-O1 1.5234 (14)

Table 2 Hydrogen-bonding geometry (Å, °) for (I) D-HA D-H HA DA D-HA N1-H2O2i 0.90 1.84 2.7147 (19) 164 N1-H3O2ii 0.90 1.81 2.7043 (19) 172 N2-H8O3ii 0.90 1.77 2.642 (2) 163 N2-H9O1iii 0.90 1.78 2.676 (2) 171 O4a-H14O1 0.95 1.90 2.840 (4) 167 O4a-H15O3ii 0.93 1.90 2.811 (4) 168 O4b-H14O1 0.93 1.90 2.752 (4) 151 O4b-H15O3ii 0.96 1.90 2.765 (4) 149 C1-H5O4aiv 0.97 2.38 3.300 (5) 159 Symmetry codes: (i) ; (ii) x,1+y,z; (iii) ; (iv) .

The water O atom was modelled as being disordered over two adjacent sites with isotropic displacement factors [d(O4aO4b) = 0.638 (5) Å; fractional site occupancies = 0.563 (14) and 0.437 (14) for O4a and O4b, respectively, with their sum constrained to unity]. The present data did not reveal H-atom sites that could be unambiguously associated with either O4a or O4b; instead, two distinct features in the difference map provided H-atom sites that were reasonable for both O4a and O4b (see Table 2). These O-H H atoms were refined by riding on O4a in their as-found positions. The N-H H atoms were found in difference maps and refined by riding in their idealized positions [d(N-H) = 0.90 Å]. The H atoms bonded to C and P were placed in calculated positions [d(C-H) = 0.97 Å; d(P-H) = 1.32 Å] and refined by riding. For all H atoms, the constraint U_iso(H) = 1.2U_eq(carrier atom) was applied.

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; molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: SHELXL97.