Poly[bis(piperazine-1,4-diium) [(μ4-cyclo-hexaphosphato)dilithium] tetrahydrate]

In the title compound, {(C4H12N2)2[Li2(P6O18)]·4H2O}n, the phosphate ring anion, located around an inversion center, adopts a chair conformation. Adjacent P6O18 rings are linked via corner-sharing by LiO4 tetrahedra, generating anionic porous {[Li2(P6O18)]4−}n layers parallel to (101). The piperazine-1,4-diium cations occupy the pores and develop hydrogen bonds with the inorganic framework. An extensive network of N—H⋯O and O—H⋯O hydrogen-bonding interactions link the components into a three-dimensional network and additional stabilization is provided by weak C—H⋯O hydrogen bonds.

Data collection: CAD-4 EXPRESS (Enraf-Nonius, 1994); cell refinement: CAD-4 EXPRESS; data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SHELXS86 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: WinGX (Farrugia, 2012 The area of framework materials continues to be of interest not only because of the wide variety of structures but also due to their potential applications in the areas of catalysis, sorption and separation processes (Mahesh et al., 2002) Natarajan, 2000. Much attention has been devoted to the synthesis of open-framework phosphates which exhibit a rich structural diversity and have been widely studied as catalysts, ion-exchangers and as positive electrode in the lithium and sodium batteries (Assani et al., 2012). Within this family of compounds, the resulting anionic frameworks, generally constructed from PO 4 tetrahedra that are vertex linked with MO n polyhedra (with n = 4, 5 and 6), generate pores and channels offering suitable environment to accommodate different other cations. The piperazine (C 4 N 2 H 10 ), which is a common heterocyclic nitrogen compound, has been indicated as excellent template for preparing microporous materials (Xu et al., 2007). The crystal structure reported here gives another illustration of this type of material. The corresponding compound, (C 4 H 12 N 2 ) 2 Li 2 P 6 O 18 .4H 2 O (I), is an organic-inorganic hybrid built of two main cyclic components, C 4 H 12 N 2 and P 6 O 18 (Fig.   1). The phosphoric rings are interconnected by the Li + cations via LiO 4 tetrahedra sharing corners to form a twodimensional inorganic framework extending along the (101) plane as shown in Fig. 2. The diprotonated (C 4 H 12 N 2 ) 2+ cations are trapped within the 10-membered ring pore of the layer, whereas the water molecules are located in the interlayer region and are grafted onto the framework oxygen atoms through hydrogen bonds (Fig. 3). The asymmetric unit of this atomic arrangement is built of one half of the P 6 O 18 ring lying on an inversion center (1/2, 1/2, 1/2), one Li + cation, two water molecules and one piperazine-1,4-diium cation. The organic and inorganic rings adopt a chair conformation with different geometrical characteristics due to their different size and flexibility. However, the P 6 O 18 ring has (P-O and O-O) distances and (O-P-O, P-O-P and P-P-P angles) comparable to those observed in other cyclohexaphosphates having the same internal inversion symmetry (Abid et al., 2011;Amri et al., 2009;Marouani et al., 2010). The LiO 4 tetrahedra is slightly distorted with Li-O distances ranging from 1.877 (4) to 1.969 (4) Å. The smallest distance between two tetrahedral centers is 5.548 (2) Å. The organic ring has for carbon atoms (C1, C2, C3 and C4) almost coplanar (r.m.s. deviation from the mean plane = 0.014 Å) and N1 and N2 displaced from the plane by 0.672 (2) and -0.663 (2) Å, respectively. These characteristics do not differ from those particular values observed in other compounds of the piperazinium despite the different constraints of their solid states (Essid et al., 2010).

Experimental
Crystals of the title compound were prepared by adding dropwise and stirring an ethanolic solution (5 mL) of piperazine (10 mmol) then an aquous solution (10 mL) of KOH (10 mmol) to an aqueous solution (10 mL) of cyclohexaphosphoric acid (5 mmol). Colourless prismatic crystals were obtained after a slow evaporation over a few days at ambient temperature. The cyclohexaphosphoric acid H 6 P 6 O 18 ,was produced from Li 6 P 6 O 18 .6H 2 O, prepared according to the procedure of Schülke and Kayser (Schülke & Kayser, 1985), through an ion-exchange resin in H-state (Amberlite IR supplementary materials sup-2 Acta Cryst. (2013). E69, m305-m306 120).

Refinement
N and C-bound H atoms were positioned geometrically (N-H = 0.90 Å, C-H = 0.97 Å) and allowed to ride on their parent atoms, with U iso (H) = 1.2 U eq (C,N). The bond distances of O-H and distance between two H atoms from each water molecules was restrained to be 0.85 and 1.37 Å with the default deviation respectively and with U iso (H) = 1.5 U eq (O).

Figure 1
An ellipsoid plot of (I) with displacement ellipsoids for non-H atoms drawn at the 30% probability level.

Poly[bis(piperazine-1,4-diium) [(µ 4 -cyclo-hexaphosphato)dilithium] tetrahydrate]
Crystal data (C 4  Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.