Crystal structure of bis(piperazin-1-ium-κN 4)bis(thiosulfato-κS)zinc(II) dihydrate

In the title compound, two thiosulfate ions coordinate to the zinc(II) ion through the terminal S atoms. The tetrahedral coordination around the ZnII ion is completed by the ligation of two N atoms of two piperazinium ions. In the crystal, a network of N—H⋯O and O—H⋯O hydrogen bonds leads to the formation of a three-dimensional supramolecular structure.

In the title compound, [Zn(C 4 H 11 N 2 ) 2 (S 2 O 3 ) 2 ]Á2H 2 O, two thiosulfate ions coordinate to the zinc(II) atom through the terminal S atoms. The tetrahedral coordination around the Zn II ion is completed by ligating to two N atoms of two piperazinium ions. The remaining two N atoms of the piperazinium ions are diprotonated and do not coordinate to the metal centre. In the crystal, however, they are involved in N-HÁ Á ÁO water and N-HÁ Á ÁOsulfato hydrogen bonds. Together, a series of N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds, involving the O atoms of the thiosulfate ions and the water molecules as acceptors and the hydrogen atoms of the piperazinium ions and the water molecules as donors, form a three-dimensional supramolecuar structure. Within this framework there are a number of intra-and intermolecular C-HÁ Á ÁO and C-HÁ Á ÁS contacts present.

Chemical context
Over the last few decades, a large number of amine-templated metal complexes and compounds with extended structures have been synthesized in the presence of a number of inorganic anions (Fé rey, 2008). One series of anions, namely the sulfur-containing oxoanions, and in particular sulfates and sulfites, are widely used in the synthesis of higher dimensional inorganic compounds because of their multidentate coordination capacity towards metal ions (Rao et al., 2006). In these examples, the anions bind to the metal cations through the oxygen atoms. The thiosulfate ion is a new example of an sulfur oxoanion used in amine-templated synthesis, although the reactivity of this ligand is less than that of the sulfate and sulfite ions. In this heteroatomic ligand, the terminal S atom, as well as the O atoms, can bind to a range of metal ions. However, the long S-S bond is unstable under acidic conditions or at high temperature. Hence, the thiosulfate anion has not, to date, been explored extensively as a network-building unit for higher dimensional structures (Paul et al., 2011). Despite these stability complications, Baggio and co-workers have synthesized a few molecular and one-dimensional structures containing thiosulfate anions that are connected to the metal through oxygen as well as sulfur atoms (Baggio et al., 1996(Baggio et al., , 1997Freire et al., 2001;Harvey et al., 2004). Our continuing synthetic efforts using the thiosulfate anion have resulted in the synthesis of some new three-dimensional structures in the family of cadmium-thiosulfate hybrid compounds formed in the presence of organic linkers (Paul et al., 2009a(Paul et al., ,b, 2010. It is noteworthy that all of the reported metal-thiosulfate compounds are synthesized in the presence of nitrogen-containing aromatic organic linkers. Aromatic ISSN 2056-9890 ligands play a dual role in metal-thiosulfate formation as they increase the dimensionality of the local structure and increase structure stabilization via secondary interactions, such as hydrogen bonds. Recently, Natarajan and co-workers (Karthik & Natarajan, 2016) have reported on some three-dimensional zinc-thiosulfate hybrid structures with aromatic N-donor organic linkers. Metal-thiosulfate compounds prepared in the presence of aliphatic amines are, however, rare (Paul, 2016) and require investigation. The title compound, is the first example of an aliphatic-amine-templated zinc thiosulfate compound. Its synthesis and crystal structure are reported on herein.

Structural commentary
The molecular structure of the title compound is illustrated in Fig. 1. In the complex, the Zn 2+ ion is coordinated by two sulfur atoms of the thiosulfate ligands (S1 and S3) and two nitrogen atoms from the piperazinium ions (N1 and N3), in an approximately tetrahedral geometry (ZnS 2 N 2 , CN = 4). The Zn-S bond lengths are 2.2927 (4) Å for Zn1-S1 and 2.3324 (4) Å for Zn1-S3. The Zn-N bond lengths are 2.0879 (13) Å for Zn1-N1 and 2.0727 (12) Å for Zn1-N3. The N/S-Zn1-S/N bond angles lie in the range 101.24 (4) to 116.79 (2) , confirming the tetrahedral nature of the zinc ions. Within the two thiosulfate ligands, the S-S bond lengths are 2.0511 (5) Å for S1-S2 and 2.0332 (5) Å for S3-S4. The S-O bond lengths vary from 1.4437 (14) to 1.4623 (13) Å , while the O-S-O angles vary from 104.53 (5) to 112.85 (10) , which is indicative of a fairly regular tetrahedral arrangement. In the molecular unit, the two thiosulfate units are bonded to the zinc(II) ion only through the terminal S atoms, and the oxygen atoms are uncoordinated. In addition, only one nitrogen atom of each piperazinium ion is bonded to the zinc(II) ion, the second being diprotonated in each case.

Figure 1
The asymmetric unit of the title compound, with atom labelling and showing 50% probability displacement ellipsoids.
There are a few examples in which zero-dimensional cadmium-thiosulfate compounds form simple dinuclear complexes, in which the thiosulfate unit is bound to the metal through both the sulfur and the oxygen atoms. As expected, the structures are stabilized through C-HÁ Á ÁO hydrogenbonding interactions andinteractions. One cadmium thiosulfate compound, bis(propane-1,3-diamine)(thiosulfato)cadmium (CSD refcode: ORUJOC), which was reported recently, was isolated in the presence of the aliphatic amine 1,3-diaminopropane (Paul, 2016). One molecular piper-azinium thiosulfate monohydrate structure has been reported, (piperazinediium thiosulfate monohydrate; CSD refcode: AROWUA; Srinivasan et al., 2011), in which the protonated aliphatic amine and thiosulfate units are linked together through extensive hydrogen bonds. It is noteworthy that there are no previous examples in the literature of zinc-thiosulfate structures that crystallize in the presence of aliphatic amines.

Synthesis and crystallization
Zn(NO 3 ) 2 Á6H 2 O (0.297 g, 1 mmol) was dissolved in 5 ml distilled water. Then (NH 4 ) 2 S 2 O 3 (0.296 g, 2 mmol) was added to the solution, which was stirred for 15 min. Piperazine (0.172 g, 2 mmol) was dissolved separately in distilled water (5 ml) and the solution poured into the initial reaction mixture until the pH was 8. The resulting solution was left undisturbed and after 1 week, colourless block-shaped crystals were obtained. The product was filtered and washed with cold water. The yield was approximately 85% based on Zn metal. Elemental analysis calculated for C 8 H 26 N 4 O 8 S 4 Zn: C 19.20, H 5.24, N 11.20%; found: C 19.27, H 5.29, N 11.16%.

Figure 2
A view along the a axis of the crystal packing of the title compound. The hydrogen bonds are shown as dashed lines (see Table 1).
were located in difference-Fourier maps and freely refined. The C-bound H atoms were included in calculated positions and refined as riding: C-H = 0.97 Å with U iso (H) = 1.2U eq (C).

Bis(piperazin-1-ium-κN 4 )bis(thiosulfato-κS)zinc(II) dihydrate
Crystal data [Zn(C 4  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.47 e Å −3 Δρ min = −0.36 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.