catena-Poly[[dichloridozinc(II)]-μ-1,4-bis(3-pyridylmethyl)piperazine]

In the title compound, [ZnCl2(C16H20N4)]n, tetrahedrally coordinated divalent Zn atoms are ligated by two Cl atoms and two N-donor atoms from two 1,4-bis(3-pyridylmethyl)piperazine (3-bpmp) ligands. The tethering 3-bpmp ligands promote the formation of [ZnCl2(3-bpmp)]n chains situated parallel to (02). These chains aggregate via C—H⋯Cl interactions to form supramolecular layers, which in turn stack to construct the three-dimensional crystal structure.


S1. Comment
In comparison to coordination polymers based on the rigid rod tether 4,4′-bipyridine, extended solids based on the hydrogen-bonding capable bis(3-pyridylmethyl)piperazine (3-bpmp) ligand are much less common (Johnston et al., 2008). The title compound was obtained during an attempt to prepare a zinc azide 3-bpmp coordination polymer.
The asymmetric unit of the title compound consists of a divalent Zn atom, two Cl atoms, and two halves of two crystallographically distinct 3-bpmp molecules. The coordination environment at Zn is a slightly distorted {ZnCl 2 N 2 } tetrahedron, with two chloro ligands and two N donor atoms from crystallographically distinct bis(3-pyridylmethyl)piperazine (3-bpmp) ligands ( Figure 1).
Neighboring Zn atoms are bridged by tethering 3-bpmp ligands to construct neutral [ZnCl 2 (3-bpmp)] n coordination polymer chains, that are oriented parallel to the (1 0 2) crystal direction. There are crystallographic inversion centres at the centroids of the piperazinyl rings within the 3-bpmp ligands. The through-ligand Zn···Zn distances within the chain motifs are 14.218 (4) and 14.259 (4) Å. These chains aggregate by C-H···Cl interactions to construct a supramolecular layer that is oriented parallel to the ac crystal planes ( Figure 2). In turn these layer motifs stack by means of crystal packing forces to establish the three-dimensional crystal structure of the title compound ( Figure 3).

S2. Experimental
Zinc chloride dihydrate and sodium azide were obtained commercially. Bis(3-pyridylmethyl)piperazine (3-bpmp) was prepared via a published procedure (Pocic et al., 2005). Zinc chloride dihydrate (0.082 g, 0.48 mmol) was dissolved in 6 ml water in a glass vial. A 2 ml aliquot of tetrahydrofuran was carefully layered on the top of the zinc chloride solution.
Above the tetrahydrofuran layer was gently placed a mixture of sodium azide (0.065 g, 1.0 mmol) and 3-bpmp (134 mg, 0.500 mmol) taken up in 5.5 ml of a 10:1 methanol:water mixture. Colourless blocks of the title compound deposited after standing at 25 °C for one week.

S3. Refinement
All H atoms bound to C atoms were placed in calculated positions, with C-H = 0.95 Å and refined in riding mode with

Figure 1
The asymmetric unit of the title compound, showing 50% probability ellipsoids and atom numbering scheme. Hydrogen atom positions are shown as sticks. Colour codes: gray Zn, green Cl, blue N, black C.

Figure 2
A layer of [ZnCl 2 (3-bpmp)] n chains in the title compound. C-H···Cl interactions are shown as dashed lines.

Figure 3
Stacking of layer motifs in the title compound.

Special details
Experimental. Reflection data were collected on a non-merohedrally twinned crystal. The twin law was determined with CELLNOW (Sheldrick, 2003). The structure was solved and refined using reflections from only the major twin component, whose reflection file was generated using TWINABS (Sheldrick, 2007). Composite reflections belonging to both twin domains were omitted from the reflection list, causing the loss of 246 reflections from the major twin component data. The data set was still 99.9% complete to 2θ of 50°. Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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.