Diaquabis{μ-1,5-bis[(pyridin-2-yl)methylidene]carbonohydrazide(1–)}di-μ-chlorido-tetrachloridotetrazinc(II)

A symmetrical dicarbonohydrazide was used to synthesize a centrosymmetric tetranuclear zinc(II) complex in which two of the zinc cations are pentacoordinated and the other two are hexacoordinated.

A tetranuclear Zn II complex, [Zn 4 (C 13 H 11 N 6 O) 2 Cl 6 (H 2 O) 2 ] or {[Zn 2 (HL)-(H 2 O)(Cl 2 )](Cl) 2 [Zn 2 (HL)(H 2 O)(Cl)]} 2 , was synthesized by mixing an equimolar amount of a methanol solution containing ZnCl 2 and a methanol solution containing the ligand H 2 L [1,5-bis(pyridin-2-ylmethylene)carbonohydrazide]. In the tetranuclear complex, each of the two ligand molecules forms a dinuclear unit that is connected to another dinuclear unit by two bridging chloride anions. In each dinuclear unit, one Zn II cation is pentacoordinated in a N 2 OCl 2 in a distorted square-pyramidal geometry, while the other Zn II cation is hexacoordinated in a N 3 OCl 2 environment with a distorted octahedral geometry. The basal plane around the pentacoordinated Zn II cation is formed by one chloride anion, one oxygen atom, one imino nitrogen atom and one pyridine nitrogen atom with the apical position occupied by a chloride anion. The basal plane of the hexacoordinated Zn II cation is formed by one chloride anion, one hydrazinyl nitrogen atom, one imino nitrogen atom and one pyridine nitrogen atom with the apical positions occupied by a water oxygen atom and a bridged chloro anion from another dinuclear unit, leading to a tetranuclear complex. A series of intramolecular C-HÁ Á ÁCl hydrogen bonds is observed in each tetranuclear unit. In the crystal, the tetranuclear units are connected by intermolecular C-HÁ Á ÁCl, C-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds, forming a planar twodimensional structure in the ac plane.

Chemical context
Symmetrical dicarbonohydrazide Schiff bases possess two cavitiess, which make them versatile. During complexation, either one or both of the cages can be occupied by a metal ion depending on the reaction conditions. The presence of an amidic bond in these molecules leads to the keto-enol tautomer, which can act in neutral or deprotonated forms. These compounds can adopt two different configurations, e.g. S-cis or S-trans, yielding different structures with the same metal cation. These ligands can coordinate to transition metals in a pentadentate or hexadentate manner (El-Gammal et al., 2012;Sow et al., 2013), as well as in the ketonic or enolic form . When the configuration of this kind of ligand is S-trans, it acts in a hexadentate fashion. In this case, the formation of a dinuclear complex with a -N,N bridge is generally observed, for example in a dinuclear copper(II) complex (Dragancea et al., 2014). The S-cis-enol configuration leads to the formation of square-grid complexes by directed ISSN 2056-9890 self-assembly (Bikas et al., 2015;Sow et al., 2013;Li et al., 2014). In these complexes, -O and -N,N atoms bridge the metal ions, which display N 4 O 2 or N 5 O octahedral environments (Shuvaev et al., 2010).

Structural commentary
The title compound is a centrosymmetric tetranuclear Zn II complex composed by two dinuclear entities. Each dinuclear entity contains one ligand molecule acting in monodeprotonated form, three bonded chloride anions, one bonded water molecule, and two Zn II cations. The two units are linked by two choride anions acting as bridges (Fig. 1). Each monodeprotonated organic molecule acts through two azomethine nitrogen atoms, two pyridine nitrogen atoms, one hydrazinyl nitrogen atom and one carbonyl oxygen atom, resulting in a hexadentate ligand. The Zn1 and Zn2 cations are situated, respectively in N 2 OCl 2 and N 3 OCl 2 coordination sites ( Fig. 1). In the structure of the complex, the two ligand molecules are arranged in the Z-E form.
The Zn1 atom is pentacoordinated by one pyridine nitrogen atom, one azomethine nitrogen atom, one oxygen atom, and two terminal chloride anions. According to the Addison (1984) index, the coordination geometry around a pentacoordinated metal center can be discussed in terms of the parameter [defined as = ( -)/60 where and are the largest values of the bond angles around the central atom]; = 0 for a perfect square pyramidal geometry while = 1 for a perfect trigonal-bipyramidal geometry. In the case of the title complex, the value of 0.1085 is indicative of a distorted square-pyramidal geometry around the Zn1 center. The equatorial plane is occupied by atoms N5, N6, Cl3, O2 while the apical position is occupied by Cl2. The angles N5-Zn1-O2 [72.76 (9)

Figure 1
A view of the title compound, showing the atom-numbering scheme. Displacement ellipsoids are plotted at the 30% probability level. Unlabelled atoms are generated by the symmetry operation 1 À x, 1 À y, Àz.
The geometry around the hexacoordinated Zn2 atom is best described as distorted octahedral. The basal plane is occupied by atoms N2, N4, N1 and Cl1 with cissoid bond angles in the range 73.95 (9)-111.20 (7) Table 1). The deviation of the angles around the Zn2 cation with respect to the valence angles for a regular octahedron (180 and 90 ) indicates that the geometry around the Zn2 ion is a distorted octahedron (Fig. 1). The fivemembered rings (NCNNZn and NCCNZn) formed by the ligand with Zn2 impose large distortions on the ideal angles of a regular octahedron with bite angles in the range 73.95 (9)-74.29 (9) .

Supramolecular features
In the crystal, numerous intermolecular O-HÁ Á ÁCl, C-HÁ Á ÁO, C-HÁ Á ÁCl and N-HÁ Á ÁO hydrogen bonds are observed (Fig. 2, Table 2). An N-HÁ Á ÁO type occurs between the oxygen atom O2 of the ligand, which acts as a proton acceptor, and the nitrogen atom of the hydrazinyl group, which acts as the proton donor. An O-HÁ Á ÁCl link is established between a water molecule in the apical position of the Zn2 ion, acting as proton donor, and a terminal chloride ions linked to Zn1 as proton acceptor. These intermolecular hydrogen bonds ensure the cohesion of the crystal, developing a planar two-dimensional structure in the ac plane. À in LOFDUH (Hoque et al., 2014), and SO 4 2À in LOFFAP (Hoque et al., 2014) and LOFFAP01 (Hoque et al., 2016). In the structure incorporating monoprotonated H 3 L + , H 2 PO 4 À is the counter-ion (LOFFIX; Hoque et al., 2014). The tetranuclear Dy 3+ complex has a [2 x 2] grid structure (DIGQER; Randell et al., 2013).

Diaquabis{µ-1,5-bis[(pyridin-2-yl)methylidene]carbonohydrazide(1-)}di-µ-chlorido-tetrachloridotetrazinc(II)
Crystal data 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )