Bis{2-hydroxyimino-N′-[1-(2-pyridyl)ethylidene]propanohydrazidato}zinc(II) dihydrate

The title compound, [Zn(C10H11N4O2)2]·2H2O, was prepared by the reaction between Zn(CH3COO)2·2H2O and 2-hydroxyimino-N′-[1-(2-pyridyl)ethylidene]propanohydrazide (Hpop). The central ZnII atom has a distorted tetragonal-bipyramidal coordination geometry formed by two amide O atoms and four N atoms of two azomethine and two pyridine groups. In the crystal, complex molecules form layers parallel to the crystallographic b direction. The layers are connected by O—H⋯N and O—H⋯O hydrogen bonds involving the solvent water molecules.


Comment
As a part of our study of coordination compounds based on oxime-containing Schiff bases we would like to present the structure of the title compound 1, Fig. 1, which is based on polynucleative strand-type ligand 2-hydroxyimino-N'-[1-(2pyridyl)ethylidene]propanohydrazone (Hpop) (Fig. 1). It has been shown previously that Hpop is able to form mono-and tetranuclear [2 x 2] grid-like assemblies with 3d-metal ions (Moroz et al., 2008a,b).
The title compound consists of neutral complex molecules and solvating water molecules. Zinc ion has a distorted tetragonal bipyramidal geometry. The coordination polyhedron is formed by two oxygen atoms from the amide groups and four nitrogen atoms belonging to two azomethine and two pyridine groups. The Zn-N and Zn-O bond lengths are comparable to previously reported zinc complexes with thiosemicarbasone and semicarbasone derivatives (Kasuga et al., (2003)), ligands with pyridine groups complexed to the metal ion (Petrusenko et al. (1997), Comba et al., (2002)) and the zinc-containing complex based on Hpop (Moroz et al., 2008b) (Table 1). The bite angles around the central atom deviate from an ideal square-planar configuration, that is a consequence of the formation of four almost flat five-membered chelate rings (Table 1). The ligands exist in complex molecule in singly charged form due to deprotonation of the amide group, C-N, C-O and N-N' bond distances are typical for deprotonated functions. In Hpop the oxime group is situated in anti-position to the amide group which was early shown in the structures of the free ligand and similiar compounds -amide derivatives of 2-hydroxyiminopropanoic acid (Onindo et al. (1995); Sliva et al. (1997a,b); Mokhir et al. (2002); Moroz et al., 2009a,b).
In the crystal packing the molecules of 1 form columns along a crystallographic direction due to hydrogen bonds and π-stacking interaction (Fig. 2). The columns are connected in 3D structure by a variety of hydrogen bonds where solvated water molecules act as donors and O and N atoms of the oxime group and O atom of the amide group of the ligand act as acceptors (Table 2).

Experimental
Zinc(II) acetate (0.011 g, 0.05 mmol) in 5 ml H 2 O was added to 10 ml of hot methanol solution of Hpop (0.022 g, 0.1 mmol) and followed by 1 ml of alkali solution (0.1 M KOH). The mixture was left for slow evaporation at room temperature. After 5 days cubic yellowish crystals of 1 suitable for X-ray analysis were obtained.

Refinement
The H 2 O hydrogen atoms were located from the difference Fourier map but constrained to ride on their parent atom, with U iso = 1.5 U eq (parent atom). Other hydrogen atoms were positioned geometrically and were also constrained to ride on their parent atoms, with C-H = 0.95-0.98 Å, and U iso = 1.2-1.5 U eq (parent atom). The highest peak is located 1.15 Å from atom H5O and the deepest hole is located 0.82 Å from atom Zn1.

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.
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 Rfactors(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.