cyclo-Tetrakis{μ-N′-[(8-oxidoquinolin-7-yl)methylidene]isonicotinohydrazidato}tetrazinc tetrahydrate

In the title compound, [Zn4(C16H10N4O2)4]·4H2O, the N′-[(8-oxidoquinolin-7-yl)methylidene]isonicotinohydrazidate (L 2−) ligand binds to the metal ions, forming stable five- and six-membered chelate rings, leaving the pyridyl groups free. The compound is a tetranuclear ZnII complex centered about a fourfold roto-inversion axis, with the ligand coordinating in the doubly deprotonated form. The ZnII atom has a distorted square-pyramidal geometry being coordinated by one N and two O-atom donors from the doubly deprotonated L 2− ligand, and by one N atom and one O-atom donor from a symmetry-related L 2− ligand. In the crystal, four symmetry-related lattice water molecules, centred about a fourfold roto-inversion axis, form a cyclic tetramer through O—H⋯O hydrogen bonds. These tetramers connect to the complex molecules through O—H⋯N hydrogen bonds, forming a chain propagating along [100]. Neighbouring molecules are linked by π–π interactions [centroid–centroid distance = 3.660 (2) Å] involving the quinolidine rings.

In the title compound, [Zn 4 (C 16 H 10 N 4 O 2 ) 4 ]Á4H 2 O, the N 0 -[(8oxidoquinolin-7-yl)methylidene]isonicotinohydrazidate (L 2À ) ligand binds to the metal ions, forming stable five-and sixmembered chelate rings, leaving the pyridyl groups free. The compound is a tetranuclear Zn II complex centered about a fourfold roto-inversion axis, with the ligand coordinating in the doubly deprotonated form. The Zn II atom has a distorted square-pyramidal geometry being coordinated by one N and two O-atom donors from the doubly deprotonated L 2À ligand, and by one N atom and one O-atom donor from a symmetryrelated L 2À ligand. In the crystal, four symmetry-related lattice water molecules, centred about a fourfold rotoinversion axis, form a cyclic tetramer through O-HÁ Á ÁO hydrogen bonds. These tetramers connect to the complex molecules through O-HÁ Á ÁN hydrogen bonds, forming a chain propagating along [100]. Neighbouring molecules are linked byinteractions [centroid-centroid distance = 3.660 (2) Å ] involving the quinolidine rings.

Experimental
Crystal data [Zn 4 (C 16 Table 1 Hydrogen-bond geometry (Å , ). The synthesis of metal-containing compounds is the first and an important step in a promising route to novel heterometallic coordination polymers (Tao et al., 2002). It is well known that the relative orientations of N donors and the variation of the bridging spacer may lead to the construction of supramolecular motifs that have not been achieved using normal linear organic ligands. The ligand N′-((8-hydroxyquinolin-7-yl)methylene)isonicotinohydrazide ligand (LH 2 ) is unsymmetrical, containing two different terminal coordinating sites, i.e. a pyridyl and a 7-hydrazinylidene-8-hydroxyquinoline chelator. The latter contains the N/O-bidentate chelating motif, which usually binds to metal ions in a deprotonated manner (Palacios et al., 2008). It was also found that this chelator binds to metal ions in preference to the pyridine N atom. This could provide a favourable coordination strategy for the synthesis of multinuclear metal-containing compounds. As part of our continuing studies of coordination compounds with bridging N-donor ligands (Dong et al., 2005), we report herein on the synthesis and crystal structure of a novel Zn II compound with free pyridyl groups.
The title compound is a tetranuclear Zn II complex, centred about a fourfold roto-inversion axis, and crystallizes as a tetrahydrate (Fig. 1). The Zn II atom has distorted square-pyramidal geometry, being coordinated by one N (N2) and two O donors (O1 and O2) from a doubly deprotonated LH 2 ligand, and one N (N1 i ) and one O donor (O2 i ) from a symmetryrelated L 2ligand [symmetry code :(i) -y + 5/4, x -3/4, -z + 9/4]. The N atoms of the pyridine rings are not involved in coordination. The dihedral angle between the pyridine and quinoline ring mean planes is 14.01 (15) (Woodward et al., 2006).

Experimental
A solution of LH 2 (5.3 mg,0.02 mmol) in MeOH (8 ml) was layered onto a solution of ZnSO 4 (5.8 mg, 0.04 mmol) in water (8 mL). The system was left for about two weeks at room temperature and yellow crystals of the title complex were obtained (yield 5.6 mg, 79%

Refinement
The C-bound H atoms were placed in geometrically idealized positions and included as riding atoms: C-H = 0.93 Å and U iso (H) = 1.2U eq (C). The water H atoms were located in a difference Fourier maps and refined with distance O-H restrained to 0.85 (2) Å and U iso (H) = 1.2U eq (O)

Special details
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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1