{5,5′-Dihydroxy-2,2′-[o-phenylenebis(nitrilomethylidyne)]diphenolato}nickel(II) dihydrate

In the title complex, [Ni(C20H14N2O4)]·2H2O, the NiII ion is in an essentially square-planar geometry involving an N2O2 atom set of the tetradentate Schiff base ligand. The Ni atom lies on a crystallographic twofold rotation axis. The asymmetric unit contains one half-molecule of the complex and a water molecule. An intermolecular O—H⋯O hydrogen bond forms a four-membered ring, producing an R 1 2(4) ring motif involving a bifurcated hydrogen bond to the phenolate O atoms of the complex molecule. In the crystal structure, molecules are linked by π–π stacking interactions, with centroid–centroid distances in the range 3.5750 (11)–3.7750 (11) Å. As a result of the twofold symmetry, the central benzene ring makes the same dihedral angle of 15.75 (9)° with the two outer benzene rings. The dihedral angle between the two hydroxyphenyl rings is 13.16 (5)°. In the crystal structure, molecules are linked into infinite one-dimensional chains by directed four-membered O—H⋯O—H interactions along the c axis and are further connected by C—H⋯O and π–π stacking into a three-dimensional network. An interesting feature of the crystal structure is the short Ni⋯O, O⋯O and N⋯N interactions which are shorter than the sum of the van der Waals radii of the relevant atoms. The crystal structure is stabilized by intermolecular O—H⋯O and C—H⋯O hydrogen bonds and by π–π stacking interactions.

In the title complex, [Ni(C 20 H 14 N 2 O 4 )]Á2H 2 O, the Ni II ion is in an essentially square-planar geometry involving an N 2 O 2 atom set of the tetradentate Schiff base ligand. The Ni atom lies on a crystallographic twofold rotation axis. The asymmetric unit contains one half-molecule of the complex and a water molecule. An intermolecular O-HÁ Á ÁO hydrogen bond forms a four-membered ring, producing an R 1 2 (4) ring motif involving a bifurcated hydrogen bond to the phenolate O atoms of the complex molecule. In the crystal structure, molecules are linked bystacking interactions, with centroid-centroid distances in the range 3.5750 (11)-3.7750 (11) Å . As a result of the twofold symmetry, the central benzene ring makes the same dihedral angle of 15.75 (9) with the two outer benzene rings. The dihedral angle between the two hydroxyphenyl rings is 13.16 (5) . In the crystal structure, molecules are linked into infinite one-dimensional chains by directed fourmembered O-HÁ Á ÁO-H interactions along the c axis and are further connected by C-HÁ Á ÁO andstacking into a three-dimensional network. An interesting feature of the crystal structure is the short NiÁ Á ÁO, OÁ Á ÁO and NÁ Á ÁN interactions which are shorter than the sum of the van der Waals radii of the relevant atoms. The crystal structure is stabilized by intermolecular O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds and bystacking interactions.
In the title compound ( Fig. 1), the Ni II ion, is in an essentially square-planar geometry involving a N 2 O 2 atom set of the tetradentate Schiff base ligand. The Ni atom lies on a crystallographic twofold rotation axis. An intermolecular O-H···O hydrogen bond forms a four-membered ring, producing an R 2 1 (4) ring motif (Bernstein et al., 1995). The bond lengths are within the normal ranges (Allen et al., 1987). The asymmetric unit contains one-half of the molecule of the complex and a water molecule. The latter shows a bifurcated hydrogen bond which is connected to the phenolato oxygen atoms of the complex. The molecule is nearly planar, with a maximum deviation from the mean plane of 0.370 (2) Å for atom C9. As a result of the twofold symmetry, the central benzene ring makes the same dihedral angle of 15.75 (9)° with the two outer benzene rings. The dihedral angle between the two hydroxy phenyl rings is 13.16 (5)°. In the crystal structure, (Fig. 2) molecules are linked into infinite one-dimensional chains by directed four-membered O-H···O-H interactions along the c axis and are furthered connected by C-H···O and π-π stacking into a three-dimensional network.
An interesting feature of the crystal structure is the short Ni···O, O···O, and N···N interactions (Table 1), which are shorter than the sum of the van der Waals radii of the relevant atoms. The short distances between the centroids of the five-and six-membered rings indicate the existence of the π-π interactions ( Table 1). The crystal structure is stabilized by intermolecular O-H···O, C-H···O hydrogen bonds (Table 2) and π-π interactions.

Experimental
A chloroform solution (40 ml) of the ligand (1 mmol, 354 mg) was added to a methanol solution (20 ml) of NiCl 2 .6H 2 O (1.05 mmol, 237 mg). The mixture was refluxed for 30 min and the resulting red precipitate was filtered, washed with cold ethanol and dried in air. Single crystals suitable for X-ray analysis were obtained from a THF solution at RT.

Refinement
The water H-atoms were located in a difference Fourier map and refined as riding on the parent atom with an isotropic displacement parameter of 1.5Ueq of the water oxygen. The hydroxyl H atoms were also located in a difference Fourier map and refined freely. The rest of the hydrogen atoms were positioned geometrically [C-H = 0.93 Å] and refined using a riding model.

Special details
Experimental. The low-temperature data was collected with the Oxford Cryosystem Cobra low-temperature attachment 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 > 2sigma(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.