{6,6′-Dimethoxy-2,2′-[2,2-dimethylpropane-1,3-diylbis(nitrilomethylidyne)]diphenolato}nickel(II) 1.78-hydrate

In the title complex, [Ni(C21H24N2O4)]·1.78H2O, the NiII ion has a slightly distorted planar geometry, coordinated by the two N and two O atoms of the tetradentate Schiff base ligand, with a mean deviation of 0.272 Å from the NiN2O2 plane. The N and O donor atoms are mutually cis. The dihedral angle between two benzene rings of the ligand is 38.86 (8)°. There are also three solvent water molecules, two of which lie across different crystallographic twofold rotation axes; one of these is partially occupied with a refined occupancy factor of 0.570 (7). The water molecules are linked together as tetramers in R 2 2(8) ring motifs, which also connect two neighbouring molecules of the complex through a network of O—H⋯O hydrogen bonds. The crystal structure is further stabilized by intermolecular C—H⋯O and C—H⋯π interactions, which link neighbouring molecules into extended chains along the b axis. Other interesting features of the crystal structure are the short intermolecular C⋯C [3.204 (3)–3.365 (3) Å] and the C⋯O [3.199 (2)–3.205 (2) Å] contacts which are shorter than the sum of the van der Waals radii of these atoms.

In the title complex, [Ni(C 21 H 24 N 2 O 4 )]Á1.78H 2 O, the Ni II ion has a slightly distorted planar geometry, coordinated by the two N and two O atoms of the tetradentate Schiff base ligand, with a mean deviation of 0.272 Å from the NiN 2 O 2 plane. The N and O donor atoms are mutually cis. The dihedral angle between two benzene rings of the ligand is 38.86 (8) . There are also three solvent water molecules, two of which lie across different crystallographic twofold rotation axes; one of these is partially occupied with a refined occupancy factor of 0.570 (7). The water molecules are linked together as tetramers in R 2 2 (8) ring motifs, which also connect two neighbouring molecules of the complex through a network of O-HÁ Á ÁO hydrogen bonds. The crystal structure is further stabilized by intermolecular C-HÁ Á ÁO and C-HÁ Á Á interactions, which link neighbouring molecules into extended chains along the b axis. Other interesting features of the crystal structure are the short intermolecular CÁ Á ÁC [3.204 (3)-3.365 (3) Å ] and the CÁ Á ÁO [3.199 (2)-3.205 (2) Å ] contacts which are shorter than the sum of the van der Waals radii of these atoms.

Comment
Schiff base complexes are some of the most important stereochemical models in transition metal coordination chemistry, with their ease of preparation and structural variations (Granovski et al., 1993). Metal derivatives of Schiff bases have been studied extensively, and copper(II) and nickel(II) complexes play a major role in both synthetic and structural research (Elmali et al., 2000;Blower, 1998;Granovski et al., 1993;Li & Chang, 1991;Shahrokhian et al., 2000). Tetradentate Schiff base metal complexes may form trans or cis planar or tetrahedral structures (Elmali et al., 2000).
In the title compound ( Fig. 1), the Ni II ion shows a slightly distorted planar geometry which is coordinated by two imine N atoms and two phenol O atoms of the tetradentate Schiff base ligand. The bond lengths (Allen et al., 1987) and angles are within normal ranges and are comparable with the related structures (Clark et al., 1968(Clark et al., , 1969(Clark et al., , 1970. The dihedral angle between two benzene rings is 38.84 (9)°.
Of the three solvent water molecules, two of them lie across different crystallographic twofold rotation axes and one of them is partially occupied with a refined occupancy factor of 0.570 (7). The water molecules are linked together as tetramers in R 2 2 (8) ring motifs which also connect two neighbouring molecules of the complex. The crystal structure is further stabilized by intermolecular C-H···O and C-H···π interactions (Table 1) Table 1) which are shorter than the sum of the van der Waals radii of these atoms.

Experimental
A chloroform solution (40 ml) of N,N'-ethylene-bis-(3-methoxy-2-hydroxysalicylaldimine) (1 mmol) 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 then filtered. After keeping the filtrate in air, deep-green needle-shaped crystals were formed at the bottom of the vessel on slow evaporation of the solvent.

Refinement
The water H-atoms were located from the difference Fourier map and constrained to refine with the parent atom with the U iso (H) = 1.5 U eq (O). The rest of the hydrogen atoms were positioned geometrically [C-H = 0.93-0.97 Å] and refined using a riding approximation model. A rotating-group model was used for the methyl groups of the methoxy substituents. Fig. 1. The assymetric unit of the title compound, showing 50% probability displacement ellipsoids and the atomic numbering. Hydrogen bonds are drawn as dashed lines.

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cyrosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1)K.
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.