1-[(4-{[(2-Oxo-1,2-dihydronaphthalen-1-ylidene)methyl]amino}anilino)methylidene]naphthalen-2(1H)-one dihydrate

The title compound, C28H20N2O2·2H2O, comprises a Schiff base molecule with an imposed inversion centre in the middle of p-phenylenediamine unit and water molecules of crystallization. In the structure, the Schiff base molecule is present as the keto–amino tautomer with a strong intramolecular N—H⋯O hydrogen bond. The Schiff base molecules and water molecules of crystallization create infinite [010] columns through O—H⋯O hydrogen bonds. Intermolecular attractions within columns are through additional π–π interactions [centroid–centroid distance = 3.352 (1) Å] between parallel Schiff base molecules. The columns are joined into infinite (011) layers through weak C—H⋯O hydrogen bonds. The layers pack in an assembly by van der Waals attractions, only being effective between bordering non-polar naphthalene ring systems.

The title compound, C 28 H 20 N 2 O 2 Á2H 2 O, comprises a Schiff base molecule with an imposed inversion centre in the middle of p-phenylenediamine unit and water molecules of crystallization. In the structure, the Schiff base molecule is present as the keto-amino tautomer with a strong intramolecular N-HÁ Á ÁO hydrogen bond. The Schiff base molecules and water molecules of crystallization create infinite [010] columns through O-HÁ Á ÁO hydrogen bonds. Intermolecular attractions within columns are through additionalinteractions [centroid-centroid distance = 3.352 (1) Å ] between parallel Schiff base molecules. The columns are joined into infinite (011) layers through weak C-HÁ Á ÁO hydrogen bonds. The layers pack in an assembly by van der Waals attractions, only being effective between bordering non-polar naphthalene ring systems.
Along with intermolecular contacts via hydrogen bonds the linking between molecules amplifies through π-π interactions with offset ( Fig. 2). Schiff base molecules arrange parallel to each other with their middle p-phenylenediamine moiety being separated at distances characteristic for layer separation in graphite. The shortest separations corresponding to the sum of van der Waals radii are: C12···C12 iii 3.362 (3) Å [(iii): -x, -y + 1, -z] and C11···C13 iv 3.374 (3) Å [(iv): x, y -1, z]. π··· π interactions are also characterised by perpendicular Cg iii ···Cg v distance 3.352 (1) Å [(v): -x, 3 -y, -z] and slippage of 2.955 Å. There is a space between each pair of neighboring columns large enough to accommodate water molecules (Fig.   3). Connection between neighboring parallel layers is accomplished through bordering non-polar naphthalene core by the standard van der Walls attractions. The rather planar Schiff base molecules of (I) reveal characteristic herringbone motif of packing arrangement (Fig. 4).
The crystals of (I), (1,4-bis(2-hydroxy-1-naphthylmethylideneamino)benzene as crystal hydrate were obtained during an unsuccessful attempt to synthesise the nickel complex of corresponding Schiff base. Schiff base itself was prepared separately in standard way by condensation of 2-hydroxy-naphtaldehyde and p-phenylenediamine in ethanol solution in molar ratio 2:1 and used as a ligand in metal complex synthesis. The 1:1 mixture of 0.1 mmol DMSO solutions of Schiff base and 0.2 mmol nickel salt, NiCl 2 . 6H 2 O was stirred under reflux for two h at 373 K. Preparation of nickel complex failed. The crystals of title compound crystallised from mother liquor after cooling to RT and mechanically separated from nickel salt.

Refinement
Hydrogen atoms were refined in two different ways.
For hydrogen atoms bonded to C and N atoms benzene type riding mode was used with C-to-H and N-to-H bond distances taken as 0.93 and 0.86 Å, respectively.
Due to somewhat higher values of anisotropic thermal parameters of O1W oxygen atom, implying to certain disorder of water molecule, bond distances O1W to H1A and H1B, respectively, as well as bond distance H1A-H1B were restrained to the values accepted for water molecule. Bond distances O1W to H1A and H1B, respectively were fixed to 0.82 (1) Å and H1A to H1B to 1.30 (1) Å and the position of hydrogen atoms were re-calculated in consecutive refinement cycles.
Isotropic thermal parameters for hydrogen atoms were estimated as 1.2 times of equivalent isotropic thermal parameter of corresponding C, N and O atoms. Fig. 1. A general overview of (I) showing numbering scheme with anisotropic thermal ellipsoids pictured at 30% probability level. Thin lines display intra N-H···O, and the two intermolecular hydrogen bonds O-H···O and C-H···O. Hydrogen atoms are drawn as spheres of arbitrary radius.    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.