1,1′-[m-Phenylenebis(nitrilomethanylylidene)]dinaphthalen-2-ol–1,1′-[m-phenylenebis(iminomethanylylidene)]dinaphthalen-2(1H)-one (0.58/0.42)

In the solid state the title Schiff base, 0.58C28H20N2O2·0.42C28H20N2O2, exists both as the keto–imino and as the enol–amino tautomer, which is manifested in the disorder of the H atom in the intramolecular hydrogen-bonded ring. The naphthalene ring systems show some distortion, which is consistent with the quinoid effect. The ratio of the enol form refined to 58 (5)%. The molecule has crystallographically imposed symmetry: a twofold axis passes through the central benzene ring. Crystals are built up of layers parallel to (010). Stacking interactions between the layers involve only standard van der Waals attraction forces between apolar groups. The alignment of the aromatic rings in neighbouring layers shows a herringbone motif. A weak C—H⋯O interaction is observed.

In the solid state the title Schiff base, 0.58C 28 H 20 N 2 O 2 Á-0.42C 28 H 20 N 2 O 2 , exists both as the keto-imino and as the enol-amino tautomer, which is manifested in the disorder of the H atom in the intramolecular hydrogen-bonded ring. The naphthalene ring systems show some distortion, which is consistent with the quinoid effect. The ratio of the enol form refined to 58 (5)%. The molecule has crystallographically imposed symmetry: a twofold axis passes through the central benzene ring. Crystals are built up of layers parallel to (010). Stacking interactions between the layers involve only standard van der Waals attraction forces between apolar groups. The alignment of the aromatic rings in neighbouring layers shows a herringbone motif. A weak C-HÁ Á ÁO interaction is observed.

Related literature
For general background to Schiff bases, see: Blagus et al. (2010). For applications of Schiff bases and derivatives as ligands, see: Herná ndez- Molina et al. (1997); Torayama et al. (1997); Elerman et al. (1998); Ganjali et al. (2008). For discussion of the quinoid effect, see: Gavranić et al. (1996Gavranić et al. ( , 1997; Friščić et al. (1998). For structures with a herringbone arrangement, see: Desiraju & Gavezzotti (1989). For standard bond lengths, see: Allen et al. (1987 Table 1 Hydrogen-bond geometry (Å , ). Data collection: CrysAlis CCD (Oxford Diffraction, 2003); cell refinement: CrysAlis RED (Oxford Diffraction, 2003); data reduction: CrysAlis RED; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999), PARST97 (Nardelli, 1995) and Mercury (Macrae et al., 2006). Financial support by the Ministry of Science, Education and Sport of the Republic of Croatia is gratefully acknowledged (grant No. 119-1193079-3069 (Blagus et al., 2010). The conformation of the free ligand is of interest for comparison with the coordinated one in a corresponding metal complex (Gavranić et al., 1997). Schiff bases derived from m-phenylenediamine can coordinate only one of their two ligand nitrogen atoms to a particular metal cation, thereby facilitating the formation of binuclear complexes, in which two coordination centres are bridged by two Schiff base molecules (Hernández-Molina, et al., 1997;Torayama et al., 1997). The environment of the coordination centre in transition metal complexes with bidentate Schiff base ligands can be modified by attaching different substituents to the ligand. Substituents have been used for tuning both steric and electronic properties, which are important for differences in structure and reactivity (Elerman et al., 1998;Ganjali et al., 2008).
The title compound is a stretched, non-planar aromatic system consisting of five aromatic and two hydrogen-bonded pseudo-aromatic rings with certain possibility of electron flow through the system (Fig. 1). The molecules are present in the crystals as disordered keto-and enol tautomers in an approximate ratio of 42 (5) : 58 (5).
The shape of the molecule can be conveniently described via three best planes, calculated through the two terminal naphthalene fragments and the central benzene ring. Because of the imposed crystallographic twofold symmetry, both naphthalene-benzene angles are 20.66 (7)°. The angle between the two naphthalene rings is 40.58 (6) of carbon-to-oxygen bond distances in quinones and phenols are 1.279 and 1.339 Å, respectively, while the corresponding carbon-to-nitrogen distances in imines and secondary amines are 1.222 and 1.362 Å, respectively (Allen et al., 1987)].
In accordance with the intermediate bond lengths, the position of the H atom in the hydrogen-bonded ring could not be determined unequivocally. Its location was represented in the δF map as a large diffuse maximum of 0.29 e/Å 3 , positioned closer to the oxygen atom O1 than to the nitrogen atom N1. The enol to ketone tautomer ratio was determined to be 0.58 (5) : 0.42 (5) by refinement of a disordered hydrogen atom model. No significant residual electron density was detected in the area of the two disordered hydrogen atom positions in the δF map at the end of the refinement.
The peculiar bond distance scheme in naphthalene, a special arrangement of shorter and longer bond lengths is very well known (Allen et al., 1987). Generally, the same naphthalene topology applies to compounds containing substituted naphthane fragments. With oxygen substitution in position 2 of naphthalene, two forms, quinoid and benzenoid (if the oxygen is protonated) can be recognised. The presence of the quionid form, i.e., the quinoid effect is the cause of the quite supplementary materials sup-2 short C3-C4 bond distance of 1.352 (3) Å in the title compound (Gavranić et al.,1996;Friščić et al., 1998). However, there is also a slightly longer than usual C-C bond present in the naphthalene fragment. The C1-C10 bond distance is 1.452 (3) Å. The corresponding bond is 1.420 (3) Å in naphthalene. There are two possible reasons for this unusual bond distance: (i) The positions of the atoms of the enol and keto tautomer do not necessarily overlap fully, which may lead to an apparent shift of C10 closer to C9, although no anomaly in the contour δF map was observed in the region of C10 carbon atom. (ii) The possibility of electron flow to the central aromatic ring and the two highly electronegative nitrogen atoms lower the electron density in the naphthalene rings and thus weaken the aromaticity in the region of the C10 atom. Shifting of the electron density from the naphthalene core may be further assistted by the fused hydrogen-bond ring.
There are 4 molecules in the unit cell of the title compound. The molecules lie in 4(c) special positions of the space group with a crystallographic 2-fold axis passing through atoms C14 and C14a, respectively. Intermolecular interactions are dominated by van der Waals forces. There is only one very weak C-H···O contact [C13···O1 i 3.414 (3) Å; (i): -x + 3/2, y, z + 1/2]. The molecular packing framework consists of infinite layers in the (010) plane having a thickness of half of the b-axis (Fig. 2). Parallel layers in a stack are connected via extremely weak C6-H6···C6 ii interactions [d(C6···C6 ii ) = 3.668 (3) Å; (ii): x + 1/2, -y + 1, -z + 1/2]. The mutual orientation of the naphthalene rings involved in this interaction creates a herringbone motif. The herringbone motif is characteristic of planar aromatic systems (Desiraju & Gavezzotti, 1989) and is stabilised by electrostatic interactions.

Experimental
The title compound was synthesized by applying the standard condensation procedure for the preparation of imino compounds. An ethanolic solution of 2-hydroxy-1-naphtaledehyde (10 mmol) and an ethanolic solution of m-phenylenediamine (5 mmol) were mixed and heated in the presence of acetic acid as catalyst. The mixture was refluxed at 331 K for 2 h. After cooling, the precipitated Schiff base was separated by vacuum filtration with a yield of 94%. A suitable single-crystal was obtained by slow recrystallization from dichloromethane solution at room temperature.  129.26, 128.10, 127.30, 123,63, 121.95, 118.94, 118.11, 112,28 (12 C, ar).

Refinement
The positional parameters and the occupancies of atoms H1 and H2, which belong to the enol and keto tautomer, respectively, were refined freely. The sum of the occupancies was constrained to 1 by useing the same SHELXL FVAR variable [i.e., occupancies of fv(2) and 1-fv(2)]. U iso values of both H1 and H2 were calculated as 120% of the equivalent isotropic thermal parameters of the corresponding heavy atoms. No electron density residues were observed after the refinement converged with a stable 58 (5) : 42 (5) occupancy ratio for H1 (enol tautomer) and H2 (keto tautomer), respectively.
All other hydrogen atoms were treated as riding atoms using instruction AFIX 43 (C-H 0.93 Å) with U iso (H) being 1.2 times the equivalent isotropic thermal parameter of corresponding carbon atom.