Two ( E )-2-({[4-(dialkylamino)phenyl]- imino}methyl)-4-nitrophenols

The slow evaporation of analytical NMR samples resulted in the formation of crystals of ( E )-2-({[4-(dimethylamino)- phenyl]imino}methyl)-4-nitrophenol, C 15 H 15 N 3 O 3 , (I), and ( E )-2-({[4-(diethylamino)phenyl]imino}methyl)-4-nitro- phenol, C 17 H 19 N 3 O 3 , (II). Despite the small structural difference between these two N -salicylideneaniline derivatives, they show different space groups and diverse molecular packing. The molecules of both compounds are close to being planar due to an intramolecular O—H (cid:2) (cid:2) (cid:2) N hydrogen bond. The 4-alkylamino-substituted benzene ring is inclined at an angle of 13.44 (19) (cid:3) in (I) and 2.57 (8) (cid:3) in (II) with respect to the 4-nitro-substituted phenol ring. Only very weak inter-molecular (cid:2) – (cid:2) stacking and C—H (cid:2) (cid:2) (cid:2) O interactions were found in these structures. Comment


Comment
(E)-2- phenyl]imino}methyl)-4-nitrophenol, (I), and (E)-2- ({[4-(diethylamino)phenyl]imino}methyl)-4-nitrophenol, (II), are products from the condensation reaction of N,N-dimethyl-p-phenylenediamine or N,N-diethyl-pphenylenediamine, respectively, with 2-hydroxy-5-nitrobenzaldehyde. These two compounds have the potential to occur in two tautomeric forms, and they were prepared in order to check whether increased acidity of OH (due to the presence of an NO 2 group in the para position) and increased basicity of the -N CH-N atom would result in the enamine form of these compounds. This study shows that they occur in the solid state in the imine form ( Figs. 1 and 2). However, in solution there is a fast H-atom exchange, observed from the chemical shifts of atoms C1, C6, C7 and N4, between the imine (with -OH) and enaminone (with -NH) forms. As previously observed with similar compounds (Gawinecki et al., 2007), the enaminone form predominates in chloroform solution. Recently, (I) was studied to obtain novel Grubbs-type bidentate Schiff base ruthenium catalysts, but its pK a value was too low for it to be utilized for that method (Drozdzak & Nishioka, 2010). Previously, the electronic properties of (I) were investigated (Avramovici et al., 1973(Avramovici et al., , 1974 and it was also studied for the preparation of organometallic Ga III complexes (Chesnut et al., 1998). To the best of our knowledge, (II) has not been discussed previously in the literature.
The crystal structure of (I) is ordered (Fig. 1), but in (II) a small static disorder exists in one ethyl group (Fig. 2), with the occupancy of the major component being 0.789 (6). The molecules of both compounds are rather planar. However, the dihedral angle between the two aromatic rings in (I) is 13.44 (19) , significantly larger than that in (II) [2.57 (8) ]. Furthermore, the dimethylamino group shows torsion angles of À13.8 (6) (C16-N15-C12-C13) and 5.2 (6) (C18-N15-C12-C11) with respect to the parent benzene ring in (I). The corresponding angles in (II) are À2.1 (3) [major component; for the minor C16B-N15-C12-C13 component the angle is 37.1 (4) ] and À14.6 (3) , respectively. Overall, the structure of (II) is more planar than that of (I) and the only significant deviation from the plane is found for the diethylamino group.
The presence of an intramolecular hydrogen bond in (I) was predicted according to previous studies on salicylideneanilines and proved by spectroscopic methods in solution about 40 years ago (Avramovici et al., 1973). Due to the conjugated system and the intramolecular hydrogen bond with an S(6) graph-set motif (Bernstein et al., 1995) in the solid state, the benzylideneamine group becomes closely planar, as demonstrated by the torsion angles of 179.7 (4) (C5-C6-C7-N8) and 0.9 (6) (C1-C6-C7-N8) between the imine bond and the phenolic ring in (I). In (II), these torsion angles are similar, with values of À178.28 (15) and 0.9 (2) , respectively.
The hydroxy H atom in both compounds was found from an electron-density map within bonding distance ($1 Å ) of the O atom, before being refined at its geometrically idealized position. Although this shows signs of the structures occurring in the imine form, reliable determination of H-atom positions is difficult. More evidence of the predominant imine form can be seen from the bond distances. The O1-C1 distance in both compounds [1.338 (5) Å in (I) and 1.334 (2) Å in (II)] is close to normal values reported for single C-O bonds in phenols and salicylideneamines (e.g. Ozeryanskii et al., 2006). Also, the N8-C7 bond is short in both compounds [1.294 (5) Å in (I) and 1.289 (2) Å in (II)], strongly indicating the existence of a conjugated C N bond, while the long C6-C7 bond [1.446 (5) Å in (I) and 1.457 (2) Å in (II)] implies a single bond. Based on these facts, the presence of an intramolecular O-HÁ Á ÁN bond (Tables 1 and 2) and the pure E isomer in both compounds are justified. These features are similar to what has been observed in related 4-dimethylamino-Nsalicylideneanilines (Filipenko et al., 1983;Aldoshin et al., 1984;Wozniak et al., 1995;Pizzala et al., 2000;Gü l et al., 2007). Whereas these structures, including the present ones, show phenol-imine forms, the related 4,5-bis(dimethylamino)-1-[(2hydroxy-5-nitrobenzylidene)amino]naphthalene is reported to have the enaminone form (Ozeryanskii et al., 2006).

Experimental
A solution of N,N-dimethyl-p-phenylenediamine (0.68 g, 5 mmol) and 2-hydroxy-5-nitrobenzaldehyde (0.83 g, 5 mmol) in ethanol (20 ml) was refluxed for 15 min. The solid which precipitated from the cooled reaction mixture was filtered off and recrystallized twice from ethanol to give crystals of (I). Compound (II) was obtained in a similar manner using N,N-diethyl-1,4-phenylenediamine (0.82 g, 5 mmol) as the starting material.
Analysis for compound (       Single crystals of both compounds suitable for X-ray diffraction were obtained by very slow evaporation of analytical samples from NMR tubes, where CDCl 3 was used as solvent. All H atoms were located from electron-density maps, but they were calculated at their idealized positions and allowed to ride on their parent atoms, with C-H = 0.95 (aromatic), 0.98 (methyl) or 0.99 Å (methylene) and O-H = 0.84 Å , and with U iso (H) = 1.2U eq (C) for aromatic or methylene groups or 1.5U eq (C,O) for methyl or hydroxy groups. The treatment of disorder in (II) required the use of similarity restraints to equalize the bonding distances N15-C16 and N15-C16B, and also the C16-C17 and C16B-C17B distances (s.u. = 0.02 Å ). Similarity restraints were also applied for the minor component to make the anisotropic displacement parameters of C16B and C17B more similar (s.u. = 0.02 Å 2 ).

Compound (I)
For both compounds, data collection: COLLECT (Bruker, 2008 The Academy of Finland is gratefully acknowledged for funding to KR (project Nos. 130629, 122350 and 140718).
Supplementary data for this paper are available from the IUCr electronic archives (Reference: LG3082). Services for accessing these data are described at the back of the journal. Table 2 Hydrogen-bond geometry (Å , ) for (II). Symmetry codes: (i) Àx þ 1; Ày; Àz þ 1; (ii) Àx þ 2; y þ 1 2 ; Àz þ 3 2 ; (iii) Àx þ 3; Ày, Àz þ 2. Table 1 Hydrogen-bond geometry (Å , ) for (I).  Special details Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(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.