Comment

Aromatic imines and their derivatives are an important group of molecules in organic chemistry, in particular those that have aryl groups bound to N or C atoms. They have been used successfully to study resonance-assisted hydrogen bonds (RAHBs; Krygowski & Stepien, 2005; Sobczyk et al., 2005). Imines substituted in the aromatic ring have offered the opportunity to study the substituent effects in RAHBs. In this context, the molecular structure of the title compound, (I) (Fig. 1), is reported here.

The single-crystal structure of the title Schiff base compound is built up by discrete molecules in the monoclinic space group P2/c, with one molecule in the asymmetric unit. A summary of bond lengths and angles is presented in Table 1. The N8-C7 and O1-C1 bond lengths are in agreement with the double-bond character for the imine ( at 1601 cm^-1) and keto-carbonyl groups ( at 1657 cm^-1), respectively (Allen, 2002). Moreover, the bond lengths in the C1-C6 ring show

clear alternation in the delocalized C2-C6 portion. The nitro group is tilted out of the mean plane of the adjacent ring by -11.1 (3)°, whereas the C4-N4 distance is in the characteristic range suggesting limited conjugation with the ring. Thus, the whole geometry is in agreement with the predominace of the oxocyclohexadienide-iminium zwitterion bonding scheme (see scheme) (Krygowski & Stepien, 2005), in close agreement with the reported configurations of p-nitrophenolates of alkali metal cations (Butt et al., 1987).

The iminium H atom (located in a difference map) is coplanar with the oxocyclohexadienide ring and on the same side of the molecule as the O atom of the keto group, allowing the formation of an intramolecular N-HO hydrogen bond (Table 2) in an S(6) motif (Bernstein et al., 1995), with dimensions in agreement with the usual values (Krygowski et al., 1997; Steiner 1998, 2002). The N8O1 distance has almost the same value as the RAHB of 2.607 (3) Å found in 3-{[(diphenoxythiophosphoryl)hydrazine]methylidene}-3,4-dihydro-2H-1-benzopyran-2,4-dione (Rybarczyk-Pirek et al., 2002). The iminium H atom is also engaged in N-HO hydrogen bonding with the O atom from the keto group of a neighbouring molecule, forming dimers with an almost square R₂²(4) motif (Fig. 2).

The first dimension of the extended structure is built up by soft C-HO interactions with the participation of one of the O atoms from the nitro group, in a monocoordinative fashion (Allen et al., 1997), as the acceptor of two hydrogen bonds (Table 2) to form an R₂¹(6) motif. The whole hydrogen-bonding scheme, makes up tapes propagating along the direction of the c axis. All C-HON and CON distances are shorter than the mean values of 2.7 (2) and 3.5 (2) Å, respectively, found in a study of nitrobenzenes (André et al., 1997), although the C-HON angles are within the accepted range [C-HO = 133 (20)°]. Even when C-HO interactions involving an NO₂ group as acceptor are half as strong as C-HO interactions (Allen et al., 1997) involving a CO group as the acceptor, they play a significant role in packing owing to their co-operative action.

The second dimension is ruled by the zwitterionic nature of (I). The anionic oxocyclohexadiene ring and the cationic iminium group from neighbouring tapes are overlapped. The iminium N and C atoms are 3.257 (3) and 3.462 (3) Å, respectively, from the centroid of the cyclohexadienide ring at (-x + 1, -y, -z). These short distances strongly suggest not only ion pairing but also a -stacking interaction between the iminium group, as acceptor, and the oxocyclohexadiene ring, as the donor of electron density (Fig. 3). These -stacking interactions crosslink pairs of the hydrogen-bonded tapes described above to develop discrete centrosymmetric ribbons which propagate along the c axis.

Experimental

Compound (I) was synthesized by amidation of ethyl 6-nitro-2-oxo-2H-chromene-3-carboxylate (0.5 g, 1.9 mmol), prepared according to Santos-Contreras et al. (2007), with 1-(2-aminoethyl)piperidine (0.26 ml, 1.9 mmol) and two drops of piperidine as catalyst in refluxing ethanol (10 ml) for 24 h. The resulting orange solution was treated with activated charcoal and evaporated to give 0.3 g of an orange solid in 57% yield. Crystals suitable for X-ray analysis were obtained by slow evaporation from a saturated ethyl acetate solution (m.p. 413 K). IR (cm^-1): (C=O) 1657, (C=N) 1601, (C-NO) 1535; ¹H NMR (300 MHz, CDCl₃): 6.90 (d, H6, ³J = 9.0 Hz), 8.16 (dd, H5, ³J = 9.0 and ⁴J = 3.0 Hz), 8.22 (d, H3, ³J = 9.0 Hz), 11.9 (b, NH), 8.31 (s, H7), 3.74 (t, 2H, ³J = 6.2 Hz, -⁺NHCH₂-), 2.64 (t, 2H, ³J = 6.2 Hz, -CH₂N), 2.46 [m, 4H, N-(CH₂)₂], 1.56 (m, 4H, -CH₂-), 1.46 (m, 2H, -CH₂-); ¹³C NMR: 173.1 (C1), 115.6 (C2), 129.7 (C3), 137.5 (C4), 129.0 (C5), 120.9 (C6), 165.3 (C7), 58.4 (C9), 53.2 (C10), 54.8 (C12), 26.2 (C13), 24.3 (C14).

The amino H atom was located in a difference Fourier map and refined with an N-H distance of 0.90 (2) Å. All other H atoms were positioned geometrically and refined using a riding model [C-H = 0.93 and 0.97 Å for aromatic and CH₂ H atoms, respectively, and U_iso(H) = 1.2U_eq(C)].

Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008) and Mercury (Macrae et al., 2006); software used to prepare material for publication: SHELXL97 and WinGX2003 (Farrugia, 1999).

---

Supplementary data for this paper are available from the IUCr electronic archives (Reference: GD3261 ). Services for accessing these data are described at the back of the journal.

---