Crystal structure and DFT study of the zwitterionic form of 3-{(E)-1-[(4-ethoxyphenyl)iminiumyl]ethyl}-6-methyl-2-oxo-2H-pyran-4-olate

The title Schiff base compound crystallizes in the zwitterionic form. The resulting iminium and hydroxyl groups are linked by a charge-assisted intramolecular N+—H⋯O− hydrogen bond, and the conformation about the C=N bond is E.


Chemical context
Hydroxy Schiff bases have been studied extensively for their biological, photochromic and thermochromic properties (Garnovskii et al., 1993;Hadjoudis et al., 2004). They can be used as potential materials for optical memory and switch devices (Zhao et al., 2007). Proton transfer in these compounds forms the basis for an explanation of the mechanisms of various biological processes where proton transfer is the rate-determining step (Lussier et al., 1987). In general, O-hydroxy Schiff bases exhibit two possible tautomeric forms, the phenol-imine (or benzenoid) and ketoamine (or quinoid) forms. Depending on the tautomers, two types of intramolecular hydrogen bonds are possible: O-HÁ Á ÁN in benzenoid and N-HÁ Á ÁO in quinoid tautomers. Ohydroxy Schiff bases have been observed in the keto form, in the enol form or in an enol/keto mixture (Nazır et al., 2000;Antonov et al., 2000) due to the H-atom transfer. Another form of the Schiff base compounds is their zwitterionic form (Ogawa & Harada, 2003). Zwitterions of Schiff bases have an ionic intramolecular hydrogen bond (N + -HÁ Á ÁO À ) and their N + -H bond lengths are longer than the normal bond length observed for neutral N-H bonds (0.87 Å ). The molecular structure of the title compound is similar to that of (E)-4hydroxy-3-[N-(4-hydroxyphenyl)ethanimidoyl]-6-methyl-2Hpyran-2-one (Djedouani et al., 2015), which also crystallizes as a zwitterion. ISSN 2056-9890

Structural commentary
The molecular structure of title compound is shown in Fig. 1. It crystallizes in the zwitterionic form, with the phenolic H atom having been transferred to the imino group. The H atom, H1N, was located in a difference-Fourier map and freely refined (N-H = 0.90 (2) Å ). The resulting iminium and hydroxy groups are linked by an intramolecular N-HÁ Á ÁO hydrogen bond forming an S(6) loop ( Fig. 1 and Table 1). The dihedral angle between the benzene (C9-C14) and pyran (O3/C2-C6) rings is 70.49 (6) . The carbon-nitrogen bond N1 C7 is 1.318 (2) Å , which agrees with values observed in related compounds . It is slightly longer than a typical C N bond [1.283 (4) Å ; Bai & Jing, 2007], but much shorter than a C-N bond. The N1-C9 bond length is 1.436 (2) Å because of resonance. The carboncarbon bond connecting the enol and imine groups exhibits intermediate distances between those of single and double bond, but being closer to the latter; C5-C7 = 1.427 (2) and C5-C6 = 1.443 (2) Å , reflecting the zwitterionic character of the title compound (Wojciechowski et al., 2003). The C4-O1 bond length [1.259 (2) Å ] is intermediate between single and double carbon-to-oxygen bond lengths (1.362 and 1.222 Å , respectively), whereas C6-O2 is 1.215 (2) Å .
The aromatic ring and dehydroacetic acid ring are in a trans position with respect to the C7 N1 bond, the dihedral angle between the two rings is 70.46 (9) and the molecular conformation is determined by the presence of the intramolecular N + -HÁ Á ÁO À hydrogen bond ( Fig. 1 and Table 1), which generates an S(6) ring motif. Similar intramolecular hydrogen bonds have been reported in other zwitterionic phenolates (Huang et al., 2006;Temel et al., 2006).

Figure 2
A view along the b axis of the crystal packing of the title compound. The hydrogen bonds are shown as dashed lines (see Table 1), and only the H atoms involved in hydrogen bonding have been included.

Figure 1
A view of the molecular structure of the title compound with the atom labelling. Displacement ellipsoids are drawn at the 50% probability level, and the intramolecular N-HÁ Á ÁO hydrogen bond (see Table 1) is shown as a dashed line.
pyran-2-one (CUGPAX; Djedouani et al., 2015). The molecular conformations of all three compounds are also determined by the presence of an intramolecular charge-assisted N + -HÁ Á ÁO À hydrogen bond (see Fig. 1 and Table 1 for the title compound), which generates an S(6) ring motif. Two of these compounds, REZMAL and CUGPAX, have a benzene ring inclined to the pyran ring by 42.25 (10) and 53.31 (11) , respectively. This is significantly different from the equivalent dihedral angle of 70.46 (9) in the title compound, which has five hydrogen bonds, two from the ethoxy group in the para position of the benzene and another from the benzene ring, which has increased the dihedral angle between the two rings.
On the other hand, CUGPAX has three hydrogen bonds and only one single bond of the hydroxy group in the para position of benzene ring, and the dihedral angle between the two rings is 53.31 (11) . REZMAL shows only two hydrogen bonds, neither of which involve benzene ring, and the dihedral angle is 42.25 (10) .

Density functional study -geometry optimization and molecular orbital calculations
Geometry optimization and molecular orbital calculations were carried out with the Guassian09 software package (Frisch et al., 2009) and the Gaussview visualization program (Dennington et al., 2007;Rassolov et al., 1998), using the threeparameter hybrid function of Becke based on the correlation function (B3LYP) of Lee et al. (1998) and Miehlich et al. (1989), with the 6-311G, 6-311G(+) and 6-311G(++) basis sets. The bond lengths, bond angles corresponding to the optimized geometry obtained using the DFT/B3LY P method are given in Table 2. The calculated C4-C5 bond distance is 1.447 Å correlates nicely with experimental value. The calculated bond lengths with B3LYP/6-311G(++) level are slightly shorter than the experimental values within 0.004-0.035 Å . The calculated bond angles C5-C4-O4 and C4-C5-C7 are close to 120 since atoms C4 and C5 have sp 2 hybridization. In general, the calculated values are in good agreement with the experimental data.
The highest occupied molecular orbitals (HOMO) and lowest unoccupied orbitals (LUMO) are named frontier orbitals (FMOs). The calculated values at the B3LYP/6-311G(++) level are presented in Table 3, and the nature of the frontier molecular orbitals for the two possible tautomeric forms, the keto-amine (NH) and the phenol-imine (OH) forms of zwitterionic forms of Schiff bases, are plotted in Fig. 3. The band-gap energy values calculated for keto-amine (NH) forms were found to be 4.297 eV, which is a large HOMO-LUMO energy gap, implying a higher molecular stability than for the phenol-imine (OH) form, which has a smaller energy gap with the difference between the HOMO and LUMO being 3.791 eV. The HOMO-LUMO energy gap is very important for the chemical activity and explains the eventual chargetransfer interaction within the molecule. Clearly, the larger HOMO-LUMO gap calculated for the keto-amine (NH) form is in agreement with the stability of the molecule in the solid state.

Synthesis and crystallization
The title compound was prepared according to a literature method (Djedouani et al., 2007). Colourless plate-like crystals were obtained by slow evaporation of a solution in ethanol.

3-{(E)-1-[(4-Ethoxyphenyl)iminiumyl]ethyl}-6-methyl-2-oxo-2H-pyran-4-olate
Crystal data Special details 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.