Synthesis and crystal structures of three Schiff bases derived from 3-formylacetylacetone and o-, m- and p-aminobenzoic acid

The condensation products of the o-, m- and p- isomers of aminobenzoic acid and 3-formylaceylacetone were synthesized and structurally characterized. As a result of the different substitution patterns, their crystal structures are governed by different types of hydrogen-bonding motifs.


Chemical context
The reaction of 3-formylacetylacetone with primary amines RNH 2 provides easy access to enamines with an aminomethylene-pentane-2,4-dione core. This approach was used for the first time as early as 1966 by Jä ger's group in order to synthesize salen-type ligands from 3-formylacetylacetone and ethylenediamine (Wolf & Jä ger, 1966). Recently, this type of ligand was applied successfully for the preparation of Fe II complexes that exhibit spin-crossover effects (Dankhoff & Weber, 2019). In a previous study, we were interested in the preparation of chiral N,O,O-ketiminate ligands from 3-formylacetylacetone and naturally occuring aminoacids (Hentsch et al., 2014) and recently, we reported on N,O,Pketiminates with additional PPh 2 functionalities (Halz et al., 2021). In this context, we studied the synthesis of Schiff bases derived from 3-formylacetylacetone and the isomeric o-, mand p-aminobenzoic acids. The corresponding crystal structures of 1, 2 and 3 are reported here. ISSN 2056-9890

Structural commentary
The ortho derivative compound 1 crystallizes in the monoclinic system, space group C2/c with Z = 8. Compound 2 (meta derivative) forms orthorhombic crystals, space group Pnma, Z = 4, and compound 3 (para derivative) crystallizes in the monoclinic space group P2 1 /c, Z = 4. Each of the three isomers 1-3 exists as the enamine tautomer with a central aminomethylene-pentane-2,4-dione structure (Figs. 1-3). The molecular structures of compounds 1 and 3 exhibit nearly planar amino-methylene-pentane-2,4-dione units, and in the case of compound 2 exact planarity is observed as the molecule resides on a crystallographic mirror plane perpendicular to the crystallographic b axis. In the case of compounds 1 and 3, there is a small torsion of the phenyl groups [1: 12.16 (6) , 3: 30.76 (8) ] with respect to the amino-methylene-pentane-2,4dione unit.

Figure 3
Molecular structure of enamine 3 showing the labelling scheme. The hydrogen bond is shown as a dashed line; displacement ellipsoids are drawn at the 50% probability level. Table 1 Selected geometric parameters (Å , ) for 1.

Figure 1
Molecular structure of enamine 1 showing the labelling scheme. Hydrogen bonds are shown as dashed lines; displacement ellipsoids are drawn at the 50% probability level.  Selected geometric parameters (Å , ) for 3. (17) The structural differences between compounds 1-3 are mainly due to individual hydrogen-bonding patterns (Tables  4-6). The presence of intramolecular N-HÁ Á ÁO-type hydrogen bonds with the amine group as hydrogen donor and the acetyl oxygen atom as acceptor is typical for aminomethylene-pentane-2,4-dione derivatives. However, as a result of the participation of the carboxyl groups, additional hydrogen-bonding patterns are formed.
In the case of the ortho derivative 1, the intramolecular S 1 1 (6) type hydrogen bond between the amino group and acetyl oxygen atom O1 is extended to a bifurcated hydrogen bridge with the carbonyl oxygen atom O4 as additional acceptor. The presence of the second hydrogen bridge leads to a significant elongation of the NÁ Á ÁO(acetyl) distance [2.631 (2) Å ] in comparison with the m-and p-derivatives 2 and 3 [2.598 (2) and 2.573 (2) Å , respectively].

Supramolecular features
For all three derivatives 1-3 the supramolecular structures in the solid state are clearly governed by the presence of intermolecular hydrogen bonds.

Figure 4
Section of the crystal structure of 1 showing the hydrogen-bonding pattern (dashed lines). Symmetry codes refer to Table 4.

Figure 5
View of the Hirshfeld surface of 1 mapped over d norm in the range À0.712 to 0.973 au showing intermolecular hydrogen bonds as green dashed lines.

Figure 6
Molecular packing of 1 in the crystal, in a view along [110].
Furthermore, the Hirshfeld surface plot hints at a weak C-HÁ Á ÁO hydrogen bond between the phenylene hydrogen atom H11 and the keto group oxygen atom O1 iii of a neighbouring chain.
As in the case of compound 1, the meta derivative 2 displays a supramolecular chain structure. The link between the Schiff base units is provided by the hydrogen atom H11 of the carboxyl group and the acetyl oxygen atom O1 i of the adjacent molecule with an O3Á Á ÁO1 i distance of 2.656 (2) Å . This connection leads to C 1 1 (10)-type chains in the a-axis direction (Fig. 7). The translational unit of the chain comprises two molecular units and the repeat distance is identical to the length of the crystallographic a axis [11.4880 (4) Å ]. In contrast to the ortho derivative, compound 2 exhibits exactly planar chains because of crystallographically imposed mirror symmetry (Fig. 8). Obviously, the planar arrangement is further stabilized by a weak C-HÁ Á ÁO hydrogen bond between the phenylene hydrogen atom H7 and the carboxyl oxygen atom O4 ii of an adjacent Schiff base unit, which is emphasized in the Hirshfeld surface plot (Fig. 9).

Figure 10
Section of the crystal structure of 3 showing the hydrogen-bonding pattern (dashed lines). Symmetry codes refer to Table 6. between H11 and the keto group oxygen atom O1 iv . Overall, this cross-linking leads to a layer structure that extends parallel to (100). The crystal packing is shown in Fig. 12.

Database survey
The Cambridge Structural Database (CSD, Version 2020.3, Groom et al., 2016) lists 22 Schiff base derivatives of 3-formylacetylacetone, all of which crystallize in the enamine form. Moreover, there are 19 Schiff base compounds derived from o-aminobenzoic acid (6 as enamine tautomers, 13 as imines), 13 from m-aminobenzoic acid (4 enamines, 9 imines) and 24 from p-aminobenzoic acid (3 enamines, 21 imines). Among the total of 53 compounds, 24 exhibit supramolecular structures based on carboxylic acid dimers with R 2 2 (8)-type hydrogen bridges, predominately in the case of the m-and pamimobenzoic acid derivatives. In the case of the o-aminobenzoic acid derivatives, 17 out of 19 compounds display intramolecular N-HÁ Á ÁO or O-HÁ Á ÁN hydrogen bridges with an S 1 1 (6) topology. Additionally, there are reports on ketoimines derived from 2,4-pentanedione and aminobenzoic acids. The corresponding o-and the p-aminobenzoic acid derivatives exist as enamines with intramolecular N-HÁ Á ÁO hydrogen bridges (Murugavel et al., 2012;Joshi et al., 2005). The crystal structure of the m-derivative has not yet been determined. Deprotonation of the aminobenzoic acid derivates was used to generate carboxylates that have been applied as ligands in transition-metal complexes (Shi & Hu, 2007) and organotin compounds (Chen et al., 2020;Baul et al., 2008Baul et al., , 2009

Synthesis and crystallization
3-Formylacetylacetone (3.0 g, 23.4 mmol) and the corresponding aminobenzoic acid (3.3 g, 24.0 mmol) were dissolved in methanol (50 ml) and stirred at room temperature for 3 h. The solid products 1-3 were isolated by filtration, washed with methanol and dried in vacuo.
Crystals suitable for single crystal X-ray diffraction of 3 were obtained from the mother liquor. In the case of compounds 1 and 2, single crystals were obtained from a slow reaction (around three days of reaction time) of a suspension of copper(II) o-or p-aminobenzoate (1.5 g in 3 ml of water) and a solution of 3-formylacetylacetone (1.0 g in 5 ml of diethyl ether).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 7. The methyl group hydrogen atoms of compound 2 and the carboxyl hydrogen atoms of compounds 2 and 3 were located from difference-Fourier maps and were refined freely. The remaining hydrogen atoms were positioned geometrically and refined using a riding model.

Funding information
We acknowledge the financial support within the funding programme Open Access Publishing by the German Research Foundation (DFG).

2-[(2-Acetyl-3-oxobut-1-en-1-yl)amino]benzoic acid (1)
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.

3-[(2-Acetyl-3-oxobut-1-en-1-yl)amino]benzoic acid (2)
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.

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.