A second monoclinic polymorph of 2-[2-(4-methoxyphenyl)hydrazinylidene]-1,3-diphenylpropane-1,3-dione

The title compound, C22H18N2O3 is the second monoclinic polymorph (P21/c) of the compound, the first being reported in space group P21 [Bertolasi et al. (1993 ▶). J. Chem. Soc. Perkin Trans. 2, pp. 2223–2228]. In the molecular structure of the title compound, the interplanar angle between the benzoyl units is 80.04 (5)°, while the corresponding angles between the phenylhydrazinylidene and benzoyl groups are 36.11 (5) and 55.77 (2)°. A strong resonance-assisted intramolecular N—H⋯O hydrogen bond is found. In the crystal, the entire supramolecular structure is constructed by weak intermolecular C—H⋯O interactions and an inter-ring π–π interaction [centroid–centroid distance = 3.6088 (8) Å].

The title compound, C 22 H 18 N 2 O 3 is the second monoclinic polymorph (P2 1 /c) of the compound, the first being reported in space group P2 1 . J. Chem. Soc. Perkin Trans. 2, pp. 2223Trans. 2, pp. -2228. In the molecular structure of the title compound, the interplanar angle between the benzoyl units is 80.04 (5) , while the corresponding angles between the phenylhydrazinylidene and benzoyl groups are 36.11 (5) and 55.77 (2) . A strong resonance-assisted intramolecular N-HÁ Á ÁO hydrogen bond is found. In the crystal, the entire supramolecular structure is constructed by weak intermolecular C-HÁ Á ÁO interactions and an inter-ringinteraction [centroid-centroid distance = 3.6088 (8) Å ].

Comment
Extensive experimental and theoretical efforts have been devoted to understand the reasons for the formation of the very strong intramolecular O-H···O hydrogen bond in β-diketoenolic systems . A model called RAHB (Resonance Assisted Hydrogen Bond) was suggested, which is essentially a synergistic mutual reinforcement of the H-bond with the π-delocalization within this heterodienic system . Besides, this concept has been applied to other heterodienic systems such as enaminones and ketohydrazones. On the other hand, in previous works we have reported the crystal structures of three β-diketohydrazones of the type 2-(2-(R-phenyl)hydrazinylidene)-1,3-diphenylpropane-1,3-dione (R = 4-Br, 4-NO 2 , 3-Cl) that contain this type of hydrogen bond (Bustos, Alvarez-Thon, Barría et al., 2011;Bustos, Alvarez-Thon, Cárcamo, Garland & Sánchez, 2011;Bustos, Alvarez-Thon, Cárcamo, Ibañez & Sánchez, 2011. We now present the crystal structure of a second monoclinic polymorph of the compound C 22 H 18 N 2 O 3 (I), described previously in the literature , prepared using similar methodology (Yao, 1964) and recrystallizing from ethanol as orange crystals. The title compound crystallizes in space group P2 1 /c whereas the previously reported structure  crystallizes in space group P2 1 . The most striking structural difference between the two polymorphs is in the methoxy group which appears rotated by 180°.
In the molecular structure of the title compound ( Fig. 1), the interplanar angle between the benzoyl units is 80.04 (5)°. The corresponding angles between the phenylhydrazinylidene and the benzoyl groups are 36.11 (5) and 55.77 (2)°, respectively.
The structure shows no conventional intermolecular hydrogen bonds and the entire supramolecular structure is constructed only by weak interactions. For the sake of clarity, the crystal packing can be described through the formation of two inversion-related dimers: firstly, a dimer is formed via weak intermolecular contacts of the type C15-H15···O3 i , and a π-π stacking interaction with a Cg···Cg i distance of 3.6088 (8) Å, where Cg is the centroid of the C16-C21 ring ( Fig. 2) [symmetry code: (i) -x + 1, -y + 1, -z + 2]. Secondly, a dimer is formed via weak contacts of the type C21-H21···O1 ii Experimental Chemicals: 1,3-diphenylpropane-1,3-dione, p-anisidine and sodium nitrite were procured from Sigma-Aldrich and sodium hydroxide, hydrochloric acid, sodium acetate and solvents from Merck. These chemicals were used without previous purification.
Procedure: A solution of 2.29 g (0.01 mole) of 1,3-diphenylpropane-1,3-dione (98%) in 100 ml of ethanol solution containing 0.4 g (0.01 mole) of sodium hydroxide was buffered by adding 4.80 g of sodium acetate trihydrate. The resulting β-diketonate solution was diluted with water to a volume of about 220 ml and stirred and cooled to -5 °C. A diazonium ion supplementary materials sup-2 solution was prepared adding 1.24 g (0.01 mole) of p-anisidine (99%) in 8 ml of hydrochloric acid (5 mol/L), cooled to -5°C , and a saturated aqueous solution containing 0.69 g (0.01 mole) of sodium nitrite was added dropwise . This solution was then added dropwise with vigorous stirring into the buffered β-diketonate solution. During the addition a yellow precipitate was formed which was filtered by suction and washed with an abundant quantity of water. Yield: 95% of crude product.
Orange single crystals suitable for X-ray studies were obtained by recrystallization from a concentrated solution of the compound in ethanol.