Synthesis, Molecular Structure and Antioxidant Activity of (E)-4-[Benzylideneamino]-1,5-dimethyl-2-phenyl-1H-pyrazol-3(2H)-one, a Schiff Base Ligand of 4-Aminoantipyrine

Abstract

In this study, the title compound, C18H17N3O (M r = 291.35), was synthesized by the condensation reaction of 4-amino-1,5-dimethyl-2-phenylpyrazole-3-one and benzaldehyde. Single-crystal X-ray diffraction data revealed that this compound adopts a trans configuration around the central C=N double bond. It crystallizes in the monoclinic, space group P21/c with a = 12.9236(17) A, b = 6.8349(9) A, c = 17.072(2) A, β = 90.316(3)°, V = 1508.0(3) A3, Z = 4, D c = 1.283 Mg/m3, F(000) = 616, μ = 0.082 mm−1, R = 0.0442, and wR = 0.0936. Two different planes exist within the molecule; e.g. the pyrazolone and benzylidene groups attached to C9 of the pyrazolone ring are almost coplanar, whereas the phenyl group attached to the N1 of the pyrazolone ring is in another plane. Density functional theory (DFT) and time-dependent DFT calculations were performed to predict the electronic structure and absorption spectra of (E)-4-[benzylideneamino]-1,5-dimethyl-2-phenyl-1H-pyrazol-3(2H)-one, a schiff base ligand of 4-aminoantipyrine using B3LYP/6-311G basis set on the AM1 optimized geometry. The predicted vibrational frequencies using the B3LYP/6-311G method were in strong agreement with the experimental IR spectra. The time dependent DFT calculations were used to evaluate the electronic absorption spectrum and three electron transition bands, which were mainly derived from the contribution of n → π* and π → π* transitions, were observed in both the experimental and predicted UV–Vis spectra. A maximum emission band at 370 nm was observed in the fluorescence spectra of the title compound. In addition, the title compound showed good DPPH antioxidant activity. The structure of title compound, C18H17N3O, had monoclinic (P21/c) symmetry and DFT studies were used to predict the electronic structure and absorption spectra using B3LYP/6-311G basis set on the AM1 optimized geometry as well as to help explain the proposed mechanism of DPPH antioxidant activity.