Synthesis of Schiff Bases Compounds from Oxamic Hydrazide: Spectroscopic Characterization, X–ray Diffraction Structure and Antioxidant Activity Study

The compounds (E)–2–amino–N'–(1–(2–hydroxyphenyl)ethylidene)–2–oxoacetohydrazide (I) and (E)–N'–(2– hydroxy–3–methoxybenzylidene)–2–amino–2–oxoacetohydrazide (II) were synthetized by the 1:1 ratio condensation reaction of oxamic hydrazide and 2–hydroxyacetophenone or o–vanillin respectively. The two compounds were characterized by physico–chemical analyses, elemental analysis, FTIR, H and C NMR spectroscopies techniques. The structure of the compound (I) was determined by single–crystal X–ray diffraction study. The compound (I) (C10H11N3O3) crystallises in the triclinic space group P–1 with the following unit cell parameters: a = 7.0399 (5) Å, b = 8.6252 (8) Å, c = 9.5474 (9) Å, a = 81.730 (3)°, b = 72.738 (3)°, g = 67.450 (3)°, V = 510.99 (8) Å, Z = 2, T = 173 (2) K, m = 0.11 mm, Dcalc = 1.438 g/cm , Rint = 0.028, Rsigma = 0.073. The oxamic hydrazide moiety of the molecule is slightly twisted as reflected by the torsion angles values of 177.2 (2)° [N1–N2–C9–C10], –171.3 (3)° [N2–C9–C10–N3], –4.6 (4)° [O2–C9–N2–N1] and 8.4 (4)° [O3–C10–C9– N2]. The intramolecular hydrogen bond O1(phenol)–H1···N1(hydrazide) which close in S (6) ring stabilized the conformation. The intermolecular hydrogen bonds, C3–H3···O1(phenol) (i: −x+1, −y, −z+1), N3(amide)–H3A···O3(amide) (ii: −x+1, −y+2, −z) and N3(amide)–H3B···O2(hydrazide) (iii: −x+1, −y+1, −z) lead to the formation of sheets parallel to ac plane. Compounds (I) and (II) showed antioxidant activities less than 10% inhibition of DPPH.


Introduction
Potentially ditopic Schiff bases prepared from oxamic or thioxamic hydrazide have been widely reported in the literature by chemists and in particular those interested in coordination chemistry [1][2][3][4][5][6]. Oxamic hydrazide has two different arms. The hydrazide arm condenses more easily than the amide arm with carbonyl [7][8][9][10]. The use of these ligands has made it possible to prepare coordination compounds with atypical structures [11][12][13]. The derivatives of oxamic hydrazide have been used for the synthesis of nucleoside [14,15]. Oxamic hydrazide have been used as coreactant in electroluminescence [16,17]. A wide variety of heterocyclic molecules with good medicinal properties are obtained starting from these oxamic precursors [18]. They are used as antibacterial agent, [19] anticonvulsant, [8] antiinflammatory [20], antituberculosis [18] and anticancer agents [4,21]. Schiff's bases are synthesized from oxamic hydrazide and used for the preparation of complexes with transition metal and lanthanide ions [9,[22][23][24][25]. The properties of these complexes are evaluated in superoxide catalysis [2,3] and magnetism [26]. It is in this perspective that we have studied these types of ligands and reported their transition metal complexes [7,27]. Continuing our work in this field, we obtained ligands (I) and (II). In the present study, we report the spectroscopic study of the two compounds and the structure of (I) obtained by X-ray diffraction.

Materials and Physical Methods
Oxamic hydrazide, 2'-hydroxyacetophenone, o-vanillin, cyclohexanol and 1,1-diphenyl-2-picrylhydrazyl (DPPH.) were of analytical reagent grade and were obtained from Sigma-Aldrich Company. All used solvents were of UV spectroscopic quality. The elemental analyses of C, H and N were recorded on a VxRio EL Instrument. FT-IR spectra were recorded in the region of 4000-400 cm -1 using a Perkin Elmer Spectrum Two FT-IR spectrometer. The UV-Visible spectra were recorded on a Perkin Elmer Lambda UV-Vis spectrophotometer. The 1 H and 13 C NMR spectra were recorded in DMSO-d 6 on a Bruker 500 MHz spectrometer at room temperature using TMS as an internal reference.

Free Radical Scavenging Antioxidant Assay
Antioxidant capacities of compound (I) were measured according to Akhtar et al. [28] method with modifications. The methanol solution of 3.8 mL DPPH• was added to test compounds (200 µL) at different concentrations. The mixture was shaken vigorously and incubated in dark for 30 min at room temperature. After the incubation time, the absorbance of the solution was measured at 517 nm by using UV-vis spectrophotometer Perkin two. The DPPH• radical scavenger effect was calculated using the following equation: Scavenging activity % control = × 100 where A control is the absorbance of the control reaction and A sample is the absorbance of the test compound. The tests were carried out in triplicate. Trolox was used as positive control.

Crystal Structure Determination
Crystals suitable for single-crystal X-ray diffraction, of the reported compound, was grown by slow evaporation of DMF solution of the compound. Details of the crystal structure solution and refinement are given in Table 1. Diffraction data were collected using a Bruker APEX-II CCD diffractometer with graphite monochromatized MoKα radiation (l = 0.71073 Å). All data were corrected for Lorentz and polarization effects. The structure was solved and refined using the Bruker SHELXTL Software Package [29]. All the structures were refined on F 2 by a full-matrix least-squares procedure using anisotropic displacement parameters for all non-hydrogen atoms [30]. H atoms of the NH groups was located in the Fourier difference maps and refined without restraints. Other H atoms were geometrically optimized and refined as riding on their carriers with Uiso(H) = 1.2Ueq(C)(1.5 for CH 3 group). Molecular graphics were generated using ORTEP-3 [31].

General Study
The synthesis of Schiff bases usually takes place in simple alcohols such as methanol, ethanol or propanol. In the synthesis of Schiff bases from oxamic hydrazide, the subject of our study, the use of these solvents leads to excessively long reaction times. In fact, for the condensation of oxamic hydrazide with carbonyl compounds, it is necessary to heat to temperature high enough to shorten the reaction time. Cyclohexanol which has a high boiling point (161.8° C) is suitable to prepare compounds (I) and (II) with short time reaction (Figure 1).
The results of elemental analysis agree with the expected formulas for the two compounds. The solid-state infrared spectrum of (I) reveals a broad band around 3383 cm -1 attributed to the OH stretching vibration and another band around 3293 cm -1 indicating the presence of NH. These two bands are present in the spectrum of compound (II) at 3375 cm -1 and 3221 cm -1 respectively [32,33]. The band due to the C=N group formed after the condensation reaction between the oxamic hydrazide and the appropriate carbonyl is pointed at 1606 cm -1 for (I) and at 1603 cm -1 for (II). The stretching vibrations due to C=O of the oxamic unit were noted at 1702 cm -1 and 1652 cm -1 for (I) and at 1704 cm -1 and 1656 cm -1 for (II) [34,35]. The shift of the second band towards the low frequencies is justified by the strong resonance of the oxalate group. The additional bands in the range [1570-1405 cm -1 ] are due to the aromatic groups.
The 1 H NMR spectra of the compounds, in DMSO-d 6 solution, are recorded. Compound (I) gives two signals characteristic of iminolisation. Indeed, the single signal designating the moiety [-C(=O)-NH 2 ] does not appear on the spectrum. The two signals at 11.428 ppm and 12.925 ppm assigned respectively to HN=C-OH and HN=C-OH are indicative of the iminolisation of the amide function of the Schiff base. The same phenomenon is observed for compound (II). The corresponding signals are pointed at 10.750 ppm and 12.360 ppm, respectively. These observations are confirmed by 13 C NMR spectra in DMSOd 6 . Compound (I) gives a signal at 159.198 ppm corresponding to the iminol carbon atom HN=C-OH. This signal is identified at 156.540 ppm in the spectrum of (II). The signals due to the hydrazide carbon atoms are at 162.232 ppm and 161.510 ppm for (I) and (II) respectively. This behavior is observed in amide-iminol tautomerism [36].

Crystal Structure
The DMF solution of compound C 10 H 11 N 3 O 3 which was left for slow evaporation for two weeks gave colorless crystals suitable for X-ray analysis. The compound crystallizes in the triclinic group P-1. The molecular structure with the atomic-labelling scheme is shown in figure  2. The crystal structure solution and refinement are given in Table 1. Selected bond distances and angles are listed in Table 2. The asymmetric unit contains one organic molecule.  [37,7]. Those distances are comparable to the values found for a derivative which has an oxalate group [38]. The bond lengths values of 1.321 (4) Å [C10-N3] and 1.340 (4) Å [C9-N2] bonds are in the normal range observed for single C-N bonds [39]. The oxamic hydrazide fragment N1/N2/C9/O2/C10/O3/N3 is planar with a maximum deviation from the least-squares plane of -0.132 (2) Table 3).

Antioxidant Activity
DPPH • is a stable free radical which becomes a stable molecule when it accepts an electron or hydrogen radical. DPPH • radical scavenging is a method widely used to evaluate the antioxidant activity of compounds [40,41]. The capacity of scavenging DPPH • radical of the two compounds (I) and (II) have been screened (Table 4). The Figure 4 shows the plots of DPPH • free radical scavenging activity (%) for the compounds (I) and (II). For compound (I), the scavenging activity increases with increasing the concentration in the range tested (50-500 mmol/L) for the two DDPH • initial concentrations. The scavenging activity of (I) varies, for the highest DPPH • (0.1014 mM) concentration, in the range 1.84±0.08 -6.32±0.05% and between 4.84±0.11 and 9.32±0.09% for the lowest DPPH • concentration (0.0507 mM). This activity is due to the NH or OH groups which can react with DPPH • radical by the typical H-abstraction reaction to form a stable radical. Radical scavenging activity of compound (II) [(1.58±0.15 -4.74±0.11% for the highest DPPH • concentration), (1.58±0.15 -4.74±0.11% for the lowest DPPH • concentration)] is slightly lower than that observed for compound (I) in the concentration range screened (Figure 4)