Synthesis and Characterization of 4-amino-N '-[(1 E )-1-(2-hydroxy-6-methyl-4-oxo-4 H -pyran -3-yl)ethylidene]benzohydrazide and its Cu(II), Ni(II), Zn(II) and Mn(II) Complexes.

. New benzohydrazone compound, 4-amino-N '-[(1 E )-1-(2-hydroxy-6-methyl-4-oxo-4 H -pyran-3-yl)ethylidene] benzohydrazide (HL 1 ) and its Cu(II), Ni(II), Zn(II) and Mn(II) complexes were synthesized. The structures of HL 1 and its complexes were elucidated by elemental analysis and IR, UV-Vis, 1 H and 13 C NMR spectroscopy and mass spectrometry. The infrared spectral data of the complexes revealed that HL 1 coordinated with the metal ions through azomethine nitrogen, enolic oxygen and amide carbonyl oxygen atoms, hence, HL 1 behaves as a monobasic tridentate ligand. UV-Vis data revealed that Zn(II) and Mn(II) complexes adopted octahedral geometry, while Cu(II) and Ni(II) complexes had five-coordinate and square-planar geometries respectively. The mass spectra data and elemental analysis values are in accordance with the calculated values for the suggested molecular formula of the complexes, a confirmation of the 1:1 ligand to metal stoichiometry in case of Cu(II) complex and 2:1 ligands to metal stoichiometry in case of the other complexes.

On the other hand, there is a growing interest in benzo hydrazides and their coordination compounds due to their physiological activities as DNA-cleaving agent (Okagu et al., 2019), bactericides and fungicides (Mangamamba et al., 2014) [22], antioxidant (Sherine & Veeramanikandan, 2017;Liu et al., 2012), anti-inflammatory and anticancer (Kendur et al., 2018). Cu(II) complexes derived from 4-hydroxybenzohydrazide exhibit potential nuclease and antibacterial activities against four pathogenic strains of bacteria; B. subtilis, P. syringae, P. aeurginosa and S. aureus (Pal et al., 2014). A series of benzohydrazide derivatives obtained from 1-chloro-3,4-dihydronaphthalene-2-carbaldehyde showed a great cytotoxicity towards prostate cancer cell lines (PC-3, LNCaP) (Arjun et al., 2018). The two combined moieties -the dehydroacetic acid and functionalized benzohydrazide would exhibit synergistic biological activities. Thus, the study of the synthesis and structures of benzohydrazones are important for new drug development. Herein, we report the synthesis and characterization of a novel benzohydrazone and it Cu(II), Ni(II), Zn(II) and Mn(II) complexes.

EXPERIMENTAL SECTION Materials
The chemicals 4-aminobenzohydrazide and sodium bicarbonate were bought from Sigma-Aldrich. Zinc chloride and nickel(II) chloride hexahydrate were purchased from Guangdong Guanghua Sci-Tech Co. Ltd. Copper(II) chloride dihydrate and manganese(II) chloride tetrahydrate were products of SCP. Dehydroacetic acid was purchased from Merck. These chemicals were used as received. Solvents methanol and DMSO were used without further purification.

Physical Methods
Infrared (IR) spectral data of the compounds were recorded on a Varian 600-IR series spectrometer (Varian Inc., USA) in the 4000 -400 cm -1 range using KBr disc. The 1 H and 13 C NMR spectra were recorded using Bruker Avance III HD 400 spectrometer (Bruker Corporation, USA) operating at 400 MHz in DMSO-d6 with TMS as an internal standard. The elemental analysis to determine the C, H, N, and S compositions of the compounds was carried out with Leco/TruSpec Micro Analytical Instrument (IR Technology Services Pvt. Ltd, Navi Mumbai, India). The electronic spectra were recorded using Pye-Unicam in DMSO as a solvent. The mass spectra were recorded with Thermo Fisher TSQ 9000 Triple Quadruple GC-MS/MS (Thermo Fisher Scientific, USA) in positive ion mode using pneumatically assisted electrospray ionization.

RESULTS AND DISCUSSION General Synthesis
The reaction of 3-acetyl-2-hydroxy-6-methyl-4Hpyran-4-one with 4-aminobenzohydrazide in the molar ratio of 1:1 in a methanolic medium gave the ligand HL1 (See the carbon atom numbering in Figure  3). The ligand was insoluble in methanol, but its warm suspension reacted with the chloride salts of Cu(II), Zn(II), Ni(II), and Mn(II) to give the complexes ( Figure 2). All the synthesized compounds were obtained in good yield (67 -76 %) and readily soluble in DMSO and ethanol. Unfortunately, no single crystals suitable for X-ray single crystal study were obtained, hence, characterization was reduced to elemental analysis, UV-Vis, FT-IR, 1 H NMR, 13 C NMR spectroscopy, and mass spectrometry. The results of the elemental analysis (Table 1) were in close agreement with the calculated values for the suggested molecular formula. The physical and analytical data of the ligands and complexes are summarised in Table 1.   The 1 H NMR ( Figure 4) spectrum of the ligand displayed a weak singlet at 11.96 ppm due to -NH proton. This signal has shifted upfield at 8.59 and 11.30 ppm in the spectra of Ni(L1)2 ( Figure 5) and Zn(L1)2 ( Figure 6) respectively due to the participation of the adjacent nitrogen atom in coordination. Signals due to aromatic protons were observed within the range 6.17-7.66 ppm in the spectra of the ligand and the complexes. A sharp singlet at 5.70 ppm in the spectrum of the ligand assigned to the olefinic proton of DHA ring has slightly shifted downfield in the complexes. The CH3-C=N and -CH3 protons appeared as sharp singlets at 2.06 and 2.61 ppm respectively in the ligand. The 13 C NMR spectrum (Figure 7) of the ligand showed significant signals that were in good agreement with the build-up of the ligand. The carbonyl carbon of the secondary amide (C9) and the pyrone ring (C3) gave signals at 166.64 and 170.24 ppm respectively. The azomethine carbon of the ligand gave signal at 153.95 ppm. In the spectra of HL1, the phenyl carbon nuclei appeared in the region between 113.03-129.70 ppm and pyran ring carbons C1, C2, C4 and C5 are characterized at 163.10, 94.63, 121.51 and 166.10 ppm respectively. The upfield signals observed in the range 17.44-19.60 ppm were assigned to methyl groups. All these signals shifted downfield in the spectra (Figure 8)

Mass Spectra and Elemental Analysis
The ESI-MS spectrum (Figure 9) of HL1 has shown the m/z signals at 303.14 and 323.93 for [M+2H] 2+ and [M+Na] + , respectively, which corresponds to its calculated molecular mass. The peak at m/z 286.90 for the ligand is due to the loss of a methyl group. In the positive mode, ESI-MS spectra (Figures 10 and 11) of Mn(L1)2 and Zn(L1)2 displayed peaks at m/z 695.56 and 705.48, respectively, corresponding to the loss of a methyl group followed by ammonia. The mass spectrum (Figure 12) of Ni(L1)2 has shown a molecular ion [M] + peak at m/z 695.28 which was in agreement with its calculated molecular mass. In the spectrum of Ni(L1)2, the loss of one methyl group is evident by the peak observed at m/z 680.51 and followed by the loss of ammonia evident from the peak at m/z 663.36. The peak observed at m/z 436.06 in the mass spectrum ( Figure 13) of Cu(L1) was assigned to [M+H] + ion. The peaks observed at m/z 607.63 and 617.48 in the spectra Zn(L1)2 and Mn(L1)2 respectively were due to the loss of benzoyl fragments suggesting that the C=N bond cleaved easily. Further, the peak at m/z 602.52 in the mass spectrum of Zn(L1)2 exactly corresponds to [M] + species of the corresponding ligand. The peak at m/z 302.80 Cu(L1) is due to [M+H] + species of the corresponding ligand, thus, confirming the ligand to metal 1:1 stoichiometry. The results of the elemental analysis (Table 1) were in good agreement with that of the proposed molecular formula of the ligand and its complexes.

Infrared Spectra of the Ligands and Metal Complexes
The IR spectrum of the ligand HL1 ( Figure 14) showed a characteristic broad band at 3466 cm −1 which was assigned to v(O-H) stretching vibration modes. In the IR spectra of the complexes, there was a 10 -56 cm −1 downward shift in v(O-H) stretching vibrations which suggested the participation of enolic oxygen in coordination with the metal ions. The shifting of the v(C-O) stretching from 1152 cm -1 in the ligand to higher wavenumber 1187 -1181 cm -1 in the spectra of the metal complexes supported the bonding of the enolic oxygen to the metal ions. Also, the IR spectra of the complexes (Figures 15-18) showed new bands at ≈3515 -3911 cm -1 due to the OH group of the coordinated water molecules. A band appeared at 1635 cm -1 in the ligand due to v(C=N) stretching modes. This band has shifted to higher wavenumbers in case of the complexes (Figure 13) due to the involvement of the nitrogen atom of the azomethine group in coordination to the metal ions (Ekennia et al., 2018). In the IR spectra of the complexes, some new bands due to v(M-N) and v(M-O) were observed at 667 -624 cm -1 and 586 -514 cm −1 respectively which supported the formation of Cu(II), Ni(II), Zn(II) and Mn(II) complexes. These bands were absent in the spectrum of the ligand. The band observed 421 cm −1 in Cu(II) complex in far-Infrared spectrum of the metal complex was assigned to M−Cl bond suggesting the existence of metal-chloride bond in the Cu(II) complex (Khan et al., 2020). Hence, from the IR spectroscopic data, it has been inferred that the ligand coordinated to the metal ions through the enolic oxygen, amide carbonyl oxygen and azomethine nitrogen atoms.

Electronic Spectra
The UV-Vis absorption spectra of the ligand and its metal complexes are shown in Figure 19. The ligand exhibited electronic absorption bands due to π → π* and n → π* transitions, ascribed to the presence of aromatic ring and carbonyl and azomethine functionalities. The ligand showed strong absorption at 312 nm attributed to intra-ligand π → π* transition. This absorption band has remained almost unchanged in the metal complexes. The n → π* transition of the carbonyl and azomethine functionalities was observed between 352 and 377 nm and suffered a blue shift upon complexation, an indication of the coordination of azomethine nitrogen to the metal ions (Al-Shaalan, 2011).   . Electronic Spectra of HL2 and its complexes CONCLUSIONS A new benzohydrazone derivative, 4-amino-N'-[(1E)-1-(2-hydroxy-6-methyl-4-oxo-4H-pyran-3yl)ethylidene]benzohydrazide (HL1) and its Cu(II), Zn(II), Ni(II) and Mn(II) complexes were synthesized and characterized. According to the analytical and spectral data, HL1 acts as a monobasic tridentate ligand coordinating to the metal ions through enolic oxygen, azomethine nitrogen and amide carbonyl oxygen except in Ni(II) complex where HL1 acts as a monobasic bidentate ligand.

CONFLICTS OF INTERESTS
The authors declare that no conflict of interest exists