N,N′-Bis[2-hydroxynaphthylidene]/[2-methoxybenzylidene]amino]oxamides and their divalent manganese complexes: Isolation, spectral characterization, morphology, antibacterial and cytotoxicity against leukemia cells

Abstract Manganese(ii) complexes of oxalic dihydrazones {N,N′-bis[2-hydroxynaphthylidene]amino]oxamide (BHO) and N,N′-bis[2-methoxybenzylidene]amino]oxamide (BMO)} have been synthesized by a general methodology. Hydrazone ligands (BHO and BMO) were obtained by the condensation of oxalic dihydrazide with 2-hydroxynaphthalene-1-carbaldehyde and 2-methoxybenzaldehyde. From the data obtained from the spectral (mass, IR, 1H-NMR, UV-Vis, ESR), magnetic and thermal measurements in addition to the elemental analyses (CHNM), the structures of ligands and their complexes have been determined. The scanning electron microscope (SEM) was used to characterize the morphology of the complex surface. The ligands coordinated with the metal center in a bi-dentate way forming binuclear Mn–BHO and mononuclear Mn–BMO complexes. The manganese complexes are proposed to have octahedral stereochemistry. The ligands and manganese(ii) complexes have been assessed for their antibacterial and antileukemia activities. The proliferation hindrance for the free ligands was enhanced upon coordination with the manganese(ii) ions.


Introduction
Choosing ligands is very significant in determining the properties of coordination compounds. A ligand framework having electronegative atoms like N and O improves the coordination outcomes. Among nitrogen-oxygen giver ligands, hydrazones have an extraordinary spot because of broad applications for medication plans, insecticides and organocatalysis [1][2][3]. Hydrazones that are recently recognized as polyfunctional ligands have a high tendency to form stable metal complexes with d-block elements like Mn(II) due to its complexing ability through keto-enol tautomerism and availability of donor sites [4]. Although many hydrazone complexes of transition metals [M = Co II , Ni II , Cu II , Mn II , Pd II , Ru II and V IV ] are being investigated [5,6], only very few papers on aryl oxalic dihydrazone complexes were published [7,8]. This may be due to the weak dissolvability of oxalic dihydrazone ligands in commonly used organic solvents. The oxalic dihydrazones (BHO and BMO) are just soluble in highly polar solvents, for example, dimethylsuphoxide (DMSO) and dimethylformamide (DMF), while insoluble in water and other common organic solvents. Obviously, this requires great effort to refine and separate their chelates. Some complexes of N,N′-bis[2-hydroxynaphthylidene]/ [2-methoxybenzylidene] amino]oxamides with Cu(II), Ni(II) and Pd(II) have been reported elsewhere [8][9][10], and this study is an expansion of the past work. In terms of biological activity, hydrazones including oxalic dihydrazone and their complexes offer a wide scope of pharmacological applications as antitumoral, antiviral and antimicrobial agents [8,[11][12][13]. Metal complexes obtained from arylhydrazones are helpful in the pharmaceutical field [14] and act as enzyme inhibitors. As hydrazones having an azomethine proton (-CH]N-) establish a significant class of compounds that may be used to develop a new drug, numerous researchers have tested their biological activities. Broad investigations have uncovered that the lone pair on the azomethine nitrogen in hydrazones is liable for their chemical and biological activities [14,15]. Throughout the years, a large number of organic/inorganic compounds have been prepared to battle pathogenic microorganisms. In this regard, the Ni(II), Co(II) and Fe(III) complexes of some drugs, e.g., chloramphenicol and ampicillin, have been synthesized and revealed antibacterial action more strongly than parent drugs [16].
Leukemia is a cancer of the blood-forming tissue, which involves the lymphatic system and the bone marrow. In individuals with leukemia, abnormal white blood cells, which do not function properly, are produced by the bone marrow. Then, the healthy blood cells in the bone marrow are crowded by these abnormal cells, reducing the number of platelets, red blood cells and healthy white blood cells. The symptoms of this disease are weakness, chills, infection, enlarged liver and bleeding, bone pain and small red spots in the skin. Scientists do not comprehend the accurate reasons for leukemia. It seems to develop from a combination of environmental and genetic factors. Viable treatment techniques for human leukemia are being worked on, but so far none of them have been seen as totally acceptable. Nevertheless, the antileukemia activity of some metal-hydrazone complexes has been reported [17,18], the synthesized hydrazones and Mn(II) complexes were not yet researched. Therefore, we examined the synthesized compounds toward leukemia; in particular, the metal complexes have been recently demonstrated as less harmful and more potent in many cases in comparison with the free ligands.
In continuation of ongoing reports on some oxalic dihydrazones and their chelates [8][9][10], the present work portrays preparation, characterization and morphological investigations of divalent manganese complexes with two hydrazones as ligands. Antimicrobial and antileukemia activities of the manganese complexes and their corresponding ligands have been researched to show the plausibility of using them in pharmacology science.

Materials and techniques
2-Hydroxynaphthalene-1-carbaldehyde, 2-methoxybenzaldehyde, manganese(II) acetate tetrahydrate and oxalic dihydrazide were acquired from Sigma-Aldrich. Solvents were utilized without any more refinement considering their highest purity. Elemental analyses, thermal (TG) and spectral (FT-IR, Mass, UV-Vis) measurements were carried out as published [9]. The 1 H-NMR spectra were obtained using a Varian mercury VX-300 NMR spectrometer at 300 MHz in DMSO-d 6 . The chemical shifts were given in δ (ppm) and determined relative to tetramethylsilane (TMS). The ESR spectra of the complexes were recorded in the powder form at room temperature utilizing Bruker-EMX-(X-bands-9.7 GHZ) spectrometer at 100 kHZ frequency, microwave power 1.008 mW and modulation/amplitude of 4 GAUSS at National Center for Radiation Research and Technology, Egypt. By utilizing high vacuum technique, the solid complexes coated using gold sputter coater (SPI-Module, USA) were observed by scanning electron microscopy (model: JSM-5500 LV; JEOL Ltd, Japan). Employing Faraday's strategy, effective magnetic moments (μ eff ) at 4 Ampere for Mn(II) complexes were estimated at room temperature (RT). were prepared by the method published in ref. [9]. Oxalic dihydrazide (0.01 mol, 1.181 g) was dissolved in 40 mL distilled water by heating on a hot plate, and 30 mL of absolute ethanol was added. This hot solution of oxalic dihydrazide was mixed with the hot absolute ethanolic solution of the chosen aldehydes (keeping the molar proportion at 1 hydrazide:2 aldehyde). Precipitation of hydrazone was observed during the increment of aldehyde solution. The reaction mixture was heated under reflux for 3 h. The resulting precipitate was filtered while hot, and abundance of dihydrazide was avoided by washing with hot water. The excess of aldehyde was removed by absolute ethanol, and the ligand was dried at 80°C for 2 h in an electric oven (m.p. of two ligands >300). The chemical formula and molecular structure of the resulting hydrazones were determined by the elemental analyses ( Table 1) and spectral ( 1 H-NMR and IR) data. Furthermore, mass spectra of BHO and BMO ( Figure 1) showed M + (molecular ion peak) at m/z (found/calculated) = 327.1/426.4 and 355.0/354.4, respectively, which coincide well with the assumed formula. Schemes 1 and 2 provide mass fragmentation pathways of the investigated ligands.

Isolation of complexes
Manganese(II) complexes were formed via the interaction between divalent manganese acetate and the ligand as follows. First, the free hydrazone (0.56 mmol, 0.2 g) was dissolved in 35 mL hot DMF, and then, methanol (40 mL) was added. Then, 25 mL methanolic solution of metal acetate was gradually added to the previous ligand solution in 1 L:2 M molar proportion. The subsequent mixture was refluxed for 3 h and then sifted on hot. The precipitate was washed using methanol and left overnight. To expel any trace of DMF solvent, the complex was washed thoroughly with methyl alcohol and propanone separately and finally dried at 80°C in an oven for 4 h (m.p. of two manganese(II) complexes >300).

Pharmacological screening 2.3.1 Antimicrobial activity
The susceptibility tests were performed by NCCLS proposals (National Committee for Clinical Lab Standards, 1993). The assessment of the restraint zone was done by the well diffusion strategy [19]. The inoculum obtained from the colonies grown overnight on an agar plate was inoculated into Mueller-Hinton broth. A sterile swab was immersed in the broth and inoculated on Mueller-Hinton agar plates. The samples were dissolved in DMSO at various concentrations (2.5, 5 and 10 mg/mL). The inhibition zone was estimated at 37°C after 24 h. Controls were obtained using DMSO.

Chemical substances
Trypan blue dye, crystal violet and DMSO solvent were purchased from Sigma (USA). One percentage of crystal violet stain was made of 50% methanol: 0.5% crystal violet (w/v) was then made up to volume with bi-distilled H 2 O and sifted through a filter paper (Whatman No. 1). RPMI-1640, Dulbecco's modified Eagle's medium (DMEM), gentamycin, L-glutamine, 0.25% trypsin-EDTA, HEPES buffer solution and fetal bovine serum were obtained from Lonza.

Cell culture
Mouse myelogenous leukemia cell line (M-NFS-60 cells) was acquired from VACSERA Tissue Culture Unit. Leukemia cells were maintained in the DMEM medium supplemented with 1% L-glutamine, HEPES buffer, 10% heat-inactivated fetal bovine serum and 50 µg/mL gentamycin at 37°C (humidified atmosphere) with 5% CO 2 two times a week.     of DMSO in the wells. After 24 h of incubation, the viable cell yield was estimated using a colorimetric method.
After the incubation period, the media were allowed to aspirate and 1% crystal violet solution was added to each plate and kept for 1/2 h. The stain was eliminated, and the microplates were washed by tap water. All wells were mixed thoroughly with 30% CH 3 COOH, and the absorbance of the microplates (at λ = 490 nm) was measured. The optical density was estimated in a reader plate (Sun Rise, TECAN, Inc., USA), and the viability percentage was calculated according to the following equation:  where ODt and ODc are the mean optical densities of wells treated and untreated with ligands/complexes, respectively. The relationship between the ligand/complex concentration and the alive cells is graphically plotted, and IC 50 was determined by Graph pad Prism program (San Diego, CA, USA) [20,21].
Ethical approval: The conducted research is not related to either human or animal use.

Results and discussion
Analytical and physical results of hydrazones ligands (BHO and BMO) and their related Mn(II) complexes (Mn-BHO and Mn-BMO) are summarized in Table 1.

IR spectroscopy
FT-IR spectral investigation of hydrazones and their manganese(II) complexes demonstrates the coordination behavior of the ligands within the chelation process.
Configuration of chosen ligands were discussed elsewhere [9] and alluded to the existence of keto forms, Figure 1

1 H-NMR spectroscopy
The NMR spectra of the ligands showed signals at various positions: in BHO ligand: 6.5-8.5 (aromatic protons, m), 9.7 (CH]N, s), 12.6 (OH, s), 12.8 (NH, s); in BMO: 3.93 (OCH 3 , s), 6.8-8.5 (aromatic protons, m), 8.9 (CH]N, s), 11.9 (OH, s), 12.3 (NH, s). Vanishing of -OH and -NH protons upon supplying of D 2 O affirmed the right assignment of these groups in the ligand spectrum. The two ligands are in harmony with staggered or syn-cis-conformation as the δCH]N, δNH and δOH resonances, each showed up as a singlet [28]. But which conformation is true? In fact, IR data information responded to this inquiry and guaranteed the staggered configuration [10].

Optical and magnetic investigations
The absorption bands of the ligands and their complexes in Nujol mull were assigned (Figure 2 and Table 2). In addition to the ligand bands, manganese complexes demonstrated d-d transitions from which the proposed geometry was predicted. The electronic information of manganese complexes was interpreted considering the two manganese(II) complexes have octahedral structures [29]. The bands ascribed to 6 A 1g (S) → 4 T 1g , 4 E g (G); 4 T 2g , 4 E g (D) and 4 T 1g (P) transitions in manganese complexes are presumed to be overlapped/masked by strong peaks of free ligands.
The values of μ eff in case of Mn-BHO and Mn-BMO complexes ( Table 1) could be reliable with the proposed octahedral geometry [30]. A significant difference between the observed effective magnetic moment (6.2, Table 1) and the pure spin

Thermogravimetric investigation
Thermal disintegration examination of the synthesized consequential solid complexes has been performed, which reveals the thermal stability as well as the type and number of solvent molecules in the proposed configuration ( Table 2). The TG thermograms of the two manganese complexes are shown in Figure 3. The Mn-BHO and Mn-BMO complexes exhibited five and four steps of degradation, respectively, concerning the gradual disintegration of metal complexes with consistence of metallic residue at final stage. TG curves illustrated that Mn-BHO and Mn-BMO begin to decompose at 300°C and 195°C, respectively, signifying that the Mn-BHO is more stable than Mn-BMO ( Figure 3).

Electron paramagnetic resonance (EPR)
The Mn-BHO and Mn-BMO solids have been analyzed by EPR spectroscopy at room temperature, and their spectra are shown in Figure 4. The ground state of the octahedral d 5 ion (spin free) is 6 A. The energy levels are at ±5/2, ±3/2 and ±1/2, and a single isotropic resonance at g = 2 is easily attained. Accordingly, the EPR spectra of manganese complexes display one wide signal for every complex with two "g" values (Mn-BHO: g ‖ = 1.88 and g ┴ = 1.93 and Mn-BMO: g ‖ = 1.85, g ┴ = 1,998). The nonappearance of hyperfine splitting for these complexes excluded any axial distortion. The average g values were calculated using the relation: Both Mn-BHO and Mn-BMO revealed similar calculated g av (1.9). The likeness in the ESR spectra and g values demonstrate that Mn-BHO and Mn-BMO take similar arrangements, i.e., octahedral geometry.
The average g value for both complexes (1.9) is almost near to free electron g value noting that the spin-orbit coupling is missing. The Mn-BHO (g ‖ = 1.88) and Mn-BMO (g ‖ = 1.85) complexes displayed g ‖ below 2.3, proposing the covalent nature of Mn-L bond, where the ionic M-L bond is characterized by g ‖ > 2.3 [31,32]. Genuinely the nitrogen and oxygen coordination in manganese complexes is in congruence with high estimations of g [33].
In the light of the prior outcomes, the manganese complexes are anticipated to have structures presented in Figure 5.

Examining electron magnifying instrument (SEM)
The SEM micrographs of Mn(II)-BHO and Mn-BMO complexes are shown in Figure 6a, b and Table 3. It is shown in the figures that the Mn-BHO complex shows a platelet-like structure, while Mn-BMO complex displays a rod-like morphology. Isolation of two complex structures ( Figure 5) is promoted from the acquisition of two SEM pictures with different morphologies (Figure 6). Clusters of larger size result from the aggregation of particles less than 100 nm in size. Average particle size of the two manganese complexes (2.5-14.5 µm) is in the diameter scope of few microns ( Table 3).

Antibacterial investigations
Antibacterial activities of ligands (BHO and BMO) and their complexes against gram-negative (Proteus, Escherichia coli) and gram-positive (Bacillus subtilis, Staphylococcus aureus) bacteria are shown in Figure 7. Free ligands showed lower activity in comparison to manganese complexes toward all the tested strains. The higher action of complexes can be illustrated in terms of chelation hypothesis [34]. On coordination, the polarity of the manganese(II) ion will be diminished to a more prominent degree because of the interaction between the ligand orbitals belong to donor groups and the positive charge of the metal. Furthermore, the complexation process will induce the delocalization of p-electrons over the chelating ring and enhances the penetration of the complex into lipid membranes and blocking of the metal binding sites in the enzymes of bacteria. In addition, the metal complex influences the breath process of the cell and hence obstructs the synthesis of proteins restricting further growth of the organisms [35]. The additional factors that enhance the action are Mn-L bond length and solubility [36].

Cytotoxic impact
Hydrazones ligands and their manganese complexes were screened against mouse myelogenous leukemia (M-NFS-60) carcinoma cells under the test conditions. The cell viability values are shown in Figure 8. Table 3 presents the concentrations of the examined samples that restrain half of cell multiplication (IC 50 ), in which the parent compounds were less cytotoxic than their complexes. In the literature, cytotoxic activity increases upon complexation, and vice versa [37,38]. In this study, coordination appeared to be a good strategy. In our data, the cytotoxic activity of hydrazones was improved upon chelation with manganese(II) ions, resulting in complexes with noteworthy cytotoxicity against M-NFS-60. This can be interpreted on the premise the manganese tie to two DNA sites of cancer cells subsequent to releasing 2H 2      Hydrazones and their Mn(II)-complexes can be isolated in a pure form. The conventional reflux strategy might be utilized for this purpose. Contrasting of theoretical and experimental percentages of elemental analyses demonstrates that the proposed formulae agree well with the chemical compositions of the isolated compounds. Consistence of the coveted complexes can be affirmed from the UV-Vis, FT-IR, ESR and magnetic data. Monitoring the color change of the ligand during the complexation process can be considered as further evidence for the formation of required complexes. Both Mn(II)-bis[2-hydroxynaphthylidene]amino]oxamide and Mn(II)-bis[2-methoxybenzylidene]amino]oxamide complexes have octahedral configurations. With the antibacterial action against four microorganisms, the outcomes suggested that the complexes are increasingly powerful and might impede protein creation of certain microorganisms. In addition, the antileukemia impact of the free hydrazones was enhanced by coordination. The Mn(II)-bis[2-hydroxynaphthylidene]amino]oxamide chelate displayed a significant cytotoxic activity against mouse myelogenous leukemia carcinoma cells compared with other examined compounds.

Conflict of interest:
Author states no conflict of interest.