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

Article abstract of DOI:10.1515/chem-2020-0044

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.

Full text of DOI:10.1515/chem-2020-0044

Open Chemistry 2020; 18: 426–437
Research Article
Ayman H. Ahmed*
N,N′-Bis[2-hydroxynaphthylidene]/[2-methoxybenzylidene]
amino]oxamides and their divalent manganese complexes:
Isolation, spectral characterization, morphology, antibacterial
and cytotoxicity against leukemia cells
https://doi.org/10.1515/chem-2020-0044
coordination outcomes. Among nitrogen–oxygen giver
ligands, hydrazones have an extraordinary spot because of
broad applications for medication plans, insecticides and
organocatalysis [1–3]. Hydrazones that are recently recog-
nized 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 = CoII,
NiII, CuII, MnII, PdII, RuII and VIV] 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 dimethylform-
amide (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 [8–10], 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 anti-
tumoral, antiviral and antimicrobial agents [8,11–13]. 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
received September 11, 2019; accepted February 18, 2020
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-hydroxynaphtha-
lene-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 co-
ordinated 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 prolifera-
tion hindrance for the free ligands was enhanced upon
coordination with the manganese(II) ions.
Keyword: hydrazone complexes, structure elucidation,
pharmacological application
1 Introduction
Choosing ligands is very significant in determining the tested their biological activities. Broad investigations have
properties of coordination compounds. A ligand framework uncovered that the lone pair on the azomethine nitrogen in
having electronegative atoms like N and O improves the 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)
* Corresponding author: Ayman H. Ahmed, Chemistry Department,
College of Science and Arts, Jouf University, Qurayat, Saudi Arabia;
Chemistry Department, Faculty of Science, Al-Azhar University, Nasr
City, Cairo, Egypt, e-mail: ahfahmi@ju.edu.sa
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].
Open Access. © 2020 Ayman H. Ahmed, published by De Gruyter.
License.
This work is licensed under the Creative Commons Attribution 4.0 Public

N,N′-Bis[2-hydroxynaphthylidene]/[2-methoxybenzylidene]amino]oxamides and their divalent manganese complexes

427
Leukemia is a cancer of the blood-forming tissue, microwave power 1.008 mW and modulation/amplitude of
which involves the lymphatic system and the bone 4 GAUSS at National Center for Radiation Research and
marrow. In individuals with leukemia, abnormal white Technology, Egypt. By utilizing high vacuum technique, the
blood cells, which do not function properly, are solid complexes coated using gold sputter coater (SPI-
produced by the bone marrow. Then, the healthy blood Module, USA) were observed by scanning electron micro-
cells in the bone marrow are crowded by these abnormal scopy (model: JSM-5500 LV; JEOL Ltd, Japan). Employing
cells, reducing the number of platelets, red blood cells Faraday’s strategy, effective magnetic moments (μeff) at
and healthy white blood cells. The symptoms of this 4 Ampere for Mn(II) complexes were estimated at room
disease are weakness, chills, infection, enlarged liver temperature (RT).
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 2.2 Preparations
techniques for human leukemia are being worked on,
but so far none of them have been seen as totally 2.2.1 Isolation of ligands (BHO and BMO)
acceptable. Nevertheless, the antileukemia activity of
some metal–hydrazone complexes has been reported N,N′-Bis[2-hydroxynaphthylidene]amino]oxamide (BHO)
[17,18], the synthesized hydrazones and Mn(II) com- and N,N′-bis[2-methoxybenzylidene]amino]oxamide (BMO)
plexes were not yet researched. Therefore, we examined were prepared by the method published in ref. [9].
the synthesized compounds toward leukemia; in parti- Oxalic dihydrazide (0.01 mol, 1.181 g) was dissolved in
cular, the metal complexes have been recently demon- 40 mL distilled water by heating on a hot plate, and
strated as less harmful and more potent in many cases in 30 mL of absolute ethanol was added. This hot
comparison with the free ligands.
solution of oxalic dihydrazide was mixed with the
In continuation of ongoing reports on some oxalic hot absolute ethanolic solution of the chosen alde-
dihydrazones and their chelates [8–10], the present work hydes (keeping the molar proportion at 1 hydrazide:2
portrays preparation, characterization and morphological aldehyde). Precipitation of hydrazone was observed
investigations of divalent manganese complexes with two during the increment of aldehyde solution. The
hydrazones as ligands. Antimicrobial and antileukemia reaction mixture was heated under reflux for 3 h. The
activities of the manganese complexes and their corre- resulting precipitate was filtered while hot, and
sponding ligands have been researched to show the abundance of dihydrazide was avoided by washing
plausibility of using them in pharmacology science.
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 (1H-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, respec-
tively, which coincide well with the assumed formula.
Schemes 1 and 2 provide mass fragmentation pathways of
the investigated ligands.
2 Experimental
2.1 Materials and techniques
2-Hydroxynaphthalene-1-carbaldehyde,
2-methoxybenz-
aldehyde, 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
2.2.2 Isolation of complexes
1
out as published [9]. The H-NMR spectra were obtained
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
using a Varian mercury VX-300 NMR spectrometer at
300 MHz in DMSO-d6. 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,

428

Ayman H. Ahmed
Table 1: Analytical and physical data of the ligands and their related Mn(II) complexes
Compound (mol. wt.)
Symbol
Color
Yield (%) Found (calcd) (%)
C
H
N
M
C
C
24H18N4O4 426.13
18H18N4O4 354.36
BHO
BMO
Yellow
White
Brown
98
78
76
68.30 (67.60) 5.11 (4.25)
12.84 (13.14)
15.47 (15.81)
—
—
59.49 (61.01)
54.19 (55.48)
5.60 (5.12)
[Mn2(BHO-H)2(H2O)4]·H2O 1060.78 Mn–BHO
[Mn(BAO-2H)(OAc)2(DMF)2] 671.56 Mn–BMO Yellowish 25
4.10 (3.99) 10.38 (10.56) 10.52 (10.36)
49.39 (50.08) 5.60 (5.40) 12.28 (12.51) 9.10 (8.18)
H
H
O
C
N
O
X
N
X
R
C
C
R
N
N
H
O
C
H
O
(Staggered)
Trans-configuration
(BHO: R=ph, X=H) and BMO: R=H, X=Me.
100%
75%
50%
25%
BHO
400
400
50
100
150
200
m/e
250
300
350
100%
BMO
75%
50%
25%
50
100
150
200
m/e
250
300
350
Figure 1: Mass spectra of the ligands (BHO and BMO).
mixture was refluxed for 3 h and then sifted on hot. 2.3.2 Cytotoxic activity on leukemia cell lines
The precipitate was washed using methanol and left
overnight. To expel any trace of DMF solvent, the 2.3.2.1 Chemical substances
complex was washed thoroughly with methyl alcohol Trypan blue dye, crystal violet and DMSO solvent were
and propanone separately and finally dried at 80°C in an purchased from Sigma (USA). One percentage of crystal
oven for 4 h (m.p. of two manganese(II) complexes >300). violet stain was made of 50% methanol: 0.5% crystal violet
(w/v) was then made up to volume with bi-distilled H2O
and sifted through a filter paper (Whatman No. 1). RPMI-
1640, Dulbecco’s modified Eagle’s medium (DMEM), genta-
mycin, L-glutamine, 0.25% trypsin–EDTA, HEPES buffer
solution and fetal bovine serum were obtained from
Lonza.
2.3 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 2.3.2.2 Cell culture
on an agar plate was inoculated into Mueller–Hinton Mouse myelogenous leukemia cell line (M-NFS-60 cells)
broth. A sterile swab was immersed in the broth and was acquired from VACSERA Tissue Culture Unit. Leukemia
inoculated on Mueller–Hinton agar plates. The samples cells were maintained in the DMEM medium supplemented
were dissolved in DMSO at various concentrations (2.5, 5 with 1% L-glutamine, HEPES buffer, 10% heat-inactivated
and 10 mg/mL). The inhibition zone was estimated at fetal bovine serum and 50 µg/mL gentamycin at 37°C
37°C after 24 h. Controls were obtained using DMSO.
(humidified atmosphere) with 5% CO2 two times a week.

N,N′-Bis[2-hydroxynaphthylidene]/[2-methoxybenzylidene]amino]oxamides and their divalent manganese complexes

429
O
C
O
C
OH
HO
H
N
NH
N
N
CH
HC
C24H18N4O4
m/e = 427.0
(R.I = 1.51%)
O
C
O
C
OH
H
N
NH
N
N
CH
HC
C24H18N4O3
m/e = 410.0
(R.I = 2.26%)
C10H8
C6H8
m/e = 79.0
(R.I = 11.31 %)
O
C
O
C
OH
m/e = 128.0
(R.I = 19.76 %)
N
NH
NH
N
CH
C13H10N4O3
m/e = 270.0
(R.I = 2.73%)
O
C
O
C
OH
OH
CH2
N
NH
NH2
•+
CH
C5H5
C13H10N3O3
m/e = 257.0
(R.I = 4.34%)
C11H9
m/e = 64.95
(R.I = 33.82 %)
m/e = 141.0
(R.I = 6.39 %)
O
C
O
C
N
NH
N
CH
C13H9N3O3
m/e = 255.0
OH
CH2
•+
(R.I = 11.45%)
C4H4
m/e = 50.0
(R.I = 88.69 %)
O
C
O
OH
N
NH
C
C11H9O
m/e = 157.0
(R.I = 100.00 %)
CH
C13H9N2O3
m/e = 242.0
(R.I = 2.02 %)
OH
O
OH
NH
CH
N
NH C
CH
C12H9N2O2
C11H9NO
m/e = 215.0
m/e = 175.0
(R.I = 50.43%)
(R.I = 56.99 %)
Scheme 1: Mass fragmentation of N,N′-bis[2-hydroxynaphthylidene]amino]oxamide (BHO).
2.3.2.3 Cytotoxicity assessment
confluent cell monolayers distributed into flat-bottomed
In a 96-well plate containing 100 µL of growth medium, microtiter plates (96 wells). The microtiter plates
concentration of 1 × 104 cells was seeded. After 24 h of incubated at 37°C for 24 h in a humidified incubator
seeding, new fresh medium with different concentra- containing 5% CO2. Without sample and with or without
tions of the sample was added. Serial twofold dilutions DMSO, the control cells were incubated. It was found
of the examined ligand/complex were added to that the examination was not influenced up to 0.1%

430

Ayman H. Ahmed
OCH3
H3CO
O
O
H
N
C
H
N
C C NH N
O
O
C
H
H
N
C18H18N4O4
m/e = 356.45
(R.I = 7.23%)
C
H
N
C C NH N
C
H
C10H9N4O2
m/e = 217.2
OCH3
(R.I = 12.88%)
O
O
O
O
H
N
H
N
C
H
N
C C NH N
C
H
C
H
N
C C NH N
C11H11N4O3
C9H8N4O2
m/e = 247.35
(R.I = 15.04%)
m/e = 204
(R.I = 8.18%)
OCH3
O
O
O
O
H
N
H
N
C
H
N
C C NH
C
H
N
C C NH N
C10H10N4O3
m/e = 234.2
(R.I = 9.19%)
C9H8N3O2
m/e = 190.6
(R.I = 9.24%)
OCH3
O
H
O
O
H
C
H
N
N
C C NH
O
C
H
N N C C
C10H10N3O3
C9H7N2O2
m/e = 175.55
(R.I = 38.89%)
m/e = 223.2
(R.I = 7.18%)
OCH3
H
O O
C C
C
H
N
N
C
O
CH
H
C5H6
C
H
N N
C8H7N2O
m/e =66.65
(R.I = 43.39%)
C7H6
m/e = 147.3
(R.I = 19.4%)
C10H9N2O3
m/e = 90.6
(R.I = 18.47%)
m/e = 202
(R.I = 8.18%)
OCH3
C
H
N NH2
O
C
C
H
N
H
C7H7N2
C
H
N N
C6H6
m/e = 119.4
(R.I = 13.18%)
C7H6N
m/e = 104.2
(R.I = 8.17%)
m/e = 77.05
(R.I = 57.52 %)
C9H9N2O2
m/e = 176.5
(R.I = 19 %)
OCH3
OCH3
OCH3
CH
OCH3
C
H
N NH
C
H
N
C8H9N2O
m/e = 149
(R.I = 14.25 %)
C7H8O
m/e = 107.15
(R.I = 18.44 %)
C8H8O
m/e = 120.4
(R.I = 7.7 %)
C8H8NO
m/e = 134.6
(R.I = 19.21 %)
Scheme 2: Mass fragmentation of N,N′-bis[2-methoxybenzylidene]amino]oxamide (BMO).
of DMSO in the wells. After 24 h of incubation, the microplates were washed by tap water. All wells were mixed
viable cell yield was estimated using a colorimetric thoroughly with 30% CH3COOH, and the absorbance of the
method.
microplates (at λ = 490 nm) was measured. The optical
After the incubation period, the media were allowed to density was estimated in a reader plate (Sun Rise, TECAN,
aspirate and 1% crystal violet solution was added to each Inc., USA), and the viability percentage was calculated
plate and kept for 1/2 h. The stain was eliminated, and the according to the following equation:

N,N′-Bis[2-hydroxynaphthylidene]/[2-methoxybenzylidene]amino]oxamides and their divalent manganese complexes

431
at 3,186 and 3,342 cm−1, respectively. The lower offset
associated with ν(OH)naphthoic demonstrated the presence of
H-bonding between the azomethine nitrogen and naphthoic
OH groups. (4) Existence of a broad band in the complex at
3,379 cm−1 shows the overlapped crystalline/coordinated
water molecules. In case of Mn–BMO, (1) the perception of
C]O groups in the IR spectrum of Mn–BMO with obscure of
ν(NH) band demonstrates that the BMO ligand favored its
keto form in the chelation process. (2) Variation of ν(C]O)
feature from strong (in ligand) to very strong (in complex)
comparative with other neighbor bands indicates input of
this group in coordination. (3) Undoubtedly, nonappearance
of ν(M–Omethoxy) with remaining NH, C]N and C–OMe
groups at similar positions ruled out contribution of the
previously mentioned groups in coordination. (4) The two
Viability percentage = [1 − (ODt/ODc)] × 100%
where ODt and ODc are the mean optical densities of wells
treated and untreated with ligands/complexes, respectively.
The relationship between the ligand/complex con-
centration and the alive cells is graphically plotted, and
IC50 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.
3 Results and discussion
Analytical and physical results of hydrazones ligands bands related to νas and νs (carboxylic) of acetate group were
(BHO and BMO) and their related Mn(II) complexes seen at 1,351 cm−1 and 1,437 cm−1, respectively. The larger
(Mn–BHO and Mn–BMO) are summarized in Table 1.
difference between their locations affirmed the coordination
of acetate as a unidentate anion through the C–O moiety of
the carboxylic group [26]. (5) Furthermore, appearance of a
new band at 1,729 cm−1 assignable to ν(C]O)DMF affirmed the
contribution of DMF solvent in coordination [27]. Surely, the
3.1 IR spectroscopy
FT-IR spectral investigation of hydrazones and their presence of new ν(M–Onaphthoic) and ν(M–Omethoxy) in
manganese(II) complexes demonstrates the coordination Mn–BHO and Mn–BMO spectra at 567 and 640 cm−1,
behavior of the ligands within the chelation process.
Configuration of chosen ligands were discussed
elsewhere [9] and alluded to the existence of keto forms,
respectively, shows the formation of Mn–L bond.
1
Figure 1 [22]. The two ligands were validated on the 3.2 H-NMR spectroscopy
premise: BHO showed bands for νOHnaphthoic
(3,476 cm−1), νNH (3,166 cm−1), νC]O (1,705 cm−1
+
The NMR spectra of the ligands showed signals at various
1,660 cm−1), νC]N (1,621 cm−1) and νC–Onaphthoic positions: in BHO ligand: 6.5–8.5 (aromatic protons, m), 9.7
(1,287 cm−1). Meanwhile, BMO uncovered νC]O (CH]N, s), 12.6 (OH, s), 12.8 (NH, s); in BMO: 3.93 (OCH3, s),
(1,653 cm−1), νNH (3,202 cm−1), νC]N (1,600 cm−1), 6.8–8.5 (aromatic protons, m), 8.9 (CH]N, s), 11.9 (OH, s),
νOHenolic (3,227 cm−1) and νC–OMe (964 cm−1) [23].
12.3 (NH, s). Vanishing of –OH and –NH protons upon
The infrared spectra of the Mn(II) complexes were supplying of D2O affirmed the right assignment of these
examined and contrasted with those of related ligands. The groups in the ligand spectrum. The two ligands are in
method of chelation and structures was assured by the harmony with staggered or syn-cis-conformation as the
following actualities: for Mn–BHO: (1) the obscure of ν(NH) δCH]N, δNH and δOH resonances, each showed up as a
with noticeable of ν(C]O)complex at 1,655 cm−1 provides an singlet [28]. But which conformation is true? In fact, IR data
indisputable proof to the coordination of metal ion with the information responded to this inquiry and guaranteed the
deprotonated NH group. (2) Also, the positive shift for staggered configuration [10].
ν(C–O)naphthoic vibration (L : 1,287 → complex: 1,299 cm−1)
gives some insight to deprotonation of both naphthoic (OH)
groups during their coordination, however remarking 3.3 Optical and magnetic investigations
ν(OH)naphthoic at lower value (3,186 cm−1) points to the
existence of naphthoic (OH) in H-bonding. (3) Splitting of The absorption bands of the ligands and their complexes in
the azomethine band in free ligand with its shift (ligand: Nujol mull were assigned (Figure 2 and Table 2). In addition
1,621 cm−1: complex: 1,600, 1,618 cm−1) justifies the sharing of to the ligand bands, manganese complexes demonstrated
this group in coordination where two dissimilar azomethine d–d transitions from which the proposed geometry was
groups are formed (Figure 5) [24,25]. This supposition is predicted. The electronic information of manganese com-
boosted from the perception of ν(OH)naphthoic and ν(OH)enolic plexes was interpreted considering the two manganese(II)

432

Ayman H. Ahmed
manganese(II) complexes (t23g⋅eg2) affirming a paramagnetic
1.0
BHO
BMO
Mn-BHO
Mn-BMO
nature.
0.8
0.6
0.4
0.2
3.4 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).
200
400
600
800
Wavelength(nm)
Figure 2: UV-Vis spectrum of free ligands and manganese(II)
complexes in Nujol mull.
complexes have octahedral structures [29]. The bands
6
4
4
4
4
4
ascribed to A1g (S) → T1g, Eg(G); T2g, Eg(D) and T1g(P)
transitions in manganese complexes are presumed to be
overlapped/masked by strong peaks of free ligands.
10.5
The values of μeff 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 (5.95) in case of Mn–BHO can be elucidated as
follows: high spin Mn–BHO complex is expected to have μ >
5.95 considering the presence of five single electrons in the
two manganese ions of [Mn2(BHO-H)2(H2O)4]·H2O complex.
Spin of these single electrons in the same direction
(Mn↑↑↑↑↑–↑↑↑↑↑Mn) gives very high magnetic moment. In
reality, the obtained magnetization is not high since nearness
of 2Mn ions near one another in the suggested configuration
may cause spin coupling (partial quenching of the spin
moments) that minimizes the magnetism. Magnetic data
indicated the presence of five unpaired d electrons in both
Mn-BHO
8.5
6.5
4.5
2.5
0.5
Mn-BMO
0
200
400
Temp. (oC)
600
800
Figure 3: TG spectrum of manganese(II) complexes.
Table 2: Electronic spectral, magnetic moment and thermal data of ligands/Mn(II) complexes
Sample
Band
position (nm)
Assignment
Configuration
μeff.
(B.M.)
Temp. (°C) TG
Assignments
Found/
calcd (%)
BHO/BMO 380–420
360–380
n → π*(C]N)
n → π*(C]O)
π → π*(C]N)
π → π*(C]O)
π → π*(Phenyl)
—
—
—
—
—
330–360
300–330
230–270
4
Mn–BHO
590
480
6A1g(S) → T1g(G)
Octahedral, D6h 6.2
Octahedral, D6h 5.12
45–150
H2Ocrystalline
2.0/1.7
4
6A1g(S) → T2g(G)
178–310
620–800
4H2Ocoordinated
6.9/6.8
70.8/71.0
Formation of 4(MnO +
MnCO3) mix.
2OAccoordinated
Formation of MnO
4
Mn–BMO 420
6A1g(S) → T1g(G)
190–280
590–800
18.0/17.4
11.1/10.6
4
378
6A1g(S) → T2g(G)

N,N′-Bis[2-hydroxynaphthylidene]/[2-methoxybenzylidene]amino]oxamides and their divalent manganese complexes

433
4
3
1 .0 x1 0
5 .0 x1 0
and g┴ = 1.93 and Mn–BMO: g‖ = 1.85, g┴ = 1,998). The
nonappearance of hyperfine splitting for these com-
plexes excluded any axial distortion. The average g
values were calculated using the relation:
Mn-BMO
0 .0
1
gav
=
3[g + 2g ]
∥
⊥
3
-5 .0 x1 0
Both Mn–BHO and Mn–BMO revealed similar calcu-
lated gav (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].
4
1 .0 x1 0
5 .0 x1 0
Mn-BHO
3
0 .0
3
-5 .0 x1 0
2 5 0 0
3 0 0 0
3 5 0 0
4 0 0 0
4 5 0 0
[G]
Figure 4: EPR spectra of solid manganese(II) complexes
In the light of the prior outcomes, the manganese
complexes are anticipated to have structures presented
in Figure 5.
3.5 Electron paramagnetic resonance (EPR)
The Mn–BHO and Mn–BMO solids have been analyzed
by EPR spectroscopy at room temperature, and their 3.6 Examining electron magnifying
spectra are shown in Figure 4. The ground state of the
instrument (SEM)
6
octahedral d5 ion (spin free) is A. The energy levels are
The SEM micrographs of Mn(II)–BHO and Mn–BMO com-
plexes 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
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
Ac
O
HO
DMF
DMF
H
H
Mn
O
C
N
C
O
O
H2
O
H
O
N
O
N
Ac
N
C
C
OH2
. H2O
HO
Mn
NH
H2
O
HN
Mn
H
H
N
OH
HO
C
N
N
HO
N
O
C
H2
N
N
O
O
O
H
H
Mn-BMO
Mn-BHO
Figure 5: Suggested structures of manganese(II) complexes.

434

Ayman H. Ahmed
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].
(a)
(b)
3.7.2 Cytotoxic impact
Hydrazones ligands and their manganese complexes
were screened against mouse myelogenous leukemia
(M-NFS-60) carcinoma cells under the test conditions.
Figure 6: SEM micrographs of manganese(II) complexes (a) Mn-BHO;
(b) Mn-BMO.
morphology. Isolation of two complex structures (Figure 5) is The cell viability values are shown in Figure 8. Table 3
promoted from the acquisition of two SEM pictures with presents the concentrations of the examined samples
different morphologies (Figure 6). Clusters of larger size result that restrain half of cell multiplication (IC50), in which
from the aggregation of particles less than 100 nm in size. the parent compounds were less cytotoxic than their
Average particle size of the two manganese complexes complexes. In the literature, cytotoxic activity increases
(2.5–14.5 µm) is in the diameter scope of few microns upon complexation, and vice versa [37,38]. In this study,
(Table 3).
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 com-
plexes 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
3.7 Pharmacological significance
3.7.1 Antibacterial investigations
Antibacterial activities of ligands (BHO and BMO) to releasing 2H2O molecules in case of Mn–BHO and
and their complexes against gram-negative (Proteus, two DMF and/or OAc in case of Mn–BMO [337] (Figure 5).
Table 3: SEM results and inhibition of M-NFS-60 cell line by ligands and their Mn(II) complexes
Compound
IC50 (µg/mL)
Concentration at which inhibition
begins (μg/mL)
SEM
Morphology
Particle size (μm)
Max.
Min.
Av.
BHO
BMO
Mn–BHO
Mn–BMO
92.1 4.5
408 9.8
58.7 2.7
183 6.1
5.9
62.5
4.2
—
—
5.0
28.5
—
—
0.066
0.057
—
—
2.5
14.5
Platelet
Rod
15.6

N,N′-Bis[2-hydroxynaphthylidene]/[2-methoxybenzylidene]amino]oxamides and their divalent manganese complexes

435
30
25
20
15
BACTRIA Gram-posiꢀve
Staphylococcus aureus
(RCMB010010)
BACTRIA Gram-posiꢀve Bacillus
subꢀlis
RCMB 015 (1) NRRL B-543
10
5
BACTRIA Gram-negaꢀve
Escherichia coli
(RCMB 010052) ATCC 25955
BACTRIA Gram-negaꢀve Proteus
vulgaris
RCMB 004 (1) ATCC 13315
0
Figure 7: Antibacterial activity for ligands and their manganese(II) complexes.
120
100
80
120
100
80
60
40
20
0
60
40
20
BHO
BMO
0
Concentration (µg/ml)
Concentration (µg/ml)
120
100
80
120
100
80
60
40
20
0
60
40
20
Mn-BHO
Mn-BMO
0
Concentration (µg/ml)
Concentration (µg/ml)
Figure 8: Cytotoxicity evaluation of ligands and their manganese(II) complexes against M-NFS-60 cell line.
The oxalic dihydrazones and their Mn(II) complexes compounds, inhibitory impact against the M-NFS-60
begin to inhibit the M-NFS-60 cell line proliferation at cells was changed by the order Mn–BHO > BHO >
concentrations listed in Table 3. Among the examined Mn–BMO > BMO.

436

Ayman H. Ahmed
4 Conclusion
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and binuclear low-spin manganese(II) complexes derived from
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[8] Ahmed AH, Hassan AM, Gumaa HA, Mohamed BH, Eraky AM.
Nickel(II)-oxaloyldihydrazone complexes: characterization,
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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 com-
pounds. 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 micro-
organisms. 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.
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