Synthesis, characterization, theoretical study and biological activity of Schiff base nanomaterial analogues

Article abstract of DOI:10.1016/j.molstruc.2019.01.007

The ligand named (H₂L: 2,2’-((1E, 1′E)-(1,3 phenylene bis (azanylylidene)) bis (methanylylidene))diphenol) was prepared from the reaction of m-phenylenediamine and 2-hydroxybenzaldehyde. A series of Co(II), Ni(II), Cu(II), Cr(III), Mn(II), Fe(I

Full text of DOI:10.1016/j.molstruc.2019.01.007

Accepted Manuscript
Synthesis, characterization, theoretical study and biological activity of Schiff base
nanomaterial analogues
G.G. Mohamed, W.H. Mahmoud, M.A. Diab, A.Z. El-Sonbati, S.Y. Abbas
PII:
S0022-2860(19)30007-9
DOI:
https://doi.org/10.1016/j.molstruc.2019.01.007
MOLSTR 26058
Reference:
To appear in: Journal of Molecular Structure
Received Date: 10 November 2018
Revised Date: 2 January 2019
Accepted Date: 2 January 2019
Please cite this article as: G.G. Mohamed, W.H. Mahmoud, M.A. Diab, A.Z. El-Sonbati, S.Y. Abbas,
Synthesis, characterization, theoretical study and biological activity of Schiff base nanomaterial
analogues, Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/j.molstruc.2019.01.007.
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ACCEPTED MANUSCRIPT
Synthesis, characterization, theoretical study and biological activity of Schiff
base nanomaterial analogues
G.G. Mohamed1, 2, W.H. Mahmoud1, M.A. Diab3, A.Z. El-Sonbati3,*, S. Y. Abbas3
1Chemistry Department, Faculty of Science, Cairo University, Giza 12613, Egypt
2 Egypt Nanotechnology Center, Cairo University, El-Sheikh Zayed, 6th October, 12588, Egypt
3Chemistry Department, Faculty of Science, Damietta University, Egypt
Abstract
The ligand named (H2L: 2,2'-((1E,1'E)-(1,3 phenylene bis (azanylylidene)) bis
(methanylylidene))diphenol) was prepared from the reaction of m-phenylenediamine and 2-
hydroxybenzaldehyde. A series of Co(II), Ni(II), Cu(II), Cr(III), Mn(II), Fe(III), Zn(II) and
Cd(II) complexes of the ligand were prepared and characterized using different physicochemical
techniques. The infrared spectral data showed that the chelation occurs through the deprotonated
phenolic oxygen and azomethine nitrogen atoms of the ligand. The data showed that the
complexes have composition of ML type. An octahedral geometry around the central metal ions
in the complexes was suggested. The molecular structures of the ligand and its metal complexes
were optimized theoretically. Scanning electron microscope micrograph proved that the ligand
and its Fe(III) complex were formed in nano size. The synthesized Schiff base ligand and its
complexes have been screened for antimicrobial activities against bacterial species and fungi. A
comparative study of the inhibition zones of the ligand and its metal complexes indicates that
most complexes exhibit higher antibacterial and antifungal effect against bacterial species than
the free ligand. The molecular docking is used to predict the efficiency of binding between Schiff
base ligand and both receptors of crystal structure of Escherichia coli (3T88) and crystal
structure of Staphylococcus aureus (3Q8U).
Keywords: Schiff base; Complexes; Biological activity; Molecular docking; Thermal analysis.
*Corresponding author: E-mail address: elsonbatisch@yahoo.com (A.Z. El-Sonbati)
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1. Introduction
Much attention has been recently paid to the study of Schiff base complexes with
transition metals of biological, environmental, industrial and pharmaceutical interests [1].
Recently, much research on macrocyclic complexes have been focused on species containing the
first-row transition metal ions and tetradentate ligands [2]. As known, the third most abundant
metal ions in human body are iron, zinc and copper and these metal ions play critical and crucial
roles in many biological processes such as RNA and DNA synthesis [3], gene expression and
enzyme regulation [4], the structural and functional enhancement of proteins [5]. Due to their
importance in biological processes, Cu(II) complexes synthesis and activity studies have been
the focus from different perspectives [6]. Moreover, Schiff base amino acid complexes of first-
row transition metals specially Cu(II) were reported to exhibit fungicidal, bactericidal, antiviral
and antitubercular activity [7]. Schiff base amino acid complexes of Cu(II) have received much
attention due to their high biological activity [8]. In some cases, Schiff bases and its metal
complexes are potentially used as anticancer drugs [9]. Moreover, these compounds show many
important biological activities such as antimicrobial and antifungal [10]. Schiff bases appear to
be important intermediates in a number of enzymatic reactions involving enzyme interaction
with an amino or a carbonyl group of a substrate. In bioinorganic chemistry, the interest in the
Schiff base complexes derives from their ability to provide synthetic models for metal-containing
sites in metalloproteins and to contribute to developments in medicinal chemistry. Thus, Schiff
bases and their complexes have a variety of applications in biological, clinical, and analytical
fields [11]. Furthermore, complexes of transition metals, which involve derivatives of
salicylaldehyde and diamine, have received considerable attention. This is because of their
potential as catalysts for the insertion of oxygen into an organic substrate [12].
Schiff base complexes have been considered to be among the most important stereo
chemical models in main group and transition metal coordination chemistry due to their
preparative accessibility and structural variety [1,13-15]. Metal complexes of Schiff bases have
a wide variety of biological applications such as antitumor, antimicrobial and antioxidant
activities. Transition metal complexes of Schiff bases are of interest due to their appreciable
biological activities and commercial importance [14-21]
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In this paper, Schiff base ligand 2,2'-((1E,1'E)-(1,3 phenylene bis (azanylylidene)) bis
(methanylylidene))diphenol) (H2L) derived from the reaction of m-phenylenediamine and 2-
hydroxybenzaldehyde was synthesized and characterized. The interaction of H2L with different
metal ions was prepared and characterized by spectroscopic and molecular docking methods. In
addition to the molecular structures of the Schiff base ligand (H2L) and its metal complexes were
optimized theoretically and the quantum chemical parameters were calculated and discussed. The
new compounds were tested for their antimicrobial and antifungal activities against different
organisms and compared with the standard antibacterial and antifungal drugs. The calculated
binding energy values of the receptors (Escherichia coli (3T88) and Staphylococcus aureu
(3Q8U)) for Schiff base ligand (H2L) compared to antimicrobial activities against Escherichia
coli (Gram (-) bacteria) and Staphylococcus aureus (Gram (+) bacteria).
2. Experimental
2.1. Materials and reagents
All chemicals used included 2-hydroxybenzaldehyde, m-phenylenediamine, CrCl3·6H2O,
MnCl2·4H2O, FeCl3, CoCl2·6H2O and CuCl2.2H2O were baught from Sigma. They were
purified by standard methods. ZnCl2 (Strem Chemicals) , CdCl2 (Sigma- Aldrich) NiCl2.6H2O
(BDH Chemical Ltd),. They were of the analytical reagent grade (AR), and of highest purity
available. The organic solvents such as absolute ethyl alcohol and dimethylformamide (DMF)
were either spectroscopic pure from BDH Chemical Ltd or purified by the recommended
methods [22].
2.2. Instruments
Microanalyses of carbon, hydrogen and nitrogen were carried out at the Microanalytical
Center, Cairo University, Egypt, using CHNS-932 (LECO) Vario Elemental Analyzer. FT-IR
spectra were recorded on a Perkin-Elmer 1650 spectrometer (4000–400 cm−1) in KBr pellets.
Electronic spectra were recorded at room temperature on a Shimadzu 3101pc spectrophotometer
as solutions in DMF. 1H NMR spectra, as a solution in DMSO- d6 were recorded on a 300 MHz
Varian-Oxford Mercury at room temperature using TMS as an internal standard.
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Mass spectra were recorded by the EI technique at 70 eV using MS-5988 GS-MS
Hewlett-Packard instrument at the Microanalytical Center, National Center for Research, Egypt.
The molar magnetic susceptibility was measured on powdered samples using the Faraday
method. The diamagnetic corrections were made by Pascal’s constant and Hg[Co(SCN)4] was
used as a calibrant. Molar conductivities of 10-3 M solutions of the solid complexes in DMF were
measured on by using Jenway 4010 conductivity meter. The scanning electron microscopic
(SEM) image of the complexes was recorded by using SEM model Quanta 250 FEG (Field
Emission Gun) attached with EDX Unit (Energy Dispersive X-ray Analyses), with accelerating
voltage 30 K.V. magnification 14x up to 1000000 and resolution for Gun.1n, National Research
Center, Egypt. The thermogravimetric analysis (TGA) was carried out from room temperature to
1000 °C using a Shimadzu TG-50H thermal analyzer. The antimicrobial activities were carried
out at the Microanalytical center, Cairo University, Egypt.
Perkin Elmer ChemBio 3D software was used in calculating the optimized geometry by HF
method with 3–21 G basis set [23-25] to obtain the most stable structure.
Molecular docking calculations were performed using Docking Server [26-28]. The ligand
atoms have Gasteiger partial charges. Rotatable bonds were defined. Non-polar hydrogen atoms
were added. Docking calculations were carried out on LYASE / LYASE INHIBITOR Crystal
structure of Escherichia coli MenB in complex with substrate analogue OSB-NCoA (3T88) and
TRANSFERASE
Crystal structure of Staphylococcus aureus nucleoside diphosphate kinase
complexed with ADP (3Q8U) proteins model [28-30].
2.3. Synthesis of ligand (H2L)
H2L Schiff base ligand was synthesized by condensation of m-phenylenediamine with 2-
hydroxybenzaldehyde (Scheme 1). A solution of 2-hydroxybenzaldehyde (74 mmol, 9.04 g, 7.74
ml) dissolved in ethanol was added dropwise to m-phenylenediamine (37 mmol, 4g) dissolved in
ethanol. The resulting mixture was stirred under reflux for about 3 h at temperature (100-150 oC)
which an orange solid compound was separated. It was filtered, recrystallized and washed with
diethylether and dried in vacuum.
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Yield 52.33 %; m.p. 130°C; yellowish green solid. Anal. Calcd. for C20H16N2O2 (%): C,
75.95; H, 5.06; N, 8.86. Found (%): C, 75.93; H, 5.05; N, 8.85. FT-IR (cm-1): phenolic υ(OH)
1
3438, azomethine υ(C=N) 1617, υ(C-N) 1386, υ(C-O) phenolic 1276. H NMR (300 MHz,
DMSO-d6, δ, ppm): 13.00 (s, 2H, OH phenolic), 9.05 (s, 2H, CH=N), 6.91-8.71 (m, 12H, Ar H).
λ
max (cm-1): 46082 and 40650 (π–π*), 30030 (n–π*).
2.4. Synthesis of complexes of H2L
The complexes were prepared by the addition of hot solution (60 °C) of the
appropriate metal chlorides (1.266 mmol) in ethanol to the hot solution (60 °C) of the ligand
(0.4g, 1.266 mmol) in a mixture of ethanol and DMF by molar ratio (2:1 v/v). The resulting
mixture was stirred under reflux for two hours whereupon the complexes precipitated. They were
collected by filtration, washed with hot ethanol and purified by washing several times with
diethyl ether. The analytical data for C, H and N were repeated twice.
[Cr(L)(Cl)(OH2)].H2O (1)
Yield 72.28 %; green solid, m.p. 118 °C. Anal. Calcd. for Cr(C20H18ClN2O4) (%):
C, 54.86; H, 4.12; N, 6.40; Cr, 11.89. Found (%): C, 54.87; H, 4.11; N, 6.41; Cr, 11.88. FT-IR
(cm-1): phenolic υ(OH) 3319, azomethine υ(C=N) 1645, υ(H2O) stretching bands of coordinated
water 932 and 896, υ(M-O) stretching bands of coordinated water 519, metal–nitrogen bond
υ(M-N) 479. Molar conductivity(10−3 M, DMSO): 43.0 ꢀ−1 cm2 mol−1. ꢀeff = 3.58 B.M.
λmax(cm-1): 46948 and 40816 (π–π*), 29850 (n–π*).
[Mn(L)(OH2)2].H2O (2)
Yield 73.82 %; yellowish green solid, m.p. 90 °C. Anal. Calcd. for
Mn(C20H20N2O5) (%): C, 56.74; H, 4.73; N, 6.62; Mn, 12.99. Found (%): C, 56.72; H, 4.74; N,
6.63; Mn, 12.98. FT-IR (cm-1): phenolic υ(OH) 3426, azomethine υ(C=N) 1647, υ(C-N) 1378,
phenolic υ(C-O) 1275, υ(H2O) stretching bands of coordinated water 960 and 869, υ(M-O)
stretching bands of coordinated water 530, metal-oxygen bond υ(M-O) 543, metal-nitrogen bond
υ(M-N) 450. Molar conductivity(10−3 M, DMSO): 33.8 ꢀ−1 cm2 mol−1. ꢀeff = 5.43 B.M. λmax (cm-
1):: 46082 and 41152 (π–π*), 29850 (n–π*).
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[Fe(L)(OH2)2].Cl (3)
Yield 60.89 %; black solid, m.p. 121 °C. Anal. Calcd. for Fe(C20H18ClN2O4) (%):
C, 54.37; H, 4.08; N, 6.34; Fe, 12.68. Found (%): C, 54.35; H, 4.10; N, 6.35; Fe, 12.67. FT-IR
(cm-1): phenolic υ(OH) 3429, azomethine υ(C=N) 1640, υ(C-N) 1371, phenolic υ(C-O) 1284,
υ(H2O) stretching bands of coordinated water 958 and 872, υ(M-O) stretching bands of
coordinated water 570, metal-oxygen bond υ(M-O) 580, metal-nitrogen bond υ(M-N) 455. Molar
conductivity(10−3 M, DMSO): 63.9 ꢀ−1 cm2 mol−1. ꢀeff = 5.17 B.M. λmax (cm-1): 46082 and 41152
(π–π*), 29850 (n–π*).
[Co(L)(OH2)2].H2O (4)
Yield 66.66 %; blue solid, m.p. 120 °C. Anal. Calcd. for Co(C20H20N2O5) (%): C,
56.22; H, 4.69; N, 6.56; Co, 13.80. Found (%): C, 56.23; H, 4.68; N, 6.57; Co, 13.79. FT-IR (cm-
1): phenolic υ(OH) 3428, azomethine υ(C=N) 1646, υ(C-N) 1382, phenolic υ(C-O) 1238, υ(H2O)
stretching bands of coordinated water 937 and 853, υ(M-O) stretching bands of coordinated
water 538, metal-oxygen bond υ(M-O) 579, metal-nitrogen bond υ(M-N) 459. Molar
conductivity(10−3 M, DMSO): 46.8 ꢀ−1 cm2 mol−1. ꢀeff = 5.18 B.M. λmax (cm-1): 46082 and 41152
(π–π*), 29850 (n–π*).
[Ni(L)(OH2)2].H2O (5)
Yield 37.05 %; yellowish green solid, m.p. 90 °C. Anal. Calcd. for Ni(C20H20N2O5)
(%): C, 56.25; H, 4.70; N, 6.56; Ni, 13.75. Found (%): C, 56.21; H, 4.71; N, 6.57; Ni, 13.77. FT-
IR (cm-1): phenolic υ(OH) 3414, azomethine υ(C=N) 1645, υ(C-N) 1365, phenolic υ(C-O) 1237,
υ(H2O) stretching bands of coordinated water 971 and 807, υ(M-O) stretching bands of
coordinated water 560, metal-oxygen bond υ(M-O) 584, metal-nitrogen bond υ(M-N) 466. Molar
conductivity(10−3 M, DMSO): 45.5 ꢀ−1 cm2 mol−1. ꢀeff = 2.76 B.M. λmax (cm-1): 46082 and 41152
(π–π*), 29850 (n–π*).
[Cu(L)(OH2)2].H2O (6)
Yield 64.12 %; black solid, m.p. >300 °C. Anal. Calcd. for Cu(C20H20N2O5) (%): C,
55.62; H, 4.63; N, 6.49; Cu, 14.72. Found (%): C, 55.61; H, 4.62; N, 6.50; Cu, 14.73. FT-IR (cm-
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1): phenolic υ(OH) 3436, azomethine υ(C=N) 1615, υ(C-N) 1383, phenolic υ(C-O) 1220, υ(H2O)
stretching bands of coordinated water 953 and 815, υ(M-O) stretching bands of coordinated
water 520, metal-oxygen bond υ(M-O) 537, metal-nitrogen bond υ(M-N) 452. Molar
conductivity(10−3 M, DMSO): 39.7 ꢀ−1 cm2 mol−1. ꢀeff = 1.84 B.M. λmax (cm-1) 46082 and 41152
(π–π*), 29850 (n–π*).
[Zn(L)(OH2)2].H2O (7)
Yield 61.11 %; yellowish green solid, m.p. 129 °C. Anal. Calcd. for
Zn(C20H20N2O5) (%): C, 55.38; H, 4.62; N, 6.46; Zn, 15.09. Found (%): C, 55.40; H, 4.63; N,
6.45; Zn, 15.07. FT-IR (cm-1): phenolic υ(OH) 3424, azomethine υ(C=N) 1651, υ(C-N) 1375,
phenolic υ(C-O) 1277, υ(H2O) stretching bands of coordinated water 962 and 862, υ(M-O)
stretching bands of coordinated water 574, metal-oxygen bond υ(M-O) 586, metal-nitrogen bond
υ(M-N) 474. Molar conductivity(10−3 M, DMSO): 18.1 ꢀ−1 cm2 mol−1. ꢀeff = dia.. 1HNMR (300
MHz, DMSO-d6, δ, ppm): 6.65-7.30 (m, 12H, Ar H), 7.95 (s, 2H, CH=N). λmax (cm-1) 46082
and 40816 (π–π*), 30120 (n–π*).
[Cd(L)(OH2)2].H2O (8)
Yield 65.82 %; yellowish green solid, m.p. 140 °C. Anal. Calcd. for
Cd(C20H20N2O5) (%): C, 49.96; H, 4.16; N, 5.83; Cd, 23.40. Found (%): C, 49.98; H, 4.17; N,
5.81; Cd, 23.38. FT-IR (cm-1): phenolic υ(OH) 3374, azomethine υ(C=N) 1617, υ(C-N) 1372,
phenolic υ(C-O) 1273, υ(H2O) stretching bands of coordinated water 978 and 862, υ(M-O)
stretching bands of coordinated water 573, metal-oxygen bond υ(M-O) 590, metal-nitrogen bond
υ(M-N) 460 Molar conductivity(10−3 M, DMSO): 7.0 ꢀ−1 cm2 mol−1. ꢀeff = dia. λmax (cm-1) 46082
and 40816 (π–π*), 30120 (n–π*).
2.5. Biological Activity
2.5.1. Antimicrobial Activity
Antimicrobial activity of the tested samples was determined using a modified Kirby-
Bauer disc diffusion method [31]. 100 ꢁl of the tested bacteria or fungi were grown in 10 ml of
fresh media until they reached a count of approximately 108 cells/ml for bacteria and 105
cells/ml for fungi [32]. 100 ꢁl of microbial suspension was spread onto agar plates corresponding
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to the broth in which they were maintained. Isolated colonies of each organism that might be
playing a pathogenic role should be selected from primary agar plates and tested for
susceptibility by disc diffusion method. Of the many media available, NCCLS recommends
Mueller-Hinton agar due to: it results in good batch-to-batch reproducibility. Disc diffusion
method for filamentous fungi tested by using approved standard method (M38-A) developed
[33]. For evaluating the susceptibilities of filamentous fungi to antifungal agent. Disc diffusion
method for yeast developed by using approved standard methode (M44-P) [34].
Plates inoculated with Gram (+) bacteria as Staphylococcus aureus and Bacillus subtilis;
Gram (-) bacteria as Pseudomonas aeruginosa, Salmonella SP. and Escherichia coli, they were
incubated at 35-37 ºC for 24-28 hours and yeast fungus as Aspergillus fumigatus and Candida
albicans incubated at 30 ºC for 24-28 hours and, then the diameters of the inhibition zones were
measured in millimeters [34]. Standard discs of Ampicillin and Gentamycin (antibacterial
agents), and Amphotericin B (antifungal agent) served as positive controls for antimicrobial
activity but filter discs impregnated with 10 ꢁl of solvent (distilled water, chloroform, DMSO)
were used as a negative control. The agar used is Meuller-Hinton agar that is rigorously tested
for composition and pH. Further the depth of the agar in the plate is a factor to be considered in
the disc diffusion method. This method is well documented and standard zones of inhibition have
been determined for susceptible and resistant values. Blank paper discs (Schleicher and Schuell,
spain) with a diameter of 8.0 mm were impregnated with 10 ꢁl of tested concentration of the
stock solutions. When a filter paper disc impregnated with a tested chemical is placed on agar,
the chemical will diffuse from the disc into the agar. This diffusion will place the chemical in the
agar only around the disc. The solubility of the chemical and its molecular size will determine
the size of the area of chemical infiltration around the disc. If an organism is placed on the agar,
it will not grow in the area around the disc if it is susceptible to the chemical. This area of no
growth around the disc is known as zone of inhibition or clear zone. For the disc diffusion, the
zone diameters were measured with slipping calipers of the national committee for clinical
laboratory standards [35]. Agar based methods such Etest and disc diffusion can be good
alternatives because they are simpler and faster than broth-based methods [36].
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3. Results and discussion
3.1. Characterization of tetradentate ligand (H2L)
The parent H2L is a yellowish green powder, soluble in various solvents. The elemental
analyses of the new H2L (Experimental part) are consistent with the proposed structure as shown
in Scheme 1.
The symmetric Schiff base ligand was prepared by condensation an appropriate amount
of 2-hydroxybenzaldehyde with m-phenylenediamine in 2:1 ratio using ethanol as solvent. The
formed new Schiff base ligand was characterized using elemental and spectral analyses.
3.2. Mass Spectrum of tetradentate ligand (H2L)
Mass spectrum of the ligand gave vital clues for elucidating the structure and to
determine the stoichiometry of the compound. The mass spectrum of the ligand shows a well
defined molecular ion peak at m/z = 316 corresponding to the [C20H16N2O2]+ ion which
coincides with the formula weight of the Schiff base.
3.3. FT-IR Spectrum of the ligand (H2L)
There is a pair of medium intensity bands present at 3200–3400 cm-1 in the FT-IR
spectrum of m-phenylenediamine corresponding to υ(NH2) stretching vibration. These bands
were disappeared in the spectrum of the Schiff base ligand. Also, no strong FT-IR absorption
band is observed at 1735 cm-1 indicating the absence of υ(C=O) group of 2-
hydroxybenzaldehyde. This indicates that the condensation of carbonyl groups of 2-
hydroxybenzaldehyde and amino groups of m-phenylenediamine has taken place in the reaction
[37]. A new band at 1617 cm-1 corresponding to azomethine υ(C=N) stretching vibration was
recorded due to this condensation reaction confirming the formation of the Schiff base ligand
[38].
3.4. 1 H NMR Spectrum of the ligand (H2L)
1H NMR spectrum of ligand at 300 MHz in DMSO-d6 exhibit a singlet at δ 9.05 ppm (s,
2H) attributed to azomethine (-CH=N-) protons. Other signals are observed in the spectrum of
ligand in the range of δ 6.91-8.71 ppm (m, 12H) due to the aromatic rings system protons. The
singlet signal at δ 13.00 ppm (s, 2H, OH) is assigned to phenolic protons [39].
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3.5. Molecular docking study of the ligand (H2L)
The purpose of studying molecular docking is to simulate the molecular recognition
process by achieving an optimized conformation for the protein and drug with relative
orientation between them such that the free energy of the overall system is minimized [1,40-43].
Molecular docking has been studied between ligand (H2L) and two types of proteins which are
receptors of the LYASE / LYASE INHIBITOR Crystal structure of Escherichia coli MenB in
complex with substrate analogue OSB-NCoA (3T88) and TRANSFERASE
Crystal
structure of Staphylococcus aureus nucleoside diphosphate kinase complexed with ADP (3Q8U).
The docking studies show an acceptable interaction between the ligand (H2L) and the receptor of
each protein as showed in Fig. 1. Fig. 2 shows HB plot curves due to the binding of the ligand
(H2L) to the proteins by hydrogen bonding interactions. The calculated binding energy values are
tabulated in Table 1. 2D plot and HB plot curves of docking with H2L ligand are shown in Fig. 3.
From Table 1, the value of estimated free energy of binding (kcal/mol) indicated that ligand
(H2L) is higher and stronger inhibitory activity against receptor target. According to the results
showed in Table 1, the receptor of Escherichia coli (3T88) shows the best interaction than the
receptor of Staphylococcus aureu (3Q8U) with the ligand (H2L).
3.6. Compositions and structures of the complexes
The proposed structure in Fig. 4 suggests that the molecular formula of the H2L is
C20H16N2O2 which upon chelation coordinates with central atom at four coordination sites and
with two water molecules (except chromium one water molecule and one chloride atom). All
complexes are colored, air stable in the solid state, having high melting points. These complexes
are soluble in DMF, DMSO and ethanol. The results of elemental analysis for the complexes
were in good agreement with the calculated values showing that the complexes have 1:1 metal–
ligand stoichiometric ratio.
3.7. Molar conductance measurements
Molar conductivity (Λm ꢀ-1mol-1 cm2) data was measured using 10-3 M solutions of the
complexes in DMF or ethanol as solvent at 25°C gave the values (43.1, 33.8, 46.8, 45.5, 39.7,
18.1 and 7 ꢀ-1mol-1 cm2) of the Cr(III), Mn(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II)
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complexes with Schiff base ligand, respectively. They are considered non-electrolytes due to the
absence of any counter ions in their structures [44]. In addition Fe(III) complex has molar
conductance value of 63.9 ꢀ-1mol-1 cm2, indicating its ionic nature and is considered as 1:1
electrolyte.
3.8. IR spectra analyses
Information about the coordination mode (metal–ligand bonding) of the Schiff bases to
metal centers was obtained by recording IR transmission spectra in the regions of 4000-400 cm-1
for each new complex and compared the data with free ligand infrared absorptions (Table 2). IR
spectra also provide information about the involving of the solution anions to bond formation
with transition metal ions. Moreover, the appearance of absorption bands in the region 3500–
3200 cm-1 is indicative of the presence of water molecules in the structure of the complexes.
Table 2 summarizes the main IR bands of the Schiff base ligand and its complexes.
The IR spectrum of the ligand displayed a strong band at 1617 cm-1 arises from the
azomethine group υ (-CH=N-) stretching frequency [45]. In the FT-IR spectra of the metal
complexes the azomethine υ (-CH=N-) band is shifted to higher or lower frequency and appeared
at 1615-1651 cm-1 [46] indicating coordination through the azomethine-nitrogen with the metal
ions. The spectrum of free ligand shows strong bands at 1276 and 1386 cm-1, respectively
assigned to υ (C-O) phenolic and υ (C-N) stretching vibrations of the ligand. However, after
complexation via oxygen and nitrogen to the metal ion, these bands were observed at the regions
of 1220-1284 cm-1 and 1365-1383 cm-1 for υ (C-O) and υ (C-N), respectively in the complexes.
The IR spectrum of the free ligand shows a broad band at 3438 cm-1 corresponding to υ (OH)
(phenolic). The presence of absorption bands at 3374-3436 cm-1 for our complexes indicates the
presence of water molecules in those complexes as a coordinated ligand, which was assumed in
the elemental analysis [47]. The new bands observed for the complexes between 537–590 cm-1
and 450–474 cm-1 were metal sensitive and are assigned to υ (M–O) and υ (M–N) [1],
respectively.
The presence of coordinated water is further confirmed by the appearance of non-ligand
bands in the region of 807-872 cm-1 and 937-978 cm-1 assignable to the rocking mode of
coordinated water [48]. The υ(M-O) stretching vibrations of coordinated water attachment to
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metal ions are found at 520-574 cm-1 for the complexes. The presence of coordinated water in all
these complexes is proved by thermal analysis. The presence of lattice water in the complexes is
also confirmed by TG and DTG analysis.
Thus, the IR data suggest that the Schiff base is a binegative tetradentate ligand bound to
the metal ions through the two deprotonated phenolic oxygen atoms and azomethine nitrogens
with ONNO donor atoms.
3.9. 1H NMR spectrum of Zn(II) complex (7)
1
A comparison of the H NMR spectrum of the complex (7) with parent ligand (Table 3)
shows that the OH phenolic signal, occurring in the spectrum of the ligand at δ 13.00 ppm,
completely disappears in the spectrum of its metal complex (7), indicating that the two OH
protons are removed by coordination with the metal ions. In addition, the imino protons bands
are significantly shifted in the complex and appeared at δ 7.95 ppm, due to coordination through
the azomethine nitrogen atoms of the ligand. Aromatic protons are shifted in the spectrum of the
complex and appeared at δ 6.65-7.30 ppm [49-51]. From 1H NMR, it is concluded that the ligand
is binegative tetradentate ligand coordinated to the metal ion through the two nitrogen atoms of
azomethine and the two deprotonated phenolic oxygen.
3.10. Mass spectrum of complex
The mass spectrum of Cd(II) complex shows a molecular ion peak at m/z = 480
corresponding to the formula [Cd(L)(H2O)2].H2O (Calcd. 480.411 g/mol). The proposed formula
of the complex was confirmed by the observed peak at 316 corresponding to the ligand moiety
[(C20H16N2O2) atomic mass 316 amu]. These data confirmed the stoichiometry of the complex as
being of [ML] type.
3.11. UV–Visible spectra
1 x 10-4 M solutions of the prepared ligand and its complexes were recorded over the
wavelength range 200-700 nm in ethanol solvent. The UV–Vis spectrum of the ligand exhibits a
band at 44843 cm-1 attributed to π → π* transition within the free ligand [1,52]. Another
absorption band observed at 26809 cm-1 for the ligand due to n → π* transition between lone-
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pair electrons of the p orbital of the N atom in the azomethine group and the O atom of the
hydroxyl group [22].
3.12. Electronic spectra and magnetic moment measurements
There are three absorption bands at 15,565, 21,861, and 27,700 cm-1in the electronic
spectrum of the Cr(III) complex shows. The three spin-allowed transitions for Cr(III) in an
4
4
4
4
4
4
octahedral field are as follows: A2g(F) → T2g(F), A2g(F) → T1g(F), and A2g(F) → T1g(P)
transitions, respectively. The magnetic moment value of Cr(III) complex is found to be 3.58
B.M. which is in agreement with the presence of three unpaired electrons [53].
The diffused reflectance spectrum of Mn(II) complex exhibits three bands at 27,124,
4
6
4
6
4
19,347 and 15,436 cm-1 which may be assigned to T1g → A1g, T2g(G) → A1g and T1g(D) →
6A1g transitions, respectively, h suggesting the octahedral geometry [54,55]. The magnetic
moment measurement of the Mn(II) complex (5.43 B.M.) lies in the range of octahedral
geometry [28,56] which due to the two coordinated water molecules.
From the diffused reflectance spectrum, it is observed that, the Fe(III) chelate exhibits a
band at 21,453 cm-1, attributed to the 6A1g → T2g(G) transition in octahedral geometry [57]. The
6A1g → 5T1g transition appears to be split into two bands at 15,426 and 13,362 cm-1. The Fe(III)
complex is found to have magnetic moment value of 5.17 B.M., suggesting octahedral geometry
involving sp3d2 hybridization in Fe(III) ion [58].
The electronic spectrum of Co(II) complex shows bands at 13,223, 15,193 and 22,153
4
4
4
4
cm-1 which are attributed to the electronic transition T1g(F) → T2g(F), T1g(F) → A2g(F) and
4T1g(F) → 4T1g(P), respectively, indicating that there is an octahedral geometry around Co(II) ion
[59,60]. The magnetic susceptibility measurement lie at 5.18 B.M. is an indicative of octahedral
geometry.
The ligand field parameters B (interelectronic repulsion of the d-electrons in complexes)
(155 cm-1), β (nephelauxetic effect) (0.16) and 10Dq (1970 cm-1) are calculate [1,28]. These
results show that the B of the d-electrons is less than that in the free ion. The β value is related
directly to covalence. The reduction of B on complex formation is caused by delocalization of
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the d-electron cloud on the ligand, which is in turn caused by the formation of covalent bond.
The data show the Co(II) complex has covalent character [28,23].
Also, the Ni(II) complex exhibits four electronic spectral bands at 26,146, 20,498, 14,593
3
3
and 13,245 cm-1 which may be assigned to charge transfer (LMCT), A2g(F) → T2g(F),
3
3
3
3A2g(F) → T1g(F) and A2g(F) → T1g(P) transitions, respectively. The position of these bands
suggested an octahedral geometry around the Ni(II) ion [61–66]. This geometry is further
supported by its magnetic susceptibility value (2.76 BM). The values of 10Dq and B are 1459
and 191 cm-1, respectively, and β is 0.18. These results show that the interelectronic repulsion of
the d-electron in a complex is less than that in free ion.
The reflectance spectrum of the Cu(II) chelate shows a broad band at 15,463 and 20,498
2
2
cm-1. The Eg and T2g states of the octahedral Cu(II) ion (d9) split under the influence of the
tetragonal distortion and the distortion can be such as to cause the three transitions 2B1g → 2B2g,
2B1g → 2Eg and 2B1g → 2A1g to remain unresolved in the spectrum [53,67]. This assignment is in
agreement with the general observation that Cu(II) d–d transitions are normally close in energy
[53]. The magnetic moment of 1.84 B.M. falls within the range normally observed for octahedral
Cu(II) complex [53, 67]. A moderately intense peak observed in the range 25,664 cm-1 is due to
ligand to metal charge transfer transition [53].
According to the empirical formulae of the Zn(II) and Cd(II) complexes, an octahedral
geometry is proposed for these complexes.
3.13. Theoretical study of the ligand (H2L) and its metal complexes
The electronic properties and geometrical structures of the ligand (H2L) and its metal
complexes were calculated by optimizing their bond lengths and bond angles. The optimized
structures of the ligand (H2L) and its metal complexes are presented in Fig. 5. Bond lengths and
bond angles of the ligand (H2L) and its metal complexes are listed in Tables S1-S8. The HOMO
and LUMO for tetradentate ligand (H2L) and its metal complexes are shown in Fig. 6. Quantum
chemical parameters of the ligand (H2L) and its metal complexes are obtained from calculations
such as energies of the highest occupied molecular orbital (EHOMO) and the lowest unoccupied
molecular orbital (ELUMO) as listed in Table 4. Additional parameters such as HOMO–LUMO
energy gap, ∆E, additional electronic charge, ∆Nmax, global softness, S, chemical potentials, Pi,
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absolute hardness, η, absolute softness, σ, global electrophilicity, ω and absolute
electronegativities, χ, are calculated using the equations [29,42,68] and listed in Table 4.
The values of ꢂE for the ligand (H2L) and its metal complexes (1, 2, 4-8) were found to be
3.063, 1.039, 0.419, 2.864, 0.631, 2.688, 2.575 and 1.920 eV, respectively.
3.14.
Thermal analyses studies
The thermal behavior of ligand and its metal complexes has been characterized (see
Supplementary data) on the basis of a thermogravimetric (TG) method to check the presence of
water molecules in the coordination sphere of metal complexes. The thermal analysis gave useful
data for the thermal stability of the metal complexes. The TG and DTG were recorded within the
temperature range from 30 to 1000 °C (Table 5).
In hydrated complexes, water could be held by bonding to the metal ion or it may be held
as lattice or crystal water. In some of the present complexes, the DTG curve showed that
dehydration occurred at relatively high temperature outside the range for lattice water. Hence,
the water molecules could be considered to be involved in bonding due to the high temperature
associated with this step [69].
The thermal decomposition process of H2L Schiff base ligand with the molecular formula
[C20H16N2O2] involves four decomposition steps. Decomposition of the ligand started at 45°C
and finished at 1000°C with four stages. The first stage of decomposition involves the loss of
C4H6O2 molecule in 45-305°C temperature range, and is accompanied by a weight loss of 27.21
% (calcd. = 27.22 %). The second stage of decomposition occurs in the 305-535°C temperature
range, corresponding to the loss of C4H4N2 molecule, and is accompanied by a weight loss of
25.30 % (calcd. = 25.31 %). The third stage involves the removal of C6H4 molecule in the
temperature range from 535-700°C, and is accompanied by a weight loss of 24.03 % (calcd. =
24.05 %). While the fourth stage involves the removal of C6H2 molecule in the temperature range
from 700-1000°C, and is accompanied by a weight loss of 23.41 % (calcd. = 23.42 %). The total
weight loss amounts to 99.95 % (calcd 100 %).
The thermal decomposition of the complex (1) occurs in the temperature interval of 25–
1000°C. TG curve of the [Cr(L)Cl(H2O)].H2O complex shows five steps of decomposition. The
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first and second stages of decomposition occur in the 25-285°C temperature range,
corresponding to the loss of HCl, N2 and 2H2O molecules, and are accompanied by a weight loss
of 22.74 % (calcd 22.96 %). While the third stage involves the removal of C4H3O0.5 molecule in
the 285–415°C temperature range, and is accompanied by a weight loss of 13.25 % (calcd 13.49
%). The next weight loss of 14.62 % (calcd. 14.63 %) at 415–590°C is attributed to the loss of
C5H4 fragment. Finally, the remaining of the ligand is lost with a weight loss of 23.21% (calc.
23.32 %) at 590–1000°C due to the loss of C8H6 organic moiety, leaving the carbon and
chromium oxide remained as residues. The total weight loss amounts to 73.82 % (calcd 74.4 %).
The TGA curve of complex (2) showed that the thermal decomposition takes place at a
temperature range 10-1000°C. The first and second decomposition steps occurred at 10-135°C
due to weight loss of 12.26 % (Calcd. 12.77 %) corresponded to the elimination of three H2O
molecules. The third stage of decomposition was due to the loss of CH4 molecule at a
temperature range 135-180°C and showed an estimated mass loss of 3.73 % (Calcd. 3.78 %),
while the fourth stage of decomposition at 180-245°C was referred to the loss of 6.02 % (Calcd.
6.15 %) corresponding to the elimination of C2H2 organic molecule. The fifth and sixth
decomposition steps occurred at 245-670°C due to weight loss of 21.72 % (Calcd. 21.75 %)
corresponding to the elimination of C5H4 and N2 molecules. The last step was referred to the loss
of 15.83 % (Calcd. 16.08 %) due to the release of C4H4O ligand molecule to give finally MnO
contaminated with carbonas residue. The total weight loss amounts to 59.56 % (Calcd. 60.53 %).
The complex (3) was thermally decomposed in three successive decomposition steps. The
first step with estimated mass loss of 22.28 % (calculated mass loss = 22.30 %) within the
temperature range 35–295°C may be attributed to the loss of 2.5H2O, HCl and NH3 molecules.
The second step occurred within the temperature range 295-680°C with an estimated mass loss
25.53 % (calculated mass loss = 25.60 %), which correspond to the loss of C8NH3 fragment. The
final step occurred within the temperature range 680-1000°C with estimated mass loss 6.72 %
(calculated mass loss = 6.79 %), which correspond to the loss of C2H6 fragment. The overall
weight loss amounted to 54.53 % (calculated mass loss = 54.69 %). The TG thermogram shows
that the complex gradually decomposed at 35-1000°C, ended with formation of contaminated
metal oxide (½Fe2O3) with carbon and they are left as a residue.
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The thermal analysis of the complex (4) suggested three water molecules per complex
compound. Two of them are coordinated with the metal ion, and the third one is water from
hydration. Thermal analysis of the complex indicated loss of weight in the following four steps:
the first step corresponds to a small weight loss in the range of 10-95°C due to loss of 1.5H2O
molecule. The weight loss amounted to 6.32 % (calculated mass loss = 6.33 %). The second step
corresponds to weight loss in the range of 95-375°C attributable to the loss of 1.5H2O and C4H2
molecules. The weight loss amounted to 17.71 % (calculated mass loss = 18.04 %). The third
step corresponds to gradual weight loss in the range of 375-630°C, which can be assigned to the
loss of C4H6N2 organic moiety. The weight loss amounted to 19.13 % (calculated mass loss =
19.20 %). The last step represents gradual weight loss from 630-1000°C, which can be assigned
to complete decomposition of the ligand moiety with a loss of C3H6O molecule with an
estimated mass loss of 13.39 % (calculated mass loss = 13.59 %). leaving the metal ion CoO
contaminated with carbon as a residue. The overall weight loss amounted to 56.55 % (calcd. =
57.16 %).
TG curve of the complex (5) indicates that the complex is thermally decomposed in four
steps in the temperature range from 15 to 1000°C. The first and second stages of decomposition
occur in the 15-440°C temperature range, corresponding to the loss of C8H2 and 3H2O molecules
of uncoordinated and coordinated water, and are accompanied by a weight loss of 35.10 %
(Calcd. 35.63 %). In the third stage of decomposition, the TG curve for the complex shows a
mass reduction in the temperature range 440-565°C which is due to weight loss of 7.43 %
(Calcd. 7.50 %) corresponding to the elimination of CH3OH organic molecule. The last step of
the thermal decomposition, which occurs in the range 565-1000°C, is assigned to the loss of CH4
and N2H4 organic molecules (observed 10.80 %) (Calcd. 11.25 %) leaving NiO contaminated
with carbon as residue. The overall weight loss amounted to 53.33 % (calcd. = 54.38 %).
The thermal decomposition process of the copper complex (6) occurs in four steps. In the
TG graph of the complex, the weight loss of 4.44 % (calc. 4.63 %) at
10-135°C corresponds
to the departure of the lattice water and the free hydrogen molecule. In the second step, the
weight loss of 18.75 % (calc. 18.77 %) at 135-410°C is associated with the separation of the two
coordinated water molecules and the organic moiety C2H7N. While the third and fourth stages
involve the removal of C5H5NO molecule in the 410-1000°C temperature range, and is
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accompanied by a weight loss of 22.01 % (calcd 22.02 %). The overall weight loss amounted to
45.20 % (calcd. = 45.42 %).
The complex (7) (gave decomposition pattern started at 10°C and finished at 1000°C with
three stages. The first stage was one step within the temperature range of 10-140°C, representing
the loss of H2O (hydrated) with a found mass loss of 4.13 % (calcd. = 4.15 %). The second stage
was also one step representing the loss of 2H2O and C2NH3 molecules with a mass loss of 17.75
% (calcd. = 17.77 %) within the temperature range 140-375°C. The final stage was also one step
representing the loss of C8H11NO molecule with a mass loss of 31.59 % (calcd. 31.61 %) within
the temperature range 375-1000°C. At the end of the thermogram, the metal oxide ZnO was the
residue, which was contaminated with carbon. The overall weight loss amounted to 53.47 %
(calcd. = 53.53 %).
The thermal decomposition process of the complex (8) occurs in three steps. In the TG
graph of the complex, the weight loss of 5.59 % (calcd. 5.62 %) at 10-225°C at the first step
corresponds to the departure of 1.5H2O molecules. The weight loss of 13.92 % (calcd. 13.95 %)
at the second step at 225-485°C is associated with the separation of 1.5H2O and C3H4 molecules,
while the last step with a weight loss of 53.35 % (calc. 53.70 %) at 485-1000°C is attributed to
loss of C14H6N2 and C3H4O organic moieties. The residual weight of 27.14 % (calc. 26.73 %)
corresponds to cadmium oxide, CdO molecule. The overall weight loss amounted to 72.86 %
(calcd. = 73.27 %).
3.15. Scanning Electron Microscope (SEM) analysis
The SEM images of free ligand H2L and its Cd(II) and Co(II) complexes showed
different morphologies as shown in Fig. 7. The SEM image of ligand seemed as irregularly
large-sized sheets with average size 88 nm. The SEM image of Co(II) complex seemed as glass
sheets crystals with average size 61 nm. The SEM image of Cd(II) complex seemed as
irregularly perpendicular walls with some rods with average size 49 nm.
3.16. Structure interpretation
From the IR spectra, it is concluded that H2L behaves as bi-negative tetradentate ligand
coordinated to the metal ions via the two deprotonated phenolic oxygen and two azomethine
nitrogen atoms. The 1H NMR spectra of the free ligand and the diamagnetic complex (7) showed
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that the deprotonated OH groups and azomethine nitrogen atoms of H2L ligand participate in
chelation. The analytical data were in good agreement with the proposed molecular formula of
the ligand and the complexes. From conductance measurements, the complexes are non-
electrolytes except Fe(III) complex, it is found to be 1:1 electrolyte. The general formulae of the
complexes are suggested as the following:
[Cr(L)(H2O)Cl] H2O, [M(L)(H2O)2] H2O, (M = Mn(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II))
and [Fe(L)(H2O)2] Cl.
On the basis of the above observations and from the magnetic and solid reflectance
measurements, octahedral geometry is suggested for the investigated complexes. The structures
of complexes are shown in Fig. 4.
3.17. Biological activity
The biological activity of the ligand and its metal complexes (Table 6) were studied in
vitro against two fungi species Aspergillus fumigates and Candida albicans, two gram positive
bacteria Staphylococcus aureus and Bacillis subtilis and three gram negative bacteria Salmonella
SP., Escherichia coli and Pseudomonas aeruginosa by using diffusion technique. The inhibition
zone ion was measured directly from the scale and compared with DMSO as blank and standard
compounds. Ampicillin and Gentamycin illin were used as reference compounds for antibacterial
activity and Amphotericin B was used as a reference compound for antifungal activity.
The increased activity of the metal chelates can be explained on the basis of chelation
theory [70, 71]. According to the concept of cell permeability, the lipid membrane surrounds the
cell favours the passage of only lipid-soluble materials due to the fact that liposolubility is
considered to be an important factor that controls antimicrobial activity. On chelation, the
polarity of the metal ion will be reduced to a greater extent due to the partial sharing of positive
charge of metal ions with donor groups and the overlap of the ligand orbital. Furthermore, it
increases the delocalization of the electrons over the whole chelate ring and enhances the
lipophilicity of the complex. This increased lipophilicity enhances the penetration of the
complexes into the lipid membrane and thus blocks the metal binding sites on enzymes of
microorganisms. These metal complexes also disturb the respiration process of the cell and thus
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block the synthesis of proteins, which restricts further growth of the organism. The variation in
the activity of different complexes against different organisms depends either on the
impermeability of the microbe’s cells or difference in the microbe’s ribosomes.
The best antimicrobial activities of the ligand (H2L) agreed its predicted best inhibitory
effect to receptors of Escherichia coli (3T88) and Staphylococcus aureus (3Q8U). These results
are found to be in good agreement with the similar results were obtained recently [72].
The bacterial growth inhibitory capacity of the complexes was compared to that of the
ligand. It was found that Co(II) and Cd(II) complexes showed the most antibacterial activities
against Staphylococcus aureus, Bacillus Subtilis, Escherichia coli and Pseudomonas aeruginosa
(Fig. 8), while Mn(II) complex showed the lowest activity against these microorganisms. The
high activity of the parent ligand and its Co(II) and Cd(II) complexes may be related by their
nano sized structures that is proven depending on SEM imaging. All complexes showed
antifungal activities higher than the ligand against Aspergillus fumigates and Candida albicans
organisms except for Fe(III) complex which have lower antifungal activity against Candida
albicans than the ligand. Of all complexes, Cd(II) showed the highest antifungal activitiy against
Candida albicans and Aspergillus fumigates.
4.
Conclusion
The data of Schiff base ligand (H2L: 2,2'-((1E,1'E)-(1,3 phenylene bis (azanylylidene))
bis (methanylylidene))diphenol) derived from m-phenylenediamine and 2-hydroxybenzaldehyde
showed that the chelation occurs through the deprotonated phenolic oxygen and azomethine
nitrogen atoms of the ligand and the complexes have composition of ML type. The nature of
bonding and the stereochemistry of the complexes were deduced from elemental analysis,
infrared spectra, magnetic moments and conductivity measurements and thermal analysis. From
magnetic moment and electronic spectral studies, all complexes showed an octahedral geometry
around the transition metal ions. The presence of coordinated water molecules in all the
complexes has been further confirmed from thermal analysis. The antibacterial studies of the
prepared compounds screened against pathogenic bacteria proved that these compounds
exhibited selective antibacterial activity. The complexes are more active as compared to their
free ligand. This implies an increase in the antibacterial activity with coordination.
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Table 1. Energy values obtained in docking calculations of the ligand (H2L) with receptors of
Escherichia coli (3T88) and Staphylococcus aureu (3Q8U).
Estimated
free energy of
binding
vdW+
bond+
desolv
Total
intercooled
Estimated
inhibition
constant
Electrostatic
Interact
surface
Receptors
Energy
(kcal/mol)
(kcal/mol)
Energy
(kcal/mol)
energy
(kcal/mol)
(Ki) (µM)
3T88
-6.44
-5.78 kcal/mol
18.99
-8.18
-0.08
-8.26
808.28
870.03
3Q8U
-7.30
kcal/mol
-0.12
kcal/mol
-7.41
kcal/mol
58.39 uM
26

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Table 2. IR spectra (4000-400 cm-1) of H2L ligand and its metal complexesa
1
2
3
4
5
6
7
8
Assignme
nt
H2L
3438br
1617sh
1386sh
1276sh
3319
1645sh
1378sh
1275sh
896,
3426br
1647sh
1378sh
1275sh
869,
3429br
1640sh
1371sh
1284sh
872,
3428br
1646sh
1382sh
1238sh
853,
3414br 3436br 3424br 3374br
1645sh 1615sh 1651sh 1617sh
1365sh 1383sh 1375sh 1372sh
1237sh 1220sh 1277sh 1273sh
OH stretching
CH=N stretching
C-N stretching
C-O phenolic
H2O stretch of
coordinated water
M-O
807,
815,
953s
537s
862,
962sh
586sh
862,
-----
-----
-----
-----
932sh
543s
960sh
543s
958sh
580s
937s
971sh
584sh
978sh
590sh
579s
M-O stretching of
coordinated water
M-N
535s
530s
570s
538s
459s
560s
520s
574s
573s
460s
479w
450w
455w
466sh
452w
474sh
aNumbers are given in Experimental part.
sh = sharp, br = broad, s = small, w = weak, m = medium
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Table 3. 1H NMR spectral data of H2L ligand and complex 7.
Chemical shift,
Compounda
Assignment
(δ) ppm
13.00
(s, 2H,OH)
9.05
(s, 2H, CH=N)
H2L
6.91-8.71
disappeared
7.95
(m, 12H, benzene rings of H2L)
(s, 2H,OH)
(s, 2H, CH=N)
Complex 7
6.65-7.30
(m, 12H, benzene rings of H2L)
aNumbers are given in Experimental part.
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Table 4. The calculated quantum chemical parameters of the ligand (H2L) and its metal
complexes
Compounda EHOMO ELUMO ꢀE
(eV) (eV) (eV)
χ
η
σ
Pi
S
ω
∆Nmax
(eV) (eV)
(eV)-1
(eV)
(eV)-1
(eV)
-7.555 -4.492 3.063 6.024 1.532 0.6529 -6.024
-10.435 -9.396 1.039 9.916 0.519 1.9249 -9.916
0.3265
11.8454
3.9331
H2L
1
0.9625
2.3866
0.3492
1.5848
0.3720
0.3883
0.5208
94.6267
210.5467
6.5072
19.0866
-9.602 -9.183 0.419 9.393 0.209 4.7733 -9.393
-5.749 -2.885 2.864 4.317 1.432 0.6983 -4.317
-8.802 -8.171 0.631 8.487 0.316 3.1696 -8.487
-6.787 -4.099 2.688 5.443 1.344 0.7440 -5.443
-7.079 -4.504 2.575 5.792 1.288 0.7767 -5.792
-6.889 -4.969 1.920 5.929 0.960 1.0417 -5.929
44.8329
3.0147
26.8986
4.0499
4.4983
6.1760
2
4
5
6
7
8
114.1374
11.0217
13.0258
18.3089
aNumbers are given in Experimental part.
29