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Introduction
Schiff bases are organic compounds resulting from the reaction of an aldehyde or a ketone with primary amines to give the imine functional group, which can coordinate with metals due to the presence of a lone pair on nitrogen atom. They may be monodentate or polydentate, depending on the number of donor atoms in the ligand [1,2]. The presence of the C=N functional group gives Schiff's bases great importance in various industrial, agricultural, and nanotechnology applications [3]. Many Schiff base complexes have been given sufficient attention by chemists all over the world [4]. Complexes derivative from Schiff bases have been widely used in biological applications, like inhibition of bacterial growth and treatment of cancerous diseases [5]. The current study depicts the coordination behavior of the complexes of Co (II), Cu (II), Cd (II), and Pt (II) that derived from the condensation of 3,3'-dimethyl-4,4'-diaminobiphenyl with salicylaldehyde and in a ratio (2:1). Studying the effect of its complexes on three types of bacteria: Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aurous and comparing their effectiveness with chloramphenicol, and gentamycin antibiotics.
Materials and Mthods
The chemical substances used were of high purity and used without purification. The chemicals: salicylaldehyde, orthotolidine, glacial acetic acid, and CoCl2.6H2O from (Sigma), CuCl2.2H2O, CdCl2, and PtCl2 (BDH), absolute ethanol, diethyl ether. Melting points were measured by an electro thermal 9300 instrument. Elemental analyses were performed using an elementar vario ELIII device. Conductivity measurements were done by conductivity meter (Cond 7Il0) with DMSO solutions, the magnetic susceptibility was measured at 25 °C using a magnetic susceptibility balance (Sherwood scientific). The FT-IR spectra carried out ALPHA II FTIR Spectrometer from Bruker Optics was measured in the range 400-4000 cm-1. The electronic spectra were measured using UV Tg +92 spectrophotometer device with a measurement range of (190-1100) with 10-3 M solutions of DMSO at 25 °C. 1H-NMR spectra were recorded in DMSO-d6 solutions using Varin 500 MHz spectrometers. Scanning electron microscope was done by a ZEISS MODEL SIGMA VP device in Iran.
Synthesis of Schiff base 3,3-dimethyl- N,N'-bis[(E)-Arylmethylidene]biphenyl-4,4'- diamine (L)
Salicylaldehyde (0.5 g, 4 mmol) was dissolved in absolute ethanol 15 mL and added dropwise to orthotolidine (0.424 g, 2 mmol) in absolute ethanol 20 mL with 5 drops of glacial acetic acid as catalyst. The mixture was refluxed for six hours at 78 °C. During the reflux time the solution volume was reduced until a light-yellow precipitate product appeared, and then the resultant product was collected, recrystallized with cooled ethanol, washed with diethyl ether, and dried at 50 ºC to produce 90% yield of L with melting point of 195-197 °C (Scheme 1).
Scheme 1: Synthesis of 3,3-dimethyl- N,N'-bis[(E)-Arylmethylidene]biphenyl-4,4'- diamine (L)
Synthesis of Complexes]M2(L)Cl4], M = Co (II), Cu (II), Cd (II), and Pt (II)
Divalent metals salt (1.0 mmol, 0.237 g, 0.170 g, 0.183 g, or 0.266 g) were dissolved in a hot mixture of water and ethanol (1:1) 25 mL. The free ligand (0.5 mmol, 0.21 g) was dissolved in hot ethanol 25 mL. Furthermore, divalent metal solutions were added gradually with stirrer to the free ligand solution and refluxed for two hours at 78°C. The resultant complexes were precipitated, filtered, and dried under vacuum to produce pure complexes in the general formula [M2(L)Cl4], where M = Co (II), Cu (II), Cd (II) or Pt (II). Moreover, physical appearance, melting point, and micro analysis for the elements, conductivity, and yield are listed in Table 1 (Scheme 2).
Scheme 2: Synthesis process of Co (II), Cu (II), Cd (II), and Pt (II) complexes
Table 1: Physical properties, analytical, and molar conductivity data of the prepared ligands and their complexes
No.
Compounds
Yield
%
Color
M.P
0C
Molar ᴧ cond./ mol-1, cm-1 hom
C% cal
H% cal
N% cal
(C%) found
(H%) found
(N%) found
1
L
90%
Light yellow
195-197
8.25
6.65 (6.68)
5.70 (5.52)
79.9 (79.7)
2
[Co2(L)Cl4]
75%
Green
278-280
10.16
4.11 (4.15)
3.52 (3.47)
49.4 (49.2)
3
[Cu2(L)Cl4]
80%
Orange
260-262
22.50
3.78 (3.76)
3.24 (3.21)
45.4 (45.29)
4
[Cd2(L)Cl4]
77%
Yellow
273-275
19.42
3.55 (3.53)
3.04 (3.10)
42.6 (42.7)
5
[Pt2(L)Cl4]
70%
Brown
290-292
23.11
2.94 (2.86)
2.52 (2.54)
35.2 (35.1)
Anti-bacterial activity
The inhibitory activity of the prepared complexes was tested against three types of bacteria. E.Coli, Pseudomonas aeruginosa, and Staphylococcus aureus. Bacterial media were inoculated in broth (inoculation medium). The medium was inoculated by incubation at 37°C for 24 hours, and then the inoculation medium containing the culture medium was added, in a sterile manner, i.e. the nutrient medium for 24 hours and mixed well to obtain a uniform distribution. Thereafter, the solution (25 ml) was poured into each Petri dish, and then left at room temperature. The wells (6 mm) were cut into agar plates. Using sterile tubes, the wells are filled with (0.1 ml) of the prepared complexes that were dissolved in dimethyl sulfoxide and left for one hour. Next, they were incubated at 37°C for 24 hours, and then the diameter of inhibition zones was read. The minimum inhibitory concentrations were determined using the serial dilution method
Results and discussion
Molar conductivity
The molar conductivity of the prepared complexes was measured at (0.001 M or 1 mM) in DMSO solvent. The lower values of conductivity ohm-1.cm2.mol-1) for all complexes indicate that the complexes are non- electrolytic, as presented in Table 1 [6].
UV-Visible spectra and magnetic properties
The electronic spectra of the [Co2(L)Cl4]complex showed three peaks at 885 nm, 397 nm, and 250 nm which are assigned to the 4A2 (F) → 4T1 (P), n→π*,and MLCTtransitions, respectively. These values confirm that the complex is tetrahedral geometry around the Co (II) [7,8]. The value of the magnetic moment 4.23 B.M shows that the tetrahedral has a high value indicate second order orbital contribution [8], as provided in Table (2). The [Cu2(L)Cl4]complex showed three peaks at 525, 290, and 260 nm which are assigned to the 2B1g→2E2g,2B1g→2A1g, n→π*, and MLCTtransitions, respectively. These values are attributed to the fact that the complex took the form of a square planar geometry around of copper (II) [9,10]. It was found that the value of the magnetic moment is (1.78) B.M and confirming that the shape is square planar geometry [8]. The spectra of the cadmium (II) complex show two bands 306 nm and 242 nm, that are attributed to n→ π*and π→ π*.The electronic spectra of [Pt2(L)Cl4] complexes exhibited bands 670, 530, and 265 transitions related to 1A1g→1T1g, 1A1g→1T2g, and MLCT charge transfer of square planar environment [11]. The UV-Vis data are indicated Table 2 (Figures 1-3).
Table 2: Electronic spectra and magnetic moments of the prepared complexes
No.
Compounds
wave length (nm)
Wave number (cm-1)
Electronic Transitions
μeff
B.M
Geometric structure
1
[Co2(L)Cl4]
885
397
250
11299
25189
40000
4A2 (F) → 4T1 (P)
MLCT
3.97
Tetrahedral
2
[Cu2(L)Cl4]
525
290
260
19048
34483
38462
2B1g→2E2g
2B1g→2A1g
LCT
1.88
Square planer
3
[Cd2(L)Cl4]
306
242
32680
41322
MLCT
Dia
Tetrahedral
4
[Pt2(L)Cl4]
760
530
265
13158
18868
37736
1A1g→1T1g
1A1g→1T2g
MLCT
Dia
Square planer
Dia=diamagnetic
Figure 1: UV-Vis spectrum of the [Co2(L)Cl4] complex
Figure 2: UV-Vis spectrum of the [Cu2(L)Cl4] complex
Figure 3: UV-Vis spectrum of the [Pt2(L)Cl4] complex
FT- IR spectral studies
The complexes showed characteristic absorption bands similar to the absorption peaks of the ligands, with a slight difference in the values which indicates chelation [12-14]. The complexes spectra displayed peaks at (1624-1626) cm−1 and (424-429) cm-1 due to γC=N of azomethine nitrogen and (M-N), respectively [15,16]. The absence of absorption bands at 3400 of phenolic hydrogen in the complexes indicates the coordination of metals with ligand and the absorption bands are shown in Table 3 (Figures 4-7).
Table 3: FT-IR data of ligands and their metal complexes
No.
Compounds
ʋ(C=H) cm-1
ʋ(C-H) ʋArom cm-1
ʋ(C-H) ʋAliph. cm-1
ʋ(O-H) cm-1
ʋ (M–N)
(azomethine) cm-1
1
L
1610 ,1584
3072
2894
3440
----
2
[Co2(L)Cl4]
1624, 1593
3072
2896
-----
416
3
[Cu2(L)Cl4]
1626,1594
3085
2896
-----
429
4
[Cd2(L)Cl4]
1625,1593
3072
2998
-----
424
5
[Pt2(L)Cl4]
1625,1574
3072
2963
------
424
Figure 4. IR spectrum of complex [Co2(L)Cl4]
Figure 5: IR spectrum of complex [Cu2(L)Cl4]
Figure 6: IR spectrum of complex [Cd2(L)Cl4]
Figure 7: IR spectrum of complex [Pt2(L)Cl4]
1H-NMR spectral studies
NMR spectroscopy measurements are very important in the diagnosis of organic and inorganic compounds, as they give important evidence to the nature of the chemical composition in solution [17]. Figure 8 depicts 1H-NMR (DMSO-500MHz) δ= 11.728 (s, 2H, 2*OH), 7.69-7.87 (m, 14H, of phenyl groups, 2.30-2.51(m, 6H, 2*CH3), and (2H, 2CH=N) (Figure 8).
Figure 8: 1H-NMR spectrum of ligand
Determination of antibacterial activity
The effect of the prepared complexes[Co2(L)Cl4], [Cu2(L)Cl4], [Cd2(L)Cl4], and [Pt2(L)Cl4] on three types of bacteria: Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa were studied, and dimethyl sulfoxide (DMSO) was used as a solvent [18,19]. A control model was performed for the solvent, which gave an activity greater than compared with the antibiotics Chloramphenicol and Gentamycin as follows:
For Escherichia coli bacteria (Gram-negative), the [Co2LCl4] complex showed low activity at concentrations of 50 and 100 µg/ml and high activity at concentrations of 150 µg/ml, while its complex [Cu2(L)Cl4] showed a high inhibitory ability from the other complexes at concentration 150 µg/ml against Pseudomonas aeruginosa. For Staphylococcus aureus, the [Pt2(L)Cl4] complex showed high inhibition activity, from other complexes [20], as indicated in Table 4 (Figure 9).
Table 4: Inhibiting activity of synthesized compounds comparison with antibiotics (inhabiting diameter mm)
Compounds
Bacteria
E.coli
Bacteria
Pseudomonas aeruginosa
Bacteria
Staphylococcus aureus
Zone of Inhibition (mm)
Conc.
(µg/m)
50
Conc.
(µg/m)
100
Conc.
(µg/m)
150
Conc.
(µg/ml)
50
Conc.
(µg/m)
100
Conc.
(µg/ml)
150
Conc.
(µg/ml)
50
Conc.
(µg/ml)
100
Conc.
(µg/ml)
150
[Co2LCl4]
24
30
55
10
12
25
11
22
38
[Cu2LCl4]
13
33
44
23
42
54
15
27
42
[Cd2LCl4]
16
25
45
7
13
26
17
24
37
[Pt2LCl4]
5
33
62
2
6
5
13
33
65
Chloramphenicol (30 mg/disc)
16
18
15
Gentamycin
(10 mg/disc)
14
13
15
Figure 9: Graphical representation of biological activities of complexes against three bacteria
Scanning electron microscopy (SEM) studies
The SEM micrographs of complexes are presented in Figure 10. The SEM image of these complexes molecules are arranged in plate-shaped structure [13]. The particle size of the prepared complexes was on the order of a few microns in diameter. Therefore, particles whose size is close to or less than 100 nm, that is agglomerates of larger size, have been also observed [7] (Figure 10).
Figure 10: SEM image of (a) [Co2(L)Cl4], (b) [Cu2(L)Cl4], (c) [Cd2(L)Cl4], and (d) [Pt2(L)Cl4] complexes
Conclusion
In this study, binuclear Schiff base complexes were prepared. The measurements which confirms the tetrahedral geometry around Co(II) and Cd(II) as well as the square planar geometry around Cu(II) and Pt(II). The molar conductance confirms the non-electrolytic of complexes. The biological effectiveness study also confirmed that the Pt(II) complex at a 150 (µ /ml) concentration had the highest effect on the E.coli and Staphylococcus aureus bacteria, and the Cu(II) complex had the best effect on the Pseudomonas aeruginosa bacteria. The SEM study showed that morphology particles of the complexes are different in size and are within the range of nanoparticles.
Disclosure Statement
No potential conflict of interest was reported by the authors.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Authors' Contributions
All authors contributed to data analysis, drafting, and revising of the paper and agreed to be responsible for all the aspects of this work
ORCID
Hasan.A.Mohammed
https://orcid.org/0000-0002-1179-7604
HOW TO CITE THIS ARTICLE
Hasan.A.Mohammed, Umeed Maaroof Ali, Qasim Rabea Abdullah. Synthesis, Characterization, Biological Activity, and Scanning Electron Microscopy Studies of Schiff Base Binuclear Complexes Co (II), Cu (II), Cd (II), and Pt (II) Derivative from Tolidine with Salicylaldehyde. Chem. Methodol., 2023, 7(7) 594-604
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