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antifungal activity and thermal behavior
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JIEC-1622; No. of Pages 8 Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx
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Some new cadmium complexes: Antibacterial/antifungal activity and thermal behavior Morteza Montazerozohori a,*, Saeedeh Zahedi a, Masoud Nasr-Esfahani a, Asghar Naghiha b a b
Department of Chemistry, Yasouj University, Yasouj 75918-74831, Iran. Department of Animal Sciences, Faculty of Agriculture, Yasouj University, Yasouj, Iran.
A R T I C L E I N F O
Article history: Received 13 July 2013 Accepted 14 October 2013 Available online xxx Keywords: Schiff base Complex Spectral In vitro Symmetric Thermodynamic
A B S T R A C T
Four new cadmium(II) Schiff base complexes formulated as CdLX2 (X = Cl, Br, I, SCN and L = bis (3(4dimethylaminophenyl)-allylidene)-1,2-diaminoethane) were synthesized. All novel compounds were identified by spectroscopic techniques including FT-IR, 1H and 13C NMR, UV–vis spectra and physicochemical methods such as molar conductivity, elemental analysis and melting point. Based on the physical and spectral evidences, the pseudo-tetrahedral geometry was proposed for all complexes. The in vitro antibacterial/antifungal activity of each compound was screened against four bacterial strains such as Staphylococcus aureus, Bacillus subtilis, Escherichia coli and Pseudomonas aeruginosa as well as two fungal pathogens such as Candida albicans and Aspergillus niger. The biological testing data showed that all compounds were antimicrobial active against both Gram-positive and Gram-negative bacteria and two fungal strains. The bactericidal/fungicidal properties of compounds showed that the cadmium complexes were more bioactive than the parent ligand. Furthermore, the thermal behavior of all compounds was studied. The ligand was completely destructed through three temperature stages. Among the cadmium complexes, the CdL(NCS)2was found as the most stable while the CdLCl2 showed the least thermal stability. Besides some activation thermodynamic parameters such as E*, DH*, DS*, DG* were calculated using thermo-grams data by Coats–Redfern equation. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Schiff base compounds are classified as a category of organic chemicals that display remarkable properties such as structural resemblance with natural biomaterials and easily flexible preparation giving those opportune structural characteristics [1]. Schiff base compounds can also be considered as a great group of effective materials in clinical and pharmaceutical chemistry due to their biological activities including anti-tumoral [2], antifungal [3–5], antibacterial [6,7], antimicrobial [8], and anthelmintic properties [9]. Furthermore, they are significant intermediates for preparation of some active biomolecules [10,11] and are applied for planning some metal complexes relevant to combinational and natural oxygen transports [12–14]. Schiff base and their complexes have attracted special attention because of their interesting characteristics such as their potency in redox reaction in bio-systems and DNA oxidation [15], photochromic traits, coordination ability to some toxic metal ions [16]. In recent years, many researches have been reported on
* Corresponding author. Tel.: +98 7412223048; fax: +98 7412223048. E-mail address: [email protected] (M. Montazerozohori).
the chemistry of the cadmium(II) complexes [17]. Cadmium(II) compounds prevent from activity of RNA polymerase in vitro [18,19], and are combined easily with biomolecules such as proteins[20]. Some biological properties of cadmium compounds, particularly its Schiff base complexes have encouraged the chemists to develop the preparation of novel cadmium coordination compounds. For instance, in 2008, Spinu et al. synthesized a bidentate Schiff base ligand entitled as N-(2-thienylmethylidene)-2-aminopyridine (TNAPY) and its pseudo-tetrahedral Cd(II) complexes. They performed the in vitro antibacterial testing against three bacterial pathogens by the disk diffusion method. The achievements showed that the metal complexes were more antibacterial active than the primary ligand [21]. In 2012, the synthesis and antibacterial activities of a Schiff base ligand and its tetrahedral Cd(II) complexes were investigated by Singh using the disk diffusion procedure. The antibacterial activities measurements revealed that the chelation enhances the biological properties against the tested bacteria [22]. In 2009, Reiss et al. synthesized a new NO-donor Schiff base named as N-(2furanylmethylene)-3-aminodibenzofuran and its Cd(II) complexes and then tested them for the antibacterial activity against three bacterial strains by the disk diffusion technique. The inhibition values displayed that all compounds prevented the
1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.10.027
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means of Metrohm-712 conductometer equipped with a dip-type conductivity cell. Thermo-gravimetric diagrams were recorded on Perkin–Elmer Pyris model with a heating rate of 10 8C min1 under nitrogen atmosphere.
bacterial growth while the antimicrobial activity was affected by the coordination of the ligand [23]. Halli et al. prepared a Schiff base ligand derived from benzofuran-2-carbohydrazide and pchloroacetophenone and its four-coordinated Cd(II) complexes and then tested the antibacterial/antifungal activity of them. In this study, it was found that the fungicidal/bactericidal activities of complexes were higher than the primary ligand [24]. Among many others, the notable biological activity of cadmium complexes led us to a new research on them. Accordingly, in continuation of our investigation on bidentate Schiff bases and their transition metal complexes [25–29], our target in this work is the synthesis, identification and the antibacterial/antifungal evaluation of a symmetric N2-donor Schiff base ligand entitled as bis(3 (4-dimethylaminophenyl)-allylidene)-1,2-diaminoethane and its new cadmium(II) complexes. Furthermore, the thermal behavior of the ligand and its cadmium complexes are described toward to each other.
2.3. Synthesis of ligand (L) The Schiff base ligand of bis(3(4-dimethylaminophenyl)-allylidene)-1,2-diaminoethane was synthesized from a condensation reaction between ethylene diamine (0.5 mmol) and 4-dimethylaminocinamaldehyde (1 mmol) in ethanol under sever stirring for 5 h. After completion of the reaction, the orange precipitate of ligand was filtered, washed and then recrystallized from dichloromethane/ ethanol solvent mixture (1:1) to afford the pure ligand in 54% yield. Some important physical and spectral (IR and UV–vis) information has been reported in Tables 1 and 2. The 1H and 13C NMR data of ligand based on Scheme 1:1H NMR (in CDCl3): 8.02 (d, 2Hb,b0 , J = 8.80 Hz), 7.37 (d, 4He,e0 , J = 8.76 Hz), 6.87 (d, 2Hd,d0 , J = 15.84 Hz), 6.73 (dd, 2Hc,c0 , J = 15.92, J = 8.80 Hz), 6.68 (d, 4Hf,f0 , J = 8.80 Hz), 3.81 (s, 4Ha,a0 ), 3.01 (s, 12Hg,g0 ) ppm. 13C NMR (in CDCl3): 164.79 (C8,80 ), 150.96 (C2,20 ), 142.34 (C6,60 ), 130.54 (C7,70 ), 128.63 (C4,40 ), 123.62 (C5,50 ), 112.04 (C3,30 ), 61.89 (C9,90 ), 40.27 (C1,10 ) ppm.
2. Experimental 2.1. Materials 4-dimethylaminocinamaldehyde, ethylene diamine, cadmium halides and other chemicals were provided by Aldrich, Merck or BDH companies. Cadmium(II) thiocynate was prepared in accordance with our previous report [30].
2.4. Synthesis of cadmium(II) complexes The synthesized Schiff base ligand dissolved in dichloromethane (0.5 mmol, 15 mL) was added to an ethanolic solution of cadmium salts (0.5 mmol, 10 mL). The mixture was severely stirred on a magnetic stirrer at room temperature for 2–3 h. After this period of time, the appeared precipitate was filtered and washed with absolute ethanol twice. Then, to obtain a pure product, the precipitates were recrystallized from dichloromethane/ethanol solvent mixture (1:1). Finally, the cadmium complexes were dried under vacuum and kept in a desiccator. Some characteristic physical and spectral data (IR and UV–vis) are tabulated in Tables 1 and 2. The 1H and 13C NMR data of cadmium complexes based on Scheme 1 are found to be as follow: [CdLCl2]: 1H NMR (in CDCl3): 8.16 (d, 2Hb,b0 , J = 9.08 Hz), 7.50 (d, 4He,e0 , J = 8.88 Hz), 7.13 (d, 2Hd,d0 , J = 15.44 Hz), 6.99 (dd, 2Hc,c0 , J = 15.26 Hz, J = 9.32 Hz), 6.69 (d, 4Hf,f0 , J = 8.92 Hz), 3.83 (s, 4Ha,a0 ), 3.07 (s, 12Hg,g0 ) ppm. 13CNMR (in CDCl3): 170.00 (C8,80 ), 152.00
2.2. Methods The FT/IR spectra of compounds were obtained by JASCO-FT/ IR680 system in the 400–4000 cm1 region using the KBr pellets. A JASCO-V570 spectrophotometer instrument was used to record the electronic absorption spectra in the chloroform and in the range of 200–800 nm. The 1H and 13C NMR spectra were recorded on a Brucker DPX FT/NMR 400 MHz spectrometer in CDCl3 using TMS as internal standard. The percent of C, H and N elements of ligand and complexes were evaluated by employing an elemental analyzer apparatus. A BUCHI B-545 instrument was applied for determining the ligand’s melting point and also the decomposition temperature of cadmium complexes. The molar conductance measurements of compounds were performed in chloroform at room temperature by Table 1 Analytical and physical data of the Schiff base ligand and its Cd(II) complexes. Run
Compound
Color
Decomposition temp. (8C)
Yield (%)
1 2 3 4 5
Ligand CdLCl2 CdLBr2 CdLI2 CdL(NCS)2
Orange Orange Orange Yellow Orange
205a 253 250 253 204
54 76 62 90 80
a
L8M (cm2 V1 M1)
Found (calc.)% C
H
N
76.7(76.97) 51.8(51.67) 43.9(44.57) 38.7(38.91) 51.9(51.78)
7.9(8.07) 5.6(5.42) 4.3(4.68) 4.2(4.08) 4.9(5.01)
14.8(14.96) 10.3(10.04) 8.4(8.66) 7.7(7.56) 14.1(13.93)
0.004 0.006 0.013 0.179 0.014
Refers to melting point.
Table 2 Vibrational (cm1) and electronic spectral data of the Schiff base (L) and its cadmium(II) complexes. Compound
n(CH)imine
n(C5 5N)
n(SCN)
n(M–N)
n(C5 5C)
l (nm), e (cm1 M1)
Ligand
2846(w), 2804(w)
–
–
2854(w), 2805(sh)
–
437(w), 475(w)
CdLBr2
2854(w), 2810(w)
–
428(w), 468(w)
CdLI2
2850(sh), 2810(w)
–
437(w), 471(w)
CdL(NCS)2
2858(w), 2810(w)
2073 (vs)
431(w), 478(w)
1441(w), 1459(w) 1440(w), 1483(w) 1439(w), 1482(w) 1438(w), 1483(w) 1437(w), 1482(w)
265(31819), 362(65239)
CdLCl2
1599 (vs) 1589 (vs) 1590 (vs) 1589 (vs) 1584 (vs)
260(21332), 416(65,600) 261(36,228), 416(81,997), 494(20,225) 242(45,342), 415(79,115) 262(23,727), 429(84,227), 503(11,097)
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9',a'
9,a 8,b
N
N
X
7,c
5
6',d' 7',c'
X
2
2.6. Minimum inhibitory concentration (MIC)
5'
4,e
H3C
inhibition zone surrounding the discs in millimeter (Table 3). DMSO has no effect on biological activity of compounds and this is considered as a negative control test.
8',b'
Cd
6,d
The MICs of antibacterial samples were evaluated by means of broth dilution method. In the serial dilution procedure, all samples were dissolved in DMSO with various concentrations from 0.975 to 5000 mg/mL in the sterilized conditions. Then, 650 mL of Muller Hinton Broth (Scharlab) and 100 mL of tested microorganism were added to each tube containing sample. After incubation of sample tubes at 37 8C for 24 h, the lowest concentration of an antibacterial sample that prevents from obvious growth of microorganism based on turbidity of mixtures is specified as the MIC [33] (Table 4).
4',e'
3,f
3',f'
2'
N
N CH3
CH3
H3C
1,g
3
1',g'
Scheme 1. Structural formula of ligand and its cadmium complexes.
2.7. Minimum bactericidal concentration (MBC)
(C2,20 , C6,60 ), 130.90 (C4,40 ), 122.10 (C5,50 ), 118.85 (C7,70 ), 111.78 (C3,30 ), 60.00 (C9,90 ), 40.13 (C1,10 ) ppm. [CdLBr2]: 1H NMR (in CDCl3): 8.17 (d, 2Hb,b0 , J = 8.92 Hz), 7.50 (d, 4He,e0 , J = 8.92 Hz), 7.13 (d, 2Hd,d0 , J = 15.36 Hz), 7.02 (dd, 2Hc,c0 , J = 15.34 Hz, J = 9.08 Hz), 6.70 (d, 4Hf,f0 , J = 8.92 Hz), 3.83 (s, 4Ha,a0 ), 3.07 (s, 12Hg,g0 ) ppm. 13C NMR (in CDCl3): 169.47 (C8,80 ), 152.98 (C2,20 ), 150.00 (C6,60 ), 130.46 (C4,40 ), 122.18 (C5,50 ), 119.82 (C7,70 ), 111.80 (C3,30 ), 59.04 (C9.90 ), 40.13 (C1,10 ) ppm. [CdLI2]: 1H NMR (in CDCl3): 8.19 (d, 2Hb,b0 , J = 8.36 Hz), 7.50 (d, 4He,e0 , J = 8.96 Hz), 7.11 (d, 4Hf,f0 , J = 7.08 Hz), 6.75 (dd, 2Hc,c0 , J = 8.96 Hz), 6.65 (d, 2Hd,d0 , J = 8.20 Hz), 3.83 (s, 4Ha,a0 ), 3.07 (s, 12Hg,g0 ) ppm. 13C NMR (in CDCl3): 169.06 (C8,80 ), 152.16 (C2,20 ),149.60 (C6,60 ), 130.35 (C4,40 ), 122.29 (C5,50 ),119.91 (C7,70 ), 111.84 (C3, 30 ), 58.92 (C9,90 ), 40.14 (C1,10 ) ppm. [CdL(NCS)2]: 1H NMR(in CDCl3): 8.01 (d, 2Hb,b0 , J = 9.64 Hz), 7.47 (d, 4He,e0 , J = 8.84 Hz), 7.41 (dd, 2Hc,c0 , J = 15.80 Hz, J = 8.40 Hz), 7.13 (d, 2Hd,d0 , J = 15.28 Hz), 6.64 (d, 4Hf,f0 , J = 8.96 Hz), 3.67 (s, 4Ha,a0 ), 3.01 (s, 12Hg,g0 ) ppm.13C NMR(in CDCl3): 179.40 (C8,80 ), 152.62 (C2,20 ), 152.24 (C6,60 ), 131.04 (C4,40 ), 128.78 (CNCS), 121.65 (C5,50 ), 117.87 (C7,70 ), 111.81 (C3,30 ), 57.41 (C9,90 ), 40.11 (C1,10 ) ppm.
The MBC is defined as the lowest concentration of a compound that exhibits any observable growth of microbe on the Muller Hinton Agar medium. In MBC screening, a loop full of above solution (the MIC solution) was sub-cultured onto the Muller Hinton Agar medium and incubated at 37 8C for 24 h [34,35] (Table 4). 2.8. Antifungal activity (in vitro) The antifungal activity screening was carried out using disk diffusion technique. The antifungal properties of the ligand and its cadmium complexes were investigated against two fungi such as Candida albicans and Aspergillus niger (local isolates). In a typical method, the discs were soaked in the compound solutions with various concentrations (1.25, 2.5 and 5 mg/mL in DMSO) and put at different positions on the Sabouraud Dextrose Agar (Oxoid, Hampshire, England) plates fecundated with 105 CFU/mL of fungal spore suspensions [36]. The plates of Candida albicans and Aspergills niger were incubated at 32 8C for 24 h and at 37 8C for 7 days respectively. After incubating, the inhibition zone (mm) that was formed around each disk was measured and recorded as antifungal activity of compound (Table 5).
2.5. Antibacterial activity (in vitro) The in vitro antibacterial activities of Schiff base ligand and its cadmium complexes were checked against four bacterial strains including Staphylococcus aureus (ATCC 6538), Bacillus subtilis (ATCC 6633), Escherichia coli (ATCC 25922) and Pseudomonas aeruginosa (ATCC 9027) by use of the disk diffusion method [31,32]. 100 mL of each bacterium including 0.5 106CFU was speared onto the Muller Hinton Agar medium by a sterile swab. All compounds were dissolved in DMSO solvent with various concentrations (0.5, 1.25 and 2.5 mg/mL). The sterile discs (6 mm in diameter) were impregnated with trial solution and placed on the surface of inoculated medium. Then, all plates were turned and incubated at 37 8C for 24 h. As a result of diffusion of the samples, the growth of bacteria was inhibited. After incubating, the antibacterial activity of compounds was determined by measuring of the diameter of
3. Results and discussion 3.1. Physical and analytical data The physical and analytical information of the Schiff base ligand and its cadmium complexes (CdLX2; X = Cl, Br, I and SCN) have been tabulated in Table 1.The ligand and its complexes are intense colored solids. Elemental analysis has shown good agreement between the theoretical and experimental values, which uphold the suggested structure in Scheme 1 and therefore the stoichiometric ratio (1:1) of metal to ligand in the complexes. The solubility tests of the compounds showed that the ligand is soluble
Table 3 Diameter of zone of inhibition (mm) by 0.5, 1.25 and 2.5 (mg/disk) of compounds. Compound
Gram-positive
Gram-negative
Bacillus subtilis
Ligand CdLCl2 CdLBr2 CdLI2 CdL(NCS)2 DMSO
Staphylococcus aureus
Pseudomonas aeruginosa
Escherichia coli
0.5
1.25
2.5
0.5
1.25
2.5
0.5
1.25
2.5
0.5
1.25
2.5
8.62 23.30 27.26 21.70 18.40 –
10.14 23.64 27.34 22.50 24.90 –
10.24 27.84 27.60 23.82 26.00 –
8.22 7.72 8.83 9.72 8.14 –
9.10 8.21 10.30 9.75 9.36 –
12.13 13.20 13.00 10.10 11.18 –
7.71 8.40 11.40 9.10 14.40 –
8.72 9.81 15.41 10.0 14.6 –
9.71 13.82 19.31 10.50 20.30 –
10.08 12.2 10.6 12.06 14.0 –
10.1 12.44 10.80 18.60 15.30 –
10.2 13.60 19.12 19.40 16.28 –
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Table 4 MIC and MBC of ligand and its cadmium complexes (in mg/mL). Gram-positive
Compound
Gram-negative
Bacillus subtilis
Ligand CdLCl2 CdLBr2 CdLI2 CdL(NCS)2
Staphylococcus aureus
Pseudomonas aeruginosa
Escherichia coli
MBC
MIC
MBC
MIC
MBC
MIC
MBC
MIC
500 500 500 500 500
250 250 250 250 250
5000 312 500 312 500
N-D N-D 125 N-D 250
500 500 78 500 15.6
N-D 125 62.5 N-D 62.5
500 7.81 62.5 125 500
125 0.975 N-D N-D 125
in chloroform and dichloromethane while is insoluble in alcohols. All cadmium complexes are soluble in some organic solvents such as dimethylformamide, dimethylsulfoxide, chloroform and dichloromethane and are insoluble in alcohols. The ligand was melted at 205 8C but the cadmium complexes were decomposed in the temperature range of 204–253 8C. The very low molar conductance values of the compounds (in chloroform solvent) at room temperature (0.004–0.179 cm2 V1 M1) were compatible with their molecular and non electrolyte nature [37]. So, it can be said that the ligand and X anions are coordinated to metal ion in inner sphere coordination. 3.2. Infra-red spectral data The infra-red spectra of ligand and its complexes were recorded in the range of 400–4000 cm1.Some important vibrational frequencies of the ligand and its complexes have been presented in Table 2. In the FT-IR spectrum of ligand, the absence of the parent aldehyde and amine vibrational frequencies at wave numbers of 1661 cm1 and (3291, 3375) cm1 respectively, and instead of them, appearance of a very strong stretching frequency at 1599 cm1confirm the formation of iminic or azomethine linkage (C5 5N) in free ligand [38]. It seems that after binding of azomethine nitrogens to the metal ion, the electron density of iminic linkage is decreased [39]. Therefore, the related vibration frequency is lowered by 9–13 cm1in the complexes spectra [40,41]. The ligand IR spectrum displays weak vibrational peaks at 2846 and 2804 cm1 that they are assigned to iminic stretching vibration (C–H). These vibrations are observed at higher wave numbers after complexation. On the other hand, there are new weak vibrations in the IR spectra of the complexes in the region of 428–478 cm1that may be attributed to M–N stretching vibration. In IR spectrum of thiocyanate complex, a new very strong vibration at 2073 cm1 can be assigned to N-bonded SCN to cadmium ion. This offer is based on previous reports on the thiocyanate complexes [42].
(Table 2, Fig. 1). The ligand electronic spectrum displayed two absorption bands. The first one was appeared at 265 nm that is assigned to p–p* transition of benzene rings. This absorption band showed a blue shift in all cadmium complexes spectra by 3–23 nm. The second absorption band as a very strong absorption is appeared at 362 nm,attributed to the p–p* transition of azomethine moieties. This peak was considerably shifted to higher wavelengths by 67 nm due to involvement of iminic nitrogens after coordination to cadmium center. Generally, in all electronic spectra of complexes, no d-d electronic transition was observed because of d10 electronic configuration of cadmium ion [43]. Furthermore, in the visible region of CdLBr2 and CdL(NCS)2 spectra, new absorption bands was appeared at 494 and 503 nm respectively that may be attributed to metal to ligand charge transfer (MLCT). 3.4. 1H and
13
C NMR spectra
A review on the recent researches reveals that the NMR spectroscopy is a very beneficial technique in affirming the skeleton and navigates of Schiff bases and their complexes [44]. The NMR spectra of the current Schiff base and its diamagnetic cadmium complexes were recorded in CDCl3as well as the tertamethylsilane (TMS) as internal standard. As typical spectra, the 1H and 13C NMR of ligand and cadmium iodide complex have been depicted in Fig. 2. The chemical shifts and the coupling constants of the different protons of compounds have been listed in the experimental section. In the 1H NMR spectrum of the ligand, the iminic protons (Hb, Hb0 ) were resonated as a doublet signal at 8.02 ppm that are red shifted to 8.16–8.19 ppm in the cadmium halide complexes and are faintly shifted to upfields (at 8.01 pm) in the case of the cadmium thiocyanate complex. The change in the chemical shifts of iminic hydrogen is related to change in the electron density of azomethine hydrogens because of the covalent linkage of azomethine nitrogens to metal ion. The ligand spectrum illustrated the signals of aromatic protons (He, He0 and Hf, Hf0 ) as doublets at 7.37 and 6.68 ppm respectively. In the spectra of all
3.3. UV–vis spectra The electronic spectra of the ligand and its complexes in chloroform solvent were obtained in the range of 200–700 nm
Table 5 Antifungal activity of compounds(1.25, 2.5 and 5 mg/disk) as diameter of zone of Inhibition (mm). Compound
Ligand CdLCl2 CdLBr2 CdLI2 CdL(NCS)2
Candida albicans
Aspergillus niger
1.25
2.5
5
1.25
2.5
5
9.50 15.0 16.0 16.3 15.5
9.78 15.20 18.36 16.48 18.12
10.00 18.72 18.50 17.50 20.90
– 13.10 12.40 14.64 13.58
– 13.56 13.10 14.70 15.40
6.70 16.00 14.90 17.40 18.44
Fig. 1. The electronic spectra of ligand and its cadmium complexes.
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Fig. 2. 1H and
5
13
C NMR of ligand and its cadmium iodide complex.
complexes, these signals were smoothly shifted to the weaker fields with respect to the free ligand except for the CdL(NCS)2 that Hf and Hf0 were upfielded (appeared at 6.64 ppm). In the ligand spectrum, olefinic hydrogens of Hd and Hd0 were observed at 6.87 ppm as a doublet due to coupling with other olefinic protons (Hc, Hc0 ). The resonance of Hc, Hc0 in ligand spectrum was observed at 6.73 ppm as a doublet of doublet because of coupling with Hb, Hb0 and Hd, Hd0 respectively. After coordination of ligand to metal, these protons were desheilded in the CdLCl2, CdLBr2 and CdL(NCS)2 complexes and therefore the related signals were upfielded. But at the CdLI2 spectrum, the doublet of Hd, Hd0 showed an upwnfield shift (at 6.65 ppm) while the doublet of doublet signal of Hc, Hc0 smoothly shifted to downfield position (at 6.75 ppm) similar to other complexes. The ligand spectrum represented the chemical shifts of aliphatic hydrogens of Ha, Ha0 and Hg, Hg0 as two singlet signals at 3.81 and 3.01 ppm respectively. In all cadmium halide complexes spectra, Ha, Ha0 and Hg, Hg0 signals were appeared at downfielded chemical shifts with respect to the free ligand. In the CdL(NCS)2 spectrum, the methylene hydrogens signal (Ha, Ha0 ) was shifted to the stronger fields (at 3.67 ppm) and the singlet signal of the methyl protons (Hg, Hg0 ) was appeared at 3.01 ppm as same as the ligand.
The 13C NMR spectrum of the ligand displayed the iminic carbon signal (C8,80 ) at 164.79 ppm [25]. This signal was shifted to the downfield position (169.06–179.40 ppm) in the cadmium complexes, offering well binding of the ligand to metal center. The other signals of ligand carbons were exhibited at 150.96 (C2,20 ), 142.34 (C6,60 ), 130.54 (C7,70 ), 128.63 (C4,40 ), 123.62 (C5,50 ), 112.04 (C3,30 ), 61.89 (C9,90 ), 40.27 (C1,10 ) ppm. All above signals smoothly shifted to the downfield regions after complex formation except for C7,70 and C5,50 and aliphatic carbons that were upfielded at all complexes spectra. In addition to the above signals, a new signal was appeared at 128.78 ppm in the CdL(NCS)2 spectra that may be attributed to the carbon of coordinated thiocyanate. Finally, the spectral and physical data of ligand and its complexes support the pseudo-tetrahedral geometry around the cadmium ion as depicted in Scheme 1. 3.5. Antibacterial bioassay (in vitro) The novel synthesized Schiff base ligand and its cadmium complexes were checked for in vitro antibacterial activity against selected bacteria inclusive E. coli, P. aeruginosa, S. aureus and B. subtilis by the disk diffiusion method [45]. The antibacterial
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3.6. Antifungal bioassay (in vitro)
20
Bacillus subsslis
15
Staphylococccus aureus
10
Pseudomonaas aeruginosa
5
Escherichia ccoli
0
Fig. 3. Plots of antibacterial activity of compounds in concentration of 2.5 mg/disk.
activity data as zone (mm) of inhibition of growth have been presented in Table 3 and plotted as shown in Fig. 3. The results revealed that the ligand and its complexes have the different degrees of prevention efficiency upon the examined bacterial growth. All complexes exhibited more antibacterial activity than free Schiff base ligand in agree with many other reports [46]. A comparison between the antibacterial activity of free ligand and its complexes displayed that CdLCl2 complex was the most active compound against B. subtilis and S. aureus bacterial strains. Therefore, it can be supposed that the CdLCl2 complex would have a very good antibacterial activity against Gram positive bacteria. In the case of S. aureus, the antibacterial activity of compounds follows the order: CdLCl2 > CdLBr2 > ligand > CdL(NCS)2 > CdLI2. So, on S. aureus bacterium, the ligand showed a moderate effect. Also the ligand was a weak compound against other bacterial strains because it had the minimum inhibition zone diameter on them. CdL(NCS)2 and CdLI2exhibited the most activity against P. aeruginosa and E. coli respectively. In addition to above achievements, all compounds were tested for the MIC and MBC technique by Mueller Hinton broth (Table 4). In broth micro-dilution method for MIC detection of compounds, it was often impossible to observe the turbidity of MIC mixtures because all sample solutions were extremely colorful. Therefore, the MBC values were evaluated in the current study. In the case of B. subtilis, all MBC values were found to be 500 mg/ml. Against S. aureus, the lowest MBC were related to CdLCl2 and CdLI2 complexes. The best bactericide compounds on P. aeruginosa and E. coli were CdL(NCS)2 and CdLCl2with MBC values of 15.6 mg/ml and 7.81 mg/ml respectively. Finally, it is to be noted that the broth micro-dilution method (MBC data) often supported the disk diffusion results so that the bactericidal effects of some complexes were found to be better as compared with the free ligand.
The antifungal activities of the Schiff base ligand and its cadmium complexes were investigated in vitro against A. niger and C. albicans fungal cultures using the disk diffusion procedure (Table 5, Fig. 4). The antifungal activity of ligand was evaluated as moderate against C. albicans. The ligand showed very low activity against A. niger in high concentration (5 mg/disk) while it had no antifungal activity against it in lower concentrations (1.25 and 2.5 mg/disk). The CdL(NCS)2 complex demonstrated the maximum inhibitory effect on A. niger and C. albicans fungal strains as comparison with free ligand. It can be found from the data that the most of complexes displayed more antibacterial/antifungal activity in compared with free ligand. A review on literature indicates that generally coordination of an organic ligand to metal centers amplifies its antimicrobial properties [47]. It seems that overlapping of the ligand and metal ion orbitals enhances the resonance of p electrons on the compound structure and therefore increases the liposolubility of the complexes with respect to free ligand [48]. An increase in lipophilicity character and therefore better diffusion in microbial membrane may be considered as notable reason for this activity improvement. Regarding the mentioned reason, the improvement of antimicrobial activity of the titled cadmium complexes with respect to free ligand is explained. It is suggested that the
120
TG%
(A)
W.L%
100
µg/min) TG%/W.L%/DTG (µ
Inhibition zones
25
DTG ug/min
80 60 40 20 0 0
200
400
600
800
1000
T(°C) TG%
TG%/W.L%/DTG(µg/min)
6
90
WL%
(B) DTG ug/min
70 50 30 10 -10 0
200
400
600
800
1000
T(°C) Fig. 4. Plots of antifungal activity of compounds in concentration of 2.5 mg/disk.
Fig. 5. The TG/DTG/DTA diagrams of cadmium bromide (A) and thiocyanate (B) complexes.
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Table 6 Thermal analysis data including temperature range, DTG peak, mass loss, proposed segment and final residue. Compound
Temp. range (8C)
Mass loss (%) exp. (calcd.)
Peak temperature (8C)
Proposed segment
Final residue exp. (calcd.)
Ligand
216–316 316–456 456–948
34.86(32.11) 52.18(53.75) 11.86(14.17)
263 372 917
C8H10N C13H17N2 C3H3N
–
CdLCl2
223–477 477–842 842–992
26.19(29.44) 52.09(50.40) 21.48(20.15)
314 642 940
C10H16N2 C14H14Cl2N2 Cd
–
CdLBr2
250–430
23.63(22.61)
302
C10H12N
CdBr 26(29.74%)
430–899
50.13(47.60)
594
C14H18BrN3
251–372
16.39(16.22)
302
C8H10N
372–767 767–993
53.44(52.38) 13.2(16.22)
506 845
C8H10I2N2 C8H10N
218–378
25.54(27.23)
261
C10H16N2 C10H10N4
378–567 567–991
28.30(30.88) 11.61(12.62)
554 575
CdLI2
CdL(NCS)2
difference in antimicrobial activities of the cadmium complexes is related to the coordinated X anions in CdLX2. The X anions with different sizes and electronegativities can affect the antimicrobial properties by steric and electronic effects [49]. When the X anion varies from chloride to bromide, thiocyanate and iodide, the size of the complex and lipophilicity are increased (as steric effect) while the order of M–N polarity and positive charge density on metal ion in the complexes (as electronic effect) is suggested to be as CdLCl2 > CdL(NCS)2 > CdLBr2 > CdLI2.The first effect leads to more diffusion but the second effect causes more interaction of complex with cell membrane. Based on our results, it seems that in the case of B. subtilis and S. aureus, interaction with cell wall is an important agent for inhibition of the growth so that more efficient interaction of more positive metal ion with phosphate groups of cell membrane has led to more inhibitory effect of CdLCl2 with respect to others. In the case of P. aeruginosa, C. albicans and A. niger, both lipophilicity character and electronic effect are suggested to be effective so that CdL(NCS)2 with suitable polarity and lipophilicity has acted as an efficient antibacterial/antifungal active compound among the complexes. Finally in the case of E. coli, diffusion of compound into cell membrane is probably responsible for antibacterial activity such that the ZnLI2 with higher lipophilicity character than others effectively prevented the bacterial growth.
Cd 17(15.18%)
CdS2 30(29.27%)
C6H4
3.7. Thermal analysis (TG and DTG) of ligand and its cadmium cadmium complexes Thermal study of Schiff base ligand and its cadmium complexes tells us: (i) the thermal stability of them, (ii) existence or absence of water molecules in their structure and (iii) general pattern for pyrolysis of the compounds. Thermal analysis (TG/DTG) of all compounds was carried out up to 1000 8C under nitrogen atmosphere at a heating rate of 10 8C/min. As typical plots, the TG/DTG diagrams of cadmium bromide and thiocyanate complexes have been illustrated in Fig. 5. The mass loss for each compound was calculated within the corresponding temperature range and the data have been tabulated in Table 6. None of the TG curves showed any weight loss below 200 8C confirming absence of water molecules inside or outside the metal coordination sphere. The ligand thermo-gram displayed that the ligand was completely decomposed in three consecutive steps (found mass loss: 100%). It is related to elimination of C8H10N, C13H17N2 and C3H3N in the 1st, 2nd and 3rd thermal steps respectively. The cadmium complexes demonstrated different thermal behaviors. The CdLCl2 complex totally destructed at the applied temperature range via three steps without any final residue. At the first step, (at 223–477 8C), 26.19% of its weight was liberated as gas. Thereafter, 52.09% of molecular weight was eliminated at
Table 7 Thermokinetic activation parameters of the thermal decomposition of ligand and its cadmium complexes. Compound
Decomposition step (8C)
E* (KJ/mol)
A* (1 S1) 1.28 10 4.09 2.80
16
DS* (KJ/mol)
DH* (KJ/mol)
DG* (KJ/mol)
5.86 10 2.40 102 2.48 102
188.74 37.91 61.52
1.57 102 1.92 102 3.57 102
Ligand
216–316 316–456 456–948
193.19 43.27 71.42
CdLCl2
223–477 477–842 842–992
83.55 56.04 73.83
4.54 104 1.21 2.21
1.61 102 2.53 102 2.5 102
78.67 48.43 63.75
1.73 102 2.80 102 3.67 102
CdLBr2
250–430 430–899
107.54 45.25
7.81 106 4.34 101
1.18 102 2.61 102
102.76 38.04
1.71 102 2.64 102
CdLI2
251–372 372–767 767–993
174.83 43.15 19.65
9.50 107 6.23 101 1.17 101
3.66 102 2.57 102 2.74 102
170.05 36.67 10.35
3.80 102 2.37 102 3.16 102
CdL(NCS)2
218–378 378–567 567–991
121.86 155.42 107.36
1.22 109 3.53 107 1.24 104
7.58 101 1.09 102 1.75 102
117.42 148.54 100.31
1.58 102 2.39 102 2.49 102
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477–842 8C which may be attributed to segments of C10H16N2 and C14H14Cl2N2 respectively. Finally, the weight loss was continued above 842 8C and the final segment removal as Cd metal was occurred. The DTG plots of CdLBr2 complex exhibited two subsequent degradation steps at 302 and 594 8C, which may be attributed to loss of C10H12N and C14H18BrN3 respectively. The thermal treatment of CdL(NCS)2 and CdLI2 complexes proposed total elimination of organic moiety of them (with an estimated mass loss: 83.03% and 65.45% respectively) via three decomposition steps. Metal component and CdS2 were suggested as residual materials respectively. The thermodynamic activation parameters of decomposition processes of compounds termed as Arrhenius constant (A), activation energy (DE*), enthalpy (DH*), entropy (DS*) and Gibbs free energy change of the decomposition (DG*) were evaluated graphically by the aid of the Coats–Redfern equation [26,50]. The obtained data have been collected in Table 7. The activation energies of decomposition have been found in the range of 19.65– 193.19 kJ/mol. The high amounts of the activation energies represent the thermal stability of the complexes. Approximately, in all temperature ranges, DS*values are negative numbers that reflect the pyrolysis processes with a lesser rate than the normal ones as seen in many literature reports. Only, in one of the evaluated temperature ranges, DS* value is a positive number that demonstrates a dissociation property of thermal decomposition. DH*and DG*quantities are totally positive values and are found to be in the ranges of 10.35–188.74 kJ/mol and 1.57–3.80 102 kJ/ mol respectively. 4. Conclusion In this paper, we described the synthesized, spectroscopic recognition, antimicrobial and thermal study of some novel cadmium complexes of a bidentate Schiff base ligand. Their antibacterial/antifungal testing showed that all of them have acceptable antibacterial/antifungal activity and it was found that the antimicrobial activity was enhanced after binding of the ligand to cadmium center. The diameter of inhibition zone (mm) expressed that CdLCl2 (at 2.5 mg/disk) is a more effective compound against B. subtilis and S. aureus while CdL(NCS)2and CdLI2 (at 2.5 mg/disk) showed more antibacterial activities against P. aeruginosa and E. coli respectively. The CdL(NCS)2 (at 5 mg/disk) was found as the best antifungal active compound with respect to others. The thermal investigation of compounds indicated that the ligand was decomposed completely via three decomposition steps. The cadmium complexes were decomposed via 2–3 temperature steps. Furthermore, the thermodynamic activation parameters of decomposition processes of compounds termed as Arrhenius constant (A), DE*, DH*, DS* and DG* of the decomposition were evaluated graphically by use of the Coats– Redfern equation Acknowledgement Partial support of this research by Yasouj University is acknowledged.
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JIEC-1622; No. of Pages 8 Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx
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Some new cadmium complexes: Antibacterial/antifungal activity and thermal behavior Morteza Montazerozohori a,*, Saeedeh Zahedi a, Masoud Nasr-Esfahani a, Asghar Naghiha b a b
Department of Chemistry, Yasouj University, Yasouj 75918-74831, Iran. Department of Animal Sciences, Faculty of Agriculture, Yasouj University, Yasouj, Iran.
A R T I C L E I N F O
Article history: Received 13 July 2013 Accepted 14 October 2013 Available online xxx Keywords: Schiff base Complex Spectral In vitro Symmetric Thermodynamic
A B S T R A C T
Four new cadmium(II) Schiff base complexes formulated as CdLX2 (X = Cl, Br, I, SCN and L = bis (3(4dimethylaminophenyl)-allylidene)-1,2-diaminoethane) were synthesized. All novel compounds were identified by spectroscopic techniques including FT-IR, 1H and 13C NMR, UV–vis spectra and physicochemical methods such as molar conductivity, elemental analysis and melting point. Based on the physical and spectral evidences, the pseudo-tetrahedral geometry was proposed for all complexes. The in vitro antibacterial/antifungal activity of each compound was screened against four bacterial strains such as Staphylococcus aureus, Bacillus subtilis, Escherichia coli and Pseudomonas aeruginosa as well as two fungal pathogens such as Candida albicans and Aspergillus niger. The biological testing data showed that all compounds were antimicrobial active against both Gram-positive and Gram-negative bacteria and two fungal strains. The bactericidal/fungicidal properties of compounds showed that the cadmium complexes were more bioactive than the parent ligand. Furthermore, the thermal behavior of all compounds was studied. The ligand was completely destructed through three temperature stages. Among the cadmium complexes, the CdL(NCS)2was found as the most stable while the CdLCl2 showed the least thermal stability. Besides some activation thermodynamic parameters such as E*, DH*, DS*, DG* were calculated using thermo-grams data by Coats–Redfern equation. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Schiff base compounds are classified as a category of organic chemicals that display remarkable properties such as structural resemblance with natural biomaterials and easily flexible preparation giving those opportune structural characteristics [1]. Schiff base compounds can also be considered as a great group of effective materials in clinical and pharmaceutical chemistry due to their biological activities including anti-tumoral [2], antifungal [3–5], antibacterial [6,7], antimicrobial [8], and anthelmintic properties [9]. Furthermore, they are significant intermediates for preparation of some active biomolecules [10,11] and are applied for planning some metal complexes relevant to combinational and natural oxygen transports [12–14]. Schiff base and their complexes have attracted special attention because of their interesting characteristics such as their potency in redox reaction in bio-systems and DNA oxidation [15], photochromic traits, coordination ability to some toxic metal ions [16]. In recent years, many researches have been reported on
* Corresponding author. Tel.: +98 7412223048; fax: +98 7412223048. E-mail address: [email protected] (M. Montazerozohori).
the chemistry of the cadmium(II) complexes [17]. Cadmium(II) compounds prevent from activity of RNA polymerase in vitro [18,19], and are combined easily with biomolecules such as proteins[20]. Some biological properties of cadmium compounds, particularly its Schiff base complexes have encouraged the chemists to develop the preparation of novel cadmium coordination compounds. For instance, in 2008, Spinu et al. synthesized a bidentate Schiff base ligand entitled as N-(2-thienylmethylidene)-2-aminopyridine (TNAPY) and its pseudo-tetrahedral Cd(II) complexes. They performed the in vitro antibacterial testing against three bacterial pathogens by the disk diffusion method. The achievements showed that the metal complexes were more antibacterial active than the primary ligand [21]. In 2012, the synthesis and antibacterial activities of a Schiff base ligand and its tetrahedral Cd(II) complexes were investigated by Singh using the disk diffusion procedure. The antibacterial activities measurements revealed that the chelation enhances the biological properties against the tested bacteria [22]. In 2009, Reiss et al. synthesized a new NO-donor Schiff base named as N-(2furanylmethylene)-3-aminodibenzofuran and its Cd(II) complexes and then tested them for the antibacterial activity against three bacterial strains by the disk diffusion technique. The inhibition values displayed that all compounds prevented the
1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.10.027
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2
means of Metrohm-712 conductometer equipped with a dip-type conductivity cell. Thermo-gravimetric diagrams were recorded on Perkin–Elmer Pyris model with a heating rate of 10 8C min1 under nitrogen atmosphere.
bacterial growth while the antimicrobial activity was affected by the coordination of the ligand [23]. Halli et al. prepared a Schiff base ligand derived from benzofuran-2-carbohydrazide and pchloroacetophenone and its four-coordinated Cd(II) complexes and then tested the antibacterial/antifungal activity of them. In this study, it was found that the fungicidal/bactericidal activities of complexes were higher than the primary ligand [24]. Among many others, the notable biological activity of cadmium complexes led us to a new research on them. Accordingly, in continuation of our investigation on bidentate Schiff bases and their transition metal complexes [25–29], our target in this work is the synthesis, identification and the antibacterial/antifungal evaluation of a symmetric N2-donor Schiff base ligand entitled as bis(3 (4-dimethylaminophenyl)-allylidene)-1,2-diaminoethane and its new cadmium(II) complexes. Furthermore, the thermal behavior of the ligand and its cadmium complexes are described toward to each other.
2.3. Synthesis of ligand (L) The Schiff base ligand of bis(3(4-dimethylaminophenyl)-allylidene)-1,2-diaminoethane was synthesized from a condensation reaction between ethylene diamine (0.5 mmol) and 4-dimethylaminocinamaldehyde (1 mmol) in ethanol under sever stirring for 5 h. After completion of the reaction, the orange precipitate of ligand was filtered, washed and then recrystallized from dichloromethane/ ethanol solvent mixture (1:1) to afford the pure ligand in 54% yield. Some important physical and spectral (IR and UV–vis) information has been reported in Tables 1 and 2. The 1H and 13C NMR data of ligand based on Scheme 1:1H NMR (in CDCl3): 8.02 (d, 2Hb,b0 , J = 8.80 Hz), 7.37 (d, 4He,e0 , J = 8.76 Hz), 6.87 (d, 2Hd,d0 , J = 15.84 Hz), 6.73 (dd, 2Hc,c0 , J = 15.92, J = 8.80 Hz), 6.68 (d, 4Hf,f0 , J = 8.80 Hz), 3.81 (s, 4Ha,a0 ), 3.01 (s, 12Hg,g0 ) ppm. 13C NMR (in CDCl3): 164.79 (C8,80 ), 150.96 (C2,20 ), 142.34 (C6,60 ), 130.54 (C7,70 ), 128.63 (C4,40 ), 123.62 (C5,50 ), 112.04 (C3,30 ), 61.89 (C9,90 ), 40.27 (C1,10 ) ppm.
2. Experimental 2.1. Materials 4-dimethylaminocinamaldehyde, ethylene diamine, cadmium halides and other chemicals were provided by Aldrich, Merck or BDH companies. Cadmium(II) thiocynate was prepared in accordance with our previous report [30].
2.4. Synthesis of cadmium(II) complexes The synthesized Schiff base ligand dissolved in dichloromethane (0.5 mmol, 15 mL) was added to an ethanolic solution of cadmium salts (0.5 mmol, 10 mL). The mixture was severely stirred on a magnetic stirrer at room temperature for 2–3 h. After this period of time, the appeared precipitate was filtered and washed with absolute ethanol twice. Then, to obtain a pure product, the precipitates were recrystallized from dichloromethane/ethanol solvent mixture (1:1). Finally, the cadmium complexes were dried under vacuum and kept in a desiccator. Some characteristic physical and spectral data (IR and UV–vis) are tabulated in Tables 1 and 2. The 1H and 13C NMR data of cadmium complexes based on Scheme 1 are found to be as follow: [CdLCl2]: 1H NMR (in CDCl3): 8.16 (d, 2Hb,b0 , J = 9.08 Hz), 7.50 (d, 4He,e0 , J = 8.88 Hz), 7.13 (d, 2Hd,d0 , J = 15.44 Hz), 6.99 (dd, 2Hc,c0 , J = 15.26 Hz, J = 9.32 Hz), 6.69 (d, 4Hf,f0 , J = 8.92 Hz), 3.83 (s, 4Ha,a0 ), 3.07 (s, 12Hg,g0 ) ppm. 13CNMR (in CDCl3): 170.00 (C8,80 ), 152.00
2.2. Methods The FT/IR spectra of compounds were obtained by JASCO-FT/ IR680 system in the 400–4000 cm1 region using the KBr pellets. A JASCO-V570 spectrophotometer instrument was used to record the electronic absorption spectra in the chloroform and in the range of 200–800 nm. The 1H and 13C NMR spectra were recorded on a Brucker DPX FT/NMR 400 MHz spectrometer in CDCl3 using TMS as internal standard. The percent of C, H and N elements of ligand and complexes were evaluated by employing an elemental analyzer apparatus. A BUCHI B-545 instrument was applied for determining the ligand’s melting point and also the decomposition temperature of cadmium complexes. The molar conductance measurements of compounds were performed in chloroform at room temperature by Table 1 Analytical and physical data of the Schiff base ligand and its Cd(II) complexes. Run
Compound
Color
Decomposition temp. (8C)
Yield (%)
1 2 3 4 5
Ligand CdLCl2 CdLBr2 CdLI2 CdL(NCS)2
Orange Orange Orange Yellow Orange
205a 253 250 253 204
54 76 62 90 80
a
L8M (cm2 V1 M1)
Found (calc.)% C
H
N
76.7(76.97) 51.8(51.67) 43.9(44.57) 38.7(38.91) 51.9(51.78)
7.9(8.07) 5.6(5.42) 4.3(4.68) 4.2(4.08) 4.9(5.01)
14.8(14.96) 10.3(10.04) 8.4(8.66) 7.7(7.56) 14.1(13.93)
0.004 0.006 0.013 0.179 0.014
Refers to melting point.
Table 2 Vibrational (cm1) and electronic spectral data of the Schiff base (L) and its cadmium(II) complexes. Compound
n(CH)imine
n(C5 5N)
n(SCN)
n(M–N)
n(C5 5C)
l (nm), e (cm1 M1)
Ligand
2846(w), 2804(w)
–
–
2854(w), 2805(sh)
–
437(w), 475(w)
CdLBr2
2854(w), 2810(w)
–
428(w), 468(w)
CdLI2
2850(sh), 2810(w)
–
437(w), 471(w)
CdL(NCS)2
2858(w), 2810(w)
2073 (vs)
431(w), 478(w)
1441(w), 1459(w) 1440(w), 1483(w) 1439(w), 1482(w) 1438(w), 1483(w) 1437(w), 1482(w)
265(31819), 362(65239)
CdLCl2
1599 (vs) 1589 (vs) 1590 (vs) 1589 (vs) 1584 (vs)
260(21332), 416(65,600) 261(36,228), 416(81,997), 494(20,225) 242(45,342), 415(79,115) 262(23,727), 429(84,227), 503(11,097)
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9',a'
9,a 8,b
N
N
X
7,c
5
6',d' 7',c'
X
2
2.6. Minimum inhibitory concentration (MIC)
5'
4,e
H3C
inhibition zone surrounding the discs in millimeter (Table 3). DMSO has no effect on biological activity of compounds and this is considered as a negative control test.
8',b'
Cd
6,d
The MICs of antibacterial samples were evaluated by means of broth dilution method. In the serial dilution procedure, all samples were dissolved in DMSO with various concentrations from 0.975 to 5000 mg/mL in the sterilized conditions. Then, 650 mL of Muller Hinton Broth (Scharlab) and 100 mL of tested microorganism were added to each tube containing sample. After incubation of sample tubes at 37 8C for 24 h, the lowest concentration of an antibacterial sample that prevents from obvious growth of microorganism based on turbidity of mixtures is specified as the MIC [33] (Table 4).
4',e'
3,f
3',f'
2'
N
N CH3
CH3
H3C
1,g
3
1',g'
Scheme 1. Structural formula of ligand and its cadmium complexes.
2.7. Minimum bactericidal concentration (MBC)
(C2,20 , C6,60 ), 130.90 (C4,40 ), 122.10 (C5,50 ), 118.85 (C7,70 ), 111.78 (C3,30 ), 60.00 (C9,90 ), 40.13 (C1,10 ) ppm. [CdLBr2]: 1H NMR (in CDCl3): 8.17 (d, 2Hb,b0 , J = 8.92 Hz), 7.50 (d, 4He,e0 , J = 8.92 Hz), 7.13 (d, 2Hd,d0 , J = 15.36 Hz), 7.02 (dd, 2Hc,c0 , J = 15.34 Hz, J = 9.08 Hz), 6.70 (d, 4Hf,f0 , J = 8.92 Hz), 3.83 (s, 4Ha,a0 ), 3.07 (s, 12Hg,g0 ) ppm. 13C NMR (in CDCl3): 169.47 (C8,80 ), 152.98 (C2,20 ), 150.00 (C6,60 ), 130.46 (C4,40 ), 122.18 (C5,50 ), 119.82 (C7,70 ), 111.80 (C3,30 ), 59.04 (C9.90 ), 40.13 (C1,10 ) ppm. [CdLI2]: 1H NMR (in CDCl3): 8.19 (d, 2Hb,b0 , J = 8.36 Hz), 7.50 (d, 4He,e0 , J = 8.96 Hz), 7.11 (d, 4Hf,f0 , J = 7.08 Hz), 6.75 (dd, 2Hc,c0 , J = 8.96 Hz), 6.65 (d, 2Hd,d0 , J = 8.20 Hz), 3.83 (s, 4Ha,a0 ), 3.07 (s, 12Hg,g0 ) ppm. 13C NMR (in CDCl3): 169.06 (C8,80 ), 152.16 (C2,20 ),149.60 (C6,60 ), 130.35 (C4,40 ), 122.29 (C5,50 ),119.91 (C7,70 ), 111.84 (C3, 30 ), 58.92 (C9,90 ), 40.14 (C1,10 ) ppm. [CdL(NCS)2]: 1H NMR(in CDCl3): 8.01 (d, 2Hb,b0 , J = 9.64 Hz), 7.47 (d, 4He,e0 , J = 8.84 Hz), 7.41 (dd, 2Hc,c0 , J = 15.80 Hz, J = 8.40 Hz), 7.13 (d, 2Hd,d0 , J = 15.28 Hz), 6.64 (d, 4Hf,f0 , J = 8.96 Hz), 3.67 (s, 4Ha,a0 ), 3.01 (s, 12Hg,g0 ) ppm.13C NMR(in CDCl3): 179.40 (C8,80 ), 152.62 (C2,20 ), 152.24 (C6,60 ), 131.04 (C4,40 ), 128.78 (CNCS), 121.65 (C5,50 ), 117.87 (C7,70 ), 111.81 (C3,30 ), 57.41 (C9,90 ), 40.11 (C1,10 ) ppm.
The MBC is defined as the lowest concentration of a compound that exhibits any observable growth of microbe on the Muller Hinton Agar medium. In MBC screening, a loop full of above solution (the MIC solution) was sub-cultured onto the Muller Hinton Agar medium and incubated at 37 8C for 24 h [34,35] (Table 4). 2.8. Antifungal activity (in vitro) The antifungal activity screening was carried out using disk diffusion technique. The antifungal properties of the ligand and its cadmium complexes were investigated against two fungi such as Candida albicans and Aspergillus niger (local isolates). In a typical method, the discs were soaked in the compound solutions with various concentrations (1.25, 2.5 and 5 mg/mL in DMSO) and put at different positions on the Sabouraud Dextrose Agar (Oxoid, Hampshire, England) plates fecundated with 105 CFU/mL of fungal spore suspensions [36]. The plates of Candida albicans and Aspergills niger were incubated at 32 8C for 24 h and at 37 8C for 7 days respectively. After incubating, the inhibition zone (mm) that was formed around each disk was measured and recorded as antifungal activity of compound (Table 5).
2.5. Antibacterial activity (in vitro) The in vitro antibacterial activities of Schiff base ligand and its cadmium complexes were checked against four bacterial strains including Staphylococcus aureus (ATCC 6538), Bacillus subtilis (ATCC 6633), Escherichia coli (ATCC 25922) and Pseudomonas aeruginosa (ATCC 9027) by use of the disk diffusion method [31,32]. 100 mL of each bacterium including 0.5 106CFU was speared onto the Muller Hinton Agar medium by a sterile swab. All compounds were dissolved in DMSO solvent with various concentrations (0.5, 1.25 and 2.5 mg/mL). The sterile discs (6 mm in diameter) were impregnated with trial solution and placed on the surface of inoculated medium. Then, all plates were turned and incubated at 37 8C for 24 h. As a result of diffusion of the samples, the growth of bacteria was inhibited. After incubating, the antibacterial activity of compounds was determined by measuring of the diameter of
3. Results and discussion 3.1. Physical and analytical data The physical and analytical information of the Schiff base ligand and its cadmium complexes (CdLX2; X = Cl, Br, I and SCN) have been tabulated in Table 1.The ligand and its complexes are intense colored solids. Elemental analysis has shown good agreement between the theoretical and experimental values, which uphold the suggested structure in Scheme 1 and therefore the stoichiometric ratio (1:1) of metal to ligand in the complexes. The solubility tests of the compounds showed that the ligand is soluble
Table 3 Diameter of zone of inhibition (mm) by 0.5, 1.25 and 2.5 (mg/disk) of compounds. Compound
Gram-positive
Gram-negative
Bacillus subtilis
Ligand CdLCl2 CdLBr2 CdLI2 CdL(NCS)2 DMSO
Staphylococcus aureus
Pseudomonas aeruginosa
Escherichia coli
0.5
1.25
2.5
0.5
1.25
2.5
0.5
1.25
2.5
0.5
1.25
2.5
8.62 23.30 27.26 21.70 18.40 –
10.14 23.64 27.34 22.50 24.90 –
10.24 27.84 27.60 23.82 26.00 –
8.22 7.72 8.83 9.72 8.14 –
9.10 8.21 10.30 9.75 9.36 –
12.13 13.20 13.00 10.10 11.18 –
7.71 8.40 11.40 9.10 14.40 –
8.72 9.81 15.41 10.0 14.6 –
9.71 13.82 19.31 10.50 20.30 –
10.08 12.2 10.6 12.06 14.0 –
10.1 12.44 10.80 18.60 15.30 –
10.2 13.60 19.12 19.40 16.28 –
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Table 4 MIC and MBC of ligand and its cadmium complexes (in mg/mL). Gram-positive
Compound
Gram-negative
Bacillus subtilis
Ligand CdLCl2 CdLBr2 CdLI2 CdL(NCS)2
Staphylococcus aureus
Pseudomonas aeruginosa
Escherichia coli
MBC
MIC
MBC
MIC
MBC
MIC
MBC
MIC
500 500 500 500 500
250 250 250 250 250
5000 312 500 312 500
N-D N-D 125 N-D 250
500 500 78 500 15.6
N-D 125 62.5 N-D 62.5
500 7.81 62.5 125 500
125 0.975 N-D N-D 125
in chloroform and dichloromethane while is insoluble in alcohols. All cadmium complexes are soluble in some organic solvents such as dimethylformamide, dimethylsulfoxide, chloroform and dichloromethane and are insoluble in alcohols. The ligand was melted at 205 8C but the cadmium complexes were decomposed in the temperature range of 204–253 8C. The very low molar conductance values of the compounds (in chloroform solvent) at room temperature (0.004–0.179 cm2 V1 M1) were compatible with their molecular and non electrolyte nature [37]. So, it can be said that the ligand and X anions are coordinated to metal ion in inner sphere coordination. 3.2. Infra-red spectral data The infra-red spectra of ligand and its complexes were recorded in the range of 400–4000 cm1.Some important vibrational frequencies of the ligand and its complexes have been presented in Table 2. In the FT-IR spectrum of ligand, the absence of the parent aldehyde and amine vibrational frequencies at wave numbers of 1661 cm1 and (3291, 3375) cm1 respectively, and instead of them, appearance of a very strong stretching frequency at 1599 cm1confirm the formation of iminic or azomethine linkage (C5 5N) in free ligand [38]. It seems that after binding of azomethine nitrogens to the metal ion, the electron density of iminic linkage is decreased [39]. Therefore, the related vibration frequency is lowered by 9–13 cm1in the complexes spectra [40,41]. The ligand IR spectrum displays weak vibrational peaks at 2846 and 2804 cm1 that they are assigned to iminic stretching vibration (C–H). These vibrations are observed at higher wave numbers after complexation. On the other hand, there are new weak vibrations in the IR spectra of the complexes in the region of 428–478 cm1that may be attributed to M–N stretching vibration. In IR spectrum of thiocyanate complex, a new very strong vibration at 2073 cm1 can be assigned to N-bonded SCN to cadmium ion. This offer is based on previous reports on the thiocyanate complexes [42].
(Table 2, Fig. 1). The ligand electronic spectrum displayed two absorption bands. The first one was appeared at 265 nm that is assigned to p–p* transition of benzene rings. This absorption band showed a blue shift in all cadmium complexes spectra by 3–23 nm. The second absorption band as a very strong absorption is appeared at 362 nm,attributed to the p–p* transition of azomethine moieties. This peak was considerably shifted to higher wavelengths by 67 nm due to involvement of iminic nitrogens after coordination to cadmium center. Generally, in all electronic spectra of complexes, no d-d electronic transition was observed because of d10 electronic configuration of cadmium ion [43]. Furthermore, in the visible region of CdLBr2 and CdL(NCS)2 spectra, new absorption bands was appeared at 494 and 503 nm respectively that may be attributed to metal to ligand charge transfer (MLCT). 3.4. 1H and
13
C NMR spectra
A review on the recent researches reveals that the NMR spectroscopy is a very beneficial technique in affirming the skeleton and navigates of Schiff bases and their complexes [44]. The NMR spectra of the current Schiff base and its diamagnetic cadmium complexes were recorded in CDCl3as well as the tertamethylsilane (TMS) as internal standard. As typical spectra, the 1H and 13C NMR of ligand and cadmium iodide complex have been depicted in Fig. 2. The chemical shifts and the coupling constants of the different protons of compounds have been listed in the experimental section. In the 1H NMR spectrum of the ligand, the iminic protons (Hb, Hb0 ) were resonated as a doublet signal at 8.02 ppm that are red shifted to 8.16–8.19 ppm in the cadmium halide complexes and are faintly shifted to upfields (at 8.01 pm) in the case of the cadmium thiocyanate complex. The change in the chemical shifts of iminic hydrogen is related to change in the electron density of azomethine hydrogens because of the covalent linkage of azomethine nitrogens to metal ion. The ligand spectrum illustrated the signals of aromatic protons (He, He0 and Hf, Hf0 ) as doublets at 7.37 and 6.68 ppm respectively. In the spectra of all
3.3. UV–vis spectra The electronic spectra of the ligand and its complexes in chloroform solvent were obtained in the range of 200–700 nm
Table 5 Antifungal activity of compounds(1.25, 2.5 and 5 mg/disk) as diameter of zone of Inhibition (mm). Compound
Ligand CdLCl2 CdLBr2 CdLI2 CdL(NCS)2
Candida albicans
Aspergillus niger
1.25
2.5
5
1.25
2.5
5
9.50 15.0 16.0 16.3 15.5
9.78 15.20 18.36 16.48 18.12
10.00 18.72 18.50 17.50 20.90
– 13.10 12.40 14.64 13.58
– 13.56 13.10 14.70 15.40
6.70 16.00 14.90 17.40 18.44
Fig. 1. The electronic spectra of ligand and its cadmium complexes.
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Fig. 2. 1H and
5
13
C NMR of ligand and its cadmium iodide complex.
complexes, these signals were smoothly shifted to the weaker fields with respect to the free ligand except for the CdL(NCS)2 that Hf and Hf0 were upfielded (appeared at 6.64 ppm). In the ligand spectrum, olefinic hydrogens of Hd and Hd0 were observed at 6.87 ppm as a doublet due to coupling with other olefinic protons (Hc, Hc0 ). The resonance of Hc, Hc0 in ligand spectrum was observed at 6.73 ppm as a doublet of doublet because of coupling with Hb, Hb0 and Hd, Hd0 respectively. After coordination of ligand to metal, these protons were desheilded in the CdLCl2, CdLBr2 and CdL(NCS)2 complexes and therefore the related signals were upfielded. But at the CdLI2 spectrum, the doublet of Hd, Hd0 showed an upwnfield shift (at 6.65 ppm) while the doublet of doublet signal of Hc, Hc0 smoothly shifted to downfield position (at 6.75 ppm) similar to other complexes. The ligand spectrum represented the chemical shifts of aliphatic hydrogens of Ha, Ha0 and Hg, Hg0 as two singlet signals at 3.81 and 3.01 ppm respectively. In all cadmium halide complexes spectra, Ha, Ha0 and Hg, Hg0 signals were appeared at downfielded chemical shifts with respect to the free ligand. In the CdL(NCS)2 spectrum, the methylene hydrogens signal (Ha, Ha0 ) was shifted to the stronger fields (at 3.67 ppm) and the singlet signal of the methyl protons (Hg, Hg0 ) was appeared at 3.01 ppm as same as the ligand.
The 13C NMR spectrum of the ligand displayed the iminic carbon signal (C8,80 ) at 164.79 ppm [25]. This signal was shifted to the downfield position (169.06–179.40 ppm) in the cadmium complexes, offering well binding of the ligand to metal center. The other signals of ligand carbons were exhibited at 150.96 (C2,20 ), 142.34 (C6,60 ), 130.54 (C7,70 ), 128.63 (C4,40 ), 123.62 (C5,50 ), 112.04 (C3,30 ), 61.89 (C9,90 ), 40.27 (C1,10 ) ppm. All above signals smoothly shifted to the downfield regions after complex formation except for C7,70 and C5,50 and aliphatic carbons that were upfielded at all complexes spectra. In addition to the above signals, a new signal was appeared at 128.78 ppm in the CdL(NCS)2 spectra that may be attributed to the carbon of coordinated thiocyanate. Finally, the spectral and physical data of ligand and its complexes support the pseudo-tetrahedral geometry around the cadmium ion as depicted in Scheme 1. 3.5. Antibacterial bioassay (in vitro) The novel synthesized Schiff base ligand and its cadmium complexes were checked for in vitro antibacterial activity against selected bacteria inclusive E. coli, P. aeruginosa, S. aureus and B. subtilis by the disk diffiusion method [45]. The antibacterial
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3.6. Antifungal bioassay (in vitro)
20
Bacillus subsslis
15
Staphylococccus aureus
10
Pseudomonaas aeruginosa
5
Escherichia ccoli
0
Fig. 3. Plots of antibacterial activity of compounds in concentration of 2.5 mg/disk.
activity data as zone (mm) of inhibition of growth have been presented in Table 3 and plotted as shown in Fig. 3. The results revealed that the ligand and its complexes have the different degrees of prevention efficiency upon the examined bacterial growth. All complexes exhibited more antibacterial activity than free Schiff base ligand in agree with many other reports [46]. A comparison between the antibacterial activity of free ligand and its complexes displayed that CdLCl2 complex was the most active compound against B. subtilis and S. aureus bacterial strains. Therefore, it can be supposed that the CdLCl2 complex would have a very good antibacterial activity against Gram positive bacteria. In the case of S. aureus, the antibacterial activity of compounds follows the order: CdLCl2 > CdLBr2 > ligand > CdL(NCS)2 > CdLI2. So, on S. aureus bacterium, the ligand showed a moderate effect. Also the ligand was a weak compound against other bacterial strains because it had the minimum inhibition zone diameter on them. CdL(NCS)2 and CdLI2exhibited the most activity against P. aeruginosa and E. coli respectively. In addition to above achievements, all compounds were tested for the MIC and MBC technique by Mueller Hinton broth (Table 4). In broth micro-dilution method for MIC detection of compounds, it was often impossible to observe the turbidity of MIC mixtures because all sample solutions were extremely colorful. Therefore, the MBC values were evaluated in the current study. In the case of B. subtilis, all MBC values were found to be 500 mg/ml. Against S. aureus, the lowest MBC were related to CdLCl2 and CdLI2 complexes. The best bactericide compounds on P. aeruginosa and E. coli were CdL(NCS)2 and CdLCl2with MBC values of 15.6 mg/ml and 7.81 mg/ml respectively. Finally, it is to be noted that the broth micro-dilution method (MBC data) often supported the disk diffusion results so that the bactericidal effects of some complexes were found to be better as compared with the free ligand.
The antifungal activities of the Schiff base ligand and its cadmium complexes were investigated in vitro against A. niger and C. albicans fungal cultures using the disk diffusion procedure (Table 5, Fig. 4). The antifungal activity of ligand was evaluated as moderate against C. albicans. The ligand showed very low activity against A. niger in high concentration (5 mg/disk) while it had no antifungal activity against it in lower concentrations (1.25 and 2.5 mg/disk). The CdL(NCS)2 complex demonstrated the maximum inhibitory effect on A. niger and C. albicans fungal strains as comparison with free ligand. It can be found from the data that the most of complexes displayed more antibacterial/antifungal activity in compared with free ligand. A review on literature indicates that generally coordination of an organic ligand to metal centers amplifies its antimicrobial properties [47]. It seems that overlapping of the ligand and metal ion orbitals enhances the resonance of p electrons on the compound structure and therefore increases the liposolubility of the complexes with respect to free ligand [48]. An increase in lipophilicity character and therefore better diffusion in microbial membrane may be considered as notable reason for this activity improvement. Regarding the mentioned reason, the improvement of antimicrobial activity of the titled cadmium complexes with respect to free ligand is explained. It is suggested that the
120
TG%
(A)
W.L%
100
µg/min) TG%/W.L%/DTG (µ
Inhibition zones
25
DTG ug/min
80 60 40 20 0 0
200
400
600
800
1000
T(°C) TG%
TG%/W.L%/DTG(µg/min)
6
90
WL%
(B) DTG ug/min
70 50 30 10 -10 0
200
400
600
800
1000
T(°C) Fig. 4. Plots of antifungal activity of compounds in concentration of 2.5 mg/disk.
Fig. 5. The TG/DTG/DTA diagrams of cadmium bromide (A) and thiocyanate (B) complexes.
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Table 6 Thermal analysis data including temperature range, DTG peak, mass loss, proposed segment and final residue. Compound
Temp. range (8C)
Mass loss (%) exp. (calcd.)
Peak temperature (8C)
Proposed segment
Final residue exp. (calcd.)
Ligand
216–316 316–456 456–948
34.86(32.11) 52.18(53.75) 11.86(14.17)
263 372 917
C8H10N C13H17N2 C3H3N
–
CdLCl2
223–477 477–842 842–992
26.19(29.44) 52.09(50.40) 21.48(20.15)
314 642 940
C10H16N2 C14H14Cl2N2 Cd
–
CdLBr2
250–430
23.63(22.61)
302
C10H12N
CdBr 26(29.74%)
430–899
50.13(47.60)
594
C14H18BrN3
251–372
16.39(16.22)
302
C8H10N
372–767 767–993
53.44(52.38) 13.2(16.22)
506 845
C8H10I2N2 C8H10N
218–378
25.54(27.23)
261
C10H16N2 C10H10N4
378–567 567–991
28.30(30.88) 11.61(12.62)
554 575
CdLI2
CdL(NCS)2
difference in antimicrobial activities of the cadmium complexes is related to the coordinated X anions in CdLX2. The X anions with different sizes and electronegativities can affect the antimicrobial properties by steric and electronic effects [49]. When the X anion varies from chloride to bromide, thiocyanate and iodide, the size of the complex and lipophilicity are increased (as steric effect) while the order of M–N polarity and positive charge density on metal ion in the complexes (as electronic effect) is suggested to be as CdLCl2 > CdL(NCS)2 > CdLBr2 > CdLI2.The first effect leads to more diffusion but the second effect causes more interaction of complex with cell membrane. Based on our results, it seems that in the case of B. subtilis and S. aureus, interaction with cell wall is an important agent for inhibition of the growth so that more efficient interaction of more positive metal ion with phosphate groups of cell membrane has led to more inhibitory effect of CdLCl2 with respect to others. In the case of P. aeruginosa, C. albicans and A. niger, both lipophilicity character and electronic effect are suggested to be effective so that CdL(NCS)2 with suitable polarity and lipophilicity has acted as an efficient antibacterial/antifungal active compound among the complexes. Finally in the case of E. coli, diffusion of compound into cell membrane is probably responsible for antibacterial activity such that the ZnLI2 with higher lipophilicity character than others effectively prevented the bacterial growth.
Cd 17(15.18%)
CdS2 30(29.27%)
C6H4
3.7. Thermal analysis (TG and DTG) of ligand and its cadmium cadmium complexes Thermal study of Schiff base ligand and its cadmium complexes tells us: (i) the thermal stability of them, (ii) existence or absence of water molecules in their structure and (iii) general pattern for pyrolysis of the compounds. Thermal analysis (TG/DTG) of all compounds was carried out up to 1000 8C under nitrogen atmosphere at a heating rate of 10 8C/min. As typical plots, the TG/DTG diagrams of cadmium bromide and thiocyanate complexes have been illustrated in Fig. 5. The mass loss for each compound was calculated within the corresponding temperature range and the data have been tabulated in Table 6. None of the TG curves showed any weight loss below 200 8C confirming absence of water molecules inside or outside the metal coordination sphere. The ligand thermo-gram displayed that the ligand was completely decomposed in three consecutive steps (found mass loss: 100%). It is related to elimination of C8H10N, C13H17N2 and C3H3N in the 1st, 2nd and 3rd thermal steps respectively. The cadmium complexes demonstrated different thermal behaviors. The CdLCl2 complex totally destructed at the applied temperature range via three steps without any final residue. At the first step, (at 223–477 8C), 26.19% of its weight was liberated as gas. Thereafter, 52.09% of molecular weight was eliminated at
Table 7 Thermokinetic activation parameters of the thermal decomposition of ligand and its cadmium complexes. Compound
Decomposition step (8C)
E* (KJ/mol)
A* (1 S1) 1.28 10 4.09 2.80
16
DS* (KJ/mol)
DH* (KJ/mol)
DG* (KJ/mol)
5.86 10 2.40 102 2.48 102
188.74 37.91 61.52
1.57 102 1.92 102 3.57 102
Ligand
216–316 316–456 456–948
193.19 43.27 71.42
CdLCl2
223–477 477–842 842–992
83.55 56.04 73.83
4.54 104 1.21 2.21
1.61 102 2.53 102 2.5 102
78.67 48.43 63.75
1.73 102 2.80 102 3.67 102
CdLBr2
250–430 430–899
107.54 45.25
7.81 106 4.34 101
1.18 102 2.61 102
102.76 38.04
1.71 102 2.64 102
CdLI2
251–372 372–767 767–993
174.83 43.15 19.65
9.50 107 6.23 101 1.17 101
3.66 102 2.57 102 2.74 102
170.05 36.67 10.35
3.80 102 2.37 102 3.16 102
CdL(NCS)2
218–378 378–567 567–991
121.86 155.42 107.36
1.22 109 3.53 107 1.24 104
7.58 101 1.09 102 1.75 102
117.42 148.54 100.31
1.58 102 2.39 102 2.49 102
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477–842 8C which may be attributed to segments of C10H16N2 and C14H14Cl2N2 respectively. Finally, the weight loss was continued above 842 8C and the final segment removal as Cd metal was occurred. The DTG plots of CdLBr2 complex exhibited two subsequent degradation steps at 302 and 594 8C, which may be attributed to loss of C10H12N and C14H18BrN3 respectively. The thermal treatment of CdL(NCS)2 and CdLI2 complexes proposed total elimination of organic moiety of them (with an estimated mass loss: 83.03% and 65.45% respectively) via three decomposition steps. Metal component and CdS2 were suggested as residual materials respectively. The thermodynamic activation parameters of decomposition processes of compounds termed as Arrhenius constant (A), activation energy (DE*), enthalpy (DH*), entropy (DS*) and Gibbs free energy change of the decomposition (DG*) were evaluated graphically by the aid of the Coats–Redfern equation [26,50]. The obtained data have been collected in Table 7. The activation energies of decomposition have been found in the range of 19.65– 193.19 kJ/mol. The high amounts of the activation energies represent the thermal stability of the complexes. Approximately, in all temperature ranges, DS*values are negative numbers that reflect the pyrolysis processes with a lesser rate than the normal ones as seen in many literature reports. Only, in one of the evaluated temperature ranges, DS* value is a positive number that demonstrates a dissociation property of thermal decomposition. DH*and DG*quantities are totally positive values and are found to be in the ranges of 10.35–188.74 kJ/mol and 1.57–3.80 102 kJ/ mol respectively. 4. Conclusion In this paper, we described the synthesized, spectroscopic recognition, antimicrobial and thermal study of some novel cadmium complexes of a bidentate Schiff base ligand. Their antibacterial/antifungal testing showed that all of them have acceptable antibacterial/antifungal activity and it was found that the antimicrobial activity was enhanced after binding of the ligand to cadmium center. The diameter of inhibition zone (mm) expressed that CdLCl2 (at 2.5 mg/disk) is a more effective compound against B. subtilis and S. aureus while CdL(NCS)2and CdLI2 (at 2.5 mg/disk) showed more antibacterial activities against P. aeruginosa and E. coli respectively. The CdL(NCS)2 (at 5 mg/disk) was found as the best antifungal active compound with respect to others. The thermal investigation of compounds indicated that the ligand was decomposed completely via three decomposition steps. The cadmium complexes were decomposed via 2–3 temperature steps. Furthermore, the thermodynamic activation parameters of decomposition processes of compounds termed as Arrhenius constant (A), DE*, DH*, DS* and DG* of the decomposition were evaluated graphically by use of the Coats– Redfern equation Acknowledgement Partial support of this research by Yasouj University is acknowledged.
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