Synthesis, spectroscopic characterization and antimicrobial studies of some metal complexes with 2-acetylpyridine phenoxyacetyl hydrazone (HAPPA)

2.1 Synthesis of the ligand

2-Acetylpyridine phenoxyacetyl hydrazone (HAPPA) Structure I) was prepared by heating equimolar amount of 2-acetylpyridine (12 mL, 0.1 mol) and phenoxyacetyl hydrazine (16.6 g, 0.1 mol) in 100 mL absolute ethanol for 1 h. On cooling, yellow crystals were separated. The product was filtered off, washed, recrystallized from absolute ethanol and finally dried in a vacuum desiccator over fused CaCl₂.

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Structure I

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2.2 Synthesis of metal chelates

The complexes, [M(HAPPA)Cl₂(H₂O)], (M = Mn(II), Co(II), Ni(II), Zn(II) and Cd(II)) and [M(HAPPA)Cl₂] (M = Cu(II) or Hg(II)) were prepared by mixing together an ethanolic solution (25 mL) of metal chloride (1 mmol) and HAPPA (0.27 g, 1 mmol) in the same solvent (25 mL). The reaction mixtures were heated under reflux for 0.5 h. The complexes, [M(APPA)OAc(H₂O)] (M = Cd(II), Ni(II), Co(II), Cu(II) and U(VI)O₂) were prepared by the same method using the corresponding metal acetate (1 mmol) in 25 mL water or ethanol. The [Pd(APPA)₂] complex was prepared by the same previous method using (2 mmol) of HAPPA in 50 mL absolute ethanol and (1 mmol) of Na₂PdCl₄ in 25 mL water.

2.3 Physical measurements

The metal and chloride contents were analyzed by the standard method (Vogel, 1989). Carbon and hydrogen were carried out at the Microanalytical Unit of Mansoura University. Magnetic moments at room temperature were determined using a Gouy balance and Hg[Co(SCN)₄] as a calibrant. IR spectra were recorded on a Mattson 5000 FTIR Spectrometer as KBr disks; electronic spectra in dimethylsulfoxide (DMSO) were obtained using an UV2-100 Unicam UV–visible spectrometer. ¹H NMR spectra were recorded on a Prucker Ac 400 Spectrometer. Molar conductances of the solid complexes (10⁻³ M) in DMSO at room temperature were measured using a type CD6NGT Tacussel Conductivity Bridge. ESR spectra were obtained on a Bruker EMX spectrometer working in the X-Band (9.78 GHz) with 100 kHz modulation frequency.

2.4 Corrosion inhibition studies

To evaluate the influence of the investigated ligand on the corrosion of aluminum in 0.5 M hydrochloric acid, aluminum was selected for this study due to its numerous industrial applications and consequently its corrosion inhibition in pickling baths is of great importance. The weight loss technique was employed. Each specimen was hung with the help of a glass hook and immersed completely in 50 mL of inhibitor solutions contained in a 150 mL beaker and kept in a thermostat at 30 °C. The experiments were carried out in duplicate. On completion of test periods 30, 60, 90, 120 and 180 min at different concentrations (5 × 10⁻⁵, 1 × 10⁻⁴, 5 × 10⁻⁴, 1 × 10⁻³ and 5 × 10⁻³ M). The specimens were removed, washed, dried and weighed. The change in weight was recorded. Therefore, the weight losses given by: ΔW = W₁ − W₂ where W₁ and W₂ are the weights of specimen before and after the reaction, respectively. The degree of dissolution, of course, was dependent on the surface area of the metal exposed, molecular size and the time of exposure; hence the amount of corrosion is given with respect to area and time. The resulting quantity, corrosion rate, is thus a fundamental measurement in corrosion science (ASTM, 1980).

2.5 Biological activity

The antimicrobial activity of the ligand and some complexes were tested as antibacterial agents. In this technique pores are made using a sterile cork paper in the solidified agar medium and an aliquot was prepared by putting 25 mL of inoculated agar into 15 cm petri-dishes and allowing them to solidify. Cups were made to receive 25 mL of the solution and allowed to diffuse and incubate at 37 °C for 24 h. Inhibition zone was measured (Gerhardt, 1981) and compared with that of gentamicin solution (commercial antibiotic, Memphis Co., Egypt, 1000 μg mL⁻¹). The experiment control was DMSO.

3.1 IR and ¹H NMR spectral studies

The infrared frequencies of the structurally important bands with their assignments of HAPPA and its metal complexes are listed in Table 2.

Table 2 IR spectral data of HAPPA and its metal complexes.

Compound	υ(NH)	υ(C⚌O)	υ(C⚌N)	υ(C⚌N) pyridine	υ(C⚌N—N⚌C)	υ(C–O)	υ(N–N)	Pyridine ring breathing mode	υ(M–O)	υ(M–N)
HAPPA	3199	1700	1620	1583	–	–	1077	995	–	–
[Mn(HAPPA)Cl2(H2O)]	3154	1675	1600	1595	–	–	1080	1019	505	420
[Cd(HAPPA)Cl2(H2O)]	3150	1675	1600	1593	–	–	1095	1016	508	420
[Ni(HAPPA)Cl2(H2O)]	3190	1662	1600	1593	–	–	1102	1024	508	433
[Co(HAPPA)Cl2(H2O)]	3150	1670	1600	1600	–	–	1080	1024	509	430
[Zn(HAPPA)Cl2(H2O)]	3190	1673	1600	1595	–	–	1091	1020	515	410
[Cu(HAPPA)Cl2]	3221	1711	1624	1602	–	–	1078	1030	–	420
[Hg(HAPPA)Cl2]	3195	1700	1518	1592	–	–	1091	1012	–	420
[Cd(APPA)OAc(H2O)]	–	–	–	1593	1565	1242	1080	1015	507	410
[Ni(APPA)OAc(H2O)]	–	–	–	1595	1563	1250	1084	1024	509	420
[Co(APPA)OAc(H2O)]	–	–	–	1592	1567	1252	1083	1024	509	426
[Cu(APPA)OAc(H2O)]	–	–	–	1595	1563	1247	1082	1021	508	426
[UO2(APPA)OAc(H2O)]	–	–	–	1593	1568	1240	1084	1015	505	416
[Pd(APPA)2]	–	–	–	1597	1570	1236	1076	1020	505	420

The IR spectrum of HAPPA reveals two bands at 1700 and 1620 cm⁻¹, attributed to υ(C⚌O) (Chundak et al., 1986) and υ(C⚌N) hydrazone, respectively. The bands observed at 1583 and 995 cm⁻¹ are assigned to υ(C⚌N) pyridine (Perlepes et al., 1989) and pyridine ring breathing mode (Rakha and Bekheit, 2000), respectively. A strong band at 1077 cm⁻¹ is attributed to υ(N—N) (Rakha, 2000), while the band at 3199 cm⁻¹ is assigned to the υ(NH) vibration. The two medium intensity bands at 610 and 430 cm⁻¹ are assigned to the pyridine ring deformation, in the plane ring deformation and out of plane deformation (Madden et al., 1970; Maddan and Nelson, 1968).

A comparison of IR spectra of HAPPA and its metal complexes (Table 2) reveals that HAPPA is a bidentate and/or tridentate ligand depending upon the metal salt used and the reaction condition.

The IR spectral data of [M(HAPPA)Cl₂(H₂O)] (M = Mn(II), Co(II), Ni(II), Zn(II) and Cd(II)), show that HAPPA behaves as a neutral tridentate ligand coordinating via the azomethine nitrogen (C⚌N) hydrazone, the carbonyl oxygen (C⚌O) and the nitrogen of the pyridine ring. This mode of chelation (Structure II) is supported by the following observations: (i) υ(C⚌O) and υ(C⚌N) hydrazone shifts to lower wavenumber, (ii) υ(C⚌N) pyridine and pyridine ring breathing mode shifts to higher wavenumber (El-Saied et al., 1991; Maurya et al., 1995), (iii) υ(N—N) shifts to higher wavenumber and (iv) the appearance of new bands in the low frequency regions at 515–490 and 433–410 cm⁻¹ assignable to υ(M—O) (Speca et al., 1974) and υ(M—N) (Beecroft et al., 1974), respectively.

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Structure II

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Also, HAPPA may act as a mononegative tridentate ligand coordinating via deprotonated carbonyl oxygen (⚌C—O⁻), azomethine nitrogen (C⚌N) hydrazone and pyridine nitrogen(C⚌N). This mode of chelation (Structure III) is found in case of [M(APPA)OAc(H₂O)] (M = Cd(II), Ni(II), Co(II), Cu(II) and U(VI)O₂) and confirmed by the following evidences: (i) the disappearance of υ(C⚌O), υ(C⚌N) hydrazone and υ(NH) with the simultaneous appearance of new bands in the range 1242–1252 and 1563–1567 cm⁻¹ assignable to υ(C-O) and υ(C⚌N-N⚌C), respectively (El-Saied et al., 1991), (ii) υ(C⚌N) pyridine and pyridine ring breathing mode shifts to higher wavenumber (El-Saied et al., 1991; Maurya et al., 1995), and (iii) the appearance of new bands in the regions 510–490 and 430–425 cm⁻¹ assignable to υ(M-O) (Speca et al., 1974) and υ(M-N) (Beecroft et al., 1974), respectively.

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Structure III

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In [M(HAPPA)Cl₂] (M = Cu(II) or Hg(II)) , HAPPA is a neutral bidentate ligand coordinating via both (C⚌N) hydrazone and (C⚌N) pyridine ring nitrogen forming five member ring including the metal atom (Structure IV). In this mode of chelation υ(C⚌N) hydrazone as well as υ(C⚌N) pyridine shift to higher wavenumber. The shifting of υ(C⚌N) hydrazone has been reported to be both to higher wavenumber (Raina and Srivastava, 1982; Sathyadevi et al., 2012; Chandra et al., 1977; Bhoon, 1983) or to lower (Mohan and Manmohan, 1982; Singh et al., 1977; Mishra et al., 1979; Mahapatra and Pujaris, 1983) wavenumber in the clase of chelating ligand containing the —N⚌C—C⚌N— grouping. This mode of complexation is supported by the following observations: (i) υ(C⚌N) pyridine and pyridine ring breathing mode shift to higher wavenumber, (ii) the appearance of new bands at 420 cm⁻¹ assignable to υ(M-N) (Beecroft et al., 1974), and (iii) υ(C⚌O) remains in the same position in Hg(II) complex and slightly shifts to higher wavenumber in Cu(II) complex, indicating that the carbonyl oxygen (C⚌O) is not taking part in coordination.

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Structure IV

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Finally, the IR spectral data of [Pd(APPA)₂], reveal that HAPPA acts as a mononegative bidentate ligand coordinating via the azomethine nitrogen (C⚌N) hydrazone and deprotonated carbonyl oxygen (⚌C—O⁻) forming five member ring including the metal atom (Structure V). This mode of coordination is suggested by the following evidences: (i) the disappearance of υ(C⚌O), υ(C⚌N) hydrazone and υ(NH) with the simultaneous appearance of new bands at 1236 and 1570 cm⁻¹ assignable to υ(C-O) and υ(C⚌N—N⚌C), respectively (El-Saied et al., 1991; Raina and Srivastava, 1982; Lorenzini et al., 1983), (ii) υ(N-N) shifts to higher wavenumber and (iii) the appearance of new bands at 505 and 420 cm⁻¹ attributable to υ(Pd-O) (Speca et al., 1974] and υ(Pd-N) (Beecroft et al., 1974), respectively.

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Structure V

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In the spectra of all the metal complexes except Pd(II) complex, the bands at 610 and 430 cm⁻¹ due to pyridine deformation shift to higher wavenumber compared to the free ligand, indicating coordination of pyridine nitrogen to the metal atoms (Singh et al., 1977; Mishra et al., 1979).

The acetate complexes show two new bands in 1518–1543 and 1440–1471 cm⁻¹ regions assignable to υ_as(O–C-O) and υ_s(O-C–O), respectively of the acetate group. The difference between these two bands indicates bidentate coordination of the acetate group (Nakamoto, 1986) as shown in Structure III.

The uranyl complex exhibits two bands at 910 and 825 cm⁻¹ assigned to υ₃ and υ₁ vibrations, respectively, of the dioxouranium ions (Hsieh et al., 1975). The υ₃ value is used for the calculation of the force constant (F) of the (O⚌U⚌O) bond by the method of McGlynn and Smith (1961). $${({\upsilon }_3)}^2={(1307)}^2(F_{\text{U}-\text{O}})/14.103$$

The calculated force constant is found to be 6.882 mdyn Å⁻¹. The value of the force constant for the uranyl complex was then substituted into the relation given by Jones (Jones, 1959) to calculate the bond distance (R_U–O): $$R_{\text{U—O}}=1.08{(F_{\text{U—O}})}^{-1/3}+1.17$$ The value of the bond distance (R_U–O) = 1.74 Å⁻¹ as well as the calculated value of the bond force, F_U–O, fall in the usual range for the uranyl complex (Rakha et al., 1996; Asadi and Shorkaei, 2013).

The ¹H NMR spectrum of HAPPA in DMSO-d6 (Fig. 1) shows two signals at δ 10.83 and δ 10.71 ppm which disappear upon adding D₂O. These signals may be assigned to the NH proton. The presence of two signals can be ascribed to the presence of amide–imide toutomerism (NH—C⚌O) $\rightleftharpoons$ (N⚌C—OH) the amide–imide ratio was found to be 1:1. The multi-signals appear in the δ 6.99–8.51 regions are assigned to the protons of phenyl and pyridyl rings. The sharp singlet observed at δ 2.5 ppm is attributed to the methyl protons. Strong evidence for the deprotonation of the enolized carbonyl (⚌C—OH) comes from the ¹H NMR spectrum of the diamagnetic [Cd(APPA)OAc(H₂O)] (Structure III) which shows the disappearance of the two signals at δ10.83 and δ 10.71 ppm due the NH proton (Fig. 2).

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Figure 1 ¹H NMR spectrum of HAPPA.

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Figure 2 ¹H NMR spectrum of [Cd(APPA)(OAc)(H₂O)].

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3.2 Electronic, ESR spectra and magnetic moment measurements

The magnetic moments, electronic absorption bands in (DMSO) and ligand filed parameters of metal complexes are reported in Table 3. The electronic spectra of [Ni(HAPPA)Cl₂(H₂O)] and [Ni(APPA)OAc(H₂O)] complexes show two bands in the 14705–15441 and 25566–26305 cm⁻¹ regions assigned to ${}^3{\text{A}}_{2_{\text{g}}}⟶{}^3{\text{T}}_{1_{\text{g}}}(\text{F})({\upsilon }_2)$ and ${}^3{\text{A}}_{2_{\text{g}}}⟶{}^3{\text{T}}_{1_{\text{g}}}(\text{P})({\upsilon }_3)$ transitions, respectively in octahedral environments around nickel(II) ion (Lever, 1984). The calculated values of the ligand filed parameters (Table 3) lie in the range reported for octahedral structure. Also, the magnetic moment values (3.1–3.2 BM) are consistent with those reported for octahedral geometry.

The electronic spectra of [Co(HAPPA)Cl₂(H₂O)] and [Co(APPA)OAc(H₂O)] complexes show two bands in the 16670–17151 and 20408–20842 cm⁻¹ regions assigned to ${}^4{\text{T}}_{1_{\text{g}}}⟶{}^4{\text{A}}_{2_{\text{g}}}$ and ${}^4{\text{T}}_{1_{\text{g}}}⟶{}^4{\text{T}}_{1_{\text{g}}}(\text{p})$ transitions, respectively. The calculated D_q, B and β values (Table 3) lie in the range reported for octahedral structure (Lewis and Wilins, 1960). The magnetic moment values (3.9–4.0 BM) are near the spin-only moment (3.87 BM) indicating that the orbital contribution was not affected.

The magnetic moment value (2.1 BM) of [Cu(HAPPA)Cl₂] and [Cu(APPA)OAc(H₂O)] complexes lies in the range measured for the d⁹ system that contains one unpaired electron and shows no Cu-Cu interaction. The electronic spectrum of [Cu(APPA)OAc(H₂O)] shows two broad bands at 14164 and 15080 cm⁻¹ assigned to ${}^2{\text{B}}_{1_{\text{g}}}⟶{}^2{\text{E}}_{\text{g}}$ and ${}^2{\text{B}}_{1_{\text{g}}}⟶{}^2{\text{A}}_{1_{\text{g}}}$ , respectively in a tetragonal distorted octahedral configurations (Sacconi, 1979). On the other hand, the electronic spectrum of [Cu(HAPPA)Cl₂] shows abroad band with a maximum at 14663 cm⁻¹ due to ${}^2{\text{B}}_{1_{\text{g}}}⟶{}^2{\text{A}}_{1_{\text{g}}}$ transition in a square planar geometry (Palaniandavar and Natarajan, 1980). Both Cu(II) complexes show bands in the 28902–29386 cm⁻¹ regions which may be due to charge transfer.

[Mn(HAPPA)Cl₂(H₂O)] complex shows a band at 20786 cm⁻¹ and strong band at 22727 cm⁻¹ which may be due to ${}^6{\text{A}}_{1_{\text{g}}}⟶{}^4{\text{T}}_{1_{\text{g}}}(\text{G})$ and ${}^6{\text{A}}_{1_{\text{g}}}⟶{}^4{\text{T}}_{2_{\text{g}}}(\text{G})$ transition, respectively, in an octahedral configuration (Lewis and Wilins, 1960) around the metal ion. Also, the value of magnetic moment (5.6 BM) lies in the range measured for the high spin d⁵ system.

The UV spectrum of the [UO₂(APPA)OAc(H₂O)] complex shows two bands at 20834 and 29210 cm⁻¹ assignable to ${}^1{\sum }_g^{+}⟶{}^2{\pi }_u$ transition of dioxouranium(VI) and charge transfer probably $n⟶{\pi }^{\ast }$ transition, respectively (Ghatlacharya and Bera, 1975).

Finally, the [Pd(APPA)₂] complex is diamagnetic and its electronic spectrum shows two bands at 14.852 and 22.406 cm⁻¹ assignable to ${}^1{\text{A}}_{1_{\text{g}}}⟶{}^1{\text{A}}_{2_{\text{g}}}$ and ${}^1{\text{A}}_{1_{\text{g}}}⟶{}^1{\text{B}}_{1_{\text{g}}}$ transition, respectively in square planar geometry, the band observed at 37.765 cm⁻¹ may be due to charge transfer (Rabar and Shinde, 1983; Bhave and Kharat, 1980).

The ESR spectrum of the square-planar [Cu(HAPPA)Cl₂] complex showed anisotropic spectrum of an axial-elondated type. It gave two g-values, $g_{||}=2.43\text{and}{g}_{\perp }=2.11$ . The small G values for [Cu(HAPPA)Cl₂] equal 3.91 indicating strong interaction between the ligand and the metal ion. Also, $g_{||}>g_{\perp }$ suggests dx²-y² ground-state (Speier et al., 1996). The spin Hamiltonian parameters of this complex were calculated (Table 4). In axial symmetry the g-values are related by the expression $G=(g_{||}-2)/(g_{\perp }-2)=4$ , where G is the exchange interaction parameter. According to Hathaway and Billing (1970), if the value of G is greater than 4, the exchange interaction between Copper(II) centers in the solid state is negligible , whereas less than 4 indicates a considerable exchange interaction. The calculated G values $(G=(g_{||}-2)/(g_{\perp }-2)=4)$ for [Cu(APPA)OAc(H₂O)] are 4.6 suggesting no copper–copper exchange interactions.

The tendency of A_|| to decrease with an increase of g_|| is an index of increasing of tetrahedral distortion in the coordination sphere of copper (Mesa et al., 1998) in order to quantify the degree of distortion of copper (II) complexes, we selected the f factor g_||/A_|| obtained from ESR spectra, which is considered as an empirical index of tetrahedral distortion (Kasumo, 2001). It ranges between 105 and 135 for square-planar complexes, depending on the nature of the coordinated atoms; a tetrahedrally-distorted structure can have much larger values (Wellman and Hulsbergen, 1978). For [Cu(HAPPA)Cl₂] the quotient is 135, supporting square-planar geometry with no appreciable tetrahedral distortion. The ratio g_||/A_|| for [Cu(APPA)OAc(H₂O)] is 138, demonstrating significant dihedral angle distortion in the xy-plane and tetrahedral distortion from square-planar geometry.

Molecular orbital coefficients, α₂ (covalent in-plane σ-bonding) and β₂ (covalent in-plane π-bonding), were calculated (Ray and Kauffman, 1990; Jayasubraman et al., 1995; Anthonisamy and Murugesan, 1998; Anthonisamy et al., 1999). $$\begin{array}{c}{\alpha }_2=-(A_{||}/0.036)+(g_{||}-2.0023)+3/7(g_{\perp }-2.0023)+0.04\hfill \\ {\beta }_2=(g_{||}-2.0023)E/-8\lambda {\alpha }_2\hfill \end{array}$$ λ = 828 cm⁻¹ for the free ion and E is the ${}^2{\text{B}}_{1_{\text{g}}}⟶{}^2{\text{A}}_{1_{\text{g}}}$ transition in plane σ-bonding α₂ = 1 indicates complete ionic character, whereas α₂ = 0.5 denotes 100% covalent bonding, with the assumption of negligibly small values of the overlap integral. The β₂ parameter gives an indication of the covalence of the in-plane π-bonding. The smaller β₂, gives an indication of the larger covalence of the bonding.

The values of α₂ and β₂ for [Cu(APPA)OAc(H₂O)] indicate that the in-plane σ-bonding and in-plane π-bonding are appreciably covalent and are consistent with very strong in-plane π-bonding, but in [Cu(HAPPA)Cl₂], the results indicate that the in-plane σ-bonding and in-plane π-bonding are appreciably ionic. These results are anticipated because there are appropriate ligand orbitals to combine with the d_xy orbital of copper(II). For square-planar geometry complexes, lower values of β₂ compared to α₂ indicate that the in-plane π-bonding is more covalent than the in-plane σ-bonding these data are consistent with reported values (Raman et al., 2001; John, 2003; Al-Hazmi et al., 2005; El-Metwally et al., 2006).

3.3 Studying the corrosion behavior of aluminum by the chemical technique

Fig. 3 shows the weight loss-time curves for the corrosion of aluminum in 0.5 M hydrochloric acid in the absence and presence of different concentrations of the investigated compound at 30 °C. As shown in this figure, by increasing the concentration of these compounds, the weight loss of aluminum samples is decreased. This means that the presence of this compound retards the corrosion. The decrease in corrosion rate of aluminum in 0.5 M hydrochloric acid in the presence of theses investigated compounds indicates that these compounds act as inhibitors (El Hosiry et al., 1972; Fouda et al., 1995). The linear variation of weight loss with time in uninhibited and inhibited 0.5 M hydrochloric acid indicated the absence of insoluble surface films during corrosion. In the absence of any surface film, the inhibitors are first adsorbed onto the metal surface and thereafter impede corrosion either by merely blocking the reaction sites (anodic and cathodic) or by altering the mechanism of the anodic and cathodic partial processes. The percentage inhibition efficiency (% IE) of the investigated compounds (Table 5) was determined using the equation: $$\%\text{IE}=\frac{\mathrm{\Delta }W-\mathrm{\Delta }{W}_i}{\mathrm{\Delta }W}\times 100$$

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Figure 3 Weight loss-time curves for the corrosion of aluminum in 0.5 M HCl in the absence and presence of different concentration of compound (HAPPA) at 30 °C.

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3.4 Antimicrobial activity bioassay

The antimicrobial activities of ligand and complexes against Bacillus thuringiensis and Pseudomonas aureginosa are summarized in Table 6. Growth inhibition zones are proportional to the antimicrobial activity of the tested compound in Fig. 4. The data suggest that Gram-Positive and Gram-Negative bacteria were affected by the tested chemicals with the strongest activity for cobalt and Mercury complexes, also, manganese for G−ve bacteria.

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Figure 4 Antimicrobial effect of HAPPA ligand and its metal complexes against G+ve and G−ve bacteria.

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