Synthesis, spectroscopic characterization and thermal studies of some divalent transition metal complexes of 8-hydroxyquinoline

2.1 Materials and methods

All the chemicals used were of analytical grade and purified by standard methods prior to use. Nutrientagar and potato dextrose agar were purchased from Hi-Media Chemicals, India. Chloride, nitrate, and sulfate metal(II)-salts were used in their hydrated form.

The elemental analysis was performed with a Vario EL CHN elemental analyzer. The FT-IR spectra were recorded on a Perkin Elmer Spectrum GX spectrophotometer using KBr pellets. The ¹H and ¹³C (APT) NMR spectra were recorded on a Bruker 400 MHz instrument using DMSO-d₆ as solvent, and TMS as internal reference standard. Diffuse electronic spectra were recorded on a Beckman DK-2A spectrophotometer using MgO as a reference. Magnetic moments were determined by the Gouy method using mercury tetrathiocyanatocobaltate(II) [HgCo(NCS)₄] as calibrant (χ_g = 1644 × 10⁻⁶ cgs units at 20 °C) and the diamagnetic correction was made using Pascal’s constant. The thermogravimetric analysis was carried out using a Perkin Elmer thermogravimetry analyzer at a heating rate of 10 °C per minute in air. The metal contents of the complexes were analyzed by EDTA titration after decomposing the organic matter with HClO₄, H₂SO₄ and HNO₃ (1:1.5:2.5) mixture (Jeffery et al., 1989). The melting point of MDMQ was checked by the standard open capillary method and is uncorrected.

The ligand MDMQ was synthesized using 5-chloromethyl-8-hydroxyquinoline hydrochloride (CMHQ) as a starting material by the modification of a reported method (Peng et al., 2007) and its metal (II) complexes were synthesized using a previously reported procedure (Shah et al., 2008). The outline for the synthesis of MDMQ and its metal complexes are shown in Schemes 1 and 2.

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Scheme 1

Synthetic route for ligand.

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Scheme 2

Synthetic route for metal complexes.

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2.2 General procedure for the synthesis of ligand (MDMQ)

MDMQ was prepared according to the method reported for CMQ-alcohol reaction (Patel and Patel, 2011). To a suspension of 5-chloromethyl-8-hydroxyquinoline hydrochloride (2.3 g, 0.01 mol), 1-methyl-1,2-dihydroquinolin-4-ol (1.60 g, 0.01 mol) in an acetone–water mixture was added. The resulting mixture was refluxed for 3 h with occasional shaking. The resulting suspension, which contained a green precipitate was made alkaline with dilute aqueous ammonia and then filtered. The solid product was collected and dried to give MDMQ (76% yield). The formation of the product was confirmed by TLC using CHCl₃:CH₃OH (80:20) mixture as the mobile phase (R_f 0.56). The product melted with decomposition at above 252 °C (uncorrected).

2.3 General procedure for the synthesis of metal complexes

A warm solution of metal(II) salt (0.01 mol, 2.91 g) in 50% aqueous formic acid was added drop by drop with continuous stirring to previously warmed solution of MDMQ (0.02 mol, 4.74 g) in 20% aqueous formic acid solution. With the proper adjustment of the pH (∼8.5) with 50% NH₄OH, the resultant mixture was further digested in a water bath for 4–5 h and centrifuged. The suspended solid was allowed to settle and collected by filtration, washed with sufficient quantity of distilled water and then with little hot ethanol and acetonitrile, then dried in vacuum desiccators over anhydrous calcium chloride. The coordination polymers are insoluble in all common organic solvents like methanol, ethanol, chloroform, acetone, benzene and dimethylsulfoxide. The yield was 80%–90%.

2.4 Preparation of the microbial culture

The antimicrobial activity of ligand and its metal complexes was investigated against bacterial strains of Escherichia coli, Bacillus subtilis, Staphylococcus aureus and a yeast strain of Saccharomyces cerevisiae, by standard microbiological parameters using the agar diffusion method (Chandra et al., 2009).The concentration of the compounds tested for the antimicrobial activity was 500 ppm. The bacterial culture was maintained on N-agar (N-broth, 2.5% w/v agar). The yeast culture was maintained on MGYP in 3%(w/v) agar.

For inoculum developments of bacterial and yeast culture, a loop of cell mass from pregrown slants was inoculated into sterile N-broth tubes containing 15 ml medium and incubated on a shaker at 150 rpm and 40 °C for 24-h to obtain sufficient cell density (i.e., 1 × 10⁸ cells/ml). Sterilized melted soft N-agar (1% in distilled water, 10.0 ml) was cooled to 40 °C and inoculated with 0.01 suspension of the test culture, mixed thoroughly and poured into the petri dish containing sterilized N-agar medium and allowed to solidify.

Two cups were prepared with the help of a sterile cup-borer on opposite ends. One was filled with 0.1 ml of test sample and other with 0.1 ml of DMSO solvent as the control. To find the minimum inhibitory concentration (MIC), all the cultures were tested for different concentrations of compound ranging from 50 to 1000 ppm. Thereafter, the plates were transferred to the refrigerator for 10 min to allow the sample diffuse out from the ditch and into the agar before the organism start growing. This is followed by incubation at 40 °C for 24 h. Next day the distance in millimeter (mm) from the ditch was measured as a parameter of inhibition.

3.1 IR Spectra

The important infrared spectral bands and their tentative assignments are discussed here. The (C–O) stretching in the free oxine molecule at 1082 cm⁻¹, shifted to higher frequencies in all the metal complexes, giving a strong absorption band at ∼1149 cm⁻¹ (Kharadi and Patel, 2009). This clearly indicates the coordination of 8-hydroxyquinoline in these complexes. The broad band at 3283 cm⁻¹ observed in the case of ligand was shifted at ∼3325 cm⁻¹, which was attributed to (O–H) of coordinated water molecule. In the investigated metal complexes, the bands observed in the regions of 3309–3324, 1267–1284, 865–875 and 710–715 cm⁻¹ are attributed to –OH stretching, bending, rocking and wagging vibrations respectively, due to the presence of water molecules. The presence of rocking band indicates the coordination nature of the water molecule (Nakamoto, 1978).

However, comparisons of IR spectra of ligand (MDMQ) and its metal (II)-coordinated complexes showed some important characteristic differences (Nakamoto, 1997). One of the significant differences to be expected was the presence of a more broadened band in the region of 2800–3500 cm⁻¹ for the chelates, as the oxygen of the –OH group of the ligand forms a coordination bond with the metal ions. It also explains the presence of coordinated water molecules (Sadasivam and Alaudeen, 2007). Another noticeable difference is that the band due to the C–N stretching vibration of 8HQ at around 1650 cm⁻¹ in the IR spectrum of MDMQ was shifted to a lower frequency, whereas the band at 1400 cm⁻¹ in the IR spectrum of MDMQ assigned to in-plane –OH deformation was shifted toward a higher frequency in the spectra of the chelates due to the formation of the M–O bond (Charles et al., 1956). This was further confirmed by a weak band at 1200 cm⁻¹ corresponding to C–O–M stretching, while bands around ∼735 and ∼550 cm⁻¹ correspond to the M → N vibration (Ilhan et al., 2009). These characteristic features of the IR studies suggest the formation of MDMQ and its metal(II) complexes. The IR spectrum of ligand and its metal complexes is shown in Fig. 1.

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Figure 1

IR spectrum of ligand and its metal complexes.

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3.2 ¹H NMR analysis

The structure of the ligand (MDMQ) was characterized by ¹H NMR spectrum in DMSO-d6 system (Fig. 2). The signal at δ = 3.08 ppm (s, 3H, –CH₃ protons) is assigned to aromatic methoxy protons. A sharp peak at δ = 9.5 ppm (s, 1H, –OH protons) is assigned to one aromatic hydroxyl proton (Kidri et al., 2004), which is confirmed by the D₂O exchange experiment. Aromatic protons are observed at 8.88 (d, 1H, H₂), 7.62 (dd, 1H, H₃), 8.50 (d, 1H, H₄), 7.35 (d, 1H, H₆), 7.08 (d, 1H, H₇), 4.96 (s, 2H, H₉),4.85(t, 1H, H₁₁),4.02(d, 2H, H₁₂), 6.72(d, 1H, H₁₃),7.15(dd, 1H, H₁₄), 6.74 (dd,1H, H₁₅), 7.95(d,1H, H₁₆).

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Figure 2

NMR spectrum of ligand.

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3.3 Diffuse electronic spectral and magnetic properties data

The diffuse electronic spectra of [Cu(MDMQ)₂(H₂O)₂] exhibited two bands at 26225 cm⁻¹ due to charge transfer and a broad band having maxima at 15585 cm⁻¹ due to the ²E_g → ²T_2g transition. The broadening of the signal might be due to Jahn–Teller distortion. The absorption bands of the diffuse electronic spectra and value of their magnetic moment favor a tetragonally distorted octahedral geometry around Cu(II) ion (Kriza et al., 2001; Vanparia et al., 2010). The [Ni(MDMQ)₂(H₂O)₂] complex shows three weak absorption bands at 9896, 16057, and 24508 cm⁻¹ corresponding to the characteristic transitions ³A_2g → ³T_2g, ³A_2g → ³T_1g(F), and ³A_2g → ³T_1g(P), while [Co(MDMQ)₂(H₂O)₂] exhibits three absorption bands at 9820, 15510, and 22150 cm⁻¹, respectively, due to ⁴T_1g(F) → ⁴T_2g(F), ⁴T_1g(F) → ⁴A_2g(F), and ⁴T_1g(F) → ⁴T_1g(P) transitions. The absorption bands of the diffuse electronic spectra and values of their magnetic moments showed an octahedral geometry around Ni(II) and Co(II) ions (Raman et al., 2005; Bailer et al., 1959). The spectra of [Mn(MDMQ)₂(H₂O)₂] showed weak bands at 16786, 18457, and 23808 cm⁻¹ assigned to the ⁶A_1g → ⁴T_1g, ⁶A_1g → ⁴T_2g, and ⁶A_1g → ⁴A_1g, ⁴E_g transitions, and magnetic moment value suggesting an octahedral geometry for the Mn(II) ion. The spectrum for [Fe(MDMQ)₂(H₂O)₂] showed bands at 18955 and 36782 cm⁻¹ assigned to the ⁵T_2g → ³E_g and ⁵T_2g → ³T_1g transitions, suggesting its octahedral geometry in support of magnetic moment value around Fe(II) ion. As the spectrum of [Zn(MDMQ)₂(H₂O)₂] was not well resolved, it was not well-interpreted even though the magnetic moment value showed that it is diamagnetic in nature as expected (Patel and Patel, 2011). The magnetic moment values of all the M(II) complexes are given in Table 1.

3.4 Thermo gravimetric analysis

Thermogravimetric analysis (TGA) for the MDMQ and its metal complexes was carried out within the temperature range of 40–900 °C in air at the heating rate of 10 °C min⁻¹ to establish their compositional differences and to ascertain the nature of associated water molecules. The thermogravimetric curves of all the compounds are presented in Fig. 3. The determined temperature ranges and corresponding percentage mass losses accompanying the changes on heating revealed the following findings.

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Figure 3

TGA of ligand and its metal complexes.

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Securitization of these data envisages that MDMQ follows single-step thermal decomposition. The initial weight loss of 1% might be due to loosely held solvent in MDMQ. The weight loss commences in the range of 150–550 °C up to 74% and further slower degradation takes place up to 900 °C.

Similarly, securitizations of the TGA data of complexes of corresponding Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II) have shown a two-step decomposition. The initial weight loss occurring might be due to the solvent molecules or loosely held moisture trapped inside the complexes, whereas the weight loss observed in the range of 125–180 °C might be attributed to the metal-coordinated two water molecules (Nikolaev et al., 1960). This also satisfies the six coordination sites of the metal ions of metal(II) complexes. For metal (II) complexes maximum weight loss (72.60%, 76.41%, 74.59%, 75.10%, 72.32% and 74.20%) was observed in the temperature range of 210–900 °C, and the remaining weights of 18.85 (878 °C), 18.80 (779 °C), 18.35 (697 °C), 19.40 (664 °C), 21.00 (631 °C) and 19.70 (784 °C) correspond to a mixture of metal oxide and some ashes as the ultimate pyrolysis products.

There was a remarkable difference in the mode of thermal degradation for MDMQ and its metal complexes. Complexes have shown fast decomposition patterns as compared with MDMQ. These results of thermal behavior can be explained by the fact that the decomposition of chelates was catalytically induced by the metal ions (DeGeiso et al., 1965).

The energy of activation (E_a) for thermal decomposition of MDMQ and its complexes was estimated by the reported method using the following equation (Brido, 1967). $$\text{ln}[\text{ln}(1/y)]=(\mathit{Ea}/\mathit{RT}+1)\text{ln}T+\text{constant}$$

The energy of activation (E_a) was computed from the slope (−E_a/R) of the plot of ln[ln (1/y)] vs (lnT), and is noted in Table 1. The energy of activation for MDMQ was found to be 14 kcal mol⁻¹, while the activation energy observed for complexes was between 8.5 and 11.2 kcal mol⁻¹.

3.5 LC–MS mass spectra

The recorded LC–MS spectrum and molecular ion peak for ligand (MDMQ) was used to confirm their molecular formula. Peak at 317.37 m/z values represents the molecular ion peak of ligand. LC–MS mass spectrum of ligand is shown in (Fig. 5).

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Figure 5

LC–MS spectrum of ligand.

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3.6 Antimicrobial activity of ligand and its metal complexes

The antimicrobial activity of ligand (MDMQ) and its metal complexes was studied against standard bacterial strains of E. coli, B. subtilis, S. aureus and a yeast strain of S. cerevisiae.

The compounds were tested at different concentrations ranging from 50 to 1000 ppm to find out the minimum concentration of the ligand and its metal complexes which inhibits the microbial growth. The minimum concentration was to be found. The inhibition of growth from ditch was measured in millimeter (mm) and the results show that all the metal complexes exhibit antimicrobial activity against one or more strain Table 2.

The ligand was found biologically active and its complexes showed significantly enhanced antibacterial activity against one or more bacterial species compared to the uncomplexed ligand. It is known that chelation tends to make the ligand more potent bacterial agent. The antimicrobial activity of compounds increases after chelation. Chelation reduces the polarity of the metal ion by partially sharing its positive charge with the donor groups (Nishant et al., 2006), increasing the lipophilic nature of the central metal ion, which in turn favors its permeation into the lipid layer of the bacterial membrane. Generally it is suggested that the chelated complexes deactivated various cellular factors viz., stability constant, molar conductivity, solubility and magnetic moment, are also responsible for increase in the antimicrobial activity of the complexes (Nishant et al., 2006).

Comparative analysis for antimicrobial study of ligand and its metal complexes are shown in Fig. 4, it is observed that some of the polychelates were more potent bactericides than the ligand. This enhancement in antibacterial activity is rationalized on the basis of Overtone’s concept, Tweedy’s chelation theory and the partial sharing of the positive charge of metal ions with donor groups (Dharmaraj et al., 2001; Chohan et al., 2004; Panchal et al., 2006). This may support the argument that some type of biomolecular binding to the metal ions or interchelation or electrostatic interactions causes the inhibition of biological synthesis and thus preventing the reproduction of organisms. The enhanced biological activity and lower toxicity of the transition metal complexes compared to the biologically active ligand make them useful in medicine and other biological applications (Kolokova et al., 2002). So it can be concluded that the complex exhibits higher antimicrobial activity than the free ligand (Raman et al., 2003).

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Figure 4

Comparative analysis for Antimicrobial activity of the ligand and its metal complexes.

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