Spectroscopic, electrochemical and biological studies of the metal complexes of the Schiff base derived from pyrrole-2-carbaldehyde and ethylenediamine

2.1 Synthesis of ligand and metal complexes

2.2 Molecular modeling

3D molecular modeling of the proposed structure of the complexes was performed using CsChem3D Ultra-11 program package. The correct stereochemistry was assured through the manipulation and modification of the molecular coordinates to obtain reasonable low energy molecular geometries. The potential energy of the molecule was the sum of the following terms (E) = E_str + E_ang + E_tor + E_vdw + E_oop + E_ele, where all Es represent the energy values corresponding to the given types of interaction (kcal/mol). The subscripts str, ang, tor, vdw, oop and ele denote bond stretching, angle bonding, torsion, deformation, vanderwaals interactions, out of plain bending and electronic interaction, respectively. The molecular mechanics describe the application of classical mechanics to the determination of molecular equilibrium structures. It enables the calculation of the total static energy of a molecule in terms of deviations from reference unstrained bond lengths, angles and torsions plus non-bonded interactions. On account of non-bonded interactions and also the chemical sense of each atom, treat the force field as a set of constants that have to be fixed by appeal to experiment or more rigorous calculations. It has been found that off-diagonal terms are usually largest when neighboring atoms are involved, and so we have to take account of non-bonded interactions, but only between next-nearest neighbors.

2.3 Biological activity: antibacterial screening

In vitro antibacterial activity of the compounds against Escherichia coli and Staphylococcus aureus was carried out using Muller–Hinton Agar media (Hi media). The activity was carried out using a paper disc method. Base plates were prepared by pouring 10 ml of autoclaved Muller–Hinton agar into sterilized petri dishes (9 mm diameter) and allowing them to settle. Molten autoclaved Muller–Hinton that had been kept at 48 °C was incubated with a broth culture of E. coli and S. aureus bacteria and then poured over the base plate. The discs were air dried and placed on the top of agar layer. The plates were incubated for 24–30 h and the inhibition zones (mm) were measured around each disc. As the organism grows, it forms a turbid layer, except in the region where the concentration of antibacterial agent is above the minimum inhibitory concentration, and a zone of inhibition is seen. The solutions of all compounds were prepared in double distilled water and chloramphenicol was used as a reference.

3.1 Spectroscopic and magnetic studies

The ¹H NMR spectrum of the ligand (Fig. 3) in DMSO showed a singlet at δ 9.40 ppm, has been assigned to the azomethine proton (HC⚌N). The peaks in the regions δ 7.1–8.1 ppm were assigned chemical shifts for the hydrogen of the symmetrical aromatic ring of the ligand. Protons on the methylene group attached to a nitrogen atom, N–CH₂–, with resonances in the region of δ 3.40–3.90 ppm as singlet pattern (Aranha et al., 2007; Kasumov et al., 2005). The peaks in the region δ 6.0–6.9 ppm were assigned to a chemical shift of pyrrole hydrogen and a singlet peak at δ 11.60 ppm due to NH of pyrrole ring (Silverstein and Webster, 2007). Metal complexes are paramagnetic; their ¹H NMR spectra could not be obtained.

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Figure 3

¹H NMR spectrum of the ligand.

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Mass spectrometry has been successfully used to investigate molecular species in solution (Sanmartin et al., 2006; Beloso et al., 2003; Singh et al., 2009). This method is particularly useful when a poorly crystalline nature of complexes prevents their X-ray characterization. The pattern of mass spectrum (Fig. 4a–c) gives an impression of the successive degradation of the target compound with the series of peaks corresponding to the various fragments. Their intensity gives an idea of stability of fragments. The recorded molecular ion peaks of the metal complexes have been used to confirm the proposed formula. The mass spectrum of the ligand having a molecular ion peak at 214{100% m/z} and having a prominent peak at 145{73% m/z} that corresponds to the [C₉H₈NO]⁺ confirms the purity of the ligand. The molecular ion peaks of Mn(II), Co(II), Ni(II) and Cu(II) – Schiff base complexes were observed at 358, 415, 391 and 414 m/z respectively, which confirm the stoichiometry of the metal complexes to be [M(L)(X)₂]·nH₂O. All the synthesized complexes containing metal ion were confirmed by good agreement between the observed and calculated molecular formula (Swamy and Pola, 2008). Elemental analysis values are in close agreement with the values calculated from molecular formulae assigned to these complexes, which is further supported by the TOF–mass studies. The molecular ion peak of the complexes by the loss of water molecules gave a fragment ion [M(L)(X)₂]⁺. All these fragments leading to the formation of the monomeric species [M(L)]⁺ which undergo demetallation to form the species [HL]⁺ are usually present in the mass spectra of these systems (Yoshida et al., 2000).The last two fragments at 214 and 146/145 are [LH]⁺ and [C₉H₈NO]⁺ of ligand peaks appearing in all the complexes showing a similar pattern of degradation.

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Figure 4

(a) Mass spectrum of complex I. (b) Mass spectrum of complex II. (c) Mass spectrum of complex III.

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The existence of numerous coordination sites in the ligand give variable binding modes, however, comparison of the infrared spectrum of the ligand and its complexes revealed that the ligand is bonded to metal ions in a quadridentate mode. The significant IR bands of the Schiff base and its metal complexes are given in Table 2. The free Schiff base ligand showed a strong band at 1641 cm⁻¹ which is characteristic of the azomethine (HC⚌N–) group (Ramesh and Maheshwaran, 2003). Coordination of the Schiff base to the metal ions through the nitrogen atom is expected to reduce electron density in the azomethine link and lower the ν_C⚌_N absorption frequency. The band due to ν_C⚌_N is shifted to lower frequencies and appears in the range of 1610–1625 cm⁻¹, indicating coordination of the azomethine nitrogen to metal ions (Keypour et al., 2009). The coordination of the azomethine nitrogen is further supported by the appearance of bands in the range of 460–530 cm⁻¹ due to ν_C⚌_N. The N–H stretching frequency at 3109 cm⁻¹ in the free ligand showed a considerable shift in all the complexes, indicating participation of this N–H group in complexes (Kumar et al., 2003). In complex I, the ν(Mn–Cl) band is observed at 364 cm⁻¹ which showed terminal rather than bridging chlorine (Nakamoto, 1978). In addition on account of the slightly modified coordination, infrared spectra of complex II show absorption bands at ca. 1265, 1090, 735, 1378, 960, 1030 cm⁻¹ due to coordinated nitrato group. These bands are assigned as ν₁, ν₂, ν₃, ν₄, ν₅, ν₆ modes respectively, and their frequencies are consistent with those associated with terminally bonded monodentate nitrato group (Addison and Sultons, 1967; Addision et al., 1971). In addition to these two weak bands with a separation of ca. 14 cm⁻¹ appear in the 1800–1700 cm⁻¹ region indicating clearly the exclusive presence of terminal monodentate nitrato group (Lever et al., 1971; Choca et al., 1972). Complexes III and IV have bands in the regions of 1615–1625 and 1403–1411 cm⁻¹ which can be assigned to ν_as(CO₂) and ν_s(CO₂) fundamental stretching bands respectively, which are in agreement with the acetate groups being monodentate and indicating the covalent nature of metal oxygen bond (Nakamoto, 1978). Complexes I, II and IV showed a broad band in the region of 3422–3502 cm⁻¹ due to (OH) from water molecules present out side the coordination sphere as lattice. This has been confirmed with thermal studies. These bands are absent in the ligand.

The electronic spectra of ligand and its metal complexes were recorded in DMSO. The spectrum of ligand consists principally of one band at ca. 23148 cm⁻¹ due to n→π^∗ transition of the azomethine group and is shifted to a longer wavelength on coordination through azomethine nitrogen (Prakash et al., 2010). The electronic spectra of the Mn(II) complex exhibit four weak intensity bands at 19280, 23265, 28260 and 38528 cm⁻¹. These bands may be assigned to the transitions, ⁶A_1g → ⁴T_1g (⁴G), ⁶A_1g → ⁴E_g, ⁴A_1g (⁴G), ⁶A_1g → ⁴E_g (D), and ⁶A_1g → ⁴T_1g (⁴P) respectively, which lie in the same range as reported for octahedrally coordinated Mn(II) ion (Kumar and Chandra, 2007). The ground state of the d⁵ system in a weak octahedral field has the spins of the d orbital which are parallel, making it a spin sextuplet. This, however, is the only sextuplet state possible, for every conceivable alteration of the electron distribution ${\text{t}}_{2\text{g}}^3{\text{e}}_{\text{g}}^2$ results in the pairing of two or four spins, thus making quartet or doublet states. Hence, all excited states of the d⁵ system have different spin multiplicity from the ground state, and transitions to them are spin forbidden. Because of weak spin–orbit interactions, such transitions are not totally absent, but they give rise only to very weak absorption bands. The magnetic moment 5.95 BM is an additional evidence for an octahedral structure of the Mn(II) complex (Figgis, 1978; Carlin, 2002; Cotton and Wilkinson, 1972). For Co(II) complex, three spin allowed transitions are: ⁴T_1g (F) → ⁴T_2g (F) (ν₁) (8412 cm⁻¹), ⁴T_1g (F) → ⁴A_2g (F) (ν₂) (≈18,000 cm⁻¹) and ⁴T_1g (F) → ⁴T_1g (P) (ν₃) (21302 cm⁻¹) (Kandaz et al., 2002). The first band appeared at 8412 cm⁻¹ usually in the infrared region, so ν₁ transition due to ⁴T_1g (F) → ⁴T_2g (F) could not be observed in the complex. The second transition is two electronic processes and therefore their bands are of low intensity. The broad band with maximum at 21302 cm⁻¹ corresponds to the highest energy ⁴T_1g (F) → ⁴T_1g (P) transition. The room temperature magnetic moment for Co(II) complex is 4.80 BM that supports an octahedral complex (Omar and Mohamed, 2005). Electronic spectra of six coordinated Ni(II) complex may possesses either O_h or D_4h symmetry. The Ni(II) complex exhibits three d-d transition bands for the transitions: ³A_2g → ³T_2g (F) (ν₁) (9732 cm⁻¹), ³A_2g → ³T_1g (F) (ν₂) (14198 cm⁻¹) and ³A_2g → ³T_1g (P) (ν₃) (21340 cm⁻¹) (Ahamed et al., 2008). The magnetic moment of the Ni(II) complex at room temperature lies at 2.78 B.M. corresponds to two unpaired electrons, and is consistent with a weak field octahedral geometry as expected. However, this magnetic moment value is lower than the spin only value for two unpaired electrons in the nickel(II) complex, indicative of some antiferromagnetic behavior (Ranford et al., 1993). The electronic spectra of six coordinated copper(II) complex (²D free ion) will split into B_1g, A_1g, B_2g and E_g levels, respectively. Thus three spin allowed transitions are expected in the visible and near IR region, but only a few examples are known, in which such bands are resolved (Figgis and Lewis, 1964; Chandra and Sharma, 2002). These bands have been assigned to the following transitions in order of increasing energy, ²B_1g → ²A_1g (dz² → dx²−y²), ²B_1g → ²B_2g (dxy → dx²−y²) and ²B_1g → ²E_g (d_xz,_yz → dx²−y²). The energy level sequence will depend on the amount of tetragonal distortion due to ligand field and Jahn–Teller effects (Lever, 1984; Wang et al., 2003; Tomilson et al., 1965). The electronic absorption spectra recorded in the range of 25,000–11,112 cm⁻¹ show a broad absorption band (λ_max = 17361 cm⁻¹). It is difficult to assign the transition of this band because it is due to mixing of all transitions. The magnetic moment of the copper complex is 1.80 BM which confirms octahedral arrangement.

EPR spectrum of the Mn(II) complex(High spin Mn(II) having ground state term S_5/2) was recorded at room temperature in DMSO solution and g_‖ (3.78), g_⊥ (1.92), A₀ (103) and g_iso (2.70) values were calculated. The combined action of spin–spin interaction and electric field gradient produces splitting of the energy levels due to second order spin orbit coupling between the A_1g ground state and the lowest level of the ⁴A_2g state (Jorgensen, 1969). Since d⁵ is an odd electron system whose ground state is a Kramer's doublet and whose degeneracy is only completely removed by a magnetic field, a resonance is readily detected even for large zero field splitting. The Mn(II) complex gives epr spectra containing the six lines arising due to the hyperfine interaction between the unpaired electron with the ⁵⁵Mn nucleus(I = 5/2). The nuclear magnetic quantum numbers, corresponding to these lines are −5/2, −3/2, −1/2, +1/2, +3/2 and +5/2 from low to high field (Jorgensen, 1969). The copper(II) ion, with d⁹ configuration, has an effective spin of S = 1/2 and is associated with a spin angular momentum, m_s = ±1/2 leading to a doubly degenerate spin state in the absence of magnetic field. In a magnetic field the degeneracy is lifted between these states and the energy difference between them is given by E = hν = gβH, where h is Planck's constant, ν is the microwave frequency for transition from ms = +½ to −½, g is the Lande splitting factor (equal to 2.0023 for a free electron), β is the Bohr magneton and H is the magnetic field. The epr spectrum of polycrystalline sample of Cu(II) at Liquid nitrogen temperature gives three g values(g₃ > g₂ > g₁) which indicate rhombic distortion is very small. The geometric parameter G, which is a measure of the exchange interaction between the copper centers in the polycrystalline compound, is calculated using the equation: G = (g_‖ − 2.0023)/(g_⊥ − 2.0023) for axial spectra and for rhombic spectra G = (g₃ − 2)/(g_⊥ − 2) and g_⊥ = (g₁ + g₂)/2. If G >4, exchange interaction is negligible if it is less than four; considerable exchange interaction is indicated in the solid complex (Hathaway and Billing, 1970). The lowest g >2.04, a spectrum can be observed for a copper(II) ion in elongated rhombic symmetry with all the axes aligned parallel and would be consistent with elongated rhombic–octahedral complexes (Hathaway and Billing, 1970). In the complex, g_‖ > g_⊥ > 2.0023 and G value within the range 2.08–4.49 are consistent with dx²−y² ground state (Bew et al., 1972). From the observed g values it is evident that the unpaired electron lies predominantly in the dx²−y² orbital with a possibility of some dz² character being mixed with it because of low symmetry. The shape of ESR lines indicates that the present complex may have octahedral geometry. In the complex, g_‖ <2.3 was observed and it indicates a fair degree of covalent character in the Cu–L bonding (Kivelson and Neiman, 1968).

3.2 Electrochemical studies

The present study is focused on the redox behavior of the Schiff base – Mn(II), Co(II), Ni(II) and Cu(II) – chelates, where the electron acceptor and electron donating properties of the ligand were reflected in the electrochemical properties. The influence of the substituent in the Schiff base moiety on the stability of above complexes was also investigated. The cyclic voltametric profile of the Mn(II) complex (Fig. 5) reveals the presence of an irreversible, anodic process (E_p = +0.79 V) and quasi-reversible reduction(E_1/2 = −1.06 V), which regenerates in the reverse scan of the oxidation of the original complex. On this basis, the oxidation may be attributed to the Mn(II)/Mn(III) couple, where the Mn(III) complex after its formation, being quite unstable in the original geometry, evolves to an unidentified differently coordinated species (fragmentation cannot be ruled out), which, however upon reduction regenerates the Mn(II) complex. The cyclic voltametric profile of the cobalt complex, displayed irreversible one-electron reduction in DMSO containing a Ag/AgCl electrode system with a peak separation value (ΔE_P = 0.224 V). The reduction wave could be assigned to metal-based Co(III)/Co(II) couples in the complex. The cathodic peak potential of the complex is located at E_pc = −0.740 V verses Ag/AgCl (Yilmaz and Kocak, 2004; Yilmaz et al., 2005; Kandaz et al., 2002). The nickel complex shows two oxidation processes. The first one is irreversible (ΔE_P1 = 0.28 V) and may be attributed to the oxidation of ligand. The second one is reversible (ΔE_P2 = 0.09 V) and consistent with a one-electron oxidation to form the mixed valence Ni(II,III) species. The voltammogram of the Cu(II) complex shows only a single reduction peak Cu(II)/ Cu(I) at −0.316 V during cathodic potential scan, whereas during the anodic potential scan, just after the reduction peak, an anodic peak is observed at 0.100 V. The separation between the peak potentials (ΔE_P = 416 mV) indicates a quasi-reversible redox process and the ratio of peak current ≈1 corresponding to a simple one electron process (Shirin and Mukherjee, 1992) assignable to the Cu(II)/Cu(I) couple. The neutral uncomplexed ligand is proved not be electroactive over the range −0.8 to +0.8 V.

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Figure 5

Cyclic voltammogram of complex I.

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The voltammogram of the complex shows a cyclic oxidation–reduction process that does not change after repeated scans. This result may be explained by the stability of the process. The result indicates the effect of substitution of the Schiff base moiety on the electrochemical behavior for different complexes. The pyrrole ring stabilizes the M(II) in different complexes. This is possibly because the pyrrole ring makes the complex more positively charged and causes it to be more easily reduced.

3.3 Powder X-ray diffraction studies

In the absence of single crystal, X-ray powder data are especially useful to deduce accurate cell parameters. The diffraction pattern reveals the crystalline nature of the complex. The indexing procedures were performed using (CCP₄, UK) Crysfire program (Shirley, 2002) giving different crystal systems with varying space groups. The density and particle size of the metal complexes have been calculated. The cell parameters of the complexes are shown in Table 3. The crystal data for complex I: monoclinic, space group C 2/c, a = 6.397563 Å, b = 6.460189 Å, c = 15.43264 Å, V = 2897.57 A³, Z = 8, crystal data for complex III: triclinic, space group P_1, a = 11.581102 Å, b = 11.905705 Å, c = 16.567397 Å, V = 1910.47 A³, Z = 4, crystal data for complex IV: triclinic, space group P₁, a = 7.089919 Å, b = 8.763800 Å, c = 12.380807Å, V = 630.23 A³, Z = 1.

3.4 Molecular modeling studies

Molecular mechanics attempts to reproduce molecular geometries, energies and other features by adjusting bond length, bond angles and torsion angles to equilibrium values that are dependent on the hybridization of an atom and its bonding scheme. In order to obtain estimates of structural details of these complexes, we have optimized the molecular structure of complexes. Energy minimization was repeated several times to find the global minimum. The energy minimization value for octahedra without restricting the structure for the metal complexes is almost same. The energy minimization values for Complex I, II, III and IV are 85.37, 79.28, 107.97 and 55.76 kcal/mol, respectively. The copper complex has minimum energy whereas nickel complex has maximum energy. It indicates that the copper complex has maximum stability compared to other metal complexes. The optimized structure of metal complexes is shown in Fig. 6a and b and their prominent bond length and angles are represented in Table 4.

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Figure 6

(a) Optimized structure of complex I. (b) Optimized structure of complex III.

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3.5 Biological studies

The bacteriological effect of the Schiff base and its metal complexes were determined against two bacteria under different concentration. The agar well – diffusion method was employed for the bacteria with respect to chloramphenicol as standard drug. The results showed that some compounds are very effective on some microorganisms (Fig. 7a and b). The nickel complex showed better activity than other metal complexes for both microorganisms. The activity of any compound is a complex combination of steric, electronic and pharmacokinetic factors. A possible explanation for the toxicity of the complexes has been postulated in the light of chelation theory. It was suggested that chelation considerably reduces the charge of the metal ion mainly because of partial sharing of its positive charge with the donor groups and possible π-electron delocalization over the whole chelate ring. This increases the lipophilic character of the metal chelate which favors its permeation through lipoid layers of cell membranes. Furthermore, the mode of action of the compounds may involve the formation of a hydrogen bond through – N⚌C group of the chelate or the ligand with the active centers of the cell constituents resulting in interference with the normal cell process. The higher bacteriotoxicity experienced by the compounds may be ascribed to the fact that the metal ions are more susceptible toward the bacterial cells than ligands (Phaniband and Dhumwad, 2007).

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Figure 7

(a) Effect of different concentrations of ligand and complexes with Escherichia coli. (b) Effect of different concentrations of ligand and complexes with Staphylococcus aureus.

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On the basis of above studies, geometries of the newly synthesized compounds have been proposed (Fig. 8). Metal complexes were found to be monomers and involved coordination through the azomethine nitrogen atoms, pyrrole nitrogen atoms and anions, giving octahedral geometry. The cell parameters of the complexes have been determined with the help of powder X-rays diffraction method. Bio-efficacy of the ligand and their metal complexes has been examined.

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Figure 8

Proposed structure of the complex.

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