INTRODUCTION:

Investigations on the complexing ability of metal ions with model ligands assist in the understanding of the function of physiological systems. Among the complexing ligands, Schiff bases have special interest due to their industrial and biological applications ^1-10. Earlier work reported that some drugs showed increased activity when administered as metal complexes rather than as organic compound^11-13.

Chromone derivatives have received great attention for their applications. These compounds exhibit a wide spectrum of biological activities including antimicrobial^14,15, antibacterial^{16, 17}, antitumor¹⁸, antifungal^19-21, antallergic²², antiviral²³, anti-inflammatory²⁴ and anticancer²⁵ activities. Additionally, chromone derivatives are essential for the synthesis of many important oxygen heterocyclic’s, pyrazoles and xanthones²⁶.

There are several reports about the coordination behaviors of transition metal ions with Schiff-base ligands derived from tryptamine (1H-indole-3-ethylenamine) and salicylaldehyde derivatives^27-31. However, no report on the synthesis of transition metal complexes with the schiff base ligand derived from tryptamine and chromone -3-carbaldehyde.

The schiff base derived from chromone-3-carbaldehyde and tryptamine has important biological applications. Hence, we have synthesized the transition metal complexes with this ligand and we have carried characterization by various spectroscopic techniques, mass, elemental analysis, molecular modeling analysis and antimicrobial studies.

EXPERIMENTAL:

Chemicals: chromone-3-carbaldehyde, tryptamine were purchased from Sigma-Aldrich chemicals and remaining all chemicals were purchased from commercial sources and used as such without purification.

Synthesis of the Schiff base ligand (L): The Schiff base ligand is prepared by condensation of chromone-3-carbaldehyde (1.74g, 0.01M) and tryptamine (1.60g, 0.01M) in absolute ethanol (25ml), by adding traces of glacial acetic acid the mixture was refluxed for 2 hrs with continuous stirring. The reddish orange color compound was separated out after 12 hrs. The compound is collected by filtration, washed several times with double distilled water, recrystallized from hot ethanol and dried in a vaccume desiccator. The melting point of the resulting Schiff base ligand (fig-1) is 98^oC. The yield is 91%.

Fig-1 Proposed structure of schiff base ligand (L)

Synthesis of metal complexes: The Schiff base ligand L (0.01M) is dissolved in hot methanol and hot methanol solution of corresponding salts (0.01M) MX₂ [where M = Cu(II), Ni(II), Co(II), Mn(II) and Zn(II) and X= chloride/acetate] were mixed together and refluxed with constant stirring for 2-3 hrs at refluxing temperature. On cooling colored solids were precipitated out. The products were filtered, washed with cold methanol, petroleum ether and dried in air and desiccator over anhydrous CaCl₂ and stored in an airtight sample bottle. All the compounds are colored and are stable when exposed to air and moisture.

Instruments used: The percentage composition of C, H, and N of complexes and ligand L were determined using micro analytical methods on Perkin Elmer 240C (USA) elemental analyzer. FT-IR spectra of the ligand and its complexes were recorded by using KBr pellets in the range 4000-400 cm^-1 on Perkin Elmer Infra red model 337. The UV-Visible spectra of the Schiff base ligand and its metal complexes were carried out in DMSO using a Shimadzu UV-1601 spectrophotometer. ¹H NMR spectrum of the ligand was recorded at 200MHz and 300MHz on Varian Gemini Unity Spectrometer using TMS as internal standard. ¹³C NMR spectra of the ligand was recorded at 100.6 MHz on Varian Gemini Spectrometer. The mass spectra of the compounds were recorded by ESI technique on VG AUTOSPEC mass spectrometer. The X-band ESR spectra was recorded with a EPR VARIAN-E-112 at RT. TGA and DTA analysis of complex was carried on Mettler Toledo Star system in the temperature range 0-1000^oC. The heating rates were controlled by 10^oC min^-1. Magnetic measurements were carried out on a Gouy balance model 7550 using Hg[Co(NCS)₄] as standard. The conductivity measurements were carried out in DMSO (10^-3M) using Digisun Electronic Digital conductivity meter, 0.01M KCl solution is used for calibration. Melting points of the ligand and decomposition temperature of complexes were determined on Polmon instrument (model No.MP-96). Molecular modeling and analysis of the compounds were carried out using Chem Office software.

Biological activity: The antibacterial activity of Schiff base ligand and its transition metal complexes were studied against four bacteria, E.coli and Edwardella. Each of the compound is dissolved in DMSO at a concentration of 1mg/ml. Paper discs of Whatmann filter paper No.1 are used after sterilization. The paper discs were saturated with 10µl of the compound dissolved in DMSO solution and were placed in Petri dishes containing nutrient agar media inoculated with the above-mentioned bacteria separately. The inhibition zone was measured in millimeters after 24 hrs incubation at 37^oC.

RESULT AND DISCUSSION:

The analytical and physical data of Schiff base ligand and its metal complexes are given in table-1. The data shows that the ligand L forms a 1:1 (M : L) complexes with Cu(II), Ni(II), Co(II), Mn(II) and Zn(II) ions. The prepared complexes are found to have the formulae [Cu(L)Cl₂], [Ni(L)Cl₂], [Co(L)Cl(H₂O)₃]Cl, [Mn(L)Cl₂(H₂O)₂] and [Zn(L)(Ac)₂] and L is Schiff base ligand, it coordinates as a neutral ligand.

Molar conductance

Molar conductance of the complexes are measured in DMSO at a concentration of 0.001 M. The observed conductance values falls in the range of 10-30 Ohm^-1cm² mol^-1 in case of Cu(II), Ni(II), Mn(II) and Zn(II) complexes, indicates that these complexes are non-electrolytes. The observed conductance value for cobalt complex is 86.4 Ohm^-1cm² mol^-1, falls in the range observed for 1:1 electrolyte³².

IR spectra and mode of bonding

The FT-IR spectral data of few important functional groups of the Schiff base ligand and its transition metal chelates are presented in table-2.

The IR spectrum of the Schiff base ligand show a very strong absorption band at 1657 cm^-1, which is characteristic of the 𝝊(C=O) of γ-pyrone. In the spectra of analyzed complexes, this absorption band has been shifted to lower region by 7-17cm^-1, indicating the coordination of the Schiff base ligand through the oxygen atom present in the 4-position of γ-pyrone ring³³.

The spectrum of free Schiff base ligand show another strong absorption band at 1617 cm^-1 assigned for the 𝝊(C=N) of azomethine group. In the spectra of analyzed complexes, this absorption band have been shifted to lower region by about 10–12 cm^-1, which confirms the coordination of azomethine group through nitrogen atom³⁴.

Table-1 Analytical and physical data of schiff base ligand L and its complexes

Compound	Empirical formula	Molecular weight	Color	Yield (%)	Melting Point in ^oC	Molar Conductance (Ohm^-1cm² mol^-1)
Schiff base ligand (L)	C₂₀H₁₆N₂O₄	316.3	Reddish orange	91	98	_
Cu(II) complex	[Cu(C₂₀H₁₆N₂O₄)Cl₂]	449	Dark green	65	120	27.2
Ni(II) complex	[Ni(C₂₀H₁₆N₂O₄)Cl₂]	445	green	69	234	29.0
Co(II) complex	[Co(C₂₀H₁₆N₂O₄)Cl(H₂O)₃]Cl	464	red	70	116	86.4
Mn(II) complex	[Mn(C₂₀H₁₆N₂O₄)Cl₂(H₂O)₂]	477	Yellowish orange	66	114	26.6
Zn(II) complex	[Zn(C₂₀H₁₆N₂O₄)(ac)₂]	500	Dark yellow	63	112	11.0

The C, H, and N analysis of Schiff base ligand and its complexes are found in good agreement with the expected values.

Table-2 Characteristic IR stretching bands of Schiff base ligand and its metal complexes in cm^-1

Compound	𝝊_C=O(γ-pyrone ring)	𝝊_C=N (azomethine)	Coordinated water	𝝊_as(OAc)	𝝊_s(OAc)	𝝊_M-O	𝝊_M-N
C₂₀H₁₆N₂O₄	1657	1617	-	-	-	-	-
[Cu(C₂₀H₁₆N₂O₄)Cl₂]	1647	1605	-	-	-	531	428
[Ni(C₂₀H₁₆N₂O₄)Cl₂]	1650	1607	-	-	-	538	425
[Co(C₂₀H₁₆N₂O₄)Cl(H₂O)₃]Cl	1640	1605	744	-	-	539	425
[Mn(C₂₀H₁₆N₂O₄)Cl₂(H₂O)₂]	1649	1606	743	-	-	530	425
[Zn(C₂₀H₁₆N₂O₄)(ac)₂]	1650	1607	-	1468	1291	529	425

Table-3 Mass spectral data of schiff base ligand and its metal chelates

Compound	Calculated mass m/Z	Obtained mass m/Z	Peak assigned
C₂₀H₁₆N₂O₄	316	317	L+H
[Cu(C₂₀H₁₆N₂O₄)Cl₂]	448.3	448.3	M
[Ni(C₂₀H₁₆N₂O₄)Cl₂]	446	445	M-1
[Co(C₂₀H₁₆N₂O₄)Cl(H₂O)₃]Cl	463	644	M+1
[Mn(C₂₀H₁₆N₂O₄)Cl₂(H₂O)₂]	477	477	M
[Zn(C₂₀H₁₆N₂O₄)(ac)₂]	499	501	M+2

The electronic spectrum of the Ni(II) complex shows three bands at 18781, 22222 and 24752 cm^-1 which are attributed to ³T₁(F) → ³T₂(F), ³T₁(F) → ³A₂(F) and ³T₁(F) → ³T₁(P) transitions⁴⁰. These transitions suggest a tetrahedral geometry of the complex. The magnetic moment value of Ni(II) complex is found to be 3.23 BM, falls within the range observed for tetrahedral complexes⁴¹.

The magnetic moment of Co(II) complex has been found to be 4.27 B.M, and it lies within the range expected for octahedral geometry. The electronic spectrum of Co(II) complex shows three peaks at 18214, 22172 and 23980 cm^-1 corresponding to ⁴T_1g(F) → ⁴T_2g(F), ⁴T_1g(F) → ⁴A_2g(F) and ⁴T_1g(F) → ⁴T_1g(F), suggesting octahedral geometry⁴².

The Mn(II) complex shows three absorption peaks at 18382 cm^-1 expected for ⁶A_1g → ⁴T_1g(S), at 22171 cm^-1 corresponding to ⁶A_1g → ⁴T_2g(G) and 24752 cm^-1 may be due to ⁶A_1g → ⁴A_1g, suggesting octahedral geometry⁴³. Further, the octahedral geometry is proposed based on magnetic moment. The magnetic moment of the complex is found to be 5.81 BM, which is less than the expected spin only magnetic momentum value.

The electronic spectrum of Zn(II) complex show one peak at 24330 cm^-1 due to charge transfer from ligand to metal. The observed magnetic moment value for this complex is zero, indicating diamagnetic nature of the complex. Based on analytical, conductance and spectral data, tetrahedral geometry is assigned to zinc complex.

ESR spectral studies:

ESR spectra of Cu(II) complex was recorded at room temperature in the polycrystalline state, on X- band at frequency of 9.1 GHZ under the magnetic field strength of 3000G.

The spectrum shows two peaks, one of small intensity towards low field region and the other of large intensity towards high field region. From the spectra the g values of the complex are g_║ (2.207) > g_⊥(2.057) >2.0023, indicating that the unpaired electron in the ground state of Cu(II) is predominantly in d_x2-y2, characteristic of square planar⁴⁴ of elongated tetragonal geometry. The g_║obtained for the Cu (II) complexes is less than 2.3 indicating covalent character of the metal ligand bond ⁴⁵. The value of exchange interaction term G, estimated from the following expression is 3.74.

G = g_|| - 2.0023/ g_⊥ - 2.0023

If G>4.0, the local tetragonal axes are aligned parallel or only slightly misaligned. If G<4.0, significant exchange coupling is present and misalignment is appreciable. The observed value for the exchange interaction term G suggests that significant exchange coupling is present and the misalignment is appreciable.

The spin-orbit coupling constant, λ value (– 587 cm–1) calculated using the relations, g_av = 1/3[g_|| + 2g_⊥] and g_av = 2(1–2λ/ 10Dq), is less than the free Cu(II) ion (–832 cm–1) which also supports covalent character⁴⁶ of M–L bond in the complex.ESR spectral data of the complex is given in table-5.

Thermal analysis:

The thermal stability of Cu(II) complex is studied by controlling heating rates 10^oC per minute under nitrogen atmosphere. Thermogram of Cu(II) complex is stable up to 250^oC, indicates the absence of lattice water as well as coordinated water. Generally in TG lattice water loses at low temperature region between 60-120^oC, where as coordinated water requires 120-250^oC. Absence of water molecule in Cu(II) complex is supported by the DTA curve, which represents weight loss by endothermic bands. The DTA of copper complex has no endothermic band in the range of 60-250^oC. Endothermic bands present at high temperature in DTA of Cu(II) complex is due to loss of organic molecules and finally metal may converts into its oxide⁴⁷. In addition to endothermic bands, the DTA curves of the complex also show exothermic bands. These bands appeared at high temperature and represent phase transition, oxidation and/or decomposition of the complex.

Molecular modeling and analysis:

Theoretical calculations (Geary, 1971; Allen et al., 1991; Rappe and Casewit, 1997) have paid a considerable attention to the characterization and inferences of geometrical optimization of the prepared complexes, therefore we could obtain the optimized geometry for each complex by computing the minimum steric energy and the theoretical physical parameter such as bond length using MM2CS Chem Office version 11.0 molecular modeling program. The steric energies of the prepared complexes are listed individually in fig-2.

Schiff base ligand (L) Copper complex

(14.80 kcal/mol) (76.36 kcal/mol)

Nickel complex Cobalt complex

(92.35 kcal/mol) (262.10 kcal/mol)

Manganese complex Zinc complex

(289.36 kcal/mol) (83.02 kcal/mol)

Fig-2 Molecular modeling’s for the proposed structures

The details of important bond lengths and bond angles as per the 3D structure of Cu (II) complex are given in the table-6. These values are obtained as a result of energy minimization of Cu (II) complex in Chem 3D Ultra 11.0 using MM2 method.

Based on the values in the table-6, it is observed that when the ligands (L) is coordinated with the copper metal ion there is an increase in the bond length in between the above mentioned atoms, which confirms the coordination of azomethine group through nitrogen and carbonyl of γ-pyrone ring through nitrogen atom with the metal ion. When the atoms are coordinated with the metal ion by donating a lone pair of electrons there is decrease of electron density on the coordinating atoms, hence bond length increases in metal complexes.

Table-6 Important bond lengths of copper complex

Atom	Bond atom	Bond length (A^o)
C(23)	N(24)	1.313
C(1)	N(24)	1.540
C(14)	O(15)	1.212
O(15)	Cu(25)	1.810
N(24)	Cu(25)	1.336
Cl(26)	Cu(25)	2.160
Cl(27)	Cu(25)	2.161

Theoretically obtained bond lengths of the ligand (L) using Chem office software are between C(1)-N(24) is 1.470, C(23)-N(24) is 1.260 and between C(14)-O(15) is 1.208 A^o.

Biological activity studies:

The antimicrobial activity of the Schiff base ligand and its metal chelates were tested against the bacteria E.coli and Edwardella by the well disc and fusion method. The test solutions were prepared in DMSO at a concentration of 1mg/ml. The zone of inhibition values were measured in millimeter after 24 hrs incubation at 37^oC. The antibacterial results are given in table-7 and fig-3.

Table-7 Antibacterial activity in millimeter (mL)

Compound	*E.coli*	*Edwardella*
C₂₀H₁₆N₂O₄	13	12
[Cu(C₂₀H₁₆N₂O₄)Cl₂]	15	12
[Ni(C₂₀H₁₆N₂O₄)Cl₂]	18	14
[Co(C₂₀H₁₆N₂O₄)Cl(H₂O)₃]Cl	13	13
[Mn(C₂₀H₁₆N₂O₄)Cl₂(H₂O)₂]	14	13
[Zn(C₂₀H₁₆N₂O₄)(ac)₂]	15	14

Antibacterial activity against Antibacterial activity against E. Coli Edwardella of Cu(II), Ni(II) of Cu(II), Ni(II) and Zn(II)

and Zn(II) complexes complexes

Figure-3 Antibacterial activity

The value in the above table indicates that the activity of the Schiff base ligand became more pronounced when coordinated with some of the metal ions. A comparative study of the growth inhibition zone values of schiff base and its complexes indicate that except manganese remaining four metal complexes exhibit higher anti bacterial activity than the free ligand and the same is indicated from the results given in the table 6. This is probably due the greater lipophilic nature of the complexes. Such increased activity of the metal chelates can be explained on the basis of Overtone’s concept and Tweedy’s chelation theory⁴⁸. According to Overtone’s concept of cell permeability, the lipid membrane that surrounds the cell favors the passage of only lipid soluble materials due to which liposolubility is considered to be an important factor that controls the anti microbial activity. On chelation, the polarity of the metal ion will be reduced to a greater extent due to the overlap of the ligand orbital and partial sharing of positive charge of metal ion with donor groups^{49, 50} Further, it increases the delocalization of the π electrons over the whole chelate ring and enhances the lipophilicity of the complex. This increased lipophilicity enhances the penetration of the complexes into lipid membrane and thus blocks the metal binding sites on enzymes of microorganisms⁵¹. These metal complexes also disturb the respiration process of the cell and thus block the synthesis of proteins, which restricts further growth of the organism. The variation in the activity of different complexes against different organisms depend either on the impermeability of the cells of the microbes or difference in ribosomes of microbial cells. Out of all nickel complex shows higher activity on the above mentioned bacteria.

CONCLUSION:

3-[(2-(1H-indole-3-yl)ethylimino)methyl]-4H-chromen-4-one (L) and its five transition metal complexes have been synthesized in 1:1 molar ratio of ligand (L) to metal ion. They have been characterized based on the analytical, spectral, thermal and magnetic measurements. The results of these investigations support the suggested structure of the metal complexes that is square planar geometry for copper complex, tetrahedral geometry for nickel and zinc complexes and octahedral geometry for manganese and cobalt complexes. Molar conductance values and analytical data suggest that the cobalt complex is 1:1 electrolyte and remaining four metal complexes are neutral in DMSO. The ESR spectrum of the copper complex suggests the partial covalent nature of the bond between metal and ligand and square planar geometry. Thermal analysis of Cu(II) complex indicates the absence of water molecules in the complex. The proposed structures of the ligand and its complexes are further studied for their energies and bond lengths theoretically using Chem office software package. Biological activity of the Schiff base ligand and its metal complexes were studied and results shows that the activity of some of the metal complexes is higher than the free Schiff base ligand.

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Received on 14.04.2011 Modified on 15.05.2011

Asian J. Research Chem. 4(7): July, 2011; Page 1043-1049

Compound	Absorption (𝝊) in cm^-1	Transition	Mag. Moment µ (B.M.)	Geometry
C₂₀H₁₆N₂O₄	25510 31847	n → Π* Π → Π*	-	-
[Cu(C₂₀H₁₆N₂O₄)Cl₂]	22172 24509	¹A_1g→ ¹B_2g ¹A_1g→ ¹A_2g	1.81	Square planar
[Ni(C₂₀H₁₆N₂O₄)Cl₂]	18781 22222 24752	³T₁(F) → ³T₂(F) ³T₁(F) → ³A₂(F) ³T₁(F) → ³T₁(P)	3.23	tetrahedral
[Co(C₂₀H₁₆N₂O₄)Cl (H₂O)₃]Cl	18214 22172 23980	⁴T_1g(F) → ⁴T_2g(F) ⁴T_1g(F) → ⁴A_2g(F) ⁴T_1g(F) → ⁴T_1g(F)	4.27	Octahedral
[Mn(C₂₀H₁₆N₂O₄)Cl₂ (H₂O)₂]	18382 22171 23980	⁶A_1g → ⁴T_1g(S) ⁶A_1g → ⁴T_2g(G) ⁶A_1g → ⁴A_1g	5.81	Octahedral
[Zn(C₂₀H₁₆N₂O₄)(ac)₂]	24330	CT	dia	tetrahedral

Complex	g_║	g_⊥	g_av	G (Axial Symmetry Parameter)	λ (cm^-1)
Cu(II) complex	2.207	2.057	2.106	3.74	-587