SYNTHESIS OF MACROCYCLIC LIGANDS AND THEIR COBALT(II) COMPLEXES SPECTRAL CHARACTRIZATION AND ANTIMICROBIAL STUDIES

SYNTHESIS OF MACROCYCLIC LIGANDS AND THEIR COBALT(II) COMPLEXES SPECTRAL CHARACTRIZATION AND ANTIMICROBIAL STUDIES

Sulekh Chandra ^*1 and Poonam Pipil ² and Swati Agarwal ³

Department of Chemistry ¹,Zakir Husain Delhi Collage (University of Delhi) JLN-Marg, New Delhi, India

Department of Chemistry ², Rajdhani College, University of Delhi, Delhi 110015, India

Department of Chemistry ³, Moti Lal Nehru College, Benito Juarez Marg, New Delhi 110021, India

ABSTRACT:  Complexes of Co(II) with macrocylic ligands i.e., 2, 3, 9, 10 tetraphenyl – 1, 4, 8, 11 – tetraazacyclotetradeca – 1, 3, 8, 10 tetraene (BDP), 2, 4, 10, 12 – tetramethyl – 1, 5, 9, 13 tetraazacyclohexadeca 1, 4, 9, 12 tetraene (ADP), 2, 3, 9, 10 tetramethyl 1, 4, 8, 11 tatraazatetredeca – 1, 3, 8, 10 tetraene (DDP) have been synthesized. All these complexes were characterized by elemental analysis, molar conductance magnetic susceptibility measurement, electronic and epr spectral studies. On the basis of elemental analysis the complexes were found to have composition CoLX2. Molar conductance measured in DMF solution indicates that complexes are non-electrolytic in nature except Co(BDP)SO4 which are 1 : 2 electrolytes. Thus the complexes may be formulated as [CoCLX2]. Magnetic moments and the electronic spectra indicate six coordinated distorted octahedral geometry for the complexes. The ligand field parameters were calculated using various energy level diagrams. In vitro synthesized compounds and metal salts have also been tested against same species of plant pathogenic fungi and bacteria in order to assess their microbial properties.

Mass, IR, Magnetic moment, EPR and Biological activity.

INTRODUCTION: Azo schiff base complexes contain both azo and azomethine groups. The azo group possess excellent donor properties and is important in coordination chemistry ^1-3 and some azo compounds have shown to possess good antibacterial activity ^4-5. Schiff bases are well known to have antifungal, antitumor and herbicidal activities ^6-11. Many metal complexes of naturally occuring porphyrins, corrins and phthalocyanines have been investigated because of their potential as dyestuffs or pigments ¹². A schiff base complexes are of great importance ¹³ in enehancing various industrial applications and in a number of biological processes such as photosynthesis and dioxygen transport ¹⁴.

Cobalt complexes, besides their presence in several stereochemical dispositions, are oxygen carriers and oxygen activators ^20-23 which interact with molecular oxygen and ultimately oxidize the complexes of Co(II) to Co(III). The best known biological function of cobalt is its involvement in the coenzyme related to vitamin
B₁₂ ^15-16.

In the present paper we report the synthesis and characterization of Schiff base complexes of cobalt (II) with 2, 3, 9, 10 tetraphenyl – 1, 4, 8, 11 – tetraazacyclotetradeca – 1, 3, 8, 10 tetraene. (BDP) 2, 4, 10, 12 – tetramethyl – 1, 5, 9, 13 – tetraazacyclohexadeca – 1, 4, 9, 12 tetraene. (ADP) 2, 3, 9, 10 tetramethyl 1, 4, 8, 11 tetraazacyclohexadeca – 1, 3, 8, 10 tetraene (DDP). Fig (1a-1c).

Experimental Section:

Materials: All the chemicals used in the present work of high purity, Anala R grade and purchased from Sigma-Aldrich. Metal salts were purchased from E. Merck and used as received. The Solvent used were either spectroscopic pure from SRL/BDH or purified by the recommended methods.

STRUCTURE OF LIGANDS (FIG 1A-1C)

1(a) 2, 3, 9, 10 tetraphenyl – 1, 4, 8, 11 – tetraazacyclotetradeca – 1, 3, 8, 10 tetraene (BDP)

1(b) 2, 4, 10, 12 – tetramethyl – 1, 5, 9, 13 tetraazacyclohexadeca 1, 4, 9, 12 tetraene (ADP),

1(c) 2, 3, 9, 10 tetramethyl 1, 4, 8, 11 tatraazatetredeca – 1, 3, 8, 10 tetraene (DDP)

Preparation of ligands:

Preparation of 2, 3, 9, 10 tetraphenyl – 1, 4, 8, 11 – tetraazacyclo tetradeca – 1, 3, 8, 10 tetraene. (BDP):

The ligand 2, 3, 9, 10 tetraphenyl – 1, 4, 8, 11 – tetraazacyclo tetradeca – 1, 3, 8, 10 tetraene. (BDP) was synthesized by refluxing an ethanolic solution of 1, 3 diaminopropane (0.5 mole) with an ethanolic solution of benzyl, (0.5 mole) in presence of ~3 mL of conc. HC1 for 3-4 hours. The resulting mixture was kept overnight, when an off-white coloured crystalline compound separated. This was then filtered, washed with ethanol, and dried over P₄O₁₀. The ligand is soluble in water and melted at 218⁰C.

Preparation of 2, 4, 10, 12 – tetramethyl – 1, 5, 9, 13 – tetraazacyclohexadeca – 1, 4, 9, 12 tetraene. (ADP):

2, 4, 10, 12 – tetramethyl – 1, 5, 9, 13 – tetraazacyclohexadeca – 1, 4, 9, 12 tetraene. (ADP) was prepared by adding an ethanolic solution of acetylacetone (0.5 mole) to an ethanolic solution of 1,3 diaminopropane (0.5 mole) in presence of ~3 mL conc. HC1 and the resulting solution was refluxed for 4 hours and kept overnight. A white crystalline compound separated on filtration which was washed with ethanol and then dried over P₄O₁₀. The compound was soluble in most organic solvents, and it’s melting point was recorded as 226⁰C.

Preparation of 2, 3, 9, 10 tetramethyl 1, 4, 8, 11 tatraazatetredeea – 1, 3, 8, 10 tetraene (DDP):

2, 3, 9, 10 tetramethyl 1, 4, 8, 11 tatraazatetredeea – 1, 3, 8, 10 tetraene (DDP) ligand was synthesized by diacetyl (0.5 mole) to an ethanolic solution of 1, 3 diaminopropane (0.5 mole) in presence of ~3 mL conc. HC1 and the resulting solution was refluxed for 4 hours and then kept overnight. Light-yellow crystals separated on filteration which were washed with ethanol and then dried over P₄O₁₀ The ligend is water soluble & melts at 221⁰C

Preparation of Co(II) complexes with BDP, ADP and DDP ligands:

The complexes of the ligands BDP, ADP and DDP ligands have been synthesized by template method because the yield of the complexes was low when the ligands were created with metal salt to form complexes.

A hot ethanolic solution of benzil/ acetylacetone / diacetyl (0.01 mole) was added to an ethanolic solution of 1, 3 diaminopropane (0.01 mole) and the resulting solution was refluxed for half and hour at ~ 40⁰C. A solution of CoX₂.nH₂O(0.005 mole, n (0-6) (X=Cl^-, NO₃^-, NCS^- in ethanol and X = ½ SO₄^2- in water) was then added to the above solution and refluxing continued for a further four to six hours. On cooling the solution, pink/brown crystalline compounds separated out. They were filtered, washed with ethanol, and dried under vacuum over P₄O₁₀.

Physical Measurement:

The C, H and N were analysed in Carlo-Erba 1106 elemental analyzer. Molar conductance was measured on an ELICO (CM82T) conductivity bridge. Magnetic susceptibility was measured at room temperature, on CAHN-2000 magnetic susceptibility balance using CuSO₄.5H₂O as a calibrant. Infrared spectra of ligands and complexes were recorded as KBr pelletson a Pekin-Elmer 1310 spectrophotometer. The electronic spectra of complexes were recorded in DMSO, on a shimadzu UV mini-1240 spectrophotometer. EPR spectra of complexes were recorded on JEOL, JES, FE3XG, EPR spectrometer. The spectra were recorded in solid as polycrystalline sample at room temperature on E₄-EPR spectrometer using the DPPH as the g-marker.

RESULTS AND DISCUSSION:

On the basis of elemental analyses (Table 1) all the complexes have the composition CoLX₂. Molar conductance of the complexes indicates that the complexes are non-electrolytic in nature, except complexes Co(BDP)SO₄. Which are 1: 2 electrloytes. Thus the complexes may be formulated as [CoLX₂] and [CoL] SO₄ respectively. Magnetic moments and the electronic spectra of the complexes discussed in succeeding paragraphs suggest six coordinated distorted octahedral geometry for the complexes.

 TABLE 1: ELEMENTAL ANALYSES AND MOLAR CONDUCTANCE DATA OF COBALT(II) COMPLEXES

  IR Spectra:

IR Spectra of all the complexes do not contain any bands that can be assigned to C=O or N-H groups. Characteristic IR bands due to phenyl groups are present in the spectra of complexes Fig (2a-2b) BDP in the region of 700-770 cm^-1. Strong bands appearing as doublets in the spectra of all the complexes around 1590-1620 cm^-1 may be assigned to v_C=N vibrations. The absence of absorptions around 3400 cm^-1 show that amino group of the diamine have reacted with the di ketone.

The phenyl ring absorptions also appear in the 1400-1600 cm^-1 region. The presence of new bands at ca. 1272, ca. 1190 and ca. 1030 cm^-1 assignable to v(C-CH₃)+v(C=C), d(CH)+v(C-CH₃) and r_r(CH₃) show that the diketone moiety is present in the complexes. The changes in the position v_C=N vibrations indicate coordination through this site. The bands at ca. 1640 and 840 cm^-1 may be assigned to NH deformation coupled with NH out of plane bending and may be due to the presence of this group in these complexes. Thus in presence of metal salts, a quadridentate macrocycle is formed which coordinate through azomethine nitrogens while pyridine nitrogen does not take part in coordination^17-18.

Bands due to anions:

Co(ADP(NCS)₂] Complex:

The IR spectrum of [Co(ADP)(NCS)₂] shows a strong band at 2075 cm^-1 thereby indicating that the thiocyanate group is N-coordinated. The absence of a band around 2140 cm^-1 shows that there is no coordination through S so both the thiocyanate groups are N-coordinated. A single shap band at 474 cm^-1 also confirms that the complex is N-bonded.

Co(DDP)SO_4:

The IR spectra Co(DDP)SO₄ shows the splitting of v₃ into three bands around 1200, 23 and 1080 cm^-1 and at 669 and 660 cm^-1 indicating the bidenate nature of the sulphate group. Similar is the case with Co(DDP)SO₄ while for the other complexes Co(BDP)SO₄, sulphate does not show coordination with the metal ion as only one strong broad band is observed around 1100 cm^-1.

Co(BDP(NO₃)₂ and Co(ADP)(NO₃)_2:

The appearance of new bands at ca. 1405(v₁), 1187(v₅) and 825 cm^-1 (v₆) in the spectra of nitrato complex of BDP indicates the monodentate nature of the nitrate group, since the separation between v₁ and v₅ is 218 cm^-1 conforming to the unidentate group, while the presence of a single band at ca. 1383 cm^-1 in the nitrato complex of ADP indicates that the nitrate group is uncoordinated.

Magnetic Moments:

Experimental magnetic moment in literature ^19-21 lie in the range 4.42 to 4.7 B.M. Magnetic moments of the complexes under study have been determined at room temperature and lie in the range of 4.95-5.04 B.M. indicating a spin quartet six coordinate octahedral Co²⁺ complexes and 4.49-4.68 B.M. for sulphato complexes indicating four coordinate geometry (Table 2).

 TABLE 2: MAGNETIC MOMENTS AND ELECTRONIC SPECTRAL BANDS OF COBALT (II)

Electronic Spectra:

Electronic spectra of the complexes under study disply two well defined spectral bands at 8700-9600 cm^-1 and 20200-21000 cm^-1 and a shoulder at 14000-14800 cm^-1 to ⁴T_1g ® ⁴T_2g (F) (v₁), ⁴T_1g ® ⁴T_1g (P) (v₃) and ⁴T_1g ® ⁴T_2g (F) (v₂) transitions

respectively, characteristic of octahedral geometry of cobalt(II) complexes. The electronic spectral bands indicates that these macrocyclic complexes have distorted octahedral geometries and might be possessing D_4th symmetr ^22-24

Co(ADP(NCS)₂] Complex:

Electronic spectra of the complexes under study disply two well defined spectral bands at 8700-9600 cm^-1 d and 20200-21000 cm^-1 and a shoulder at 14000-14800 cm^-1 to ⁴T_1g ® ⁴T_2g (F) (v₁), ⁴T_1g ® ⁴T_1g (P) (v₃) and ⁴T_1g ® ⁴T_2g (F) (v₂) transitions respectively, characteristic of octahedral geometry of cobalt(II) complexes. The electronic spectral bands indicates that these macrocyclic complexes have distorted octahedral geometries and might be possessing D_4th symmetry ^22-24

Co(DDP)SO₄ :

The three electronic transitions occur from the ground state. ⁴ A₂. The first v₁ is rarely observed while v₂ is usually wide and apeears in the near IR region and v₃ is intense and broad occurring in the region 15000–21000 cm^-1.

Co(BDP(NO₃)₂ and Co(ADP)(NO₃)₂:

The electronic spectra of all the nitroto complexes show three bands around 8700, 14000, and 20500 cm^-1, which are characteristic of octahedral cobalt(II) complexes.

Ligand Field Parameters:

Various ligand field parameters Dq, B and b have been calculated and given in Table 3. Dq values have been evaluated by using Orgel energy level diagram³¹ Nephelauxetic parameters b has been calculated by using equation.

b =

Where B (free ion) ²⁵ is 1120 cm^-1. The value of b in the present study indicates appreciable covalent character.

TABLE 3: LIGAND FIELD PARAMETERS AND ESR SPECTRAL DATA OF COBALT(II) COMPLEXES

 EPR Spectra:

EPR spectra of the polycrystalline complexes under study ²⁵ were recorded at liquid nitrogen temperature Fig (3a). Since, the rapid spin lattice relaxation of Co²⁺ broadens the lines at higher

temperatures, g-values are presented in Table 3. The large deviation of the g values from the spin only value (g = 2.0023) is due to the large angular momentum contribution, this result accords with the magnetic susceptibilities and electronic spectra as discussed earlier.

FIG 3a: ESR SPECTRUM OF [Co (DDP) (SO₄)₂] COMPLEX

Thus, on the basis of magnetic susceptibility measurement, molar conductance measureent, IR, electronic and EPR spectral studies and the subsequent discussion for the complexes given

above, the following structure may be proposed for the 6-coordinate complexes (Fig. 4a-4c) while for the 4-coordinate complexes the structure are proposed as (Fig. 4d).

Biological Study:

The antimicrobial screeniung data show that the metal chelates exhibit a higher inibitory effect than the free ligand and metal salts. The increased activity of the metal chelates can be explained based on the chelation theory. The chelation reduces the polarity of the metal atom mainly because of the partial sharing of its positive charge with the donor groups and possible p electron delocalization within the whole chelation ring.

The chelation ring increases the lipophillic nature of the central atom which subsequently favours its permeation through the lipid layer of the cell memberane. The enhanced activity of the complexes can also be explained on the basis of their high solubility fineness of the particles, size of the metal ion and the presence of bulkier organic moieties. The mode of action may involve the formation of a hydrogen bond through the azomethane nitrogen atom with the active centers of the cell constituents, resulting in interference with the normal cell prosess. The variation in the effectiveness of different compounds against different organisms depends either on the impermeability of the cells of the microbes or difference in ribosomes of microbial cells. It has also been proposed that concentration plays a vital role in increasing the degree of inhibition; as the concentration increases, the activity increases ^26-30. The results of fungicidal screening (Figure 3) show that all the complexes are highly active as compared to free ligand and metal salt against all the fungal species.

CONCLUSION: The present study reveled six coordinated octahedral geometry for the Co(II) complexes. All the ligands act as a tetradentatc manner coordinating through four nitrogens of the azomethine groups in N N N N fashion moreover, the fungicial data reveal that the complexes were superior to the free ligand in the inhibition of the tested fungi it is proposed that concentration plays a vital role in increasing the degree of inhibition, the activity increased with increasing concentration of the complexes.

 ACKNOWLEDGEMENT: We are thankful to UGC New Delhi for financial Assistance and I.I.T. Bobay for recording EPR spectra.

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How to cite this article:

Chandra S and Pipil P: Synthesis of Macrocyclic Ligands and their Cobalt(II) Complexes Spectral Charactrization and Antimicrobial Studies. Int J Pharm Sci Res 2015; 6(3): 1258-65.doi: 10.13040/IJPSR.0975-8232.6(3).1258-65.

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