Synthesis and Characterization of a New Bidentate Ligand 5-Substituted-( 2-methyl-5-nitro-1-imidazomethyl )-1 , 3 , 4-oxadiazole-2-thione and its Metal Complexes of Ag ( I ) , Cu ( II ) and Zn ( II )

A new ligand (2-methyl-5-nitro-1-imidazomethyl)-1,3,4-oxadiazole2-thione (L) and its Ag(I),Cu(II) and Zn(II) complexes were synthesized. The authenticity of the ligand and its transition metal complexes were established by elemental analyses, conductance and magnetic susceptibility measurements, as well as spectroscopic (IR, H NMR, electronic, mass and ESR) and thermal studies. The IR spectral studies revealed the existence thiol-thione tautamerism in the ligand molecule. The magnetic and electronic spectral studies suggest an octahedral geometry for Cu(II) and Zn(II) complexes. The ligand acts as a bidentate coordinating through the N-3 nitrogen and the exocyclic sulfur atoms of oxadiazole rings. Antimicrobial screening of the ligand and its metal complexes were determined against the bacteria Escherichia coli and Salmonella paratyphi A.


Introduction
1,3,4-Oxadiazoles are of considerable pharmaceutical and material interest [1][2][3] .They have been shown to possess muscle relaxant, anti mitotic, analgesic, anti-inflammatory, anticonvulsive, diuretic and anti-emetic properties [1][2][3] .They also possess tranquilizing, antitubercular, hypoglycemic, herbicidal, antiviral, amoebicidal, insecticidal, hypnotic and sedative activities.Some material applications of 1,3,4-oxadiazole derivatives lie in the field of liquid crystals 5 .Oxadiazole derivatives, which belong to an important group of heterocyclic compounds, have been the subject of extensive study in the recent past.Numerous reports have highlighted their chemistry and use [1][2][3] .Diverse biological activities, such as anti-tuberculostatic, anti inflammatory, analgesic, antipyretic and anticonvulsant 4 , have been found to be associated with oxadiazole derivatives 5 .For this reason our aim was to synthesize various 1,3,4-oxadiazole-2-thione derivatives to make notable contributions to this class of heterocyclic compounds.We report the synthesis and characterization of some 5-substituted-1,3,4-oxadiazole-2-thiones using the synthetic procedure based on the ring closure reactions of appropriate acid hydrazides with carbon disulphide.The heterocyclic thiones represent an important type of compound in the field of coordination chemistry because of their potential multifunctional donor sites, viz either exocyclic sulfur or endocyclic nitrogen [6][7] .There are a few reports on the metal complexes of 5-substituted-1,3,4-oxadiazole-2-thione 4 .To the best of my knowledge I am reporting for the first time metal complexes based on 1,3,4-oxadiazole-2-thione containing 2-methyl-5-nitroimidazole moiety (Figure 1).

Figure 1
Influenced by these various properties of 1,3,4-oxadiazoles and their complexes, a programme to design and study the title ligand (Figure 1), which has an appropriate architecture to interact directly with metal centre was commenced.In order to reveal the conformational changes of the newly synthesized ligand upon coordination with transition metals, a study on the synthesis, characterization and spectroscopic evidence for their structural elucidation was undertaken.
Carbon, hydrogen and nitrogen contents were estimated on a truspec model microanalyser.By following standard procedures 8 , the complexes were analyzed for their metal content.For metal ion determination, a solution of the complex was first prepared by decomposing a known amount of complex with a mixture of conc.nitric acid and conc.Sulfuric acid 8 .The digestion was continued with repeated addition of acid mixture to ensure complete decomposition of organic matter in the complex.The residue was extracted with distilled water and the metal contents were determined by standard methods.The copper content was determined by volumetric analysis 8 (Iodometric titrations), zinc by EDTA method 9 and Silver content by gravimetric analysis 8 .The sulfur was estimated as BaSO 4 by gravimetric method 8 and chloride was estimated by volumetric method 8 using standard AgNO 3 solution.
The molar conductance measurements were made in DMSO with 10 -3 M solutions 9 using an EI Conductivity meter, type CC-01 with a cell constant of 1.02.The molar conductance values and magnetic susceptibility data are included in Table 2.
Magnetic susceptibility measurements were performed at room temperature on a Gouy balance using Hg[Co(NCS) 4 ] as the calibrant 9 .Electronic spectra were recorded on a Shimadzu 1700 UV-Vis spectrophotometer using 10 -5 M solution of ligand and complexes in DMSO solvent.IR spectra were recorded in the 4,000-400 cm -1 region(for the ligand) IR spectrophotometer PE FTIR and in 4000-200 cm -1 region (for metal complexes) on Nicolet instrument model MAGNA 550 spectrophotometer as KBr pellets. 1 H NMR spectra were recorded in DMSO-d 6 on Bruker AV III NMR spectrometer using TMS as internal reference.The EI-Mass spectrum was recorded on JEOL GC -MATE GC--MS Spectrometer using 70 eV with a source temperature of 150 0 C. The ESR spectra were recorded on a Varian E-112 ESR spectrometer, x-band microwave frequency (9.5 GHz) with sensitivity of 5x10 10 Δ H spins.The thermal studies of the complexes were performed on a NETZSCH TGA/DTA/DSC Thermal analyzer under a nitrogen atmosphere at a heating rate of 10 °K/min in the temperature range of 50 to 1400 °C.

Preparation of metal complexes of MNIMOT
To a refluxing solution of ligand (L) MNIMOT (4 mmole, 0.97 g) in 18 mL methanol and 2 mL DMF a hot solution of CuCl 2 .2H 2 O (2mmole, 0.34 g) in 15 mL methanol was added with constant stirring.A light green color precipitate was separated.The reaction mixture was further refluxed for 30 minutes.It was cooled, filtered, washed with hot water, warm methanol and finally with ether.The complex was dried at 120 0 C and was cooled in a desiccators.The complexes of AgNO 3 and Zn(CH 3 COO) 2 3H 2 O were prepared by following the similar procedure 9 .

Results and Discussion
Examination of elemental analyses data, given in Table

Infrared spectra
The IR spectra of the ligand (L) and complexes have been recorded and the probable assignments are given in the Table 3.The IR spectra of the ligand shows two bands at 2753 cm -1 (SH group) and 1159 cm 1 (C=S group) respectively which confirms the existence of thiol-thione tautamerism in the ligand molecule.The IR band appearing at 2753 cm -1 (SH) in the ligand molecule has disappeared in the complexes indicating the presence of ligand molecule only in thione form in the complexes.The IR spectra of complexes indicate that the ligand behaves as a bidentate and co-ordinate to the metal via C-NH and C=S groups.In the ligand, band appearing at 3434 cm -1 due to NH stretching either decreased or increased in the complexes.In the complexes of Cu(II) and Zn(II) of it has increased to 3458 cm -1 and 3444 cm -1 respectively.Whereas in complexes of Ag(I) it has decreased to 3429 cm -1 .This shows the involvement of N-atom at position 3 in the bond formation with the metal ions.The shifting of C=S stretching vibration to the lower wave number i.e. 1153,1152 and 1136 cm -1 respectively compared to the free ligand i.e. 1159 cm -1 is indicative of participation of thio carbonyl group(C=S) in coordination.The new bands appearing in the complexes are shown in Table 4.The bands at 535 cm -1 420 cm -1 , 360 cm -1 and 345 cm -1 are assigned for M-N, M-O and M-Cl and M-S stretching frequencies respectively which is indicative of metal-ligand bond formation.---420 --

H NMR spectra
The proposed structure of the ligand molecule is shown in (Figure 1).The 1 H NMR signals have been tabulated in Table 5.The CH 3 proton shows signal at δ 2.39-2.51and N-CH2 proton at δ5.52.The singlet observed at δ 8.4 ppm is due to the imidazole proton.A peak expected for SH proton of 1,3,4-oxadiazole-thiol (4a) is either shifted to the higher region or has disappeared because of proton exchange occurring in the thiol-thione tautomerism.
The formation of the ligand is further confirmed by the mass spectra which have a molecular ion peak at m/z 241; which is equivalent to its molecular weight.Signals observed in the complexes at region of δ 8.39-8.42due to the imidazole proton either remained unaffected or shifted slightly downfield with reference to those of the parent ligand and the position of signal due to N-CH 2 protons almost remained unaffected in the complexes.In the metal complexes of Cu(II) and Zn(II) additional peaks have made their appearance.In the complex(A) the peak at 2.5 δppm is due to the protons of coordinated H 2 O molecule.
Similarly a new peak appearing at 2.5δ ppm in the complex (B) is due to the methyl protons of coordinated CH 3 COO -ions.These observations support for assigning structure to the complexes.

Mass spectrum of ligand [L]
The mass spectrum of the ligand exhibits a molecular ion peak [M+1] at m/z 241 which is equivalent to its molecular weight.The fragmentation peaks, at m/z 167, 142 and 128 are ascribed due to the cleavage of C 6 H 6 N 4 O 2 ,C 5 H 6 N 3 O 2 and C 4 H 4 N 3 O 2 fragments respectively.

Mass spectrum of Cu(II) complex
The mass spectrum of the complex [CuL 2 Cl.H 2 O].Cl.H 2 O shows a molecular ion peak at m/z 652 which is equivalent to molecular mass of the complex.This complex shows a fragment ion peak with loss of hydrated water molecule at m/z 634; this fragment on further losing chlorine atom gives fragment ion peak at m/z 599.This fragment on losing coordinated water molecule and chlorine atom gives fragment ion peak at m/z 546 and this on demetallation gave another fragment ion peak for [L 2 ] + at m/z 483.

Electronic spectra and magnetic properties
The electronic spectra of the ligand and complexes were recorded in 10 -5 M solution in DMSO.The probable assignments are included in Table 6.The Electronic spectra of Cu(II) complex display two prominent bands 12 .A low intensity broad band around 12987 cm -1 and 16393 cm -1 is assignable to 2 T2g 2 Eg transition 12 .Another high intensity band at 25562 cm -1 is due to ligand -metal charge transfer.On the basis of electronic spectra distorted octahedral geometry around Cu(II) ion is suggested.The Cu(II) complex showed magnetic moment 1.75 B M, is slightly higher than the spin-only value 1.73 B M expected for one unpaired electron, which offers possibility of an octahedral geometry 13 .

ESR spectra
The ESR spectrum of copper(II) complex [Cu(II)L 2 Cl.H 2 O].Cl.H 2 O.At RT exhibits three different g-values indicating magnetic anisotropy in the complex.The distorted structure of the complex is supported by its ESR spectrum recorded in solid state both RT and LNT, which shows anisotropic effects with g-tensor as g 1 , g 2 and g 3 respectively at 2.2222, 2.1345 and 2.0341 in consistent with earlier report 14 .The trend g 1 (2.30)> g 2 (2.27) >g 3 (2.05), for the complex suggest a rhombic octahedral geometry for the complex.

Thermogravimetric analysis
The heating rates were suitably controlled at 10 0 C min -1 under nitrogen atmosphere and the weight loss was measured from the ambient temperature up to 1400 0 C. The thermo gravimetric studies reveal that no water molecules associated with the ligand.The decomposition of organic matter starts around 220 0 C. The thermogram of ligand showed first mass loss of 14% (cal.13.6%) at 219.3 0 C which is due to loss S-atom of ligand molecule.The second mass loss of 21% occurred at 264.4 0 C is due to the loss of NO 2group.After this TGA curve of ligand molecule showed maximum mass loss of 60% and is almost removed as a volatile components at about 750 0 C. The TGA curves of the complexes showed that the initial mass loss occurring within 100-120 0 C range is interpreted as loss of moisture and hydrated water molecules during the chelate drying process and the second weight loss at around 200 o C range is due to loss of coordinated water molecules.
The TGA curve of [Cu(II)L 2 Cl.H 2 O].Cl.H 2 O complex showed first mass loss of 2.7% at 76.7 0 C due to the loss hydrated water molecule, the second mass loss of 23% at 252.8 0 C is due to the loss of 2-methyl-5-nitroimidazole and chlorine atom.The third mass loss of 5.4% occurring at 343.4 0 C is due to the loss of coordinated chlorine atom.The fourth stage mass loss occurring at about 700 0 C attributed to the removal organic components from the complex.After this the thermogam is almost horizontal due the formation of metal oxide.Similarly the TGA curves of [Zn(II)L 2 (CH 3 COO) 2 ].3H 2 O complex showed first mass loss of 7.9% (cal=7.5%)at 89.9 0 C due to the loss of three hydrated water molecules.The second mass loss of 36% occurring at 315 0 C is due to the decomposition of two (2-methyl-5-nitroimidazole) moieties.The third mass loss of 8.8% is due to the loss of one coordinated acetate ion.The final mass loss 13% at 798 0 C is due to the conversion Zn metal ion into ZnO.

Biological activities
The antimicrobial activity of the ligand and the complexes were determined by the disc diffusion technique [15][16][17] .The compounds were screened in vitro against.Escherichia coli and Salmonella paratyphi A. A 1 mg/mL solution in DMSO was used.The standard used was gentamycin sulphate.Media Used: Peptone-10 g, NaCl-10 g and Yeast extract 5 g, Agar 20 g in 1000 mL of distilled water.
Initially, the stock cultures of bacteria were revived by inoculating in broth media and grown at 37 ºC for 18 h.The agar plates of the above media were prepared and wells were made in the plate.Each plate was inoculated with 18 h old cultures (100 μL, 10 -4 cfu) and spread evenly on the plate.After 20 min, the wells were filled with of compound at different concentrations.The control wells with Gentamycin were also prepared.All the plates were incubated at 37 ºC for 24 h and the diameter of inhibition zone were noted.The Table 7 contains diameter of inhibition zones (in cm).

Conclusion
The ligand molecule acts as a bidentate ligand.The spectroscopic results show the involvement of C=N(at position 3) and C=S(at position 2) groups in coordination to the central metal ion.On the basis of various techniques such as IR, 1 H.NMR, Mass, ESR, UV-Vis, Molar conductance and thermal studies used for the characterization of metalcomplexes a rhombic octahedral geometry for Cu(II) complex and octahedral geometry for Zn(II) complex is proposed.In view of the foregoing discussions, the high melting points and insolubility in common organic solvents, we have assigned following probable structure of the complexes (Figure 2).

Table 1 .
.1 for the complexes [CuL 2 Cl.H 2 O] Cl.H 2 O and [ZnL 2 (CH 3 COO) 2 ].3H 2 O indicate their 1:2 (metal:ligand) stoichiometry.Whereas for the complex AgLNO 3 it is 1:1.They is insoluble in chloroform, carbon tetrachloride, methanol and ethanol but soluble in DMF and DMSO.The molar conductivity data obtained in DMSO suggest that the complex is [CuL 2 Cl.H 2 O]Cl.H 2 O 1:1 electrolyte and other two complexes are non-electrolytes in nature.Analytical data of MNIMOT(L) and its metal complexes

Table 2 .
Molar conductance and Magnetic properties of metal complexes

Table 3 .
Major IR bands of ligand(MNIMOT) and its metal complexes

Table 4 .
IR bands of metal complexes.

Table 5 .
1H NMR Spectra of ligand (MNIMOT) and its metal complexes

Table 6 .
UV-Vis Spectra of free ligand and its metal complexes in 10 -5 M solution in DMSO solvent

Table 7 .
Results of Antibacterial activities of ligand (MNIMOT) and its metal complexes