SYNTHESIS, SPECTROSCOPIC, THERMAL AND BIOLOGICAL STUDIES ON MANGANESE(III), IRON(III) AND COBALT(III) COMPLEXES WITH BIS(MERCAPTO AZOLES)

Complexes of bis(mercapto azoles) i.e. bis(mercapto triazoles), bis(mercapto thiadiazoles) and bis(mercapto oxadiazoles) (LH 2 ) with manganese(III), iron(III) and cobalt(III) have been prepared in methanol in the presence of sodium hydroxide and binuclear products of the type Na[M(L) 2 (H 2 O) 2 ] have been isolated. Tentative structural conclusions are drawn for these complexes based upon elemental analyses, electrical conductance, magnetic moment and spectral (electronic, infrared and 1 H NMR) data. The thermal stability and mode of decomposition for the complexes have been studied by TG, DTA and DSC techniques. The antifungal, antiviral and antibacterial


Experimental
The solvents were purchased from Merck and metal salts were purchased from Aldrich. Buchi 530 apparatus was used to determine the melting points. Elemental analysis was performed with a Vario EL III Carlo Erba 1105 CHN analyser and elemental (C, H, N) analysis indicated that calculated and observed values were within acceptable limits. IR spectra were recorded in KBr using Shimadzu 8201 PC model spectrophotometer while NMR spectra were recorded in DMSO-d 6 solvent by a Bruker DRX-300 spectrometer using tetramethylsilane as an internal reference. Thermogravimetric analysis (TGA) data of the complexes were studied using a Perkin Elmer-STA 6000 thermal analyser instrument.
The ligands were prepared as reported earlier [8].

Results And Discussion:-
Manganese(III) acetate, iron(III) chloride and cobalt(III) acetate react with bis(mercaptoazoles) (LH 2 ) viz., bis(mercaptotriazoles), bis(mercapto-oxadiazoles) or bis(mercaptothiadiazoles) in 1:2 molar ratio, respectively, in methanol the presence of sodium hydroxide to binuclear products of type Na[M(L) 2  The physical properties and the analytical data of the complexes are given in Tables 1-6. The complexes are coloured solids and are soluble in dimethylformamide and dimethylsulphoxide. The electrical conductance measurements in dimethylformamide are consistent with their non-electrolytic nature. The presence of coordinated water molecules in these complexes has been inferred from thermogravimetric study, which show weight loss at ca. 160 o C corresponding to two water molecules.

Magnetic Moment
At room temperature magnetic moment of the powdered polycrystalline manganese(III) complexes is observed between 4.80 -4.90 B.M. which is in excellent agreement with the spin only moment (4.90 μ B ) for a high spin d 4 (S = 2) configuration normally observed for mononuclear manganese(III) complexes [9]. The magnetic moments of iron(III) complexes lie around 5.0-5.2 B.M. at room temperature which are lower than the usual value (5.90 B.M.) Cobalt(III) complexes are diamagnetic.

Electronic Spectra
The electronic spectral bands of manganese(III), iron(III) and cobalt(III) complexes are given in Table 7.
The visible spectra of manganese(III) complexes have almost identical features containing multiple absorption bands mostly in the form of shoulders. In DMF solution two prominent bands associated with d-d transitions are observed in the 15,220-16,200 and 18,250-19,100 cm -1 range along with a high intensity band at ca 21500-22360 cm -1 .
Consequently three spin-allowed d-d transitions 4 B 1g → 5 A 1g , 5 B 1g → 5 B 2g and 4 B 1g → 5 E g are possible in this type of system [9].
The electronic spectrum of iron(III) complexes shows several bands at 12,200-12880, 19,200-19,940 and 24100-24,990 cm -1 which may be assignment to transitions 6 A 1g → 4 T 1g , 6 A 1g → 4 T 2g and 6 A 1g → 4 A 1g , E g . The positions of these bands indicate [9] that these complexes are octahedral in nature. In addition to these bands, a charge-transfer band at ca. 32500 cm -1 is also observed which may be assigned to Fe (e g   * (ligand) transition. The magnetic moment and the ligand field parameters indicate that it is of the spin-free octahedral type.
a) The electronic spectra of cobalt(III) complexes show bands in the regions 16,000-16,450, 19,590-22,610 and 24,215-25,400 cm -1 . The positions of these bands resemble those reported [9] for other six-coordinated cobalt(III) complexes and may be assigned to 1 A 1g → 3 T 2g , 1 A 1g → 1 T 1g , 1 A 1g → 1 T 2g transitions, respectively. The appearance of three bands in cobalt(III) complexes indicates a low symmetry ligand field. b)

Infrared Spectra
The assignments of infrared spectral bands of the ligands and the complexes are based on earlier studies on similar ligands [10][11][12][13][14][15][16]. The infrared spectra of bis(mercaptotriazoles), bis(mercaptooxadiazoles) and bis(mercaptothiadiazoles) show one weak band at 2480-2550 cm -1 due to the -SH group vibration. However, in the spectra of complexes the band disappears indicating the coordination through sulphur after deprotonation. This is further supported 4 by the appearance of band at ca. 340-380 cm -1 , assignable to (M-S). A strong band in the region of 1585-1560 cm -1 in the ligands is characteristics [14,15] of (C=N) (ring) group. However, in the complexes the position of this band remains at the same position, indicating the non-coordination of the ring nitrogen to metal ion. The infrared spectra of bis(mercaptotriazoles) show one band at 3150 cm -1 assignable [17] to (N-H). The bands due to (C-O-C) in bis(mercaptooxadiazoles) appear at ca. 1290 cm -1 (symmetric) and 1350 cm -1 (asymmetric); while bis(mercaptothiadiazoles) show band at ca. 660-650 cm -1 due to (C-S) [16]. The position of infrared bands due to phenyl and heterocyclic (triazole, oxadiazole or thiadiazole) ring do not change in the complexes indicating the noncoordination of nitrogen (triazole ring), oxygen (oxadiazole ring) or sulphur (thiadiazole ring) atoms. The complexes show one strong band at ca. 3400 cm -1 which may be due to coordinated water molecule. This is further confirmed by the appearance of medium band at ca. 500 cm -1 , assignable to (M-O).

Proton Magnetic Resonance Spectra
The proton magnetic resonance spectra of ligands and their corresponding cobalt(III) complexes were recorded in DMSO-d 6 . The intensities of all the resonance lines were determined by planimetric integration. The signal due to -SH proton appears at ca.  8.8-9.0 in the ligands which disappears in the corresponding cobalt(III) complexes.
Thus, on the basis of elemental analyses, electrical conductance and spectral data, the following structure (IV) is tentatively proposed for chromium(III), manganese(III), iron(III) and cobalt(III) complexes. Antifungal Activity Sugarcane, a major source of sugar and important cash crop, is extensively grown worldwide. Diseases are one of the major constraints in the profitable cultivation of sugarcane [18,19]. About one hundred diseases of sugarcane caused by virus, fungi, bacteria, phytoplasma and nematodes have been reported from different parts of the world [20] and 84 diseases have been reported from India. Fungal diseases are causing alarming losses in sugarcane production of India.
Among fungal diseases red-rot by Colletotrichum falcatum [21], wilt caused by Fusarium moniliforme and Fusarium oxysporum [22] , pineapple caused by Ceraticystis paradox and leaf spots caused by Curvularia lunata, Curvularia pallescence, Epicocum nigrim and Periconia atropurpuria are the main diseases which are responsible for loss in sugarcane production in India. Among these, fungi Colletotrichum falcatum (Red rot) and Fusarium oxysporum (fusarium sett or stem rot, pokkah boeng, knife cut, and wilt) are the most important pathogens that may be transmitted primarily by planting infected setts. Curvularia pallescence is the important foliar pathogens of sugarcane, sometimes causing enormous losses to sugar industry. Different chemical compounds have been used as chemotherapeutants to control sugarcane diseases. A large number of inorganic compounds of mercury, lead, copper and sulphur have been used as fungicides to prevent fungal diseases of sugarcane. However several of these inorganic compounds are dangerous to human beings and host plants [23]. The increasing consciousness of the hazards involved in the use of such fungicides has reinforced the need for more efficient and harmless compounds. A number of synthetic organic compounds are now known to be useful in the control of various fungal diseases in plants [24,25].
Antifungal effect of all prepared complexes were studied against three pathogenic fungal strains by agar plate technique; the antifungal activity outcome were recorded as percentage of inhibtion and compared with standard drug Fluconazole. For each compound 1% standard solution was prepared in DMSO and 1mL of the solution was mixed with 9mL of the solvent. Three concentration viz. 10, 100, 1000 ppm stock solutions were prepared for each compound and 1 mL of each concentration solution was mixed with 9 mL of agar medium in sterilized petriplates. After the medium was prepared, fungus strain was inoculated in the center of each plate and assay plates were incubated at 29⁰ 2⁰C for seven days.
Metal complexes containing bis(mercapto azoles) also showed (Tables 8-10) promising results in inhibiting the mycelial growth of all the test fungi. Complexes with bis(mercapto thiadiazoles) are found to be more active than complexes with bis(mercapto triazoles) which in turn show better activity than bis(mercapto oxadiazoles). Manganese(III) complex with 1,2-bis(5-mercapto-1,3,4-thiadiazol-2-yl)butane (MThBH 2 ) is found to be 100% inhibitory to mycelial growth for all the species of fungi at 1000 ppm concentration. Other derivatives also show promising antifungal results against all test fungi from 35% to 92% at 1000 ppm concentration.
The minimum inhibitory concentration (MIC) of the two most active manganese(III) complexes were determined. Three concentrations 1000, 2000 and 3000 ppm of each test compound with respect to the culture medium were prepared. The fungistatic/ fungicidal nature of the active chemicals was determined in three replicates using the test fungi following the procedure of Garbour and Houston [26]. This was done by observing if revival of growth of the inhibition mycelial disks occurred following transfer to a chemical-free medium.

Screening Data Conclusions
1. There is significant alteration in the antifungal activity with the change in the nature of organic group attached to metal ion. 2. Metal complexes are found to be more active than their corresponding ligands. In other words, chelation increases the fungicidal activity. 3. For any particular species of fungus, manganese(III) derivatives show better activity than iron(III) derivatives which in turn show better activity than cobalt(III) derivatives. Chromium(III) complexes show least activity as compared to manganese(III), iron(III) and cobalt(III) derivatives. 4. The activity decreases on dilution.