INTRODUCTION

Multimetallic complexes have received much attention in connection with their potential relevance to bioinorganic chemistry.1 The polydentate and compartamental ligands can form such multimetallic clusters by binding two or more metal centres in close proximity. The homomultimetallic molecular complexes are of interest in areas like multimetallic enzymes, homogeneous catalysis and heterogeneous catalysis. Many enzymes in biological system are homo-metallic that perform their biological function by redox co-operativity.2 The toxicological importance of the ligands containing –N-C=S moiety and their metal complexes has been well established as potential antifungal and antibacterial agent.3 Therefore, the studies of such bioactive ligands and their complexation behaviour have received much importance now-a-days. In continuation of our consistent effort towards synthesis and characterization of such type of complex,4,5 we report here the synthesis and characterization of hither to unknown homobinuclear complexes with the title ligands, bispt, fispt, pmispt and tmispt obtained by replacing one H-atom of methylene group at the 5th position of the thiohydantoin moiety in 3-benzilidine/furfurylidene (pyridyl/thienyl-2’-methylene) imino-2-thiohydantoin by p-sulphonamido phenyl azo group as given in Fig. 1 with UO2(VI), ZrO (IV) and Th(IV) ions. This is our continuing investigation on the coordination chemistry of multidentate ligands containing NOS donors.6-9

Fig. 1.

EXPERIMENTAL

Material and Method

Reagent grade chemicals were used in the present study. The solvents were purified before use by standard process. The starting materials such as 3-benzylidine/furfurylidine/(pyridyl/thienyl-2’-methylene) imino-2-thiohydantoin by the method reported earlier.10

Preparation of Ligands (L), L=bispt/fispt/pmispt/tmispt: Sulphanilamide (0.1 mol) dissolved in glacial acetic acid and water (25 mL) was diazotized with NaNO3 in an ice bath. The resulting dilute solution was added gradually with stirring to previously cooled solution of 3-benzylidine immino-2-thiohydantoin (0.1 mol) in NaOH kept in ice bath. After addition, the mixture was mechanically stirred for half an hour during which the yellow colour azo dye separated out. It was filtered and recrystallized from ethanol. The ligands fispt, pmispt and tmispt were prepared by adopting the similar procedure as adopted for bispt from 3-furylidene(pyridyl/thienyl-2’-methylene) imino-2-thiohydantoin respectively.

Preparation of the complexes of the type [M2L(NO3)n(H2O)m] where M=UO2(VI), ZrO(IV), Th(IV): A hot ethanolic solution of ligand(L) was mixed with corresponding hydrated UO2(VI)/ZrO(IV)/Th(IV) nitrates dissolved in minimum volume of ethanol in 1:2 molar proportion and the pH of the resulting solution was adjusted to ~6.5-7 by drop wise addition of dilute ammonia. UO2(VI) complexes separated out immediately while complexes of ZrO(IV) and Th(IV) were obtained only after refluxing for 7 to 8 hours on a water bath. In each case microcrystalline coloured complex was obtained which was filtered off washed several times with ethanol followed by ether and finally dried in vacuo over fused CaCl2.

Analysis and Physical measurements: The metal contents in the complexes were determined gravimetrically following standard procedure.11 A weighed quantity of compound (0.2~0.3 g) was treated with a few drops of conc. H2SO4 and 1mL of conc. HNO3. It was heated till the organic matter decomposed and sulphur trioxide fumes came out. The same process was repeated two to three times to decompose the substance completely. Then it was dissolve in water and the resulting solution was used for analysis of metal ions. Uranium, zirconium and thorium were precipitated as ammonium diuranate, zirconium mandelate and thorium oxalate followed by sufficient ignition to their respective oxide as U3O8, ZrO2 and ThO2. Sulphur was determined as BaSO4. The molar conductance measurements were carried out at room temperature with a Toshniwal conductivity Bridge (model CL-01-06, cell constant 0.5 cm-1) using 1×10-3M solution of the complexes in DMSO. Carbon, hydrogen and nitrogen contents of the complexes were determined by using a MLW-CHN micro analyser. FTIR spectra in KBr pallets were recorded on a Varian FTIR spectrophotometer, Australia. The electronic spectra of the complexes in DMSO were recorded on a Perkin-Elmer spectrophotometer. Thermogravimetric analyses were done by Netzch-429 thermoanalyzer. The 1H-NMR spectra of the complexes were recorded in DMSO-d6 medium on JEOL, GSX-400 model equipment.

RESULTS AND DISCUSSION

The complexes were formulated from the analytical data and molar conductance data support the suggested formulae (Table 1). The complexes are highly coloured and insoluble in water and common organic solvent such as ethanol, methanol, acetone, CCl4, CHCl3, benzene and ether but moderately soluble in highly coordinating solvents such as DMF and DMSO. They are diamagnetic, non hygroscopic, highly stable under normal conditions and all of them decompose above 250 ℃. The molar conductance data values in DMSO for the complexes indicate them to be non-electrolyte in nature. However the conductivity values are higher than those expected for non-electrolytes probably due to partial solvolysis of the complexes in DMSO medium.12

Table 1.aOhm-1 cm2 mole-1

IR Spectra

The introduction of p-sulphonamido azophenyl group at 5th position of 2-thiohydantoin moiety of 3-benzylidine/furfurylidine/(pyridyl/thienyl-2’ methylene)-2-thiohydantoin did not cause any shift of the band position due to νN-H (~3400 cm-1) and thioimido-I (~1450 cm-1), II (~1320 cm-1), III (~1080 cm-1), IV (~840 cm-1), νC=O (~1710 cm-1) and azomethine νC=N (~1640 cm-1) of thiohydantoin moiety. Only additional band system of three strong bands at ~3300, ~1360 and ~1175 cm-1 due to sulphonamido group13 and a band of medium intensity at ~1600 cm-1 due to stretching vibration of azo(-N=N-) group were observed in the IR spectrum of the ligands. On comparing IR spectra of the ligands with complexes, stretching bands corresponding to azomethine, νC=O group, azo group and thioimido (NH) groups were found to be invariable shifted by 20-30 cm-1 towards lower frequency region indicating the co-ordination of azomethine nitrogen, carbonyl oxygen, azonitrogen and thioimidic nitrogen to the metal ions consequently the band occurring at ~1050 cm-1 due to νNN of the ligand is blue shifted to ~1070 cm-1 in all the complexes. The presence of furan ring in fispt is also indicated by the appearance of band at 1500 cm-1 due to νC-O-C group of furan ring.14 The position of this band is shifted to lower frequency region in the metal complexes suggesting participation of oxygen atom of furan ring in complexation.

In addition to thioimide bands and a band due to azomethine ν(C=N), a system of bands at ~1580, ~1430, ~1000 and ~600 cm-1 corrosponding to antisymmetric ring stretching, symmetric ring stretching, ring breathing and in plane deformation mode of pyridine nucleus were observed in IR spectrum of the ligand pmispt.14 It became difficult to arrive at any conclusion from the bands occurring at finger print region due to overlapping nature, regarding the co-ordination behaviour of ring nitrogen atom of pyridine moiety, however the shifting of band at 600 cm-1 to high frequency region by 20-30 cm-1 in the present complexes suggested the involvement of pyridine ring nitrogen atom to the metal ion.15 The ligand tmispt contains both the thiophene ring and thioimide groups. All the bands due to above groups were not observed in the IR spectrum of tmispt. Instead IR spectrum of this ligand only exhibits bands at ~1440, ~1310, ~1070 cm-1 assignable to thioimide I, II and III bands respectively being overlap with ring stretching vibration of thiophene ring, where as the bands observed at ~840 cm-1 and 800 cm-1 may be assigned due to thioimide IV band and νC-S-C vibration respectively.13 The characterstics IR bands due to νC-S-C (thiophene ring) shifted their position to lower frequency region thereby indicating the coordination of ring sulphur atom to the metal ion.14 The IR spectra of all the complexes show additional bands at ~1390 and ~1020 cm-1 which unambiguously suggest the presence of coordinated nitrate anions in unidentate manner corresponding to ν2 and ν4 mode of vibration of coordinated nitrate ion under C2v symmetry.16 The presence of nitrate ion in the coordination sphere of all the complexes has also been supported by non-electrolytic nature of complex in DMSO. Besides the bands observed at ~3500(S), ~880(m) cm-1 may assigned to ν(OH), and ρ(r) of co-ordinated water, which is further confirmed by thermal analysis.

The occurrence of new bands at ~530-515 and ~460-450 cm-1 in the spectra of the complexes further suggest the coordination of metal ions through oxygen and nitrogen respectively. The uranyl complexes exhibit a strong band a ~940-900 cm-1 and the medium intensity band at ~835-820 cm-1 assignable to νas(O=U=O) and νs(O=U=O) mode respectively.17 The zirconyl complexes exhibit one strong band in the region ~900-870 cm-1 which can be attributed to the ν(Zr=O) indicating the presence of (Zr=O)2+ moiety in these complexes.18

Thermal analysis

All these complexes follow the same pattern of thermal decomposition. The thermogram of the complexes exhibited the characteristics of co-ordinate water. The complexes remain almost unaffected up to ~140 ℃. There after a slight depression up to ~200 ℃ is observed. The weight loss at this temperature is equivalent to four water molecule for bispt complexes and three water molecules for other complexes indicating them to be coordinated water19 in conformity with our earlier observations from analytical and IR spectral investigation. Simultaneous elimination of coordinated water suggests them to be in the same chemical environment. The anhydrous complexes remain stable up to 400 ℃ and there after the complexes shows rapid degradation presumably due to decomposition of organic constituents of the complexes molecules as indicated by the steep fall in the percentage of weight loss. The decomposition temperature varies for different complexes as shown in Table 2.

Table 2.Important features of thermo gravimetric analysis (TGA)

Electronic spectra

The electronic spectra of complexes in DMSO are quite similar. The present system of ligands where 2-thiohydantion is coupled at 3-position with benzylidene/furfurylidene/(pyridyl/thienyl-2’-methylene) imino group and at 5 position with-p-sulphonamido-phenyl azo group display only two bands corresponding to λmax, 280-300 nm and 350-400 nm assignable to π-π* and n-π* transition respectively indicating extensive conjugation. The UO2(VI) complexes display mainly one weak band at 440 nm and a highly intense band in the range ~300 nm which may be due to 1Σg+→3πu transitions and charge transfer being overlapped with π–π* transition respectively.20 It may be noted that the band occurring at 370 nm is due to uranyl moiety because of apical oxygen →ƒo(U) transition is being merged with the ligand band due to n→π* transition as evident from broadness and intensity. The electronic spectra of ZrO(IV) and Th(IV) complexes exhibit only highly intensive additional band in the region 370-400 nm which may be due to charge transfer besides the ligand bands.

1H NMR spectra

The 1H NMR spectra of the ligands as well as the complexes are recorded in DMSO-d6 medium. The 1H NMR spectra of the ligands show a complex broad multiplet at δ 8.4-7.3 ppm corresponding to nine aromatic protons for bispt, While the ligands like fispt, pmispt and tmispt show addinational multipletes at δ 7.5-6.20 ppm corresponding to three protons of furan moiety of fispt, at δ 8.0-8.7 ppm corresponding to four aromatic protons for pyridine moiety of pmispt and at δ 7.78-7.22 ppm corresponding to three protons of thiophene moiety of tmispt, besides the multiplet at δ 8.7-8.5 ppm due to four phenyl ring protons.21 The former multipletes undergo downfield shift to some extent probably due to involvement of ring O-atom, ring N-atom, ring S-atom in complextation, because of lowering of electron density in the ring system as evidenced from IR investigation while the latter multiplet remains practically unalterted.

Another set of three distinct 1H NMR peaks were observed at δ ~9.5 ppm (1H), δ ~9.1 ppm (1H) and at δ ~3.42 ppm in the present system of ligands corresponding to N=C-H proton, -NH proton and -CH protons (thiohydantoin moiety). The signals due to H-C=N proton and NH proton show a downfield shift by δ 0.3~0.4 ppm indicating the coordination of azomethine nitrogen and amidic nitrogen to the metal ions. However, the position of peaks due to –CH proton does not change appreciably because of the coordination of the ligand with metal ion. Besides an additional peak at δ ~3.5 ppm is observed in all the complexes indicating the presence of coordinated water molecules.22

Fig. 2.(M=UO22+, ZrO2+) (UO2)2(bispt)(NO3)4(H2O)4 (ZrO)2(bispt)(NO3)4(H2O)4.

Fig. 3.(Th)2(bispt)(NO3)8(H2O)4.

Fig. 4.(M=UO22+, ZrO2+)(X=O,S).

Fig. 5.(Th)2(fispt)(NO3)8(H2O)3(Th)2(tmispt)(NO3)4(H2O)3(X=O,S).

Fig. 6.(UO2)2(pmispt)(NO3)4(H2O)3 (ZrO)2(pmispt)(NO3)4(H2O)3(M=UO22+, ZrO2+).

Fig. 7.(Th)2(pmispt)(NO3)4(H2O)3.

Fungicidal Screening

The antifungal activity of ligand bispt, fispt, pmispt, tmispt and zirconium complexes were tested against the organism Aspergillus niger, Aspergillus flavus by a typical procedure,23 well diffusion method24 method. A well was made on NCCLS approved standard potato dextrose agar medium inoculated with micro organism. The well was filled with the test solutions prepared by dissolving the complex in DMSO using a micropipette and the plates were incubated at 35 ℃ for five days. During this period the test solution diffused affected the growth of the inoculated bacteria. The % of inhibition was calculated as

% of inhibition=100(P-Q)/P

Where Q=Area of colony growth with test sample

P=Area of colony growth without test sample

The results obtained have been presented in the Table 3 as given below.

Table 3.Fungi toxicity of the ligands and Zr(IV) complexes

The results of antifungal screening indicate that the complexes are more active than the free ligand. Increased activity of the complexes can be explained on the basis of chelation theory, there by increasing delocalization of π electron in the chelating ring.25