SYNTHESIS , CHARACTERIZATION AND ANTIFUNGAL STUDIES ON CHROMIUM ( III ) , MANGANESE ( III ) , IRON ( III ) AND COBALT ( III ) COMPLEXES WITH THIOHYDANTOINS

A. K. Srivastava. A series of metal complexes of Cr(III), Mn(III), Fe(III) and Co(III) with 1aryl-2-thiohydantoin having composition [M(L)3] (M=Cr(III), Mn(III), Fe(III) and Co(III); LH= 1-phenl/o-methylphenyl/o-chlorophenyl/obromophenyl-2-thiohydantoin) are prepared and characterized on the basis of elemental analyses, electrical conductance, magnetic moment, spectral (electronic, IR, PMR) data. The magnetic moment and electronic spectral data suggest an octahedral geometry around the central metal ion. The molar conductance of complexes is an indicative of their non-electrolytic nature. IR and PMR data indicate that the ligands act as bidentate chelating agents, coordinating through thiolsulphur and azomethine nitrogen. The antifungal activity screening against Colletotrichum falcatum, Fusarium oxysporum and Curvularia pallescence show that the complexes are more potent in comparison with free ligands

Tautomeric equilibria may be further modified in solution by changes in pH. In highly acidic media, fully protonated species (a) are likely and may appear to have all coordination sites blocked. Alternatively, because imido (NH) protons are labile species such as (a) are also potentially ambidentate with p ka values varying from 5 to 11, deprotonation is possible over a wide pH range. The resultant thiolate anion (d) is capable of monodentate, bidentate or bridging coordination behaviour and of involving either or both, sulphur and nitrogen in these processes. Thus, the coordination behaviour of hetrocyclic thione molecule depends upon reaction conditions, nature of metal ion and pH of the medium. Extensive use has been made of spectroscopic methods in determining the coordination character of hetrocyclic thione molecules.
The stimulus for much of the research into the coordination chemistry of hetrocyclic thione donors stems from their wide ranging applications 21,22 in analytical chemistry, in polymers and plastics, in biological systems, in metal finishing and electroplating industries etc.

Structure of Ligands:-
The structures of various 1-aryl-2-thiohydantoin are given below- All these complexes are coloured . They are thermally stable but decompose in the temperature range 120-275 o C. They are quite stable in air. Conductance measurements in dimethylformamide reveal that they are essentially nonelectrolytes.

Magnetic Moments:-
The chromium complexes show the magnetic moments in the range 3.80-3.90 at room temperature which is in accord with the observed spin-only value having no TIP contribution, suggesting octahedral environment around chromium ion.  In the present ligands, these bands appear at cα. 1500, 1400-1350, 1050 and 880-840 cm -1 respectively. These bands are expected to be affected differently by different modes of coordination after complexation with the metal ions 1,18 . The appearance of these bands indicate the existence of ligands in the thione form in the solid state. In the solution (basic) infrared spectra of the ligands, all these bands were absent but one weak band at cα. 2480 cm -1 appears, assignable to  (S-H). These facts indicate the possibility of thione thiol tautomerism in the solution. In the spectra of the complexes all thioamide bands as well as (S-H) disappear, suggesting coordination of the sulphur atom to the metal ion in the thiol form. The  (M-S) vibration appears at 400-370 cm -1 . The infrared spectra of 1-ary-2thiohydantoin ligands show bands at cα. 3300 cm -1 , which may be assigned to  (N-H). In the complexes, this band disappears. In the solution I.R. spectra of ligands, one band appears at cα. 1600-1580 cm -1 , assignable to (C=N), formed as a result of thione thiol tautomerism. In the complexes, this band shifts to lower frequency which may be taken as an evidence for the coordination of azomethine nitrogen [31][32][33][34][35] . This is further confirmed by the appearance of new bands around 450-420 cm -1 assignable to  (M-N). A sharp band appearing at 1720-1700cm -1 , due to  (C=O) in the spectra of ligands, appears at the same place in the spectra of the complexes, ruling out the possibility of coordination through the carbonyl group.
Thus, it becomes evident that the ligands act as bidentate chelating agents, coordinating through thiol sulphur and azomethine nitrogen.

Proton Magnetic Resonance Spectra:-
The proton NMR spectra of cobalt(III) complexes were recorded in deuterated chloroform and dimethylsulphoxide; the intensities of all the resonance lines were determined by planimetric integration . In general, a slight low field shift of the resonance signals of various protons (R) in these complexes, in comparison with the respective proton signals in free ligands, may be attributed to deshielding upon coordination. The spectra of 1-aryl-2-thiohydantoins show one sharp signal at δ 4.8-4.92 due to NH proton which disappears in the corresponding complexes. The signals due to aromatic ring appear in the range δ 7.25-7.48.

Structure of Complexes:-
On the basis of elemental analysis, electrical conductance measurements and spectral features, the following structures are tentatively proposed for [M(L) 3 ], respectively.

Fungicidal Activity:-
The newly synthesized complexes of chromium(III), manganese(III), iron(III) and cobalt(III) derivatives containing thiohydantoins were screened for their fungitoxic properties against Colletorichum falcatum, Fusarium oxysporum and Curvularia pallescence ( all parasitic on sugarcane).Most of the complexes displayed significant fungitoxicity. The metal complexes containing thiohydantoins show promising results in inhibiting the mycelial growth of all the test fungi. In this series, the best activity was recorded with manganese(III) complex containing 1-o-chlorophenyl-2-thiohydantoin. This compound showed activity upto 78.5% against C. falcatum, 75.2% against F.oxysporum and 76.4% against C.pallescence at 1000 ppm concentration. Other derivatives showed inhibition from 22.9% to 75% against all test fungi at 1000 pm concentration.