Synthesis, Spectral and Antimicrobial Studies of Bis(cyclopentadienyl)titanium(IV) Derivatives with Schiff Bases Derived from 2-Amino-5-phenyl-1,3,4-thiadiazole

The reactions of bis(cyclopentadienyl)titanium(IV) dichloride with Schiff bases derived by condensing 2- amino-5-phenyl-1,3,4-thiadiazole with benzaldehyde (SPT), 4-nitrobenzaldehyde (SNT), 4-methoxybenzaldehyde (SMT), 2-hydroxybenzaldehyde (SSTH) or 2-hydroxyacetophenone (SATH) have been studied in refluxing tetrahydrofuran and complexes of types [Cp2TiCl(SB)]Cl (SB= SPT, SNT or SMT) and [Cp2Ti(SB')]Cl (SB'H= SSTH or SATH) have been isolated. Tentative structural conclusions are drawn for these reaction products based upon elemental analyses, electrical conductance, magnetic moment and spectral (UV-vis, IR and 1H NMR) data. Studies were conducted to assess the growth-inhibiting potential of the complexes synthesized, and the ligands, against various fungal and bacterial strains.


INTRODUCTION
One aspect of the use of coordination compounds of transition metals is their application as biologically active substances/1/. The attention of investigators has focused increasingly on bio-coordination compounds, which can be used as biologically active preparations in medicine and agriculture. The complexes prepared for use in agriculture are, as a rule, more effective at lower concentrations than the metal ions and organic molecules, which enter into their composition. The anticancer and antiviral activities of metal complexes have also been studied. It has also been observed that a small structural change in the ligand may lead to the enhanced activity of the metal complexes. Among the various classes of biologically active coordination compounds, complexes with thiadiazoles as ligands have attracted attention.

EXPERIMENTAL
All the reactions were carried out under strictly anhydrous conditions. Tetrahydrofuran (Merck) was dried by sodium wire overnight and then refluxed until it gave a blue colour with benzophenone. The ligands, Schiff bases, were prepared as mentioned in the literature/11/. Bis(cyclopentadienyl)titanium(IV) dichloride was prepared /12/ by heating CpNa with TIC14 in a N2 atmosphere. Elemental analyses and physical measurements were made as noted earlier/13/.
The brown coloured complex so obtained was filtered, washed with anhydrous tetrahydrofuran and dried in vacuo. The complex was identified to be [(CsI-15)2TiCI(CsH1N3S)]C1. Yield 58% The same procedure was adopted for the synthesis of other Schiff bases (derived by the condensation of 2-amino-5-phenyl-l,3,4-thiadiazole and 4-nitrobenzaldehyde (SNT) or 4-methoxy benzaldehyde (SMT)) derivatives of bis(cyclopentadienyl)titanium(IV). For the sake of brevity, these reactions are summarized in Table 1. The analytical data of the products are given in Table 1. Table 1 Reactions of bis(cyclopentadienyl)titanium(IV) dichloride with Schiff bases derived from 2-amino-5-  To the mixture of bis(cyclopentadienyl)titanium(IV) dichloride (0.01 mol) and Schiff base (SSTH) (0.01 mol) was added anhydrous dichloromethane (60 cm3) followed by 45 hours reflux. The volume of the solution was reduced to about 10 cm3. Dry petroleum ether (b.p. 60-80 , 20 cm) was added to it and the solution was allowed to stand for one hour, which resulted in yellow crystals of the product. The complex, thus obtained, was filtered and dried in oven at 80C. The complex was found to be [(CsHs)-Ti(C.sHj,N3SO)]CI. Yield 72%.

titanium(IV) Derivatives
The same procedure was adopted for the synthesis of bis(cyclopentadienyl)titanium(IV) derivatives with Schiff bases derived from 2-amino-5-phenyl-l,3,4-thiadiazole and o-hydroxy acetophenone (SATH). For the sake of brevity, such reactions are summarized in Table 1. The analytical data of the products are given in Table 1.

RESULTS AND DISCUSSION
A systematic study of the reactions of bis(cyclopentadienyl)titanium(IV) dichloride with neutral (SB) and monobasic (SB'H) Schiff bases in (1:1)

Electronic spectra
The electronic spectra of all Schiff base derivatives in nujol show bands in the regions 23,500--24,200, 30,000 and 34,800 cm-. The first band may be assigned /14/ to the charge-transfer band and its in accordance with their (n-1)dns electronic configuration. The second and third bands (also appear in ligands) are assigned to w-* transitions of the azomethine linkages.

Infrared spectra
The significant infrared spectral bands of the complexes are given in Table 2. A comparison of the characteristic infrared absorption bands of Schiff bases with those of the corresponding titanium(IV) derivatives reveals the following important features. (a) The infrared spectra of the ligands show bands at ca. 1650-1635 and 1580-1565 cm -l which may be assigned/2/to be v(C=N) (azomethine group) and v(C=N) (ring), respectively. The first band is shifted towards the lower frequency region (20-30 cm-) in the titanium(IV) complexes indicating the coordination of the azomethine nitrogen atom of the Schiff bases. However, the second band vC=N (ring) is found to split into two; one almost located at the original position indicating non-coordinated vC=N, and the other shifted to lower frequency at 1550-1540 cm -, arising due to coordinated vC=N mode. The splitting of the vC=N (ring) absorption band suggests that only one of the ring nitrogen is involved in coordination and other is free and non-coordinated. This is further supported by a new band at ca. 450-440 cmassignable/15/to v(Ti-N) in the complexes.

H NMR spectra
The different proton magnetic resonance signals (recorded in DMSO-d6) are given in Table 3. The intensities of all the resonance lines were determined by planimetric integration. Comparing the spectra of ligands and their corresponding complexes, one can derive the following conclusions.
(a) A signal in all Schiff base derivatives at 5 6.5-6.8 may be assigned to the protons of the cyclopentadienyl ring. The appearance of a single, sharp cyclopentadienyl resonance is attributed/18/to the rapid rotation of the cyclopentadienyl ring around the metal ring axis. (b) All the ligands show signal at 5 7.1-7.5 integrating for aromatic group protons. This signal shows slight shift upon co-ordination.
(c) The signal observed at 5 9.4-9.8 in the spectra of ligands, derived from salicylaldehyde and ohydroxyacetophene may be assigned to the phenolic protons. This signal disappears in the spectra of their corresponding complexes indicating the coordination through deprotonation of phenolic oxygen atoms.
(d) The proton signal due to the CH=N group appears at 5 8.3 in the ligands. In the complexes, this signal shifts downfield which is probably due to the donation of the lone pair of electrons by the azomethine nitrogen to the titanium atom. (a) The compounds show significant toxicity at 1000 ppm conc. Against all species of fungi. However, the complexes are more active than ligands, which may be owing to the chelation and the presence of sulphur atom.
(b) The activity decreases on dilution.
(e) The best activity was noted for complex with ligand derived from 4-methoxybenzaldehyde. This indicates that the presence of methoxy group imparts the fungicidal power in these series of compounds.
(d) The ligands and complexes are more active against A. niger than A. alternata and H. oryzae.
The variation in the effectiveness of different biocidal agents against different organisms, as suggested by Lawrence et al. /20/ depends either on the impermeability of the cells or differences in ribosomes of antimicrobial agent.

Antibacterial activity
The antibacterial activity of the complexes together with parent ligands has been screened against Grampositive Bacillus subtilis and Gram-negative Escherichia coli by the paper disk plate method/21/at 1000 ppm conc. The inhibition zone (mm) around each disk was measured after 24 h and the results of these studies are listed in Table 5. It is clear from the screening data that complexes are_slightly more toxic than the parent ligands. Table 5 Antibacterial activity of Schiff bases and their corresponding bis(cyclopentadienyl)titanium(IV) complexes Compound Diameter of inhibition zone (mm) B. Subtilis E. Coli SPT 10 12 [CpETiCI(SPT)]CI