SYNTHESIS, SPECTRAL AND SOL-GEL BEHAVIOR OF MIXED LIGAND COMPLEXES OF TITANIUM(IV) WITH OXYGEN, NITROGEN AND SULFUR DONOR LIGANDS

A new route to synthesize nano-sized Ti(IV) mixed ligand complexes have been investigated by the reaction of titanium(IV) chloride with ammonium salts of dithiophosphate and 3(2'-hydroxyphenyl)-5-(4substituted phenyl) pyrazolines. The resultant complex is then treated with H2S gas to get sulfur bridged dimer of Ti(IV) complex, a precursor of TiS2. The morphology of the complexes was studied by employing XRD which shows that all the complexes are amorphous solid. Molecular weight measurements, elemental analysis in conjugation with spectroscopic (IR, H NMR, C NMR and P NMR) studies revealed the dimeric nature of the complexes in which pyrazoline and dithiophosphate are bidentate. Scanning electron microscopic image and XRD indicate that the particles are in the nano range (50 nm). Putting all the facts together, coordination number six is proposed for titanium with octahedral geometry.


INTRODUCTION
Titanium proves to be an excellent corrosion-resistance material in many environments as it forms a protective oxide layer on its surface [1,2]. The high tensile strength, light weight and excellent corrosion resistant make the titanium a useful alloying agent for many parts of highspeed aircraft, motorbikes, ships and missiles [3][4][5]. Titanium being a biocompatible material found application in prosthetic devices [6,7].
The Ti(IV) complexes with nitrogen, oxygen and sulfur donor ligands have received considerable attention due to their widespread utilization as an active precursor for making TiO 2 and TiS 2 [8]. Owing to the hard acid character of titanium, the synthesis of its simple thiolates was not possible. Attempts have been made to reduce the acidic strength of titanium metal centre by attaching electron-rich ligands such as dialkyl nitrogen and cyclopentadienyl which then forms a stable complex with soft bases [9]. The highly sensitive nature of titanium complexes towards hydrolysis reduces its activity towards different applications [10]. Available reports showed that the addition of bulky electron-rich ligands to Ti metal centre increases the resistance of metal complexes towards hydrolysis [11,12].
The excellent biological activity of sulfur containing transition metal complexes makes them interesting [13]. Several reports are available on alkylene and O,O'-dialkyl dithiophosphate derivatives of Ag(I), Zr(IV), Fe(II) and Cu(II) [14,15]. Carmalt et al. [9] reported titanium pyridine and pyridine thiolates as a precursor for the production of titanium disulfide. Ti(IV) has been extensively used for the polymerization of ethylene and propylene [16,17]. Salen-Ti(IV) complex has been effectively employed in the controlled polymerization of D,L-lactic acid [18]. Park et al. [19] designed and synthesized a new class of green colored titanium complexes with a dithiolate ligand for LCD and TFT panels. The first non-platinum anticancer drug exhibiting excellent efficacy was titanium based titanocene dichloride and budotitane [20]. Later on, The corresponding ammonium salts of the synthesized dithiophosphoric acids have been prepared by passing dry ammonia gas through their benzene solutions (Eq. 3-4). The structure of ammonium salt of substituted dithiophosphate ligands are shown in Figure 1.

Synthesis of substituted pyrazoline ligands
Substituted pyrazoline ligands were synthesized by reported procedure [38].
(a) Synthesis of substituted 2'-hydroxychalcone. A hot solution of sodium hydroxide was added to a mixture of o-hydroxyacetophenone and substituted benzaldehyde in ethanol. The mixture was stirred at room temperature for 6-8 hours. The sodium salt of the chalcone was obtained as dark yellow thick mass. It was cooled in ice and neutralized with aqueous acetic acid (50%). The yellow solid separated was filtered and washed with water before drying. Crystallization from ethanol yielded yellow needles.
(b) Synthesis of substituted pyrazoline. A mixture of substituted 2'-hydroxychalcone and hydrazine hydrate in ethanol was refluxed for 3-4 hours. It was allowed to cool at room temperature. A white crystalline solid thus obtained was separated, washed with water and dried. Recrystallization with ethanol afforded white crystals of pyrazoline. The structure of substituted pyrazoline ligand is shown in Figure 2.

Synthesis of TiCl 2 (C 15 H 12 N 2 OX)(RO) 2 PS 2
A benzene solution of pyrazoline (1.21 g, 5.10 mmol) was added dropwise with constant stirring to the titanium tetrachloride (0.96 g, 5.11 mmol) suspension at room temperature. To ensure the completion of reaction, the reaction mixture was stirred for 2-3 hours. To the above reaction mixture, the solution of ammonium salt of dithiophosphate in methanol was added dropwise under constant stirring for 3-4 hours. The by-product (NH 4 Cl) was filtered off using alkoxy funnel. A reddish-brown solid compound was obtained (1.76 g, 88%) after removal of the volatiles from the filtrate under reduced pressure. The same procedure was adopted for the synthesis of all the compounds .
The two-step reaction scheme is proposed for the synthesis of mixed ligand titanium complexes of the general formula TiCl 2 (C 15 H 12 N 2 OX)(RO) 2 PS 2 ] (Eq. 5-6).

RESULTS AND DISCUSSION
All the synthesized compounds are non-hygroscopic orange-colored solid which are stable at room temperature. They are easily soluble in coordinating solvents (THF, DMSO and DMF) as well as in common organic solvents (benzene, chloroform and methanol). The proposed stoichiometries of the synthesized compounds are in good agreement with the elemental analysis (H, C, N, S, Cl, and Ti) data reported in Table 1.

Spectral analysis of Ti 2 (C 15 H 12 N 2 OX) 2 [(RO) 4 P 2 S 6 ]
Infrared spectral data analysis The medium intensity band observed at 3346-3325 cm -1 could be assigned to vibrations corresponding to [N-H] stretching [39] while the spectral bands in the region 1624-1604 cm -1 are due to the [C=N] stretching vibration [40]. As compared to free pyrazoline the v[C=N] stretching in all the synthesized compounds is observed to be shifted to the lower wavenumber. This suggests that the imino nitrogen of pyrazoline is coordinated to a metal centre. The complete absence of a signal at ~3080 cm -1 in synthesized metal complexes, which is due to (O-H) stretching originally present in pyrazoline ligands suggests that the oxygen is covalently bonded to Ti metal. This is further confirmed by the appearance of the band in the region 485-460 cm -1 corresponding to [Ti-O] stretching vibration. The bands present in 824-899 cm -1 and 1078-1050 cm -1 region has been assigned respectively to [P-O-(C)] [41,42] and [(P)-O-C] [43,44]. The new bands of medium intensity observed in the region 549-529 cm -1 may be assigned to [P-S] stretching modes [45].
In comparison to free ligands, the appearance of two new bands in 335-321 cm -1 and 302-290 cm -1 region corresponds to [Ti-S] stretching vibrations. Splitting of bands into two regions indicates that two types of sulfur are present in the molecule, one is terminal sulfur and another is bridging sulfur. The appearance of bands in the region 396-380 cm -1 has been ascribed to vibrations corresponding to [Ti-N] stretching [46]. The IR data of synthesized complexes are compiled in Table 2.

H NMR spectra analysis
The 1 H NMR spectra of synthesized mixed ligand complexes, recorded in CDCl 3 exhibit characteristic signals (Table 3). In the region δ 7.42-6.39 ppm, a very complex pattern may be assigned to the aromatic protons of ligand pyrazoline [47]. The pyrazoline ligand exhibits a characteristic peak at δ~11.00 ppm due to hydroxyl protons, the absence of that particular peak in the 1 H NMR spectra of the metal complex suggests that the hydroxyl oxygen atom is bonded to Ti metal. A broad singlet peak observed at δ 5.37-4.86 ppm may be attributed to the N-H group (primarily at δ 5.40-4.90 ppm in free pyrazoline) indicating that the -NH group is not involved in metal complex formation [47]. The bands at 3.82-3.07 and 2.25-2.02 ppm could be ascribed, respectively to -CH and -CH 2 groups. The band at δ 5.54-4.19 ppm for -OCH 2 and at δ 4.94-4.21 ppm for -OCH group and bands for methyl group are observed at δ 1.10-0.90 ppm. The complex pattern observed at δ 7.21-7.04 ppm may be due to the skeletal protons of the phenyl ring. The hydrogen atom calculated through the integrations ratio suggests that two of the dithiophosphate ligands and two pyrazoline ligands are present in synthesized mixed ligand complexes.

P NMR spectra analysis
The synthesized compounds exhibit only one signal for the phosphorus atoms in protondecoupled 31 P NMR spectra. The 31 P NMR signals of Ti dichlorodithio-compounds are obtained at δ = 90.0 ppm while that of synthesized Ti mixed ligand complexes are observed at δ = 110.0-91.3 ppm. The downfield shifting of the signal due to dithiophosphato phosphorus atom at about δ15.0 ppm confirms the bidentate nature of the ligand [48]. Although two phosphorus atoms are there only one signal is obtained, indicates a similar environment for both the phosphorus atom (Table 3).   13 C and 31 P) data are summarized in Table 3.

FAB Mass spectra analysis
The FAB mass spectra of the synthesized metal complexes have been recorded to determine the molecular weight. The molecular ion peak confirms that the metal complexes exist in dimeric form. FAB mass spectra of compound numbers 6, 12, 18 and 24 with different substituted pyrazoline ligands in each series have been reported in Table 4.

XRD and SEM studies
These crystalline/amorphous natures of the complexes have been examined through XRD. The morphology of the complexes was studied by employing XRD which shows that all the complexes are amorphous solid. The average diameter of the complexes has been calculated using "Debye Scherrer" expression (Eq. 9).
Particle size = D = 0.9 λ / β cos θ B (9) where, λ is the X-ray wavelength (1.5418Å), β is corrected band broadening (full width at half maxima), θ B is the diffraction angle, D is the average nanocrystal domain diameter. The value of full width at half maximum intensity (β) and corresponding diffraction angle (θ B ) is calculated using an X-ray diffractogram. The average particle size thus obtained was found to be in the range of 41-62 nm, which is further confirmed by the SEM studies (  Figure 3 and Figure 4, respectively.   Molecular weight measurement, elemental and spectral analysis confirms the dimeric nature of the synthesized metal complexes and proposes octahedral geometry ( Figure 5).

CONCLUSION
The present study describes the new route for the synthesis of Ti(IV) mixed ligand complexes with dithiophosphate and substituted pyrazoline ligands.Molecular weight measurements, elemental analysis in conjugation with spectroscopic (IR, 1 HNMR, 13 C NMR and 31 P NMR) studies reveal thedimeric nature of the complexes in which pyrazoline and dithiophosphate are bidentate. Scanning electron microscopic image and XRD indicate that the particles are in the nano range (50 nm). Coordination number six is proposed for titanium with octahedral