Synthesis , Characterization , and Photophysical Studies of Some Novel Ruthenium ( II ) Polypyridine Complexes Derived from Benzothiazolyl hydrazones

A series of five benzothiazolylhydrazone ligands 3a–3e and their seven novel Ru(II) polypyridine complexes 4a–4d, 5a, 5d, 5e of the type [Ru(N–N) 2 (L)]Cl 2 , where (N–N) 2 is 2,2-bipyridine (bpy) 2 or 1,10-phenanthroline (phen) 2 and (L) is ligands 3a– 3e, have been synthesized and characterized by elemental analysis, IR, H NMR, electronic absorption, and emission spectral studies. The interpretation of the analytical data revealed that the ligands coordinate with the metal ion in a bidentate fashion through azomethine nitrogen and phenolic oxygen to form complexes. These ligands and complexes also exhibit absorption and luminescence properties.


Introduction
In the last two decades, Ru(II) complexes have been widely studied in the field of inorganic chemistry [1][2][3][4][5].The polypyridine complexes of Ru(II) have been studied extensively with the point of interest being their rich and well characterized photophysics [6,7].In recent years, transition metal complexes have been used extensively in the fore front of fields such as probes of DNA structure, DNAdependent electron transfer, and site-specific DNA cleavage [8][9][10].The area of metal complexes with hydrazones has been investigated intensively during the last years regarding their pharmacological applications such as tuberculostatic, antitumor, antibacterial, and antifungal agents [10][11][12].The thiazole and benzothiazole hydrazone represent a very interesting class of ligands, due to additional donor sites: nitrogen, sulphur, or oxygen atom which introduce a wide range in their coordinative and pharmaceutical properties [10,[13][14][15].

Experimental
2.1.Materials and Methods.All reagents and solvents were purchased commercially and were used as received.2-Mercaptobenzothiazole, hydrazine hydrate, acetophenone, and RuCl 3 ⋅ 3H 2 O were obtained from SD Fine-chem limited (India).The compounds 3a-3e [13], [Ru(bpy) 2 Cl 2 ] ⋅ nH 2 O and [Ru(phen) 2 Cl 2 ] ⋅ nH 2 O, and the metal complexes 4a-4d, 5a, 5d, and 5e [16] were synthesized according to the literature methods.Elemental analysis was carried out on a PerkinElmer 240Q elemental analyzer.The infrared spectra were recorded on a Shimadzu FTIR-8400 spectrometer in KBr pellets. 1 H NMR spectra were recorded on a 500 MHz Bruker FT-NMR spectrometer using CDCl 3 , and all chemical shifts are given relative to TMS.UV-Vis spectra were recorded on a Shimadzu UV-1601 spectrophotometer, and the emission spectra were recorded on a shimadzu RF-5000 luminescence spectrophotometer at room temperature.Thermogravimetric analysis was performed on a computer-controlled TA Instrument 2050 TGA analyzer into alumina pans at a heating rate of 10 ∘ C/min.

Synthesis of Ligands.
The ligands utilized in the present investigation were synthesized by the condensation reaction 2-hydrazinobenzothiazole with substituted acetophenones.2-Hydrazinobenzothiazole was prepared by the addition of hydrazine hydrate (1 mmol) to the warm solution of 2mercaptobenzothiazole (1 mmol) in absolute ethyl alcohol (10 mL), and the mixture was refluxed on a water bath for 4 hours or until H 2 S gas ceased to evolve.The reaction mixture was concentrated and cooled.The white needle-like crystals, separated were filtered, and recrystallized from ethanol.
To a warm ethanolic solution of 2-hydrazinobenzothiazole (1 mmol, in 10 mL) an ethanolic solution of respective acetophenones (1 mmol, in 10 mL) was added, and the mixture was refluxed for 4 hours on a water bath, the precipitate was obtained; it was filtered, washed, and recrystallized from ethanol, as shown in Scheme 1.

Synthesis of the Complexes.
A hot ethanolic solution of the respective ligands 3a-3e (1 mmol, in 10 mL) was mixed with an ethanolic solution of respective hydrated Ru(II) 2,2  bipyridyl chloride or Ru(II) 1,10 phenanthroline chloride (1 mmol in 10 mL), and the mixture was refluxed on a water bath for 4 hours, and then cooled to room temperature.On cooling, a colored solid product was formed.
The solid was filtered, washed with ethanol, then ether, and dried.Crystallization from ethanol gave the desired metal complexes shown in Schemes 2 and 3.

Results and Discussion
The five benzothizolyl ligands 3a-3e were synthesized by utilizing the 2-hydrazinobenzothiazole via condensation reaction with substituted acetophenones.These synthesized ligands 3a-3e were further reacted with hydrated Ru(II) 2,2  bipyridine chloride or Ru(II) 1,10 phenanthroline chloride to obtain polypyridine complexes 4a-4d, 5a, 5d, and 5e.The structures of the synthesized ligands and their complexes were confirmed on the basis of physical, analytical and spectral data mentioned in Tables 1 and 2.

Characterization of the Ligands and Complexes
3.1.1.IR and 1 H NMR Spectra.The IR and 1 H NMR spectral data of the ligands 3a-3e and complexes 4a-4d, 5a, 5d, and 5e are listed in Table 2.The IR spectrum of all the ligands 3a-3e exhibits a broad strong band at 3400-3350 cm −1 , due to the phenolic OH group stretching vibration which disappears in the spectra of the complexes, indicating the coordination of these ligands through the phenolic oxygen [12,13,15].It is also supported by the shift to lower wave numbers of the band from 1467-1450 cm −1 , to 1438-1413 cm −1 due to the stretching vibration of C-O phenolic when compared to the uncomplexed ligands [3,14,15,17].In all the five ligands 3a-3e, a strong band at 1600-1590 cm −1 due to the azomethine group is observed.On complexation, this band was shifted to as lower frequency in the 1564-1404 cm −1 region indicating the coordination of the azomethine nitrogen atom to the metal ion [13,15,18].It is further confirmed by the downward shift of the bands due to ](N-H), ](N-N), ](C-N, cyclic), and ](C-S-C) [14,17,19] and the appearance of new additional bands in  International Journal of Inorganic Chemistry   the regions 1105-1056 cm −1 and 768-736 cm −1 due to phen/ bpy ring -C-H and -C=N stretching vibrations, respectively [20,21].Again, the bands appeared in the range 650 cm −1 were attributed to coordinated water molecule and bands observed due to ]Ru-N and ]Ru-O stretching vibrations in the range of 520-495 cm −1 and 450-418 cm −1 , respectively [17,[22][23][24][25].
In the 1 H NMR spectrum of the ligands 3a-3e, a singlet observed at  12-14 ppm for one proton corresponds to the  Phenolic of the hydrazone ring.The metal complexes of these ligands did not show any proton signal to the phenolic OH range which suggested that the phenolic oxygen participated in the coordination, after complete deprotonation [5,14,26].In addition to this, the singlet observed at  7 ppm for one proton in 3a-3e is assigned to the NH proton of benzothiazolylhydrazone. Two multiplet signals observed around  7-8 ppm for four protons and three protons of ligands 3a, 3e and four protons of ligands 3b, 3c, and 3d were assigned to aromatic protons [5,12,27].
In addition to these, one singlet peak observed at  2.40 ppm for three protons in 3a-3e is assigned to methyl group attached to the hydrazone ring.A singlet peak observed at  2.31 ppm for three protons in 3e is assigned to methyl group attached to the phenyl ring [12,20,27].In the 1 H NMR spectra of the all complexes, the NH protons, aromatic protons, and the protons of methyl group of ligands are shifted downfield due to coordination to the metal ions [5,12,26,27].Two multiplet signals of eight protons observed in the spectrum of the complexes 4a-4d at  7.58 ppm and  8.31 ppm were assigned to bipyridyl protons, and the signals at  7.35-7.58ppm and  7.80 ppm in the spectrum of complexes 5a, 5d, and 5e were assigned to phen protons [20,21,28].Thus, all the protons were found to be in their expected region.The conclusions drawn from these studies further support the mode of bonding discussed in their IR spectra.

UV-Vis Absorption and Emission Spectral Study.
The UV-vis absorption spectra of ligands 3a, 3d, and 3e and metal complexes 4a, 5a, and 5e are shown in Figures 1 and 2, and the spectral data are listed in Table 3.
In the UV-vis absorption spectra of ligands 3a, 3d, and 3e, the absorption at 330, 323, and 323 nm is attributed to - * transitions involving the benzothiazolylhydrazone [10,29].The band observed at 370 nm is assigned to the - * transition of azomethine (C=N) group [30].The other absorption bands of the ligands are due to various - * and  - * transitions [5,14,27,29].In the UV-vis absorption spectra of complexes and the bands at 322, 332, and 323 nm should receive a dominant contribution from transition involving the ligands 3a, 3d, 3e, respectively [6,14,28].In particular, the bands with 290-300 nm range should receive a dominant contribution from - * transition involving bpy and phen moieties [7,[31][32][33].The other weak absorption bands of the complexes in the UV region are expected to include contributions from spin-allowed metal to ligand charge transfer, MLCT [5,27,29].The emission spectra of ligands 3a, 3d, and 3e and metal complex 4a in chloroform at room temperature are shown in Figures 3 and 4, and the relevant data is mentioned in Table 3.The luminescence of the ligands 3a, 3d, and 3e are assigned to the benzothiazolylhydrazone centered - * level [4,[34][35][36].The ligands 3a, 3d, and 3e showed the emission bands at 550 nm, 545 nm, and 535 nm, respectively.The metal complexes 4a, 5a, and 5e showed the emission bands at 540 nm, 620 nm, and 625 nm, respectively.The complexes of Ru(II) are probably the most investigated inorganic luminophores [31,37].The metal complexes studied here exhibit relatively intense luminescence.

Thermogravimetric Analysis Study.
The TG curves of the complexes showed an endothermic peak at 160-200 ∘ C due to the loss of coordinated water [37], which is in agreement with the results obtained from the IR spectra.The endothermic peaks observed at 340 ∘ C correspond to the removal of the anionic chloride [38].All the complexes lose the organic ligand in a large exothermic process, in the range of 400-800 ∘ C [39].On the basis of analytical, thermogravimetrical, IR, 1 H NMR, analysis electronic absorption and emission spectral data, it can be concluded that the ligands 3a-3e act as bidentate NO donor in all the complexes.

Conclusion
We have investigated the synthesis of seven novel polypyridine complexes of Ru(II) with five benzothiazolylhydrazone ligands.The analytical, thermogravimetrical, IR, 1 H NMR, electronic absorption and emission spectral data revealed that the ligands 3a-3e coordinate with the metal ions in a bidentate fashion through azomethine nitrogen and phenolic oxygen to form complexes 4a-4d, 5a, 5d, and 5e.

Table 1 :
Physical and analytical data of ligands and their complexes.

Table 2 :
IR and 1 H NMR spectral data of ligands and their complexes.

Table 3 :
Absorption and emission spectral data of ligands and complexes.