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

Authors: Shaikh Khaled; Mohammed Zamir Ahmed; Firdous G. Khan; Shaikh Kabeer Ahmed
Journal: International Journal of Inorganic Chemistry
 (2013)

---

## Abstract

A series of five benzothiazolylhydrazone ligands3a–3e and their seven novel Ru(II) polypyridine complexes 4a–4d, 5a, 5d, 5e of the type [Ru(N–N)2(L)]Cl2, 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, 1H 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.


---

## Body

## 1. Introduction

In the last two decades, Ru(II) complexes have been widely studied in the field of inorganic chemistry [1–5]. 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, DNA-dependent electron transfer, and site-specific DNA cleavage [8–10]. 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 [10–12]. 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, 13–15].In the context of this, we synthesized and characterized five ligands 2-(2′-hydroxy-3′-iodo-5′-chloro acetophenyl) benzothiazolylhydrazone3a, 2-(2′,4′-dihydroxy acetophenyl) benzothiazolylhydrazone 3b, 2-(2′,5′-dihydroxy acetophenyl) benzothiazolylhydrazone 3c, 2-(2′-hydroxy-5′-chloro acetophenyl) benzothiazolylhydrazone 3d, 2-(2′-hydroxy-4′-methyl-5′-chloro acetophenyl) benzothialylhydrazone 3e, and their seven Ru(II) polypyridine complexes: [Ru(bpy)23a]Cl2  4a, [Ru(bpy)23b]Cl2  4b, [Ru(bpy)23c]Cl2  4c, [Ru(bpy)23d]Cl2  4d, [Ru(phen)23a]Cl25a, [Ru(phen)23d]Cl2  5d, and [Ru(phen)23e]Cl2  5e.


## 2. Experimental

### 2.1. Materials and Methods

All reagents and solvents were purchased commercially and were used as received. 2-Mercaptobenzothiazole, hydrazine hydrate, acetophenone, and RuCl3 · 3H2O were obtained from SD Fine-chem limited (India). The compounds 3a–3e [13], [Ru(bpy)2Cl2] · nH2O and [Ru(phen)2Cl2] · 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. 1H NMR spectra were recorded on a 500 MHz Bruker FT-NMR spectrometer using CDCl3, 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.


### 2.2. 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 2-mercaptobenzothiazole (1 mmol) in absolute ethyl alcohol (10 mL), and the mixture was refluxed on a water bath for 4 hours or until H2S 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.Scheme 1
Synthetic route of ligands3a–3e.


### 2.3. Synthesis of the Complexes

A hot ethanolic solution of the respective ligands3a–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.Scheme 2
Synthesis of complexes4a–4d.Scheme 3
Synthesis of complexes5a, 5d, and 5e.


## 2.1. Materials and Methods

All reagents and solvents were purchased commercially and were used as received. 2-Mercaptobenzothiazole, hydrazine hydrate, acetophenone, and RuCl3 · 3H2O were obtained from SD Fine-chem limited (India). The compounds 3a–3e [13], [Ru(bpy)2Cl2] · nH2O and [Ru(phen)2Cl2] · 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. 1H NMR spectra were recorded on a 500 MHz Bruker FT-NMR spectrometer using CDCl3, 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.


## 2.2. 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 2-mercaptobenzothiazole (1 mmol) in absolute ethyl alcohol (10 mL), and the mixture was refluxed on a water bath for 4 hours or until H2S 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.Scheme 1
Synthetic route of ligands3a–3e.


## 2.3. Synthesis of the Complexes

A hot ethanolic solution of the respective ligands3a–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.Scheme 2
Synthesis of complexes4a–4d.Scheme 3
Synthesis of complexes5a, 5d, and 5e.


## 3. Results and Discussion

The five benzothizolyl ligands3a–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.Table 1
Physical and analytical data of ligands and their complexes.
CC
Yield (%)
Colour
MP (°C)
Empirical formula
Elemental analysis found (Calcd.) (%)
C
H
N
S
Ru
3a
90
Pale brown
225
C15H11N3OSClI
40.59
2.44
9.43
7.18
(40.63
2.48
9.48
7.22)
3b
90
Yellow
185
C15H13N3O2S
60.16
4.29
14.01
10.67
(60.20
4.34
14.04
10.70)
3c
85
Dirty green
160
C15H13N3O2S
60.15
4.29
13.98
10.68
(60.20
4.34
14.04
10.70)
3d
90
Dirty yellow
220
C15H12N3OSCl
56.72
3.75
13.19
10.05
(56.78
3.78
13.24
10.09)
3e
90
Pale yellow
210
C16H14N3OSCl
57.97
4.19
12.61
9.62
(58.00
4.22
12.65
9.66)
4a
90
Dirty yellow
224
RuC35H26N7OCl3SI
45.29
2.76
10.54
3.41
10.84
(45.35
2.80
10.58
3.45
10.90)
4b
90
Black
270
RuC35H28N7O2Cl2S
53.64
3.54
12.49
4.02
12.86
(53.70
3.58
12.53
4.09
12.91)
4c
85
Brown
210
RuC35H28N7O2Cl2S
53.64
3.53
12.49
4.05
12.87
(53.70
3.58
12.53
4.09
12.91)
4d
90
Dark brown
310
RuC35H27N7OCl3S
52.46
3.32
12.20
3.95
12.58
(52.50
3.37
12.25
4.00
12.62)
5a
85
Brown
225
RuC39H26N7OSCl3I
47.98
2.61
10.01
3.24
10.30
(48.04
2.66
10.06
3.28
10.36)
5d
90
Dark brown
290
RuC39H27N7OSCl3
55.12
3.12
11.50
3.74
11.86
(55.18
3.18
11.55
3.77
11.91)
5e
90
Brown
215
RuC40H29N7OSCl3I
48.49
2.89
9.84
3.18
10.16
(48.53
2.93
9.90
3.23
10.21)
CC: compound code.Table 2
IR and1H NMR spectral data of ligands and their complexes.
CC
IR (KBr, Cm−1), ν
1H NMR (CDCl3, 500 MHz, ppm), δ
3a
3400 (OH), 3195 (N–H), 1596 (C=N), 1590 (C=N, cyclic), 1510 (N–N), 1450 (C–O), 1280 (C–N), 860 (C–S)
2.49 (s, 3H, CH3), 7.00 (s, 1H, NH), 7.21 (m, 2H, ArH), 7.45 (m, 4H, ArH), 13.94 (s, 1H, br, OH)
3b
3352 (OH), 3174 (N–H), 1598 (C=N), 1552 (C=N, cyclic), 1512 (N–N), 1467 (C–O), 1274 (C–N), 863 (C–S)
2.45 (s, 3H, CH3), 7.00 (s, 1H, NH), 7.19 (m, 3H, ArH), 7.37 (s, 4H, ArH), 12.60 (s, 2H, br, two OH)
3c
3350 (OH), 3198 (N–H), 1590 (C=N), 1588 (C=N, cyclic), 1510 (N–N), 1452 (C–O), 1280 (C–N), 860 (C–S)
2.40 (s, 3H, CH3), 6.90 (s, 1H, NH), 7.10 (m, 3H, ArH), 7.21 (m, 4H, ArH), 12.01 (s, 2H, br, two OH)
3d
3400 (OH), 3204 (N–H), 1595 (C=N), 1585 (C=N, cyclic), 1500 (N–N), 1465 (C–O), 1280 (C–N), 860 (C–S)
2.45 (s, 3H, CH3), 7.00 (s, 1H, NH), 7.19 (m, 3H, ArH), 7.37 (s, 4H, ArH), 12.60 (s, 1H, br, OH)
3e
3400 (OH), 3190 (N–H), 1600 (C=N), 1590 (C=N, cyclic), 1510 (N–N), 1456 (C–O), 1280 (C–N), 860 (C–S)
2.41 (d, 6H, two CH3), 6.90 (s, 1H, NH), 7.30 (m, 7H, ArH), 12.30 (s, 1H, br, OH)
4a
3198 (N–H), 1598 (C=N, cyclic), 1404 (C=N), 1502 (N–N), 1430 (C–O), 1282 (C–N), 858 (C–S), 745 (bpy ring), 651 (H2O), 495 (Ru–N), 418 (Ru–O)
2.41 (s, 3H, CH3), 6.81 (s, 1H, NH), 7.05 (m, 3H, ArH), 7.41 (m, 4H, ArH), 7.58 (m, 8H, bpy protons), 7.81 (m, 8H, bpy, protons)
4b
3380 (OH), 3200 (N–H), 1594 (C=N, cyclic), 1450 (C=N), 1504 (N–N), 1438 (C–O), 1280 (C–N), 860 (C–S), 740 (bpy ring), 650 (H2O), 498 (Ru–N), 450 (Ru–O)
2.41 (s, 3H, CH3), 6.93 (s, 1H, NH), 7.10 (m, 3H, ArH), 7.34 (m, 4H, ArH), 7.58 (m, 8H, bpy protons), 7.81 (m, 8H, bpy protons), 13.92 (s, 1H, br, OH)
4c
3363 (OH), 3194 (N–H), 1593 (C=N, cyclic), 1468 (C=N), 1505 (N–N), 1422 (C–O), 1280 (C–N), 860 (C–S), 736 (bpy ring), 650 (H2O), 495 (Ru–N), 418 (Ru–O)
2.39 (s, 3H, CH3), 6.92 (s, 1H, NH), 7.10 (m, 3H, ArH), 7.33 (m, 4H, ArH), 7.58 (m, 8H, bpy protons), 7.81 (m, 8H, bpy protons), 13.94 (s, 1H, br, OH)
4d
3208 (N–H), 1602 (C=N, cyclic), 1466 (C=N), 1508 (N–N), 1415 (C–O), 1285 (C–N), 856 (C–S), 768 (bpy ring), 651 (H2O), 520 (Ru–N), 445 (Ru–O)
2.41 (s, 3H, CH3), 6.93 (s, 1H, NH), 7.21 (m, 3H, ArH), 7.43 (m, 4H, ArH), 7.58 (m, 8H, bpy protons), 7.81 (m, 8H, bpy protons)
5a
3196 (N–H), 1602 (C=N, cyclic), 1465 (C=N), 1502 (N–N), 1413 (C–O), 1282 (C–N), 1056 (phen ring), 650 (H2O), 510 (Ru–N), 438 (Ru–O)
2.43 (s, 3H, CH3), 6.87 (s, 1H, NH) 7.11 (m, 3H, ArH), 7.41 (m, 4H, ArH), 7.58 (m, 8H, phen protons), 7.80 (m, 8H, phen protons)
5d
3205 (N–H), 1602 (C=N, cyclic), 1564 (C=N), 1503 (N–N), 1428 (C–O), 1278 (C–N), 862 (C–S), 1056 (phen ring), 651 (H2O), 515 (Ru–N), 419 (Ru–O)
2.41 (s, 3H, CH3), 6.89 (s, 1H, NH), 7.11 (m, 3H, ArH), 7.41 (m, 4H, ArH), 7.55 (m, 8H, phen protons), 7.80 (m, 8H, phen protons)
5e
3208 (N–H), 1594 (C=N, cyclic), 1465 (C=N), 1500 (N–N), 1423 (C–O), 1289 (C–N), 858 (C–S), 1105 (phen ring), 650 (H2O), 511 (Ru–N), 423 (Ru–O)
2.31 (s, 3H, CH3), 2.46 (s, 3H, CH3), 7.03 (s, 1H, NH), 7.14 (m, 2H, ArH), 7.17 (s, 4H, ArH), 7.58 (m, 8H, phen protons), 7.80 (m, 8H, phen protons), 12.67 (s, 1H, br, OH)
CC: compound code.


### 3.1. Characterization of the Ligands and Complexes

#### 3.1.1. IR and1H NMR Spectra

The IR and1H 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 ligands3a–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 ν(N–H), ν(N–N), ν(C–N, cyclic),  and ν(C–S–C) [14, 17, 19] and the appearance of new additional bands in 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 νRu–N and νRu–O stretching vibrations in the range of 520–495 cm−1 and 450–418 cm−1, respectively [17, 22–25].In the1H NMR spectrum of the ligands 3a–3e, a singlet observed at δ 12–14 ppm for one proton corresponds to the δ 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 δ 7 ppm for one proton in 3a–3e is assigned to the NH proton of benzothiazolylhydrazone. Two multiplet signals observed around δ 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 atδ 2.40 ppm for three protons in 3a–3e is assigned to methyl group attached to the hydrazone ring. A singlet peak observed at δ 2.31 ppm for three protons in 3e is assigned to methyl group attached to the phenyl ring [12, 20, 27]. In the 1H 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 δ 7.58 ppm and δ 8.31 ppm were assigned to bipyridyl protons, and the signals at δ 7.35–7.58 ppm and δ 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.


#### 3.1.2. UV-Vis Absorption and Emission Spectral Study

The UV-vis absorption spectra of ligands3a, 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.Table 3
Absorption and emission spectral data of ligands and complexes.
Compounds
Absorptionλ nm
Emissionλ nm
3a
330, 364, 370, 384
550
3d
307, 323, 331, 361, 389
545
3e
323, 345, 359, 376
535
4a
300, 322, 337, 364, 369, 384
540
5a
297, 314, 332
620
5e
298, 312, 323
625Figure 1
UV-Vis  Absorption spectrum of  Ligands3a, 3d, and 3e.Figure 2
UV-Vis  Absorption spectrum of  complexes4a, 5a, and 5e.In the UV-vis absorption spectra of ligands3a, 3d, and 3e, the absorption at 330, 323, and 323 nm is attributed to π-π* transitions involving the benzothiazolylhydrazone [10, 29]. The band observed at 370 nm is assigned to the π-π* transition of azomethine (C=N) group [30]. The other absorption bands of the ligands are due to various n-π* and π-π* 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 π-π* transition involving bpy and phen moieties [7, 31–33]. 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 π-π* level [4, 34–36]. 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. In particular, emissions of complexes should originate from Ru-bpy and Ru-phen CT state [6, 9, 29, 37]. The metal complexes exhibited the usual MLCT emission [5, 37].Figure 3
Emission spectrum of ligands3a, 3d, and 3e.Figure 4
Emission spectrum of complex4a.


#### 3.1.3. 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, 1H 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.


## 3.1. Characterization of the Ligands and Complexes

### 3.1.1. IR and1H NMR Spectra

The IR and1H 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 ligands3a–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 ν(N–H), ν(N–N), ν(C–N, cyclic),  and ν(C–S–C) [14, 17, 19] and the appearance of new additional bands in 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 νRu–N and νRu–O stretching vibrations in the range of 520–495 cm−1 and 450–418 cm−1, respectively [17, 22–25].In the1H NMR spectrum of the ligands 3a–3e, a singlet observed at δ 12–14 ppm for one proton corresponds to the δ 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 δ 7 ppm for one proton in 3a–3e is assigned to the NH proton of benzothiazolylhydrazone. Two multiplet signals observed around δ 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 atδ 2.40 ppm for three protons in 3a–3e is assigned to methyl group attached to the hydrazone ring. A singlet peak observed at δ 2.31 ppm for three protons in 3e is assigned to methyl group attached to the phenyl ring [12, 20, 27]. In the 1H 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 δ 7.58 ppm and δ 8.31 ppm were assigned to bipyridyl protons, and the signals at δ 7.35–7.58 ppm and δ 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.


### 3.1.2. UV-Vis Absorption and Emission Spectral Study

The UV-vis absorption spectra of ligands3a, 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.Table 3
Absorption and emission spectral data of ligands and complexes.
Compounds
Absorptionλ nm
Emissionλ nm
3a
330, 364, 370, 384
550
3d
307, 323, 331, 361, 389
545
3e
323, 345, 359, 376
535
4a
300, 322, 337, 364, 369, 384
540
5a
297, 314, 332
620
5e
298, 312, 323
625Figure 1
UV-Vis  Absorption spectrum of  Ligands3a, 3d, and 3e.Figure 2
UV-Vis  Absorption spectrum of  complexes4a, 5a, and 5e.In the UV-vis absorption spectra of ligands3a, 3d, and 3e, the absorption at 330, 323, and 323 nm is attributed to π-π* transitions involving the benzothiazolylhydrazone [10, 29]. The band observed at 370 nm is assigned to the π-π* transition of azomethine (C=N) group [30]. The other absorption bands of the ligands are due to various n-π* and π-π* 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 π-π* transition involving bpy and phen moieties [7, 31–33]. 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 π-π* level [4, 34–36]. 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. In particular, emissions of complexes should originate from Ru-bpy and Ru-phen CT state [6, 9, 29, 37]. The metal complexes exhibited the usual MLCT emission [5, 37].Figure 3
Emission spectrum of ligands3a, 3d, and 3e.Figure 4
Emission spectrum of complex4a.


### 3.1.3. 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, 1H 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.


## 3.1.1. IR and1H NMR Spectra

The IR and1H 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 ligands3a–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 ν(N–H), ν(N–N), ν(C–N, cyclic),  and ν(C–S–C) [14, 17, 19] and the appearance of new additional bands in 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 νRu–N and νRu–O stretching vibrations in the range of 520–495 cm−1 and 450–418 cm−1, respectively [17, 22–25].In the1H NMR spectrum of the ligands 3a–3e, a singlet observed at δ 12–14 ppm for one proton corresponds to the δ 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 δ 7 ppm for one proton in 3a–3e is assigned to the NH proton of benzothiazolylhydrazone. Two multiplet signals observed around δ 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 atδ 2.40 ppm for three protons in 3a–3e is assigned to methyl group attached to the hydrazone ring. A singlet peak observed at δ 2.31 ppm for three protons in 3e is assigned to methyl group attached to the phenyl ring [12, 20, 27]. In the 1H 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 δ 7.58 ppm and δ 8.31 ppm were assigned to bipyridyl protons, and the signals at δ 7.35–7.58 ppm and δ 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.


## 3.1.2. UV-Vis Absorption and Emission Spectral Study

The UV-vis absorption spectra of ligands3a, 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.Table 3
Absorption and emission spectral data of ligands and complexes.
Compounds
Absorptionλ nm
Emissionλ nm
3a
330, 364, 370, 384
550
3d
307, 323, 331, 361, 389
545
3e
323, 345, 359, 376
535
4a
300, 322, 337, 364, 369, 384
540
5a
297, 314, 332
620
5e
298, 312, 323
625Figure 1
UV-Vis  Absorption spectrum of  Ligands3a, 3d, and 3e.Figure 2
UV-Vis  Absorption spectrum of  complexes4a, 5a, and 5e.In the UV-vis absorption spectra of ligands3a, 3d, and 3e, the absorption at 330, 323, and 323 nm is attributed to π-π* transitions involving the benzothiazolylhydrazone [10, 29]. The band observed at 370 nm is assigned to the π-π* transition of azomethine (C=N) group [30]. The other absorption bands of the ligands are due to various n-π* and π-π* 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 π-π* transition involving bpy and phen moieties [7, 31–33]. 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 π-π* level [4, 34–36]. 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. In particular, emissions of complexes should originate from Ru-bpy and Ru-phen CT state [6, 9, 29, 37]. The metal complexes exhibited the usual MLCT emission [5, 37].Figure 3
Emission spectrum of ligands3a, 3d, and 3e.Figure 4
Emission spectrum of complex4a.


## 3.1.3. 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, 1H 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.


## 4. Conclusion

We have investigated the synthesis of seven novel polypyridine complexes of Ru(II) with five benzothiazolylhydrazone ligands. The analytical, thermogravimetrical, IR,1H 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.



---

*Source: 212435-2013-10-02.xml*