International Journal of Inorganic ChemistryVolume 2013 (2013), Article ID 212435, 7 pageshttp://dx.doi.org/10.1155/2013/212435

Research Article

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

Shaikh Khaled,¹ Mohammed Zamir Ahmed,¹ Firdous G. Khan,² and Shaikh Kabeer Ahmed³

¹Organic Chemistry Research Laboratory, Yeshwant Mahavidyalaya, Nanded, Maharashtra 431602, India²School of Chemical Sciences, Swami Ramanand Teerth Marathwada University, Nanded, Maharashtra 431606, India³P.G. Department of Chemistry, Sir Sayyed College of Arts, Commerce & Science, Aurangabad, Maharashtra 431001, India

Received 30 April 2013; Revised 19 July 2013; Accepted 3 August 2013

Academic Editor: Wolfgang Linert

Copyright © 2013 Shaikh Khaled et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

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)]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.

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) benzothiazolylhydrazone 3a, 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 ligands 3a–3e.

2.3. 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.

Scheme 2: Synthesis of complexes 4a–4d.

Scheme 3: Synthesis of complexes 5a, 5d, and 5e.

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.

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 and ¹H NMR spectral data of ligands and their complexes.

CC
IR (KBr, Cm⁻¹), ν
1H NMR (CDCl₃, 500 MHz, ppm),

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 and ¹H NMR Spectra

The IR and ¹H 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 ν(N–H), (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 the ¹H 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 at δ 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 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.

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

Compounds
Absorption  nm
Emission  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
625

Figure 1: UV-Vis Absorption spectrum of Ligands 3a, 3d, and 3e.

Figure 2: UV-Vis Absorption spectrum of complexes 4a, 5a, and 5e.

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, 31–33]. 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, 34–36]. 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 ligands 3a, 3d, and 3e.

Figure 4: Emission spectrum of complex 4a.

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.

Acknowledgments

The authors are thankful to the Principal of Yeshwant Mahavidyalaya, Nanded, for providing necessary facilities for the research work. One of the authors, Shaikh Khaled, is also thankful to NCL, Pune, for providing spectral analysis data.

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