Synthesis, Characterization, Spectral Properties and Electrochemical of Compounds trans-[Ru(NH)₃L(bpa)]²⁺

Wagner Batista dos Santos^1*, Marcio Adriano Sousa Chagas¹, K.M.D. de Sousa¹, Daniel Tizo Costa², Luiz Alfredo Pavanin³

¹Institute of Exact and Earth Sciences, Federal University of Mato Grosso, Cuiabá, Brazil.
²FESURV, University of Rio Verde, Rio Verde, Brazil.
³Chemical Institute, Federal University of Uberlandia, Uberlandia, Brazil.

DOI: 10.4236/ojic.2016.62009                

Abstract

n this work, we present synthesis of the compounds trans-[Ru(NH₃)₄L(bpa)]²⁺ where L is pyridine ligands: pyridine (py), isonicotinamide (isn), 4-acetylpyridine (4-acpy) and 4-picoline (4-pic) and 1,2-bis (4-pyridyl) ethane (bpa), their characterization by UV-visible spectroscopy and electrochemical properties. This series shows intense bands in the region between 400 and 515 nm, allocated bands charge transfer (MLCT), the influence of substituents on the pyridine ring (4-acpy and isn), and the interaction between the ligand and the metal, causing a second MLCT band, which is lighter and has more energy. The compound is characterized by spectroscopy by Fourier transform infrared spectroscopy (FTIR). The displacement observed in the symmetrical stretching of ν_s(CCN) group in the complex compared with the ν_s(CCN) group in the free ligand is indicative of coordination of the pyridine group to the Ruthenium (II) metallic center. The electrochemical data (cyclic voltammetry) show that reversibility criteria are well defined and formal E_f potential, indicating the influence of the pyridine ring substituent.

Keywords

Ruthenium, Pyridine Ligands, MLCT Bands

Received 23 December 2016; accepted 26 April 2016; published 29 April 2016

1. Introduction

In recent years, work with the amines ruthenium (II) and (III) has been further developed in different areas, such as anti-tumor treatment [1] - [3] , antibacterial agents [4] , and photochemical properties [5] , among others. Trans- tetra ammine ruthenium (II) shows intense band charge transfer (MLCT) and an intimate dependency on the position of the band with the ligand [6] - [11] . In previous studies with complexes obtained with ruthenium coordinated to heterocyclic nitrogenous ligands [6] - [9] , an effective capacity load back donation between the metal and the binder is shown, opening up the possibility of using of these systems in several processes induced by light [12] - [15] . However for use of these properties induced by lighting the knowledge of the electrochemical properties of the compounds especially redoxes are necessary. Studies conducted by Lim et al. [16] and Matsubara et al. [17] with amines of ruthenium have shown that the change of a molecule of ammonia in [Ru(NH₃)₆]^+3/+2 or water in [Ru(NH₃)₅H₂O]^+3/+2, by an unsaturated ligand L, such as pyridines caused an increase in the value of the formal potential (E_f), for the compound. This increase is attributed to the p and s interactions of ligand L with metal; thus, for ruthenium, unsaturated ligands stabilize the lowest oxidation state. Chatt et al. [18] observe the effect of the ligands on the metal atom, similar to that observed for aromatic compounds using cyclic voltammetry. These studies show that the higher the reception capability of electrons p is allied to the smaller capacity s of ligand L, the greater the value of the formal charge E_f will be. This work presents the synthesis, and spectroscopic and electrochemical properties, highlighting the influence of the substituent on the pyridine ring binder, in the charge transfer bands, and in the formal potential and reversibility criteria presented by the synthesized compounds.

2. Experimental

Synthesis: The synthesis of the isomers trans-[Ru(NH₃)₄L L’]²⁺ where L ≠ L’ (L’ = bpa; L = 4-acpy, py, isn and 4-pic) was adapted from previously reports [6] - [8] [19] .

2.1. Synthesis of trans-[Ru(NH₃)₄SO₄L’]²⁺

Here, 600 mg of trans-[Ru(NH₃)₄SO₂Cl]Cl (synthesized as previously described in the literature [6] - [8] [19] ) was suspended in 11 ml of water, followed by the slow addition of 347 mg of sodium bicarbonate. Then, the ligand (L = 4-acpy, py, ISN, 4-pic) was added in excess. The final solution was filtered and about 5 mL of concentrated HCl was added to the filtrate. The mixture was placed in an ice bath to precipitate and the solid obtained was filtered, rinsed with ethanol and ether and air dried (brown solid in all ligands). The brown solid obtained was dissolved in approximately 50 ml of a solution of 0.1 mol・L⁻¹ HCl, followed by the addition of a few drops of H₂O₂, until the color changed (pale yellow to brown). Then, 500 ml of acetone was added, resulting in the precipitation of a light yellow solid. After cooling, the solid was collected by filtration, washed with ethanol and ether and air dried. The medium yield was 60%.

2.2. Synthesis of trans-[Ru(NH₃)₄LL’]²⁺

Here, 180 mg trans-[Ru(NH₃)₄SO₄L]Cl (L = isn, 4-pic, 4-acpy and py) was dissolved in 2.0 ml of previously distilled and deaerated water. Argon was bubbled for 15 minutes in this yellow solution and then zinc amalgam was added. This solution remained under an inert atmosphere for about 20 minutes. During the reaction, the solution turned to an orange-red color. In parallel, 440 mg of the ligand 1,2-bis (4-pyridyl) ethane (BPA) was dissolved in the lowest portion of distilled and deaerated water possible, by addition to the initial solution. The resulting mixture was left under constant argon bubbling for 2 hours, protected from light. After this period, the resulting solution was filtered over a freshly prepared saturated solution of NaBF₄ and filtered on a 1:1 basis (NaBF₄ 1 g/1 ml of water), and then taken to a refrigerator to crystallize. The resulting mixture was filtered and the crystals washed with ethanol, ether and dried in a vacuum. The medium yield was 50%.

2.3. Elemental Analysis

Elemental analysis of carbon, nitrogen and hydrogen was performed at the Chemistry Institute of the University of São Paulo. Trans-[Ru(NH₃)₄(py)(bpa)](BF₄)₂・NaBF₄・5H₂O: calcd.: C, 25.29; N, 12.15; H, 4.83. found.: C, 24.96; N, 13.21; H, 4.17. Trans-[Ru(NH₃)₄(isn)(bpa)](BF₄)₂・NaBF₄・3H₂O: calcd.: C, 26.56; N, 13.77; H, 4.42. found.: C, 26.73; N, 13.40; H, 4.14. Trans-[Ru(NH₃)₄(4-pic)(bpa)](BF₄)₂・2NaBF₄. H₂O: calcd.: C, 25.20; N, 11.43; H, 3.88. found.: C, 25.12; N, 12.00; H, 4.12. Trans-[Ru(NH₃)₄(4-acpy)(bpa)](BF₄)₂・NaBF₄.: calcd.: C, 30.11; N, 12.94; H, 4.12. found.: C, 30.80; N, 13.92; H, 4.50.

3. Results and Discussion

3.1. Absorption Spectra (UV-Vis)

The spectroscopic monitoring for the compounds obtained is shown in Figures 1-4. We observed the formation of charge-transfer bands (MLCT-1) as well as the appearance of a second charge transfer band for the compounds trans-[Ru(NH₃)₄(isn)(bpa)]²⁺ and trans-[Ru(NH₃)₄(4-acpy)(bpa)]²⁺. Due to the structural similarity of py and 4-pic binders, it is possible to observe the initial existence of 4 well-defined peaks, where the first band observed between 200 and 300 nm is attributed to internal transitions of the ligand (IL) π − π* in analogy to other ruthenium amines [6] [19] [20] . The energies and intensity observed are similar to those of the free ligands, but shifted to higher energies. With the development of the reaction (Figure 1), the decrease in intensity of this band is observed (IL) indicating the greater interaction of ligands with the metal [6] [19] [20] . It can be observed that the bands at 319 nm, 424 nm and 520 nm are characteristic of the compound trans-[Ru(NH₃)₄SO₂L’]²⁺, where L = py

Figure 1. Qualitative spectrum monitoring the synthesis of the compound trans-[Ru (NH₃)₄(py)(bpa)]⁺².

Figure 2. Qualitative spectrum monitoring the synthesis of compound trans-[Ru (NH₃)₄(4-pic)(bpa)]⁺².

Figure 3. Qualitative spectrum monitoring the synthesis of compound trans-[Ru(NH₃) ₄(isn)(bpa)^]+2.

Figure 4. Qualitative spectrum monitoring the synthesis of compound trans-[Ru (NH₃)₄(4-acpy)(bpa)]⁺².

converged to only one band at 422 nm with the addition of bpa ligand; this may be attributed to the replacement process of bpa to the SO₂ ligand. Similar remarks can be made for the compound obtained for ligand L’ = 4-pic (Figure 2). The final intense bands observed at 422 (L = py) and 424 (L = 4-pic), where e is in the order of 10⁴ mol⁻¹・cm⁻¹ ml in the visible region, have been attributed to electronic transitions between molecular orbital centered on metal (orbital t_2g) and a molecular orbital centered ligand (p^* of ligand) [19] [21] (Figure 5(a)). Already for the compounds trans-[Ru(NH₃)₄(isn)(bpa)]²⁺ and trans-[Ru(NH₃)₄(4-acpy)(bpa)]²⁺, bands were obtained at 275, 325 and 400 nm (isn) and 285 and 340 nm (4-acpy). The spectral behavior reveals that for compound trans-[Ru(NH₃)₄(isn)(bpa)]²⁺, a displacement of the bands observed at 325 and 400 nm occurs as a result of substitution of the SO₂ by bpa ligand with maintenance of the band (IL) in the region of 275 nm, which is characteristic of the ligand (isn) (Figure 3); also, there is displacement of the bands 325 nm to 366 nm and 400 nm to 474 nm, which is attributed to electronic transitions between molecular orbitals centered on metal (orbital t_2g) and molecular orbital centered ligands (p^* ligand). The spectral behavior of the compound trans- [Ru(NH₃)₄ (4-acpy)(bpa)]²⁺ (Figure 4) exhibits a shift of the band at 285 nm (IL) to 275 nm and 340 nm to 382 nm, resulting from replacement of the ligand SO₂ by bpa. It also shows the appearance of an intense band at 510 nm attributed to electronic transitions between the molecular orbital centered on the metal (t_2g orbital) and the ligand (p^* ligand). The literature shows [6] [9] [19] that the trans-tetra ammine of ruthenium (II) may present more than one absorption band in the visible region when L ≠ L’, and second band MLCT is weak [20] . Comparing the ligands involved in this series of compounds, it can be observed that the pyridinic ligands, (isn and/or 4-acpy) show the pyridine ring substituent groups that change the charge density on the metal; this change is evidenced by the presence of MLCT bands with low energy (366 and 382 nm) (Figure 5(b)).

Table 1 summarizes the spectroscopic properties of the complex trans-[Ru(NH₃)₄L(L’)]⁺² reported here.

3.2. Absorption Spectra (IR)

Electronic spectroscopy in the medium infrared region: The absorption spectrum in the infrared region was used to characterize the ligands coordinated to the metal according to the energy of their vibrations Figure 6. Table 2 shows the values identified are highlighted to vibrational groups and their respective wave numbers of complex trans-[Ru(NH₃)(bpa)(L)]⁺² where L = py, isn 4-acpy and 4-pic in comparison to the experimental vibrational spectra of free ligands. The displacement observed in the symmetric stretch of the ν_s(CCN) group of the complex compared to ν_s(CCN) group in the free ligand is indicative of coordination of the pyridine group to the Ru²⁺ metallic centre [22] . It was also noted that there is no significant displacements at asymmetric and symmetric stretching mode of the carbonyl group of free ligand (4-acpy and isn) compared to complex. Meanwhile, the rise of a second stretching is observed in the complexes assigned to ν_s(C = C) group of bpa ligand. It was not observed asymmetric and symmetric stretching displacement of ν_s(C = C) group of the pyridine ring between free ligand and the complex for the obtained compounds with pyridine (py), and 4-picoline (4-pic) which shows that there was no interaction at this site. The intense band observed around 1070 cm⁻¹, and attributed to the asymmetric stretching of (bpa) group, present in all of the compounds as well as the band seen in 3400 cm⁻¹ range.

3.3. Cyclic Voltammetry

From the cyclic voltammograms obtained, the parameters are: i_pa, i_pc, E_pa, E_pc, E_1/2a, and E_1/2c; these are listed in

(a)(b)

Figure 5. Quantitative electronic spectra of the compounds trans-[Ru(NH₃)₄L(bpa)]⁺² where (a) L = py and 4-pic and (b) L = isn and 4-acpy, in water concentration of 4.84 × 10⁻⁵ mol・L⁻¹.

Table 1. spectroscopic properties of the compounds trans-[Ru(NH₃)₄L(bpa)]⁺² and analogues trans-[Ru(NH₃)₄LL’]⁺² and [Ru(NH₃)₅L]⁺² in aqueous solution.

^*This work.

(a)(b)(c)(d)(e)

Figure 6. Spectrum in the infrared region for the complex trans-[Ru(NH₃)₄(L)(bpa)]^{+ 2} in KBr.

Table 3. The results obtained for the potential has an uncertainty of the order of 10 mV, and for all compounds studied here, generally fit the criteria of reversibility [25] - [29] . The potential anodic and cathodic peaks for compounds are within the experimental error and are independent of the potential sweep speed. Comparing the spectral data with voltammetry, as shown in Table 4, there is a decrease in the formal potential (E_f), similar to the decreased energy observed in the electronic spectra of the monomers. This decrease can be attributed to an increase in the electron captor capacity of the substituent groups on the aromatic rings of ligands that occurs in

Table 2. FTIR experimental for free ligand and complex trans-[Ru(NH₃)(bpa)(L)]⁺².

^arefer [23] ; ^bfree gas-phase [24] .

Table 3. Electrochemical parameters and observed relationships for compounds trans-[Ru(NH₃)₄(L)(bpa)]²⁺ to 0.5 mmol/L NH₄PF₆, T = 25˚C and potential (±10 mV).

Table 4. Comparison of the formal potential and band metal ligand charge transfer complexes of ruthenium trans- tetrammines with “donor number” in various solvents.

solvents (Donor Number), leading to an increase in the formal potential. Already in dimethylsulfoxide solvent, systems have shown evidence of electrochemical reactions, which can be attributed to the basicity of the solvent employed (high Gutmann donor number [30] ), making it possible to replace the pyridinic ligands.

4. Conclusion

The spectroscopic characterizations using UV-vis characterization reveal the presence of band charge transfer (MLCT) with e values in the order of 10⁴ mol⁻¹ L・cm⁻¹, which is a very important characteristic for cases in which light is a fundamental role. The infrared analysis (Table 2) shows that the CCN symmetric stretching of pyridine ring is shifted to lower frequency band at 1274 cm⁻¹, 1276 cm⁻¹, 1276 cm⁻¹ and 1348 cm⁻¹ due to the coordination of the nitrogen with the metal. The analysis carried out by cyclic voltammetry reveals the system’s reversibility, accrediting the compounds studied here to the dyes used in photoregenerative cells.

Acknowledgements

The authors thank the Brazilian agencies CAPES and FAPEMAT, for the material and financial support.

NOTES

^*Corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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