Advances in Physical ChemistryVolume 2011 (2011), Article ID 157484, 11 pagesdoi:10.1155/2011/157484

Research Article

Synthesis, Characterization, and Electrochemical Study of Tetradentate Ruthenium-Schiff Base Complexes: Dioxygen Activation with a Cytochrome P450 Model Using 1- or 2-Methylimidazole as Axial Bases

Ali Ourari,¹ Mostefa Khelafi,¹ Djouhra Aggoun,¹ Gilles Bouet,² and Mustayeen A. Khan²

¹Laboratoire d'Électrochimie, d'Ingénierie Moléculaire et de Catalyse Rédox (LEIMCR), Faculté des Sciences de l'Ingénieur, Université Ferhat Abbas, Sétif 19000, Algeria²SONAS, EA 921, Faculté de Pharmacie, Université d’Angers 16 Boulevard Daviers, 49045 Angers Cedex 01, France

Received 18 September 2010; Revised 21 December 2010; Accepted 22 March 2011

Academic Editor: Milan M. Jaksic

Copyright © 2011 Ali Ourari et al. Ali Ourari 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.

Salicylaldehyde, 2-hydroxyacetophenone, and 3,5-dichlorosalicylaldehyde react with 1,2-diaminoethane to give three symmetrical Schiff bases H₂L¹, H₂L², and H₂L³, respectively. With Ru(III) ions, these ligands lead to three complexes: Ru(III)ClL¹ (), Ru(III)ClL² (), and Ru(III)ClL³ (). The purity of these compounds was estimated by TLC technique and microanalysis while their structures were supported by the usual spectroscopic methods such as NMR, infrared, and electronic spectra. The cyclic voltammetry in acetonitrile showed irreversible waves for all three ligands. Under the same experimental conditions, it was proved that the ruthenium is coordinated in the three complexes , , and showing quasireversible redox systems. The behavior of these complexes and their comparison with cytochrome P450 are investigated using them as catalysts in the presence of molecular oxygen with an apical nitrogen base: 1- or 2-methylimidazole.

1. Introduction

During the last decade, the tetradentate Schiff base complexes of transition metals have been extensively studied mainly with manganese, iron, cobalt, copper, and nickel [1–5] while those of ruthenium are relatively less studied [6]. Thus, it was demonstrated that such materials might be involved in several applications such as in catalysis [7] or electrocatalysis [8] by activating many small molecules like carbon dioxide [9] or molecular oxygen [10]. Here, we focus on the electrochemical study and the activation of molecular oxygen using the tetradentate ruthenium-Schiff base complexes, in order to substantially reduce its reduction potential, leading to electrocatalytic systems operating at lower potential values. So, the metabolic role of cytochrome P450 is well known [11] and its metabolic cycle is established [12]. In some steps, molecular dioxygen fixation, water elimination, and peroxo high oxidation state metal (i.e., FeIV) and so forth, are involved In all cases, the active form of the coordinated iron ion is axially linked to a sulfur protein. Therefore, a large number of cytochrome P450 oxocomplexes, including the nonporphyrinic ligands such as Schiff base complexes, have been tested for their catalytic activity [13–17] and their electrocatalytic properties [8, 17–26]. Thus, new catalysts deriving from symmetrical tetradentate ruthenium-Schiff base complexes seem to be more and more attractive and efficient in catalysis [27, 28]. These new electrocatalytic systems have been less studied than those currently applied in catalysis [6].

Manganese(III), iron(III), cobalt(II), and ruthenium(III)-Schiff base complexes are mainly used in indirect electroreductions of halohydrocarbons [29, 30] or molecular oxygen [31–36] in order to epoxide olefins or to oxidize hydrocarbons. These reactions using electrocatalytic systems could be performed in homogeneous as well as in heterogeneous catalysis. This second case concerns the elaboration of modified electrodes (ME) by covalent grafting of pyrrole or thiophene moieties via the etherification [37] or esterification [38] reactions involving a phenolic group. These materials for electrodes may also be used as amperometric sensors to detect various species: molecular dioxygen [39–42] and biomolecules like glucose [43–45]. Several studies have demonstrated the high efficiency of these electrocatalytic systems when operating in heterogeneous catalysis in a chemical-electrochemical pathway using modified electrodes based mainly upon organic polymer films containing the complex species covalently grafted as catalyst [35, 38, 46].

In this work, three complexes with ruthenium(III) ions are synthesized (Scheme 1) and characterized. Our main aim is to study these complexes as catalysts by cyclic voltammetry in the presence of molecular oxygen and of two axial bases; 1-methylimidazole and its position isomer 2-methylimidazole. Thus, the electrophile character on the metallic center increases when the basicity of the axial base is higher leading to a prompt reaction between oxygen and metal. This electrocatalytic system model of cytochrome P450 is mainly studied in the activation of small molecules among them the molecular oxygen. This study might also be supported by further applications such as epoxidation of olefins or oxidation of hydrocarbons or for other applications.

Figure 1: Voltammograms of complex 3; 10⁻³ M in CH₃CN with TBAP 10⁻¹ M obtained at scan rate 25 mV s⁻¹. (1) Under nitrogen without 1-methylimidazole and then with oxygen; (2) under nitrogen in presence of 1-methylimidazole and then with oxygen.

In this case, the redox systems of all these complexes appear always before those of oxygen but their values are very slightly shifted to the more cathodic potentials by about 33–55 mV. For example, the nearer value is that of the complex 1 which is observed at −0.660 instead of −0.622 V under nitrogen atmosphere. So, under the same experimental conditions and under oxygen atmosphere, the superoxide species are exclusively produced as reported by Sawyer et al. [60]. For this reason, all the complexes 1, 2, and 3 (Table 1:  mV) may be considered as catalysts activating the molecular oxygen like P450 cytochrome [18]. The shape of the voltammograms obtained from these Ru(III)-complexes is represented in Figure 1.

On these voltammograms, it was observed that the half-wave potential values remain stable under nitrogen or oxygen atmosphere noting that the electroreduction of the metal-superoxo species occurred at the same potential of their formation as it was already indicated. When varying the scan rate, the cathodic peak currents () decrease at low sweep rates leading to the lower ratios 1.60 (1), 2.57 (2) and 1.76 (3) at 10 mV s⁻¹ while, at scan rates moderately higher (100 mV s⁻¹), they are 3.06, 3.27, and 3.48, respectively. Considering the reversibility of oxygenation reaction for these species, this behavior may be approached to their corresponding manganese(III)-superoxo species [22–25].

4.2.2. Complexes with 1-Methylimidazole

First, under nitrogen atmosphere, the addition of 10⁻² M of nitrogen base like 1-Me-im., acting as an axial ligand, leads to a well-defined redox system without any significant displacement of values. There is, also, a neat enhancement of the peak currents and after the addition of a nitrogen base as 1-methylimidazole [63]. This nitrogen base coordinated to the central metal ion enhances the coordination number and produces an improvement of the electrophilic character on the metal center, and its further reaction with molecular oxygen becomes easier.

Under oxygen atmosphere, the shape of the voltammograms is quite similar to those obtained without 1-methylimidazole except some differences in their wave intensities exhibiting a relative high reactivity of the ruthenium(III)-complexes towards oxygen molecules as illustrated in Figures 1 and 3. Thus, the wave enhancement (3) was noted when comparing these voltammograms with those recorded in the same experimental conditions without nitrogen base. This enhancement involving the reaction of the Ru(II) species with oxygen (3) was estimated to be 10–13% according to the nature of the complex. This behavior is also accompanied by the diminishing of the wave up to 28–37% suggesting an accelerating of the chemical reaction between Ru(II) species and oxygen (Figure 5, way (a)) when their coordination number is increased by an addition of an amount of 1-methylimidazole (10 equivalents).

The electrocatalytic currents of these three complexes were represented by the ratio / values and are 1.75 (1), 2.42 (2), and 1.90 (3). It should be noted that these ratios were calculated by summing the cathodic current () values obtained in the presence of 1-methylimidazole under oxygen atmosphere. The greater ratios are mainly obtained from an excess of redox properties induced by the σ donor effect of substituents on the metal centre such as those of the two methyl groups, symmetrically grafted on the complex (2) Ru(III)-[7,7′-(Me)₂Salen].

Under these experimental conditions (Figure 2) and Table 3, all the voltammograms of the complexes give almost similar results. Thus, the redox system of the metallic center Ru(II)/Ru(III) was studied and the electrochemical characteristics obtained as peak-to-peak separation potential values, and ratio are consistent with a quasireversible redox couple [61, 62].

(a)

(b)

Figure 2: Curves of complex 3; 10⁻³ M in CH₃CN solutions 10⁻¹ M TBAP. (a) 1-methylimidazole; (b) 2-methylimidazole.

Figure 3: Voltammograms of complex 3; 10⁻³ M in CH₃CN with TBAP 10⁻¹ M obtained at scan rate 25 mV s⁻¹: (1) Under nitrogen and then oxygen; (2) in presence of 1-methylimidazole under oxygen; (3) in presence of 2-methylimidazole under oxygen atmosphere.

4.2.3. Complexes with 2-Methylimidazole

The addition of 10⁻² M of 2-methylimidazole to the Ru(III) complex 10⁻³ M, acting as an axial ligand, induces the same effects as 1-methylimidazole but with a higher speed of the chemical reaction (3) and the disappearance of reduction wave (5) to the benefit of reduction wave (3) as shown in Figure 3.

This expresses the relative high reactivity of the ruthenium(II) complex molecules towards oxygen when 1-methylimidazole is replaced with 2-methylimidazole. This result is observed with all our Ru(III)-complexes. This relative reactivity induced by the use of 1- or 2-methylimidazole as apical bases indicates a higher electrophilic character on the metal center leading to a greater accelerating of the oxygen consumption by the Ru(II) species (2). To our knowledge, these results were not reported in the literature, seeing that these complexes were not studied by cyclic voltammetry in the presence of different nitrogen bases. In this case, when adding 1- or 2-Me-im., the speed of formation of the superoxo or peroxo species (peroxidase form) is significantly increased as expressed by the ratios in Table 4. Finally, it must be noted that these oxygenation reactions of ruthenium complexes are reversible when the solutions are bubbled in argon or nitrogen atmosphere, as reported in the literature [22–25, 63].

Table 4: Electrochemical characteristics of the Ru(III)-complexes 1, 2, and 3 in presence of 1- and 2-methylimidazole under N₂ then under O₂ atmosphere.

Complex
(mV)a
(mV)
(μA)
(μA)

b

Electrochemical characteristics of the Ru(III)-complexes

With 1-methylimidazole

1
580 (622)
70 (70)
6.4 (10.0)
— (1.2)
0.75
1.75

2
520 (565)
75 (75)
5.6 (11.6)
— (2.0)
0.82
2.42

3
500 (507)
65 (65)
8.8 (14.8)
— (2.0)
0.86
1.90

With 2-methylimidazole

1
580 (622)
70 (70)
6.6 (16.6)
— (0.0)
0.80
2.51

2
520 (565)
70 (70)
6.4 (20.9)
— (0.0)
0.82
3.26

3
500 (507)
75 (75)
9.6 (32.5)
— (0.0)
0.88
2.45

; values obtained under nitrogen.; values given between brackets are obtained under oxygen.

These results may be connected with the high relative basicity of 1- and 2-methylimidazole as previously reported in literature [17]. Moreover, we have also determined the number of electrons that are involved in these electrochemical reactions. There are few reports in the literature on these reactions with molecular oxygen, reduced to hydrogen peroxide or water via a two- or four-electron transfer on chemically modified electrodes. The main papers were published by Horwitz et al. [21] and Taqui Khan et al. [47], Anson et al. [64–66], and Chang et al. [67–69]. Here, we have determined the number of electrons for our ruthenium complexes using coulometry after electrolysis under oxygen atmosphere.

4.3. Coulometry Study

For this study, we have proceeded as reported in the literature [47]. Thus, our three complexes were electrolyzed at controlled potentials in a three-compartment cell (acetonitrile solution) in presence of 1-methylimidazole as an apical base. Before each experiment, a pre-electrolysis was achieved for at least one half hour under nitrogen atmosphere in order to eliminate any electroactive impurities. The number of electrons was finally calculated from the following Faraday equation: where is the number of electrons involved in the reaction, and the amount of electricity, , and are, respectively, the mass of the complex and its molar mass. The results are given in Table 5.

Table 5: Number of electrons involved in the electrolyses of Ru(III)-complexes under oxygen atmosphere in presence of 1-methylimidazole.

Determination of number of electrons for waves and

Complex
Number of electrons
Number of electrons

1
1.50
0.44

2
1.60
0.36

3
1.50
0.46

These results suggest that the major part of the compound (75%) was electroreduced according to the first reduction wave and only 25% were electrolyzed following the second reduction wave . When the 2-methylimidazole is used as an apical base, the complex is exclusively electrolyzed according to the first reduction wave with high reactivity of the Ru(II) species towards dioxygen molecules which were subsequently electroreduced by a monoelectronic transfer process (3). At this level, it seems us important to note this particular behavior, characterizing the high reactivity of Ru(III)-Schiff base complexes when they are compared to their analogs such as the iron(III)- [63] and Mn(III)-complexes [22–25] with which the metal-oxo (M=O) species are almost exclusively formed from the electroreduced M(II)-species.

4.4. Complexes with 1- or 2-Methylimidazole and Benzoic Anhydride

Benzoic anhydride reacts with superoxo intermediates, giving an oxo compound with elimination of one oxygen atom according to the following equation [6, 21, 36, 47]:

When benzoic anhydride is added to the acetonitrile solution of a complex and Me-im, we note a significant enhancement of the cathodic peak current . In the mean time, the anodic peak current is clearly diminishing, showing a loss of reversibility of the Ru(III)/Ru(II) redox system. This is due to an important consumption of the two superoxo species (LnRu(III)–O– + 1e⁻ and LnRu(IV)–O– + 2e⁻) according to (7) and (8) where they are transformed to the corresponding metal-oxo species. As soon as these superoxo species are produced, the electrophilic intermediates (i.e., C₆H₅–CO⁺) react faster to produce the metal-oxo species [6, 21, 47]. Of course, an important enhancement of the two cathodic peak currents was observed at each one of the both reduction waves and as previously indicated. These two waves explain the formation of the ruthenium-oxo species, which could be further involved in the epoxidation of olefins or oxidation of hydrocarbons (Figure 4).

(a)

(b)

Figure 4: Voltammograms of complex 2; 10⁻³ M in CH₃CN with TBAP 10⁻¹ M obtained in presence of nitrogen base 10⁻² M and benzoic anhydride 10⁻¹ M under nitrogen and then oxygen atmosphere at scan rate 25 mV s⁻¹. (a) 1-methylimidazole; (b) 2-methylimidazole, (full line: with pure benzoic anhydride; dotted line: with benzoic anhydride containing 2-3% of its acid).

Figure 5: Proposed scheme for the electrocatalytic epoxidation of olefins or oxidation of hydrocarbons.

These ruthenium-oxo species are formed at the same potential values than the corresponding superoxo species (  and  ). At this important step, the ruthenium-oxo species could transfer their oxygen atom to the substrate leading to an efficient electrocatalytic cycle or they are simply electrochemically reduced to recover their initial form of the ruthenium(III)-complex giving an inefficient electrocatalytic cycle as illustrated in Figure 5 [21–25].

The enhancement of the cathodic peak current (O₂) expresses a catalytic activity towards oxygen as given by the ratio. Figure 4 showed clearly that the is not influenced by the traces of benzoic acid as observed for their corresponding Manganese(III)-complexes where the electrocatalytic efficiency was seriously affected [22–25]. The values plotted versus the square root of sweep rates (Figure 6) shows also higher ratios of at lower scan rates, suggesting more time to achieve the chemical reaction with oxygen as illustrated in Figure 6.

Figure 6: Curves for complex 3; (10⁻³ M, acetonitrile). (a) 1-methylimidazole, (b) 2-methylimidazole.

Their shapes are typical of a high electrocatalytic activity towards the reaction with oxygen, to produce the metal-oxo derivatives which might transfer their oxygen atom to a substrate in the oxidation reactions of hydrocarbons. Thus, as it can be observed in Table 6, showing systematically higher values for the ratios, mainly with 2-MeIm. or 1-Me-im. (10⁻² M) in the presence of benzoic anhydride (10⁻¹ M) as activator, expressing the formation of ruthenium-oxo species from the superoxo ones.

Table 6: Electrochemical characteristics giving the electrocatalytic currents .

Complex
(mV s⁻¹)
− (mV/SCE)
(μA)
(μA)

Electrochemical characteristics of the Ru(III)-complexes

With 1-methylimidazole

2
10
680
080.00a
03.50
22.85

100
710
086.00a
12.00
07.16

3
10
600
060.00a
06.00
10.00

100
660
067.00a
16.00
04.20

With 2-methylimidazole

2
10
680
140.00
03.80
36.84

100
710
136.00
12.80
10.62

3
10
600
075.00
06.40
11.72

100
660
073.00
18.70
04.00

.

Under these experimental conditions and with 1-MeIm. or 2-MeIm., the cathodic peak current ratios are remarkably increased particularly at lower scan rates (10 mV s⁻¹). This improvement at lower scan rates (10 mV s⁻¹) implies a sufficient time to achieve the chemical reaction between ruthenium species and oxygen. These results allow to conclude that the best electrocatalytic currents are obtained, as well, at lower scan rates (10 mV s⁻¹) 22.85 (1-MeIm.) and 36.84 (2-MeIm.) for the complex (2). These current ratios are fairly higher without any comparison to those obtained with their analogous of manganese, iron, and cobalt already mentioned. Regarding the complex 3, it shows current ratios relatively lower, but they should be more efficient towards olefins epoxidation for their robustness, acquired by the chlorine atoms grafted on the ligand as yet reported in the literature for the polyhalogenated Schiff base [22–25, 63] or porphyrinic complexes [70, 71]. Thus, the Ru(III) and Ru(II) species may develop an important and particular reactivity towards molecular oxygen as in the diverse systems of P450 cytochrome [72].

5. Conclusion

In this work, we have showed that the Ru(III)-species develop a particular reactivity towards molecular oxygen in comparison to their corresponding analogs species of manganese(III), iron(III), and cobalt(III). These latter complexes exhibit a reactivity quite similar towards oxygen when they are in their divalent form M(II). In this case, it was also revealed that the formation of byproducts such as μ-oxodimers are advantageously avoided with the Ru(III)-complexes. In the presence of benzoic anhydride, the electrocatalytic currents are remarkably exalted particularly when 2-methylimidazole is used and they are seven times greater than their Mn(III)Cl-Salen analogs. Therefore, these Ru(III)-complexes can work as good catalysts in biomimetic oxidation or as oxygen sensors operating with high sensitivity.

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