Introduction

Metallo-salen complexes have been extensively used as catalysts for a broad range of transition-metal-catalyzed reactions including epoxidation of olefins, hydroxylation, lactide polymerization and asymmetric ring opening of epoxides [1]. The oxidative nature of metallo-salens has also been exploited for the development of novel chemical nucleases [2,3,4,5,6]. Various metallo-salen complexes are also capable of hydrolytic cleavage of DNA and RNA [7]. As DNA-interacting molecules find potential applications in anti-tumor therapy, intense research efforts are currently being invested toward the development of novel DNA/RNA modifiers and understanding their molecular mechanisms of action [8,9,10].

Oxalic acid is the simplest dicarboxylic acid, with the formula HO2CCO2H. It is a much stronger acid than acetic acid and is also a strong reducing agent [11]. Its conjugate base, the oxalate dianion (C2O42−), is a chelating agent for many metals, for example platinum(II) in the drug oxaliplatin [12]. Oxalic acid and oxalates can be oxidized by permanganate in an autocatalytic reaction [13]. Redox reactions involving oxalic acid have been studied [11, 14, 15].

In spite of these and numerous other uses, the redox reaction of the complex, [Fe(Salphen)]+ (salphen = bis(salicylidene)phenylenediamine), with oxalic acid has not yet been reported. The present study has therefore been carried out to obtain kinetic data with a view to gaining insight into the mechanism of this redox reaction.

Materials and methods

All the reagents used were of Analar grade. Reaction rates were monitored by following the decrease in absorbance of the reaction mixture at 435 nm on a CORNING colorimeter 253. Conductivity measurements were taken with a HANNA HI 4321 conductivity meter. Oxalic acid (JHD) was used as the reducing agent, while sodium chloride (M&B) was used to maintain the ionic strength of the reaction medium.

Bis(salicylidene)phenylenediamine and the N,N′-phenylenebis(salicylideneiminato)iron(III) complex [Fe(Salphen)]Cl were synthesized and characterized according to the published procedures [16]. The structure of the complex is shown in Scheme 1. The Schiff base was prepared by refluxing o-phenylenediamine (BDH, 1.71 g, 15.8 mmol) with salicylaldehyde (Merck, 3.3 ml, 31.6 mmol) in methanol (Merck, 30 ml) for 1 h. The precipitate was collected by filtration after cooling, washed with methanol and dried in a desiccator: yield 4.29 g (86%).

Scheme 1
scheme 1

Structure of N,N′-phenylenebis(salicylideneiminato)iron(III)

Full size image

The complex was prepared by stirring a mixture of the Schiff base (0.78 g, 2.5 mmol) with anhydrous ferric chloride (SureChem, 0.4 g, 2.5 mmol) in methanol (20 ml) at 60 °C for 30 min, then keeping the mixture at room temperature overnight to precipitate out the complex. The product was recrystallized from methanol and dried in a desiccator: yield 0.47 g (47%).

The molar conductivity of the complex was determined in 2.0 × 10−4 mol dm−3 (DMSO:H2O; 1:4 v/v) solution as 145 S cm2 mol−1 with specific conductance of 29.0 × 10−6 S cm−1, consistent with a 1:1 electrolyte. The existence of [Fe(salen)]+ species in DMSO–H2O (4:1 v/v) and CH3CN–H2O (1:1 v/v) solvent systems has been reported previously [17, 18]. Furthermore, Kurahashi and co-workers reported that the ESI mass spectrum of H2O coordinated iron(III)salen perchlorate in solution gave a single signal corresponding to [Fe(salen)]+ with loss of both H2O and ClO4− [19].

The stoichiometry of the reaction was determined by spectrometric titration using the mole ratio method. The stoichiometry was evaluated from a plot of absorbance against mole ratio [20]. The kinetic studies were carried out under pseudo-first-order conditions with [H2C2O4] in excess over [Fe(Salphen)+] at 435 nm, I = 0.2 coulomb2 mol dm−3, T = 26 ± 1 °C. Pseudo-first-order rate plots of log (At − A∞) versus time were drawn (where A∞ and At are the absorbance at the end of the reaction and at time t), and from the slopes of the plots, the pseudo-first-order rate constants (k1) were determined. The second-order rate constants (k2) were obtained from Eq. 1.

$$k_{2} = \, k_{1} /\left[ {{\text{H}}_{2} {\text{C}}_{2} {\text{O}}_{4} } \right]$$
(1)

The effect of [H+] on the reaction rate was investigated by varying the [H+] between 1.0 × 10−5 and 1.0 × 10−4 mol dm−3 (using HCl), while [Fe(Salphen)+] and [H2C2O4] were kept constant at 2.0 × 10−4 mol dm−3 and 6.0 × 10−3 mol dm−3, respectively, at 26 ± 1 °C and I = 0.2 coulomb2 mol dm−3 [21]. The effect of varying the ionic strength of the reaction medium on the rate of the reaction was investigated in the range of 0.18–0.40 coulomb2 mol dm−3, while the concentrations of the reactants were kept constant at 26 ± 1 °C. The effects of added cation and anion were investigated for [X] = 1.0–6.0 × 10−3 mol dm−3 ([X] = Mg2+ or AcO−) at constant [Fe(Salphen)+], [H2C2O4] and ionic strength. The influence of temperature on the reaction rates was studied in the range of 298–313 K, and thermodynamic parameters were determined at constant [Fe(Salphen)+], [H2C2O4] and ionic strength.

Spectra of the reaction mixture were recorded after commencement of the reaction and were compared with the spectra of the complex alone over a wavelength range of 400–700 nm. A Michaelis–Menten-type plot of 1/k1 versus 1/[H2C2O4] was also made. A test for free radicals was made by the addition of acrylamide followed by excess methanol to partially reacted mixtures of [Fe(Salphen)]+ and H2C2O4 [22].

Results and discussion

From the stoichiometric studies, the mole ratio of the reaction was found to be 2:3 (Fig. 1) and can therefore be represented by the equation:

Fig. 1
figure 1

Plot of Absorbance versus mole ratio for the redox reaction of [Fe(Salphen)]+ and H2C2O4 at [Fe(Salphen)+] = 2.0 × 10−4 mol dm−3, µ = 0.2 coulomb2 mol dm−3, T = 26 ± 1 °C and λmax = 435 nm

Full size image
$$2\left[ {\text{Fe(Salphen)}} \right]^{ + } + \, 3{\text{H}}_{2} {\text{C}}_{2} {\text{O}}_{4} + {\text{ O}}_{2} \to 2\left[ {\text{Fe(Salphen)}} \right] \, + \, 6{\text{CO}}_{2} + \, 2{\text{H}}_{2} {\text{O }} + \, 2{\text{H}}^{ + }$$
(2)

A stoichiometry of 1:4 has been reported in the reaction between chromic and oxalic acids [11].

In the kinetic analysis, a plot of log(At − A∞) against time t gave a straight line graph, suggesting that the reaction is first order with respect to [Fe(Salphen)+] (Fig. 2). The order of the reaction with respect to [H2C2O4] was determined by plotting log k1 against log [H2C2O4]. The slope of the resulting straight line was 1.04 (Fig. 3). The value of the second-order rate constant k2 was fairly constant for different [H2C2O4] and ionic strengths (Table 1). The rate equation for the reaction can be represented by Eq. 3.

Fig. 2
figure 2

Typical pseudo-first-order plot for the redox reaction of [Fe(Salphen)]+ and H2C2O4 at [Fe(Salphen)+] = 2.0 × 10−4 mol dm−3, H2C2O4 = 6.0 × 10−3 mol dm−3, µ = 0.2 coulomb2 mol dm−3, T = 26 ± 1 °C and λmax = 435 nm

Full size image
Fig. 3
figure 3

Plot of log k1 versus log [H2C2O4] for the redox reaction of [Fe(Salphen)]+ and H2C2O4 at [Fe(Salphen)+] = 2.0 × 10−4 mol dm−3, µ = 0.2 coulomb2 mol dm−3, T = 26 ± 1 °C and λmax = 435 nm

Full size image
Table 1 Pseudo-first-order and second-order rate constants for the reaction of [Fe(Salphen)]+ with H2C2O4 at [Fe(Salphen)+] = 2.0 × 10−4 mol dm−3, T = 26 ± 1 °C and λmax = 435 nm
Full size table
$$- {\text{d}}\left[ {{\text{Fe(Salphen)}}^{ + } } \right]/{\text{d}}t \, = k_{2} \left[ {{\text{Fe(Salphen)}}^{ + } } \right]\left[ {{\text{H}}_{2} {\text{C}}_{2} {\text{O}}_{4} } \right]$$
(3)

where k2 = 7.29 ± 0.157 × 10−1 dm3 mol−1 s−1.

The rate of reaction was found to be independent of [H+], as shown in Table 2. This observation suggests that the undissociated oxalic acid HO2CCO2H is involved in the reaction [11, 14]. The results in Table 1 show that variations in the ionic strength of the reaction medium had no effect on the rate. A plot of log k2 against √I gave a slope of zero, suggesting a negligible Brønsted–Debye salt effect [23]. This implies that the reaction proceeds via an interaction between uncharged forms of the reactants [24].

Table 2 Effect of acid concentration on the rate of reaction of [Fe(Salphen)]+ with H2C2O4 at [Fe(Salphen)+] = 2.0 × 10−4 mol dm−3, H2C2O4 = 6.0 × 10−3 mol dm−3, µ = 0.2 coulomb2 mol dm−3, T = 26 ± 1 °C and λmax = 435 nm
Full size table

The reaction rate was also unaffected by the presence of added ions (Mg2+ and AcO−, Table 3). This result suggests that the reaction follows an inner-sphere mechanism.

Table 3 Effect of added ions on the rate of reaction of [Fe(Salphen)]+ with H2C2O4 at [Fe(Salphen)+] = 2.0 × 10−4 mol dm−3, H2C2O4 = 6.0 × 10−3 mol dm−3, µ = 0.2 coulomb2 mol dm−3, T = 26 ± 1 °C and λmax = 435 nm
Full size table

We next considered whether an intermediate complex is involved in the reaction. There was no shift in λmax (435 nm) when the spectrum of the reaction mixture was compared with that of [Fe(Salphen)+]. The lack of spectrophotometric evidence for the formation of intermediate complex may suggest an outer sphere mechanism. However, a Michaelis–Menten-type plot of 1/k1 versus 1/[H2C2O4] was linear with a positive intercept, suggesting the participation of an intermediate complex (Fig. 4) and a possible inner-sphere mechanism.

Fig. 4
figure 4

Michaelis-Menten plot for the redox reaction of [Fe(Salphen)]+ and H2C2O4 at [Fe(Salphen)+] = 2.0 × 10−4 mol dm−3, µ = 0.2 coulomb2 mol dm−3, T = 26 ± 1 °C and λmax = 435 nm

Full size image

Addition of acrylamide to the partially reacted solution to serve as a radical scavenger in the presence of large excess of methanol did not produce a gelatinous precipitate. This indicates that the involvement of free radicals in the reaction is unlikely.

The results of temperature dependence experiments are presented in Table 4. A large negative value of ∆Sǂ indicates that the species in the activated complex are more ordered, which is evidence for an associative mechanism and an inner-sphere mechanism of electron transfer.

Table 4 Temperature dependence of the rate constants and activation parameters for the reaction of [Fe(Salphen)]+ with H2C2O4 at [Fe(Salphen)+] = 2.0 × 10−4 mol dm−3, H2C2O4 = 6.0 × 10−3 mol dm−3, µ = 0.2 coulomb2 mol dm−3 and λmax = 435 nm
Full size table

We have analyzed the reaction products as follows. On completion of the reaction, the presence of Fe2+ as the reduction product of Fe3+ was confirmed by mixing the reaction solution with KMnO4 solution. The formation of a brown precipitate indicated the presence of Fe2+ [25]. The presence of CO2, the oxidation product of H2C2O4, was confirmed using lime water (Ca(OH)2) which turned milky [26].

On the basis of the results obtained from this investigation, the following reaction scheme is proposed for this reaction:

$$\left[ {\text{Fe(Salphen)}} \right]^{ + } + {\text{ H}}_{2} {\text{C}}_{2} {\text{O}}_{4} \mathop{\longrightarrow}\limits^{{k_{3} }}\left[ {{\text{Fe(Salphen)}}^{ + } //{\text{H}}_{2} {\text{C}}_{2} {\text{O}}_{4} } \right]$$
(4)
$$\left[ {{\text{Fe(Salphen)}}^{ + } //{\text{H}}_{2} {\text{C}}_{2} {\text{O}}_{4} } \right]\mathop{\longrightarrow}\limits^{{k_{4} }}\left[ {\text{Fe(Salphen)}} \right] \, + \, 2{\text{CO}}_{2} + \, 2{\text{H}}^{ + } + {\text{ e}}^{ - }$$
(5)
$$\left[ {\text{Fe(Salphen)}} \right]^{ + } + \, 2{\text{H}}_{2} {\text{C}}_{2} {\text{O}}_{4} + {\text{ O}}_{2} + {\text{ e}}^{ - } \mathop{\longrightarrow}\limits^{{k_{5} }}\left[ {\text{Fe(Salphen)}} \right] \, + \, 4{\text{CO}}_{2} + \, 2{\text{H}}_{2} {\text{O}}$$
(6)

For this mechanism,

$${\text{Rate }} = \, k_{4} \left[ {{\text{Fe(Salphen)}}^{ + } //{\text{H}}_{2} {\text{C}}_{2} {\text{O}}_{4} } \right]$$
(7)

Applying the steady-state approximation for the intermediate complex [Fe(Salphen)+//H2C2O4],

$$k_{3} \left[ {{\text{Fe(Salphen)}}^{ + } } \right]\left[ {{\text{H}}_{2} {\text{C}}_{2} {\text{O}}_{4} } \right] \, - \, k_{ - 3} \left[ {{\text{Fe(Salphen)}}^{ + } //{\text{H}}_{2} {\text{C}}_{2} {\text{O}}_{4} } \right] \, - \, k_{4} \left[ {{\text{Fe(Salphen)}}^{ + } //{\text{H}}_{2} {\text{C}}_{2} {\text{O}}_{4} } \right] \, = \, 0$$
(8)

Then

$$\left[ {{\text{Fe(Salphen)}}^{ + } //{\text{H}}_{2} {\text{C}}_{2} {\text{O}}_{4} } \right] \, = \, k_{3} \left[ {{\text{Fe(Salphen)}}^{ + } } \right]\left[ {{\text{H}}_{2} {\text{C}}_{2} {\text{O}}_{4} } \right]/\left( {k_{ - 3} + \, k_{4} } \right)$$
(9)

Substituting Eq. (9) into Eq. (7),

$${\text{Rate }} = \, k_{3} k_{4} \left[ {{\text{Fe(Salphen)}}^{ + } } \right]\left[ {{\text{H}}_{2} {\text{C}}_{2} {\text{O}}_{4} } \right]/\left( {k_{ - 3} + \, k_{4} } \right)$$
(10)

Equation (10) is analogous to Eq. (3), where k2 = k3k4/(k−3 + k4) = 7.29 ± 0.157 × 10−1 dm3 mol−1 s−1.

Conclusion

The redox reaction between N,N′-phenylenebis-(salicylideneiminato)iron(III) and oxalic acid in mixed aqueous medium (DMSO:H2O; 1:4) showed a stoichiometry of 2:3. The reaction is second order overall. The rate of the reaction is acid independent and displayed zero Brønsted–Debye salt effect. Kinetic investigations showed evidence for the formation of an intermediate complex. Based on these observations, an inner-sphere mechanism is proposed as the most plausible mechanistic pathway for this reaction.