Aquation reaction of iminodiacetate complex of oxidovanadium(IV) with 2,2’-bipyridine induced by Fe(III) ions: Kinetic studies

Introduction

Aquation is a type of substitution reaction. During the aquation process, the ligand is substituted by water molecules. Recently, interest in the kinetics of the complexation reaction of transition metal compounds has increased.^1–8 For many coordination compounds, the aquation requires the introduction of a special promoter. The substitution reaction of complex compounds with special promoters may occur in acidic (the promoters are H⁺ ions) or basic (the promoters are OH^– ions) media or may be induced with Fe³⁺ ions.^1–8

Polycarboxylic anions are a group of organic compounds which are used in coordination chemistry as ligands,^1–20 because they stabilize metal ions in solutions ¹⁴ and, moreover, they are noncytotoxic. ¹⁵ Furthermore, the polycarboxylate coordination compounds of cobalt(II), nickel(II),^12,16,18 oxidovanadium(IV),^15,17 and copper(II)^19,20 have interesting antioxidant, antibacterial, and antifungal activities.

The kinetics of the aquation reaction of oxydiacetate complex compounds of nickel(II), cobalt(II), and chromium(III) induced by Fe(III) ions have been known in the literature.^6–8 Knowledge of the mechanisms of substitution in coordination compounds is important to understand processes involving the activity of enzymes activated by metal ions or the transport of metal ions across the cell membrane. ²¹

In this work, for the first time, we try to describe the kinetics of the aquation reaction of oxidovanadium(IV) complex. Consequently, the aquation of [VO(ida)(bipy)]·2H₂O (VO(ida)(bipy)) (Figure 1) induced by Fe³⁺ ions has been investigated. The values of the reaction rate constants were calculated for the spontaneous step of the reaction as well as for the step of reaction induced by Fe³⁺ ions. Finally, the mechanism for the aquation of VO(ida)(bipy), induced by Fe(III) ions, has been proposed.

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Figure 1.

2,2’-Bipyridyl(iminodiacetato)oxidovanadium(IV) dihydrate.

Experimental analysis

Results and discussion

The kinetics of the aquation of the VO(ida)(bipy) complex induced by Fe³⁺ ions were studied at five temperatures, within the range of 293.15–313.15 K, in the five promoter concentrations in the range of 2 × 10^–4 to 8 × 10^–4 mol L^–1, whereas the concentration of the coordination compound, that is, VO(ida)(bipy), was constant (1 × 10^–3 mol L^–1). Fe³⁺ acts as a promoter because [Fe(ida)(H₂O)₃]⁺ has the very high stability constant (logβ₁ is 10.72). ²³ The values of the observable reaction rate constants (k_obs) increased with the Fe³⁺ concentration and the temperature (Table 1), which was consistent with the formation of the proposed intermediate complex (Figure 2). The values of the observable rate constants were calculated using the “Glint” program by the global analysis method based on the A → B model reaction, which may be described using equation (1)

V = d C B dt = k · C A ; k = k obs

(1)

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Table 1.

The promoter concentration in the 2 × 10^–4 to 8 × 10^–4 mol L^–1 range of the Fe³⁺ ions dependence of the observable rate constants k_obs (s^–1) of the aquation reaction investigated at five temperatures (together with standard deviations).

Concentration of Fe3+ ions(mol L–1)	Temperature (K)
	293.15	298.15	303.15	308.15	313.15
2 × 10–4	2.68 × 10–4 ±3.76 × 10–7	3.97 × 10–4 ±5.93 × 10–6	4.27 × 10–4 ±9.78 × 10–6	4.86 × 10–4 ±1.35 × 10–5	5.45 × 10–4 ±4.97 × 10–6
4 × 10–4	3.39 × 10–4 ±3.44 × 10–7	4.38 × 10–4 ±3.78 × 10–7	4.79 × 10–4 ±7.80 × 10–7	5.00 × 10–4 ±1.79 × 10–6	6.04 × 10–4 ±2.20 × 10–6
6 × 10–4	4.51 × 10–4 ±4.48 × 10–7	5.13 × 10–4 ±9.33 × 10–7	6.32 × 10–4 ±1.93 × 10–6	6.77 × 10–4 ±2.70 × 10–6	7.20 × 10–4 ±2.62 × 10–6
8 × 10–4	5.06 × 10–4 ±7.35 × 10–7	6.46 × 10–4 ±8.88 × 10–7	7.15 × 10–4 ±1.33 × 10–6	7.33 × 10–4 ±1.01 × 10–6	8.00 × 10–4 ±2.84 × 10–6

The concentration of VO(ida)(bipy) = 1 × 10^–3 mol L^–1, ionic strength I = 1.0 mol L^–1, and pH = 2.5.

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Figure 2.

The proposed mechanism of the substitution reaction of the iminodiacetate anion in VO(ida)(bipy) induced by [Fe(H₂O)₆]³⁺.

where A is the substrate concentration, B is the product concentration, k is the reaction rate constant, k_obs is the observed rate constant, V is the reaction rate, and t is the time period.

The values of the first-order rate constants as a function of the [Fe(H₂O)₆]³⁺ concentration occur as linear dependences at the five temperatures studied. These results are compatible with equation (2):

k obs = ( k + k 1 K { [ Fe ( H 2 O ) 6 ] 3 + } ) ( 1 + K { [ Fe ( H 2 O ) 6 ] 3 + } )

(2)

where k is the rate constant of spontaneous reaction in the absence of Fe³⁺, k₁ is the interchange rate constant for a process in which the intermediate undergoes decomposition to give the products, and K is the precursor formation constant describing a fast pre-equilibrium step between the reactants and the intermediate product (a heterodinuclear complex compound with increased coordination number in comparison to VO(ida)(bipy)). However, in the case of the above equation, a limitation is observed because if 1 >> K{[Fe(H₂O)₆]³⁺}, this equation is simplified to k_obs = k + k₁K{[Fe(H₂O)₆]³⁺}. Analyzing the dependence of the values of k_obs on the Fe³⁺ concentration (Table 1), it can be concluded that for the Fe³⁺ concentration equal to 0 mol L^–1, the values of the observable rate constants are low, but are not equal to zero. This means that the spontaneous aquation reaction occurs simultaneously with that promoted by Fe³⁺ ions. However, this process is too slow to be accurately measurable.

VO(ida)(bipy) in aqueous solutions undergoes the aquation reaction induced by Fe³⁺ ions, resulting in the substitution of the iminodiacetate anion by three water molecules. Based on the analysis of the kinetic relationships, it can be proposed that the investigated reaction proceeds in two parallel paths (Figure 2). The first path is promoter-independent (k) and is likely to proceed according to a dissociative interchange mechanism, where dissociation of the VO(ida)(bipy) is present. The second path consists of two steps. The first one is fast and reversible (K). It depends on the concentration of Fe(III) and results in the formation of a binuclear intermediate complex. In the second, the rate-controlling step, which is the decomposition of the intermediate, takes place (k₁), resulting in the formation of the [VO(bipy)(H₂O)₃]²⁺ and [Fe(ida)(H₂O)₃]⁺ species.

The values of the spontaneous reaction rate constant k and the apparent secondary rate constants k ′ 1 = k 1 × K for the aquation of VO(ida)(bipy) at five temperatures within the range of 293.15–313.15 K are collected and shown in Table 2. It has been concluded that the values of the rate constants are lower for the spontaneous step of the aquation than the values for the step dependent on the promoter concentration.

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Table 2.

The values of the rate constant k for the spontaneous reaction and the apparent secondary rate constant k ′ 1 = k 1 × K (mol L^–1 s^–1) for the aquation of VO(ida)(bipy) at five temperatures.

Temperature (K)	k ′ 1 × 1 0 2 (mol L–1 s–1)	k × 102 (s–1)
293.15	4.15 ± 0.33	1.85 ± 0.02
298.15	4.10 ± 0.80	2.95 ± 0.04
303.15	5.10 ± 0.65	3.10 ± 0.04
308.15	4.50 ± 1.11	3.75 ± 0.06
313.15	4.35 ± 0.46	4.50 ± 0.03

The concentration of VO(ida)(bipy) = 1 × 10^–3 mol L^–1, ionic strength I = 1.0 mol L^–1 (NaNO₃), and pH = 2.5.

The analysis of the results presented in Table 2 shows that the values of k ′ 1 barely changes over the temperature range, implying that the activation energy equals to zero. Furthermore, the values of the activation energy could not be calculated due to the lack of linearity in the dependence resulting from the Arrhenius equation.

Conclusion

The aquation of VO(ida)(bipy) has been monitored spectrophotometrically in the presence of the Fe³⁺ ions as a promoter. It has been found that the values of the observable rate constants k_obs increase proportionally to the concentration of the promoter over the entire range of temperatures studied (293.15–313.15 K), which is consistent with the theory of the active complex. The values of the rate constants are lower for the spontaneous step of the aquation than for the step dependent on the promoter. The dissociation of the intermediate binuclear complex has the greatest impact on the rate of the aquation of VO(ida)(bipy) induced by Fe(III) ions. The mechanism for this reaction has been proposed. The first path is promoter-independent and is likely to proceed according to a dissociative interchange mechanism. The second path consists of two steps. In the first step (the reversible part of the aquation reaction dependent on the Fe³⁺ ions), a dinuclear intermediate product is formed, and in the next step, the products of the reaction are created - [VO(bipy)(H₂O)₃]²⁺ and [Fe(ida)(H₂O)₃]⁺.