Kinetics and mechanism of incorporation of zinc(II) into tetrakis(1-methylpyridium-4-yl)porphyrin in aqueous solution

2.1 Reagents and materials

5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin, [H₂TMPyP]⁴⁺, was purchased as a tosylate from Dojindo Chemical Institute, Kumamoto, Japan. The porphyrin solution was prepared by dissolving 68.18 mg of [H₂TMPyP]⁴⁺ tetratosylate in 50 mL of distilled water. The standardization of the porphyrin solution was done by spectrophotometric titration (molar ratio method) with a standard copper(II) solution (Makino and Itoh, 1981; Nyarko et al., 2004b; Habib et al., 2004). Zinc solutions were prepared by dissolving ZnCl₂ in aqueous solution and their concentrations were determined by atomic absorption spectrophotometry (Perkin Elmer, AAanalyst 200). Sodium chloride, sodium nitrate, sodium hydroxide and hydrochloric acid were purchased from Merck, Germany, used as obtained without further purification. Tetracation zinc(II) porphyrin, [ZnTMPyP]⁴⁺, was prepared and absorption spectra were recorded in water at pH 10.30 containing 0.1 M NaNO₃. Absorption maximum (λ_max) and molar extinction coefficient (ε) of the prepared [ZnTMPyP]⁴⁺ complex were 437 nm and 181 × 10³ M⁻¹cm⁻¹ respectively (Anula et al., 2006). Buffer solutions were prepared with acetic acid and sodium acetate. Distilled water was used throughout the experiment.

2.2 Speciation of Zn²⁺ complexes

A series of Zn²⁺_(aq) solutions of 5.00 × 10⁻³ M at various pH ranging from 2.28 to 11.30 in 50 mL volumetric flask were prepared separately. The ionic strength was adjusted to 0.10 M by adding NaNO₃. Either HCl or NaOH was used to adjust the solution pH in the acetate buffer ([Acetate] = 0.02 M). A double beam UV–vis spectrophotometer (SHIMADZU, Model UV-1800) was used to record the spectra of Zn²⁺ species in the wavelength range from 250 to 400 nm. A series of spectrophotometric titrations was carried out within a range of acetate buffer concentrations from 0 to 1.00 × 10⁻² M with Zn²⁺ ion under the same experimental conditions. Results show that the acetate buffer has negligible complexing abilities with the Zn²⁺ species. The pH of the solutions was measured by using a pH meter (HANNA HI 2211).

2.3 Kinetics of Zn²⁺ incorporation into [H₂TMPyP]⁴⁺

The kinetic runs were carried out under pseudo-first order condition within a pH range from 4.07 to 11.55 at a constant ionic strength, I = 0.10 M (NaNO₃). The concentrations of H₂TMPyP⁴⁺ was 1.24 × 10⁻⁵ M and Zn²⁺ was varied within a range from 0.25 × 10⁻³ to 5.00 × 10⁻³ M. The metalloporphyrin was prepared in a 1-cm cell compartment, maintained at 25 ± 0.1 °C, by mixing the porphyrin solution with the Zn²⁺ solution. These solutions were pre-equilibrated at the reaction temperature. The change in the absorbance was recorded with time at 422 nm (λ_max of [H₂TMPyP]⁴⁺) by using a double-beam UV–Vis spectrophotometer (SHIMADZU, Model UV-1800). Two isosbestic points, 430 and 480 nm, were found in the visible region as the porphyrin reacted with Zn²⁺ to form the [ZnTMPyP]⁴⁺ complex I (Scheme 1). The spectral pattern of the formation of [ZnTMPyP]⁴⁺ complex with time is shown in Fig. 1. The existence of the isosbestic points indicates that the free base porphyrin and the metalloporphyrin are the only absorbing species present. Plots of ln(A_t-A_∞) vs time were linear over two to three half-lives and the slope of such plots yielded the observed rate constants (k_obs). Rate constants for the reactions were determined at different zinc concentrations, pH values and various ionic strengths. The duplicate runs under the same experimental conditions agreed within 5% error. The addition of acetate did not affect the spectra of the porphyrin. The pH of the solutions was measured by using a pH meter (Hanna HI 2211).

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Fig. 1

Spectral pattern of formation of Zn(II)TMPyP⁴⁺complex at 25 ± 1 °C. I = 0.1 M (NaNO₃). [Zn²⁺] = 1.00 × 10⁻³ M; [H₂TMPyP⁴⁺] = 1.24 × 10⁻⁵ M. Absorbance of [H₂TMPyP]⁴⁺ (λ_max = 422 nm) decreases with time. The formation time are as follows: (1): 0; (2): 5; (3) 10; (4): 15; (5): 30; (6): 50; (7): 70; (8): 100 and (9): 140 min respectively.

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3.1 Speciation of Zn²⁺

In aqueous solution, Zn²⁺ ion predominantly exists as an octahedral hexaaqua complex ion, [Zn(H₂O)₆]²⁺, particularly in acidic pH (Burgess, 1978). However, aqueous solutions of zinc salts are mildly acidic because the aqua-ions take part in hydrolysis reaction with a pK_a value about 9, thereby in formation of [Zn(H₂O)₅(OH)]⁺ and H⁺ ions (Baes and Mesmer, 1976). Speciation diagram provides the presence of a specific reactive species at a particular solution pH that is required to establish a reaction mechanism through studying the kinetics of a particular reaction. A speciation diagram of Zn²⁺-species with different solution pH has been constructed by Gallios and Vaclavikova (2008) using a visual MINTEQ program (Choi et al., 2013). The speciation diagram is shown in Fig. 2. As seen from the Fig. 2, aqua Zn²⁺ (Zn²⁺_(aq)) species is predominantly present within a pH range from 4 to 8.5, however, aqua-monohydroxo Zn²⁺, [Zn(OH)⁺_(aq)], species exists with its maximum level at pH 8.5. The zinc-aqua-dihydroxo, [Zn(OH)_2(aq)], species is mostly distributed within a pH range from 9.0 to 11.0 and its maximum level is observed at pH ∼ 10.0. With further increase the solution pH, zinc-trihydroxo, [Zn(OH)₃⁻] and zinc-tetrahydroxo, [Zn(OH)₄²⁻], species are formed and predominantly present at pH > 11.5 (Fig. 2).

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Fig. 2

Speciation diagram of Zn²⁺ species at different solution pH. The diagram is reproduced under permission from the Royal Society of Chemistry (Choi et al., 2013).

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The UV–Vis absorption spectra of zinc(II) chloride (1.0 × 10⁻³ M) with different solution pH are shown in the supplementary Fig. S-1. As seen from the supplementary Fig. S-1, the hexaaqua octahedral Zn²⁺, [Zn(H₂O)₆]²⁺, ion exhibits a UV–Vis spectrum centered at λ_max = 302 nm at pH 2.28. With increasing the solution pH from 2.28 to 10.30, a slight hypochromicity without any red/blue shift is observed (supplementary Fig. S-1). This result suggests that the water molecules are strongly bound to the Zn²⁺ ion even at higher solution pH, 12.0 or 13.0 and successively displaced by the hydroxo (OH^–) ligand as a function of solution pH. Therefore, it may conclude that the hexaaqua Zn²⁺ species, [Zn(H₂O)₆]²⁺, co-exists with [Zn(H₂O)_6-n(OH)_n]²⁻ⁿ [n = 1, ….,6] in a pH range from 2 to 14 according to the following equations ([Zn(H₂O)₆]²⁺ is written as Zn²⁺_(aq) for simplicity) (Choi et al., 2013):

(2)

(3)

(4)

3.2 Kinetics of formation of [ZnTMPyP]⁴⁺ complex

The kinetics of the formation of [ZnTMPyP]⁴⁺ complex in aqueous medium at 25 ± 1 °C in I = 0.10 M NaNO₃ was studied spectrophotometrically by monitoring the decrease of absorbance of the free base porphyrin, [H₂TMPyP]⁴⁺, at λ_max = 422 nm with time. Fig. 1 shows the spectral pattern of formation of the [ZnTMPyP]⁴⁺complex with time. The observed rate constants (k_obs) for the reactions between [H₂TMPyP]⁴⁺ and Zn²⁺ ion at various solution pH were determined by plotting ln(A_t-A_∞) vs time.

The rate of formation of the [ZnTMPyP]⁴⁺ complex is first order in porphyrin that followed by the equation:

In order to investigate the reactivity of different Zn²⁺-species under the present experimental conditions, the observed rate constants (k_obs) were measured at various solution pH. The rate constants (k_obs) are plotted as a function of the solution pH as shown in Fig. 3. Results show that the observed rate constant increases with increasing the solution pH and goes to its maximum value at pH = 10.30 and then slows down with further increase the pH. As seen from the Fig. 3, the increasing trend of the rate constant is very slow until the solution pH is reached at 7.44 and then increases sharply with pH. From this observation, it is concluded that the reacting species of the Zn²⁺ ion is hexaaqua Zn²⁺, [Zn(H₂O)₆]²⁺ written as Zn²⁺_(aq), at low pH which is one of the less reactive among the [Zn(H₂O)_6-n(OH)_n]²⁻ⁿ [n = 1, ….,6] species. At pH ∼ 8.0, the main Zn²⁺ species is Zn²⁺_(aq) and changes to aqua-monohydroxo, [Zn(H₂O)₅(OH)⁺], aqua-dihydroxo, [Zn(H₂O)₄(OH)₂], aqua-trihydroxo, [Zn(H₂O)₃(OH)₃⁻], and aqua-tetrahydroxo, [Zn(H₂O)₂(OH)₄²⁻], species with increasing the solution pH as mentioned in equations 1–4 (Gallios and Vaclavikova, 2008; Choi et al., 2013).

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Fig. 3

Dependence of the observed rate constant, k_obs, as a function of solution pH ranging from 4.07 to 11.55 in I = 0.10 M (NaNO₃) at 25 ± 1 °C. [Zn²⁺] = 2.50 × 10⁻³ M; [H₂TMPyP⁴⁺] = 1.24 × 10⁻⁵ M.

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As mentioned above, Zn²⁺ ion predominantly exists as its hexaaqua, [Zn(H₂O)₆]²⁺, species within a pH range from 4.0 to 7.44 (Fig. 2), and showed the lowest reactivity in incorporation into the free base porphyrin, [H₂TMPyP]⁴⁺ (Fig. 3). However, the reactivity of the Zn²⁺ ion towards [H₂TMPyP]⁴⁺ increases slightly as the presence of aqua-monohydroxo Zn²⁺, Zn(OH)⁺_(aq), species increases (Fig. 3). According to the speciation diagram, the Zn(OH)⁺_(aq) species is distributed with the aqua-dihydroxo Zn²⁺, [Zn(H₂O)₄(OH)₂] simplified as Zn(OH)_2(aq), at solution pH 8.5 (Fig. 2). It has been reported that the reactivity of the Zn(OH)⁺_(aq) species supposed to be higher than that of Zn(OH)_2(aq) species (Thompson and Krishnamurthy, 1979; Sutter and Hambright, 1993), however, the overall distribution of the aqua-monohydroxo Zn²⁺, Zn(OH)⁺_(aq), species in the studied pH range is significantly low (Fig. 2). The low distribution of the Zn(OH)⁺_(aq) species at pH 8.5 is responsible for the poorer reactivity compared to that of Zn(OH)_2(aq) species towards the H₂TMPyP⁴⁺ (Fig. 3). On the other hand, distribution of the aqua-dihydroxo Zn²⁺, Zn(OH)_2(aq), species is extremely high within a pH range from 9.0 to 11.0 and its maximum value is observed at pH ∼ 10.30 (Fig. 2). Accordingly, the Zn(OH)_2(aq) species exhibits the highest reactivity among the [Zn(H₂O)_6-n(OH)_n]ⁿ⁻² species, towards H₂TMPyP⁴⁺ at solution pH ∼ 10.30 (Fig. 3). With further increase the solution pH, the trihydroxo Zn²⁺, Zn(OH)₃⁻, species becomes the main species and eventually its reactivity goes more slow down towards the free base porphyrin, H₂TMPyP⁴⁺, however, the negatively charged Zn(OH)₃⁻ species is supposed to be easy approached to the tetracationic H₂TMPyP⁴⁺ from electrostatic force of attraction point of view. This is because the Zn(OH)₃⁻ species phases out from the aqueous system due to its low solubility, thereby resulting in the presence of its little amount in aqueous phase (Choi et al., 2013). Moreover, the extremely low concentration of the Zn(OH)₃⁻_(aq) along with the Zn(OH)₄²⁻_(aq) species at pH ∼ 11.0 is also responsible for their less and/or least reactivity towards the free base porphyrin, H₂TMPyP⁴⁺ (Figs. 2 and 3).

Therefore, the rate of incorporation of Zn²⁺ into the [H₂TMPyP]⁴⁺ (for simplicity the [H₂TMPyP]⁴⁺ written as H₂P⁴⁺) is expressed by the following equations:

(7)

(8)

(9)

(10)

Due to the least reactivity of the tetrahydroxozincate(II), Zn(OH)₄²⁻_(aq), species towards porphyrin and its few percent at pH = 10.30, Eq. (10) can thus be ignored, accordingly Eqs. (6)–(9) can be considered in calculating the k_obs.

According to the speciation equilibria (equs. 1–4) and pH dependent observed rate constants, k_obs, (Fig. 3), the individual rate constant in Eq. (11) was calculated by averaging the observed rate constants. These are as follows: k₁ = (0.20 ± 0.10) × 10⁻¹; k₂ = (0.70 ± 0.50) × 10⁻¹; k₃ = (2.60 ± 0.60) × 10⁻¹, k₄ = (1.20 ± 0.40) × 10⁻¹ M⁻¹s⁻¹ at 25 ± 1 °C. Results show that the aqua-dihydroxo Zn²⁺, Zn(OH)_2(aq), species shows the highest reactivity among the Zn²⁺ species towards the free base porphyrin [H₂TMPyP]⁴⁺ where k₃ = 2.60 ± 0.60 × 10⁻¹ M⁻¹s⁻¹. The enhanced reactivity of the zinc-aqua-dihydroxo, Zn(OH)_2(aq), species towards H₂TMPyP⁴⁺ is comparable to the increased rate constant of Zn(OH)₂ with hematoporphyrin IX (Paquette and Zador, 1978). The reactivity of the Zn²⁺ ion with hematoporphyrin IX decreases with increasing number of the OH^– group coordinated to the Zn²⁺ ion and found the following sequence: k_Zn(OH)2⁻>k_Zn(OH)3⁻>k_Zn(OH)4⁻ (Paquette and Zador, 1978). Cabbiness and Margerum (1970) also reported that the trihydroxocupriate(II), [Cu(OH)₃]⁻, species shows higher reactivity than the tetrahydroxocupriate(II), [Cu(OH)₄]²⁻, with the hematoporphyrin IX, i.e., k_Cu(OH)3⁻ > k_Cu(OH)4²⁻. In other reports, it has been explained that the [ZnOH]⁺ species showed enhanced reactivity towards non-N-substituted porphyrins compared to that of Zn²⁺ ion where k_ZnOH⁺ > k_Zn²⁺ (Hambright and Chock, 1974; Paquette and Zador, 1978; Thompson and Krishnamurthy, 1979; Sutter and Hambright, 1993). The increased rate constants have been ascribed to the formation of hydrogen bonding between the active oxygen of the hydroxyl ligand and the pyrrolic hydrogen atom of the free base porphyrin. Tabata and Ishimi (1997) found enhanced reactivity for the ZnOH⁺ species with the N-p-nitrobenzyl-5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin. The better reactivity has also been elucidated due to the formation of hydrogen bonding between the oxygen atom of the hydroxo-ligand and the pyrrolic hydrogen atom of the porphyrin ring. In our previous study, we also found the enhanced reactivity of monohydroxotrichloroaurate(III), AuCl₃(OH)⁻, species towards the [H₂TMPyP]⁴⁺ where k_AuCl3(OH)⁻ > k_AuCl4⁻ > k_AuCl2(OH)2⁻ > k_AuCl(OH)3⁻ (Habib et al., 2004). In another investigation, monohydroxocopper(II), [Cu(OH)]⁺, species shows much more reactivity with [H₂TMPyP]⁴⁺ compared to its aqua ion, Cu²⁺_(aq), i.e., k_Cu(OH)⁺ ≫ k_Cu²⁺_(aq) (Schneider, 1975). The marked reactivities of the oxygen containing copper species also indicated the formation of hydrogen bonding between the oxygen atom of the hydroxo-ligand of monohydroxocopper(II) and the pyrrolic hydrogen of the [H₂TMPyP]⁴⁺. Consequently, the formation of hydrogen bonding between the oxygen atom of OH^– group and the pyrrolic hydrogen atom facilitates an easy approach of the Zn²⁺ species to the porphyrin core, thereby resulting in obtaining the higher reactivity of the Zn²⁺ ion. On the contrary, the aqua-Zn²⁺, Zn²⁺_(aq), species is lack of hydroxo (OH^–) ligands thus might have less approaching ability to the porphyrin core that is responsible of its least reactivity towards the H₂TMPyP⁴⁺ and found the lowest value of the rate constant: k₁ = 0.20 ± 0.10 × 10⁻¹ M⁻¹s⁻¹.

Therefore it is concluded that the presence of hydroxyl group, OH^–, is responsible for the enhanced reactivity of the Zn²⁺ species towards the free base porphyrin, [H₂TMPyP]⁴⁺. This is due to the formation of hydrogen bonding between the oxygen atom of the OH^– group and the pyrrolic hydrogen atom of the porphyrin ring. The formation of hydrogen bonding facilitates an easy approach of the hydroxo-speceis of Zn²⁺ ion, Zn(OH)_n^2–, to the porphyrin core while aqua-species, Zn²⁺_(aq), might have less approaching ability to the porphyrin core that is responsible of its least reactivity. The less reactivity of the trihydroxozincate(II), Zn(OH)₃⁻, species towards the H₂TMPyP⁴⁺ can be explained by taking the following factors into account: (i) presence of small amount of the Zn(OH)₃⁻ species at pH ∼ 11.25 due to its poor solubility in aqueous medium (Choi et al., 2013) and (ii) slower displacement of the second OH^– ligand (Habib et al., 2004). The trihydroxozincate(II), Zn(OH)₃⁻, species is anionic while the free base porphyrin is tetracationic, accordingly an electrostatic force of attraction will be developed between the oppositely charged species. Such easy approach should enhance the reactivity of the Zn(OH)₃⁻ species towards the cationic H₂TMPyP⁴⁺, however, the low concentration of the Zn(OH)₃⁻ species and slower displacement of the bound second OH^– ligand causes its less reactivity with the H₂TMPyP⁴⁺ although the first OH^– ligand is responsible for the hydrogen bonding with the pyrrolic hydrogen.

3.3 Effect of Zn²⁺ concentration on the observed rate constants (k_obs)

Attempts have also been taken to study the rate of reaction of formation of ZnTMPyP⁴⁺as a function of concentration of Zn²⁺ at pH = 10.30. The observed rate constant (k_obs) was obtained by plotting the ln(A_t-A_α) vs time. The obtained values of the observed rate constants (k_obs) as a function of concentration of Zn²⁺ are shown in Fig. 4. The dependence of the rate on the concentration of Zn²⁺ (as shown in Fig. 4) being linear (r² = 0.997) with a small intercept almost at the origin shows that the reaction is first order with respect to both the concentrations of Zn²⁺ and porphyrin.

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Fig. 4

Plot of the observed rate constants (k_obs) for the reaction of [H₂TMPyP]⁴⁺ as a function of concentrations of Zn²⁺ in I = 0.10 M (NaNO₃) at 25 ± 1 °C. [(H₂TMPyP)⁴⁺] = 1.24 × 10⁻⁵ M; pH = 10.30.

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The formation rate constant (k_f) for [Zn(OH)_2(aq)]/[H₂(TMPyP)]⁴⁺ was found to be 1.265 × 10⁻¹ M⁻¹s⁻¹ at I = 0.10 M, NaNO₃ (25 ± 1 °C). However, Turay and Hambright (1980) studied the kinetics of Co²⁺ and Ni²⁺ with the [H₂(TMPyP)]⁴⁺ and they reported the rate constants were to be 1.10 × 10⁻³ and 2.10 × 10⁻⁶ M⁻¹s⁻¹ respectively in I = 0.50 M (NaNO₃) (25 °C). A slow reaction would be expected for Zn²⁺ species, but the rate constant of [Zn(OH)_2(aq)] seems to be relatively large due to the formation of hydrogen bonding between the oxygen atom of the Zn(OH)_2(aq) species and the pyrrolic hydrogen atom of the porphyrin ring. Moreover, Habib et al. (2004) reported high value of the formation rate constant for the reaction between [AuCl₃(OH)]⁻ and [H₂(TMPyP)]⁴⁺ was to be 3.83 × 10⁻¹ M⁻¹s⁻¹ for I = 0.10 M (NaNO₃) at 25 °C while Au³⁺ (d⁸) ion seems kinetically inert (Pearson, 1963). The high reactivity of the [AuCl₃(OH)]⁻ species for the cationic porphyrin, [H₂(TMPyP)]⁴⁺, is due to the negative charge and hydrogen bonding effects of the [AuCl₃(OH)]⁻ species.

3.4 Effect of ionic strength on the observed rate constants (k_obs)

The effect of ionic strength on the reaction rate was investigated by carrying out the reaction of formation of the [ZnTMPyP]⁴⁺ complex within a range from 0 to 10.0 × 10⁻² M NaNO₃ under the same experimental conditions at solution pH 10.30. The observed rate constant (k_obs) was obtained by plotting the ln(A_t-A_α) vs time at different ionic strengths. The decrease of the observed rate constants (k_obs) with ionic strength (Fig. 5) indicates that the Zn²⁺ species are negative that reduces the activity of the reactive species, thus resulting in slower the reaction rate.

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Fig. 5

Effect of ionic strength on the observed rate constant, (k_obs), at solution pH 10.30. [(H₂TMPyP)⁴⁺] = 1.24 × 10⁻⁵ M; [Zn²⁺] = 2.50 × 10⁻³ M; Temperature: 25 ± 1 °C.

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Hambright (2002) has also investigated the variation of the rate constants of the incorporation of Zn²⁺ into [H₂(TMPyP)]⁴⁺ with ionic strengths. By using the Bronsted-Bjerrum equation, the apparent net charge of the [H₂TMPyP]⁴⁺ was found to be + 1.4. Since the direct reaction of Zn²⁺ with [H₂TMPyP]⁴⁺ seems to be difficult, so nitrate ions reduce the repulsive force between Zn²⁺ and [H₂TMPyP]⁴⁺ by interacting with the positively charged porphyrintetracation, forming cation–anion aggregates of lower positive charge. Therefore, the apparent charge of [H₂TMPyP]⁴⁺ may be much lower than the actual charge of the [H₂TMPyP]⁴⁺ ion in the course of the reaction with Zn²⁺ compared to that of [Zn(OH)_2(aq)]. According to the Fuoss equation, the effect of ionic strength on the observed reaction rate constants divulged that the net charge on the porphyrin periphery is calculated to be +3.4.