Catalase-like activity of the polyoxovanadate anion [Mn^IVV₁₃O₃₈]⁷⁻: a mechanistic study

Abstract

The anion [Mn^IVV₁₃O₃₈]⁷⁻, 1, in aqueous acetate buffer catalyses the disproportionation of hydrogen peroxide into oxygen and water (TON > 200) by a Mn^IV/II cycle, which operates via quick formation of diperoxovanadate at the surface of 1.

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Polyoxometalates (POMs) belong to a large class of metal–oxygen cluster anions mostly consisting of group 5 or group 6 transition metals in their higher oxidation states. These compounds are very useful in manifold applications such as catalysis,¹ nanotechnology,² medicine³ and materials science.⁴ Many examples of polyoxomolybdates and polyoxotungstates are available, but polyoxovanadates are rather scant in comparison, and challenges prevail in detailed study of mechanism of their reactivities. One example of such heteropolyvanadates is the 1 : 13 heteropoly anion [Mn^IVV₁₃O₃₈]⁷⁻1. This complex was first synthesized in 1970 by Flynn and Pope⁵ and was characterized by IR spectra,⁶ TG/DTA curve⁶ (Fig. S1 and S2 in the ESI† agree well with those reported for 1) and X-ray crystallography.^7a,b This polyoxovanadate anion containing a MnO₆ octahedra inside (see Fig. 1) shows in vitro inhibitory effect on human tumor cells,⁸ has some anti-influenza activity,⁹ and can catalyse the mineralization of phenols with 30% aqueous H₂O₂ as the oxidizing agent.¹⁰ Nevertheless the mechanism of its catalytic activity is unknown to any degree of certainty. In the present study we observed that 1 catalyses the decomposition of H₂O₂ to H₂O and O₂ (catalase like activity).

Fig. 1 A ball–stick model of K₇[MnV₁₃O₃₈]·18H₂O. The blue, red and black balls correspond to vanadium, manganese and oxygen respectively.

Herein we describe a detailed mechanism of interaction of H₂O₂ with 1, anticipating its relevance to the mechanism of POM catalysed oxidation of organic substrates by H₂O₂.

According to stoichiometric measurement 2 moles of H₂O₂ disproportionate into 1 mole of O₂ and 2 moles of H₂O (see Table S2 in ESI†). Evolution of O₂ was measured by downward displacement of water. The measured volume of the gas collected was corrected to NTP as usual. The reaction thus appears to be:

Consumption of H₂O₂ was also studied spectrophotometrically¹¹ at 400 nm by forming a titanium(IV) peroxo complex (ε = 935 M⁻¹ cm⁻¹) (see Fig. S3 in ESI†). At pH = 3.8, 1 × 10⁻³ mol dm⁻³ of H₂O₂ was decomposed by 5 × 10⁻⁵ mol dm⁻³ of 1, within three hours.

All the kinetics were done at a suitably chosen wavelength of 450 nm where the diperoxovanadate species does not interfere.^12a The uncatalyzed decomposition of H₂O₂ is insignificant under the reaction conditions (pH 3.5–5.4, acetate buffer medium, I = 0.5 M, NaClO₄). When H₂O₂ is added in excess over 1, the absorbance of the reaction mixture first decreased and then slowly increased to almost 50% of the initial value (see Fig. S4 and S5 in ESI†). As the total course of the reaction is very complex, we adopted the initial rate method. For evaluation of the initial rate, absorbance vs. time data were fitted to a polynomial equation of order three. The second coefficient (with a negative sign) extracted from the plot was then divided by the ε value (2790 M⁻¹ cm⁻¹) of the complex to find out the initial rate (V_i).

At fixed [H₂O₂] (1 × 10⁻³ mol dm⁻³) V_i increases linearly with increasing concentration of H⁺ (slope = 0.67 s⁻¹, intercept = 3.9 × 10⁻⁸ mol dm⁻³ s⁻¹, see Table S1 and Fig. S6, ESI†) while 1/V_i varies linearly with 1/[H₂O₂]² for the entire range of pH studied (at pH = 3.5, slope = 1.8 × 10⁻⁴ mol dm⁻³ s, intercept = 1.54 × 10³ mol⁻¹ dm³ s, and at pH = 4.3, slope = 4 × 10⁻⁵ mol dm⁻³ s, intercept = 3.5 × 10² mol⁻¹ dm³ s, see Table S1 and Fig. S7 in ESI†). The rate is not affected by the media ionic strength.

The enhancement of the initial rate with increasing [H⁺] indicates mono-protonation of complex 1 (see Complex protonation step of Scheme 1), while the linear dependence of 1/V_i on 1/[H₂O₂]² is explicable from the formation of diperoxovanadate (dpv) species. The three vanadium atoms residing in the same plane as that of Mn^IV have two oxo groups each.^7b So a diperoxovanadate species can be generated by replacing the two oxo groups attached to any of these three V^V atoms by two peroxo groups. A new peak appearing at −688 ppm (see Fig. S8a, ESI†) in the ⁵¹V NMR spectra, taken immediately after addition of H₂O₂ to the reaction mixture, also accounts for the formation of a diperoxovanadate^12a,b intermediate.‡ The NMR spectrum of the reaction mixture becomes identical (see Fig. S8b, ESI†) to that of 1 after consumption of all the H₂O₂. This fact leads us to conclude that the polyoxovanadate cage structure remains the same at the end of the reaction. The whole cycle repeats, i.e. the signal at −688 ppm reappears, if fresh portions of H₂O₂ are added, then disappears again after total consumption of the added H₂O₂.

Scheme 1

EPR spectra of the reaction mixture consisting of six hyperfine lines (g = 2.1) (see Fig. S9 in ESI†), corresponding to octahedral Mn^II,¹³ account for the reduction of the Mn^IV center to Mn^II without any change in the structure of 1. The EPR spectra of the complex show six lines around g = 2.1 and a broad peak in the low magnetic field region which is consistent with the reported EPR spectra of octahedral Mn^IV complexes (see the inset of Fig. S9, ESI†).¹⁴ Mn^IV is also present with Mn^II during the decomposition of hydrogen peroxide. The complex can decompose 100 fold or more H₂O₂ without degrading its polyoxometalate framework. Thus it may be presumed that a Mn^IV/II cycle operates to catalyse the disproportionation of H₂O₂ into water and oxygen. In accordance with the above inferences a reaction scheme (Scheme 1) for the catalytic process can be proposed.

Cyclic voltammetry study (−1.6 V to +1.6 V, 50 mV s⁻¹ scan rate, using SCE as reference electrode) of the complex (1 × 10⁻³ mol dm⁻³) in acetate buffer medium (pH = 3.8) shows that there is one quasi-reversible redox couple with E_1/2 = +0.175 V. A very small reduction peak at +0.79 V also appears with two major irreversible reduction peaks at −0.5 V and at −1.02 V in the cyclic voltammetry diagram of the complex. Three irreversible oxidation peaks appear at −0.72 V, +0.0738 V and at +0.95 V (see Fig. S10a in the ESI†). These peaks can be attributed to the V^V/IV redox couple for different types of vanadium in the complex.¹⁵ Upon addition of H₂O₂ the quasi-reversible peaks with E_1/2 = +0.175 V vanish and current intensity of the irreversible oxidation peak at +0.95 V increases (see Fig. S10b in the ESI†). Formation of diperoxovanadate species could be attributed for such an outcome. The cyclic voltammetry diagram of H₂O₂ under the same conditions shows no certain peaks (see Fig. S10c in the ESI†). Though EPR spectra taken after treating the buffer solution of complex with H₂O₂ suggest formation of Mn^II, cyclic voltammetry gives no clear sign of the Mn^IV/II redox couple. So we can conclude that either the Mn^IV/II redox couple has been shaded¹⁶ by the V^V/IV redox couple or the Mn^IV center of the Mn^IVO₆ octahedra residing inside the polyoxovanadate cage is electrochemically inaccessible in the studied potential window. EPR spectra were taken after bulk electrolysis of the complex solution (pH = 3.8, acetate buffer) at +0.6 V, 0 V and −0.6 V. In each case the EPR spectrum consists of 8 hyperfine lines characterising the formation of V^IV species (see Fig. S11 in the ESI†),¹⁷ which leads us to conclude that all the peaks in the CV diagram correspond to the V^V/IV redox system. But this does not rule out the possibility of the Mn^IV/II redox couple being shaded, as even if the Mn^IV center was reduced to Mn^II, it would be hard to distinguish the six line EPR spectra around g = 2.1 for Mn(II) in the presence of V^IV species for which an eight line splitting around g = 2 is observed in EPR.¹⁸ Spectrophotometric titration of the complex with ascorbic acid (AA_red) shows 1 : 5 stoichiometry (see Fig. S12 in ESI†). The stoichiometry can be explained using eqn (2) and (3).

[Mn^IVV₁₃O₃₈] + AA_red ⇌ [Mn^IIV₁₃O₃₈] + AA_ox (2)

[Mn^IVV₁₃O₃₈] + 3AA_red ⇌ Mn²⁺ + [V₁₀O₂₈]⁶⁻ + VO²⁺ + 3AA_ox (3)

From the information obtained from all kinetic data we deduced an expression for the initial rate showing dependence of V_i on [H⁺] and [H₂O₂] (see ESI† for the derivation).

V_i = {(k₁K₁[H₂O₂]² + k₂K_1HK_H[H⁺][H₂O₂]²)T_c}/(1 + K₁[H₂O₂]²) (4)

It is very interesting to note that we are able to show the importance of the transition metal, embedded inside the polyoxovanadate cage, in controlling its catalytic activity.

In summary we have shown that the heteropolyanion, 1, in aqueous acetate buffer (pH = 3.5–5.4) catalyses the decomposition of H₂O₂ to O₂ and H₂O. The polyoxovanadate cage, instead of acting as a barrier, transfers electrons to the otherwise inaccessible Mn^IV center by forming a diperoxovanadate intermediate, and drives a Mn^IV/II catalytic cycle without any structural change to the Keggin complex. Though EPR, NMR, and kinetic data led us to presume about the Mn^IV/II cycle, unambiguous determination of the Mn^IV centered redox process was not possible.

Sanchita Chakrabarty thanks University Grants Commission (UGC, New Delhi, India) for the JRF award. We thankfully acknowledge the department of chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, West Bengal, for recording the EPR spectra. SC thanks Dr Saurabh Das of J.U. for helpful discussion and Dr Jnan Prakash Naskar of J.U. for helping with the CV experiments. We are also grateful to Arnab Kumar Maity of Indian Institute of Technology, Kharagpur, for recording the ⁵¹V NMR spectra.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cy20163a

‡ The NMR study has not been done quantitatively as VOCl₃, which is the commonly used internal standard for ⁵¹V NMR, degrades into V₂O₅ on standing in aqueous medium.

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