Electron-transfer properties of a nonheme manganese(IV)–oxo complex acting as a stronger one-electron oxidant than the iron(IV)–oxo analogue

Abstract

Electron-transfer properties of a nonheme Mn(IV)–oxo complex, [(Bn-TPEN)Mn^IV(O)]²⁺, reveals that Mn(IV)–oxo complex acts as a stronger one-electron oxidant than the Fe(IV)–oxo analogue. As a result, an electron transfer process in N-dealkylation has been detected by a transient radical cation intermediate, para-Me-DMA˙⁺, in the oxidation of para-Me-DMA by [(Bn-TPEN)Mn^IV(O)]²⁺.

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High-valent metal–oxo complexes play pivotal roles as reactive intermediates in a wide range of heme (e.g. cytochromes P450 and peroxidases)¹ and nonheme metalloenzymes² as well as in their biomimetic catalysts.³ In particular, manganese–oxo complexes have attracted much attention recently as key intermediates in oxygen-evolving complex (OEC) in photosystem II,⁴ in which four-electron oxidation of H₂O to O₂ is efficiently catalyzed. Although extensive efforts have been devoted to elucidate the structural and electronic properties as well as the reactivities of manganese–oxo complexes in oxidation reactions,^5–9 it is quite important to understand the factors that control the redox reactivity of the intermediates since electron transfer is the most fundamental process among a variety of redox reactions, including hydrogen atom transfer (HAT) and oxygen atom transfer (OAT). With regard to nonheme iron(IV)–oxo complexes, electron-transfer properties have been reported in relation to the redox reactivity.¹⁰ However, electron-transfer properties of manganese(IV)–oxo complexes have yet to be studied in comparison with those of nonheme iron(IV)–oxo analogues.

We report herein thermodynamic and kinetic data for electron transfer from a series of electron donors to a nonheme Mn(IV)–oxo complex, [(Bn-TPEN)Mn^IV(O)]²⁺ (1, Bn-TPEN = N-benzyl-N,N′,N′-tris(2-pyridylmethyl)-1,2-diaminoethane) (Fig. 1).¹¹ The driving force dependence of the electron-transfer rate is analyzed in light of the Marcus theory of electron transfer,¹² leading to the evaluation of the fundamental electron-transfer properties, such as the reorganization energy and the one-electron reduction potential of 1 in comparison with those of the Fe(IV)–oxo analogue, [(Bn-TPEN)Fe^IV(O)]²⁺. The reactivity of 1 is also investigated in N-dealkylation of N,N-dialkylamines to compare its reactivity with that of nonheme Fe(IV)–oxo species.

Fig. 1 (a) Bn-TPEN ligand and (b) DFT-optimized structure of [(Bn-TPEN)Mn^IV(O)]²⁺ (1).¹¹

1 was prepared by the reaction of [(Bn-TPEN)Mn^II]²⁺ with four equiv. of iodosylbenzene (PhIO) in trifluoroethanol (CF₃CH₂OH) according to the reported procedure.¹¹ The iron(IV)–oxo analogue, [(Bn-TPEN)Fe^IV(O)]²⁺, was also prepared by the literature method.^10a We then performed electron-transfer reactions with a series of electron donors, such as ferrocene (Fc) and its derivatives, and the metal–oxo complexes, such as 1 and [(Bn-TPEN)Fe^IV(O)]²⁺, in a 1 : 1 (v/v) mixture of CF₃CH₂OH and acetonitrile (MeCN) at 273 K. The solvent mixture was chosen to optimize the solubility of electron donors and the stability of 1 and [(Bn-TPEN)Fe^IV(O)]²⁺; both the metal–oxo complexes are quite stable at 273 K under the reaction conditions. Although no electron-transfer from dibromoferrocene (Br₂Fc) to [(Bn-TPEN)Fe^IV(O)]²⁺ occurred at 273 K, we observed the occurrence of electron transfer from Br₂Fc to 1 under the identical reaction conditions (see Fig. 2), where the NIR absorption band at 1040 nm due to 1 disappears,¹¹ accompanied by the appearance of the absorption band at 700 nm due to a dibromoferrocenium ion (Br₂Fc⁺). This observation suggests that 1 is a stronger one-electron oxidant than [(Bn-TPEN)Fe^IV(O)]²⁺. The electron transfer from Br₂Fc to 1 is also found to be in equilibrium [eqn (1)], where the final concentration of Br₂Fc⁺ produced increases with an increase in the initial concentration of Br₂Fc (see Fig. 3). The equilibrium constant (K_et) is determined to be 23 at 273 K by fitting the plot in ESI,† Fig. S1.

The apparent one-electron reduction potential, E_red, of 1 is determined from the K_et value and the E_ox value of Br₂Fc (0.71 V versus SCE: see ESI,† Table S1) using the Nernst equation [eqn (2)] to be 0.78 V.¹³ This E_red value of 1 is significantly more positive than the E_red value of [(Bn-TPEN)Fe^IV(O)]²⁺ (0.49 V versus SCE),^10a which is consistent with the result that no electron-transfer from Br₂Fc to [(Bn-TPEN)Fe^IV(O)]²⁺ occurs because the electron transfer is endergonic (ΔG_et = 0.22 eV). The higher E_red value of 1 than those of the corresponding nonheme Fe^IV(O) complexes was also reported for the case of Mn^IV(O) porphyrins with electron-donating and withdrawing substituents, which have significantly higher E_red values (E_red = 0.95–1.20 V versus SCE)¹⁴ than the E_red value of an Fe^IV(O) porphyrin (Horseradish peroxidase compound II: 0.62 V versus SCE).^15,16

E_red = E_ox + (RT/F)ln K_et (2)

Rates of electron transfer from Br₂Fc to 1 were determined from the increase in the absorption band at 700 nm due to Br₂Fc⁺ (see Fig. 2a). The electron-transfer rates obeyed pseudo-first-order kinetics in a large excess of Br₂Fc (Fig. 2b). The pseudo-first-order rate constants (k_obs) increased linearly with increasing concentration of Br₂Fc (ESI,† Fig. S2). The second-order rate constant of the electron transfer (k_et) was determined from the slope of the linear plot of k_obsversus concentration of Br₂Fc to be 3.1 × 10 M⁻¹ s⁻¹. Similarly, the k_et values of electron-transfer from a series of electron donors to 1 were determined, and the k_et values obtained are listed in Table S1 (also see ESI,† Fig. S2 and S3),†¹³ together with the E_ox values of electron donors and the driving force of electron transfer, which was determined using eqn (3), where e is the elementary charge. The k_et value for dimethylferrocene is obtained by the kinetic measurement carried out under second-order conditions because the reaction is too fast to follow even with stopped-flow equipment under pseudo-first-order conditions (ESI,† Fig. S3).

−ΔG_et (eV) = e(E_red − E_ox) (3)

The driving force dependence of the electron-transfer rate constants is shown in Fig. 4, where the log k_et values are plotted against the −ΔG_et values. The driving force dependence of k_et is well fitted by the solid line in Fig. 4 in light of the Marcus theory of adiabatic outer-sphere electron-transfer [eqn (4)], where Z is the collision frequency taken as 1 × 10¹¹ M⁻¹ s⁻¹, λ is the reorganization energy of electron transfer, k_B is the Boltzmann constant, and T is the absolute temperature.^10,12 The λ value is determined to be 2.37 eV, which is the best fit value of eqn (4) and this value is compared with λ values of iron(IV)–oxo complexes in Table S2 (ESI†) where the E_red values are also listed.¹⁰

k_et = Zexp[−(λ/4)(1 + ΔG_et/λ)²/k_BT] (4)

The λ value of the electron-transfer reduction of 1 (2.37 eV) is similar to those determined for the electron-transfer reduction of nonheme iron(IV)–oxo complexes. This indicates that one-electron reduction of high-valent metal–oxo complexes generally requires large reorganization energy probably due to significant elongation of metal–oxo bonds upon a one-electron reduction. 1 has the most positive E_red value as compared with those of iron(IV)–oxo complexes, suggesting that 1 is the strongest one-electron oxidant among nonheme metal–oxo complexes reported so far.

Fig. 2 (a) Absorption spectral changes observed in electron transfer from Br₂Fc (2.5 × 10⁻³ M) to 1 (2.5 × 10⁻⁴ M) in CF₃CH₂OH–MeCN (1 : 1 v/v) at 273 K. (b) Time profiles of the absorbance at 700 nm (blue circles) and 1040 nm (red circles) due to the formation of Br₂Fc⁺ and disappearance of 1, respectively.

Fig. 3 Plot of concentration of Br₂Fc⁺ produced in electron transfer from Br₂Fc to 1versus initial concentration of Br₂Fc, [Br₂Fc]₀.

Fig. 4 Driving force of the ET (−ΔG_et) dependence of rate constants (log k_et) from one-electron donors (1: Br₂Fc, 2: acetylferrocene, 3: bromoferrocene, 4: ferrocene, 5: dimethylferrocene) to [(Bn-TPEN)Mn^IV(O)]²⁺ in CF₃CH₂OH–MeCN (1 : 1 v/v) at 273 K. The gray line is the Marcus line calculated with λ value of 2.37 eV.

We also investigated the reactivity of 1 in N-dealkylation and then compared it with that of an iron(IV)–oxo complex, [(N4Py)Fe^IV(O)]²⁺.^10d,17,18 We examined the oxidation of para-methyl-N,N-dimethylaniline (p-Me-DMA; E_ox = 0.69 V versus SCE) by 1. The N-dealkylation of DMA derivatives by metal–oxo species is known to consist of an electron-transfer coupled with a proton transfer from DMA derivatives to metal–oxo species.^10d,17,19 In the case of oxidation of p-Me-DMA by [(N4Py)Fe^IV(O)]²⁺, the intermediate DMA˙⁺ resulting from an electron-transfer process is not detected because the initial ET process is the rate-determining step.^10d,17 In contrast, addition of a p-Me-DMA to a deaerated CF₃CH₂OH–MeCN (1 : 1 v/v) solution of 1 resulted in the immediate generation of a transient absorption band at λ_max = 460 nm (Fig. 5) The absorption band was identical to that of the p-Me-DMA˙⁺, given by the reaction of p-Me-DMA (2.5 × 10⁻³ M) with cerium(IV) ammonium nitrate (2.5 × 10⁻³ M) in CF₃CH₂OH–MeCN (1 : 1 v/v) at 273 K (ESI,† Fig. S4). The transient absorption band of p-Me-DMA˙⁺ appears, accompanied by a decrease in the absorption band at 725 nm²⁰ due to 1 (Fig. 5b). Electron-transfer from p-Me-DMA to 1 proceeds via fast electron-transfer from DMA to 1 to produce p-Me-DMA˙⁺, followed by slower proton transfer from p-Me-DMA˙⁺ to [(Bn-TPEN)Mn^III(O)]⁺ in a stepwise manner. The initial electron-transfer becomes energetically feasible due to the highly positive E_red value of 1. The rate constant was evaluated to be 2.6 × 10² M⁻¹ s⁻¹ at 273 K. Thus, to the best of our knowledge, this is the first example of detecting the radical cation intermediate in electron-transfer reactions of metal–oxo complexes without acids or metal ion effect.

Fig. 5 (a) Optical spectral changes in the reaction of 1 (2.5 × 10⁻⁴ M) and p-Me-DMA (2.5 × 10⁻³ M) in CF₃CH₂OH–MeCN (1 : 1 v/v) at 273 K. (b) The time course of the decay of 1 (blue line) and the formation and decay of p-Me-DMA˙⁺ (red line) monitored at 725 nm and 460 nm, respectively.

In summary, we have determined for the first time the fundamental electron-transfer properties of a nonheme Mn^IV(O) complex, which is featured by a highly positive one-electron reduction potential, as compared with those of nonheme Fe^IV(O) complexes. As a result of the highly positive one-electron reduction potential, the occurrence of electron transfer in the N-dealkylation of DMA derivatives has been confirmed by the detection of the radical cation intermediate, p-Me-DMA˙⁺, in the oxidation of p-Me-DMA by the nonheme Mn^IV(O) complex.

The work at EWU was supported by NRF/MEST of Korea through CRI (to W.N.), GRL (2010-00353) (to W.N.), and WCU (R31-2008-000-10010-0) (to W.N. and S.F.). The work at OU was supported by a Grant-in-Aid (No. 20108010 to S.F.) and a Global COE program, “the Global Education and Research Centre for Bio-Environmental Chemistry” from MEXT, Japan (to S.F.). Y.M. appreciates support from the Global COE program and Grant-in-Aid for JSPS fellowship for young scientists.

Notes and references

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11. The manganese(IV)–oxo species could be generated by use of a stoichiometric amount of PhIO. However, 4 equiv. of PhIO was used in this work to shorten the reaction time and also to make sure the complete conversion to the manganese(IV) species; see: X. Wu, M. S. Seo, K. M. Davis, Y.-M. Lee, J. Chen, K.-B. Cho, Y. N. Pushkar and W. Nam, J. Am. Chem. Soc., 2011, 133, 20088.
12. R. A. Marcus, Angew. Chem., Int. Ed. Engl., 1993, 32, 1111.
13. Oxidation potentials of electron donors in CF₃CH₂OH–MeCN (1 : 1 v/v) were determined in this work (see Table S1 in ESI†).
14. S. Fukuzumi, T. Mizuno and T. Ojiri, Chem.–Eur. J., 2012 , in press.
15. S. Fukuzui, Coord. Chem. Rev. DOI:10.1016/j.ccr.2012.07.021.
16. The reason why the E_red value of [(N2Py2)Fe^IV(O)]²⁺ is higher than those of other nonheme Fe^IV(O) complexes may result from the less electron-donating coordination of the tetradentate N2Py2 ligand as compared with pentadentate ligands of other nonheme Fe^IV(O) complexes.
17. K. Nehru, M. S. Seo, J. Kim and W. Nam, Inorg. Chem., 2007, 46, 293.
18. It has been reported that [(Bn-TPEN)Fe^IV(O)]²⁺ exhibits similar reactivity to [(N4Py)Fe^IV(O)]²⁺; see ref. 10a.
19. P. R. Ortiz de Montellano and J. J. De Voss, Nat. Prod. Rep., 2002, 19, 477.
20. Due to the range limitation of a stopped-flow spectrometer (350–750 nm), the absorbance at 725 nm was monitored.

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Footnote

† Electronic supplementary information (ESI) available: Experimental and kinetic details. See DOI: 10.1039/c2cc36291k

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