Mononuclear Copper Complex Catalyzed Four-Electron Reduction of Oxygen

Abstract

A mononuclear Cu^II complex acts as an efficient catalyst for four-electron reduction of O₂ to H₂O by a ferrocene derivative via formation of the dinuclear Cu^II peroxo complex that is further reduced in the presence of protons by a ferrocene derivative to regenerate the Cu^II complex.

Cytochrome c oxidases (CcOs), with a bimetallic active-site consisting of a heme a and Cu (Fe_a3/Cu_B), are the terminal enzymes of respiratory chains, catalyzing the reduction of molecular oxygen to water by the soluble electron carrier, cytochrome c.^1,2 Synthetic Fe_a3/Cu_B analogs have attracted significant attention, because the four-electron reduction of O₂ is not only of great biological interest,^3,4 but also of technological significance such as in fuel cells.^5,6 Multicopper oxidases such as laccase also activate oxygen at a site containing a three-plus-one arrangement of 4 Cu atoms, exhibiting remarkable electroactivity for the four-electron reduction of oxygen at potentials approaching 1.2 V (vs RHE).⁷ Such electrocatalytic reduction of O₂ has frequently been used to probe the catalytic reactivity of synthetic CcO model complexes^3–5 and some copper (only) complexes have also been investigated.^8–10 However, there has been no report on the copper complex catalyzed four-electron reduction of O₂ employing one-electron reductants in homogeneous solution; such situations are amenable to systematic studies which provide considerable mechanistic insights.¹¹

We report herein that a copper complex [(tmpa)Cu^II](ClO₄)₂ (1: tmpa = tris(2-pyridylmethyl)amine)¹² efficiently catalyzes the four-electron reduction of O₂ by one-electron reductants such as ferrocene derivatives in the presence of HClO₄ in acetone. As described below, the catalytic mechanism is clarified based on kinetic studies and detection of reactive intermediates.

The addition of a catalytic amount of 1 to an O₂-saturated acetone solution of decamethylferrocene (Fc^*) and HClO₄ results in the efficient oxidation of Fc^* by O₂ to afford ferrocenium cation (Fc^*+) (see Supporting Information for the experimental section). Figure 1 shows the spectral changes obtained following stepwise addition of HClO₄ to this solution. For each time period, the concentration of Fc^*+ (λ_max = 380 and 780 nm)¹¹ immediately formed is the same as the concentration of HClO₄ added. The reduced product of O₂ is confirmed to be H₂O based on the detection of H₂ ¹⁸O by ¹⁸O-labeled O₂ experiments (Figure S1). It has also been confirmed that no H₂O₂ is detected via iodometric titration experiments (Figure S2).¹³ When more than four equiv. of Fc^* relative to O₂ (i.e., limiting [O₂]) were employed, only four equiv. Fc^*+ were formed in the presence of four equiv. HClO₄ (Figure S3).¹⁴ Thus, the stoichiometry of the catalytic reduction of O₂ by Fc^* is given by eq 1.¹⁵

$$4{\text{Fc}}^{*}+{\mathrm{O}}_2+4{\mathrm{H}}^{+}\stackrel{{[(\text{tmpa}){\text{Cu}}^{\text{II}}]}^{2+}}{\to }4{\text{Fc}}^{{*}_{+}}+2{\mathrm{H}}_2\mathrm{O}$$ (1)

Figure 1.

UV-vis spectral change in four-electron reduction of O₂ by Fc^* (1.5 mM) with 1 (9.0 × 10⁻⁵ M) in the presence of HClO₄ in acetone at 298 K. Inset shows the change in absorbance at 380 and 780 nm due to Fc^*+ by stepwise addition of HClO₄ (0.18 – 1.44 mM) to an O₂-saturated acetone solution ([O₂] = 11 mM) of Fc^* and 1.

The time profile of the four-electron reduction of O₂ with Fc^* catalyzed by 1 in the presence of HClO₄ in acetone at 298 K was examined by stopped-flow measurements. Figure 2a shows the observed absorption spectral change during the catalytic reaction. Under the conditions employed with relative concentrations of reagents as given in the Figure 2 caption, it is only after Fc^*+ (λ_max = 780 nm) is completely formed that the peroxo species, [(tmpa)Cu^II(O₂)Cu^II(tmpa)]²⁺ (2: λ_max = 520 nm)¹⁶ starts to be produced.¹⁷ This is more clearly seen in Figure 2b, the time profiles for the absorbance at 780 nm due to Fc^*+, by comparison to the absorbance at 520 nm due to 2. Because the concentration of HClO₄ is smaller than that of Fc^*, HClO₄ has been consumed when the reaction is over. It is well established that [(tmpa)Cu^I]⁺ reacts with O₂ affording the superoxo species [(tmpa)Cu^II(O₂ ⁻)]⁺ which reacts rapidly with [(tmpa)Cu^I]⁺ to produce the peroxo species 2.¹⁶ Thus, electron-transfer reduction of 1 by Fc^* with O₂ but without HClO₄ affords 2. This is the reason why 2 starts to appear only after HClO₄ is all consumed. The stoichiometry of the reaction of Fc^* with 1 and O₂ is given by eq 2.

$$2{\text{Fc}}^{*}+2{[(\text{tmpa}){\text{Cu}}^{\text{II}}]}^{2+}+{\mathrm{O}}_2\to 2{\text{Fc}}^{{*}_{+}}+{[(\text{tmpa}){\text{Cu}}^{\text{II}}({\mathrm{O}}_2){\text{Cu}}^{\text{II}}(\text{tmpa})]}^{2+}$$ (2)

Figure 2.

(a) Formation of the peroxo species 2 (λ_max = 520 nm) in electron transfer from Fc^* (1.0 mM) to 1 (0.12 mM) in the presence of HClO₄ (0.35 mM) in aerated acetone at 298 K. (b) Time profile of the absorbance at 520 nm (●) and 780 nm (○) due to 2 and Fc^*+, respectively.

The rate of formation of Fc^*+ in Figure 2b appears to be constant with respect to the concentration of Fc^*+, when the concentration of Fc^* is in large excess compared to that of HClO₄. The constant rate (M s⁻¹) increases linearly with increasing concentration of 1 and Fc^* (Figure S5). The second-order rate constant (k_obs) is determined to be (1.1 ± 0.1) × 10⁵ M⁻¹ s⁻¹ from the slope of Figure S5, which is divided by the initial concentration of Fc^* (1.0 mM).

The rate of formation of Fc^*+, accompanied by formation of 2 via electron transfer from Fc^* to 1 with O₂, was also determined without HClO₄, obeying pseudo-first-order kinetics, when the concentration of Fc^* is much larger than that of 1 (Figure S6). This rate constant increases linearly with concentration of Fc^*. From the slope of the linear plot, the second-order rate constant (k'_obs) for formation of Fc^*+ without HClO₄ under single turnover conditions is determined to be (4.9 ± 0.4) × 10⁴ M⁻¹ s⁻¹ (Figure S6). This value is one-half as compared to the rate constant under catalytic conditions with HClO₄. This is quite consistent with the stoichiometries of the catalytic reaction (eq 1) and the single turnover reaction (eq 2), because an additional equiv. of Fc^*+ is formed in the presence of HClO₄ under the catalytic conditions following formation of one equiv. of Fc^*+ under the single turnover reaction (see Supporting Information for the kinetic analysis).

No further reduction of the peroxo species 2 occurs without an acid. However, the addition of HClO₄ facilitates electron-transfer and the further two-electron reduction of 2 to produce two equiv. of Fc^*+, accompanied by regeneration of 1. This was confirmed by low temperature measurements (Figure S7). Thus, the overall catalytic cycle is given in Scheme 1. The initial electron transfer from Fc^* to 1 and reaction with O₂ affords the two-electron reduction of oxygen to produce the peroxo species that can be further reduced in the presence of HClO₄ to facilitate the four-electron reduction of O₂ to H₂O by Fc^* (Scheme 1).¹⁸

Scheme 1.

In summary, a copper complex 1 acts as an effective catalyst for the four-electron reduction of O₂ by one-electron reductants such as Fc^* in the presence of an acid. The present study opens a new approach and the use of copper ion to develop efficient catalysts for the four-electron reduction of O₂, because the catalytic activity and stability of intermediates can certainly be controlled and tuned by variation of ligands for copper ion.