Catalytic two-electron reduction of dioxygen catalysed by metal-free [14]triphyrin(2.1.1)

Abstract

The catalytic two-electron reduction of dioxygen (O₂) by octamethylferrocene (Me₈Fc) occurs with a metal-free triphyrin (HTrip) in the presence of perchloric acid (HClO₄) in benzonitrile (PhCN) at 298 K to yield Me₈Fc⁺ and H₂O₂. Detailed kinetic analysis has revealed that the catalytic two-electron reduction of O₂ by Me₈Fc with HTrip proceeds via proton-coupled electron transfer from Me₈Fc to HTrip to produce H₃Trip˙⁺, followed by a second electron transfer from Me₈Fc to H₃Trip˙⁺ to produce H₃Trip, which is oxidized by O₂via formation of the H₃Trip/O₂ complex to yield H₂O₂. The rate-determining step in the catalytic cycle is hydrogen atom transfer from H₃Trip to O₂ in the H₃Trip/O₂ complex to produce the radical pair (H₃Trip˙⁺ HO₂˙) as an intermediate, which was detected as a triplet EPR signal with fine-structure by the EPR measurements at low temperature. The distance between the two unpaired electrons in the radical pair was determined to be 4.9 Å from the zero-field splitting constant (D).

Chart 1. Structure of HTrip.

Fig. 1. (a) Absorption spectral changes of HTrip (2.0 × 10^–5 M) upon the addition of HClO₄ in air-saturated PhCN at 298 K. (b) Absorbance change profile at 565 nm.

Fig. 2. Cyclic voltammograms (upper) and differential pulse voltammograms (lower) of deaerated PhCN solutions of HTrip (1.0 × 10^–3 M) recorded in the presence of TBAPF₆ (0.10 M) (a) without HClO₄ and (b) with HClO₄ (1.0 × 10^–2 M); sweep rate: 100 mV s^–1 for CV and 4 mV s^–1 for DPV.

Scheme 1

Fig. 3. (a) Absorption spectral changes upon addition of Me₈Fc (1.0 × 10^–3 and 2.0 × 10^–3 M) to a deaerated PhCN solution of H₂Trip⁺ (1.0 × 10^–3 M) in the presence of HClO₄ (1.0 × 10^–3 M) at 298 K in a quartz cuvette (light path length = 1 mm) (black); absorption spectral change upon addition of HClO₄ (1.0 × 10^–3 M) to the solution indicated by the black line (red); absorption spectral change upon addition of HClO₄ (1.0 × 10^–3 M) to the solution indicated by the red line (blue). (b) Absorption change at 738 nm upon addition of various concentrations of Me₈Fc and HClO₄.

Fig. 4. Time profiles of absorbance at 738 nm due to H₃Trip in the reduction of H₂Trip⁺ (2.5 × 10^–5 M) (a) by various concentrations of Me₈Fc in the presence of HClO₄ (3.0 × 10^–4 M) and (b) by Me₈Fc (2.0 × 10^–3 M) in the presence of various concentrations of HClO₄ in deaerated PhCN at 298 K.

Fig. 5. (a) Plot of k_obsvs. [Me₈Fc] for the reduction of H₂Trip⁺ (2.5 × 10^–5 M) by various concentrations of Me₈Fc in the presence of HClO₄ (3.0 × 10^–4 M) in PhCN at 298 K. (b) Plot of k_obsvs. [HClO₄] for the reduction of H₂Trip⁺ (2.5 × 10^–5 M) by Me₈Fc (2.0 × 10^–3 M) in the presence of various concentrations of HClO₄ in deaerated PhCN at 298 K.

Scheme 2

Fig. 6. (a) Absorption spectral changes produced by electron transfer from Me₈Fc (1.0 × 10^–4 M) to HTrip (2.5 × 10^–5 M) in the presence of HClO₄ (1.0 × 10^–4 M) in deaerated PhCN at 298 K. (b) Absorption spectral changes upon introducing O₂ to a deaerated PhCN solution of (a). The red and green lines show the spectrum of H₃Trip before and after introducing O₂ by O₂ gas bubbling, respectively. The blue line shows the spectrum due to precursor complex. Insets show absorption time profiles.

Fig. 7. (a) Absorption spectral changes in the two-electron reduction of O₂ (9.4 × 10^–4 M) by Me₈Fc (1.0 × 10^–2 M) with HTrip (5.0 × 10^–5 M) in the presence of HClO₄ (1.0 × 10^–2 M) in PhCN at 298 K. The black and red lines show the spectra before and after addition of Me₈Fc, respectively. The dotted line is the absorbance at 750 nm due to 1.9 × 10^–3 M of Me₈Fc⁺. (b) Time profile of absorbance at 750 nm due to Me₈Fc⁺. Inset shows first-order plot.

Scheme 3

Fig. 8. Plots of (a) k_obsvs. [HTrip] for the two-electron reduction of O₂ (9.4 × 10^–4 M) by Me₂Fc (1.0 × 10^–2 M) with various concentrations of HTrip in the presence of HClO₄ (1.0 × 10^–2 M) in PhCN; (b) k_obsvs. [HClO₄] for the two-electron reduction of O₂ (9.4 × 10^–4 M) by Me₈Fc (1.0 × 10^–2 M) with HTrip (5.0 × 10^–5 M) in PhCN at 298 K; (c) k_obsvs. [Me₈Fc] for the two-electron reduction of O₂ (9.4 × 10^–4 M) by various concentrations of Me₈Fc with HTrip (5.0 × 10^–5 M) in the presence of HClO₄ (1.0 × 10^–2 M) in PhCN at 298 K; and (d) k_obsvs. [O₂] for the two-electron reduction of O₂ by Me₈Fc (1.0 × 10^–2 M) with HTrip (5.0 × 10^–5 M) in the presence of HClO₄ (1.0 × 10^–2 M) in PhCN at 298 K.

Fig. 9. EPR spectrum observed after the reduction of HTrip (1.0 × 10^–3 M) by (BNA)₂ (2.0 × 10^–3 M) in the presence of HClO₄ (1.0 × 10^–3 M) in aerated PhCN under photoirradiation using a high-pressure Hg lamp (1000 W) measured at 80 K. Experimental conditions: Microwave frequency 9.0 GHz, microwave power 1.0 mW, modulation frequency 100 kHz, and modulation width 10 G. (b) Optimized structure of H₃Trip/O₂ calculated by DFT with calculated spin-density values given in parentheses at the UB3LYP/6-31G(d) level of theory.