Factors that control catalytic two- versus four-electron reduction of dioxygen by copper complexes

Abstract

The selective two-electron reduction of O(2) by one-electron reductants such as decamethylferrocene (Fc*) and octamethylferrocene (Me(8)Fc) is efficiently catalyzed by a binuclear Cu(II) complex [Cu(II)(2)(LO)(OH)](2+) (D1) {LO is a binucleating ligand with copper-bridging phenolate moiety} in the presence of trifluoroacetic acid (HOTF) in acetone. The protonation of the hydroxide group of [Cu(II)(2)(LO)(OH)](2+) with HOTF to produce [Cu(II)(2)(LO)(OTF)](2+) (D1-OTF) makes it possible for this to be reduced by 2 equiv of Fc* via a two-step electron-transfer sequence. Reactions of the fully reduced complex [Cu(I)(2)(LO)](+) (D3) with O(2) in the presence of HOTF led to the low-temperature detection of the absorption spectra due to the peroxo complex [Cu(II)(2)(LO)(OO)] (D) and the protonated hydroperoxo complex [Cu(II)(2)(LO)(OOH)](2+) (D4). No further Fc* reduction of D4 occurs, and it is instead further protonated by HOTF to yield H(2)O(2) accompanied by regeneration of [Cu(II)(2)(LO)(OTF)](2+) (D1-OTF), thus completing the catalytic cycle for the two-electron reduction of O(2) by Fc*. Kinetic studies on the formation of Fc*(+) under catalytic conditions as well as for separate examination of the electron transfer from Fc* to D1-OTF reveal there are two important reaction pathways operating. One is a rate-determining second reduction of D1-OTF, thus electron transfer from Fc* to a mixed-valent intermediate [Cu(II)Cu(I)(LO)](2+) (D2), which leads to [Cu(I)(2)(LO)](+) that is coupled with O(2) binding to produce [Cu(II)(2)(LO)(OO)](+) (D). The other involves direct reaction of O(2) with the mixed-valent compound D2 followed by rapid Fc* reduction of a putative superoxo-dicopper(II) species thus formed, producing D.

Figure 1

UV-vis spectral changes observed in the two-electron reduction of O₂ catalyzed by [Cu^II₂(LO)(OH)] (D1) (0.040 mM) with Fc^* (1.0 mM) in the presence of HOTF (3.0 mM) in O₂-saturated acetone ([O₂] = 11.0 mM) at 223 K. The Inset shows the time profile of the absorbance at 780 nm due to Fc^*+.

Figure 2

(a) Plot of the pseudo-first-order rate constants (k_obs) vs concentrations of D1 (black line) to determine second-order rate constant (k_cat) for the two-electron reduction of O₂ catalyzed by D1 with Fc^* (1.0 mM) in the presence of TFA (3.0 mM) in O₂-saturated acetone ([O₂] = 11.0 mM) at 223 K. Red line shows plot of k_obs vs concentrations of TFA in the two-electron reduction of O₂ catalyzed by D1 (0.10 mM) with Fc^* (1.0 mM) in the presence of TFA in O₂-saturated acetone ([O₂] = 11.0 mM) at 223 K. (b) Plot of k_cat vs concentrations of O₂ in the two-electron reduction of O₂ catalyzed by D1 (0.10 mM) with Fc^* (1.0 mM) in the presence of TFA (3.0 mM) in acetone at 223 K.

Figure 3

(a) UV-visible spectral changes of [Cu^II₂(LO)(OH)] (D1) (0.20 mM) in the presence of HOTF (0.0 – 28.0 mM) in acetone at 298 K. (b) Plot of α⁻¹ − 1 vs {[HOTF]₀ − (1 − α)[D1]₀} to determine the equilibrium constants (K_eq) in the protonation of D1 upon addition of TFA (0.0 – 28.0 mM) into the solution of D1 (0.20 mM) in acetone at 298 K.

Figure 4

(a) Cyclic voltammograms and differential pulse voltammograms (DPV) of D1 (2.0 mM) in the absence of HOTF in deaerated acetone at 233 K. (b) CV and DPV of D1 (2.0 mM) in the presence of HOTF (50 mM) in deaerated (black), air-saturated (red) and O2-saturated (blue) acetone at 233 K. TBAPF₆ (0.20 M) was used as an electrolyte.

Figure 5

UV-vis spectral changes observed in the electron transfer from Fc^* (0.40 mM) to [Cu^II₂(LO)(OH)] (D1) (0.10 mM) in the presence of HOTF (2.0 mM) at 203 K. The Inset shows the time profile of the absorbance at 780 nm due to Fc^*+, showing that 1 equiv Fc^*+ is formed in the first step, i.e., first phase. Note: the nonzero absorbance at 780 nm before the mixing of D1 and Fc^* is due to d-d transition of D1 complex, causing a large increase when the initial spectrum was recorded. This absorbance was subtracted from the total absorbance to give the absorbance due to Fc^*+ to determine the rate constant.

Figure 6

Plot of the pseudo-first-order rate constants (k_obs) vs concentrations of Fc^* in the first electron transfer from Fc^* to [Cu^II₂(LO)(OH)] (D1) (0.10 mM) to determine the k_et1 value in the presence of HOTF (2.0 mM) in acetone at 203 K.

Figure 7

UV-vis spectral changes observed in the electron transfer from Fc^* (1.2 mM) to [Cu^IICu^I(LO)]²⁺ (D2) (0.10 mM), formed as described in the text, in the presence of HOTF (2.0 mM) at 213 K. The Inset shows the time profile of the absorbance at 780 nm due to Fc^*+. Note: the nonzero absorbance at 780 nm before the mixing of D2 and Fc^* is due to a d-d transition of this complex and this causes a large increase in recording of the first spectrum. For practical reasons this absorbance is subtracted from the total absorbance to give the absorbance due to Fc^*+ to calculate the rate constant.

Figure 8

EPR spectrum of [Cu^IICu^I(LO)]²⁺ (D2) (1.0 mM) recorded in acetone at 5 K. D2 was generated in the reaction of D1 (1.0 mM) and Fc^* (1.0 mM) in the presence of HOTF (5.0 mM) in acetone at 298 K. The experimental parameters: microwave frequency = 9.6483 GHz, microwave power = 1.0 mW, and modulation frequency = 100 kHz.

Figure 9

Plot of the pseudo-first-order rate constants (k_obs) vs concentrations of Fc^* in the second electron transfer from Fc^* to [Cu^IICu^I(LO)]²⁺ (D2) (0.10 mM) to determine the k_et2 value in the presence of HOTF (2.0 mM) in acetone at 213 K.

Figure 10

UV-vis spectral changes and time profiles of (a) [Cu^I₂(LO)]⁺ (D3) (0.070 mM) after O₂ introduction demonstrating the generation of [Cu^II₂(LO)(OOH)]²⁺ (D4) at 395 nm and (b) D3 (0.070 mM) after O₂ introduction and addition of 1 equiv HOTF (0.070 mM) in acetone at 193 K.

Figure 11

(a) Full formation of the [Cu^II₂(LO)(OOH)]²⁺ (D4) in an acetone solution containing [Cu^I₂(LO)]⁺ (D3) (0.070 mM) and HOTF (0.070 mM) after O₂ introduction at 193 K. Inset shows the absorbance change at 395 nm due to the generated hydroperoxo species. (b) Addition of excess Fc^* (0.28 mM) (red spectrum) to the hydroperoxo species generated (black spectrum) in acetone at 193 K.

Figure 12

(a) UV-visible spectral changes of [Cu^II₂(LO)(OOH)]²⁺ (D4) (0.10 mM) in the presence of HOTF (0.10 – 3.0 mM) in acetone at 193 K. (b) Absorbance changes at 395 nm as a function of HOTF concentration.

Figure 13

(a) UV-visible spectral changes resulted from introduction of O₂ at 193 K into an acetone solution of [Cu^IICu^I(LO)]²⁺ (D2) (green spectrum) produced from room temperature mixing of [Cu^II₂(LO)(OH)] (D1) (0.10 mM) and Fc^* (0.10 mM) in the presence of HOTF (1.0 mM). The Inset shows the time profile of the absorbance at 395 nm due to the [Cu^II₂(LO)(OOH)]²⁺ (D4) generated (red spectrum).

Figure 14

Optimized structure with LUMO orbital of [Cu^II₂(LO)(OOH)]²⁺ (D4) calculated by DFT B3LYP/lanl2dz basis set.

Figure 15

Optimized structures with LUMO of (a) [Cu^II₂(N3)(O₂)]²⁺ calculated and (b) [(BzPY1)Cu^III(BzPY1)(O)₂Cu^III(BzPY1)]²⁺ by DFT B3LYP/Lanl2dz.

Scheme 1

Scheme 2

Scheme 3

Reaction sequence deduced for the catalytic two-electron two-proton reduction of O₂ to H₂O₂. The initial catalyst is [Cu^II₂(LO)(OH)](SbF₆)₂ (D1) and the reaction is carried out in acetone solution using decamethylferrocene (Fc^*) as reductant and trifluoroacetic acid (HOTF) as proton source. See text for further details, including the “short-circuit” path from D2 to D.

Scheme 4