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. 2013 Mar 13;135(10):4018-26.
doi: 10.1021/ja312256u. Epub 2013 Feb 26.

Acid-induced mechanism change and overpotential decrease in dioxygen reduction catalysis with a dinuclear copper complex (VSports app下载)

Dipanwita Das  1 , Yong-Min Lee, Kei Ohkubo, Wonwoo Nam, Kenneth D Karlin, Shunichi Fukuzumi
Affiliations

Acid-induced mechanism change and overpotential decrease in dioxygen reduction catalysis with a dinuclear copper complex (V体育安卓版)

Dipanwita Das et al. J Am Chem Soc. .

Abstract

Catalytic four-electron reduction of O2 by ferrocene (Fc) and 1,1'-dimethylferrocene (Me2Fc) occurs efficiently with a dinuclear copper(II) complex [Cu(II)2(XYLO)(OH)](2+) (1), where XYLO is a m-xylene-linked bis[(2-(2-pyridyl)ethyl)amine] dinucleating ligand with copper-bridging phenolate moiety], in the presence of perchloric acid (HClO4) in acetone at 298 K. The hydroxide and phenoxo group in [Cu(II)2(XYLO)(OH)](2+) (1) undergo protonation with HClO4 to produce [Cu(II)2(XYLOH)](4+) (2) where the two copper centers become independent and the reduction potential shifts from -0. 68 V vs SCE in the absence of HClO4 to 0. 47 V; this makes possible the use of relatively weak one-electron reductants such as Fc and Me2Fc, significantly reducing the effective overpotential in the catalytic O2-reduction reaction. The mechanism of the reaction has been clarified on the basis of kinetic studies on the overall catalytic reaction as well as each step in the catalytic cycle and also by low-temperature detection of intermediates. The O2-binding to the fully reduced complex [Cu(I)2(XYLOH)](2+) (3) results in the reversible formation of the hydroperoxo complex ([Cu(II)2(XYLO)(OOH)](2+)) (4), followed by proton-coupled electron-transfer (PCET) reduction to complete the overall O2-to-2H2O catalytic conversion VSports手机版. .

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Figures

Figure 1
Figure 1
UV-vis spectral changes observed in the four-electron reduction of O2(1.0 mM) by Me2Fc (6.0 mM) with HClO4 (40 mm) catalyzed by 1 (0.20 mM) in acetone at 298 K. Inset shows the time profile of the absorbance at 650 nm due to Me2Fc+.
Figure 2
Figure 2
(a) Time profiles of the absorbance at 650 nm due to Me2Fc+ in the four-electron reduction of O2 catalyzed by 1 (0.080 mM (green), 0.10 mM (black), 0.12 mM (red), 0.20 mM (dark yellow) and 0.25 mM (blue)) with Me2Fc (4.0 mM) in the presence of HClO4 (40 mM) in an air-saturated ([O2] = 2.2 mM) acetone solution at 298 K. (b) Plot of kobs vs [1] for the four-electron reduction of O2 catalyzed by 1 with Me2Fc (4.0 mM) in the presence of HClO4 (40 mM) in an air-saturated ([O2] = 2.2 mM) acetone solution at 298 K. (c) Plot of kobs vs [HClO4] for the four-electron reduction of O2 by Me2Fc (4.0 mM) catalyzed by 1 (0.12 mM) in an acetone solution containing O2 (2.2 mM) at 298 K. (d) Plot of kobs vs [O2] for the four-electron reduction of O2 catalyzed by 1 (0.20 mM) with Me2Fc (3.2 mM) in the presence of HClO4 (40 mM) in an acetone solution at 298 K.
Figure 3
Figure 3
(a) UV-visible spectral changes of [CuII2(XYLO)(OH)](PF6)2 (1) (0.20 mM) upon addition of HClO4 (0.0–6.0 mM) in acetone at 298 K. (b) Absorbance changes at 378 nm as a function of HClO4 concentration.
Figure 4
Figure 4
X-band EPR spectra of [CuII2(XYLO)(OH)](PF6)2 (1) (1.0 mM) in the (a) absence and (b) presence of HClO4(5.0 mM) recorded in acetone at 5 K. The experimental parameters: microwave frequency = 9.646 GHz, microwave power = 1.0 mW and modulation frequency = 100 kHz.
Figure 5
Figure 5
Cyclic voltammograms (CV, solid line) and differential pulse voltammograms (DPV, dotted line) of 1 (2.0 mM) in the (a) absence and (b) presence of HClO4 (50 mM) in deaerated acetone at 298 K. TBAPF6 (0.20 M) was used as an electrolyte.
Figure 6
Figure 6
(a) Plot of kobs vs [Me2Fc] in the electron transfer from Me2Fc to [CuII2(XYLO)(OH)](PF6)2 (1) (0.10 mM) in presence of HClO4(40 mM) in acetone at 298 K. (b) Plot of kobs vs [Fc] in the electron transfer from Fc to 1 (0.10 mM) in presence of HClO4(40 mM) in acetone at 298 K.
Figure 7
Figure 7
Formation of the hydroperoxo complex, [CuII2(XYLO)(OOH)]2+ (λmax= 395 nm) in the reaction of [CuI2(XYL-OH)]2+(0.11 mM) with O2 in acetone at 193 K.
Figure 8
Figure 8
(a) UV-visible spectra indicating the reversible nature of dioxygen binding to [Cu2I(XYLOH)]2+(3). Bubbling O2 into an acetone solution of 3 produces [Cu2II(XYLO)(OOH)]2+ (4) at 193 K (black, solid line) (inset zoom view). Increasing the temperature up to 223 K produces dark yellow solid spectrum. After cooling to 193 K again gives black dotted spectrum. (b) van’t Hoff plot to determine the activation parameters, enthalpy and entropy, in the dioxygen binding to [Cu2I(XYLOH)]2+ in acetone.
Figure 9
Figure 9
Formation of Fc*+ by addition of Fc* (0.25 mM) and HClO4 (40 mM) to the hydroperoxo complex (0.060 mM) (generated by O2 bubbling to the solution of [Cu2I(XYLOH)]2+ (0.060 mM)) at 193 K.
Figure 10
Figure 10
(a) UV-vis spectral changes observed in the reaction of H2O2 (0.033 mM) with [Cu2I(OH)]2+ (0.025 mM) in acetone at 298 K. Inset shows the time profile monitored at 378 nm due to the formation of [Cu2II(XYLO)(OH)]2+. (b) Plot of kobs vs [H2O2] in the reaction of H2O2 with [Cu2I(OH)]2+ (0.025 mM) in acetone at 298 K.
Figure 11
Figure 11
EPR spectra of (a) [CuII2(XYLO)(OH)](PF6)2 (1) (0.040 mM) with HClO4(40 mM) and (b, c) the reaction solution of 1 (0.040 mM) with Me2Fc (10 mM) in the presence of HClO4(40 mM) in O2-saturated acetone at 298 K [(b) during the reaction and (c) after completion of the reaction]. Spectra were recorded at 20 K. The experimental parameters: microwave frequency = 9.654 GHz, microwave power = 1.0 mW and modulation frequency = 100 kHz.
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