Research paperFour-electron reduction of dioxygen to water by a trinuclear copper complex
Graphical abstract
A trinuclear copper complex 1 performs the catalytic 4e−/4H+ reduction of dioxygen to water with a rate determining O2-binding step. The mechanism of the O2-reduction by the mononuclear complex, in contrast, does not involve O2 binding in the rate determining step.
Introduction
The catalytic four-electron reduction of dioxygen (O2) to water has tremendous technological significance, particularly in fuel cell applications [1]. For example, in fuel cells, the four-electron reduction of O2 is catalyzed at the cathode by platinum impregnated in carbon [2]. The high loadings of this precious metal that are required to achieve appreciable reactivity have prompted considerable activity in the development of catalysts based on nonprecious metals [3]. In biology, multicopper oxidases, such as laccase and bilirubin oxidase, activate dioxygen at an active site containing a three-plus-one arrangement of four Cu atoms, exhibiting remarkable activity for the four-electron/four-proton reduction of O2 to H2O [4]. Inspired by multicopper oxidases, various copper complexes have been investigated to examine the (electro)catalytic activity for the two-electron/two-proton reduction of O2 to H2O2 as well as the four-electron/four-proton reduction of O2 to H2O [5]. However, in contrast to nature, which mainly uses polynuclear copper centers for the activation of dioxygen, the artificial catalysts are mainly restricted to mono- and di-nuclear copper centers only. Although, mononuclear diamine complexes have been shown to self-assemble upon reaction with O2 to form a CuII2CuIII(μ3-O)2 species, [6] reduction of these trinuclear cores led to dinuclear species, thereby, hindering attempts for developing catalytic versions of this reaction. An alternate strategy has been the employment of trinucleating frameworks [7]. Most systems investigated are rather flexible and do not facilitate cooperative trinuclear reactivity. The only exception is the trinuclear CuI3Y complex [8] supported by a ligand framework that was selectively templated by yttrium(III) binding to a tripodal [O3N4]-trisphenoxide-trisimine-amine moiety. This complex showed cooperative O2 binding to generate a CuII2CuIII(μ3-O) core; however, no catalytic reduction of dioxygen to water was reported.
In our effort to rationalize nature’s preference of employing trinuclear copper centers in the reduction of dioxygen, we now report the synthesis and characterization of a novel trinuclear ligand, tateb, (Scheme 1; tateb = 1,3,5-tris-3,3-iminobis(N,N-dimethylpropylamine)-2,4,6-triethylbenzene) and its Cu(I) complex [Cu3(tateb)](OTf)3 (1), which performs a catalytic 4e−/4H+ reduction of O2 to water in the presence of decamethylferrocene (Fc∗) as a one-electron reductant and trifluoroacetic acid (TFA) as a proton source.
The mechanisms of the reaction are elucidated on the basis of detailed kinetic studies of the overall catalytic reactions as well as individual catalytic steps and the characterization of various copper-dioxygen intermediates, which are formed during the catalytic cycle.
Section snippets
Synthesis and characterization of the tateb ligand and its copper complex, 1
The generation of the tateb ligand was performed via coupling of the commercially available triamine (3,3′-iminobis(N,N-dimethylpropylamine)) ligand with 1,3,5-triethyl-2,4,6-bromomethyl-benzene in a single step in moderate yields (41%). Notably, the three triamine functional groups at the 2,4,6-positions are enforced to positioning at the same side of the benzene ring by the 1,3,5-triethylbenzene spacer, as evident from the 1H NMR spectrum of tateb (Fig. 1). Combination of tateb with 1.5
Conclusions
In summary, we report the synthesis of a trinuclear copper complex 1 that can act as a catalyst for the 4e−/4H+ reduction of dioxygen to water. To the best of our knowledge complex 1 represents the only example of a biomimetic model of the tri-nuclear copper center in multicopper oxidases. Detailed spectroscopic and kinetic studies provide evidence for a cooperative activation of dioxygen during the 1-catalyzed reduction of dioxygen to water; the dioxygen binding step is nevertheless the
General techniques and chemicals employed
All studies and measurements were performed in the Institut für Chemie at the Humboldt Universität zu Berlin, unless otherwise stated. All the chemicals employed were purchased from SIGMA-ALDRICH and used without further purification unless required. Anhydrous solvents were purchased from CARL-ROTH GMBH under the tradename ROTIDRY (>99.5%, <50 ppm H2O) and degassed before use. Deuterated solvents were purchased from EURISO-TOP.
Compounds sensitive to air or water were prepared and handled under
Acknowledgements
Financial support from the Deutsche Forschungsgemeinschaft (Cluster of Excellence “Unifying Concepts in Catalysis”; EXC 314-2) is gratefully acknowledged. K.R. also thanks the Heisenberg-Program of the Deutsche Forschungsgemeinschaft for financial support. J.E. thanks NTU for funding. XAS measurements on SSRL beamline 9-3 were made possible by the US Department of Energy Office of Science (contract DE-AC02-76SF00515 to SLAC National Accelerator Laboratory) and the National Institutes of Health (
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