Nonaqueous Catalytic Water Oxidation

Chen, Zuofeng; Concepcion, Javier J.; Luo, Hanlin; Hull, Jonathan F.; Paul, Amit; Meyer, Thomas J.

doi:10.1021/ja107347n

Cited by 146 publications

(159 citation statements)

References 25 publications

(17 reference statements)

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“…It shows that the nucleophilic attack proceeds via a concerted mechanism, in which the formation of the O-O bond and the deprotonation of the solvent water molecule happen concurrently (Fig. S2), in analogy with what has been proposed for single-center Ru complexes (28).…”

Section: Resultssupporting

confidence: 73%

Water oxidation surface mechanisms replicated by a totally inorganic tetraruthenium–oxo molecular complex

Piccinin

Sartorel

Aquilanti

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

106

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Solar-to-fuel energy conversion relies on the invention of efficient catalysts enabling water oxidation through low-energy pathways. Our aerobic life is based on this strategy, mastered by the natural Photosystem II enzyme, using a tetranuclear Mn-oxo complex as oxygen evolving center. Within artificial devices, water can be oxidized efficiently on tailored metal-oxide surfaces such as RuO 2 . The quest for catalyst optimization in vitro is plagued by the elusive description of the active sites on bulk oxides. Although molecular mimics of the natural catalyst have been proposed, they generally suffer from oxidative degradation under multiturnover regime. Here we investigate a nano-sized Ru 4 -polyoxometalate standing as an efficient artificial catalyst featuring a totally inorganic molecular structure with enhanced stability. Experimental and computational evidence reported herein indicates that this is a unique molecular species mimicking oxygenic RuO 2 surfaces. Ru 4 -polyoxometalate bridges the gap between homogeneous and heterogeneous water oxidation catalysis, leading to a breakthrough system. Density functional theory calculations show that the catalytic efficiency stems from the optimal distribution of the free energy cost to form reaction intermediates, in analogy with metal-oxide catalysts, thus providing a unifying picture for the two realms of water oxidation catalysis. These correlations among the mechanism of reaction, thermodynamic efficiency, and local structure of the active sites provide the key guidelines for the rational design of superior molecular catalysts and composite materials designed with a bottom-up approach and atomic control.artificial photosynthesis | ab initio simulations | X-ray absorption spectroscopy | electrocatalysis P hotocatalytic water splitting offers a bioinspired strategy for replacing fossil fuels with clean energy vectors (1-3). The overall reaction entails a sequence of light-promoted electron and proton transfers coupled with cleavage and formation of molecular bonds, ultimately splitting H 2 O molecules into O 2 and H 2 . The application of such technology for a viable solar-fuel economy is at the forefront of a very intense research effort. The main issue is the design and optimization of innovative catalysts enabling the half reaction of water oxidation (2H 2 O → O 2 + 4H + + 4e − ) (1) at low overpotential, high turnover frequencies, and longterm operation stability. Considering its high thermodynamic cost [E 0 = −1.23 V at pH = 0 vs. normal hydrogen electrode (NHE)] and mechanistic complexity, water oxidation catalysis (WOC) poses severe challenges for artificial photosynthesis applications.In plants, water oxidation is catalyzed by the Mn 4 CaO 4 oxygen evolving complex of Photosystem II (PSII) enzymes. The natural catalyst exhibits a functional asset of four redox active metal centers with adjacent μ-oxo bridges to enable water oxidation with a maximal turnover frequency of 400 s −1 per O 2 molecule (4). The drawback lies in the intrinsic weakness of the biol...

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Section: Resultssupporting

confidence: 73%

Water oxidation surface mechanisms replicated by a totally inorganic tetraruthenium–oxo molecular complex

Piccinin

Sartorel

Aquilanti

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

106

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“…Significant efforts have been undertaken to improve the activity and stability of homogeneous catalysts, as well as developing new ones [99,100]. The overall process for water splitting can best be described as two half reactions: water reduction [101] and water oxidation [102]. EPR spectroscopy has been used as a valuable tool in detecting radical intermediates involved in these reactions, especially for 1 st row transition metal-based catalysts.…”

Section: Photocatalytic Water Splittingmentioning

confidence: 99%

EPR Spectroscopy as a Tool in Homogeneous Catalysis Research

et al. 2015

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In the context of homogeneous catalysis, openshell systems are often quite challenging to characterize. Nuclear magnetic resonance (NMR) spectroscopy is the most frequently applied tool to characterize organometallic compounds, but NMR spectra are usually broad, difficult to interpret and often futile for the study of paramagnetic compounds. As such, electron paramagnetic resonance (EPR) has proven itself as a useful spectroscopic technique to characterize paramagnetic complexes and reactive intermediates. EPR spectroscopy is a particularly useful tool to investigate their electronic structures, which is fundamental to understand their reactivity. This paper describes some selected examples of studies where EPR spectroscopy has been useful for the characterization of open-shell organometallic complexes. The paper concentrates in particular on systems where EPR spectroscopy has proven useful to understand catalytic reaction mechanisms involving paramagnetic organometallic catalysts. The expediency of EPR spectroscopy in the study of organometallic chemistry and homogenous catalysis is contextualized in the introductory Sect. 1. Section 2 of the review focusses on examples of C-C and C-N bond formation reactions, with an emphasis on catalytic reactions where ligand/substrate non-innocence plays an important role. Both carbon and nitrogen centered radicals have been shown to play an important role in these reactions. A few selected examples of catalytic alcohol oxidation proceeding via related N-centered ligand radicals are included in this section as well. Section 3 covers examples of the use of EPR spectroscopy to study important commercial ethylene oligomerization and polymerization processes. In Sect. 4 the use of EPR spectroscopy to understand the mechanisms of Atom Transfer Radical Polymerization is discussed. While this review focusses predominantly on the application of EPR spectroscopy in mechanistic studies of C-C and C-N bond formation reactions mediated by organometallic catalysts, a few selected examples describing the application of EPR spectroscopy in other catalytic reactions such as water splitting, photo-catalysis, photo-redox-catalysis and related reactions in which metal initiated (free) radical formation plays a role are included as well. EPR spectroscopic investigation in this area of research are dominated by EPR spectroscopic studies in isotropic solution, including spin trapping experiments. These reactions are highlighted in Sect. 5. EPR spectroscopic studies have proven useful to discern the correct oxidation states of the active catalysts and also to determine the effective concentrations of the active species. EPR is definitely a spectroscopic technique that is indispensable in understanding the reactivity of paramagnetic complexes and in conjunction with other advanced techniques such as X-ray absorption spectroscopy and pulsed laser polymerization it will continue to be a very practical tool.

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“…1). Two different PDI structures are considered: The first includes 3,5-di-t-butylphenoxy groups at the 1,7 "bay region" positions, while the second generation of this compound uses 3,5-bis(trifluoromethyl)phenyl groups to make the PDI a more powerful photooxidant, as well as a 3-pentyl tail at one imide position to enhance solubility in highly polar organic solvents relevant to solar fuels applications, like acetonitrile or propylene carbonate (26). While both PDI variants undergo electron transfer with the iridium complex, the more highly oxidizing 1Ã PDI excited state in 2 will likely be necessary to access the putative higher-valent iridium(V) state implicated in the proposed catalytic cycle (21).…”

mentioning

confidence: 99%

Ultrafast photodriven intramolecular electron transfer from an iridium-based water-oxidation catalyst to perylene diimide derivatives

Vagnini

Smeigh

Blakemore

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

122

106

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Photodriving the activity of water-oxidation catalysts is a critical step toward generating fuel from sunlight. The design of a system with optimal energetics and kinetics requires a mechanistic understanding of the single-electron transfer events in catalyst activation. To this end, we report here the synthesis and photophysical characterization of two covalently bound chromophore-catalyst electron transfer dyads, in which the dyes are derivatives of the strong photooxidant perylene-3,4:9,10-bis(dicarboximide) (PDI) and the molecular catalyst is the Cp Ã IrðppyÞCl metal complex, where ppy ¼ 2-phenylpyridine. Photoexcitation of the PDI in each dyad results in reduction of the chromophore to PDI •− in less than 10 ps, a process that outcompetes any generation of 3Ã PDI by spin-orbit-induced intersystem crossing. Biexponential charge recombination largely to the PDI-Ir(III) ground state is suggestive of multiple populations of the PDI •− -IrðIVÞ ion-pair, whose relative abundance varies with solvent polarity. Electrochemical studies of the dyads show strong irreversible oxidation current similar to that seen for model catalysts, indicating that the catalytic integrity of the metal complex is maintained upon attachment to the high molecular weight photosensitizer.photoinduced electron transfer | solar fuels | ultrafast optical spectroscopy | water oxidation A rtificial photosynthetic systems for solar fuels generation must integrate the functions of light harvesting, charge separation, and catalysis, with water as the source of electrons for reductive fuel-forming chemistry (1-5). The design of a lightdriven water-splitting system based on molecular catalysts requires a fundamental understanding of the individual electron transfer steps involved in multielectron catalyst activation. To investigate the energetic and kinetic demands of coupling photodriven charge separation and catalysis, many research groups have studied the photophysical properties of covalently linked redox-active organic dyes and transition metal complexes (6-16). In several cases, electron transfer to or from the metal has been observed and characterized, though energy transfer and intersystem crossing to the chromophore triplet state can be significant competing processes. In this work, we use derivatives of perylene-3,4∶9,10-bis(dicarboximide) (PDI) linked to an iridium complex of the type Cp Ã IrðN-CÞX to demonstrate light-driven single-electron oxidation of a highly active molecular catalyst precursor for water oxidation.Derivatives of PDI are useful organic chromophores for solar device applications (5,17,18). Their high molar absorptivity, stability, low cost, ease of synthetic manipulation, and often advantageous self-assembly properties have led to their use in molecular electronics and a variety of solar energy conversion systems (19). For solar fuels applications, the mild reduction potentials of PDI derivatives make their excited states powerful photooxidants, though there are only a few literature examples in which 1Ã PDI is used to ...

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Nonaqueous Catalytic Water Oxidation

Cited by 146 publications

References 25 publications

Water oxidation surface mechanisms replicated by a totally inorganic tetraruthenium–oxo molecular complex

Water oxidation surface mechanisms replicated by a totally inorganic tetraruthenium–oxo molecular complex

EPR Spectroscopy as a Tool in Homogeneous Catalysis Research

Ultrafast photodriven intramolecular electron transfer from an iridium-based water-oxidation catalyst to perylene diimide derivatives

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