Spectroscopic, Electrochemical and Computational Characterisation of Ru Species Involved in Catalytic Water Oxidation: Evidence for a [Ru(V) (O)(Py2 (Me) tacn)] Intermediate

Heterogenization of Molecular Water Oxidation Catalysts in Electrodes for (Photo)Electrochemical Water Oxidation

Water oxidation is still one of the most important challenges to develop efficient artificial photosynthetic devices. In recent decades, the development and study of molecular complexes for water oxidation have allowed insight into the principles governing catalytic activity and the mechanism as well as establish ligand design guidelines to improve performance. However, their durability and long-term stability compromise the performance of molecular-based artificial photosynthetic devices. In this context, heterogenization of molecular water oxidation catalysts on electrode surfaces has emerged as a promising approach for efficient long-lasting water oxidation for artificial photosynthetic devices. This review covers the state of the art of strategies for the heterogenization of molecular water oxidation catalysts onto electrodes for (photo)electrochemical water oxidation. An overview and description of the main binding strategies are provided explaining the advantages of each strategy and their scope. Moreover, selected examples are discussed together with the the differences in activity and stability between the homogeneous and the heterogenized system when reported. Finally, the common design principles for efficient (photo)electrocatalytic performance summarized.

XAS and EPR in Situ Observation of Ru(V) Oxo Intermediate in a Ru Water Oxidation Complex**

In this study, we combine in situ spectroelectrochemistry coupled with electron paramagnetic resonance (EPR) and X‐ray absorption spectroscopies (XAS) to investigate a molecular Ru‐based water oxidation catalyst bearing a polypyridinic backbone [Ru^II(OH₂)(Py₂Metacn)]²⁺. Although high valent key intermediate species arising in catalytic cycles of this family of compounds have remain elusive due to the lack of additional anionic ligands that could potentially stabilize them, mechanistic studies performed on this system proposed a water nucleophilic attack (WNA) mechanism for the O−O bond formation. Employing in situ experimental conditions and complementary spectroscopic techniques allowed to observe intermediates that provide support for a WNA mechanism, including for the first time a Ru(V) oxo intermediate based on the Py₂Metacn ligand, in agreement with the previously proposed mechanism.

Highly selective oxidation of organic contaminants in the RuIII-activated peroxymonosulfate process: The dominance of RuVO species

Ruthenium (Ru^III)-activated peroxymonosulfate (the Ru^III/PMS process) is one of the most efficient PMS-based advanced oxidation processes for the abatement of organic contaminants. Here we interestingly found that phenyl methyl sulfoxide (PMSO) was significantly oxidized to phenyl methyl sulfone (PMSO₂) in the Ru^III/PMS process at the pH range of 3.0-6.0, with the conversion ratio of ΔPMSO to ΔPMSO₂ was close to 100%, which favored the dominance of high-valent ruthenium-oxo species (Ru^VO) instead of the widely-recognized radicals (i.e, hydroxyl radical and sulfate radical). Scavenging experiments further indicated that Ru^VO was unreactive to tert-butyl alcohol, but could be scavenged by methanol and dimethyl sulfoxide. Besides, sulfamethoxazole, acetaminophen, carbamazepine, diclofenac, 2,4,6-trichlorophenol were readily degraded in the Ru^III/PMS process, but atrazine, ibuprofen, benzoic acid and 4-nitrobenzoic acid were barely removed, suggesting the high selectivity of Ru^VO species. This study enriched the understandings on the mechanism of Ru^III-mediated PMS activation and the nature of Ru^VO species.

Isolation of a Ru(IV) side-on peroxo intermediate in the water oxidation reaction

The electrons that nature uses to reduce CO₂ during photosynthesis come from water oxidation at the oxygen-evolving complex of photosystem II. Molecular catalysts have served as models to understand its mechanism, in particular the O-O bond-forming reaction, which is still not fully understood. Here we report a Ru(IV) side-on peroxo complex that serves as a 'missing link' for the species that form after the rate-determining O-O bond-forming step. The Ru(IV) side-on peroxo complex (η²-1^iv-OO) is generated from the isolated Ru(IV) oxo complex (1^iv=O) in the presence of an excess of oxidant. The oxidation (IV) and spin state (singlet) of η²-1^iv-OO were determined by a combination of experimental and theoretical studies. ¹⁸O- and ²H-labelling studies evidence the direct evolution of O₂ through the nucleophilic attack of a H₂O molecule on the highly electrophilic metal-oxo species via the formation of η²-1^iv-OO. These studies demonstrate water nucleophilic attack as a viable mechanism for O-O bond formation, as previously proposed based on indirect evidence.

Redox-Active Ligand Assisted Catalytic Water Oxidation by a RuIV =O Intermediate

Water splitting is one of the most promising solutions for storing solar energy in a chemical bond. Water oxidation is still the bottleneck step because of its inherent difficulty and the limited understanding of the O-O bond formation mechanism. Molecular catalysts provide a platform for understanding this process in depth and have received wide attention since the first Ru-based catalyst was reported in 1982. Ru^V =O is considered a key intermediate to initiate the O-O bond formation through either a water nucleophilic attack (WNA) pathway or a bimolecular coupling (I2M) pathway. Herein, we report a Ru-based catalyst that displays water oxidation reactivity with Ru^IV =(O) with the help of a redox-active ligand at pH 7.0. The results of electrochemical studies and DFT calculations disclose that ligand oxidation could significantly improve the reactivity of Ru^IV =O toward water oxidation. Under these conditions, sustained water oxidation catalysis occurs at reasonable rates with low overpotential (ca. 183 mV).

Design of an inherently-stable water oxidation catalyst

While molecular water-oxidation catalysts are remarkably rapid, oxidative and hydrolytic processes in water can convert their active transition metals to colloidal metal oxides or hydroxides that, while quite reactive, are insoluble or susceptible to precipitation. In response, we propose using oxidatively-inert ligands to harness the metal oxides themselves. This approach is demonstrated by covalently attaching entirely inorganic oxo-donor ligands (polyoxometalates) to 3-nm hematite cores, giving soluble anionic structures, highly resistant to aggregation, yet thermodynamically stable to oxidation and hydrolysis. Using orthoperiodate (at pH 8), and no added photosensitizers, the hematite-core complex catalyzes visible-light driven water oxidation for seven days (7600 turnovers) with no decrease in activity, far exceeding the documented lifetimes of molecular catalysts under turnover conditions in water. As such, a fundamental limitation of molecular complexes is entirely bypassed by using coordination chemistry to harness a transition-metal oxide as the reactive center of an inherently stable, homogeneous water-oxidation catalyst.

A current challenge in the development of molecular water oxidation catalysts is to overcome their inherent susceptibilities to oxidative or hydrolytic degradation under turnover conditions in water. Here, the authors design an inherently-stable water oxidation catalyst using oxidatively-inert ligands to harness a reactive metal oxide.

Pushing the photodelivery of nitric oxide to the visible: are {FeNO}7 complexes good candidates?

Photodelivery of NO requires stable compounds which can be made reactive by irradiation with (visible) light. Traditional {MNO}⁶ complexes require a substantial ligand design to shift their absorption spectra to the appropriate region of the electromagnetic spectrum. [Fe((CH₂Py₂)₂Me[9]aneN₃)(NO)](BF₄)₂ is a new {FeNO}⁷ octahedral coordination compound, which is thermally and air-stable in solution. Illumination with a 450 nm light source induces significant photodetachment of the coordinated NO (ϕ_NO = 0.52 mol einstein^-1), suggesting that {FeNO}⁷ compounds can be in fact suitable compounds for therapeutic NO-photorelease.

Synthesis, characterization and solution behavior of a systematic series of pentapyridyl-supported RuII complexes: comparison to bimetallic analogs

A series of RuII complexes stabilized with the pentapyridyl ligand Py5Me2 (Py5Me2 = 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine) and with an axial X ligand (X = Cl-, H2O, N3-, MeCN) were prepared and characterized in the solid state and in non-aqueous solution. The cyclic voltammograms of these complexes in MeCN reflect a reversible substitution of the axial X ligand with MeCN. Irreversible ligand substitution of [(Py5Me2)RuN3]+ is also observed in propylene carbonate, but only at oxidizing potentials that decompose the azide ligand. The monometallic chloride and azide species are compared with analogous Ru2 metal-metal bonded complexes, which have been reported to undergo irreversible chloride dissociation upon reduction.

Mechanistic Investigation of Water Oxidation Catalyzed by Uniform, Assembled MnO Nanoparticles

The development of active water oxidation catalysts is critical to achieve high efficiency in overall water splitting. Recently, sub-10 nm-sized monodispersed partially oxidized manganese oxide nanoparticles were shown to exhibit not only superior catalytic performance for oxygen evolution, but also unique electrokinetics, as compared to their bulk counterparts. In the present work, the water-oxidizing mechanism of partially oxidized MnO nanoparticles was investigated using integrated in situ spectroscopic and electrokinetic analyses. We successfully demonstrated that, in contrast to previously reported manganese (Mn)-based catalysts, Mn(III) species are stably generated on the surface of MnO nanoparticles via a proton-coupled electron transfer pathway. Furthermore, we confirmed as to MnO nanoparticles that the one-electron oxidation step from Mn(II) to Mn(III) is no longer the rate-determining step for water oxidation and that Mn(IV)═O species are generated as reaction intermediates during catalysis.

O–O bond formation in ruthenium-catalyzed water oxidation: single-site nucleophilic attack vs. O–O radical coupling

A review of water oxidation by ruthenium-based molecular catalysts, with emphasis on the mechanism of O–O bond formation.