A Ru-Hbpp-based water-oxidation catalyst anchored on rutile TiO2

Molecular Mechanism of the Mononuclear Copper Complex-Catalyzed Water Oxidation from Cluster-Continuum Model Calculations

Cluster-continuum model calculations were conducted to decipher the mechanism of water oxidation catalyzed by a mononuclear copper complex. Among various O-O bond formation mechanisms investigated in this study, the most favorable pathway involved the nucleophilic attack of OH^- onto the ^.+ L-Cu^II -OH^- intermediate. During such process, the initial binding of OH^- to the proximity of ^.+ L-Cu^II -OH^- would result in the spontaneous oxidation of OH^- , leading to OH⋅ radical and Cu^II -OH^- species. The further O-O coupling between OH⋅ radical and Cu^II -OH^- was associated with a barrier of 14.8 kcal mol^-1 , leading to the formation of H₂ O₂ intermediate. Notably, the formation of "Cu^III -O^.- " species, a widely proposed active species for O-O bond formation, was found to be thermodynamically unfavorable and could be bypassed during the catalytic reactions. On the basis the present calculations, a catalytic cycle of the mononuclear copper complex-catalyzed water oxidation was proposed.

Mechanistic Insight into the O2 Evolution Catalyzed by Copper Complexes with Tetra- and Pentadentate Ligands

The mononuclear complexes ([(bztpen)Cu] (BF₄)₂ (bztpen = N-benzyl-N,N',N'-tris (pyridin-2-yl methyl ethylenediamine))) and ([(dbzbpen)Cu(OH₂)] (BF₄)₂ (dbzbpen = N,N'-dibenzyl-N,N'-bis(pyridin-2-ylmethyl) ethylenediamine)) have been reported as water oxidation catalysts in basic medium (pH = 11.5). We explore the O₂ evolution process catalyzed by these copper catalysts with various ligands (L) by applying the first-principles molecular dynamics simulations. First, the oxidation of catalysts to the metal-oxo intermediates [LCu(O)]²⁺ occurs through the proton-coupled electron transfer (PCET) process. These intermediates are involved in the oxygen-oxygen bond formation through the water-nucleophilic addition process. Here, we have considered two types of oxygen-oxygen bond formation. The first one is the transfer of the hydroxide of the water molecule to the Cu═O moiety; the proton transfer to the solvent leads to the formation of the peroxide complex ([LCu(OOH)]⁺). The other is the formation of the hydrogen peroxide complex ([LCu(HOOH)]²⁺) by the transfer of proton and hydroxide of the water molecule to the metal-oxo intermediate. The formation of the peroxide complex requires less activation free energy than hydrogen peroxide formation for both catalysts. We found two transition states in the well-tempered metadynamics simulations: one for proton transfer and another for hydroxide transfer. In both cases, the proton transfer requires higher free energy. Following the formation of the oxygen-oxygen bond, we study the release of the dioxygen molecule. The formed peroxide and hydrogen peroxide complexes are converted into the superoxide complex ([LCu(OO)]²⁺) through the transfer of proton, electron, and PCET processes. The superoxide complex releases an oxygen molecule upon the addition of a water molecule. The free energy of activation for the release of the dioxygen molecule is lesser than that of the oxygen-oxygen bond formation. When we observe the entire water oxidation process, the oxygen-oxygen bond formation is the rate-determining step. We calculated the rates of reaction by using the Eyring equation and found them to be close to the experimental values.

A Calix[4]arene-Based Cyclic Dinuclear Ruthenium Complex for Light-Driven Catalytic Water Oxidation

A cyclic dinuclear ruthenium(bda) (bda: 2,2'-bipyridine-6,6'-dicarboxylate) complex equipped with oligo(ethylene glycol)-functionalized axial calix[4]arene ligands has been synthesized for homogenous catalytic water oxidation. This novel Ru(bda) macrocycle showed significantly increased catalytic activity in chemical and photocatalytic water oxidation compared to the archetype mononuclear reference [Ru(bda)(pic)2 ]. Kinetic investigations, including kinetic isotope effect studies, disclosed a unimolecular water nucleophilic attack mechanism of this novel dinuclear water oxidation catalyst (WOC) under the involvement of the second coordination sphere. Photocatalytic water oxidation with this cyclic dinuclear Ru complex using [Ru(bpy)3 ]Cl2 as a standard photosensitizer revealed a turnover frequency of 15.5 s-1 and a turnover number of 460. This so far highest photocatalytic performance reported for a Ru(bda) complex underlines the potential of this water-soluble WOC for artificial photosynthesis.

Ruthenium Nanoparticles for Catalytic Water Splitting

Both global warming and limited fossil resources make the transition from fossil to solar fuels an urgent matter. In this regard, the splitting of water activated by sunlight is a sustainable and carbon-free new energy conversion scheme able to produce efficient technological devices. The availability of appropriate catalysts is essential for the proper kinetics of the two key processes involved, namely, the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). During the last decade, ruthenium nanoparticle derivatives have emerged as true potential substitutes for the state-of-the-art platinum and iridium oxide species for the HER and OER, respectively. Thus, after a summary of the most common methods for catalyst benchmarking, this review covers the most significant developments of ruthenium-based nanoparticles used as catalysts for the water-splitting process. Furthermore, the key factors that govern the catalytic performance of these nanocatalysts are discussed in view of future research directions.

Artificial photosynthesis: opportunities and challenges of molecular catalysts

Molecular catalysis plays an essential role in both natural and artificial photosynthesis (AP). However, the field of molecular catalysis for AP has gradually declined in recent years because of doubt about the long-term stability of molecular-catalyst-based devices. This review summarizes the development history of molecular-catalyst-based AP, including the fundamentals of AP, molecular catalysts for water oxidation, proton reduction and CO2 reduction, and molecular-catalyst-based AP devices, and it provides an analysis of the advantages, challenges, and stability of molecular catalysts. With this review, we aim to highlight the following points: (i) an investigation on molecular catalysis is one of the most promising ways to obtain atom-efficient catalysts with outstanding intrinsic activities; (ii) effective heterogenization of molecular catalysts is currently the primary challenge for the application of molecular catalysis in AP devices; (iii) development of molecular catalysts is a promising way to solve the problems of catalysis involved in practical solar fuel production. In molecular-catalysis-based AP, much has been attained, but more challenges remain with regard to long-term stability and heterogenization techniques.

Production of solar chemicals: gaining selectivity with hybrid molecule/semiconductor assemblies

Research on the production of solar fuels and chemicals has rocketed over the past decade, with a wide variety of systems proposed to harvest solar energy and drive chemical reactions. In this Feature Article we have focused on hybrid molecule/semiconductor assemblies in both powder and supported materials, summarising recent systems and highlighting the enormous possibilities offered by such assemblies to carry out highly demanding chemical reactions with industrial impact. Of relevance is the higher selectivity obtained in visible light-driven organic transformations when using molecular catalysts compared to photocatalytic materials.

How to make an efficient and robust molecular catalyst for water oxidation

The key factors to design an efficient and rugged molecular water oxidation catalyst (Mol-WOC) are reviewed and discussed.

Water oxidation catalysis with ligand substituted Ru–bpp type complexes

The influence of electronic effects over Ru–bpp water oxidation catalysts.

Incorporation of a ruthenium–bis(pyridine)pyrazolate (Ru–bpp) water oxidation catalyst in a hexametallic macrocycle

New terpyridine-functionalised analogues of the in,in-[{Ru^II(trpy)}₂(μ-bpp)(H₂O)₂]³⁺ water oxidation catalyst (bpp = bis-(2-pyridyl)pyrazolate) have been synthesised and used to create a hexametallic {Fe₂Ru₄} macrocycle.

Oxygen-oxygen bond formation pathways promoted by ruthenium complexes

The photoproduction of hydrogen from water and sunlight represents an attractive means of artificial energy conversion for a world still largely dependent on fossil fuels. A practical technology for producing sun-derived hydrogen remains an unachieved goal, however, and is dependent on developing a better understanding of the key reaction, the oxidation of water to dioxygen. The molecular complexity of this process is such that sophisticated transition metal complexes, which can access low-energy reaction pathways, are considered essential as catalysts. Complexes based on Mn, Co, Ir, and Ru have been described recently; a variety of ligands and nuclearities that comprise many complex topologies have been developed, but very few of them have been studied from a mechanistic perspective. One step in particular needs to be understood and better characterized for the transition-metal-catalyzed oxidation of water to dioxygen, namely, the circumstances under which the formation of O-O bonds can occur. Although there is a large body of work related to the formation of C-C bonds promoted by metal complexes, the analogous literature for O-O bond formation is practically nonexistent and just beginning to emerge. In this Account, we describe the sparse literature existing on this topic, focusing on the Ru-aqua complexes. These complexes are capable of reaching high oxidation states as a result of the sequential and simultaneous loss of protons and electrons. A solvent water molecule may or may not participate in the formation of the O-O bond; accordingly, the two main pathways are named (i) solvent water nucleophilic attack (WNA) and (ii) interaction of two M-O units (I2M). Most of the complexes described belong to the WNA class, including a variety of mononuclear and polynuclear complexes containing one or several Ru-O units. A common feature of these complexes is the generation of formal oxidation states as high as Ru(V) and Ru(VI), which render the oxygen atom of the Ru-O group highly electrophilic. On the other hand, only one symmetric dinuclear complex that undergoes an intramolecular O-O bond formation step has been described for the I2M class; it has a formal oxidation state of Ru(IV). A special section is devoted to Ru-OH(2) complexes that contain redox active ligands, such as the chelating quinone. These ligands are capable of undergoing reversible redox processes and thus generate a complex but fascinating electron-transfer process between the metal and the ligand. Despite the intrinsic experimental difficulties in determining reaction mechanisms, progress with these Ru complexes is now beginning to be reported. An understanding of recent successes, as well as pitfalls, is essential in the search for a practical water oxidation catalyst.