Preparation and study of a family of dinuclear Ru(II) complexes that catalyze the decomposition of water

Plight of CORMs: The unreliability of four commercially available CO-releasing molecules, CORM-2, CORM-3, CORM-A1, and CORM-401, in studying CO biology

Carbon monoxide (CO) is an endogenously produced gaseous signaling molecule with demonstrated pharmacological effects. In studying CO biology, three delivery forms have been used: CO gas, CO in solution, and CO donors of various types. Among the CO donors, four carbonyl complexes with either a transition metal ion or borane (BH3) (termed CO-releasing molecules or CORMs) have played the most prominent roles appearing in over 650 publications. These are CORM-2, CORM-3, CORM-A1, and CORM-401. Intriguingly, there have been unique biology findings that were only observed with these CORMs, but not CO gas; yet these properties were often attributed to CO, raising puzzling questions as to why CO source would make such a fundamental difference in terms of CO biology. Recent years have seen a large number of reports of chemical reactivity (e.g., catalase-like activity, reaction with thiol, and reduction of NAD(P)+) and demonstrated CO-independent biological activity for these four CORMs. Further, CORM-A1 releases CO in an idiosyncratic fashion; CO release from CORM-401 is strongly influenced or even dependent on reaction with an oxidant and/or a nucleophile; CORM-2 mostly releases CO2, not CO, after a water-gas shift reaction except in the presence of a strong nucleophile; and CORM-3 does not release CO except in the presence of a strong nucleophile. All these beg the question as to what constitutes an appropriate CO donor for studying CO biology. This review critically summarizes literature findings related to these aspects, with the aim of helping result interpretation when using these CORMs and development of essential criteria for an appropriate donor for studying CO biology.

Bio-Inspired Molecular Catalysts for Water Oxidation

The catalytic tetranuclear manganese-calcium-oxo cluster in the photosynthetic reaction center, photosystem II, provides an excellent blueprint for light-driven water oxidation in nature. The water oxidation reaction has attracted intense interest due to its potential as a renewable, clean, and environmentally benign source of energy production. Inspired by the oxygen-evolving complex of photosystem II, a large of number of highly innovative synthetic bio-inspired molecular catalysts are being developed that incorporate relatively cheap and abundant metals such as Mn, Fe, Co, Ni, and Cu, as well as Ru and Ir, in their design. In this review, we briefly discuss the historic milestones that have been achieved in the development of transition metal catalysts and focus on a detailed description of recent progress in the field.

Revisiting Dinuclear Ruthenium Water Oxidation Catalysts: Effect of Bridging Ligand Architecture on Catalytic Activity

An attractive catalytic pathway for the conversion of water to oxygen would involve two metal oxide centers combining in a constructive sense to make O═O. This prospect makes the study of certain dinuclear transition metal complexes particularly attractive. In this work, we describe the design and synthesis of two symmetrical bis-tridentate polypyridine ligands 6 and 12 that bind two Ru^II centers at a separation of 3.6 Å in 7 and 5.7 Å in 13. In the presence of Ce^IV at pH = 1, these systems oxidize water with the system having the more proximal metals being more reactive. In the case of the more proximal metal centers, the bridging ligand is a 3,6-disubstituted pyridazine which, under the influence of Ce^IV, cleaves into two [Ru(bpc)(pic)₂CH₃CN]⁺ fragments (14) which then function as the actual catalyst (bpc = 2,2'-bipyridine-6-carboxylate, pic = 4-methylpyridine). The second dinuclear catalyst contains a central pyrimidine ring which is less sensitive to oxidative decay and hence less reactive. Caution is advised in the use of Ce^IV as a sacrificial electron acceptor due to unexpected oxidative decay of the catalyst.

Mononuclear Ruthenium-Based Water Oxidation Catalyst Supported by Anionic, Redox-Non-Innocent Ligand: Heterometallic O-O Bond Formation via Radical Coupling Pathway

Cerium(IV)-driven water oxidation catalysis mediated by a mononuclear ruthenium(III) complex, [Ru(L)(pic)₃] (H₃L = 2,2'-iminodibenzoic acid, pic = 4-methylpyridine), has been demonstrated in this work. The mechanistic details of water oxidation have been investigated by the combined use of spectroscopy, electrochemistry, kinetic analysis, and computational studies. It was found that water oxidation proceeds via formal high-valent Ru^VII species. The capability of accessing such a high-valent state is derived from the non-innocent behavior of the anionic tridentate ligand frame which helps in accumulation of oxidative equivalents in cooperation with metal center. This metal-ligand cooperation facilitates the multi-electron-transfer reaction such as water oxidation. Kinetic analysis suggests water oxidation at a single site of Ru where O-O bond formation occurs via radical-radical coupling pathway between the oxygen atom of ruthenium-oxo species and the oxygen atom of the hydroxocerium(IV) ion.

X-ray Photoelectron Fingerprints of High-Valence Ruthenium-Oxo Complexes along the Oxidation Reaction Pathway in an Aqueous Environment

Recent advances in operando-synchrotron-based X-ray techniques are making it possible to address fundamental questions related to complex proton-coupled electron transfer reactions, for instance, the electrocatalytic water splitting process. However, it is still a grand challenge to assess the ability of the different techniques to characterize the relevant intermediates, with minimal interference on the reaction mechanism. To this end, we have developed a novel methodology employing X-ray photoelectron spectroscopy (XPS) in connection with the liquid-jet approach to probe the electrochemical properties of a model electrocatalyst, [Ru^II(bpy)₂(py)(OH₂)]²⁺, in an aqueous environment. There is a unique fingerprint of the extremely important higher-valence ruthenium-oxo species in the XPS spectra along the oxidation reaction pathway. Furthermore, a sequential method combining quantum mechanics and molecular mechanics is used to illuminate the underlying physical chemistry of such systems. This study provides the basis for the future development of in-operando XPS techniques for water oxidation reactions.

Recent Advances in the Development of Molecular Catalyst-Based Anodes for Water Oxidation toward Artificial Photosynthesis

Catalytic water oxidation represents a bottleneck for developing artificial photosynthetic systems that store solar energy as renewable fuels. A variety of molecular water oxidation catalysts (WOCs) have been reported over the last two decades. In view of their applications in artificial photosynthesis devices, it is essential to immobilize molecular catalysts onto the surfaces of conducting/semiconducting supports for fabricating efficient and stable water oxidation anodes/photoanodes. Molecular WOC-based anodes are essential for developing photovoltaic artificial photosynthesis devices and, moreover, the performance of molecular WOC on anodes will provide important insight into designing extended molecular WOC-based photoanodes for photoelectrochemical (PEC) water oxidation. This Review concerns recent progress in the development of molecular WOC-based anodes over the last two decades and looks at the prospects for using such anodes in artificial photosynthesis.

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.

Ru-bda: Unique Molecular Water-Oxidation Catalysts with Distortion Induced Open Site and Negatively Charged Ligands

A water-oxidation catalyst with high intrinsic activity is the foundation for developing any type of water-splitting device. To celebrate its 10 years anniversary, in this Perspective we focus on the state-of-the-art molecular water-oxidation catalysts (MWOCs), the Ru-bda series (bda = 2,2'-bipyridine-6,6'-dicarboxylate), to offer strategies for the design and synthesis of more advanced MWOCs. The O-O bond formation mechanisms, derivatives, applications, and reasons behind the outstanding catalytic activities of Ru-bda catalysts are summarized and discussed. The excellent performance of the Ru-bda catalyst is owing to its unique structural features: the distortion induced 7-coordination and the carboxylate ligands with coordination flexibility, proton-transfer function as well as small steric hindrance. Inspired by the Ru-bda catalysts, we emphasize that the introduction of negatively charged groups, such as the carboxylate group, into ligands is an effective strategy to lower the onset potential of MWOCs. Moreover, distortion of the regular configuration of a transition metal complex by ligand design to generate a wide open site as the catalytic site for binding the substrate as an extra-coordination is proposed as a new concept for the design of efficient molecular catalysts. These inspirations can be expected to play a great role in not only water-oxidation catalysis but also other small molecule activation and conversion reactions involving artificial photosynthesis, such as CO₂ reduction and N₂ fixation reactions.

An Overview of Significant Achievements in Ruthenium-Based Molecular Water Oxidation Catalysis

Fossil fuels (coal, oil, natural gas) are becoming increasingly disfavored as long-term energy options due to concerns of scarcity and environmental consequences (e.g., release of anthropogenic CO₂). Hydrogen gas, on the other hand, has gained popularity as a clean-burning fuel because the only byproduct from its reaction with O₂ is H₂O. In recent decades, hydrogen derived from water splitting has been a topic of extensive research. The bottleneck of the water splitting reaction is the difficult water oxidation step (2H₂O → O₂ + 4H⁺ + 4e⁻), which requires an effective and robust catalyst to overcome its high kinetic barrier. Research in water oxidation by molecular ruthenium catalysts enjoys a rich history spanning nearly 40 years. As the diversity of novel ligands continues to widen, the relationship between ligand geometry or electronics, and catalyst activity is undoubtedly becoming clearer. The present review highlights, in the authors’ opinion, some of the most impactful discoveries in the field and explores the evolution of ligand design that has led to the current state of the art.

Photoisomerization and thermal isomerization of ruthenium aqua complexes with chloro-substituted asymmetric bidentate ligands

A series of ruthenium complexes with chloro-substituted bidentate ligands, proximal-[Ru(tpy)(Cl-pyqu)L] n+ [n = 1 for L = Cl, and n = 2 for L = OH2, tpy = 2,2';6',2''-terpyridine, pyqu = 2-(2'-pyridyl)quinoline] were synthesized and their reversible photoisomerizations and thermal isomerizations were investigated experimentally. The crystal structures of the complexes indicated that introduction of a chloro substituent at the 4- or 4'-position of the pyqu ligand did not change the structure around the metal center from that of the non-substituted complex, proximal-[Ru(tpy)(pyqu)L] n+. In contrast, the 6'-substituted complexes had sterically hindered environments around the metal center. The ruthenium aqua complexes showed reversible photoisomerization between the proximal and distal isomers. The quantum yield for photoisomerization of the 6'-substituted ruthenium aqua complex was almost twice as large as those of the other derivatives. This is explained by weakening of the ligand field on the ruthenium center by introduction of a chloro substituent at the 6'-position. Thermal back isomerization from the distal isomer to the proximal one was observed for the 6'-substituted complex, but such reactions were not observed for the other derivatives. The steric hindrance in the 6'-substituted aqua complex enhanced both thermal isomerization and photoisomerization.

Promoting proton coupled electron transfer in redox catalysts through molecular design

Mini-review on using the secondary coordination sphere to facilitate multi-electron, multi-proton catalysis.

Photochemical oxidation of water catalysed by cyclometalated Ir(iii) complexes bearing Schiff-base ligands

Synthesis, characterization and photochemical oxidation of water catalysed by cyclometalated Ir(iii) complexes bearing Schiff-base ligands in the presence of Na₂S₂O₈ and [Ru(bpy)₃]²⁺ as a PS has been demonstrated.

Synthesis, crystal structure and water oxidation activity of [Ru(terpy)(bipy)Cl]+ complexes: influence of ancillary ligands on O2 generation

Synthesis of four new complexes [Ru^II(MeMPTP)(bpy)Cl]PF₆ (1), [Ru^II(MeMPTP)(dmbpy)Cl]PF₆ (2), [Ru^II(MeMPTP)(dmdcbpy)Cl]PF₆ (3) and [Ru^II(MeMPTP)(Pic)₂Cl]PF₆ (4) and their catalytic activity for chemical oxidation of water into O₂ generation has been demonstrated.

Uncovering the Role of Oxygen Atom Transfer in Ru-Based Catalytic Water Oxidation

The realization of artificial photosynthesis carries the promise of cheap and abundant energy, however, significant advances in the rational design of water oxidation catalysts are required. Detailed information on the structure of the catalyst under reaction conditions and mechanisms of O-O bond formation should be obtained. Here, we used a combination of electron paramagnetic resonance (EPR), stopped flow freeze quench on a millisecond-second time scale, X-ray absorption (XAS), resonance Raman (RR) spectroscopy, and density functional theory (DFT) to follow the dynamics of the Ru-based single site catalyst, [Ru^II(NPM)(4-pic)₂(H₂O)]²⁺ (NPM = 4-t-butyl-2,6-di(1',8'-naphthyrid-2'-yl)pyridine, pic = 4-picoline), under the water oxidation conditions. We report a unique EPR signal with g-tensor, g_x = 2.30, g_y = 2.18, and g_z = 1.83 which allowed us to observe fast dynamics of oxygen atom transfer from the Ru^IV═O oxo species to the uncoordinated nitrogen of the NPM ligand. In few seconds, the NPM ligand modification results in [Ru^III(NPM-NO)(4-pic)₂(H₂O)]³⁺ and [Ru^III(NPM-NO,NO)(4-pic)₂]³⁺ complexes. A proposed [Ru^V(NPM)(4-pic)₂═O]³⁺ intermediate was not detected under the tested conditions. We demonstrate that while the proximal base might be beneficial in O-O bond formation via nucleophilic water attack on an oxo species as shown by DFT, the noncoordinating nitrogen is impractical as a base in water oxidation catalysts due to its facile conversion to the N-O group. This study opens new horizons for understanding the real structure of Ru catalysts under water oxidation conditions and points toward the need to further investigate the role of the N-O ligand in promoting water oxidation catalysis.

Mononuclear ruthenium polypyridine complexes that catalyze water oxidation

Over the past decade, significant advances have been made in the development of molecular water oxidation catalysts (WOCs) in the context of developing a system that would accomplish artificial photosynthesis. Mononuclear ruthenium complexes with polypyridine ligands have drawn considerable attention in this regard, due to their high catalytic activity and relatively simple structure. In this perspective review, we will discuss mononuclear Ru polypyridine WOCs by organizing them into four groups according to their ligand environments. Each group will be discussed with regard to three fundamental questions: first, how does the catalyst initiate O-O bond formation? Second, which step in the catalytic cycle is rate-determining? Third, how efficient is the catalyst according to the specific descriptors such as turnover frequency? All discussion is based on the high-valent ruthenium intermediates that are proposed in the catalytic cycle according to experimental observation and theoretical simulation. Two fundamental mechanisms are set forth. An acid-base mechanism that involves the attack of a water molecule on the oxo of a high valent Ru 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 O species to form the O-O bond. Subsequent steps lead to dissociation of O₂ and rehydration of the metal center. A second mechanism involves the formation of a Ru-O˙ radical species, two of which then couple to form a Ru-O-O-Ru species that can release O₂ afterwards. The acid-base mechanism appears to be more common and mechanistic differences could result from variation directly related to polypyridine ligand structures. Understanding how electronic, steric, and conformational properties can effect catalyst performance will lead to the rational design of more effective WOCs with not only ruthenium but also other transition metals.

Dinuclear first-row transition metal complexes with a naphthyridine-based dinucleating ligand

A series of dinuclear and tetranuclear first-row transition metal complexes were synthesized with the dinucleating ligand 2,7-bis(di(2-pyridyl)fluoromethyl)-1,8-naphthyridine (DPFN). The coordination pocket and rigidity of the DPFN ligand enforces pseudo-octahedral geometries about the metal centers that contain chloro, hydroxo, and aqua bridging ligands forming a "diamond" shaped configuration with metal-metal distances varying from 2.7826(5) to 3.2410(11) Å. Each metal center in the dinuclear complexes has an additional open coordination site that accommodates terminal ligands in a syn geometry of particular interest in catalyst design. The complexes are characterized by electronic spectroscopy, electrochemistry and potentiometric titration methods.

Synthesis, Electrochemistry, and Excited-State Properties of Three Ru(II) Quaterpyridine Complexes

The complexes [Ru(qpy)LL'](2+) (qpy = 2,2':6',2″:6″,2‴-quaterpyridine), with 1: L = acetonitrile, L'= chloride; 2: L = L'= acetonitrile; and 3: L = L'= vinylpyridine, have been prepared from [Ru(qpy) (Cl)2]. Their absorption spectra in CH3CN exhibit broad metal-to-ligand charge transfer (MLCT) absorptions arising from overlapping (1)A1 → (1)MLCT transitions. Photoluminescence is not observed at room temperature, but all three are weakly emissive in 4:1 ethanol/methanol glasses at 77 K with broad, featureless emissions observed between 600 and 1000 nm consistent with MLCT phosphorescence. Cyclic voltammograms in CH3CN reveal the expected Ru(III/II) redox couples. In 0.1 M trifluoroacetic acid (TFA), 1 and 2 undergo aquation to give [Ru(II)(qpy)(OH2)2](2+), as evidenced by the appearance of waves for the couples [Ru(III)(qpy)(OH2)2](3+)/[Ru(II)(qpy)(OH2)2](2+), [Ru(IV)(qpy)(O)(OH2)](2+)/[Ru(III)(qpy)(OH2)2](3+), and [Ru(VI)(qpy)(O)2](2+)/[Ru(IV)(qpy)(O)(OH2)](2+) in cyclic voltammograms.

Efficient water oxidation with electromodified Langmuir–Blodgett films of procatalytic [CoIII(N2O3)] metallosurfactants on electrodes

Electromodified Langmuir–Blodgett films of a cobalt-based metallosurfactant support water oxidation at turnover numbers of up to 54 000 h⁻¹.

Water oxidation catalysis with ligand substituted Ru–bpp type complexes

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

[RuV(NCN-Me)(bpy)(O)]3+ mediated efficient photo-driven water oxidation

The complex [Ru(NCN-Me)(bpy)H₂O](PF₆)₂ acts as an active catalyst for the photo-driven oxidation of water, when employed with [Ru(bpy)₃]²⁺ as photosensitizer and Na₂S₂O₈ as sacrificial electron acceptor at pH 6.5 phosphate buffer, with a TON of 130.

Molecular ruthenium water oxidation catalysts carrying non-innocent ligands: mechanistic insight through structure–activity relationships and quantum chemical calculations

Herein is highlighted how structure–activity relationships can be used to provide mechanistic insight into H₂O oxidation catalysis.

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.

New insights into the selectivity of four 1,10-phenanthroline-derived ligands toward the separation of trivalent actinides and lanthanides: a DFT based comparison study

To understand the correlation between the chemical structures of N-ligands and Ln/An separation capability, we selected four phenanthroline-derived ligands as examples to probe why some ligands work whereas others fail.

New pyridazine-based binuclear nickel(ii), copper(ii) and zinc(ii) complexes as prospective anticancer agents

The complexes containing diethylamino substituents exhibit promising cytotoxicity against the estrogen receptor (ER) negative MDA-MB-231 breast cancer cell line.

A computational study of the mechanism for water oxidation by (bpc)(bpy)Ru(II)OH2

A mechanistic study on the catalytic cycle water oxidation with 1 [(bpc)(bpy)RuIIOH2]+ (Hbpc = 2,2′-bipyridine-6-carboxylic acid, bpy = 2,2′-bipyridine) is described in this paper. Stepwise oxidation via proton-coupled electron transfer gives 3 [(bpc)(bpy)RuIVO]+. An active 4 [(bpc)(bpy)RuVO]2+, which is involved in the OO bond formation is generated from further 1e− oxidation of 3. Another different possible reaction at 4 was investigated and new destructive paths involving overoxidation of the metal were identified. The most viable path for OO bond formation via a water nucleophilic attack at the oxo of 4 is found to be the rate-determining step in this water oxidation catalytic cycle, and the hydro-peroxo 6 [(bpc)(bpy)RuIIIOOH]+ is generated accompanied with a proton transfer. The super-oxo 7side-on [(bpc)(bpy)RuIVOO]+ and 8side-on [(bpc)(bpy)RuVOO]2+, both low spin species, are generated by further oxidations of 6. Through an intersystem crossing they can transform to their high spin states, 9end-on [(bpc)(bpy)RuIVOO]+ and 12end-on [(bpc)(bpy)RuVOO]2+, respectively. Following a dissociative pathway O2 is readily generated from both 9end-on and 12end-on.

Dinuclear manganese complexes for water oxidation: evaluation of electronic effects and catalytic activity

During recent years significant progress has been made towards the realization of a sustainable and carbon-neutral energy economy. One promising approach is photochemical splitting of H2O into O2 and solar fuels, such as H2. However, the bottleneck in such artificial photosynthetic schemes is the H2O oxidation half reaction where more efficient catalysts are required that lower the kinetic barrier for this process. In particular catalysts based on earth-abundant metals are highly attractive compared to catalysts comprised of noble metals. We have now synthesized a library of dinuclear Mn2(II,III) catalysts for H2O oxidation and studied how the incorporation of different substituents affected the electronics and catalytic efficiency. It was found that the incorporation of a distal carboxyl group into the ligand scaffold resulted in a catalyst with increased catalytic activity, most likely because of the fact that the distal group is able to promote proton-coupled electron transfer (PCET) from the high-valent Mn species, thus facilitating O-O bond formation.

Well-defined palladium nanoparticles supported on siliceous mesocellular foam as heterogeneous catalysts for the oxidation of water

Herein, we describe the use of Pd nanoparticles immobilized on an amino-functionalized siliceous mesocellular foam for the catalytic oxidation of H₂O. The Pd nanocatalyst proved to be capable of mediating the four-electron oxidation of H₂O to O₂, both chemically and photochemically. The Pd nanocatalyst is easy to prepare and shows high chemical stability, low leaching, and recyclability. Together with its promising catalytic activity, these features make the Pd nanocatalyst of potential interest for future sustainable solar-fuel production.

Ruthenium catalysts for water oxidation involving tetradentate polypyridine-type ligands

A series of Ru^II complexes that behave as water oxidation catalysts were prepared involving a tetradentate equatorial ligand and two 4-substituted pyridines as the axial ligands. Two of these complexes were derived from 2,9-di-(pyrid-2′-yl)-1,10-phenanthroline (dpp) and examine the effect of incorporating electron-donating amino and bulky t-butyl groups on catalytic activity. A third complex replaced the two distal pyridines with N-methylimidazoles that are more electron-donating than the pyridines of dpp and potentially stabilize higher oxidation states of the metal. The tetradentate ligand 2-(pyrid-2′-yl)-6-(1′′,10′′-phenanthrol-2′′-yl)pyridine (bpy–phen), possessing a bonding cavity similar to dpp, was also prepared. The Ru^II complex of this ligand does not have two rotatable pyridines in the equatorial plane and thus shows different flexibility from the [Ru(dpp)] complexes. All the complexes showed activity towards water oxidation. Investigation of their catalytic behavior and electrochemical properties suggests that they may follow the same catalytic pathway as the prototype [Ru(dpp)pic₂]²⁺ involving a seven-coordinated [Ru^IV(O)] intermediate. The influence of coordination geometry on catalytic performance is analyzed and discussed.

Oxo-bridge scenario behind single-site water-oxidation catalysts

High-oxidation-state decay of mononuclear complexes [RuTB(H2O)](2+) (X(2+), where B = 2,2'-bpy or bpy for X = 1; B = 5,5'-F2-bpy for X = 2; B = 6,6'-F2-bpy for X = 3; T = 2,2':6',2″-terpyridine) oxidized with a large excess of Ce(IV) generates a manifold of polynuclear oxo-bridged complexes. These include the following complexes: (a) dinuclear [TB-Ru(IV)-O-Ru(IV)-(T)(O)OH2](2+) (1-dn(4+)), [TB-Ru(III)-O-Ru(III)-T(MeCN)2](4+) (1-dn-N(4+)), and {[Ru(III)(trpy)(bpy)]2(μ-O)}(4+) (1-dm(4+)); (b) trinuclear {[Ru(III)(trpy)(bpy)(μ-O)]2Ru(IV)(trpy)(H2O)}(ClO4)5(6+) (1-tr(6+)) and {[Ru(III)(trpy)(bpy)(μ-O)]2Ru(IV)(pic)2}(ClO4)4 (1-tr-P(4+), where P is the 2-pyridinecarboxylate anion); and (c) tetranuclear [TB-Ru(III)-O-TRu(IV)(H2O)-O-TRu(IV)(H2O)-O-Ru(III)-TB](8+) (1-tn(8+)), [TB-Ru(III)-O-TRu(IV)(AcO)-O-TRu(IV)(AcO)-O-Ru(III)-TB](6+) (1-tn-Ac(6+)), and [TB-Ru(II)-O-TRu(IV)(MeCN)-O-TRu(IV)(MeCN)-O-Ru(II)-TB](6+) (1-tn-N(6+)). These complexes have been characterized structurally by single-crystal X-ray diffraction analysis, and their structural properties were correlated with their electronic structures. Dinuclear complex 1-dm(4+) has been further characterized by spectroscopic and electrochemical techniques. Addition of excess Ce(IV) to 1-dm(4+) generates dioxygen in a catalytic manner. However, resonance Raman spectroscopy points to the in situ formation of 1-dn(4+) as the active species.

Spectroscopic analysis of catalytic water oxidation by [Ru(II)(bpy)(tpy)H2O]2+ suggests that Ru(V)═O is not a rate-limiting intermediate

Modern chemistry's grand challenge is to significantly improve catalysts for water splitting. Further progress requires detailed spectroscopic and computational characterization of catalytic mechanisms. We analyzed one of the most studied homogeneous single-site Ru catalysts, [Ru(II)(bpy)(tpy)H2O](2+) (where bpy = 2,2'-bipyridine, tpy = 2,2';6',2″-terpyridine). Our results reveal that the [Ru(V)(bpy)(tpy)═O](3+) intermediate, reportedly detected in catalytic mixtures as a rate-limiting intermediate in water activation, is not present as such. Using a combination of electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy, we demonstrate that 95% of the Ru complex in the catalytic steady state is of the form [Ru(IV)(bpy)(tpy)═O](2+). [Ru(V)(bpy)(tpy)═O](3+) was not observed, and according to density functional theory (DFT) analysis, it might be thermodynamically inaccessible at our experimental conditions. A reaction product with unique EPR spectrum was detected in reaction mixtures at about 5% and assigned to Ru(III)-peroxo species with (-OOH or -OO- ligands). We also analyzed the [Ru(II)(bpy)(tpy)Cl](+) catalyst precursor and confirmed that this molecule is not a catalyst and its oxidation past Ru(III) state is impeded by a lack of proton-coupled electron transfer. Ru-Cl exchange with water is required to form active catalysts with the Ru-H2O fragment. [Ru(II)(bpy)(tpy)H2O](2+) is the simplest representative of a larger class of water oxidation catalysts with neutral, nitrogen containing heterocycles. We expect this class of catalysts to work mechanistically in a similar fashion via [Ru(IV)(bpy)(tpy)═O](2+) intermediate unless more electronegative (oxygen containing) ligands are introduced in the Ru coordination sphere, allowing the formation of more oxidized Ru(V) intermediate.

Surface immobilization of a tetra-ruthenium substituted polyoxometalate water oxidation catalyst through the employment of conducting polypyrrole and the layer-by-layer (LBL) technique

A tetra Ru-substituted polyoxometalate Na10[{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2] (Ru4POM) has been successfully immobilised onto glassy carbon electrodes and indium tin oxide (ITO) coated glass slides through the employment of a conducting polypyrrole matrix and the layer-by-layer (LBL) technique. The resulting Ru4POM doped polypyrrole films showed stable redox behavior associated with the Ru centres within the Ru4POM, whereas, the POM's tungsten-oxo redox centres were not accessible. The films showed pH dependent redox behavior within the pH range 2-5 whilst exhibiting excellent stability towards redox cycling. The layer-by-layer assembly was constructed onto poly(diallyldimethylammonium chloride) (PDDA) modified carbon electrodes by alternate depositions of Ru4POM and a Ru(II) metallodendrimer. The resulting Ru4POM assemblies showed stable redox behavior for the redox processes associated with Ru4POM in the pH range 2-5. The charge transfer resistance of the LBL films was calculated through AC-Impedance. Surface characterization of both the polymer and LBL Ru4POM films was carried out using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Initial investigations into the ability of the Ru4POM LBL films to electrocatalytically oxidise water at pH 7 have also been conducted.