Synergistic Effects of Metals in a Promising RuII−PtII Assembly for a Combined Anticancer Approach: Theoretical Exploration of the Photophysical Properties

Ru^II?Pt^II complexes are a class of bioactive molecules of interest as anticancer agents that combine a light‐absorbing chromophore with a cisplatin‐like unit. The results of a DFT and TDDFT investigation of a Ru^II complex and its conjugate with a cis‐PtCl₂ moiety reveal that a synergistic effect of the metals makes the assembly a promising multitarget anticancer drug. Inspection of type I and type II photoreactions and spin–orbit coupling computations reveals that the cis‐PtCl₂ moiety improves the photophysical properties of the Ru^II chromophore, ensuring efficient singlet oxygen generation and making the assembly suitable for photodynamic therapy. At the same time, the Ru^II chromophore promotes a new alternative activation mechanism of the Pt^II ligand via a triplet metal‐to‐ligand charge transfer (³M LCT) state, before reaching the biological target. The importance of the supramolecular architecture is accurately derived, opening interesting new perspectives on the use of bimetallic Ru^II?Pt^II assemblies in a combined anticancer approach.     

Synthesis,characterization, and micellization studies of coil‐rod‐coil and ABA ruthenium(II) terpyridine assemblies with π‐conjugated electron acceptor systems

A series of Ru^II heterodinuclear complexes of ABA ‐type with electron‐deficient bis‐terpyridines as building blocks was synthesized by (R‐tpy)Ru^IIICl₃ complexation. These compounds were characterized by NMR spectroscopy, MALDI‐TOF, ESI‐TOF mass spectrometry, and elemental analysis. The results were compared with a coil‐rod‐coil Ru^II metallo‐supramolecular copolymer, which was synthesized by bis‐complex formation between a hydrophilic ω‐terpyridine poly(ethylene glycol) Ru^II mono‐complex and a hydrophobic bis‐terpyridine‐functionalized rigid core. This amphiphilic Ru^II triblock copolymer showed self‐assembly to clusters and micelles in aqueous solution, which was studied by transmission electron microscopy and dynamic light scattering. Applying velocity sedimentation experiments the number of amphiphilic Ru^II ABA triblock copolymer molecules within the micelles could be estimated. Finally, the photophysical properties of the Ru^II supramolecular assemblies were investigated by UV–vis spectroscopy. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

A pyrazine-bridged trimer of oxo-centered triruthenium–carbonyl clusters and the supramolecular assembly built from multiple noncovalent contacts

The crystal structure of a new pyrazine-bridged trimer of oxo-centered triruthenium–carbonyl clusters formulated as [Ru₃O(EtCOO)₆(CO)(pyrazine)]₂[Ru₃O(EtCOO)₆(CO)(μ-pyrazine)₂] ( 1 ) has been unequivocally determined by single-crystal X-ray diffraction (SC-XRD) analysis at 100 K. A supramolecular cyclic assembly of two trimers was formed via intermolecular CO⋯CO contacts, which is further assembled into a three-dimensional orthogonal layer-by-layer stack via hydrogen bonds.     

Structural Diversity and Magnetic Properties of Hybrid Ruthenium Halide Perovskites and Related Compounds

There has been a great deal of recent interest in extended compounds containing Ru³⁺ and Ru⁴⁺ in light of their range of unusual physical properties. Many of these properties are displayed in compounds with the perovskite and related structures. Here we report an array of structurally diverse hybrid ruthenium halide perovskites and related compounds: MA₂RuX₆ (X=Cl or Br), MA₂MRuX₆ (M=Na, K or Ag; X=Cl or Br) and MA₃Ru₂X₉ (X=Br) based upon the use of methylammonium (MA=CH₃NH₃⁺) on the perovskite A site. The compounds MA₂RuX₆ with Ru⁴⁺ crystallize in the trigonal space group and can be described as vacancy‐ordered double‐perovskites. The ordered compounds MA₂MRuX₆ with M⁺ and Ru³⁺ crystallize in a structure related to BaNiO₃ with alternating MX₆ and RuX₆ face‐shared octahedra forming linear chains in the trigonal space group. The compound MA₃Ru₂Br₉ crystallizes in the orthorhombic Cmcm space group and displays pairs of face‐sharing octahedra forming isolated Ru₂Br₉ moieties with very short Ru–Ru contacts of 2.789 Å. The structural details, including the role of hydrogen bonding and dimensionality, as well as the optical and magnetic properties of these compounds are described. The magnetic behavior of all three classes of compounds is influenced by spin–orbit coupling and their temperature‐dependent behavior has been compared with the predictions of the appropriate Kotani models.     

IR,Raman, and Surface‐enhanced Raman Spectroscopic Study on Triruthenium Dipyridylamide Diruthenium Nickel Dipyridylamide Family: Metal‐metal Bonding and Structures

We report the infrared, Raman, and surface‐enhanced Raman scattering (SERS) spectra of triruthenium dipyridylamido complexes and of diruthenium mixed nickel metal‐string complexes. From the results of analysis on the vibrational modes, we assigned their vibrational frequencies and structures. The infrared band at 323–326 cm^?1 is assigned to the Ru₃ asymmetric stretching mode for [Ru₃(dpa)₄Cl₂]^0–2+. In these complexes we observed no Raman band corresponding to the Ru₃ symmetric stretching mode although this mode is expected to have substantial Raman intensity. There is no frequency shift in the Ru₃ asymmetric stretching modes for the complexes with varied oxidational states. No splitting in Raman spectra for the pyridyl breathing line indicates similar bonding environment for both pyridyls in dpa^– , thus a delocalized structure in the [Ru₃]^6–8+ unit is proposed. For Ru₃(dpa)₄(CN)₂ complex series, we assign the infrared band at 302 cm^?1 to the Ru₃ asymmetric stretching mode and the weak Raman line at 285 cm^?1 to the Ru₃ symmetric stretching. Coordination to the strong axial ligand CN^– weakens the Ru‐Ru bonding. For the diruthenium nickel complex [Ru₂Ni(dpa)₄Cl₂]^0–1+, the diruthenium stretching mode ν_Ru‐Ru is assigned to the intense band at 327 and 333 cm^?1 in the Raman spectra for the neutral and oxidized forms, respectively. This implies a strong Ru‐Ru metal‐metal bonding.     

Supramolecular cluster catalysis: facts and problems

By checking the chemistry underlying the concept of “supramolecular cluster catalysis” we identified two major errors in our publications related to this topic, which are essentially due to contamination problems. (1) The conversion of the “closed” cluster cation [H₃Ru₃(C₆H₆)(C₆Me₆)₂(O)]⁺ (1) into the “open” cluster cation [H₂Ru₃(C₆H₆)(C₆Me₆)₂(O)(OH)]⁺ (2), which we had ascribed to a reaction with water in the presence of ethylbenzene is simply an oxidation reaction which occurs in the presence of air. (2) The higher catalytic activity observed with ethylbenzene, which we had erroneously attributed to the “open” cluster cation [H₂Ru₃(C₆H₆)(C₆Me₆)₂(O)(OH)]⁺ (2), was due to the formation of RuO₂ · nH₂O, caused by a hydroperoxide contamination present in ethylbenzene.     

Electron-Rich Diruthenium Complexes with π-Extended Alkenyl Ligands and Their F4TCNQ Charge-Transfer Salts**

The synthesis of dinuclear ruthenium alkenyl complexes with {Ru(CO)(PⁱPr₃)₂(L)} entities (L=Cl⁻ in complexes Ru₂-3 and Ru₂-7 ; L=acetylacetonate (acac⁻) in complexes Ru₂-4 and Ru₂-8 ) and with π-conjugated 2,7-divinylphenanthrenediyl ( Ru₂-3 , Ru₂-4 ) or 5,8-divinylquinoxalinediyl ( Ru₂-7 , Ru₂-8 ) as bridging ligands are reported. The bridging ligands are laterally π-extended by anellating a pyrene ( Ru₂-7 , Ru₂-8 ) or a 6,7-benzoquinoxaline ( Ru₂-3 , Ru₂-4 ) π-perimeter. This was done with the hope that the open π-faces of the electron-rich complexes will foster association with planar electron acceptors via π-stacking. The dinuclear complexes were subjected to cyclic and square-wave voltammetry and were characterized in all accessible redox states by IR, UV/Vis/NIR and, where applicable, by EPR spectroscopy. These studies signified the one-electron oxidized forms of divinylphenylene-bridged complexes Ru₂-7 , Ru₂-8 as intrinsically delocalized mixed-valent species, and those of complexes Ru₂-3 and Ru₂-4 with the longer divinylphenanthrenediyl linker as partially localized on the IR, yet delocalized on the EPR timescale. The more electron-rich acac⁻ congeners formed non-conductive 1 : 1 charge-transfer (CT) salts on treatment with the F₄TCNQ electron acceptor. All spectroscopic techniques confirmed the presence of pairs of complex radical cations and F₄TCNQ^.− radical anions in these CT salts, but produced no firm evidence for the relevance of π-stacking to their formation and properties.     

Redox‐Controlled Exchange Bias in a Supramolecular Chain of Fe4 Single‐Molecule Magnets

Tetrairon(III) single‐molecule magnets [Fe₄(pPy)₂(dpm)₆] ( 1 ) (H₃pPy=2‐(hydroxymethyl)‐2‐(pyridin‐4‐yl)propane‐1,3‐diol, Hdpm=dipivaloylmethane) have been deliberately organized into supramolecular chains by reaction with Ru^IIRu^II or Ru^IIRu^III paddlewheel complexes. The products [Fe₄(pPy)₂(dpm)₆][Ru₂(OAc)₄](BF₄)_x with x=0 ( 2 a ) or x=1 ( 2 b ) differ in the electron count on the paramagnetic diruthenium bridges and display hysteresis loops of substantially different shape. Owing to their large easy‐plane anisotropy, the s=1 diruthenium(II,II) units in 2 a act as effective s_eff=0 spins and lead to negligible intrachain communication. By contrast, the mixed‐valent bridges (s=3/2, s_eff=1/2) in 2 b introduce a significant exchange bias, with concomitant enhancement of the remnant magnetization. Our results suggest the possibility to use electron transfer to tune intermolecular communication in redox‐responsive arrays of SMMs.     

Crystal Structures and Magnetic Properties of 6H-Perovskites Ba3MRu2O9 (M=Y, In, La, Sm, Eu, and Lu)

Magnetic properties of quaternary oxides Ba₃MRu₂O₉ (M=Y, In, La, Sm, Eu, and Lu) are reported. Rietveld analyses of the X-ray diffraction data indicate that they adopt the 6H-perovskite structure and have the valence state of Ba₃M³⁺ Ru^4.5+₂O₉. All compounds are nonmetallic at least over the temperature range of 100-400 K. The magnetic susceptibilities show a broad maximum at 135-370 K except for the La compound, which shows a plateau around 22 K. In addition, another magnetic anomaly is observed at 4.5-12.5 K by the magnetic susceptibility and specific heat measurements for any compound. It is considered that this magnetic behavior is ascribed to the antiferromagnetic coupling between two Ru ions in a Ru₂O₉ dimer and to the magnetic interaction between the Ru₂O₉ dimers.     

Model oxygen-evolving center composed of polymer membrane and dimer ruthenium complex

The in situ spectrocyclic voltammetric investigations of the dimeric ruthenium complex used for water oxidation, [(bpy)₂(H₂O)Ru–O–Ru(H₂O)(bpy)₂]⁴⁺ (H₂O–Ru^III–Ru^III–OH₂), were carried out in a homogeneous aqueous solution and in a Nafion membrane under different pH conditions. The in situ absorption spectra recorded for the dimer show that the dimer H₂O–Ru^III–Ru^III–OH₂ complex underwent reactions initially to give the detectable H₂O–Ru^III–Ru^IV–OH and H₂O–Ru^III–Ru^IV–OH₂ complexes, and at higher positive potentials, this oxidized dimer underwent further oxidation to produce a presumably higher oxidation state Ru^V–Ru^V complex. Since this Ru^V–Ru^V complex is reduced rapidly by water molecules to H₂O–Ru^III–Ru^IV–OH₂, it could not be detected by absorption spectrum. Independent of the pH conditions and homogeneous solution/Nafion membrane systems, the dimer Ru^III–Ru^IV was detected at higher potentials, suggesting that the dimer complex acts as a three-electron oxidation catalyst. However, in the Nafion membrane system it was suggested that the dimer complex may act as a four-electron oxidation catalyst. While the dimer complex was stable under oxidation conditions, the reduction of the dimer Ru^III–Ru^III to Ru^II–Ru^II led to decomposition, yielding the monomeric cis-[Ru(bpy)₂(H₂O)₂]²⁺.     

A paramagnetic precursor for polymeric supramolecular assemblies based on multiply bonded dimetal units: μ‐acetato‐aceto­nitrile­tris(μ‐N,N′‐di­phenyl­formamidinato)­di­ruthenium tetra­fluoro­borate di­chloro­methane hemisolvate

The title compound, [Ru₂(C₁₃H₁₁N₂)₃(C₂H₃O₂)(C₂H₃N)]BF₄·0.5CH₂Cl₂ or [Ru₂(μ‐DPhF)₃(μ‐O₂CMe)(MeCN)]BF₄·0.5CH₂Cl₂, where DPhF is N,N′‐diphenyl­formamidinate, crystallized as dark‐blue block‐shaped crystals. In the unit cell, the diruthenium cation lies on a general position, and the BF₄⁻ anions reside on two independent special positions with crystallographic twofold symmetry. Disorder was observed for one of the phenyl groups in the formamidinate ligand, the axial aceto­nitrile mol­ecule and the interstitial di­chloro­methane mol­ecule. The compound, which exhibits a long Ru—Ru bond of 2.4131 (5) Å, is the first {Ru₂}⁵⁺ formamidin­ate species that is both equatorially and axially functionalized so that it can be used as a precursor for polymeric paramagnetic supramolecular assemblies.     

Magnetic and Spectral Properties of Heterotrinuclear Acetates [Ru2Co(μ3-O)(μ-CH3CO2)6(Py)3]n+ (n = 0 or 1)

The preparation and physical properties of the new heterotrinuclear acetates, [Ru₂CO(μ₃-O)(μ-CH₃CO₂)₆(Py)₃] (Ru₂Co(II)) and [Ru₂Co(μ₃-O)(μ-CH₃CO₂)₆(Py)₃)l₃ (Ru₂Co(III), Py = pyridine), are reported. Three reversible one-electron-redox waves are observed at 1.19, 0.40, and ?1.24 V vs Ag/Ag⁺ electrode for Ru₂Co(lI) in CH₂Cl₂. The complexes of Ru₂Co(II) and Ru₂Co(III) show an intense visible absorption at 570 (? 5950 M^?1 cm^?1) and 551 nm (? 7240 M^?1 cm^?1), respectively. The magnetic susceptibilities of both complexes were also measured from 4.2 to 300 K. The resulting least-squares fit parameters for Ru₂Co(II) areJ_RuCo = ?9 cm^?1, J_RuRu = ?22 cm^?1, g_Co, = g_Ru= 2.19.     

A Highly Oxidizing and Isolable Oxoruthenium(V) Complex [RuV(N4O)(O)]2+: Electronic Structure,Redox Properties,and Oxidation Reactions Investigated by DFT Calculations

The electronic structure and redox properties of the highly oxidizing, isolable Ru^V?O complex [Ru^V(N₄O)(O)]²⁺, its oxidation reactions with saturated alkanes (cyclohexane and methane) and inorganic substrates (hydrochloric acid and water), and its intermolecular coupling reaction have been examined by DFT calculations. The oxidation reactions with cyclohexane and methane proceed through hydrogen atom transfer in a transition state with a calculated free energy barrier of 10.8 and 23.8 kcal mol^?1, respectively. The overall free energy activation barrier (ΔG^≠=25.5 kcal mol^?1) of oxidation of hydrochloric acid can be decomposed into two parts: the formation of [Ru^III(N₄O)(HOCl)]²⁺ (ΔG=15.0 kcal mol^?1) and the substitution of HOCl by a water molecule (ΔG^≠=10.5 kcal mol^?1). For water oxidation, nucleophilic attack on Ru^V?O by water, leading to O? O bond formation, has a free energy barrier of 24.0 kcal mol^?1, the major component of which comes from the cleavage of the H? OH bond of water. Intermolecular self‐coupling of two molecules of [Ru^V(N₄O)(O)]²⁺ leads to the [(N₄O)Ru^IV? O₂? Ru^III(N₄O)]⁴⁺ complex with a calculated free energy barrier of 12.0 kcal mol^?1.     

Self‐Assembled Amphiphilic Water Oxidation Catalysts: Control of O−O Bond Formation Pathways by Different Aggregation Patterns

The oxidation of water to molecular oxygen is the key step to realize water splitting from both biological and chemical perspective. In an effort to understand how water oxidation occurs on a molecular level, a large number of molecular catalysts have been synthesized to find an easy access to higher oxidation states as well as their capacity to make O?O bond. However, most of them function in a mixture of organic solvent and water and the O?O bond formation pathway is still a subject of intense debate. Herein, we design the first amphiphilic Ru‐bda (H₂bda=2,2′‐bipyridine‐6,6′‐dicarboxylic acid) water oxidation catalysts (WOCs) of formula [Ru^II(bda)(4‐OTEG‐pyridine)₂] ( 1 , OTEG=OCH₂CH₂OCH₂CH₂OCH₃) and [Ru^II(bda)(PySO₃Na)₂] ( 2 , PySO₃^?=pyridine‐3‐sulfonate), which possess good solubility in water. Dynamic light scattering (DLS), scanning electron microscope (SEM), critical aggregation concentration (CAC) experiments and product analysis demonstrate that they enable to self‐assemble in water and form the O?O bond through different routes even though they have the same bda^2? backbone. This work illustrates for the first time that the O?O bond formation pathway can be regulated by the interaction of ancillary ligands at supramolecular level.     

In Situ Tracking of Dynamic NO Capture through a Crystal-to-Crystal Transformation from a Gate-Open-Type Chain Porous Coordination Polymer to a NO-Adducted Discrete Isomer

Optimal control of gas adsorption properties in metal–organic frameworks (MOFs) or porous coordination polymers (PCPs) remains a great challenge in the field of materials science. An efficient strategy to capture electron-acceptor-type gas molecules such as nitrogen monooxide (NO) is to use host–guest interactions by utilizing electron-donor-type MOFs/PCPs as host frameworks. Herein, we focus on a highly electron-donating chain compound by using the paddlewheel-type [Ru₂^II,II] complex [Ru₂(2,4,5-Me₃PhCO₂)₄] (2,4,5-Me₃PhCO₂⁻=2,4,5-trimethylbenzoate) with the phenazine (phz) linker: [Ru₂(2,4,5-Me₃PhCO₂)₄(phz)] ( 1 ). Compound 1 exhibited a specific gated adsorption for NO under gas pressures greater than 60 kPa at 121 K, which finally resulted in approximately seven molar equivalents being taken up at 100 kPa followed by four molar equivalents remaining under vacuum at 121 K; its Rh isomorph ( 2 ) with weaker donation ability was inactive for NO. When the sample of 1 ⊃4 NO was heated to room temperature, the compound underwent a crystal-to-crystal phase transition to give [Ru₂(2,4,5-Me₃PhCO₂)₄(NO)₂](phz) ( 1 -NO), involving a post-synthetic nitrosylation on the [Ru₂] unit, accompanied by an eventful site-exchange with phz. This drastic event, which is dependent on the NO pressure, temperature, and time, was coherently monitored by using several different in situ techniques, revealing that the stabilization of NO molecules in nanosized pores dynamically and stepwisely occurred with the support of strong electronic/magnetic host–guest interactions.     

Synthesis and spectral studies on heterobimetallic complexes of manganese and ruthenium derived from N-(2-hydroxynaphthalen-1-yl)methylenebenzoylhydrazide

The complex [Mn^IV(napbh)₂] (napbhH₂ = N-(2-hydroxynaphthalen-1-yl)methylenebenzoylhydrazide) reacts with activated ruthenium(III) chloride in methanol in 1 : 1.2 molar ratio under reflux, giving heterobimetallic complexes, [Mn^IV(napbh)₂Ru^IIICl₃(H₂O)] · [Ru^III(napbhH)Cl₂(H₂O)] reacts with Mn(OAc)₂·4H₂O in methanol in 1 : 1.2 molar ratio under reflux to give [Ru^III(napbhH)Cl₂(H₂O)Mn^II(OAc)₂]. Replacement of aquo in these heterobimetallic complexes has been observed when the reactions are carried out in the presence of pyridine (py), 3-picoline (3-pic), or 4-picoline (4-pic). The molar conductances for these complexes in DMF indicates 1 : 1 electrolytes. Magnetic moment values suggest that these heterobimetallic complexes contain Mn^IV and Ru^III or Ru^III and Mn^II in the same structural unit. Electronic spectral studies suggest six coordinate metal ions. IR spectra reveal that the napbhH₂ ligand coordinates in its enol form to Mn^IV and bridges to Ru^III and in the keto form to Ru^III and bridging to Mn^II.     

Self-Assembly of Supramolecular Architectures Using Chlorotetra(Pyrrole-2-Carboxylato)Diruthenium Molecules as Building Blocks

Two new complexes, based on the unit Ru₂Cl(μ-O₂CC₄H₄N)₄ (1) (O₂CC₄H₄N = pyrrole-2-carboxylate), Ru₂Cl(μ-O₂CC₄H₄N)₄(H₂O)·4H₂O [1(H ₂ O)·4H ₂ O], and Ru₂Cl(μ-O₂CC₄H₄N)₄(Me₂CO) [1(Me ₂ CO)], are synthesized and structurally characterized. The physical properties of these complexes are studied and compared with those previously reported for Ru₂Cl(μ-O₂CC₄H₄N)₄(thf)·thf·H₂O [1(thf)·thf·H ₂ O]. The nature of the solvent molecule bonded to the axial position of the dimetallic unit determines the supramolecular interactions leading to different arrangements in the solid state. The presence of NH groups in the pyrrolic rings favours the existence of hydrogen bond interactions that are present in the three complexes. In addition, complex 1(Me ₂ CO) shows π–π stacking interactions through pyrrolic rings of different dimetallic units. Dedicated to the memory of Prof. F. Albert Cotton.     

Mercury-bridged,nitrosyl substituted,triruthenium carbonyl clusters: Synthesis and x-ray crystal structure of [Ru3(NO)(CO)10]2Hg

The neutral mixed-metal cluster [Ru₃(NO)(CO)₁₀]₂Hg has been prepared by the reaction of the [Ru₃(NO)(CO)₁₀], with HgCl₂. An X-ray crystal structure shows that the mercury atom links two Ru₃ triangular units by bridging an RuRu edge of each unit. The dihedral angle between the two Ru₂Hg triangles is 27.6°. In each Ru₃ triangle a nitrosyl ligand bridges the same RuRu edge as the bridging Hg atom while the ten carbonyl groups are all terminal.     

Near‐Infrared Light‐Driven Hydrogen Evolution from Water Using a Polypyridyl Triruthenium Photosensitizer

In order to realize artificial photosynthetic devices for splitting water to H₂ and O₂ (2 H₂O+hν→2 H₂+O₂), it is desirable to use a wider wavelength range of light that extends to a lower energy region of the solar spectrum. Here we report a triruthenium photosensitizer [Ru₃(dmbpy)₆(μ‐HAT)]⁶⁺ (dmbpy=4,4′‐dimethyl‐2,2′‐bipyridine, HAT=1,4,5,8,9,12‐hexaazatriphenylene), which absorbs near‐infrared light up to 800 nm based on its metal‐to‐ligand charge transfer (¹MLCT) transition. Importantly, [Ru₃(dmbpy)₆(μ‐HAT)]⁶⁺ is found to be the first example of a photosensitizer which can drive H₂ evolution under the illumination of near‐infrared light above 700 nm. The electrochemical and photochemical studies reveal that the reductive quenching within the ion‐pair adducts of [Ru₃(dmbpy)₆(μ‐HAT)]⁶⁺ and ascorbate anions affords a singly reduced form of [Ru₃(dmbpy)₆(μ‐HAT)]⁶⁺, which is used as a reducing equivalent in the subsequent water reduction process.     

Spectroscopic,Electrochemical and Computational Characterisation of Ru Species Involved in Catalytic Water Oxidation: Evidence for a [RuV(O)(Py2Metacn)] Intermediate

A new family of ruthenium complexes based on the N‐pentadentate ligand Py₂^Metacn (N‐methyl‐N′,N′′‐bis(2‐picolyl)‐1,4,7‐triazacyclononane) has been synthesised and its catalytic activity has been studied in the water‐oxidation (WO) reaction. We have used chemical oxidants (ceric ammonium nitrate and NaIO₄) to generate the WO intermediates [Ru^II(OH₂)(Py₂^Metacn)]²⁺, [Ru^III(OH₂)(Py₂^Metacn)]³⁺, [Ru^III(OH)(Py₂^Metacn)]²⁺ and [Ru^IV(O)(Py₂^Metacn)]²⁺, which have been characterised spectroscopically. Their relative redox and pH stability in water has been studied by using UV/Vis and NMR spectroscopies, HRMS and spectroelectrochemistry. [Ru^IV(O)(Py₂^Metacn)]²⁺ has a long half‐life (>48 h) in water. The catalytic cycle of WO has been elucidated by using kinetic, spectroscopic, ¹⁸O‐labelling and theoretical studies, and the conclusion is that the rate‐determining step is a single‐site water nucleophilic attack on a metal‐oxo species. Moreover, [Ru^IV(O)(Py₂^Metacn)]²⁺ is proposed to be the resting state under catalytic conditions. By monitoring Ce^IV consumption, we found that the O₂ evolution rate is redox‐controlled and independent of the initial concentration of Ce^IV. Based on these facts, we propose herein that [Ru^IV(O)(Py₂^Metacn)]²⁺ is oxidised to [Ru^V(O)(Py₂^Metacn)]²⁺ prior to attack by a water molecule to give [Ru^III(OOH)(Py₂^Metacn)]²⁺. Finally, it is shown that the difference in WO reactivity between the homologous iron and ruthenium [M(OH₂)(Py₂^Metacn)]²⁺ (M=Ru, Fe) complexes is due to the difference in the redox stability of the key M^V(O) intermediate. These results contribute to a better understanding of the WO mechanism and the differences between iron and ruthenium complexes in WO reactions.