Electronic Structure of the Water Oxidation Catalyst cis,cis-[(bpy)2(H2O)RuIIIORuIII(OH2)(bpy)2]4+, The Blue Dimer

Structural and electronic forms of doubly oxido/Pz and triply oxido/(Pz)2 bridged mixed valent and isovalent diruthenium complexes (Pz = pyrazolate)

The article reported on the diastereomeric dinuclear mixed-valent complexes [(acac)₂Ru(III)(μ-O)(μ-Pz^R)Ru(IV)(acac)₂] (R = H, Me, meso: ΔΛ, 1a-1c; rac: ΔΔ/ΛΛ, 2a-2c) and rac-[(acac)₂Ru(III)(μ-O)(μ-Iz)Ru(IV)(acac)₂], (2d) (HPz = pyrazole, HIz = indazole, acac = acetylacetonate). Moreover, the diruthenium(II,II) complexes [(HPz)₃Ru(II)(μ-O)(μ-Pz)₂ Ru(II)(HPz)₃] (3a) and [(HIz)₃Ru(II)(μ-O)(μ-Iz)₂Ru(II)(HIz)₃] (3d) were presented. The analogous form of 3a, i.e., [(HPz)₂(Pz)Ru(III)(μ-O)(μ-Pz)₂Ru(III)(Pz)(HPz)₂], was previously reported. Single crystal X-ray structures of 1a-1c/2a-2d and representative 3a showed their molecular forms, including the diastereomeric nature of the former. The Ru-O-Ru angle decreased appreciably on switching from doubly bridged 1 and 2 (128-135°) to triply bridged 3a (114°). Both series of complexes displayed rhombic symmetry in their EPR spectra, with g₁ and g₂ being very similar for 1a-1c with an almost axial look. The mixed-valence complex with a Ru(III)Ru(IV) (S = 1/2) state of 1 and 2 would lead to iso-valence complexes of Ru(III)Ru(III) and Ru(IV)Ru(IV) with an EPR inactive state by one electron redox reaction. On the other hand, metal based {Ru(II)Ru(II)/Ru(II)Ru(III), 3a/3a⁺} and terminal ligand (HPz/HPz^-, 3a/3a^-) based redox processes displayed anisotropic and free radical EPR, respectively. An IVCT (intervalence charge transfer) band was found for the delocalised mixed valent 1 and 2 {Ru(III)Ru(IV)} or 3a⁺ {Ru(II)Ru(III)} in the NIR region. The intense metal-to-ligand charge transfer (MLCT) transitions of 1-3 in the visible region varied systematically as a function of the metal oxidation state.

Amphiphilic Oxo-Bridged Ruthenium "Green Dimer" for Water Oxidation

In 1982, an oxo-bridged dinuclear ruthenium(III) complex, known as “blue dimer,” was discovered to be active for water oxidation. In this work, a new amphiphilic ruthenium “green dimer” 2, obtained from an amphiphilic mononuclear Ru(bda) (N-OTEG) (L1) (1; N-OTEG = 4-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-pyridine; L1 = vinylpyridine) is reported. An array of mechanistic studies identifies “green dimer” 2 as a mixed valence of Ru^II-O-Ru^III oxo-bridged structure. Bearing the same bda^2- and amphiphilic axial ligands, monomer 1 and green dimer 2 can be reversibly converted by ascorbic acid and oxygen, respectively, in aqueous solution. More importantly, the oxo-bridged “green dimer” 2 was found to take water nucleophilic attack for oxygen evolution, in contrast to monomer 1 via radical coupling pathway for O-O bond formation. This is the first report of an amphiphilic oxo-bridged catalyst, which possesses a new oxygen evolution pathway of Ru-bda catalysts.

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Green dimer (Ru^II-O-Ru^III), referring to “blue dimer” of Ru^III-O-Ru^III, is disclosed

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The first amphiphilic μ-oxido-bridged catalyst is reported active for water oxidation

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The oxo-bridged “green dimer” 2 takes water nucleophilic attack for O-O bond formation

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This is the first Ru-bda catalyst, which possesses a new oxygen evolution pathway

Ruthenium Trichloride Catalyst in Water: Ru Colloids versus Ru Dimer Characterization Investigations

An easy-to-prepare ruthenium catalyst obtained from ruthenium(III) trichloride in water demonstrates efficient performances in the oxidation of several cycloalkanes with high selectivity toward the ketone. In this work, several physicochemical techniques were used to demonstrate the real nature of the ruthenium salt still unknown in water and to define the active species for this Csp³-H bond functionalization. From transmission electron microscopy analyses corroborated by SAXS analyses, spherical nanoobjects were observed with an average diameter of 1.75 nm, thus being in favor of the formation of reduced species. However, further investigations, based on X-ray scattering and absorption analyses, showed no evidence of the presence of a metallic Ru-Ru bond, proof of zerovalent nanoparticles, but the existence of Ru-O and Ru-Cl bonds, and thus the formation of a water-soluble complex. The EXAFS (extended X-ray absorption fine structure) spectra revealed the presence of an oxygen-bridged diruthenium complex [Ru(OH) _xCl_{3- x}]₂(μ-O) with a high oxidation state in agreement with catalytic results. This study constitutes a significant advance to determine the true nature of the RuCl₃·3H₂O salt in water and proves once again the invasive nature of the electron beam in microscopy experiments, routinely used in nanochemistry.

Electrochemical and Resonance Raman Spectroscopic Studies of Water-Oxidizing Ruthenium Terpyridyl-Bipyridyl Complexes

The irreversible conversion of single-site water-oxidation catalysts (WOC) into more rugged catalysts structurally related to [(trpy)(5,5'-X₂ -bpy)Ru^IV (μ-O)Ru^IV (trpy)(O)(H₂ O)]⁴⁺ (X=H, 1-dn⁴⁺ ; X=F, 2-dn⁴⁺ ; bpy=2,2'-bipyridine; trpy=2,2':6',2"-terpyridine) represents a critical issue in the development of active and durable WOCs. In this work, the electrochemical and acid-base properties of 1-dn⁴⁺ and 2-dn⁴⁺ were evaluated. In situ resonance Raman spectroscopy was employed to characterize the species formed upon the stoichiometric oxidation of the single-site catalysts and demonstrated the formation of high-oxidation-state mononuclear Ru=O and RuO-O complexes. Under turnover conditions, the dinuclear intermediates, 1-dn⁴⁺ and 2-dn⁴⁺ as well as the previously proposed [Ru^VI (trpy)(O)₂ (H₂ O)]²⁺ complex (3²⁺ ) are formed. Complex 3²⁺ is a pivotal intermediate that provides access to the formation of dinuclear species. Single-crystal X-ray diffraction analysis of the isolated complex [Ru^IV (O)(trpy)(5,5'-F₂ -bpy)]²⁺ reveals a clear elongation of the Ru-N bond trans to the oxido ligand that documents the weakness of this bond, which promotes the release of the bpy ligand and the subsequent formation of 3²⁺ .

Dinuclear nitrido-bridged ruthenium complexes bearing diimine ligands

Nitrido-bridged ruthenium complexes are synthesized via ligand substitution reactions and evaluated for mitochondrial calcium uptake inhibition.

Synthesis and properties of new mononuclear Ru(ii)-based photocatalysts containing 4,4′-diphenyl-2,2′-bipyridyl ligands

Almost colourless trans-Ru^IICl₂(N^N)(CO)₂ (N^N = a derivative of 4,4′-diphenyl-2,2′-bipyridyl) complexes are reasonably effective photocatalytic oxidants in combination with a photosensitizer and sacrificial oxidant.

How a [Co(IV) a bond and a half O](2+) fragment oxidizes water: involvement of a biradicaloid [Co(II)-(⋅O⋅)](2+) species in forming the O-O bond

The mechanism of water oxidation performed by a recently discovered cobalt complex [Co(Py5)(OH2)](ClO4)2 (1; Py5=2,6-(bis(bis-2-pyridyl)-methoxymethane)pyridine) was examined using quantum chemical models based on density functional theory. The computer models were first benchmarked against the experimental cyclic voltammetry data to identify the catalytically competent resting state of the catalyst, which was thought to contain a Co(IV) -oxyl complex. The electronic structure calculations suggest that the low-spin doublet state is energetically most favorable, but the catalytically most active species is the intermediate-spin quartet complex that is almost isoenergetic with the doublet state. The electronic structure of the quartet state shows significant spin polarization on the terminal oxygen atom, which is consistent with an intramolecular electron transfer from the oxygen to the metal. Based on the calculated spin densities, the formally [Co(IV) a bond and a half O] can be viewed as a biradicaloid [Co(II)-(⋅O⋅)](2+), that is, a cobalt-oxene moiety. This electronic structure is reminiscent of many other systems where similar electronic patterns were proposed to be responsible for the oxidative reactivity. In this context, this first-row transition-metal system constitutes a logical extension, because the oxyl-radical character is maximized by using the more easily accessible high-spin configurations in which two half-filled Co-dπ orbitals can work in concert to maximize the oxyl-radical character to ultimately afford a new reactive intermediate that can be characterized as carrying a biradicaloid oxene moiety with a formal oxidation state of zero. This conceptual proposal for the catalytically active species provides a plausible rationale for the remarkable oxidative reactivity.

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.

Coupled states in dinitrofluorene: relationships between ground state and excited state mixed valence

The electronic absorption spectrum of 9,9-dimethyl-2,7-dinitrofluorene radical anion in HMPA displays both a NIR intervalence charge transfer and a visible excited state mixed valence transition. These transitions contain a similar vibronic progression resulting from molecular orbitals that are common to both transitions. Vibrational frequency and intensity data are acquired from the resonance Raman spectrum and used to calculate a best fit for the absorption spectrum. The normal coordinate distortions are analyzed in terms of the electronic changes for both transitions to explain their similarity. The Raman scattering intensity decreases at lower excitation wavelength as a result of Raman de-enhancement caused by interference between neighboring excited states.

Three distinct redox states of an oxo-bridged dinuclear ruthenium complex

A series of [{(terpy)(bpy)Ru}(μ-O){Ru(bpy)(terpy)}](n+) ([RuORu](n+), terpy=2,2';6',2''-terpyridine, bpy=2,2'-bipyridine) was systematically synthesized and characterized in three distinct redox states (n=3, 4, and 5 for Ru(II,III)2, Ru(III,III)2, and Ru(III,IV)2, respectively). The crystal structures of [RuORu](n+) (n=3, 4, 5) in all three redox states were successfully determined. X-ray crystallography showed that the Ru-O distances and the Ru-O-Ru angles are mainly regulated by the oxidation states of the ruthenium centers. X-ray crystallography and ESR spectra clearly revealed the detailed electronic structures of two mixed-valence complexes, [Ru(III)ORu(IV)](5+) and [Ru(II)ORu(III)](3+), in which each unpaired electron is completely delocalized across the oxo-bridged dinuclear core. These findings allow us to understand the systematic changes in structure and electronic state that accompany the changes in the redox state.

The electronic nature of terminal oxo ligands in transition-metal complexes: ambiphilic reactivity of oxorhenium species

The synthesis of the Lewis acid-base adducts of B(C6F5)3 and BF3 with [DAAmRe(O)(X)] DAAm = N,N-bis(2-arylaminoethyl)methylamine; aryl = C6F5 (X = Me, 1, COCH3, 2, Cl, 3) as well as their diamidopyridine (DAP) (DAP=(2,6-bis((mesitylamino)methyl)pyridine) analogues, [DAPRe(O)(X)] (X = Me, 4, Cl, 5, I, 6, and COCH3,7), are described. In these complexes the terminal oxo ligands act as nucleophiles. In addition we also show that stoichiometric reactions between 3 and triarylphosphine (PAr3) result in the formation of triarylphosphine oxide (OPAr3). The electronic dependence of this reaction was studied by comparing the rates of oxygen atom transfer for various para-substituted triaryl phosphines in the presence of CO. From these experiments a reaction constant ρ = -0.29 was obtained from the Hammett plot. This suggests that the oxygen atom transfer reaction is consistent with nucleophilic attack of phosphorus on an electrophilic metal oxo. To the best of our knowledge, these are the first examples of mono-oxo d(2) metal complexes in which the oxo ligand exhibits ambiphilic reactivity.

Competition between Ultrafast Energy Flow and Electron Transfer in a Ru(II)-Loaded Polyfluorene Light-Harvesting Polymer

This Letter describes the synthesis and photophysical characterization of a Ru(II) assembly consisting of metal polypyridyl complexes linked together by a polyfluorene scaffold. Unlike many scaffolds incorporating saturated linkages, the conjugated polymer in this system acts as a functional light-harvesting component. Conformational disorder breaks the conjugation in the polymer backbone, resulting in a chain composed of many chromophore units, whose relative energies depend on the segment lengths. Photoexcitation of the polyfluorene by a femtosecond laser pulse results in the excitation of polyfluorene, which then undergoes direct energy transfer to the pendant Ru(II) complexes, producing Ru(II)* excited states within 500 fs after photoexcitation. Femtosecond transient absorption data show the presence of electron transfer from PF* to Ru(II) to form charge-separated (CS) products within 1-2 ps. The decay of the oxidized and reduced products, PF(+•) and Ru(I), through back electron transfer are followed using picosecond transient absorption methods.