Catalysis of Mononuclear Aquaruthenium Complexes in Oxygen Evolution from Water: A New Radical Coupling Path using Hydroxocerium(IV) Species

The mechanism of O₂ evolution from water catalyzed by a series of mononuclear aquaruthenium complexes, [Ru(terpy)(bpy)(OH₂)]²⁺, [Ru(tmtacn)(R₂bpy)(OH₂)]²⁺ (R=H, Me, and OMe; R₂bpy=4,4′‐disubstituted‐2,2′‐bipyridines), and [Ru(tpzm)(R₂bpy)(OH₂)]²⁺ (R=H, Me, and OMe), is investigated, where terpy=2,2′:6′,2′′‐terpyridine, bpy=2,2′‐bipyridine, tmtacn=1,4,7‐trimethyl‐1,4,7‐triazacyclononane, and tpzm=tris(1‐pyrazolyl)methane. The kinetics of O₂ evolution is investigated as a function of either the catalyst concentration or the oxidant concentration by employing Ce(NH₄)₂(NO₃)₆ as an oxidant; these catalysts can be classified into two groups that have different rate laws for O₂ evolution. In one class, the rate of O₂ evolution is linear to both the catalyst and Ce⁴⁺ concentrations, as briefly reported for [Ru(terpy)(bpy)(OH₂)]²⁺ (S. Masaoka, K. Sakai, Chem. Lett. 2009 , 38, 182). For the other class, [Ru(tmtacn)(R₂bpy)(OH₂)]²⁺, the rate of O₂ evolution is quadratic to the catalyst concentration and independent of the Ce⁴⁺ concentration. Moreover, the singlet biradical character of the hydroxocerium(IV) ion was realized by experimental and DFT investigations. These results indicate that the radical coupling between the oxygen atoms of a Ru^V?O species and a hydroxocerium(IV) ion is the key step for the catalysis of [Ru(terpy)(bpy)(OH₂)]²⁺ and [Ru(tpzm)(R₂bpy)(OH₂)]²⁺, while the well‐known oxo‐oxo radical coupling among two Ru^V?O species proceeds in the catalysis of [Ru(tmtacn)(R₂bpy)(OH₂)]²⁺. This is the first report demonstrating that the radical character provided by the hydroxocerium(IV) ion plays a crucial role in the catalysis of such ruthenium complexes in the evolution of O₂ from water.     

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⁺.     

Controlling Metal–Ligand–Metal Oxidation State Combinations by Ancillary Ligand (L) Variation in the Redox Systems [L2Ru(μ‐boptz)RuL2]n,boptz=3,6‐bis(2‐oxidophenyl)‐1,2,4,5‐tetrazine,and L=acetylacetonate, 2,2′‐bipyridine,or 2‐phenylazopyridine

The new compounds [(acac)₂Ru(μ‐boptz)Ru(acac)₂] ( 1 ), [(bpy)₂Ru(μ‐boptz)Ru(bpy)₂](ClO₄)₂ ( 2 ‐(ClO₄)₂), and [(pap)₂Ru(μ‐boptz)Ru(pap)₂](ClO₄)₂ ( 3 ‐(ClO₄)₂) were obtained from 3,6‐bis(2‐hydroxyphenyl)‐1,2,4,5‐tetrazine (H₂boptz), the crystal structure analysis of which is reported. Compound 1 contains two antiferromagnetically coupled (J=?36.7 cm^?1) Ru^III centers. We have investigated the role of both the donor and acceptor functions containing the boptz^2? bridging ligand in combination with the electronically different ancillary ligands (donating acac^?, moderately π‐accepting bpy, and strongly π‐accepting pap; acac=acetylacetonate, bpy=2,2′‐bipyridine pap=2‐phenylazopyridine) by using cyclic voltammetry, spectroelectrochemistry and electron paramagnetic resonance (EPR) spectroscopy for several in situ accessible redox states. We found that metal–ligand–metal oxidation state combinations remain invariant to ancillary ligand change in some instances; however, three isoelectronic paramagnetic cores Ru(μ‐boptz)Ru showed remarkable differences. The excellent tolerance of the bpy co ‐ ligand for both Ru^III and Ru^II is demonstrated by the adoption of the mixed ‐ valent form in [L₂Ru(μ‐boptz)RuL₂]³⁺, L=bpy, whereas the corresponding system with pap stabilizes the Ru^II states to yield a phenoxyl radical ligand and the compound with L=acac^? contains two Ru^III centers connected by a tetrazine radical‐anion bridge.     

Oxygen Evolution Catalyzed by a Mononuclear Ruthenium Complex Bearing Pendant SO3− Groups

Rational molecular design of catalytic systems capable of smooth O? O bond formation is critical to the development of efficient catalysts for water oxidation. A new ruthenium complex was developed, which bears pendant SO₃^? groups in the secondary coordination sphere: [Ru(terpy)(bpyms)(OH₂)] (terpy=2,2′:6′,2′′‐terpyridine, bpyms=2,2′‐bipyridine‐5,5′‐bis(methanesulfonate)). Water oxidation driven by a Ce⁴⁺ oxidant is distinctly accelerated upon introduction of the pendant SO₃^? groups in comparisons to the parent catalyst, [Ru(terpy)(bpy)(OH₂)]²⁺ (bpy=2,2′‐bipyridine). Spectroscopic, electrochemical, and crystallographic investigations concluded that the pendant SO₃^? groups promote the formation of an O? O bond via the secondary coordination sphere on the catalyst, whereas the influence of the pendant SO₃^? groups on the electronic structure of the [Ru(terpy)(bpy)(OH₂)]²⁺ core is negligible. The results of this work indicate that modification of the secondary coordination sphere is a valuable strategy for the design of water oxidation catalysts.     

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.     

Near‐IR Absorbing Ruthenium Complexes of Non‐Innocent 6,12‐Di(pyridin‐2‐yl)indolo[3,2‐b]carbazole: Variation as a Function of Co‐Ligands

This article deals with the hitherto unexplored metal complexes of deprotonated 6,12‐di(pyridin‐2‐yl)‐5,11‐dihydroindolo[3,2‐b]carbazole (H₂L). The synthesis and structural, optical, electrochemical characterization of dimeric [{Ru^III(acac)₂}₂(μ‐L^.?)]ClO₄ ([ 1 ]ClO₄, S=1/2), [{Ru^II(bpy)₂}₂(μ‐L^.?)](ClO₄)₃ ([ 2 ](ClO₄)₃, S=1/2), [{Ru^II(pap)₂}₂(μ‐L^2?)](ClO₄)₂ ([ 4 ](ClO₄)₂, S=0), and monomeric [(bpy)₂Ru^II(HL^?)]ClO₄ ([ 3 ]ClO₄, S=0), [(pap)₂Ru^II(HL^?)]ClO₄ ([ 5 ]ClO₄, S=0) (acac=σ‐donating acetylacetonate, bpy=moderately π‐accepting 2,2’‐bipyridine, pap=strongly π‐accepting 2‐phenylazopyridine) are reported. The radical and dianionic states of deprotonated L in isolated dimeric 1 ⁺/ 2 ³⁺ and 4 ²⁺, respectively, could be attributed to the varying electronic features of the ancillary (acac, bpy, and pap) ligands, as was reflected in their redox potentials. Perturbation of the energy level of the deprotonated L or HL upon coordination with {Ru(acac)₂}, {Ru(bpy)₂}, or {Ru(pap)₂} led to the smaller energy gap in the frontier molecular orbitals (FMO), resulting in bathochromically shifted NIR absorption bands (800–2000 nm) in the accessible redox states of the complexes, which varied to some extent as a function of the ancillary ligands. Spectroelectrochemical (UV/Vis/NIR, EPR) studies along with DFT/TD‐DFT calculations revealed (i) involvement of deprotonated L or HL in the oxidation processes owing to its redox non‐innocent potential and (ii) metal (Ru^III/Ru^II) or bpy/pap dominated reduction processes in 1 ⁺ or 2 ²⁺/ 3 ⁺/ 4 ²⁺/ 5 ⁺, respectively.     

Catalytic Four‐Electron Oxidation of Water by Intramolecular Coupling of the Oxo Ligands of a Bis(ruthenium–bipyridine) Complex

A bis(ruthenium–bipyridine) complex bridged by 1,8‐bis(2,2′:6′,2′′‐terpyrid‐4′‐yl)anthracene (btpyan), [Ru₂(μ‐Cl)(bpy)₂(btpyan)](BF₄)₃ ([ 1 ](BF₄)₃; bpy=2,2′‐bipyridine), was prepared. The cyclic voltammogram of [ 1 ](BF₄)₃ in water at pH 1.0 displayed two reversible [Ru^II,Ru^II]³⁺/[Ru^II,Ru^III]⁴⁺ and [Ru^II,Ru^III]⁴⁺/[Ru^III,Ru^III]⁵⁺ redox couples at E_1/2(1)=+0.61 and E_1/2(2)=+0.80 V (vs. Ag/AgCl), respectively, and an irreversible anodic peak at around E=+1.2 V followed by a strong anodic currents as a result of the oxidation of water. The controlled potential electrolysis of [ 1 ]³⁺ ions at E=+1.60 V in water at pH 2.6 (buffered with H₃PO₄/NaH₂PO₄) catalytically evolved dioxygen. Immediately after the electrolysis of the [ 1 ]³⁺ ion in H₂¹⁶O at E=+1.40 V, the resultant solution displayed two resonance Raman bands at $\tilde \nu $ =442 and 824 cm^‐1. These bands shifted to $\tilde \nu $ =426 and 780 cm^?1, respectively, when the same electrolysis was conducted in H₂¹⁸O. The chemical oxidation of the [ 1 ]³⁺ ion by using a Ce^IV species in H₂¹⁶O and H₂¹⁸O also exhibited the same resonance Raman spectra. The observed isotope frequency shifts (Δ$\tilde \nu $ =16 and 44 cm^?1) fully fit the calculated ones based on the Ru? O and O? O stretching modes, respectively. The first successful identification of the metal? O? O? metal stretching band in the oxidation of water indicates that the oxygen–oxygen bond at the stage prior to the evolution of O₂ is formed through the intramolecular coupling of two Ru–oxo groups derived from the [ 1 ]³⁺ ion.     

Density Functional Theory Calculations on Oxidative CC Bond Cleavage and NO Bond Formation of [RuII(bpy)2(diamine)]2+ via Reactive Ruthenium Imide Intermediates

DFT calculations are performed on [Ru^II(bpy)₂(tmen)]²⁺ ( M1 , tmen=2,3‐dimethyl‐2,3‐butanediamine) and [Ru^II(bpy)₂(heda)]²⁺ ( M2 , heda=2,5‐dimethyl‐2,5‐hexanediamine), and on the oxidation reactions of M1 to give the C?C bond cleavage product [Ru^II(bpy)₂(NH=CMe₂)₂]²⁺ ( M3 ) and the N?O bond formation product [Ru^II(bpy)₂(ONCMe₂CMe₂NO)]²⁺ ( M4 ). The calculated geometrical parameters and oxidation potentials are in good agreement with the experimental data. As revealed by the DFT calculations, [Ru^II(bpy)₂(tmen)]²⁺ ( M1 ) can undergo oxidative deprotonation to generate Ru‐bis(imide) [Ru(bpy)₂(tmen‐4 H)]⁺ ( A ) or Ru‐imide/amide [Ru(bpy)₂(tmen‐3 H)]²⁺ ( A′ ) intermediates. Both A and A′ are prone to C?C bond cleavage, with low reaction barriers (ΔG^≠) of 6.8 and 2.9 kcal mol^?1 for their doublet spin states ² A and ² A′ , respectively. The calculated reaction barrier for the nucleophilic attack of water molecules on ² A′ is relatively high (14.2 kcal mol^?1). These calculation results are in agreement with the formation of the Ru^II‐bis(imine) complex M3 from the electrochemical oxidation of M1 in aqueous solution. The oxidation of M1 with Ce^IV in aqueous solution to afford the Ru^II‐dinitrosoalkane complex M4 is proposed to proceed by attack of the cerium oxidant on the ruthenium imide intermediate. The findings of ESI‐MS experiments are consistent with the generation of a ruthenium imide intermediate in the course of the oxidation.     

Tuning Excited State Isomerization Dynamics through Ground State Structural Changes in Analogous Ruthenium and Osmium Sulfoxide Complexes

The complexes [Ru(bpy)₂(pyESO)](PF₆)₂ and [Os(bpy)₂(pyESO)](PF₆)₂, in which bpy is 2,2′‐bipyridine and pyESO is 2‐((isopropylsulfinyl)ethyl)pyridine, were prepared and studied by ¹H NMR, UV–visible and ultrafast transient absorption spectroscopy, as well as by electrochemical methods. Crystals suitable for X‐ray structural analysis were grown for [Ru(bpy)₂(pyESO)](PF₆)₂. Cyclic voltammograms of both complexes provide evidence for S→O and O→S isomerization as these voltammograms are described by an ECEC (electrochemical‐chemical electrochemical‐chemical) mechanism in which isomerization follows Ru²⁺ oxidation and Ru³⁺ reduction. The S‐ and O‐bonded Ru^3+/2+ couples appear at 1.30 and 0.76 V versus Ag/AgCl in propylene carbonate. For [Os(bpy)₂(pyESO)](PF₆)₂, these couples appear at 0.97 and 0.32 V versus Ag/AgCl in acetonitrile, respectively. Charge‐transfer excitation of [Ru(bpy)₂(pyESO)](PF₆)₂ results in a significant change in the absorption spectrum. The S‐bonded isomer of [Ru(bpy)₂(pyESO)]²⁺ features a lowest energy absorption maximum at 390 nm and the O‐bonded isomer absorbs at 480 nm. The quantum yield of isomerization in [Ru(bpy)₂(pyESO)]²⁺ was found to be 0.58 in propylene carbonate and 0.86 in dichloroethane solution. Femtosecond transient absorption spectroscopic measurements were collected for both complexes, revealing time constants of isomerizations of 81 ps (propylene carbonate) and 47 ps (dichloroethane) in [Ru(bpy)₂(pyESO)]²⁺. These data and a model for the isomerizing complex are presented. A striking conclusion from this analysis is that expansion of the chelate ring by a single methylene leads to an increase in the isomerization time constant by nearly two orders of magnitude.     

Spectrocyclic Voltammetry of Ruthenium Purple Electrodeposited on a WO3/Tris(2,2′-bipyridine)-ruthenium(II)/Polymer Hybrid Film

Summary: Electrochemical reactions of Ruthenium purple, Feequation/tex2gif-stack-1.gif[Ru^II(CN)₆]₃ (RP; Fe^III-Ru^II) were studied using a spectrocyclic voltammetry (SCV) technique. The SCV measurement for an RP film coated on an ITO electrode showed a reversible redox between RP and Ruthenium white (RW; Fe^II-Ru^II) at 0.14 V vs saturated calomel reference electrode (SCE). An RP film was electrodeposited on a hybrid film of tungsten trioxide (WO₃)/tris(2,2′-bipyridine)ruthenium(II) ([Ru(bpy)₃]²⁺; bpy = 2,2′-bipyridine)/poly(sodium 4-styrenesulfonate) (PSS) (denoted as WRP film) from a colloidal solution containing 0.5 mM FeCl₃, 0.5 mM K₄[Ru(CN)₆] and 40 mM KCl using a potentiodynamic multi-sweep technique. In a cyclic voltammogram (CV) of a WRP/RP film, a redox response was observed at 0.61 V in addition to essential redox responses of WRP hybrid film (a [Ru(bpy)₃]²⁺/[Ru(bpy)₃]³⁺ redox at 1.03 V and a H_xWO₃/WO₃ redox below 0.09 V), but a redox response of RW/RP was not observed at 0.14 V. The SCV measurement for the WRP/RP film suggested that the redox response at 0.61 V is attributed to a redox of [Ru(bpy)₃]²⁺/[Ru(bpy)₃]³⁺ interacted electrostatically with RP. It also showed that RW is oxidized to RP via [Ru(bpy)₃]²⁺/[Ru(bpy)₃]³⁺ redox and RP is reversibly reduced to RW via H_xWO₃/WO₃ redox. This unique geared electrochemical reaction for the WRP/RP film leads to a hysteresis property of an RW/RP redox.     

Systematic tuning of the reactivity of [RuII(terpy)(N^N)Cl]Cl complexes

Abstract

The substitution behavior of the [Ru^II(terpy)(ampy)Cl]Cl (terpy = 2,2′:6′,2′′-terpyridine, ampy = 2-(aminomethyl)pyridine) complex in water with several bio-relevant ligands such as chloride, thiourea and N,N′-dimethylthiourea, was investigated and compared with the reactivity of the [Ru^II(terpy)(bipy)Cl]Cl and [Ru^II(terpy)(en)Cl]Cl (bipy =2,2′-bipyridine and en?=?ethylenediamine) complexes. Earlier results have shown that the reactivity and pK_a values of Ru(II) complexes can be tuned by a systematic variation of electronic effects provided by bidentate spectator chelates. The reactivity of both the chlorido and aqua derivatives of the studied Ru(II) complexes increases in the order [Ru^II(terpy)(bipy)X]^+/2+?<?[Ru^II(terpy)(ampy)X]^+/2+?<?[Ru^II(terpy)(en)X]^+/2+. This finding can be accounted for in terms of π back-bonding effects provided by the pyridine ligands. The activation parameters for all the studied reactions support an associative interchange substitution mechanism.     

Evaluation of the Oxo‐bridged Dinuclear Ruthenium Ammine Complex as Redox Mediator in an Electrochemical Biosensor

The mediation of electron‐transfer by oxo‐bridged dinuclear ruthenium ammine [(bpy)₂(NH₃)Ru^III(µ‐O)Ru^III(NH₃)(bpy)₂]⁴⁺ for the oxidation of glucose was investigated by cyclic voltammetry. These ruthenium (III) complexes exhibit appropriate redox potentials of 0.131–0.09 V vs. SCE to act as electron‐transfer mediators. The plot of anodic current vs. the glucose concentration was linear in the concentration range between 2.52×10^?5 and 1.00×10^?4 mol L^?1. Moreover, the apparent Michaelis‐Menten kinetic (K_M^app) and the catalytic (K_cat) constants were 8.757×10^?6 mol L^?1 and 1,956 s^?1, respectively, demonstrating the efficiency of the ruthenium dinuclear oxo‐complex [(bpy)₂(NH₃)Ru^III(µ‐O)Ru^III(NH₃)(bpy)₂]⁴⁺ as mediator of redox electron‐transfer.     

A Ruthenium(III)–Oxyl Complex Bearing Strong Radical Character

Proton‐coupled electron‐transfer oxidation of a Ru^II?OH₂ complex, having an N‐heterocyclic carbene ligand, gives a Ru^III?O^. species, which has an electronically equivalent structure of the Ru^IV=O species, in an acidic aqueous solution. The Ru^III?O^. complex was characterized by spectroscopic methods and DFT calculations. The oxidation state of the Ru center was shown to be close to +3; the Ru?O bond showed a lower‐energy Raman scattering at 732 cm^?1 and the Ru?O bond length was estimated to be 1.77(1) Å. The Ru^III?O^. complex exhibits high reactivity in substrate oxidation under catalytic conditions; particularly, benzaldehyde and the derivatives are oxidized to the corresponding benzoic acid through C?H abstraction from the formyl group by the Ru^III?O^. complex bearing a strong radical character as the active species.     

Application of a Structure/Oxidation‐State Correlation to Complexes of Bridging Azo Ligands

Based on data from more than 40 crystal structures of metal complexes with azo‐based bridging ligands (2,2′‐azobispyridine, 2,2′‐azobis(5‐chloropyrimidine), azodicarbonyl derivatives), a correlation between the N? N bond lengths (d_NN) and the oxidation state of the ligand (neutral, neutral/back‐donating, radical‐anionic, dianionic) was derived. This correlation was applied to the analysis of four ruthenium compounds of 2,2′‐azobispyridine (abpy), that is, the new asymmetrical rac‐[(acac)₂Ru1(μ‐abpy)Ru2(bpy)₂](ClO₄)₂ ([ 1 ](ClO₄)₂), [Ru(acac)₂(abpy)] ( 2 ), [Ru(bpy)₂(abpy)](ClO₄)₂ ([ 3 ](ClO₄)₂), and meso‐[(bpy)₂Ru(μ‐abpy)Ru(bpy)₂](ClO₄)₃ ([ 4 ](ClO₄)₃; acac^?=2,4‐pentanedionato, bpy=2,2′‐bipyridine). In agreement with DFT calculations, both mononuclear species 2 and 3 ²⁺ can be described as ruthenium(II) complexes of unreduced abpy⁰, with 1.295(5)<d_NN<1.320(3) Å, thereby exhibiting effects from π back‐donation. However, the abpy ligand in both the asymmetrical diamagnetic compound 1 ²⁺ (d_NN=1.374(6) Å) and the symmetrical compound 4 ³⁺ (d_NN=1.360(7), 1.368(8) Å) must be formulated as abpy^.?. Remarkably, the addition of [Ru^II(bpy)₂]²⁺ to mononuclear [Ru^II(acac)₂(abpy⁰)] induces intracomplex electron‐transfer under participation of the noninnocent abpy bridge to yield rac‐[(acac)₂Ru1^III(μ‐abpy^.?)Ru2^II(bpy)₂]²⁺ ( 1 ²⁺) with strong antiferromagnetic coupling between abpy^.? and Ru^III (DFT (B3LYP/LANL2DZ/6‐31G*)‐calculated triplet–singlet energy separation E_S=1?E_S=0=11739 cm^?1). Stepwise one‐electron transfer was studied for compound 1 ⁿ, n=1?, 0, 1+, 2+, 3+, by UV/Vis/NIR spectroelectrochemistry, EPR spectroscopy, and by DFT calculations. Whereas the first oxidation of compound 1 ²⁺ was found to mainly involve the central ligand to produce an (abpy⁰)‐bridged Class I mixed‐valent Ru1^IIIRu2^II species, the first reduction of compound 1 ²⁺ affected both the bridge and Ru1 atom to form a radical complex ( 1 ⁺), with considerable metal participation in the spin‐distribution. Further reduction moves the spin towards the {Ru2(bpy)₂} entity.     

Metal‐catalyzed reversible conversion between chemical and electrical energy designed towards a sustainable society

Proton dissociation of an aqua‐Ru‐quinone complex, [Ru(trpy)(q)(OH₂)]²⁺ (trpy = 2,2′ : 6′,2″‐terpyridine, q = 3,5‐di‐t‐butylquinone) proceeded in two steps (pK_a = 5.5 and ca. 10.5). The first step simply produced [Ru(trpy)(q)(OH)]⁺, while the second one gave an unusual oxyl radical complex, [Ru(trpy)(sq)(O^?.)]⁰ (sq = 3,5‐di‐t‐butylsemiquinone), owing to an intramolecular electron transfer from the resultant O^2? to q. A dinuclear Ru complex bridged by an anthracene framework, [Ru₂(btpyan)(q)₂(OH)₂]²⁺ (btpyan = 1,8‐bis(2,2′‐terpyridyl)anthracene), was prepared to place two Ru(trpy)(q)(OH) groups at a close distance. Deprotonation of the two hydroxy protons of [Ru₂(btpyan)(q)₂(OH)₂]²⁺ generated two oxyl radical Ru‐O^?. groups, which worked as a precursor for O₂ evolution in the oxidation of water. The [Ru₂(btpyan)(q)₂(OH)₂](SbF₆)₂ modified ITO electrode effectively catalyzed four‐electron oxidation of water to evolve O₂ (TON = 33500) under electrolysis at +1.70 V in H₂O (pH 4.0). Various physical measurements and DFT calculations indicated that a radical coupling between two Ru(sq)(O^?.) groups forms a (cat)Ru‐O‐O‐Ru(sq) (cat = 3,5‐di‐t‐butylcathechol) framework with a μ‐superoxo bond. Successive removal of four electrons from the cat, sq, and superoxo groups of [Ru₂(btpyan)(cat)(sq)(μ‐O₂^?)]⁰ assisted with an attack of two water (or OH^?) to Ru centers, which causes smooth O₂ evolution with regeneration of [Ru₂(btpyan)(q)₂(OH)₂]²⁺. Deprotonation of an Ru‐quinone‐ammonia complex also gave the corresponding Ru‐semiquinone‐aminyl radical. The oxidized form of the latter showed a high catalytic activity towards the oxidation of methanol in the presence of base. Three complexes, [Ru(bpy)₂(CO)₂]²⁺, [Ru(bpy)₂(CO)(C(O)OH)]⁺, and [Ru(bpy)₂(CO)(CO₂)]⁰ exist as an equilibrium mixture in water. Treatment of [Ru(bpy)₂(CO)₂]²⁺ with BH₄^? gave [Ru(bpy)₂(CO)(C(O)H)]⁺, [Ru(bpy)₂(CO)(CH₂OH)]⁺, and [Ru(bpy)₂(CO)(OH₂)]²⁺ with generation of CH₃OH in aqueous conditions. Based on these results, a reasonable catalytic pathway from CO₂ to CH₃OH in electro‐ and photochemical CO₂ reduction is proposed. A new pbn (pbn = 2‐pyridylbenzo[b]‐1,5‐naphthyridine) ligand was designed as a renewable hydride donor for the six‐electron reduction of CO₂. A series of [Ru(bpy)_3‐n(pbn)_n]²⁺ (n = 1, 2, 3) complexes undergoes photochemical two‐ (n = 1), four‐ (n = 2), and six‐electron reductions (n = 3) under irradiation of visible light in the presence of N(CH₂CH₂OH)₃. © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 169–186; 2009: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.200800039     

Water‐Splitting Electrocatalysis in Acid Conditions Using Ruthenate‐Iridate Pyrochlores

The pyrochlore solid solution (Na_0.33Ce_0.67)₂(Ir_1?xRu_x)₂O₇ (0≤x≤1), containing B‐site Ru^IV and Ir^IV is prepared by hydrothermal synthesis and used as a catalyst layer for electrochemical oxygen evolution from water at pH<7. The materials have atomically mixed Ru and Ir and their nanocrystalline form allows effective fabrication of electrode coatings with improved charge densities over a typical (Ru,Ir)O₂ catalyst. An in situ study of the catalyst layers using XANES spectroscopy at the Ir L_III and Ru K edges shows that both Ru and Ir participate in redox chemistry at oxygen evolution conditions and that Ru is more active than Ir, being oxidized by almost one oxidation state at maximum applied potential, with no evidence for ruthenate or iridate in +6 or higher oxidation states.     

An Unusually Delocalized Mixed‐Valence State of a Cyanidometal‐Bridged Compound Induced by Thermal Electron Transfer

The heterometallic complexes trans ‐[Cp(dppe)FeNCRu(o ‐bpy)CNFe(dppe)Cp][PF₆]_n ( 1 [PF₆]_n , n =2, 3, 4; o ‐bpy=1,2‐bis(2,2′‐bipyridyl‐6‐yl)ethane, dppe=1,2‐bis(diphenylphosphino)ethane, Cp=1,3‐cyclopentadiene) in three distinct states have been synthesized and fully characterized. 1 ³⁺[PF₆]₃ and 1 ⁴⁺[PF₆]₄ are the one‐ and two‐electron oxidation products of 1 ²⁺[PF₆]₂, respectively. The investigated results suggest that 1 [PF₆]₃ is a Class II mixed valence compound. 1 [PF₆]₄ after a thermal treatment at 400 K shows an unusually delocalized mixed valence state of [Fe^III‐NC‐Ru^III‐CN‐Fe^II], which is induced by electron transfer from the central Ru^II to the terminal Fe^III in 1 [PF₆]₄, which was confirmed by IR spectroscopy, magnetic data, and EPR and Mössbauer spectroscopy.     

Efficient Electronic Communication of Two Ruthenium Centers through a Rigid Ditopic N‐Heterocyclic Carbene Linker

A ditopic benzobis(carbene) ligand precursor was prepared that contained a chelating pyridyl moiety to ensure co‐planarity of the carbene ligand and the coordination plane of a bound octahedral metal center. Bimetallic ruthenium complexes comprising this ditopic ligand [L₄Ru‐C,N‐bbi‐C,N‐RuL₄] were obtained by a transmetalation methodology (C,N‐bbi‐C,N=benzobis(N‐pyridyl‐N′‐methyl‐imidazolylidene). The two metal centers are electronically decoupled when the ruthenium is in a pseudotetrahedral geometry imparted by a cymene spectator ligand (L₄=[(cym)Cl]). Ligand exchange of the Cl^?/cymene ligands for two bipyridine or four MeCN ligands induced a change of the coordination geometry to octahedral. As a consequence, the ruthenium centers, separated through space by more than 10 Å, become electronically coupled, which is evidenced by two distinctly different metal‐centered oxidation processes that are separated by 134 mV (L₄=[(bpy)₂]; bpy=2,2′‐bipyridine) and 244 mV (L₄=[(MeCN)₄]), respectively. Hush analysis of the intervalence charge‐transfer bands in the mixed‐valent species indicates substantial valence delocalization in both complexes (delocalization parameter Γ=0.41 and 0.37 in the bpy and MeCN complexes, respectively). Spectroelectrochemical measurements further indicated that the mixed‐valent Ru^II/Ru^III species and the fully oxidized Ru^III/Ru^III complexes gradually decompose when bound to MeCN ligands, whereas the bpy spectators significantly enhance the stability. These results demonstrate the efficiency of carbenes and, in particular, of the bbi ligand scaffold for mediating electron transfer and for the fabrication of molecular redox switches. Moreover, the relevance of spectator ligands is emphasized for tailoring the degree of electronic communication through the benzobis(carbene) linker.     

Contrasting effects of redox potentials on the rate constants of oxidation and hydrolysis reactions in ruthenium complexes

We report in this work the rate constant of oxidation by peroxydisulfate of the ammine ruthenium center in [(bpy)₂Ru(μ-5-CNphen)Ru(NH₃)₅]⁴⁺ (bpy?=?2,2′-bipyridine and 5-CNphen?=?5-cyano-1,10-phenanthroline) and the rate constant of hydrolysis of coordinated acetonitrile in [Ru(TPTZ)(bpy)(CH₃CN)]²⁺ (TPTZ = 2,4,6-tris(2-pyridil)-1,3,5-triazine). With these data and literature values of related reactions, we establish the existence of contrasting effects of redox potentials of Ru^3+/2+ couples on the rates of both processes.     

Chemical and Photochemical Water Oxidation Catalyzed by Mononuclear Ruthenium Complexes with a Negatively Charged Tridentate Ligand

Two mononuclear ruthenium complexes [RuL(pic)₃] ( 1 ) and [RuL(bpy)(pic)] ( 2 ) (H₂L=2,6‐pyridinedicarboxylic acid, pic=4‐picoline, bpy=2,2′‐bipyridine) have been synthesized and fully characterized. Both complexes could promote water oxidation chemically and photochemically. Compared with other known ruthenium‐based water oxidation catalysts using [Ce(NH₄)₂(NO₃)₆] (Ce^IV) as the oxidant in solution at pH 1.0, complex 1 is one of the most active catalysts yet reported with an initial rate of 0.23 turnover s^?1. Under acidic conditions, the equatorial 4‐picoline in complex 1 dissociates first. In addition, ligand exchange in 1 occurs when the Ru^III state is reached. Based on the above observations and MS measurements of the intermediates during water oxidation by 1 using Ce^IV as oxidant, [RuL(pic)₂(H₂O)]⁺ is proposed as the real water oxidation catalyst.