Visible‐Light‐Driven Water Oxidation by a Molecular Manganese Vanadium Oxide Cluster

Photosynthetic water oxidation in plants occurs at an inorganic calcium manganese oxo cluster, which is known as the oxygen evolving complex (OEC), in photosystem II. Herein, we report a synthetic OEC model based on a molecular manganese vanadium oxide cluster, [Mn₄V₄O₁₇(OAc)₃]^3?. The compound is based on a [Mn₄O₄]⁶⁺ cubane core, which catalyzes the homogeneous, visible‐light‐driven oxidation of water to molecular oxygen and is stabilized by a tripodal [V₄O₁₃]^6? polyoxovanadate and three acetate ligands. When combined with the photosensitizer [Ru(bpy)₃]²⁺ and the oxidant persulfate, visible‐light‐driven water oxidation with turnover numbers of approximately 1150 and turnover frequencies of about 1.75 s^?1 is observed. Electrochemical, mass‐spectrometric, and spectroscopic studies provide insight into the cluster stability and reactivity. This compound could serve as a model for the molecular structure and reactivity of the OEC and for heterogeneous metal oxide water‐oxidation catalysts.     

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.     

Two‐Photon Chemistry in Ruthenium 2,2′‐Bipyridyl‐Functionalized Single‐Wall Carbon Nanotubes

Ruthenium polypyridyl complexes are widely used as light harvesters in dye‐sensitized solar cells. Since one of the potential applications of single‐wall carbon nanotubes (SWCNTs) and their derived materials is their use as active components in organic and hybrid solar cells, the study of the photochemistry of SWCNTs with tethered ruthenium polypyridyl complexes is important. A water‐soluble ruthenium tris(bipyridyl) complex linked through peptidic bonds to SWCNTs (Ru‐SWCNTs) was prepared by radical addition of thiol‐terminated SWCNT to a terminal C?C double bond of a bipyridyl ligand of the ruthenium tris(bipyridyl) complex. The resulting macromolecular Ru‐SWCNT (≈500 nm, 15.6 % ruthenium complex content) was water‐soluble and was characterized by using TEM, thermogravimetric analysis, chemical analysis, and optical spectroscopy. The emission of Ru‐SWCNT is 1.6 times weaker than that of a mixture of [Ru(bpy)₃]²⁺ and SWCNT of similar concentration. Time‐resolved absorption optical spectroscopy allows the detection of the [Ru(bpy)₃]²⁺‐excited triplet and [Ru(bpy)₃]⁺. The laser flash studies reveal that Ru‐SWCNT exhibits an unprecedented two‐photon process that is enabled by the semiconducting properties of the SWCNT. Thus, the effect of the excitation wavelength and laser power on the transient spectra indicate that upon excitation of two [Ru(bpy)₃]²⁺ complexes of Ru‐SWCNT, a disproportionation process occurs leading to delayed formation of [Ru(bpy)₃]⁺ and the performance of the SWCNT as a semiconductor. This two‐photon delayed [Ru(bpy)₃]⁺ generation is not observed in the photolysis of [Ru(bpy)₃]³⁺; SWCNT acts as an electron wire or electron relay in the disproportionation of two [Ru(bpy)₃]²⁺ triplets in a process that illustrates that the SWCNT plays a key role in the process. We propose a mechanism for this two‐photon disproportionation compatible with i) the need for high laser flux, ii) the long lifetime of the [Ru(bpy)₃]²⁺ triplets, iii) the semiconducting properties of the SWNT, and iv) the energy of the HOMO/LUMO levels involved.     

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,III₂ , Ru^III,III₂ , and Ru^III,IV₂ , 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^IIIORu^IV]⁵⁺ and [Ru^IIORu^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.     

Photochemical,Electrochemical, and Photoelectrochemical Water Oxidation Catalyzed by Water‐Soluble Mononuclear Ruthenium Complexes

Two mononuclear ruthenium complexes [Ru(H₂tcbp)(isoq)₂] ( 1 ) and [Ru(H₂tcbp)(pic)₂] ( 2 ) (H₄tcbp=4,4′,6,6′‐tetracarboxy‐2,2′‐bipyridine, isoq=isoquinoline, pic=4‐picoline) are synthesized and fully characterized. Two spare carboxyl groups on the 4,4′‐positions are introduced to enhance the solubility of 1 and 2 in water and to simultaneously allow them to tether to the electrode surface by an ester linkage. The photochemical, electrochemical, and photoelectrochemical water oxidation performance of 1 in neutral aqueous solution is investigated. Under electrochemical conditions, water oxidation is conducted on the deposited indium‐tin‐oxide anode, and a turnover number higher than 15,000 per water oxidation catalyst (WOC) 1 is obtained during 10 h of electrolysis under 1.42 V vs. NHE, corresponding to a turnover frequency of 0.41 s^?1. The low overpotential (0.17 V) of electrochemical water oxidation for 1 in the homogeneous solution enables water oxidation under visible light by using [Ru(bpy)₃]²⁺ ( P1 ) (bpy=2,2′‐bipyridine) or [Ru(bpy)₂(4,4′‐(COOEt)₂‐bpy)]²⁺ ( P2 ) as a photosensitizer. In a three‐component system containing 1 or 2 as a light‐driven WOC, P1 or P2 as a photosensitizer, and Na₂S₂O₈ or [CoCl(NH₃)₅]Cl₂ as a sacrificial electron acceptor, a high turnover frequency of 0.81 s^?1 and a turnover number of up to 600 for 1 under different catalytic conditions are achieved. In a photoelectrochemical system, the WOC 1 and photosensitizer are immobilized together on the photoanode. The electrons efficiently transfer from the WOC to the photogenerated oxidizing photosensitizer, and a high photocurrent density of 85 μA cm^?2 is obtained by applying 0.3 V bias vs. NHE.     

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     

Polymerization of acrylamide photoinitiated by tris(2,2′‐bipyridine)ruthenium(II)–amine in aqueous solution: Effect of the amine structure

The photopolymerization of acrylamide (AA) initiated by the metallic complex tris(2,2′‐bipyridine)ruthenium(II) [Ru(bpy)₃⁺²] in the presence of aliphatic and aromatic amines as co‐initiators was investigated in aqueous solution. Aromatic amines, which are good quenchers of the emission of the metal‐to‐ligand‐charge‐transfer excited state of the complex, are more effective co‐initiators than those that do not quench the luminescence of Ru(bpy)₃⁺², such as aliphatic amines and aniline. Laser‐flash photolysis experiments show the presence of the reduced form of the complex, Ru(bpy)₃⁺¹, for all the amines investigated. For aliphatic amines, the yield of Ru(bpy)₃⁺¹ increases with temperature, and on the basis of these experiments, a metal‐centered excited state is proposed as the reactive intermediate in the reaction with these amines. The decay of the transient Ru(bpy)₃⁺¹ is faster in the presence of AA. This may be understood by an electron‐transfer process from Ru(bpy)₃⁺¹ to AA, regenerating Ru(bpy)₃⁺² and producing the radical anion of AA. It is proposed that this radical anion protonates in a fast process to give the neutral AA radical, initiating in this way the polymerization chain. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 4265–4273, 2001     

Nanoscale Metal–Organic Layers for Deeply Penetrating X‐ray‐Induced Photodynamic Therapy

We report the rational design of metal–organic layers (MOLs) that are built from [Hf₆O₄(OH)₄(HCO₂)₆] secondary building units (SBUs) and Ir[bpy(ppy)₂]⁺‐ or [Ru(bpy)₃]²⁺‐derived tricarboxylate ligands (Hf‐BPY‐Ir or Hf‐BPY‐Ru; bpy=2,2′‐bipyridine, ppy=2‐phenylpyridine) and their applications in X‐ray‐induced photodynamic therapy (X‐PDT) of colon cancer. Heavy Hf atoms in the SBUs efficiently absorb X‐rays and transfer energy to Ir[bpy(ppy)₂]⁺ or [Ru(bpy)₃]²⁺ moieties to induce PDT by generating reactive oxygen species (ROS). The ability of X‐rays to penetrate deeply into tissue and efficient ROS diffusion through ultrathin 2D MOLs (ca. 1.2 nm) enable highly effective X‐PDT to afford superb anticancer efficacy.     

Assessing a Spectroelectrochemical Sensor’s Performance for Detecting [Ru(bpy)3]2+ in Natural and Treated Water

A spectroelectrochemical sensor that combines three modes of selectivity in a single device was evaluated in natural and treated water samples using tris‐(2,2′‐bipyridyl) ruthenium(II) dichloride hexahydrate, [Ru(bpy)₃]²⁺, as a model analyte. The sensor was an optically transparent indium tin oxide (ITO) electrode coated with a thin film of partially sulfonated polystyrene‐block‐poly(ethylene‐ran‐butylene)‐block‐polystyrene (SSEBS). As the potential of the ITO electrode was cycled from +0.7 to +1.3 V, the analyte changed from the colored [Ru(bpy)₃]²⁺ complex to colorless [Ru(bpy)₃]³⁺ complex and the change in absorbance at 450 nm was used as the optical signal for quantification. Calibration curves were obtained for [Ru(bpy)₃]²⁺ in natural well water, river water and treated tap water with detection limits of 108, 139 and 264 nM, respectively. A standard addition method was developed to determine an ‘unknown’ spike addition concentration of [Ru(bpy)₃]²⁺ in well water. The spectroelectrochemical sensor determined the concentration of [Ru(bpy)₃]²⁺ spiked into a sample of Hanford well water to be 0.39±0.03 µM versus the actual concentration of 0.40 µM.     

Hydrogen‐Atom Abstraction Reactions by Manganese(V)– and Manganese(IV)–Oxo Porphyrin Complexes in Aqueous Solution

High‐valent manganese(IV or V)–oxo porphyrins are considered as reactive intermediates in the oxidation of organic substrates by manganese porphyrin catalysts. We have generated Mn^V– and Mn^IV–oxo porphyrins in basic aqueous solution and investigated their reactivities in C? H bond activation of hydrocarbons. We now report that Mn^V– and Mn^IV–oxo porphyrins are capable of activating C? H bonds of alkylaromatics, with the reactivity order of Mn^V–oxo>Mn^IV–oxo; the reactivity of a Mn^V–oxo complex is 150 times greater than that of a Mn^IV–oxo complex in the oxidation of xanthene. The C? H bond activation of alkylaromatics by the Mn^V– and Mn^IV–oxo porphyrins is proposed to occur through a hydrogen‐atom abstraction, based on the observations of a good linear correlation between the reaction rates and the C? H bond dissociation energy (BDE) of substrates and high kinetic isotope effect (KIE) values in the oxidation of xanthene and dihydroanthracene (DHA). We have demonstrated that the disproportionation of Mn^IV–oxo porphyrins to Mn^V–oxo and Mn^III porphyrins is not a feasible pathway in basic aqueous solution and that Mn^IV–oxo porphyrins are able to abstract hydrogen atoms from alkylaromatics. The C? H bond activation of alkylaromatics by Mn^V– and Mn^IV–oxo species proceeds through a one‐electron process, in which a Mn^IV–‐oxo porphyrin is formed as a product in the C? H bond activation by a Mn^V–oxo porphyrin, followed by a further reaction of the Mn^IV–oxo porphyrin with substrates that results in the formation of a Mn^III porphyrin complex. This result is in contrast to the oxidation of sulfides by the Mn^V–oxo porphyrin, in which the oxidation of thioanisole by the Mn^V–oxo complex produces the starting Mn^III porphyrin and thioanisole oxide. This result indicates that the oxidation of sulfides by the Mn^V–oxo species occurs by means of a two‐electron oxidation process. In contrast, a Mn^IV–oxo porphyrin complex is not capable of oxidizing sulfides due to a low oxidizing power in basic aqueous solution.     

Mechanistic Study of a Photocatalyzed CS Bond Formation Involving Alkyl/Aryl Thiosulfate

This study presents thioether construction involving alkyl/aryl thiosulfates and diazonium salt catalyzed by visible‐light‐excited [Ru(bpy)₃Cl₂] at room temperature in 44–86 % yield. Electron paramagnetic resonance studies found that thiosulfate radical formation was promoted by K₂CO₃. Conversely, radicals generated from BnSH or BnSSBn (Bn=benzyl) were clearly suppressed, demonstrating the special property of thiosulfate in this system. Transient absorption spectra confirmed the electron‐transfer process between [Ru(bpy)₃Cl₂] and 4‐MeO‐phenyl diazonium salt, which occurred with a rate constant of 1.69×10⁹ M ^?1 s^?1. The corresponding radical trapping product was confirmed by X‐ray diffraction. The full reaction mechanism was determined together with emission quenching data. Furthermore, this system efficiently avoided the over‐oxidation of sulfide caused by H₂O in the photoexcited system containing Ru²⁺. Both aryl and heteroaryl diazonium salts with various electronic properties were investigated for synthetic compatibility. Both alkyl‐ and aryl‐substituted thiosulfates could be used as substrates. Notably, pharmaceutical derivatives afforded late‐stage sulfuration smoothly under mild conditions.     

Synthesis,Characterization, Redox Properties,and Photodynamics of Donor–Acceptor Nanohybrids Composed of Size‐Controlled Cup‐Shaped Nanocarbons and Porphyrins

Cup‐shaped nanocarbons (CNC) generated by the electron‐transfer reduction of cup‐stacked carbon nanotubes have been functionalized with porphyrins (H₂P) as light‐capturing chromophores. The resulting donor–acceptor nanohybrid has been characterized by thermogravimetric analysis (TGA), Raman and IR spectroscopy, transmission electron microscopy, elemental analysis, and UV/Vis spectroscopy. The weight of the porphyrins attached to the cup‐shaped nanocarbons was determined as 20 % by TGA and elemental analysis. The UV/Vis absorption spectrum of CNC? (H₂P)_n in DMF agrees well with that obtained by the superposition of reference porphyrin (ref‐H₂P) and cup‐shaped nanocarbons. The photoexcitation of the CNC? (H₂P)_n nanohybrid results in formation of the charge‐separated (CS) state to attain the longest CS lifetime (0.64±0.01 ms) ever reported for donor–acceptor nanohybrids, which may arise from efficient electron migration following the charge separation. The formation of a radical ion pair was detected directly by electron spin resonance (ESR) measurements under photoirradiation of CNC? (H₂P)_n with a high‐pressure mercury lamp in frozen DMF at 153 K. The observed ESR signal at g=2.0044 agrees with that of ref‐H₂P^.+ produced by one‐electron oxidation with [Ru(bpy)₃]³⁺ in deaerated CHCl₃, indicating the formation of H₂P^.+. The electron‐acceptor ability of the reference CNC compound (ref‐CNC) was also examined by the electron‐transfer reduction of ref‐CNC by a series of semiquinone radical anions.     

Distance Dependence of Bidirectional Concerted Proton–Electron Transfer in Phenol‐Ru(2,2′‐bipyridine)32+ Dyads

Proton‐coupled electron transfer (PCET) was investigated in three covalent donor–bridge–acceptor molecules with different bridge lengths. Upon photoexcitation of their Ru(bpy)₃²⁺ (bpy=2,2′‐bipyridine) photosensitizer in acetonitrile, intramolecular long‐range electron transfer from a phenolic unit to Ru(bpy)₃²⁺ occurs in concert with release of the phenolic proton to pyrrolidine base. The kinetics of this bidirectional concerted proton–electron transfer (CPET) reaction were studied as a function of phenol–Ru(bpy)₃²⁺ distance by increasing the number of bridging p‐xylene units. A distance decay constant (β) of 0.67±0.23 Å^?1 was determined. The distance dependence of the rates for CPET is thus not significantly steeper than that for ordinary (i.e., not proton coupled) electron transfer across the same bridges, despite the concerted motion of oppositely charged particles into different directions. Long‐range bidirectional CPET is an important reaction in many proteins and plays a key role in photosynthesis; our results are relevant in the context of photoinduced separation of protons and electrons as a means of light‐to‐chemical energy conversion. This is the first determination of β for a bidirectional CPET reaction.     

Light‐Harvesting Photocatalysis for Water Oxidation Using Mesoporous Organosilica

An organic‐based photocatalysis system for water oxidation, with visible‐light harvesting antennae, was constructed using periodic mesoporous organosilica (PMO). PMO containing acridone groups in the framework (Acd‐PMO), a visible‐light harvesting antenna, was supported with [Ru^II(bpy)₃²⁺] complex (bpy=2,2′‐bipyridyl) coupled with iridium oxide (IrO_x) particles in the mesochannels as photosensitizer and catalyst, respectively. Acd‐PMO absorbed visible light and funneled the light energy into the Ru complex in the mesochannels through excitation energy transfer. The excited state of Ru complex is oxidatively quenched by a sacrificial oxidant (Na₂S₂O₈) to form Ru³⁺ species. The Ru³⁺ species extracts an electron from IrO_x to oxidize water for oxygen production. The reaction quantum yield was 0.34 %, which was improved to 0.68 or 1.2 % by the modifications of PMO. A unique sequence of reactions mimicking natural photosystem II, 1) light‐harvesting, 2) charge separation, and 3) oxygen generation, were realized for the first time by using the light‐harvesting PMO.     

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.     

Lewis Acid Coupled Electron Transfer of Metal–Oxygen Intermediates

Redox‐inactive metal ions and Brønsted acids that function as Lewis acids play pivotal roles in modulating the redox reactivity of metal–oxygen intermediates, such as metal–oxo and metal–peroxo complexes. The mechanisms of the oxidative C?H bond cleavage of toluene derivatives, sulfoxidation of thioanisole derivatives, and epoxidation of styrene derivatives by mononuclear nonheme iron(IV)–oxo complexes in the presence of triflic acid (HOTf) and Sc(OTf)₃ have been unified as rate‐determining electron transfer coupled with binding of Lewis acids (HOTf and Sc(OTf)₃) by iron(III)–oxo complexes. All logarithms of the observed second‐order rate constants of Lewis acid‐promoted oxidative C?H bond cleavage, sulfoxidation, and epoxidation reactions of iron(IV)–oxo complexes exhibit remarkably unified correlations with the driving forces of proton‐coupled electron transfer (PCET) and metal ion‐coupled electron transfer (MCET) in light of the Marcus theory of electron transfer when the differences in the formation constants of precursor complexes were taken into account. The binding of HOTf and Sc(OTf)₃ to the metal–oxo moiety has been confirmed for Mn^IV–oxo complexes. The enhancement of the electron‐transfer reactivity of metal–oxo complexes by binding of Lewis acids increases with increasing the Lewis acidity of redox‐inactive metal ions. Metal ions can also bind to mononuclear nonheme iron(III)–peroxo complexes, resulting in acceleration of the electron‐transfer reduction but deceleration of the electron‐transfer oxidation. Such a control on the reactivity of metal–oxygen intermediates by binding of Lewis acids provides valuable insight into the role of Ca²⁺ in the oxidation of water to dioxygen by the oxygen‐evolving complex in photosystem II.     

[Ru(bpy)3]2+-trivalent titanium. A photochemical system with hydrogen production from aqueous solutions by visible light

Molecular hydrogen, detected by gas-chromatographic and mass-spectrometric measurements, was obtained by irradiating with visible light aqueous hydrochloridic solutions of [Ru(bpy)₃]²⁺ and trivalent titanium. The active species is the ³CT of [Ru(bpy)₃]²⁺, which is quenched by Ti(III). The suggested mechanism is an electron transfer with Ti(II) formation. The back reaction between [Ru(bpy)₃]³⁺ and Ti(II) is hindered by the very fast competitive reaction of Ti(II) (not stable in acid aqueous solutions) with H⁺, carrying to hydrogen evolution.     

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.     

Light‐Driven Water Splitting Mediated by Photogenerated Bromine

Light‐driven water splitting was achieved using a dye‐sensitized mesoporous oxide film and the oxidation of bromide (Br^?) to bromine (Br₂) or tribromide (Br₃^?). The chemical oxidant (Br₂ or Br₃^?) is formed during illumination at the photoanode and used as a sacrificial oxidant to drive a water oxidation catalyst (WOC), here demonstrated using [Ru(bda)(pic)₂], ( 1 ; pic=picoline, bda=2,2′‐bipyridine‐6,6′‐dicarboxylate). The photochemical oxidation of bromide produces a chemical oxidant with a potential of 1.09 V vs. NHE for the Br₂/Br^? couple or 1.05 V vs. NHE for the Br₃^?/Br^? couple, which is sufficient to drive water oxidation at 1 (Ru^V/IV≈1.0 V vs. NHE at pH 5.6). At pH 5.6, using a 0.2 m acetate buffer containing 40 mm LiBr and the [Ru(4,4′‐PO₃H₂‐bpy)(bpy)₂]²⁺ ( RuP ²⁺, bpy=2,2′‐bipyridine) chromophore dye on a SnO₂/TiO₂ core–shell electrode resulted in a photocurrent density of around 1.2 mA cm^?2 under approximately 1 Sun illumination and a Faradaic efficiency upon addition of 1 of 77 % for oxygen evolution.