“…The designation of localized oxidation states in this and related mixed-valence intermediates is a convenience. There is evidence for strong electronic coupling across the -oxo bridge, and oxidation states may be delocalized, (O)Ru 4.5 Ru 4.5 (O) 3ϩ (31,32).…”
Light-driven water oxidation occurs in oxygenic photosynthesis in photosystem II and provides redox equivalents directed to photosystem I, in which carbon dioxide is reduced. Water oxidation is also essential in artificial photosynthesis and solar fuelforming reactions, such as water splitting into hydrogen and oxygen (2 H2O ؉ 4 h 3 O2 ؉ 2 H2) or water reduction of CO2 to methanol (2 H2O ؉ CO2 ؉ 6 h 3 CH3OH ؉ 3/2 O2), or hydrocarbons, which could provide clean, renewable energy. The ''blue ruthenium dimer,'' cis,cis-[(bpy)2(H2O)Ru III ORu III (OH2)(bpy)2] 4؉ , was the first well characterized molecule to catalyze water oxidation. On the basis of recent insight into the mechanism, we have devised a strategy for enhancing catalytic rates by using kinetically facile electron-transfer mediators. Rate enhancements by factors of up to Ϸ30 have been obtained, and preliminary electrochemical experiments have demonstrated that mediator-assisted electrocatalytic water oxidation is also attainable.catalysis ͉ redox mediator ͉ electron transfer W ater oxidation is a key reaction in photosynthesis, the basis for most of life as we know it (1-8). It is also a central reaction in artificial photosynthesis, an example being solardriven splitting of water into hydrogen and oxygen, 2 H 2 O 3 O 2 ϩ 2 H 2 (9-11). In natural photosynthesis, water oxidation occurs at photosystem II (PSII) through the Kok cycle after absorption of four photons. Detailed insight into how this reaction occurs is emerging on the basis of theoretical and spectroscopic studies and recent x-ray diffraction and extended x-ray absorption fine structure results to 3.0-Å resolution (12)(13)(14).Given the demands of the half reaction, 2 H 2 O 3 O 2 ϩ 4 H ϩ ϩ 4e Ϫ , with requirements for both 4e Ϫ /4H ϩ loss and OOO bond formation, water oxidation is difficult to achieve at a single catalyst site or cluster. In addition to PSII, water oxidation is also catalyzed by the ruthenium ''blue dimer'' cis,cis-[(bpy) 2 (H 2 O)Ru III ORu III (OH 2 )(bpy) 2 ] 4ϩ (bpy is 2,2Ј-bipyridine) and structurally related derivatives (15-21) (Fig. 1). Other catalysts based on iridium and ruthenium complexes and in rutheniumcontaining polyoxometalates have been reported recently (22-25).The low oxidation state Ru III -O-Ru III form of the ruthenium blue dimer undergoes oxidative activation by proton-coupled electron transfer (PCET) in which stepwise loss of electrons and protons occurs. PCET is essential, because it allows for the buildup of multiple oxidative equivalents at a single site or cluster without building up positive charge (21, 26). As shown by the results of pH-dependent electrochemical studies (16), loss of 4e Ϫ /4H ϩ occurs to give a reactive, transient intermediate followed by O 2 evolution.In the absence of a serendipitous discovery, mechanistic knowledge is required for the design of robust, long-lived catalysts for water oxidation. Such knowledge is also important for the microscopic reverse reaction, oxygen reduction to water, which occurs at the cathode (reducing e...
“…The designation of localized oxidation states in this and related mixed-valence intermediates is a convenience. There is evidence for strong electronic coupling across the -oxo bridge, and oxidation states may be delocalized, (O)Ru 4.5 Ru 4.5 (O) 3ϩ (31,32).…”
Light-driven water oxidation occurs in oxygenic photosynthesis in photosystem II and provides redox equivalents directed to photosystem I, in which carbon dioxide is reduced. Water oxidation is also essential in artificial photosynthesis and solar fuelforming reactions, such as water splitting into hydrogen and oxygen (2 H2O ؉ 4 h 3 O2 ؉ 2 H2) or water reduction of CO2 to methanol (2 H2O ؉ CO2 ؉ 6 h 3 CH3OH ؉ 3/2 O2), or hydrocarbons, which could provide clean, renewable energy. The ''blue ruthenium dimer,'' cis,cis-[(bpy)2(H2O)Ru III ORu III (OH2)(bpy)2] 4؉ , was the first well characterized molecule to catalyze water oxidation. On the basis of recent insight into the mechanism, we have devised a strategy for enhancing catalytic rates by using kinetically facile electron-transfer mediators. Rate enhancements by factors of up to Ϸ30 have been obtained, and preliminary electrochemical experiments have demonstrated that mediator-assisted electrocatalytic water oxidation is also attainable.catalysis ͉ redox mediator ͉ electron transfer W ater oxidation is a key reaction in photosynthesis, the basis for most of life as we know it (1-8). It is also a central reaction in artificial photosynthesis, an example being solardriven splitting of water into hydrogen and oxygen, 2 H 2 O 3 O 2 ϩ 2 H 2 (9-11). In natural photosynthesis, water oxidation occurs at photosystem II (PSII) through the Kok cycle after absorption of four photons. Detailed insight into how this reaction occurs is emerging on the basis of theoretical and spectroscopic studies and recent x-ray diffraction and extended x-ray absorption fine structure results to 3.0-Å resolution (12)(13)(14).Given the demands of the half reaction, 2 H 2 O 3 O 2 ϩ 4 H ϩ ϩ 4e Ϫ , with requirements for both 4e Ϫ /4H ϩ loss and OOO bond formation, water oxidation is difficult to achieve at a single catalyst site or cluster. In addition to PSII, water oxidation is also catalyzed by the ruthenium ''blue dimer'' cis,cis-[(bpy) 2 (H 2 O)Ru III ORu III (OH 2 )(bpy) 2 ] 4ϩ (bpy is 2,2Ј-bipyridine) and structurally related derivatives (15-21) (Fig. 1). Other catalysts based on iridium and ruthenium complexes and in rutheniumcontaining polyoxometalates have been reported recently (22-25).The low oxidation state Ru III -O-Ru III form of the ruthenium blue dimer undergoes oxidative activation by proton-coupled electron transfer (PCET) in which stepwise loss of electrons and protons occurs. PCET is essential, because it allows for the buildup of multiple oxidative equivalents at a single site or cluster without building up positive charge (21, 26). As shown by the results of pH-dependent electrochemical studies (16), loss of 4e Ϫ /4H ϩ occurs to give a reactive, transient intermediate followed by O 2 evolution.In the absence of a serendipitous discovery, mechanistic knowledge is required for the design of robust, long-lived catalysts for water oxidation. Such knowledge is also important for the microscopic reverse reaction, oxygen reduction to water, which occurs at the cathode (reducing e...
“…The molecular structure of a second-generation approach to the design of water oxidation catalysts is shown in Figure . It is cis , cis -[(bpy) 2 (OH)Ru IV −O−Ru III (H 2 O)(bpy) 2 ] 4+ , the one-electron-oxidized form of the blue dimer precursor, cis , cis -[(bpy) 2 (H 2 O)Ru III −O−Ru III (H 2 O)(bpy) 2 ] 4+ . ,− They are two members of the 4e - /4H + oxidation sequence shown in Figure which ultimately leads to water oxidation. This scheme was established through a series of painstaking experiments involving stopped-flow kinetics measurements and the spectroscopic identification of intermediates. ,− 45a 6 ORTEP diagram and labeling scheme for the cation [(bpy) 2 (H 2 O)Ru III −O−Ru IV (OH)(bpy) 2 ] 4+ .…”
There is a remarkable redox chemistry of higher oxidation state M(IV)-M(VI) polypyridyl complexes of Ru and Os. They are accessible by proton loss and formation of oxo or nitrido ligands, examples being cis-[RuIV(bpy)2(py)(O)]2+ (RuIV=O2+, bpy=2,2'-bipyridine, and py=pyridine) and trans-[OsVI(tpy)(Cl)2(N)]+ (tpy=2,2':6',2' '-terpyridine). Metal-oxo or metal-nitrido multiple bonding stabilizes the higher oxidation states and greatly influences reactivity. O-atom transfer, hydride transfer, epoxidation, C-H insertion, and proton-coupled electron-transfer mechanisms have been identified in the oxidation of organics by RuIV=O2+. The Ru-O multiple bond inhibits electron transfer and promotes complex mechanisms. Both O atoms can be used for O-atom transfer by trans-[RuVI(tpy)(O)2(S)]2+ (S=CH3CN or H2O). Four-electron, four-proton oxidation of cis,cis-[(bpy)2(H2O)RuIII-O-RuIII(H2O)(bpy)2]4+ occurs to give cis,cis-[(bpy)2(O)RuV-O-RuV(O)(bpy)2]4+ which rapidly evolves O2. Oxidation of NH3 in trans-[OsII(tpy)(Cl)2(NH3)] gives trans-[OsVI(tpy)(Cl)2(N)]+ through a series of one-electron intermediates. It and related nitrido complexes undergo formal N- transfer analogous to O-atom transfer by RuIV=O2+. With secondary amines, the products are the hydrazido complexes, cis- and trans-[OsV(L3)(Cl)2(NNR2)]+ (L3=tpy or tpm and NR2-=morpholide, piperidide, or diethylamide). Reactions with aryl thiols and secondary phosphines give the analogous adducts cis- and trans-[OsIV(tpy)(Cl)2(NS(H)(C6H4Me))]+ and fac-[OsIV(Tp)(Cl)2(NP(H)(Et2))]. In dry CH3CN, all have an extensive multiple oxidation state chemistry based on couples from Os(VI/V) to Os(III/II). In acidic solution, the OsIV adducts are protonated, e.g., trans-[OsIV(tpy)(Cl)2(N(H)N(CH2)4O)]+, and undergo proton-coupled electron transfer to quinone to give OsV, e.g., trans-[OsV(tpy)(Cl)2(NN(CH2)4O)]+ and hydroquinone. These reactions occur with giant H/D kinetic isotope effects of up to 421 based on O-H, N-H, S-H, or P-H bonds. Reaction with azide ion has provided the first example of the terminal N4(2-) ligand in mer-[OsIV(bpy)(Cl)3(NalphaNbetaNgammaNdelta)]-. With CN-, the adduct mer-[OsIV(bpy)(Cl)3(NCN)]- has an extensive, reversible redox chemistry and undergoes NCN(2-) transfer to PPh3 and olefins. Coordination to Os also promotes ligand-based reactivity. The sulfoximido complex trans-[OsIV(tpy)(Cl)2(NS(O)-p-C6H4Me)] undergoes loss of O2 with added acid and O-atom transfer to trans-stilbene and PPh3. There is a reversible two-electron/two-proton, ligand-based acetonitrilo/imino couple in cis-[OsIV(tpy)(NCCH3)(Cl)(p-NSC6H4Me)]+. It undergoes reversible reactions with aldehydes and ketones to give the corresponding alcohols.
“…Ru IV ORu IV is the most strongly oxidizing intermediate in the series of blue dimer intermediates involved in water oxidation. 51 The dramatic electrode response to further oxidation of the blue dimer on an ITO electrode surface-derivatized by binding Ru-(bpy) 2 (4,4 0 (PO 3 H 2 ) 2 bpy) 2+ (ITO-Ru 2+ ) is also shown in Fig. 3.…”
Section: The Search For Water Oxidation Catalysts Pcet Appearsmentioning
Water oxidation is a key half reaction in energy conversion schemes based on solar fuels and targets such as light driven water splitting or carbon dioxide reduction into CO, other oxygenates, or hydrocarbons. Carrying out these reactions at rates that exceed the rate of solar insolation for the extended periods of time required for useful applications presents a major challenge. Water oxidation is the key ''other'' half reaction in these schemes and it is dominated by PCET given its multi-electron, multi-proton character, 2H 2 O / O 2 + 4e À + 4H + . Identification of PCET was an offshoot of experiments designed to investigate energy conversion by electron transfer quenching of molecular excited states. The concepts ''redox potential leveling'' and concerted electron-proton transfer came from measurements on stepwise oxidation of cis-Ru II (bpy) 2 (py)(OH 2 ) 2+ to Ru IV (bpy) 2 (py)(O) 2+ . The Ru ''blue dimer'', cis,cis-, was the first designed catalyst for water oxidation. It undergoes oxidative activation by PCET to give the transient (bpy, O-atom attack on water to give a peroxidic intermediate, and further oxidation and O 2 release. More recently, a class of single site water oxidation catalysts has been identified, e.g., Ru(tpy)(bpm)(OH 2 ) 2+ (tpy is 2,2 0 :6 0 ,2 00terpyridine; bpm is 2,2 0 -bipyrimidine). They undergo stepwise PCET oxidation to Ru IV ¼O 2+ or Ru V (O) 3+ followed by O-atom transfer with formation of peroxidic intermediates which undergo further oxidation and O 2 release. PCET plays a key role in the three zones of water oxidation reactivity: oxidative activation, O/O bond formation, oxidation and O 2 release from peroxidic intermediates. Similar schemes have been identified for electrocatalytic water oxidation on oxide electrode surfaces based on phosphonated derivatives such as [Ru(Mebimpy)(4,4 0 -(PO 3 H 2 CH 2 ) 2 bpy)(OH 2 )] 2+ . A PCET barrier to Ru III -OH 2+ / Ru IV ¼O 2+ oxidation arises from the large difference in pK a values between Ru III -OH 2+ and Ru IV (OH) 3+ . On oxide surfaces this oxidation occurs by multiple pathways. Kinetic, mechanistic, and DFT results on single site catalysts reveal a new pathway for the O/O bond forming step (Atom-Proton Transfer, APT), significant rate enhancements by added proton acceptor bases, and accelerated water oxidation in propylene carbonate as solvent with water added as a stoichiometric reagent. Lessons learned about water oxidation and the role of PCET and concerted pathways appear to have direct relevance for water oxidation in Photosystem II (PSII) with PSII a spectacular example of PCET in action. This includes a key role for Multiple Site-Electron Proton Transfer in oxidative activation of the Oxygen Evolving Complex (OEC) in the S 0 / S 1 transition in the Kok cycle.
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