Nucleophilic Attack of Hydroxide on a MnV Oxo Complex: A Model of the O−O Bond Formation in the Oxygen Evolving Complex of Photosystem II

A Bifunctional Electrocatalyst for Oxygen Evolution and Oxygen Reduction Reactions in Water

Oxygen reduction and water oxidation are two key processes in fuel cell applications. The oxidation of water to dioxygen is a 4 H(+)/4 e(-) process, while oxygen can be fully reduced to water by a 4 e(-)/4 H(+) process or partially reduced by fewer electrons to reactive oxygen species such as H2O2 and O2(-). We demonstrate that a novel manganese corrole complex behaves as a bifunctional catalyst for both the electrocatalytic generation of dioxygen as well as the reduction of dioxygen in aqueous media. Furthermore, our combined kinetic, spectroscopic, and electrochemical study of manganese corroles adsorbed on different electrode materials (down to a submolecular level) reveals mechanistic details of the oxygen evolution and reduction processes.

A Bifunctional Electrocatalyst for Oxygen Evolution and Oxygen Reduction Reactions in Water

Oxygen reduction and water oxidation are two key processes in fuel cell applications. The oxidation of water to dioxygen is a 4 H⁺/4 e^- process, while oxygen can be fully reduced to water by a 4 e^-/4 H⁺ process or partially reduced by fewer electrons to reactive oxygen species such as H₂O₂ and O₂^-. We demonstrate that a novel manganese corrole complex behaves as a bifunctional catalyst for both the electrocatalytic generation of dioxygen as well as the reduction of dioxygen in aqueous media. Furthermore, our combined kinetic, spectroscopic, and electrochemical study of manganese corroles adsorbed on different electrode materials (down to a submolecular level) reveals mechanistic details of the oxygen evolution and reduction processes.

Molecular ruthenium water oxidation catalysts carrying non-innocent ligands: mechanistic insight through structure–activity relationships and quantum chemical calculations

Herein is highlighted how structure–activity relationships can be used to provide mechanistic insight into H₂O oxidation catalysis.

Water oxidation using earth-abundant transition metal catalysts: opportunities and challenges

Catalysts for the oxidation of water are a vital component of solar energy to fuel conversion technologies. This Perspective summarizes recent advances in the field of designing homogeneous water oxidation catalysts (WOCs) based on Mn, Fe, Co and Cu.

Dinuclear cobalt(iii) and mixed valence trinuclear MnIII–MnII–MnIII complexes with a tripodal bridging pyridylaminophenol ligand

Two new complexes with the tripodal N-(2-pyridyl-methyl)-N,N-bis-[2′-hydroxy-5′-methyl-benzyl]-amine, one Co^III dimer and a linear mixed valence Mn^III–Mn^II–Mn^III have been synthesized and characterized.

How does ammonia bind to the oxygen-evolving complex in the S2state of photosynthetic water oxidation? Theoretical support and implications for the W1 substitution mechanism

The mechanistic study shows that NH₃substitutes W1 rather than O5 of the OEC in the S₂state and leaves in the S₄′ state.

Artificial synthetic Mn(IV)Ca-oxido complexes mimic the oxygen-evolving complex in photosystem II

A novel family of heteronuclear Mn(IV)Ca-oxido complexes containing Mn(IV)Ca-oxido cuboidal moieties and reactive water molecules on Ca(2+) have been synthesized and characterized to mimic the oxygen-evolving complex (OEC) of photosystem II (PSII) in nature.

A nonheme manganese(IV)-oxo species generated in photocatalytic reaction using water as an oxygen source

A nonheme manganese(IV)-oxo complex, [Mn(IV)(O)(BQCN)](2+), was generated in the photochemical and chemical oxidation of [Mn(II)(BQCN)](2+) with water as an oxygen source, respectively. The photocatalytic oxidation of organic substrates, such as alcohol and sulfide, by [Mn(II)(BQCN)](2+) has been demonstrated in both neutral and acidic media.

Reverse Catalase Reaction: Dioxygen Activation via Two-Electron Transfer from Hydroxide to Dioxygen Mediated By a Manganese(III) Salen Complex

Although atmospheric dioxygen is regarded as the most ideal oxidant, O2 activation for use in oxygenation reactions intrinsically requires a costly sacrificial reductant. The present study investigated the use of aqueous alkaline solution for O2 activation. A manganese(III) salen complex, Mn(III)(salen)(Cl), in toluene reacts with aqueous KOH solution under aerobic conditions, which yields a di-μ-oxo dimanganese(IV) salen complex, [Mn(IV)(salen)]2(μ-O)2. The (18)O isotope experiments show that (18)O2 is indeed activated to give [Mn(IV)(salen)]2(μ-(18)O)2 via a peroxide intermediate. Interestingly, the (18)OH(-) ion in H2(18)O was also incorporated to yield [Mn(IV)(salen)]2(μ-(18)O)2, which implies that a peroxide species is also generated from (18)OH(-). The addition of benzyl alcohol as a stoichiometric reductant selectively inhibits the (18)O incorporation from (18)OH(-), indicating that the reaction of Mn(III)(salen)(Cl) with OH(-) supplies the electrons for O2 reduction. The conversion of both O2 and OH(-) to a peroxide species is exactly the reverse of a catalase-like reaction, which has a great potential as the most efficient O2 activation. Mechanistic investigations revealed that the reaction of Mn(III)(salen)(Cl) with OH(-) generates a transient species with strong reducing ability, which effects the reduction of O2 by means of a manganese(II) intermediate.

Synthesis and Antiviral Activity Evaluation of Nitroporphyrins and Nitrocorroles as Potential Agents against Human Cytomegalovirus Infection

Different nitroporphyrinoid derivatives were synthesized and studied as potential agents against human Cytomegalovirus. Interestingly, two nitrocorroles display strong activity against human Cytomegalovirus with IC 50 < 0.5 μM. These compounds also possess antiproliferative activities without detected in vivo toxicity. Therefore, nitrocorroles appear for the first time as potential active compounds that can be applied in human health.

Water oxidation catalysed by iron complex of N,N'-dimethyl-2,11-diaza[3,3](2,6)pyridinophane. Spectroscopy of iron-oxo intermediates and density functional theory calculations

The macrocyclic [Fe^III(L1)Cl₂]⁺ (1, L1 = N,N'-dimethyl-2,11-diaza[3,3](2,6)pyridinophane) complex is an active catalyst for the oxidation of water to oxygen using [NH₄]₂[Ce^IV(NO₃)₆] (CAN), NaIO₄, or Oxone as the oxidant. The mechanism of 1-catalysed water oxidation was examined by spectroscopic methods and by ¹⁸O-labelling experiments, revealing that Fe^IV 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 O and/or Fe^V 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 O species are likely to be involved in the reaction. The redox behaviour of 1 and these high-valent Fe 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 O species of L1 has been examined by both cyclic voltammetry and density functional theory (DFT) calculations. In aqueous solutions, the cyclic voltammograms of 1 at different pH show a pH-dependent reversible couple (E_1/2 = +0.46 V vs. SCE at pH 1) and an irreversible anodic wave (E_pa = +1.18 V vs. SCE at pH 1) assigned to the Fe^III/Fe^II couple and the Fe^III to Fe^IV oxidation, respectively. DFT calculations showed that the E value of the half reaction involving [Fe^V(L1)(O)(OH)]²⁺/[Fe^IV(L1)(O)(OH₂)]²⁺ is +1.42 V vs. SCE at pH 1. Using CAN as the oxidant at pH 1, the formation of an Fe^IV 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 O reaction intermediate was suggested by ESI-MS and UV-vis absorption spectroscopic measurements, and the rate of oxygen evolution was linearly dependent on the concentrations of both 1 and CAN. Using NaIO₄ or Oxone as the oxidant at pH 1, the rate of oxygen evolution was linearly dependent on the concentration of 1, and a reactive Fe^V 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 O species with formula [Fe^V(L1)(O)₂]⁺ generated by oxidation with NaIO₄ or Oxone was suggested by ESI-MS measurements. DFT calculations revealed that [Fe^V(L1)(O)₂]⁺ is capable of oxidizing water to oxygen with a reaction barrier of 15.7 kcal mol^-1.

A new copper species based on an azo-compound utilized as a homogeneous catalyst for water oxidation

A new azo-complex [(L)Cu(II)(NO3)] [L = (E)-3-(pyridin-2-yldiazenyl)naphthalen-2-ol (HL)], was prepared via a one-pot synthetic method at 60 °C and was structurally characterized by IR, EA, PXRD and single crystal X-ray diffraction. In addition, TGA studies indicated that the complex was stable in air. The redox properties were determined by cyclic voltammetry, which revealed that the complex could be utilized as a catalyst for water oxidation under mild conditions. Subsequently, the complex was employed as a catalyst to take part in water oxidation reaction in the presence of a Ce(IV) salt utilized as an oxidant at pH 11 in PBS (Phosphate Buffered Saline) solution. The results suggested that the catalyst exhibited a high stability and activity toward water oxidation reaction under these conditions with an initial TOF of 4.0 kPa h(-1). Calculation methodology was performed to study the mechanism of the reaction, which revealed that in this catalytic process, the initial oxidation of Cu(II) to Cu(III) occurred by the formation of an intermediate "Cu(III)-O-O-Cu(III)". The formation of this intermediate, resulted in a release of oxygen and closing of the catalytic cycle.

Dinuclear manganese complexes for water oxidation: evaluation of electronic effects and catalytic activity

During recent years significant progress has been made towards the realization of a sustainable and carbon-neutral energy economy. One promising approach is photochemical splitting of H2O into O2 and solar fuels, such as H2. However, the bottleneck in such artificial photosynthetic schemes is the H2O oxidation half reaction where more efficient catalysts are required that lower the kinetic barrier for this process. In particular catalysts based on earth-abundant metals are highly attractive compared to catalysts comprised of noble metals. We have now synthesized a library of dinuclear Mn2(II,III) catalysts for H2O oxidation and studied how the incorporation of different substituents affected the electronics and catalytic efficiency. It was found that the incorporation of a distal carboxyl group into the ligand scaffold resulted in a catalyst with increased catalytic activity, most likely because of the fact that the distal group is able to promote proton-coupled electron transfer (PCET) from the high-valent Mn species, thus facilitating O-O bond formation.

Well-defined palladium nanoparticles supported on siliceous mesocellular foam as heterogeneous catalysts for the oxidation of water

Herein, we describe the use of Pd nanoparticles immobilized on an amino-functionalized siliceous mesocellular foam for the catalytic oxidation of H₂O. The Pd nanocatalyst proved to be capable of mediating the four-electron oxidation of H₂O to O₂, both chemically and photochemically. The Pd nanocatalyst is easy to prepare and shows high chemical stability, low leaching, and recyclability. Together with its promising catalytic activity, these features make the Pd nanocatalyst of potential interest for future sustainable solar-fuel production.

Efficient electrochemical water oxidation catalysis by nanostructured Mn2O3

Hydrothermally synthesized Mn₂O₃nanorods have been demonstrated as an efficient electrochemical water oxidation catalyst over MnO₂despite the fact that later has a higher surface area.

Earth-abundant metal complexes as catalysts for water oxidation; is it homogeneous or heterogeneous?

This minireview focuses on the aspects that determine whether particular catalysts for the oxidation of water are homogeneous or heterogeneous.

Mechanism for O-O bond formation in a biomimetic tetranuclear manganese cluster--A density functional theory study

Density functional theory calculations have been used to study the reaction mechanism of water oxidation catalyzed by a tetranuclear Mn-oxo cluster Mn4O4L6 (L=(C6H4)2PO4(-)). It is proposed that the O-O bond formation mechanism is different in the gas phase and in a water solution. In the gas phase, upon phosphate ligand dissociation triggered by light absorption, the O-O bond formation starting with both the Mn4(III,III,IV,IV) and Mn4(III,IV,IV,IV) oxidation states has to take place via direct coupling of two bridging oxo groups. The calculated barriers are 42.3 and 37.1 kcal/mol, respectively, and there is an endergonicity of more than 10 kcal/mol. Additional photons are needed to overcome these large barriers. In water solution, water binding to the two vacant sites of the Mn ions, again after phosphate dissociation triggered by light absorption, is thermodynamically and kinetically very favorable. The catalytic cycle is suggested to start from the Mn4(III,III,III,IV) oxidation state. The removal of three electrons and three protons leads to the formation of a Mn4(III,IV,IV,IV)-oxyl radical complex. The O-O bond formation then proceeds via a nucleophilic attack of water on the Mn(IV)-oxyl radical assisted by a Mn-bound hydroxide that abstracts a proton during the attack. This step was calculated to be rate-limiting with a total barrier of 29.2 kcal/mol. This is followed by proton-coupled electron transfer, O2 release, and water binding to start the next catalytic cycle.

Photosystem II like water oxidation mechanism in a bioinspired tetranuclear manganese complex

The synthesis of Mn-based catalysts to mimic the structural and catalytic properties of the oxygen-evolving complex in photosystem II is a long-standing goal for researchers. An interesting result in this field came with the synthesis of a Mn complex that enables water oxidation driven by the mild single-electron oxidant [Ru(bpy)3](3+). On the basis of hybrid density functional calculations, we herein propose a water oxidation mechanism for this bioinspired Mn catalyst, where the crucial O-O bond formation proceeds from the formal Mn4(IV,IV,IV,V) state by direct coupling of a Mn(IV)-bound terminal oxyl radical and a di-Mn bridging oxo group, a mechanism quite similar to the presently leading suggestion for the natural system. Of importance here is that the designed ligand is shown to be redox-active and can therefore store redox equivalents during the catalytic transitions, thereby alleviating the redox processes at the Mn centers.

Preparation and Properties of an MnIV-Hydroxide Complex: Proton and Electron Transfer at a Mononuclear Manganese Site and its Relationship to the Oxygen Evolving Complex within Photosystem II

Photosynthetic water oxidation is catalyzed by a Mn4O5Ca cluster with an unprecedented arrangement of metal ions in which a single manganese center is bonded to a distorted Mn3O4Ca cubane-like structure. Several mechanistic proposals describe the unique manganese center as a site for water binding and subsequent formation of a high valent Mn-oxo center that reacts with a M-OH unit (M = Mn or CaII) to form the O-O bond. The conversion of low valent Mn-OHn (n = 1,2) to a Mn-oxo species requires that a single manganese site be able to accommodate several oxidation states as the water ligand is deprotonated. To study these processes, the preparation and characterization of a new monomeric MnIV-OH complex is described. The MnIV-OH complex completes a series of well characterized Mn-OH and Mn-oxo complexes containing the same primary and secondary coordination spheres; this work thus demonstrates that a single ligand can support mononuclear Mn complexes spanning four different oxidation states (II through V) with oxo and hydroxo ligands that are derived from water. Moreover, we have completed a thermodynamic analysis based on this series of manganese complexes to predict the formation of high valent Mn-oxo species; we demonstrated that the conversion of a MnIV-OH species to a MnV-oxo complex would likely occur via a stepwise proton transfer-electron transfer mechanism. The large dissociation energy for the MnIVO-H bond (~95 kcal/mol) diminished the likelihood that other pathways are operative within a biological context. Furthermore, these studies showed that reactions between Mn-OH and Mn-oxo complexes lead to non-productive, one-electron processes suggesting that initial O-O bond formation with the OEC does not involve an Mn-OH unit.

Ligand modification transforms a catalase mimic into a water oxidation catalyst

The catalytic reactivity of the high-spin Mn(II) pyridinophane complexes [(Py2NR2)Mn(H2O)2](2+) (R=H, Me, tBu) toward O2 formation is reported. With small macrocycle N-substituents (R=H, Me), the complexes catalytically disproportionate H2O2 in aqueous solution; with a bulky substituent (R=tBu), this catalytic reaction is shut down, but the complex becomes active for aqueous electrocatalytic H2O oxidation. Control experiments are in support of a homogeneous molecular catalyst and preliminary mechanistic studies suggest that the catalyst is mononuclear. This ligand-controlled switch in catalytic reactivity has implications for the design of new manganese-based water oxidation catalysts.

Substrate-water exchange in photosystem II is arrested before dioxygen formation

Light-driven oxidation of water into dioxygen, catalysed by the oxygen-evolving complex (OEC) in photosystem II, is essential for life on Earth and provides the blueprint for devices for producing fuel from sunlight. Although the structure of the OEC is known at atomic level for its dark-stable state, the mechanism by which water is oxidized remains unsettled. Important mechanistic information was gained in the past two decades by mass spectrometric studies of the H₂¹⁸O/H₂¹⁶O substrate–water exchange in the four (semi) stable redox states of the OEC. However, until now such data were not attainable in the transient states formed immediately before the O–O bond formation. Using modified photosystem II complexes displaying up to 40-fold slower O₂ production rates, we show here that in the transient state the substrate–water exchange is dramatically slowed as compared with the earlier S states. This further constrains the possible sites for substrate–water binding in photosystem II.

The oxygen-evolving complex of photosystem II converts water into oxygen during photosynthesis, but how this process occurs is not yet fully understood. Here, the authors use modified complexes with reduced reaction rates to study the process of oxygen evolution in more detail.

Regulating proton-coupled electron transfer for efficient water splitting by manganese oxides at neutral pH

Manganese oxides have been extensively investigated as model systems for the oxygen-evolving complex of photosystem II. However, most bioinspired catalysts are inefficient at neutral pH and functional similarity to the oxygen-evolving complex has been rarely achieved with manganese. Here we report the regulation of proton-coupled electron transfer involved in water oxidation by manganese oxides. Pyridine and its derivatives, which have pK_a values intermediate to the water ligand bound to manganese(II) and manganese(III), are used as proton-coupled electron transfer induction reagents. The induction of concerted proton-coupled electron transfer is demonstrated by the detection of deuterium kinetic isotope effects and compliance of the reactions with the libido rule. Although proton-coupled electron transfer regulation is essential for the facial redox change of manganese in photosystem II, most manganese oxides impair these regulatory mechanisms. Thus, the present findings may provide a new design rationale for functional analogues of the oxygen-evolving complex for efficient water splitting at neutral pH.

Manganese oxides are extensively investigated as analogues of nature's oxygen-evolving complex, but they rarely function at neutral pH. Here, the authors investigate the induction and regulation of the proton-coupled electron-transfer mechanism involved in water oxidation by manganese oxides.

Electrocatalytic water oxidation by a monomeric amidate-ligated Fe(III)-aqua complex

The six-coordinate Fe(III)-aqua complex [Fe(III)(dpaq)(H2O)](2+) (1, dpaq is 2-[bis(pyridine-2-ylmethyl)]amino-N-quinolin-8-yl-acetamido) is an electrocatalyst for water oxidation in propylene carbonate-water mixtures. An electrochemical kinetics study has revealed that water oxidation occurs by oxidation to Fe(V)(O)(2+) followed by a reaction first order in catalyst and added water, respectively, with ko = 0.035(4) M(-1) s(-1) by the single-site mechanism found previously for Ru and Ir water oxidation catalysts. Sustained water oxidation catalysis occurs at a high surface area electrode to give O2 through at least 29 turnovers over an 15 h electrolysis period with a 45% Faradaic yield and no observable decomposition of the catalyst.

Cobalt selenium oxohalides: catalysts for water oxidation

Two new oxohalides Co4Se3O9Cl2 and Co3Se4O10Cl2 have been synthesized by solid state reactions. They crystallize in the orthorhombic space group Pnma and the monoclinic space group C2/m respectively. The crystal structure of the two compounds are made up of similar building blocks; Co4Se3O9Cl2 is made up of [CoO4Cl2], [CoO5Cl] and [SeO3] polyhedra and Co3Se4O10Cl2 is made up of [CoO4Cl2] and [SeO3] polyhedra. As several Co-containing compounds have proved to be good catalysts for water oxidation, the activities of the two new compounds were compared with the previously found oxohalide Co5Se4O12Cl2 in reference to CoO and CoCl2. The one electron oxidant Ru(bpy)3(3+) was used as oxidizing species in a phosphate buffer and it was found that the activities of the oxohalide species were in between CoO and CoCl2. The roles of Cl(-) and PO4(3-) ions are discussed.

Artificial photosynthesis: from nanosecond electron transfer to catalytic water oxidation

Human society faces a fundamental challenge as energy consumption is projected to increase due to population and economic growth as fossil fuel resources decrease. Therefore the transition to alternative and sustainable energy sources is of the utmost importance. The conversion of solar energy into chemical energy, by splitting H2O to generate molecular O2 and H2, could contribute to solving the global energy problem. Developing such a system will require the combination of several complicated processes, such as light-harvesting, charge separation, electron transfer, H2O oxidation, and reduction of the generated protons. The primary processes of charge separation and catalysis, which occur in the natural photosynthetic machinery, provide us with an excellent blueprint for the design of such systems. This Account describes our efforts to construct supramolecular assemblies capable of carrying out photoinduced electron transfer and to develop artificial water oxidation catalysts (WOCs). Early work in our group focused on linking a ruthenium chromophore to a manganese-based oxidation catalyst. When we incorporated a tyrosine unit into these supramolecular assemblies, we could observe fast intramolecular electron transfer from the manganese centers, via the tyrosine moiety, to the photooxidized ruthenium center, which clearly resembles the processes occurring in the natural system. Although we demonstrated multi-electron transfer in our artificial systems, the bottleneck proved to be the stability of the WOCs. Researchers have developed a number of WOCs, but the majority can only catalyze H2O oxidation in the presence of strong oxidants such as Ce(IV), which is difficult to generate photochemically. By contrast, illumination of ruthenium(II) photosensitizers in the presence of a sacrificial acceptor generates [Ru(bpy)3](3+)-type oxidants. Their oxidation potentials are significantly lower than that of Ce(IV), but our group recently showed that incorporating negatively charged groups into the ligand backbone could decrease the oxidation potential of the catalysts and, at the same time, decrease the potential for H2O oxidation. This permitted us to develop both ruthenium- and manganese-based WOCs that can operate under neutral conditions, driven by the mild oxidant [Ru(bpy)3](3+). Many hurdles to the development of viable systems for the production of solar fuels remain. However, the combination of important features from the natural photosynthetic machinery and novel artificial components adds insights into the complicated catalytic processes that are involved in splitting H2O.

Ammonia binding to the oxygen-evolving complex of photosystem II identifies the solvent-exchangeable oxygen bridge (μ-oxo) of the manganese tetramer

The assignment of the two substrate water sites of the tetra-manganese penta-oxygen calcium (Mn4O5Ca) cluster of photosystem II is essential for the elucidation of the mechanism of biological O-O bond formation and the subsequent design of bio-inspired water-splitting catalysts. We recently demonstrated using pulsed EPR spectroscopy that one of the five oxygen bridges (μ-oxo) exchanges unusually rapidly with bulk water and is thus a likely candidate for one of the substrates. Ammonia, a water analog, was previously shown to bind to the Mn4O5Ca cluster, potentially displacing a water/substrate ligand [Britt RD, et al. (1989) J Am Chem Soc 111(10):3522-3532]. Here we show by a combination of EPR and time-resolved membrane inlet mass spectrometry that the binding of ammonia perturbs the exchangeable μ-oxo bridge without drastically altering the binding/exchange kinetics of the two substrates. In combination with broken-symmetry density functional theory, our results show that (i) the exchangable μ-oxo bridge is O5 {using the labeling of the current crystal structure [Umena Y, et al. (2011) Nature 473(7345):55-60]}; (ii) ammonia displaces a water ligand to the outer manganese (MnA4-W1); and (iii) as W1 is trans to O5, ammonia binding elongates the MnA4-O5 bond, leading to the perturbation of the μ-oxo bridge resonance and to a small change in the water exchange rates. These experimental results support O-O bond formation between O5 and possibly an oxyl radical as proposed by Siegbahn and exclude W1 as the second substrate water.

Biological water oxidation

Photosystem II (PSII), a multisubunit pigment-protein supercomplex found in cyanobacteria, algae, and plants, catalyzes a unique reaction in nature: the light-driven oxidation of water. Remarkable recent advances in the structural analysis of PSII now give a detailed picture of the static supercomplex on the molecular level. These data provide a solid foundation for future functional studies, in particular the mechanism of water oxidation and oxygen release. The catalytic core of the PSII is a tetramanganese-calcium cluster (Mn₄O₅Ca), commonly referred to as the oxygen-evolving complex (OEC). The function of the OEC rests on its ability to cycle through five metastable states (Si, i = 0-4), transiently storing four oxidizing equivalents, and in so doing, facilitates the four electron water splitting reaction. While the latest crystallographic model of PSII gives an atomic picture of the OEC, the exact connectivity within the inorganic core and the S-state(s) that the X-ray model represents remain uncertain. In this Account, we describe our joint experimental and theoretical efforts to eliminate these ambiguities by combining the X-ray data with spectroscopic constraints and introducing computational modeling. We are developing quantum chemical methods to predict electron paramagnetic resonance (EPR) parameters for transition metal clusters, especially focusing on spin-projection approaches combined with density functional theory (DFT) calculations. We aim to resolve the geometric and electronic structures of all S-states, correlating their structural features with spectroscopic observations to elucidate reactivity. The sequence of manganese oxidations and concomitant charge compensation events via proton transfer allow us to rationalize the multielectron S-state cycle. EPR spectroscopy combined with theoretical calculations provides a unique window into the tetramangenese complex, in particular its protonation states and metal ligand sphere evolution, far beyond the scope of static techniques such as X-ray crystallography. This approach has led, for example, to a detailed understanding of the EPR signals in the S₂-state of the OEC in terms of two interconvertible, isoenergetic structures. These two structures differ in their valence distribution and spin multiplicity, which has important consequences for substrate binding and may explain its low barrier exchange with solvent water. New experimental techniques and innovative sample preparations are beginning to unravel the complex sequence of substrate uptake/inclusion, which is coupled to proton release. The introduction of specific site perturbations, such as replacing Ca²⁺ with Sr²⁺, provides discrete information about the ligand environment of the individual Mn ions. In this way, we have identified a potential open coordination site for one Mn center, which may serve as a substrate binding site in the higher S-states, such as S₃ and S₄. In addition, we can now monitor the binding of the substrate water in the lower S-states (S₁ and S₂) using new EPR-detected NMR spectroscopies. These studies provided the first evidence that one of the substrates is subsumed into the complex itself and forms an oxo-bridge between two Mn ions. This result places important new restrictions on the mechanism of O-O bond formation. These new insights from nature's water splitting catalyst provide important criteria for the rational design of bioinspired synthetic catalysts.

Insights into Ru-based molecular water oxidation catalysts: electronic and noncovalent-interaction effects on their catalytic activities

A series of Ru-bda water oxidation catalysts [Ru(bda)L2] (H2bda = 2,2'-bipyridine-6,6'-dicarboxylic acid; L = [HNEt3][3-SO3-pyridine], 1; 4-(EtOOC)-pyridine, 2; 4-bromopyridine, 3; pyridine, 4; 4-methoxypyridine, 5; 4-(Me2N)-pyridine, 6; 4-[Ph(CH2)3]-pyridine, 7) were synthesized with electron-donating/-withdrawing groups and hydrophilic/hydrophobic groups in the axial ligands. These complexes were characterized by (1)H NMR spectroscopy, high-resolution mass spectrometry, elemental analysis, and electrochemistry. In addition, complexes 1 and 6 were further identified by single crystal X-ray crystallography, revealing a highly distorted octahedral configuration of the Ru coordination sphere. All of these complexes are highly active toward Ce(IV)-driven (Ce(IV) = Ce(NH4)2(NO3)6) water oxidation with oxygen evolution rates up to 119 mols of O2 per mole of catalyst per second. Their structure-activity relationship was investigated. Electron-withdrawing and noncovalent interactions (attraction) exhibit positive effect on the catalytic activity of Ru-bda catalysts.