Water oxidation at base metal molecular catalysts

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Water oxidation at base metal molecular catalysts Julio Lloret-Fillola,b,*, Miquel Costasc,* a

Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Tarragona, Spain Catalan Institution for Research and Advanced Studies (ICREA), Barcelona, Spain c Departament de Quı´mica, Institut de Quı´mica Computacional i Cata`lisi (IQCC), Universitat de Girona, Girona, Spain *Corresponding authors: e-mail address: [email protected]; [email protected] b

Contents 1. Introduction 1.1 Water oxidation in nature. The oxygen evolving complex 1.2 Synthetic models for the natural water oxidation reaction 1.3 Oxidants in water oxidation reactions 1.4 General mechanisms for water oxidation 1.5 Typology of catalysts 2. Model well-defined water oxidation catalysts 2.1 Manganese water oxidation catalysts 2.2 Water oxidation with molecular iron catalysts 2.3 Cobalt water oxidation catalysts 2.4 Nickel-based water oxidation catalysts 2.5 Copper-based water oxidation catalysts 3. Conclusion and outlook Acknowledgments References Further reading

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1. Introduction 1.1 Water oxidation in nature. The oxygen evolving complex In nature, the oxidation of water occurs at the Oxygen Evolving Complex (OEC).1 The OEC contains a Mn4O4Ca cluster finally responsible for the oxidation of the water molecule to dioxygen. The most recent and accurate crystallographically determined structure of the protein complex Advances in Organometallic Chemistry ISSN 0065-3055 https://doi.org/10.1016/bs.adomc.2019.02.003

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2019 Elsevier Inc. All rights reserved.

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˚ , which permits to obtain a Photosystem II (PSII) has a resolution of 1.9 A reasonably well-defined picture of the Mn4O4Ca cluster as a distorted cubane-like Mn3O4Ca complex linked to the forth Mn center via an oxo bridge.2 This cluster is an impressive water oxidation catalyst; it produces oxygen at outstanding rates in the range of 100–300 O2 molecules
s1, with an estimated TON of about half a million and an overpotential lower than 200 mV.3 However, the D1 protein,4 responsible for the binding of the secondary plastoquinone,5 at the PSII is rapidly photodamaged and must be replaced every 30 min by a new copy.6 The reaction mechanism is known as the Kok cycle (Fig. 1), and consists of four different stages (from S0 to S4) connected via a light-dependent 1e oxidation process, and a fifth light-independent O2 release step (S4 ➔ S0),3b,7–9 The resting state is S1 and has been spectroscopically

Fig. 1 Kok cycle and possible intermediate structures for the different O–O bond formation pathways (acid-base or direct coupling).7,8 WNA, water nucleophilic attack; DC, direct coupling; XFEL, X-ray free-electron laser.8b

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characterized to contain 2 MnIII and 2 MnIV centers. States S0 and S2 are also well-characterized as MnIII 3 MnIV and MnIII MnIV 3 . S3 is proposed to have a MnIV 4 cluster.7 Further 1e oxidation generates S4, where OdO bond formation takes place. At this point triplet O2 is liberated and S0 regenerated. The exact nature of S3 and S4 is still under debate but two scenarios are most commonly considered; either it contains a Mn(V) center, or a Mn(IV)-oxyl radical.8 Two possible mechanisms for OdO bond formation are also considered; the first one entails a nucleophilic attack of a Ca2+ bound water molecule on an electrophilic μ-oxo bridge. Alternatively, OdO formation may take place through the coupling of two metal-oxyl radicals. This scenario requires binding of an additional water molecule to the cluster at the S3-state, specifically at the open binding site of a Mn center that results from cleaving a Mn-O-Mn unit.8a,10 This additional water substrate forms the oxyl radical in S4, which reacts with the oxygen atom at the adjacent Mn2CaO vertex.7 An important aspect of this cycle is that accumulation of each oxidant equivalent is balanced with proton release. As a result, the four oxidation steps occur within only 1 V,3 and high oxidation potentials are not needed to reach the higher oxidation states of the cluster necessary for OdO bond formation. This feature represents a valuable lesson for the development of artificial catalysts for multielectronic processes. Proton coupled electron transfer (PCET) processes represent valuable fundamental reactions that permit to reach high oxidation states with relatively low red-ox potentials. This aspect is important because the oxidation potential needed to generate the high valent species responsible for OdO bond formation is often the major contribution to the overpotential and kinetic barriers of the overall water oxidation reaction. In addition, lower oxidation potentials also reduce potential oxidative degradation pathways.

1.2 Synthetic models for the natural water oxidation reaction The inherent complexity associated with the multiprotonic and multielectronic nature of the water oxidation reaction, and the structural and reactivity complexity of the Mn4OCa center represent major challenges for catalysis and inorganic chemistry.2,11 However, basic chemical features of the natural water oxidation reaction serve as guideline for the design of artificial catalysts. Furthermore, the extensive research efforts devoted to the discovery of water oxidation catalysts since the pioneered ruthenium complexes have shown important mechanistic and design aspects.

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The Ru-blue dimer first described by Meyer12 initiated an intense search for synthetic water oxidation catalysts. A large quantity of ruthenium catalysts have been described since13 and some recently described catalysts, such as Sun mononuclear Ru-bpa complex,3a catalyze the reaction at rates that exceed that of PSII. Closer in time, Ir catalysts have proven to be extraordinarily efficient,14 providing TONs in the order of a million.15 First row transition metals constitute a desirable target for catalyst development because these metals are earth abundant, thus they have the potential to be employed in massive scale, as it will be needed in artificial photosynthesis.16 However, they present potential problems; in first place, first row transition metals form more labile metal-ligand bonds than the heavier metals counterparts. This is specially accentuated in low valent, and high spin complexes. A second problem is their paramagnetic nature, which complicates their characterization. The last issue is that they tend to require very high oxidation potentials to reach high oxidation states. All these elements make catalyst design a prime aspect in the development of molecular catalysts with these metals. Discovered water oxidation catalysts have shown that the reaction can be catalyzed with relatively simple coordination complexes. It is now wellestablished that several metals and ligand architectures can engage in water oxidation reactions. These molecular catalysts are important because they provide lessons that facilitate rational design of novel more active and robust catalysts, and permit the realization of deep mechanistic studies of the challenging catalytic water oxidation reaction. In the current chapter, we review some of the most significant systems that rely in first row transition metals (Fig. 2).

1.3 Oxidants in water oxidation reactions In nature, water oxidation is thermodynamically driven by light. However, the complexity of coupling efficient light absorption with the generation of metal species eventually capable of catalyzing water oxidation, makes artificial light driven water oxidation complicated. Most often, the process is separated into several fundamental and simpler problems; efficient light absorption and effective use of this energy in generating species capable of oxidizing water is per se a research target.17 On the other hand, the design of catalysts that can oxidize water employing chemical oxidants or electrochemical oxidation constitutes a second line of work. Two types of oxidants are employed in WO reactions; oxo-transfer agents such as hypochlorite (HClO, 1.39 V vs SCE), Oxone (SO5  , 1.85 V vs SCE) and periodate (at low pH H5IO6, 1.60 V vs SCE), and

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Fig. 2 Timeline of a selection of the most representative WOCs based on 1st-row transition metal complexes. TON, TOF (s1), relevant discovery, oxidation method the corresponding authors are included. Echem-driven stands for electrochemical driven water oxidation, and Photo-Echem is used for a system powered by a combination of light and electrical current.

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electron-transfer (ET) agents like [Ce(NO3)6](NH4)2 (CAN, 1.72 V vs SCE), [Co(OH2)6]3+ (1.92 V vs SCE), and [Ru(bpy)3]3+ (bpy ¼ 2,20 bipyridine, 1.29 V vs SCE).17a,18 ET agents are regarded as particularly suitable from a mechanistic perspective. Oxygen atoms are not present in the oxidant and consequently, O2 formation must arise from oxidation of two water molecules, formally analogous to the PSII reaction. However, anions like NO3  may, in rare cases, be a source of oxygen atoms.19 CAN has been extensively studied in water oxidation reactions with ruthenium and iridium complexes, presumably because the compound is commercially available, and it is not expensive, but its high acidity cannot be tolerated by most first row transition metal complexes. Additionally, the potential non-innocent nature of CeIV in establishing M-O-Ce bonds with metal oxo groups should be also considered in mechanistic studies.20 In the case of oxo-transfer agents, one of the oxygen atoms of evolved O2 should originate from the oxidant, and the second comes from water. However, one should consider that oxo donors like OCl, IO4  , PhIO, as well as metal-oxo species, engage in oxygen exchange reactions with water molecules, and this process may be accelerated in the presence of transition metal ions. These processes need to be considered when drawing mechanistic interpretations. The most conflictive oxidants are peroxides, because an OdO bond is already present, and in these cases, there is always the risk that peroxide disproportionation and not water oxidation is the origin of O2. In any case, isotopic analyses can provide a definitive answer on whether a reaction is a genuine water oxidation reaction or a peroxide disproportionation. Electrochemical methods have increased in a very important manner the discovery of water oxidation catalysts based on first row transition metals. Electrochemical oxidations impose milder reaction conditions than chemical oxidants, which in general, operate in acidic conditions that can induce hydrolytic damage of the catalysts. Therefore, despite there are only few first-row transition metal complexes that can operate with chemical oxidants, the number of electrocatalysts is very high. On the other hand, electrochemical oxidations are convenient because they can be straightforwardly performed at different pH’s and they provide information about the overpotentials needed to perform the reaction. On the other hand, electrochemical oxidations require the use of an electrolyte and most often buffers, which may also interact with the catalyst, and may not be innocent with respect to the chemistry. Furthermore, in general, elucidation of the species involved in electrochemical oxidations is very difficult because spectroscopic characterization is very challenging.21

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1.4 General mechanisms for water oxidation Despite the typology of coordination complexes that can catalyze water oxidation is quite diverse, the OdO bond formation step occurs by a limited number of reaction mechanisms (Scheme 1).

Scheme 1 Basic mechanistic schemes for O–O bond formation.

(a). The most common mechanism is the nucleophilic attack (NA) of a water or hydroxide entity over a high valent metal-oxo species. This reaction takes place at a single metal center. The reaction results in (hydro)peroxide species concomitant to the reduction of the metal center. Basic orbital occupancy considerations indicate that for some late transition metals the bond order of the metal-oxo bond cannot be larger than 1. In these cases, metal-oxyl and high valent metal-hydroxide species may be also susceptible to undergo nucleophilic attack by water. (b). Bimolecular coupling (BC) of two metal-oxo species. This mechanism requires two metal centers, is much less frequent and requires proper orientation of the two metal-oxo species to promote OdO bond formation. The presence of radical character in the oxo ligand has been considered important for promoting this reaction.

1.5 Typology of catalysts Coordination complexes that catalyze the oxidation of water need to be resistant to oxidizing conditions. This limits the typology of ligands that can be employed. The most common are; (a). Deprotonated polyamides are highly basic and provide stabilization to high oxidation states. In addition, they are quite resistant toward oxidation. The prototypical cases are tetraamido macrocyclic ligands (TAML’s) and related polyamides, which have been used in iron,

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(b).

(c).

(d).

(e).

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cobalt and copper complexes. A limitation of the complexes with this type of ligands is that they are quite sensitive to hydrolysis, specially under acidic conditions. A second group of complexes are those based on aminopyridine ligands. Polyamine and polypyridyl complexes can be considered subcases of this class of compounds. The stability of these ligands against oxidation is largely dependent on the metal and the lability of the complex. While bound to the metal, the amine moiety remains protected, but upon detachment, they become an easily oxidizable site. A third numerous group of complexes are polyoxometallates. These are purely inorganic compounds, non-sensitive to oxidation conditions. On the other hand, they tend to suffer from hydrolytic instability, leaching transition metals in the media. Metalloporphyrins constitute a group of complexes that receive use in water oxidation reactions. The ability of the complexes to reach high valent metal oxo species make them suitable for catalyzing oxidation reactions. However, porphyrins are quite electron rich systems and tend to be sensitive to oxidation. Carboxylate clusters, especially those based on cobalt and manganese receive use as water oxidation catalysts. However, because of the lability of the metal-carboxylate bonds, their integrity in water oxidation reactions is generally a concern.

2. Model well-defined water oxidation catalysts This Chapter covers water oxidation catalysis with first row transition metal complexes. The work does not aim to be comprehensive but instead an effort has been placed in discussing the most recent systems, while also discussing the most remarkable classical examples.

2.1 Manganese water oxidation catalysts Manganese is the metal that nature selected to effectively drive the water oxidation reaction by the CaMn3O4 cluster of the OEC at the PSII and therefore the metal of choice for the development of biomimetic systems. In this regard, intensive synthetic efforts have been devoted to obtain valuable water oxidation catalysts based of manganese clusters, but also mononuclear coordination complexes. Despite the efforts, our understanding of the basis to obtain catalytically activity Mn clusters is rather limited. Only a reduced number of complexes with cubane structure present catalytic

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activity. Moreover, the low catalytic activity observed with reported systems and the effectivity of manganese oxides (MnOx) as water oxidation catalysts, questions the molecular origin of the water oxidation activity. Another important aspect under debate in the OEC is the role of the calcium ion. The preparation of structural analogs of the OEC containing a calcium atom may help to clarify its role. But a synthetically equivalent OEC with catalytic activity has not been accomplished. In this regard, coordination complexes have special value to clarify the particularities of the manganese chemistry in high oxidation states in the OdO bond formation context. 2.1.1 Catalytic water oxidation with manganese coordination complexes One of the most extensively studied manganese complex as water oxidation catalyst is the mixed-balance dimeric [MnIII,IV (μ-O)2(terpy)2](NO3)3 (ter2 py ¼ 2,20 :60 ,200 -terpyridine). The complex reacts with OCl or oxone to produce 4 TON and >50 TON of O2, respectively.22 Mechanistic studies suggest a very complex reaction picture (Scheme 2). Binding of oxone at the MnIII or at the MnIV site results in productive and unproductive cycles, respectively. In the first case, binding at the MnIII site forms [(terpy)(SO5)MnIII(μ-O)2MnIV(terpy)]3+, which subsequently evolves to [(terpy) MnV(O)(μ-O)2MnIV(terpy)]3+. This complex reacts with water, forming

Scheme 2 Water oxidation mechanism proposed for [Mn2(μ-O)2(terpy)2(H2O)2]3+ (terpy ¼ 2,20 :6,200 -terpyridine). The proposed intermediate responsible for the O–O bond formation and release has been highlighted.23

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[(terpy)MnIII(OOH)(μ-O)2MnIV(terpy)]2+ which eventually evolves O2.23 Isotopic analysis shows incorporation of 18O from H218O into the evolved O2,23 in agreement with the proposed mechanism. Furthermore, DFT calculations show that the mechanism is plausible from the energetical point of view. An interesting aspect that arises from the computational analysis of the water oxidizing species is that the MnV]O unit is best described as a MnIV-oxyl radical species. A drawback of this system from a mechanistic perspective is that the use of CAN as a single electron oxidant destroys the [MnIII,IV (μ-O)2(terpy)2(H2O)2]3+ 2 complex, presumably because the strong acidic conditions imposed by this oxidant leads to ligand decoordination. The use of this oxidant (CAN) is preferred to avoid parallel sources of O2 not arising from H2O. However, when the complex is adsorbed onto layered materials such as kaolin or mica, it appears to be stable upon reaction with CAN, as confirmed by EXAFS analysis, and shows water oxidation activity (14 TON).24 A very elegant system from mechanistic and ligand design perspectives is the 1,2-arene ligated bis-Mn-porphyrin complexes described by Naruta (Scheme 3). In aqueous acetonitrile solutions containing NH4OH the

Scheme 3 Selected Mn based water oxidation catalysts. Top: Diagram of dimeric Mn-porphyrin complexes that act as electrochemical water oxidation catalysts and the proposed mechanism.25 Bottom, left: Pyridophane complexes employed in H2O2 disproportionation and water oxidation.25 Bottom, middle: The first homogeneous Mn-WOC working with single electron oxidants and the X-ray crystal structure of the dimeric form at 50% probability level of the biomimetic complex.26 Bottom right: schematic diagram for a proposed mechanism of O–O bond formation in PhII.7

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complexes are water oxidation electrocatalysts (up to 9.2 TON) at a potential above 1.4 V vs SHE. The O2 production rate was found linearly dependent on dimer concentration, and monomeric Mn-porphyrin analogs did not promote water oxidation. These aspects strongly suggest that water oxidation requires the synergistic operation of the two manganese centers, and the origin of the water oxidation activity is proposed to be based on the rigid spatial disposition of two MnV]O units, which is proposed to promote intramolecular OdO bond formation (MnIV-O-O-MnIV) that then liberates O2.25 So far there is a single example of a homogeneous manganese water oxidation catalyst that can perform the reaction with an outer sphere single electron transfer oxidant ([Ru(bpy)3]3+ (bpy ¼ 2,20 -bipyridine)). The mixed valence complex [MnII,III 2 (dCIP)(OMe)(CH3CO2)] (dCIP ¼ 2-(3-(7-carboxy1H-3λ4-benzol[d]imidazol-2-yl)-2-hydroxyphenyl)-1H-benzo[d]imidazole4-carboxylic acid) contains a dinuclear manganese center, where the two metal ions are bridged by a phenoxide, a methoxide and an acetate ligand (Scheme 3). The coordination sphere of each of the manganese centers is filled with a nitrogen atom from a benzylimidazole ligand and a carboxylate oxygen from the ligand. The dinuclear unit crystalizes as a tetrameric species, reminiscent of the OEC cluster, that is held together by carboxylate bridging of the Mn atoms of two different dimers. The highly negatively charged donor set is an important feature because it substantially reduces the redox potentials required for reaching the active high oxidation states. Consequently, they can be generated under mild conditions and are more stable. The complex indeed carries out the WO in phosphate buffer with a TON of 25 and TOF of 0.027 s1. Control experiments showed that the combination of the ligand and Mn(OAc)2, in the same experimental conditions, does not produce O2, and the reaction does not show induction period. This data strongly suggest that MnOx are not the WO catalyst in this system. Furthermore, isotopic labeling analyses show that the two oxygen atoms of evolved Ox originate from water. The catalyst is competent for the light-driven process with a TON of 4 (Na2S2O8 as sacrificial oxidant and [Ru(bpy)3]2+ or [Ru(bpy)2(4,4’-CO2Et-bpy)](PF6)2 as photosensitizers). A computational analysis on the reaction mechanism for water oxidation of this complex initiates the reaction from a MnIII 3 MnIV species, better described as a MnIII 4 associated to a ligand radical.27 Four PCET processes generate a MnIII Mn3 IV species, where the two ligands are 1e oxidized and have radical character. Intramolecular proton transfer generates then a Mn(μ-O)2MnIV-O that attacks a bridging oxo ligand of the second

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Mn(μ-O)2Mn core (see Scheme 3), leading to OdO bond formation. This reaction mechanism has close similarities to one of the proposed mechanisms for the OEC and gives important lessons about key aspects of the natural reaction; the presence of a terminally bound water molecule is necessary for eventually generating a reactive terminal metal-oxo species. The reactive character of a bridging oxo ligand is also an important feature because computations suggest that this path is favorable when compared to a nucleophilic attack toward a water molecule. Finally, the red-ox non-innocence of the ligand helps to the storage of charge and oxidation power without the need to rely in very high and therefore reactive metal oxidation states. Several mononuclear manganese complexes have been shown to form high valent manganese oxo species upon reaction with oxidants such as peroxides, Oxone or PhIO.28 These high valent species are potential oxidants of the water molecule. However, clear cut evidence for the reaction remains to be shown. The fact that peroxides are employed as oxidants makes mechanistic interpretations quite complicate because peroxide disproportionation is also a common source of O2. Among them, a particularly interesting system is the family of manganese complexes with pyridophane ligands [(Py2NR2)Mn(H2O)2]2+ shown in Scheme 3.25 Complexes where R ¼ H or Me disproportionate H2O2 in aqueous solutions. Instead, the complex with R ¼ tBu does not show this catalase behavior and is an electrocatalyst for water oxidation at pH 12.2. Oxidation occurs at 1.23 V vs NHE, with faradaic efficiencies of 74–81%. TON’s between 16 and 24 were obtained. A series of mechanistic studies suggest that the reaction is catalyzed by a molecular catalyst and not manganese oxides formed after decomposition of the initial complex; in first place nanoparticles were not observed with DLS analyses, no spectral changes were observed after electrolysis, and catalytic activity does not increase with repeated scans, as it will be the case if active MnOx deposits were formed in the electrode. Furthermore, EDX experiments did not show accumulation of Mn deposits at the electrode. Finally, the intensity of the electrocatalytic wave is linearly dependent on manganese catalyst concentration, suggesting that OdO formation takes place at a single manganese center. 2.1.2 Water oxidation with polyphosphate and polycarboxylate manganese complexes Polymetallic manganese complexes with oxide and carboxylate bridges are readily accessible complexes that have received major interest in the field of molecular magnetism. They also bear resemblance with the manganese

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cluster of the OEC. In these complexes, strong electronic communication between multiple manganese atoms can occur. This electronic communication may be envisioned as a useful element to permit accumulation of oxidation equivalents, which may eventually be used in multielectronic reactions such as water oxidation. This type of complexes has been explored as water oxidation catalysts. The integrity of these structures in aqueous solutions, under water oxidation conditions, combined with the fact that manganese oxides (MnOx’s) also catalyze the reaction, constitute critical issues that complicate the analysis of these reactions. A paradigmatic case is tetramanganese complexes L6Mn4O4 (L ¼ diarylphosphinate ligand, ðp-R-C6 H4 Þ2 PO2  (R ¼ H, alkyl, OMe; Fig. 3)), which constitute pioneering models of the OEC. By supporting these clusters in Nafion, photoelectrochemical driven water oxidation was observed at 1.0–1.4 V vs SHE. After photoelectrolysis for 65 h, a net charge equivalent to 1000

Fig. 3 Proposed structural rearrangement of the L6Mn4O4 cluster during water oxidation. Only one phosphinate ligand is shown in the catalytic cycle to emphasize the coordination/decoordination process. a Photoelectrochemical TON obtained for the L6Mn4O4 cluster supported on Nafion after 65 h at 1.4 V.

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turnovers per cluster was measured.29 It was proposed that upon absorption of light, O2 is produced, generating a “pinned butterfly” structure that will bind two water molecules and extrude four protons, regenerating the cubane structure. However, Spiccia and coworkers recently showed that L6Mn4O4 clusters supported on Nafion are transformed into Mn2+ oxides (birnessite), which in turn is electro-oxidized into MnIII/IV-Ox nanoparticles (NP), which are proven to be the species responsible for water oxidation.30 An interesting recent example is a water soluble polymanganese-oxo cluster with general formula [Mn12O12(O2CC6H3(OH)2)16(H2O)4].31 The complex can be considered biomimetic because it contains a central ½MnIV 4 O4  cube that resembles the OEC. Of notice, the cluster can undergo several reversible 1e oxidations, accumulating positive charge by generating manganese atoms in high oxidation states. The cluster also contains water binding sites, which can be important for enabling water oxidation catalysis. Most remarkably, the complex is a water oxidation electrocatalyst at pH 6, with a small overpotential of 334 mV. After bulk electrolysis for 5 h at 1.21 V vs NHE, 15.5 TON of O2 were measured. Control experiments show that there is not deposition of catalytically active material into the electrode, suggesting that the reaction takes place in the homogeneous phase. Despite CV experiments provide strong support that the complex is stable in water solution under oxidation conditions, partial degradation occurs over time (5 h) as evidenced by a bulk electrolysis experiment performed during 5 h, which shows 91% production of O2 in the first 3 h, and then a plateau of O2 production. Significant changes in the UV–Vis spectra of the solution further indicates partial degradation of the catalysts. An additional aspect that suggests catalyst degradation is the Faradaic efficiency of 77.9% after the initial 20 min of electrolysis. The authors indicate that a second product is generated, presumably CO2 from catalyst decomposition. The groups of Scandola, Bonchio, and Kortz also reported the synthesis of Mn clusters that are catalytically active. In this case, the polyoxometalate  III 6 Mn 3 MnIV O3 ðCH3 COOÞ3 ðA-α-SiW9 O34 Þ (Mn4POM) was able to accumulate multiple 1e oxidations, resembling the OEC. Nanosecond laser flash photolysis showed that the tetramanganese cluster MnIII/MnIV evolves through five electronic states (S0 ➔ S4) like PSII to finally release an O2 molecule.32 Under illumination the Mn4POM forms up to 5.2 TON of O2 and quantum efficiency of 1.7% when the [Ru(bpy)3]2+ is used as photoredox catalyst and Na2S2O8 in NaHCO3/Na2SiF6 buffer (pH 5.2).

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2.2 Water oxidation with molecular iron catalysts Iron is a particularly attractive element for designing catalysts. It is abundant and highly available and has minimum environmental impact.33 Furthermore, iron can exist in multiple oxidation and spin states, and exhibits a very rich chemistry. High valent iron-oxo species are very powerful oxidants, and are the substrate oxidizing species in a number of enzymes.34 Elizarova and co-workers in the early 1980s35 and Kaneko and co-workers in 1998,36 provided first evidence of iron catalyzed water oxidation. A major breakthrough in the field was the report by Bernhard and Collins in 2009 describing fast catalytic oxidation with a molecular iron catalyst based on tetraamido macrocyclic ligands (TAML’s).37 Since then, different families of coordination complexes have been described as water oxidation catalysts.38 In parallel, iron-oxides are emerging as powerful water oxidation materials.39 We have organized the iron catalysts in four groups; iron complexes based on tetraanionic tetraamido ligands, monoiron complexes with aminopyridine ligands, oxo-bridged diiron complexes and polyiron complexes. 2.2.1 Iron catalysts with tetraanionic tetraamido macrocyclic ligands Tetraanionic tetraamido macrocyclic ligands (TAML’s) provide effective charge stabilization of transition metals in high oxidation states. In addition, this class of ligands are quite robust against oxidation. Thanks to these properties, a number of high valent first row transition metal complexes bearing this class of ligands have been prepared.40 Most notable, mononuclear iron(V)-oxo complexes have been described, and evidence for species with a further 1e oxidation has been gained.41 In addition, iron-TAML complexes and related have been extensively used in oxidation catalysis and cleaning of waters via oxidative degradation of pollutants.40a In their initial report, Collins and Berhard showed that Fe-TAML complexes, in aqueous solutions (Scheme 4) catalyze fast oxidation of water upon

Scheme 4 Schematic representation of the Fe-TAML and biuret Fe-bTAML complexes studied in water oxidation.

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reaction with CAN or NaIO4 as sacrificial oxidants.37 O2 is generated with a high TOF that depends on the nature of the ligand. A value of 1.3 s1 is obtained for the fastest catalyst of the series, but the system undergoes fast deactivation (20 s), producing low TONs (16). The O2 evolution rate is first order in concentration of the catalyst, suggesting that water oxidation takes place at a monoiron site. UV–Vis monitoring of the reactions indicate that FeV(O) species are formed under the catalytic conditions, although it is not demonstrated that these species are the finally responsible for water oxidation. Instead, a computation study (DFT) performed by Cramer and coworkers suggests that the active oxidant is formed via one electron oxidation of the FeV(O) species to generate [(TAML+%)-FeV ¼O], with performs water oxidation via nucleophilic attack of a molecule over the FeV(O). The electronic structure of these species deserve special consideration because the TAML ligand is red-ox non innocent and one electron oxidized, ligated to a FeV(O) center responsible for the OdO bond formation via a water nucleophilic attack.42 The rapid deactivation of TAML-Fe complexes, presumably via hydrolysis, prevents electrocatalytic WO activity in a homogeneous phase. However, immobilization in Nafion provides materials that show electrocatalytic WO.43 Oxygen was produced when applying a static current of 5 mA with a Faradaic yield of 45% at an estimate TOF-O2 of 0.081 s1. Oxidation of the carbon based supporting material accounts for the reduced Faradaic yield. Modification of the TAML macrocyclic frame by replacing a carbon site by an amine produces complexes with enhanced hydrolytic stability. In addition, the corresponding FeV(O) complex is stable at room temperature. Biuret-modified Fe-TAML complexes (Fe-bTAML, Scheme 4) catalyze photochemical WO in aqueous solutions.44 Upon irradiation at 440 nm and employing [Ru(bpy)3]2+ as photosensitizer, fast WO (TON ¼ 220, TOF 0.76 s1) can be accomplished by using Na2S2O8 as terminal oxidant. Spectroscopic monitoring of the reaction reveals the rapid formation of an μ-oxo-FeIV dimer, which then evolves into a FeV(O) species. Neither the μ-oxo-FeIV dimer nor the FeV(O) monomer are kinetically competent to oxidize water and, thus, as earlier observed for the parent Fe-TAML complexes, further oxidation of the FeV(O) appears necessary for generating the water oxidizing species. 2.2.2 Iron catalysts with aminopyridine ligands Iron complexes with aminopyridine ligands have emerged as powerful oxidation catalysts for the oxidation of organic molecules.45 Some of these are

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also efficient water oxidation catalysts employing chemical oxidants.46 The activity of this class of catalysts depends on the nature of the ligand, which determines its stability against hydrolytic and oxidative decomposition, but also impacts on the ability of the complexes to reach the high oxidation states required for water oxidation to occur. Optimum catalysts contain tetradentate ligands and leave two cis-labile sites. In aqueous solution, upon reaction with chemical oxidants such as CAN or periodate, they produce O2, and the efficiency of the process is largely dependent on the nature of the catalyst. Complexes with bidentate, tridentate, pentadentate, or tetradentate ligands that leave trans-labile sites are virtually inactive. The complexes [Fe(OTf )2(Pytacn)] (Pytacn¼ 1,4-dimethyl-7-(2-pyridylmethyl)-1,4,7-triazacyclononane, OTf ¼ trifluoromethanesulfonate anion) and [Fe(OTf )2(mcp)] (mcp ¼ N,N 0 -dimethyl-N,N 0 -bis(2-pyridylmethyl)cyclohexane-trans-1,2-diamine) (Scheme 5) have been studied in most detail because they exhibit remarkable stability against hydrolytic and oxidative decomposition under water oxidation conditions when chemical oxidants are employed (at low pH’s). Water oxidation with chemical oxidants impose quite drastic experimental conditions; solutions of CAN and NaIO4 have a pH 1 and 2, respectively, and both are very strong oxidants, especially in acidic media (E° 1.7 V at pH 0 for CeIV/CeIII in CAN, and E°  1.6 V at pH 0 for IO4/IO3). Consequently, the number of first row transition metal complexes that can stand such conditions is very limited. [Fe(OTf )2(Pytacn)] and [Fe(OTf )2(mcp)] are quite unique complexes in this regard. Both of them generate disolvato cations [Fe(H2O)2(L)]2+ (L ¼ Pytacn and mcp) when dissolved in water. The high spin nature makes ferrous complexes labile, and slow hydrolysis takes place when dissolved in acidic water in the absence of

Scheme 5 Differentiation between active (left) and non-active (right) structures for water oxidation with iron complexes bearing neutral aminopyridine ligands.

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an oxidant. However, when they are rapidly subjected to water oxidation conditions, ferric and oxoiron(IV) species form. These are much more stable under acidic and oxidant conditions, enabling sustained catalytic activity. [Fe(OTf )2(mcp)] is among the most efficient water oxidation catalysts based on 1st row transition metals described to date, reaching TON values up to 360 (TOFmax ¼ 0.23 s1) and >1000 (TOFmax ¼ 0.06 s1) when using CAN (pH 1) and NaIO4 (pH 2) as chemical oxidant, respectively.46a Unfortunately, these complexes are poorly competent in electrocatalytic or photochemically driven water oxidation processes.47 In the latter case, under basic conditions [FeCl2(mcp)] undergoes rapid decomposition forming iron-oxide nanoparticles that are then catalytically competent.17c,48 Reaction mechanisms of [Fe(OTf )2(L)], L ¼ Pytacn and mcp with CAN have been throughtfully explored. Reactions conducted in the presence of H218O show that evolved O2 originates from water. Diffraction light scattering (DLS) experiments show no evidence for nanoparticle formation, and negligible CO2 formation takes place.46a This data strongly suggests that water oxidation takes place at molecular complexes where the ligand has not been degraded. Quite indicative, monitoring the reactions by UV–Vis and MS shows that [FeIV(O)(S)(L)]2+ (S being presumably water) are formed under catalytic reactions and their presence is ligated to O2 evolution. Once these species disappear, O2 evolution ceases, even when excess of oxidant is present. Kinetic analyses provide a detailed picture of the reaction mechanism. Rate constants were determined by monitoring O2 evolution (by manometry and by GC) and CAN consumption (by UV–Vis). The three different analyses result in congruent rates, which indicate that the kinetic models are solid. In first place, the kinetic data shows that [FeIV(O)(S)(L)]2+ are not kinetically competent to react with water, an observation that was further supported by DFT calculations.46b,49 Instead reaction rates exhibit a first order dependence on [FeIV¼O] and on CAN when the latter is used in slight excess. However, using large amounts of oxidant (CAN >20 equiv.) leads to a saturation of the reaction rates. Under these conditions, a new species accumulate in solution, which can be readily distinguished by their UV–Vis spectrum.46b In catalytic reactions performed with the [Fe(OTf )2(mcp)] complex, the new intermediate could be further characterized by HRMS and resonance Raman experiments as an heterometallic [FeIV(O) (μ-O)CeIV] species. This heterometallic species can be understood as an inner sphere intermediate in the single-electron oxidation of the FeIV(O) species by CeIV to form a FeV(O)(OH) species.46c Of interest, related RudCe heterometallic adducts find precedent in the literature.50

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The FeV(O)(OH) species is then responsible for the OdO bond formation via a water nucleophilic attack of the water molecule over the terminal oxo ligand (Scheme 6). The mechanism of OdO formation was validated by a DFT study, which also highlights the key role of the hydroxide ligand, which binds and orients the incoming water molecule substrate toward the reactive Fe]O unit. Computations also show that the oxidation of FeIV(O)(OH2) by CeIV to form FeV(O)(OH) is exergonic because the reaction entails a proton coupled electron transfer (PCET) process. However, single-electron oxidation of FeIV(O) complexes with ligands that do not permit the PCET process, such as pentadentate amines, require much larger, inaccessibly high red-ox potentials.

Scheme 6 Mechanism postulated for water oxidation with iron complexes bearing tetradentate aminopyridine ligands.

Kinetic and spectroscopic analyses of catalytic water oxidation with the series of complexes [Fe(OTf )2(X,YPytacn)], where X and Y are substitutions at positions 2 and 4, respectively, of the pyridine indicate that this family of complexes follows an analogous mechanistic scheme.46b Under catalytic

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conditions, reversible initial binding of CeIV to the iron(IV)-oxo species is followed by an irreversible step, presumably entailing inner sphere electron transfer to generate a reactive FeV(O)(OH) intermediate that is finally responsible for the water oxidation. Equilibrium constants for the CeIV binding systematically increase as the electron donating ability of Y increases, suggesting that the FeIV]O becomes more basic/nucleophilic. Instead, the irreversible step becomes faster in the opposite direction, for the complexes with more electron-withdrawing groups, suggesting the generation of a more reactive FeV species. Substitution at position 2 have a dramatic effect in reducing the activity of the catalysts, presumably because the pyridine-Fe bond becomes longer and weaker, and the corresponding complexes may be more susceptible to hydrolytic decomposition. Variations on the mechanistic scheme delineated above may result from changes in the nature of the complexes. In the case of the iron pyridophane complex [Fe(LN4Py2)Cl2]+ (LN4Py2 ¼ N,N0 -dimethyl-2,11-diaza[3,3] (2,6)pyridinophane), reaction with CAN does not produce a detectable iron-cerium complex. DFT calculations and mass spectrometry studies suggest that an FeIV(O)(OH) evolves to FeV(O)(O) (S ¼ 3/2), which engages in OdO bond formation through a water nucleophilic attack (WNA) mechanism.46g Complexes bearing ligands that can be regarded as variations of the structure of the basic mcp tetraamine have been explored. These include different diamine backbones, changes of pyridines by other heterocycles, and incorporation of different functionalities in the pyridines.46d,e,51 In each case, the resulting complex shows reduced activity when compared with the [Fe(OTf )2(mcp)] case,46e and the reduced catalytic behavior can be readily explained by noticing that these variants either contain easily oxidizable groups or reduced stability against hydrolysis.48a,52 Besides aminopyridine type of ligands, aliphatic polyamines are also quite common ligands in coordination chemistry and have been shown to support iron complexes with water oxidation activity. Most notable, the ferric complex bearing the cross-bridged cyclam ligand (4,11-dimethyl-1,4,8,11tetraazabicyclo[6.6.2]hexadecane) catalyzes water oxidation with NaIO4, reaching a maximum TON and TOF of 1030 and 0.028 s1, respectively.53 Iron complexes with polyamine type of ligands exhibit modest activity in electrocatalytic water oxidation, and it has been difficult to establish if the activity originates from the molecular catalysts, or from iron-oxides that form after initial oxidation of the ligand. Careful analysis of the electrocatalytic activity of a series of complexes has been put forward; on-line mass spectrometry in combination with classical electroanalytical

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techniques have been used to obtain key information about the reactivity such as the onset potential of water oxidation for a series of iron complexes.52,54 In this study, the presumed electrocatalytic wave was analyzed at the same time as O2 and CO2 evolution (by MS). The data convincingly shows that cis-[Fe(Cl)2(cyclam)]Cl displays a electrocatalytic water oxidation activity; O2 was produced before the onset of CO2 production, which appears as a minor process, takes place, indicating that electrocatalytic water oxidation takes place before complex decomposition occurs. 2.2.3 Water oxidation with an anionic polyamine ligand While iron complexes with polyamine ligands exhibit better water oxidation activity with chemical oxidants than electrocatalytically, the ferric complex [FeIII(dpaq)(H2O)]2+ (dpaq¼ 2-[bis(pyridine-2-ylmethyl)]amino-N-quinolin8-yl-acetamido, Scheme 7), constitutes a remarkable case of the opposite behavior.55

Scheme 7 Proposed cycle for the catalytic water oxidation activity exhibited by [FeIII(dpaq)(H2O)]2+.

The complex appears to be inactive in aqueous solutions employing CAN as oxidant. Instead, in propylene carbonate–water (8%) mixtures the complex exhibits electrocatalytic behavior. The CV in dry propylene

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carbonate shows a quasi-reversible 1e wave at 0.38 V vs NHE that is attributed to the FeIII(OH2)/FeII(OH)2 pair and a second, irreversible two e process assigned to the FeV(O)/FeIII(OH2) process. When the same analysis is performed in propylene carbonate–water (8%) mixtures, a current increase is observed, indicative of electrocatalytic water oxidation. Kinetic analysis of the electrocatalytic behavior suggests formation of the FeV(O) species is then followed by rate determining bimolecular water oxidation (first order in catalyst and first order in water). A side-on peroxide species is proposed to be formed, in analogy to the mechanism of polypyridyl-Ru catalysts. Bulk electrolysis at 1.58 V vs NHE results in sustained water oxidation catalysis yielding 29 turnovers of O2 over a 15 h electrolysis experiment with a 45% Faradaic yield. However, no significant electrocatalytic activity in aqueous solution is observed over an extended pH range. 2.2.4 Water oxidation at oxo-bridged diiron complexes Oxo-bridged diferric complexes are very common in iron coordination chemistry. Study of their catalytic activity in water oxidation reactions is difficult because oxo-bridged diferric complexes dissociate to monoferric species under acidic conditions, and water oxidation with chemical oxidants occurs in highly acidic solutions. Different examples with the archetypical TPA (TPA ¼ tris-(2-methylpyridyl)amine) ligand have been explored (Scheme 8). [(TPA)2Fe2(μ-O)(OH2)2]4+, [(TPA)2Fe2(μ-O)(μ-SO4)]2+

Scheme 8 Water oxidation catalysts based on oxo-bridged diiron complexes.

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and [(TPA)2μFe2(-O)(μ-OAc)]3+ have been studied with CAN, NaIO4 and Oxone, respectively, as oxidant.46f,56 In all of the cases evidence is provided that chemistry takes place at the molecular catalysts, but the nature of the active catalytic intermediates was not investigated. Particularly interesting is the oxo-bridged diiron complex [(H2O)(ppq)Fe(μ-O)Fe(Cl)(ppq)]3+, where each center contains a planar tetradentate polypyridyl ligand (Scheme 8). The complex reacts with CAN, producing O2 at a remarkably fast rate (TOF ¼ 7920 h1).57 The complex also exhibits electrocatalytic behavior. CV’s indicate that the complex undergoes a twoelectron oxidation from H2O–FeIIIFeIII to H2O–FeIVFeIV which is then proposed to isomerize to an O]FeVFeIII species, finally responsible for the water oxidation. Spectroscopic (UV–Vis) monitoring of the reaction suggests that the diferric complex retains its integrity in solution and a comparison with a monoferric complex with the same ligand demonstrates a superior catalytic competence of the differric catalyst. 2.2.5 Water oxidation at a multiiron complex The discovery of the extraordinary electrocatalytic water oxidation activity of a multiiron complex has represented a major breakthrough for the field of  3+ water oxidation.58 The pentairon complex FeII 4 FeIII ðμ3  OÞðμ  LÞ6 (Scheme 9) contains a Fe5O core where the iron centers are strongly electronically coupled via a central oxo-bridge. The five iron atoms are arranged in the vertexes of an axially elongated trigonal bipyramid. The iron atoms in the central Fe3O core are five coordinated, while the two apical centers are hexacoordinated. In dry acetonitrile, the CV of the complex shows one reversible reduction wave (E1/2 ¼ 0.55 V vs ferrocene) and four chemically and electrochemically reversible one-electron waves at E1/2 ¼ 0.13, 0.30, 0.68 and 1.08 V. These waves correspond to single electron processes where the iron centers of the FeII 5 complex are sequentially oxidized up to the FeIII 5 state. In the presence of water, the last oxidation wave transforms into a large electrocatalytic wave, corresponding to catalytic water oxidation. Bulk electrolysis at 1.42 V vs Fc produces O2 with a 96% Faradaic yield. Most remarkably, kinetic analyses indicate that the complex exhibits a very large TOF, 1900 s1, which results in an estimated TON after 120 min of 1–10  106. These values are over 1000 times larger than any previously described iron catalysts, and even bypass the values of the OEC (100–400 s1). On the downside, these extraordinarily large numbers are obtained at a very large overpotential.

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Scheme 9 Schematic diagram of the Fe5O complex that performs electrocatalytic water oxidation, and its proposed mechanism.

The catalytic cycle proposed for this complex is shown in Scheme 9. The reaction entails a series of four single electron transfer processes. Two water molecules are proposed to bind at the all-ferric state. Internal electron transfer from the basal to apical centers, and deprotonation of the bound water molecules produces two terminal iron(IV)-oxo moieties in close space and proper arrangement to engage in OdO formation. Several aspects of this complex deserve special consideration; in first place, positive charge is built in an electronically delocalized polymetallic complex, avoiding the generation of localized, very reactive high valent metal centers. In second place, electronic coupling appears to be key for facilitating several steps on the reaction mechanism; for example, generation of the ferryl moieties by internal electron transfer. In third place, two metal

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centers are coordinatively unsaturated and have binding positions available for the incoming water substrate to bind. Interestingly, the binding sites are closely disposed and suitably oriented for promoting O


O interaction. All these factors combine to make charge accumulation and OdO formation very fast and efficient. Actually, most of these aspects are also considered key elements in the OEC. A single negative aspect of this catalyst, which is also an important difference regarding the OEC, is that oxidation takes place at a large overpotential, a factor that may be addressed by proper ligand design.

2.3 Cobalt water oxidation catalysts Simple Co2+ ions, cobalt oxides and derivatives (or cobalt oxophosphate salts “CoPi”) shows the remarkable property of being active catalysts for water oxidation at moderate overpotentials under neutral to basic conditions.59 Multinuclear cobalt clusters have been used as models to shed light on the water oxidation mechanism on heterogeneous materials. Likewise, cobalt complexes based on polyoxometalates,60 porphyrins, phthalocyanins, corroles, TAML,44b polypyridines,61 polyamines,62 polypyridinamines,63 and salen64 ligands have shown catalytic WO activity. Although an intrinsically high catalytic activity of a metal (metal oxide) is a desirable property for the development of efficient water oxidation catalysts, it also makes difficult to study the mechanism of homogeneous catalysts. A great effort is needed to assign the catalytic outcome to a specific catalytic species, and care should be taken when analyzing the mechanism. We will focus the attention in representative examples among the large number of cobalt compounds WO active, since an exhaustive review goes beyond the objective of this chapter. 2.3.1 Cobalt Polyoxometalates Polyoxometalates as ligands have the advantage of their inherent stability against oxidation. However, metal complexes based of polyoxometalates are hydrolytically instable leaching metal in solution, which can form active WO nanoparticles. In this regard, in 2010 Craig L. Hill and coworkers proposed the use of cobalt complexes based on carbon-free polytungstate ligands (polyoxometalates such as PxWxOx and SixWxOx) to carry out the water oxidation (Fig. 4).60a They found that [Co4(H2O)2(PW9O34)2]10 (Fig. 4, 1-POM-Co) produces O2 at high rate (TOF of 5 s1, 75 TON) when dissolved in phosphate buffer (pH 8) and using [Ru(bpy)3]3+ as sacrificial oxidant. UV–Vis and 31P NMR spectra proved the stability of the compound in solution and recyclability was proved by IR. The addition

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Fig. 4 Representations of selected CoPOM WOCs. 1-POM-Co [Co4(H2O)2(PW9O34)2]10, 2-POM-Co [CoIIICoII(H2O)W11O39]7, 3-POM-Co and 4-POM-Co isomers of [{Co4(μ-OH) (H2O)3}60f(Si2W19O70)]11, 5-POM-Co {Co9(H2O)6(OH)3(HPO4)2(PW9O34)3}16 6-POM-Co {Co4(H2O)2(SiW9O34)2
nH2O}10, crystal obtained from another source65 7-POM-Co [CoMo6O24H6]3, and 8-POM-Co [Co2Mo10O38H4]6. Oxygen is in red, cobalt in blue, phosphorous in gray (figure 80) and polyhedra CoO6 (2-POM-Co), WO6 and MoO6 are dark blue, gray and light blue (7-POM-Co, 8-POM-Co), respectively. Si and P are drawn as translucent orange polyhedra. a [Ru(bpy)3]3+ was the SO. b Light driven WO (Na2S2O8, [Ru(bpy)3]2+). c TOF was calculated graphically. d NaOCl was the SO and TON were obtained by 4 cumulative additions of SO. Adapted from reference Lv H, Geletii YV, Zhao C, et al. Polyoxometalate water oxidation catalysts and the production of green fuel. Chem Soc Rev. 2012;41:7572 with permission from the Royal Society of Chemistry.

of bpy to the catalytic medium, chelating possible cobalt free species, decreased the O2 yield from 67% to 48%, indicating 1-POM-Co as the dominant catalyst. However, the evidences did not rule out the possibility of secondary catalytic species (1-POM-Co fragment or heterogeneous CoOx). The light-driven oxidation yielded a turnover of 224 (O2 yield, (100x [O2]/[SO]) ¼ 45%), with an initial quantum yield of 30% (O2 produced/absorbed photons, in %).66 Under these conditions, the activity was limited by the depletion of the sacrificial oxidant (persulfate). No evidence of particle formation was observed by DLS and HRMS data showed that the compound was stable (pH 6–10), in favor of a homogeneous

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catalytic process. By 17O NMR spectroscopy labeling studies, they suggested that the specific binding mode of the water ligand in the protected cavity of this 1-POM-Co can be key for the water oxidation.60b Several following up studies by the groups of Finke, Hill and Scandola showed the hydrolysis of the 1-POM-Co, illustrating the difficulty to identify the real nature of the active catalyst in solution.60c,h,k–m Finke and Hill groups found that hydrolysis of the 1-POM-Co was strongly dependent on the reaction conditions and thus it is very important to determine which catalyst is the legitimate under the conditions employed. A potential dual homogeneous-heterogeneous catalytic behavior of 1-POM-Co was also remarked, on the nanosecond laser flash photolysis carried out by Scandola and coworkers.1 Only freshly prepared solutions showed that the hole scavenging from RuðbpyÞ3 3+ in the 1-POM-Co. Co(NO3)2 was not able to reduce [Ru(bpy)3]3+ at the 0–100 ms time scale. The formation of the catalytic species was postulated to arise from the starting 1-POM-Co, but evolved during time.60m,67 Other polyoxometalate examples have also been reported as active compounds for the oxygen production. The Keggin-type POM K7[CoIIICoII(H2O)W11O39] (Fig. 4, 2-POM-Co) was found active for the light-driven (TONmax ¼ 360, TOFmax ¼ 0.5 s1) and thermal (TONmax ¼ 15, TOFmax ¼ 0.06 s1) water oxidation, with an O2 yield of 30% and 60%, respectively.60i No nanoparticles were detected (by DLS, CV, FT-IR, EDX, laser flash photolysis) and the catalyst was recyclable, showing similar activities as the freshly prepared sample. The electrochemical onset for the water oxidation was observed at 0.97 V, easily accessible for the hole-scavenging (1.12 V vs SHE was reported for [Ru(bpy)3]3+), favoring the homogeneity of the mechanism.60i [{Co4(μ-OH)(H2O)3} (Si2W19O70)]11 (Fig. 4, two isomers (3-POM-Co and 4-POM-Co) in 1:1 ratio)68 Under illuminated conditions ([Ru(bpy)3]2+ and S2 O8 2 , pH 9), 80 TON were reported, with an initial TOF of 0.1 s1 and an O2 yield around 24%. Another relevant WO catalyst was reported by Galan-Mascaro´s and coworkers, consisting in a triple Keggin-type POM containing a nonacobalt core ({Co9(H2O)6(OH)3(HPO4)2(PW9O34)3}16 (Co9POM, Fig. 4, 5-POM-Co).60e In this study, electrolysis of the Co9POM (in pH 7 1

RuðbpyÞ3 3+ is generated by quenching RuðbpyÞ3 2 + ∗ (excited state achieved by a 355 nm laser) with Na2S2O8 in a <10 ns timeframe and the reaction with the catalyst (the recovery of the RuðbpyÞ3 2+ , chromophore at 450nm) is monitored in the ms timescale 450 nm.

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NaPi buffer) at 1.41 V vs SHE resulted in the formation of an electrodeposited film, containing cobalt and phosphorous, which maintains its activity when transferred to a control buffered solution. Because heterogeneous catalysis were not completely excluded in the electrochemical studies, subsequently authors used the insoluble Cs+ salt of Co9POM as a doping agent into a carbon electrode, showing a very robust catalytic process under a wide range of pH.60j Control experiments using 125 μM of Co3O4 showed two to three times less activity than 2 μM of Co9POM, thus reinforcing the catalytic activity of the polyoxomethalate (Table 1). [Co4(H2O)2(SiW9O34)2]12 (Fig. 4, 6-POM-Co) was also tested as   2 + WOC under light-driven conditions RuðbpyÞ3 and S2 O8 2 . TON of 24 and TOF of 0.4 s1 (at 20 μM and 42 μM of catalyst, respectively) were reported.60d A family of Co-POM with molybdenum ([CoMo6O24H6]3 and [Co2Mo10O38H4]6) (Fig. 4, 7-POM-Co and 8-POM-Co) were also found photochemically active and TON of 107 and 154 were obtained after 30 min of irradiation.69 Under their experimental conditions, TOF for Table 1 Representative examples of catalytic water oxidation with cobalt-based POM’s. Experimental Evidence for homogeneity Ref. POM TOF (s2 1) TON conditions

1-POM-Co

5

+

PSRu 3 as oxidant, NaPi (pH 8)

UV–Vis and 31P NMR and IR proved stability

No nanoparticle 224 Light driven O2 yield ¼ 45% Θ ¼ 30% according to DLS

1-POM-Co 2-POM-Co

75

0.5

3-POM-Co/ 0.1

360 Light driven

80

Light driven at pH 9

60a

66

No nanoparticle 60i according to DLS, CV, FT-IR, EDX, laser flash photolysis 68

O2 yield 24%

4-POM-Co

Light driven

60d

6-POM-Co

0.4

24

7-POM-Co

0.11

107 Light driven

69

8-POM-Co

0.16

154 Light driven

69

Phosphate buffer (NaPi), sodium borate buffer (NaBi) [Ru(bpy)3]2+(PSRu2+), [Ru(bpy)3]3+(PSRu3+), Θ ¼ quantum yield, Light-driven condition: [Ru(bpy)3]2+, Na2S2O8 and irradiation.

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7-POM-Co and 8-POM-Co (0.11 and 0.16 s1) were found higher than those for Co4POM (77) (TOF ¼ 0.08 s1). Heptanuclear cobalt POMs Na12[{CoII7AsIII 6 O9(OH)6}-(A-α70 SiW9O34)2]
8H2O, [{(B-α-PW9O34)Co3(OH)(H2O)2(O3PC(O) (C3H6NH3)PO3)}2Co]14.71 and heptanuclear cluster 72 [CoII5 CoIII (mdea) (N ) (CH CN)(OH) (H O)
4ClO ] (H mdea ¼ N2 4 3 4 3 2 2 2 4 2 methyldiethanolamine) were also found active in WOC but only Na12[{CoII7AsIII 6 O9(OH)6}-(A-α-SiW9O34)2]
8H2O was claimed being resilient under electrocatalytic WO conditions as confirmed by ESI-MS. Under photocatalytic WO conditions ([POM] ¼ 20 μM, [[Ru(bpy)3] Cl2] ¼ 1 mM, [Na2S2O8] ¼ 5 mM and sodium borate buffer 80 mM; O2 yield 38%, TON 115, TOF 0.114 s1), no nanoparticles were detected after the catalytic test by DLS measurements. The catalyst was recycled using acetone as precipitation method and analyzed by FT-IR and X-ray photoelectron spectra (XPS) indicating that the structure is maintained intact during catalysis. 2.3.2 Cobalt complexes with planar macrocyclic ligands: Porphyrin, phthalocyanin, corrole and TAMLs ligands Since first examples of cobalt phthalocyanines or porphyrines as water oxidation catalysts, reported by Elizarova et al.73 detailed studies have showed its potential. For instance, cobalt phthalocyanine (Co-Pc) supported on ITO showed very efficient water splitting at an applied bias potential of 0.5 V vs SHE (η ¼ 0.08 V, 2000 TON in 1 h).74 In 2013 Takanabe and Rodionov groups expanded the water electro-oxidation to perfluorinated cobalt phthalocyanine immobilized in fluorine-doped tin oxide (FTO).75 The onset potential was found at 1.7 V vs. RHE. Homogeneity was based on spectroscopic (UV–Vis, Raman), electrochemical (long-term controlledpotential electrolysis, >8 h, TON >1  105 and TOF about 3.6 s1), and inhibition experiments (EDTA). Cobalt porphyrins such as CoII(TDMImp(OH2)), Co-TM4PyP, II Co (TPhP) and CoII(TBrPhP) (Scheme 10) among others also catalyze the oxidation of water.76,77 For the later, the activity was enhanced at pH about 9.0,77b with Faraday yields close to 100% for 2 h of electrolysis at 1.3 V (0.5 M Bi, pH 9.2). The accepted mechanism involves the accumulation of two positive charges on the CoIII (resting state), pointing toward a cation radical CoIV% as the active species, suggesting + P-CoIV-O as the catalytically competent intermediate. Sakai and coworkers (Scheme 10) studied the light-driven WO with Co-porphyrin (CodPo) systems ([Ru(bpy)3]3+ as PS, S2 O8 2 as SO),77a without any evidences of NP formation (DLS).

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Scheme 10 Corrole and porphyrin cobalt complexes for water oxidation. TOF reported here were all under electrochemical conditions.

Second order on catalyst and DFT calculations supported a direct coupling mechanism via CoIV oxyl species. Nevertheless, the oxidation state of the cobalt in the rate determining step remained unclear.77c Sun and co-workers re-investigated the cobalt porphyrin for electrocatalytic water oxidation on FTO surface and reported that the cobalt tetraphenylporphyrin promptly decomposed to form CoOx as active species (borate buffer at pH 9.2). This finding is supported by electrochemistry and synchrotron-based photoelectron spectroscopy (1000 eV XPS, SOXPES). Traditional techniques such as UV–Vis, scanning electron microscopy (SEM), energydispersive X-ray spectroscopy (EDS) are not sensitive enough and gives a false negative. This study shows how difficult is to characterize ultratrace metal oxides films that are the true catalysts.78 Nocera’s group introduced the cobalt “xanthene-hangman” complex derived of trianionic corrole type ligands, [CoIII(bpfxc)] (Scheme 10). The

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complex immobilized with Nafion, is active at 1.6 V vs SHE (pH 7).79 The authors argued that the hanging moiety (the acid-base functionality placed over the face of the corrole) assists in the intramolecular proton transfers by pre-organizing the water molecule within the system,80 while the simple [CoIII(tpfc)(py)2] corrole, studied by Du, Lai, Cao, has lower WO rate (0.2 s1 at 1.6 V vs SHE pH 7, NaPi).81 UV–Vis, ESI-MS, SEM, and EDX analysis supported the molecularity of the catalytic system derived from the initial complex (precatalyst) upon dissociation of the pyridine group from the cobalt.82 Immobilization of pyrene-modified [CoIII(tpfc)(py)2] on multiwall carbon nanotubes (MWCN) had an increase in the current density with respect to the unmodified one (11.9 versus 7.5 mA cm2 at 1.55 V, 0.1 M pH 7.0 phosphate buffer).83 Biuret-modified tetraamidomacrocycle (formally a tetraanionc ligand) cobalt complex [CoIII-bTAML] has also been found competent to catalyze the electrochemical water oxidation.44b The irreversible peak at 1.5 V was ascribed to the catalytic water oxidation and it was found to be pH independent, but with a KIE (kobsH2O/kobsD2O) of 8.6. Therefore, it was associated to a CoIV(O)/CoV(O) redox process with operation of the atom proton transfer (APT) mechanism during water oxidation. 2.3.3 Coordination compounds based on poly-amine/pyridyl ligands The study reported by Berlinguette and coworkers on [CoII(PY5)(OH2)] (ClO4)2 (Scheme 11) showed the complications on studying polypyridyl cobalt complexes.84 [CoII(PY5)(OH2)](ClO4)2 shows an irreversible oxidation at 1.4 V (at pH 9.2) assigned to the formation of the catalytically active [CoIVOH]3+ species with a WO rate (kcat) of 79 s1. Kinetics showed first order on cobalt and on [OH], suggesting that the rate determining step (RDS) is the electrophilic attack of a high valent [CoIV-OH]3+ to [OH], with an observed KIE of 4.7.85 The pH stability of the complex ranged from 7.6 to 10.3. Under these electrochemical conditions film deposition was not observed.84 In addition the pH-dependent differences between the complex and Co2+(aq) suggested that the molecular compound was responsible for the main part of the observed current. However, diluted Co2+ (aq) showed a similar behavior and therefore metal oxide species could also be responsible in part for the catalytic activity. Later on, electrocatalytic studies on a similar complex ([CoII(PY5OH)(Cl)](BF4)) revealed equivalent features. Under photochemical conditions a TON of 51 was found ([Ru(bpy)3]2+/Na2S2O8 at pH 8, borate buffer). Nanoparticles as WOC were discarded in the 7.5–10 pH range based on DLS experiments, but at pH > 10 nanoparticles formation was evidenced.61a

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Scheme 11 Polyamino cobalt complexes.

Lau and co-workers reported two cobalt complexes based ligands formed by the fusion of pyridines at 2 position, the mononuclear [CoII(qpy)(OH2)2+] (Scheme 11, qpy ¼ 2,20 :60 ,200 :600 ,2000 -quaterpyridine)86 and dinuclear [(Co)2(spy)2(ClO4)4] (Scheme 11), spy¼ 2,20 :60 ,200 :6000 ,2000 :6000 ,20000 :60000 ,200000 sexipyridine).61b The versatile [CoII(qpy)(OH2)2+] was the first dual WO and WR catalyst, which under photochemical conditions led to 335 TON O2 after 1.5 h irradiation at 457 nm ([Ru(bpy)3]2+ and Na2S2O8 at pH 8, borate buffer).86 In the case of the helicoidal cobalt dimer [(Co)2(spy)2(ClO4)4] the O2 production increased with the catalyst concentration up to 2 μM leading a maximum TOM of 442 after 3 h of irradiation and a maximum TOF of 1.9 s1. In both cases DLS analysis did not support the presence of NP. The importance of the ligand to stabilize the metal complex avoiding the metal leaching is illustrated by the report of Nam, Fukuzumi and co-workers on cobalt complexes based of polyamine ligands (Scheme 11).87 In this regard, hydrolytically instable cobalt complexes can serve as precatalysts

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for electro- and photo-water oxidation.88 Under photocatalytic conditions at pH 7–10, complexes [CoII(Me6tren)(OH2)]2+, [CoIII(Cp*)(bpy)(OH2)]2+, [CoII(12-TMC)]2+, and [CoII(13-TMC)]2+ (Me6tren ¼ tris(N,N0 5 dimethylaminoethyl)amine, Cp* ¼ η -pentamethylcyclopentadientyl, 12TMC ¼ 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane, 13-TMC¼ 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclotriecane) form a heterogeneous material after 3 min of irradiation. DLS and TEM studies showed the formation of NP sizes depending on the precatalyst used (20–200 nm). Likewise, [Co(cyclam)(ClO4)] ClO4 (cyclam ¼ 1,4,8,11-tetraazacyclotetradecane) served as precursor for fabrication of modified electrode with a water oxidation activity of 6.5 mA cm2 at an overpotential of 580 mV at pH 12 with 98% Faraday yield.62 Closely related systems are the cobalt complexes based on aminopyridine ligands such as tris(2-pyridylmethyl)amine (TPA) ligand.63a–c In this case, the catalytic activity is also related with the formation of cobalt oxide nanoparticles.63d Electrodeposition, titration experiments (bpy), EDX and XPS showed that cobalt oxides deposited over the electrode were the responsible for WO. An important outcome is that TEM was sensitive enough to show the presence of nanoparticles but DLS was not. Extreme care should be taken to ensure that the catalytic species are well-define complexes.61c 2.3.4 Salen type cobalt complexes First report by F. Scandola, A. Sartorel, and co-workers reported on the Salophen-based cobalt complexes (Scheme 12, CoSlp, Slp ¼ N,N0 -bis (salicylaldehyde)-1,2-phenylenediamine). At pH 7.1 (NaPi buffer) showed

Scheme 12 Selected Schiff base type cobalt complexes for the water oxidation.

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light-driven water oxidation activity (17 TON, [Ru(bpy)3]2+/Na2S2O8),64a with initial lag-time. Authors supports the homogeneous catalysis based on the low overpotential observed (η ¼ 0.3 V, vs η ¼ 0.5–0.6 V for water oxidation with other Co2+ based catalysts including cobalt oxides) and the catalyst stability confirmed by UV–Vis, DLS and EPR. Laser flash photolysis pointed toward CoIV]O as the active species since a double electron transfer (ET) per CoII was detected. In addition, the first ET rate was remarkably higher than for a Co4O4 core.60f The same authors also studied the Co(II)-Ru(II) dyad covalently assembled (Scheme 12),64b which showed a dissociation equilibria generating free CoII aqua-ions. The simpler Co(Sl) was also effective for light-driven WOC, (854 TON and TOF of 6.4 s1, quantum yield ¼ 38.6%, borate buffer, pH 9). Under chemical conditions, using [Ru(bpy)3]3+ as chemical oxidant, a TON of 194 and TOF of 2.0 s1 were reported. After the illumination, different-sized NP were detected, together with the presence of CO2 in the gas phase. A precipitate was isolated after few minutes of irradiation and XPS revealed the absence of Co(II) on its surface, differing from the results reported by Fukuzumi et al. (vida supra). The isolated solid (probably Co(III) inorganic species) remained catalytically active for the light-driven WO (lower than the freshly compound), indicating that Co(Sl) acts as a precatalyst.89 The group of Verpoort depicted a different light-driven water oxidation behavior of salophen complexes depending of the pH.90 Under basic conditions cobalt salophen complexes acts as precatalysts decomposing to form cobalt hydroxide nanoparticles as the true catalyst. In contrast, at pH 7 no evidences of formation of nanoparticles were found. Ones more this report illustrate the delicate nature of the catalysts and the narrow window for investigating the mechanism of water oxidation. In this regard, cobalt salen type cobalt complexes have been explored as convenient precursors of cobalt films over electrodes.64c,90,91 2.3.5 Cobalt oxide clusters Initially Mn cluster have served as synthetic models for OEC in PSII, but more attention revived Co4O4 cubanes since Nocera and coworkers reported on the presence of Co4O4 cubane-like motives in the active CoOx and its capacity of self-assembling and self-healing during electrochemical water oxidation by Co2+ ions in NaPi buffered water.92 The Co4O4 cubical

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motif has served as a template to develop molecular WOCs providing a rational design of well-defined M4O4 clusters to fine-tuning of their reactivity, toward gaining insight toward both the mechanism and the fundaments as well as their connection with the oxygen evolving complex (OEC).60f,78,93 First informative reports on the activity of molecular Co4O4 core were provided by the groups of Dismukes and Scandola, Campagna and Bonchio,60f,93a A CoIII 4 O4(CH3CO2)4(py)4 (Scheme 13) was found photochemically active in the presence of [Ru(bpy)3]2+ and S2 O8 2 with TON of 40 and a TOFmax of 0.02 s1 after 1 h of irradiation.93a Using a Co2+ concentration equal to the photodecomposition observed for Co4O4 no O2 evolution was observed. Moreover, it was found different lag times and kinetic profiles for Co4O4 and Co2+, supporting the presence of different active species,60f,93b Additionally, Sun and co-workers studied the reactivity of the CoIII 4 O4 ðCH3 CO2 Þ4 ðpX PyÞ4 ðX ¼ Me, CF3 , CNÞ clusters immobilized on a Nafion film-coated fluorine-doped tin oxide (FTO) electrode and a α-Fe2O3 photoanode as surface catalysts for WO.93s In both cases, with the FTO/Nafion/Co4O4 electrode (230 μA/cm2 for 10 h in neutral phosphate buffer at 1.2 V applied vs Ag/AgCl) and with the photoanode α-Fe2O3/Nafion/Co4O4 under visible-light (200 μA/cm2 at 0.5 V applied vs Ag/AgCl) the water oxidation capacity is consistent with previous reports. Moreover, all the experiments suggested that the system is maintained

Scheme 13 Selected multinuclear cobalt complexes for the water oxidation.

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stable for long operation times, and that the catalytic activity depends on the ligands employed and increase in the following order: Me < CF3 < CN. Authors concluded that the supported CoIII 4 O4 ðCH3 CO2 Þ4 ðpX PyÞ4 ðX ¼ Me, CF3 , CNÞ clusters where maintained as molecular catalysts for the studied time scale.93s Same authors report on the use of supramolecular assemblies to improve the photocatalytic performance in a factor of five respect the multicomponent system.93a,b Nocera and co-workers found that the activity reported previously III Co4 O4(Ac)4(p-XPy)4 (X ¼ H, CO2Me) emanate from a Co(II) impurity.93t p-X The catalytic activity of fleshly prepared and purified CoIII Py)4 4 O4(Ac)4( 93t samples was an order of magnitude lower. Furthermore Sartorel, Natali p-X and co-workers reported that the CoIII Py)4 pristine sample 4 O4(Ac)4( evolves in aqueous media increasing the WO activity, most probably due to ligand displacement.93q Nonetheless later on, Dismukes, Tilley and Nocera groups proposed the CoIII 4 O4 complexes as molecular models to understand the OdO bond formation capacity of CoOx.93q,t,v,z,94 On the other hand, Maschmeyer and co-workers studied the evolution of the cubane cluster indicating that the aged samples formed CoOx similar to that of aged “CoPi.”93l,y

2.4 Nickel-based water oxidation catalysts Nickel oxides are emerging as excellent heterogeneous catalysts for electrocatalytic water oxidation, with the potential of being integrated into useful devices.95 Water oxidation with nickel complexes may provide details at a molecular scale of the mechanisms of these heterogeneous reactions, and has fueled the interest for these compounds. However, the number of examples is still quite limited. Since NiOx are very effective water oxidation catalysts, special care has been devoted in these examples to demonstrate that the observed activity originates from molecular catalysts and not by deposition of NiOx on the electrodes. 2.4.1 Coordination compounds containing N-based ligands for electrocatalytic water oxidation A pioneer example was the macrocyclic complex [Ni(meso-L)]2+ (Scheme 14).96 The complex contains a macrocyclic ligand (meso-L) that in the solid state enforces a square planar nickel center. In water solution, two water molecules bind in the axial positions, forming the octahedrally

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Scheme 14 Top: Nickel complexes used as electrocatalysts for water oxidation. Bottom: Mechanism of electrocatalytic water oxidation proposed for [Ni(meso-L) (H2O)2](ClO4)2.

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coordinated complex, [Ni(meso-L)(H2O)2]2+. In phosphate, carbonate or acetate buffer, at neutral pH, the complex catalyzes oxidation of water at a relatively low overpotential (Ep,a ¼ 1.41 V vs NHE). The cyclic voltammetry of the complex exhibits a first irreversible wave at Ep,a ¼0.87 V vs NHE corresponding to PCET oxidation from [NiII(mesoL)(H2O)2]2+ to [NiIII(meso-L)(OH)(H2O)]2+ and a second electrocatalytic wave at Ep,a ¼ 1.41 V vs NHE that is proposed to arise from PCET from [NiIII(meso-L)(OH)(H2O)]2+ to [NiIV(meso-L)(O)(H2O)]2+ or [NiIII(meso-L)(O
)(H2O)]2+, which is then responsible for the water oxidation reaction. The proposal that the two redox events correspond to PCET processes arises from the fact that they are pH dependent, changing 59 mM per pH unit. Interestingly, the onset potential (Ep,o) for water oxidation emerges at ca. 0.99 V vs NHE, with an overpotential of only ca. 170 mV. The mechanism was explored with DFT methods. The study shows that the complex isomerizes from a non-reactive trans-(H2O)Ni(O) to a reactive cis-Ni(OH)2 form, and then OdO bond formation occurs via a HO-OH coupling, resulting in a [NiII(meso-L)(H2O2)]2+ intermediate that is further oxidized to produce O2. Building on the proposal that two cis-labile sites are a crucial element in the OdO bond formation step, electrocatalytic water oxidation was also explored with the nickel complexes [Ni(men)(H2O)2]2+ (men¼ N, N 0 -dimethyl-N,N 0 -bis(2-pyridylmethyl)ethane-1,2-diamine and [Ni(mcp) (H2O)2]2+ based on tetradentate aminopyridine ligands.97 Both complexes exhibit electrocatalytic behavior at a low overpotential, very much resembling [Ni(meso-L)(H2O)2](ClO4)2, but they are less active. Two more recent reports question the necessity of two cis-sites for OdO bond formation. The nickel complex [Ni(Py5)(Cl)]+ (Scheme 14) contains a pentapyridine ligand, but it shows electrocatalytic behavior98 in phosphate buffer. Water oxidation is proposed to occur via a nucleophilic attack of the water molecule on a high valent NiV-oxo species, the OdO bond formation being assisted by proton transfer to HPO4 2 . The reaction rate is accelerated by HPO4 2 , which acts as proton acceptor, reaching high reaction rates (1820 s1). The Ni-porphyrin complex [Ni(PorphPyMe)]4+.99 is also an electrocatalyst that effectively operates in a pH range of 2.0–8.0, exhibiting catalytic behavior at low overpotentials (onset at 1.0 V at neutral pH). The mechanism was investigated by CV, by determining KIE’s and by

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DFT. The crucial OdO bond formation is proposed to occur via reaction of a Ni(III)–O
species that undergoes nucleophilic attack by a water molecule, coupled with a hydrogen atom transfer from the water molecule to a second water molecule or a base (acetate of phosphate).99

2.5 Copper-based water oxidation catalysts A number of copper complexes have lately emerged as effective water oxidation catalysts. The typology of ligands and the nuclearity of the complexes is quite diverse and includes aminopyridine,100 polypyridines,101 peptides102 and amides103 in one side, and monomers, dimers, POM’s104 and even tetrametallic cubane clusters105 in the other. In general, the complexes only operate as electrocatalysts and require basic conditions. The nature of the buffer is usually very important for activity, and in some cases there is mounting evidence that the buffer anions may have a role as ligands, generating active copper catalysts. The lability of the CuII ion makes the corresponding complexes very labile, often dynamic equilibria takes place in solution and the paramagnetism of the species makes spectroscopic characterization in aqueous solution difficult. In addition, decomposition of the copper complexes generates copper oxide/hydroxide species that are catalytically competent,106 and therefore, elucidation of the real catalytic species may be difficult. Pioneer examples of copper-catalyzed water oxidation35,73 employed CuCl2, Cu(bpy)2Cl2 and Cu(bpy)3Cl2 as catalysts and RuðbpyÞ3 3+ as oxidant. Since then, reports have been based on electrocatalytic oxidation. Mayer and coworkers found that [(bpy)Cu(μ-OH)]22+ is a catalyst for electrochemical water oxidation under basic conditions. Spectroscopic (EPR) and electrochemical analyses (CV) led to the conclusion that the complex mostly dissociates into [(bpy)Cu(OH)2] under catalytically relevant conditions, which is then the real catalyst. Deposition of the catalyst or copper-oxides in the electrode were discarded, but rapid decomposition of the catalyst was noticed, and modest TON (30) could only be obtained. A second generation of improved bpy based catalysts was created by introducing hydroxyl groups in positions 6,60 of the bipyridyl ligand. The modification of the ligand intended to facilitate PCET during catalysis (the bipy-OH ligand can acts as a red-ox non-innocent, H-atom donor),

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and reduce the oxidation potentials of the copper center (the hydroxyl ligand is electron donor), facilitating access to the reactive oxidized species.102b Effectively, the overpotential for electrocatalytic water oxidation is reduced by 200 mV compared to the unsubstituted system, and is substantially smaller than the overpotential exhibited by the parent 4,40 -hydroxy-bipyridine system. The 6-hydroxyl substituted bpy system is also more robust; >90% of the ligand can be recovered after acidification, and a TON of 400 can be reached. An apparent TOF of 0.4 s1 is calculated. The triglycylglycine CuII complex [(TGG)CuII-OH2]2 also shows electrocatalytic water oxidation behavior. The onset for water oxidation is at 1.1 V (vs SHE in NaPi, pH 11), and the catalyst is stable for several hours, until pH decreases as a result of water oxidation.102a Kinetics agree with a single site process, and a mechanism resembling those operating in Ru based systems has being proposed. A formal CuIV]O active species is proposed to be involved in a rate-determining OdO bond formation process, generating a CuII(OOH) or CuII(H2O2) transient, that is further oxidized to form [CuIII-O2
] which evolves O2 and closes the catalytic cycle. A single example of a copper catalyst that operates under neutral conditions has been described.100 A dicopper complex bearing a dinucleating naphthyridine based ligand shows electrocatalytic water oxidation at neutral pH. CV’s show an electrocatalytic wave at 1.6 V vs NHE at pH 7. The onset of catalytic activity is pH dependent, 50 mV per pH unit, suggesting a PCET process. Bulk electrolysis shows that catalytic activity occurs with high Faradaic efficiency (98%). The reaction mechanism for this complex has been explored by DFT methods, suggesting that one of the pyridine arms of the ligand assist OdO bond formation by initial H-bonding with a hydroxyl ligand, and subsequently directing the oxygen atom toward a reactive oxo-bridge. Electrocatalytic water oxidation with a family of monocopper complexes bearing tetra-anionic tetradentate amidate ligands related to TAML’s has been described (Scheme 15).103,107 The series of mononuclear square planar Cu(II) complexes undergo a first metal based CuIII/CuII oxidation and a second pH dependent ligand based single electron oxidation that is associated with an electrocatalytic wave. Interestingly, the overpotential for water oxidation is dependent on the electron donating nature of the ligand, and it decreases as the ligand becomes more electron-donating. On the basis of DFT calculations, a singular mechanistic analysis has been proposed. The active species is proposed to be a [L%CuIII-OH] species,

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Water oxidation catalyzed by first row transition metal complexes

A

B

C

D

Scheme 15 Top: Schematic representation of the Cu-WOCs, [(xbpy)Cu(μ-OH)]22+ (left, A) and [(TGG)CuII-OH2]2 (B, right) and the mechanistic proposal for the electrochemical acid-base water oxidation.102b Bottom: Dicopper naphthyridine complex (left, C)100 and copper complexes with tetraanionic amidate ligands (right, D).103

which undergoes attack by a hydroxyl ligand, forming a (HO-OH)%– radical anion fragment with a partial OdO bond (computed OdO distance is ˚ ). The (HO-OH)%– is hydrogen bonded to the [(L)CuIII] complex. 2.3 A A second electron transfer leads to a [(L)CuII(HOOH)] species, where the HOOH is hydrogen bound to the CuII complex. Further 2e oxidation evolves O2, two protons, and closes the catalytic cycle.

3. Conclusion and outlook Water oxidation with earth abundant metals has been developed quite extensively during the last decade. Complexes with virtually any first-row transition metal have shown catalytic competence. Clarification of the reaction mechanisms is still quite premature. The lability of the metals, their paramagnetic nature and the high reactivity of the oxidizing species involved in this chemistry makes the study quite challenging. On top of that, a number of examples exist where the initial complexes are transformed into active metal-oxides. This is not unusual for the traditional metals employed in water oxidations such as ruthenium or iridium, but the lability of first

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row transition metals makes this problem more acute. Not unexpectedly, the harsh reaction conditions imposed by chemical oxidants limits the number of complexes very much that could operate with these oxidants. However, the increasing use of electrocatalytic methods have opened the reaction very much to a number of complexes. A side problem is that identification of the species in electrocatalysis is quite more difficult that under chemical oxidants conditions. More interesting is the recent design of polymetallic systems that can accumulate charge without reaching high oxidation states, resembling the principles and reaction mechanisms of PhII. The activity and mechanisms of action of these systems promise to deliver important lessons toward the development of catalysts for multielectronic reactions.

Acknowledgments We acknowledge the Spanish Ministry of Science CTQ2015-70795-P (M.C.), CTQ201680038-R (J.L.-F.). M.C. thanks Generalitat de Catalunya for an ICREA Academia Award and 2017SGR 264. J.L.-F. thanks the European Commission for ERC-CG-2014-648,304 project. Financial support from the ICIQ Foundation and CELLEX Foundation through the CELLEX-ICIQ Starting Career Program is gratefully acknowledged.

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2H2O. Polyhedron. 1998;17:1343. Smith PF, Hunt L, Laursen AB, et al. Water oxidation by the [Co4O4(OAc)4(py)4]+ cubium is initiated by OH– addition. J Am Chem Soc. 2015;137:15460. (a) Gong M, Li YG, Wang HL, et al. An advanced Ni–Fe layered double hydroxide electrocatalyst for water oxidation. J Am Chem Soc. 2013;135:8452. (b) Subbaraman R, Tripkovic D, Chang KC, et al. Trends in activity for the water electrolyser reactions on 3d M(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts. Nat Mater. 2012;11:550. Zhang M, Zhang MT, Hou C, Ke ZF, Lu TB. Homogeneous electrocatalytic water oxidation at neutral pH by a robust macrocyclic nickel(II) complex. Angew Chem Int Ed Engl. 2014;53:13042. (a) Wang JW, Zhang XQ, Huang HH, Lu TB. A nickel(II) complex as a homogeneous electrocatalyst for water oxidation at neutral pH: dual role of HPO42 in catalysis. ChemCatChem. 2016;8:3287. (b) Luo GY, Huang HH, Wang JW, Lu TB. Further investigation of a nickel-based homogeneous water oxidation catalyst with two cis labile sites. ChemSusChem. 2016;9:485. Wang L, Duan LL, Ambre RB, et al. A nickel (II) PY5 complex as an electrocatalyst for water oxidation. J Catal. 2016;335:72.

ARTICLE IN PRESS 52

Julio Lloret-Fillol and Miquel Costas

99. Han YZ, Wu YZ, Lai WZ, Cao R. Electrocatalytic water oxidation by a water-soluble nickel porphyrin complex at neutral pH with low overpotential. Inorg Chem. 2015;54:5604. 100. Su X-J, Gao M, Jiao L, et al. Electrocatalytic water oxidation by a dinuclear copper complex in a neutral aqueous solution. Angew Chem Int Ed. 2015;54:4909. 101. Barnett SM, Goldberg KI, Mayer JM. A soluble copper-bipyridine water-oxidation electrocatalyst. Nat Chem. 2012;4:498. 102. (a) Zhang M-T, Chen Z, Kang P, Meyer TJ. Electrocatalytic water oxidation with a copper(II) polypeptide complex. J Am Chem Soc. 2013;135:2048. (b) Zhang T, Wang C, Liu S, Wang J-L, Lin W. A biomimetic copper water oxidation catalyst with low overpotential. J Am Chem Soc. 2014;136:273. (c) Gerlach DL, Bhagan S, Cruce AA, et al. Studies of the pathways open to copper water oxidation catalysts containing proximal hydroxy groups during basic electrocatalysis. Inorg Chem. 2014;53:12689. 103. Garrido-Barros P, Funes-Ardoiz I, Drouet S, Benet-Buchholz J, Maseras F, Llobet A. Redox non-innocent ligand controls water oxidation overpotential in a new family of mononuclear Cu-based efficient catalysts. J Am Chem Soc. 2015;137:6758. 104. Yu L, Lin J, Zheng M, Chen M, Ding Y. Homogeneous electrocatalytic water oxidation at neutral pH by a robust trinuclear copper(II)-substituted polyoxometalate. Chem Commun. 2018;54:354. 105. Jiang X, Li J, Yang B, et al. A bio-inspired Cu4 O4 cubane: effective molecular catalysts for electrocatalytic water oxidation in aqueous solution. Angew Chem Int Ed Engl. 2018;57:7850. 106. (a) Najafpour MM, Ebrahimi F, Safdari R, Ghobadi MZ, Tavahodi M, Rafighi P. New findings and the current controversies for water oxidation by a copper(II)-azo complex: homogeneous or heterogeneous? Dalton Trans. 2015;44:15435. (b) Lu C, Du J, Su X-J, et al. Cu(II) aliphatic diamine complexes for both heterogeneous and homogeneous water oxidation catalysis in basic and neutral solutions. ACS Catal. 2015;6:77. 107. Funes-Ardoiz I, Garrido-Barros P, Llobet A, Maseras F. Single electron transfer steps in water oxidation catalysis. Redefining the mechanistic scenario. ACS Catal. 2017;7:1712.

Further reading 108. Karlsson EA, Lee B-L, A˚kermark T, et al. Photosensitized water oxidation by use of a bioinspired manganese catalyst. Angew Chem Int Ed Engl. 2011;50:11715.