CHEMSUSCHEM MINIREVIEWS DOI: 10.1002/cssc.201402322

Progress in Base-Metal Water Oxidation Catalysis Alexander Rene Parent*[a] and Ken Sakai*[a, b, c] This minireview provides a brief overview of the progress that has been made in developing homogeneous water oxidation catalysts based on base metals (manganese, iron, cobalt, nickel, and copper) from the 1990s to mid-2014. The impact of each contribution is analyzed, and opportunities for further improvement are noted. In addition, the relative stabilities of the

base-metal catalysts that have been reported are compared to illustrate the importance of developing more robust catalytic systems by using these metals. This manuscript is intended to provide a firm foundation for researchers entering the field of water oxidation based on base metals and a useful reference for those currently involved in the field.

1. Introduction Over the past decade, increased awareness of the impact of rising carbon dioxide emissions on the earth’s climate has led to increased research into alternatives to fossil fuel based energy.[1–5] Of the multiple potential renewable energy sources, only solar energy has the potential to meet global energy demands.[1, 2] To enable solar-energy harvesting on a global scale, the intermittency of the energy supplied by the sun must be addressed.[1, 2] The most promising method for storing solar energy is in the form of chemical bonds through the generation of chemical fuels.[2, 5, 6] During the conversion of solar energy into a chemical fuel, a feedstock, such as carbon dioxide or proton, is reduced into a fuel, such as methane or H2 gas. To supply the electrons necessary for the reduction of the feedstock into a fuel, a concurrent oxidation reaction is required.[2] Only water is sufficiently abundant to serve as a source of electrons for the production of solar fuels. Indeed, nature uses water as its electron source to drive natural photosynthesis.[5] Highly active water oxidation catalysts (WOCs) are required to promote the efficient extraction of electrons from water and their combination with feedstocks to form fuels (Scheme 1). Recently, a number of stable and efficient WOCs based on second- and third-row transition metals have been reported.[7–12] On the other hand, substantially less progress has been made when using first-row transition metals. Critically, to be deployed on a global scale, a WOC must be based on materials [a] Dr. A. R. Parent, Prof. K. Sakai International Institute for Carbon-Neutral Energy Research (WPI-I2CNER) Kyushu University, Motooka 744, Nishi-ku Fukuoka 819-0395 (Japan) E-mail: [email protected] [email protected] [b] Prof. K. Sakai Department of Chemistry, Faculty of Sciences Kyushu University, 6-10-1 Hakozaki, Higashi-ku Fukuoka 812-8581 (Japan) [c] Prof. K. Sakai Center for Molecular Systems (CMS), Kyushu University Motooka 744, Nishi-ku, Fukuoka 819-0395 (Japan)

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Scheme 1. Schematic illustration of a solar fuel-generation cell.

sufficiently abundant to scale to global energy demands; this provides severe constraints on the use of second- and thirdrow transition metals in a practical device.[1] A number of general reviews on WOCs have been published recently.[4, 5, 13–16] In addition, several specialized reviews on the mechanisms of water oxidation (WO),[17, 18] and a review of attempts to mimic the structure of the natural oxygen-evolving complex (OEC),[19] have been recently published. Herein, we focus on attempts to overcome the low abundance of secondand third-row transition metals through the design of homogeneous catalysts based on first-row transition metals. This field has recently become the subject of intense research as a method of decreasing the cost of artificial photosynthetic cells. Analysis of the advantages and limitations of these systems is also presented, along with potential future directions for research in this area. 1.1. Methods for studying WOCs There are two general methods for studying WOCs. The first is direct electrochemical studies on the WOC in question to generate species capable of driving WO. Electrochemical studies have a major advantage in that the system studied closely resembles the conditions that would be found in a solar fuelChemSusChem 2014, 7, 2070 – 2080


CHEMSUSCHEM MINIREVIEWS generation cell (Scheme 1). Electrochemical studies of WOCs are often highly challenging and require significant expertise for proper interpretation of the results. For this reason, sacrificial oxidants are commonly used to determine kinetic parameters during WO.[20] Moreover, the introduction of sacrificial oxidants changes the reaction solution from what would be used in an actual device; this must be kept in mind when interpreting the results of such studies.

Scheme 2. Water nucleophilic attack (WNA) and oxyl radical coupling (ORC) pathways for O O bond formation.

1.2. Mechanisms of catalytic WO Two basic mechanisms have been proposed for WO to O2 (Scheme 2).[14, 17, 18] In the first mechanism, called water nucleophilic attack (WNA), a highly oxidized oxo or oxyl is attacked by solvent water, resulting in a two-electron oxidation process to form a hydroperoxo intermediate, which can then be further oxidized to release O2. In a variation of this mechanism, under alkaline conditions, hydroxide may serve as the nucleophile that attacks the oxo or oxyl. The second mechanism by which WO can be catalyzed is ORC.[14, 17, 18] In ORC, two one- or two-electron-oxidized oxylcontaining species couple to form an O O bond in either Alexander Rene Parent received his B.A. in chemistry from Clark University in 2008 after studying molecular magnetism in the lab of Prof. Mark Turnbull. He received his Ph.D. from Yale University for working on water oxidation catalysis under the supervision of Profs. Gary W. Brudvig and Robert H. Crabtree. He is currently working as a Postdoctoral Research Associate at the International Institute for Carbon Neutral Energy Research (WPI-I2CNER) at Kyushu University. His research interests include small-molecule activation, catalysis, reaction mechanisms, and coordination chemistry. Ken Sakai received his PhD in 1993 from Waseda University. From 1991 to 1999, he was an Assistant Professor at Seikei University. In 1999, he became a lecturer at the Tokyo University of Science, where he became an Associate Professor in 2003 before becoming a Full Professor at Kyushu University (current position) in 2004. He started to hold his second professor position at the Center of Molecular Systems of Kyushu University in 2011. He also became a principal investigator of the International Institute for Carbon-Neutral Energy Research, Kyushu University, in 2012, where his studies aiming at the development of artificial photosynthetic devices are strongly promoted by the worldwide demand to establish a society running with clean/recyclable energy.

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a two- or four-electron process. Commonly, ORC occurs between identical oxyl species (symmetric ORC), which may be either terminal or bridging.[7, 21] Coupling between two distinct oxyl species (asymmetric ORC) has also been reported. Specifically, when using cerium(IV) ammonium nitrate (CAN) to drive WO with ruthenium-based WOCs, the cerium(IV) center can impart radical character to the oxygen atoms of bound hydroxide or nitrate, allowing them to participate in ORC.[22–25]

2. Manganese Apart from a preliminary report on WO catalyzed by a vanadium-based polyoxometalate (POM),[26] manganese is the lightest first-row transition metal to have been successfully used to synthesize a WOC, and has the distinction of being the metal chosen by nature to enable natural photosynthesis.[27] Much early research into WO focused on developing manganese complexes capable of mimicking the photosynthetic activity of plants; the first functional systems were developed by Naruta et al. and Brudvig and co-workers in the mid-1990s.[28, 29] 2.1. Manganese terpy dimer One of the first manganese complexes shown to function as a WOC when driven chemically, and indeed one of the first artificial WOCs, is the manganese terpyridyl (terpy) dimer (Figure 1).[29] First characterized by using both NaOCl and KHSO5 (trade name Oxone),[30] it was later found that hypochlorite underwent rapid oxygen exchange with water, and thus, subsequent studies focused primarily on the reaction of the terpy dimer with Oxone.[31] Under low Oxone concentrations, 18O-labeling studies showed that the produced O2 resulted from WNA on a MnIV oxyl formed by O-atom transfer from Oxone to the MnIII center. At higher Oxone concentrations, electrophilic attack by Oxone on the MnIV oxyl begins to compete with nucleophilic attack by water (Scheme 3).[32] Nevertheless, only Oxone is able to drive WO catalytically with the terpy dimer. If the one-electron oxidant CAN is introduced as a terminal oxidant, only substoichiometric amounts of oxygen are generated.[33] This lack of activity is attributed to the rapid degradation of the complex into free manganese(II) at the low pH required by CAN.[34] This significant drawback was addressed by Yagi and Narita by intercalating the terpy dimer between the anionic layers of montmorillonite MK10 clay, which allowed WO to be driven catalytically with the one-electron oxidant CAN,[35] and, more ChemSusChem 2014, 7, 2070 – 2080



it was found to exhibit WO activity when driven either chemically or electrochemically.[41] DFT calculations showed that, upon chemical or electrochemical oxidation, the tetramer split into two MnIV,IV dimers, one of which contained a terminal oxyl. This oxyl then participated in WNA to form the O O bond in the same manner as the terpy dimer (Scheme 2).[41] Incorporation of the terpy dimer into a metal–organic framework (MOF) improves the stability of the catalyst when driven with Oxone. This increased stability indicates that interdimer interactions play a role in the degradation of the catalyst. These interdimer interactions are prevented by isolation of the complex in the pores of the MOF; thus improving its durability. Degradation mediated by interdimer interactions provides an explanation for the much lower turnover numbers (TONs) seen with the terpy tetramer relative to the terpy dimer in solution (Table 1).[42] A recent extended X-ray absorption fine structure (EXAFS) study has suggested that the true active catalyst when the terpy tetramer is deposited on montmorillonite MK10 clay is Figure 1. Structures of homogeneous manganese WOCs discussed herein. mcbpen = N-methyl-N’-carboxymethylheterogeneous manganese N,N’-bis(2-pyridylmethyl)ethane-1,2-diamine, TPHPN = N,N,N’,N’-tetrakis(2-pyridylmethyl)-2-hydroxy-1,3-propanediaoxide formed in situ upon reacmine, bpmp = 2,6-bis{[N,N-di(2-pyridinemethyl)amino]methyl}-4-methylphenol. tion with CAN.[43] It is not clear whether these results are applicable to the MOF-based system as well, but demonstrates that caution is needed when studying heterogenized homogeneous WOCs. The terpy dimer has proven extremely valuable as a demonstration that simple base-metal coordination complexes can serve as WOCs. Mechanistic studies clearly show that O O Scheme 3. Electrophilic attack of Oxone on the manganese(IV) oxyl of the bond formation occurs through WNA, and the catalyst proterpy dimer versus nucleophilic attack of water. vides a valuable model for natural photosynthesis. Unfortunately, the manganese(II) oxidation state formed in the catalytic cycle of the terpy dimer is highly labile, leading to rapid catrecently, with photogenerated [Ru(bpy)3]3 + (bpy = 2,2’-bipyralyst decomposition.[33] Although this decomposition can be indine).[36, 37] Further studies on this system revealed that WO was [38, 39] second order in catalyst on the clay. hibited by immobilization of the WOC on solid substrates, manganese inevitably escapes from the ligand, which leads to These findings led to renewed investigations into the WO loss of catalytic activity after only a few dozen TONs (Table 1). activity of a related terpy tetramer ([MnIV4O5(terpy)4(H2O)2]This lability issue is a recurring theme in base-metal WOCs, (ClO4)6 ; Figure 1). Originally reported in 2004,[40] the ability of and is one of the major hurdles in their development. this complex to catalyze WO was not studied until 2012, when  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Table 1. TONs for homogeneous base-metal WOCs.[a] WOC


Measurement method[b]


[Mn(terpy)] dimer [Mn(terpy)] dimer [Mn(terpy)] dimer on clay [Mn(terpy)]tetramer [Mn(mcbpen)] dinuclear Mn–porphyrin Mn4Ca tetramer [Fe(TAML)] [Fe(mcp)] [Fe(mcp)] [Fe(dpaq)] Co4POM Co4VPOM Co4Si2POM Co4SiPOM Co9POM CoPOM Co2POM CoIIICoIIPOM Co–porphyrin Ni5POM [Cu(bpy)(OH)2] [Cu(bpyOH)(OH)2] [Cu(TGG)(OH2)]

 60 4 13.5 3.18 15 000 9.2 1000 > 29 > 1000 750  80 24 > 800 107 154 361 121.8  60 > 30 > 400 > 13

0.5 mm WOC, 50.0 mm KHSO5, 230 mm pH 4.3 acetate buffer 12.5 mm WOC, 70 mm NaOCl, pH 8.6 46 mm WOC, 100 mm CAN 70 mm WOC, 1.6 V vs. SCE, pH 1.5 40 mm WOC, 3.2 m tBuOOH 1 mm WOC, 1.8 V vs. Ag/Ag + , 5 % water in MeCN 11 mm WOC, 1.7 mm KHSO5 975 mm WOC, 182 mm CAN 12.5 mm WOC, 125 mm CAN 12.5 mm WOC, 125 mm NaIO4, adjusted to pH 2 with HOTf 200 mm WOC, 0.5 m LiClO4, 1.58 V vs. NHE, 8 % water in propylene carbonate 120 nm WOC, 2.4 mm [RuIII(bpy)3], 30 mm phosphate, 30 mm borate buffer pH 8 2.0 mm WOC, 1.0 mm [RuII(bpy)3], 5.0 mm Na2S2O8, 80 mm borate buffer pH 9 10.0 mm WOC, 1.0 mm [RuII(bpy)3], 5 mm Na2S2O8, 25 mm borate buffer pH 9 12 mm WOC, 1 mm [RuII(bpy)3], 5 mm Na2S2O8, 20 mm Na2SiF6 buffer pH 5.8 100 mm WOC, 100 mm NaClO, 0.9 m phosphate buffer pH 8 3.6 mm WOC, 60 mm [RuII(bpy)3], 3 mm Na2S2O8, 100 mm borate buffer pH 8 1.9 mm WOC, 60 mm [RuII(bpy)3], 3 mm Na2S2O8, 100 mm borate buffer pH 8 1 mm WOC, 1.0 mm [RuII(bpy)3], 5.0 mm Na2S2O8, 80 mm borate buffer pH 9 10 mm WOC, 1 mm [RuII(bpy)3], 5 mm Na2S2O8, 100 mm phosphate buffer pH 11 2 mm WOC, 1 mm [RuII(bpy)3], 5 mm Na2S2O8, 80 mm borate buffer pH 8 1 mm WOC, 100 mm acetate buffer, 1.35 V vs. NHE, pH 12.5 1 mm WOC, 100 mm acetate buffer, 1.135 V vs. NHE, pH 12.4 2 mm WOC, 250 mm phosphate buffer, 1.3 V vs. NHE, pH 11

[31] [30] [35] [41] [45] [28] [58] [61] [64] [64] [71] [73] [85] [75] [76] [80] [81] [81] [86] [21] [93] [94] [95] [96]

[a] SCE = standard calomel electrode; TAML = tetraamido macrocyclic ligand; mcp = N,N’-dimethyl-N,N’-bis(2-pyridylmethyl)cyclohexane-trans-1,2-diamine; Tf = triflyl; dpaq = 2-[bis(pyridine-2-ylmethyl)]amino-N-quinolin-8-yl-acetamide; NHE = normal hydrogen electrode; bpyOH = 6,6’-dihydroxy-2,2’-bipyridine; TGG = triglycylglycine. [b] Chemical oxidant or electrochemical potential, and species concentrations.

2.2. [Mn(mcbpen)] [Mn(mcbpen)] (Figure 1) was identified as a WOC by McKenzie and co-workers in 2005.[44, 45] This WOC evolves exclusively 34O2 when driven with either tBuOOH or CAN in 18O-labeled water. In the case of tBuOOH, this result indicates that the O2 generated results from the reaction of one water-derived 18O atom and one tBuOOH-derived 16O atom, which demonstrates that tBuOOH serves as an oxo-transfer reagent with this WOC. In the case of CAN, only nitrate is available as a source of 16O in solution, which indicates that the catalytic cycle involves the reduction of nitrate. Similar involvement of nitrate from CAN has also been seen in the catalytic cycle of ruthenium-based WOCs.[46, 47] Initially, the mechanism of WO by this WOC was proposed to involve the coupling of two bridging oxo groups to form a bridging peroxo species. Anderlund and co-workers, however, were able to isolate this species and found that it did not evolve oxygen either spontaneously or when exposed to CAN; this demonstrated that an alternate mechanism operated.[48] To address these findings, a revised mechanism was proposed based on the results of calculations, in which a manganese(IV) oxyl species undergoes O O bond formation through WNA, as proposed for the terpy dimer.[49] The WO activity reported for [Mn(mcbpen)] inspired additional investigations into the possible WO activity of related complexes. By using a derivative of mcbpen, Anderlund and co-workers reported a tetranuclear manganese WOC capable of being driven with both Oxone and CAN.[50] Brudvig and co 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

workers used a similar ligand set to investigate the role of the anionic nature of the ligand, and determined that introducing an anionic donor moiety strongly enhanced WO activity.[51] [Mn(mcbpen)] functions as a WOC for a remarkable duration when driven with tBuOOH (Table 1). This is attributable to the stability of the manganese(II) resting state. Because manganese(II) is tightly chelated by the mcbpen ligand, inactivation by formation of manganese(II) solvate and free ligand is avoided. However, caution is required upon applying the findings from this WOC, because WO driven by CAN with this WOC requires reduction of nitrate in solution, which greatly complicates the interpretation of WO by this WOC. 2.3. Manganese Schiff base complexes Bidentate manganese Schiff base complexes were originally reported to serve as precursors for heterogeneous WOCs by Najafpour and Boghaeiin 2009.[52] Later Gonzales-Riopedre et al. reported that tetradentate Schiff base ligands afforded homogeneous manganese WOCs (Figure 1).[53] Their complex was capable of WO when driven with photoexcited benzoquinone, which was reduced to hydroquinone in the process. This method is derived from earlier studies on stoichiometric WO promoted by manganese Schiff base complexes.[54] A special characteristic of using benzoquinone as an oxidant is its ability to abstract an electron and proton concurrently, which allows it to drive WO by complexes that may not otherwise exhibit catalytic activity. Indeed, the Schiff base complex reported did not catalyze WO with tert-butylbenzoquinone; ChemSusChem 2014, 7, 2070 – 2080


CHEMSUSCHEM MINIREVIEWS this demonstrated the necessity of proton-coupled electron transfer (PCET) enabled by benzoquinone. Although not strictly a WOC because only one equivalent of water can be oxidized by the complex, the mechanism of WO by this complex provides valuable information on the design of manganese WOCs. 2.4. Manganese complexes with multinucleating ligands Seeking to develop improved WOCs, several groups have turned to multinucleating ligands, that is, ligands with two metal-binding sites. The first complex of this type to show WO activity was a linked manganese–porphyrin dimer reported by Naruta et al. (Figure 1).[28] A solution of this complex in acetonitrile generated O2 upon electrochemical oxidation in the presence of water and tetrabutylammonium hydroxide. Under high pH conditions, a MnV,V dimer was isolated, which then released a stoichiometric amount of O2 upon addition of acid.[55] More recently, Styring and co-workers reported a systematic screening of the ability of binuclear manganese complexes to serve as WOCs, and were able to report WO by several complexes previously unknown as WOCs (Figure 1).[56] In addition, the WOCs were compared with several literature WOCs under the same reaction conditions to allow direct comparison of their WO activities. Following this report, membrane inlet mass spectrometry (MIMS) was used to measure the extent of 18O incorporation in O2 produced by the catalysts when driven with Oxone in 18O-labeled water.[57] Most of the WOCs studied showed half-label incorporation, which indicated that one oxygen atom resulted from the oxidant and the other was derived from bulk water. On the other hand, [Mn2(bpmp)] shows full incorporation of labeled water, which suggests an alternative WO mechanism for this WOC.[57] To more closely model the Mn3Ca cubane motif found in the OEC achieved in natural photosynthesis, several groups have synthesized tetranuclear Mn3Ca complexes. A review of these complexes was recently published;[19] however, the only system of this type found to evolve O2 was a Mn3CaNa complex reported by Reedijk and co-workers in early 2011 (Figure 1); this produced small amounts of O2 upon the addition of various chemical oxidants.[58] The use of multinucleating ligands has allowed the characterization of several proposed catalytic intermediates, and thus, a more complete understanding of the mechanism of WO.[28, 57] Surprisingly, manganese complexes created with dinucleating ligands have not proven to be more robust or catalytically active than those formed from simpler ligand scaffolds (Table 1). Multinucleating ligands thus have yet to live up to their promise as advanced scaffolds and their further development is expected to yield improved WOCs.

3. Iron Iron-based WOCs have been reported in the literature only recently, and their catalytic mechanism and degradation pathways remain poorly understood. The first report of WO catalysis by iron dates to the 1980s: simple iron, cobalt, and copper– phthalocyanine and –porphyrin complexes were reported to  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim generate O2 from water catalytically upon reaction with [Ru(bpy)3]3 + .[59] Moreover, the mechanism of WO was not well characterized. Later reports by the same group suggested that the active WOCs in these systems were heterogeneous species generated in situ.[60] The first homogeneous iron-based WOC was reported by the Bernhard and Collins groups in 2010, wherein an [Fe(TAML)] system, modified to be highly resistant to oxidative damage, exhibited WO activity similar to those based on second- and third-row transition metals when driven with CAN.[61] Unfortunately, despite extensive modification of the ligand, TONs of less than 20 are achieved by this WOC (Table 1). The mechanism of WO by this WOC was investigated by UV/Vis spectroscopy when driven with sodium periodate. Under neutral conditions, a strong absorbance at l  770 nm assigned as a mono-m-oxo-bridged FeIV,IV dimer was observed, along with slow O2 production. Moreover, upon reaction with periodate under acidic conditions, only a bleach in the absorption spectrum is observed, with no O2 generation.[61] These results suggested that dinuclear species were not involved in WO by this WOC. Initial computational studies on this WOC suggested the formation of a formally iron(VI) center with one of the oxidizing equivalents localized on the TAML ligand, followed by WNA with concerted deprotonation of the attacking water by bulk solvent to form an iron(IV) peroxyl species.[62] An alternative mechanism based on computation was also proposed by the Siegbahn group, wherein nitrate attack on the formally FeVI center competed with WNA at the O O bond-forming step, and both pathways contributed to the observed O2 production.[63] Further experimental studies are required to determine the extent to which each of these pathways contributes to O2 generation by this WOC. Following this work, Lloret-Fillol et al. reported a family of 4N non-heme iron complexes, which served as WOCs, with [Fe(mcp)] showing the highest activity for WO (Figure 2).[64] In this family, it was found that two open binding sites located cis to one another were required to realize WO activity, in contrast to the trans binding sites of the [Fe(TAML)] species discussed above. Dramatic effects on the WO rate and the stability of the WOC were found upon ligand modification, allowing for the development of an iron-based WOC with TONs in the order of hundreds when driven with CAN. In addition, WO with these WOCs can also be driven with sodium periodate, which leads to decreased activity, but much higher TONs (in excess of 1000; Table 1). Given the ease of varying the ligand scaffold of this family of WOCs and their relatively high stability, they have been the focus of significant mechanistic studies to further improve their characteristics as WOCs. One early report showed that under alkaline conditions these WOCs rapidly formed iron oxide nanoparticles, which then served as heterogeneous WOCs; in contrast to their homogeneous behavior under more acidic conditions.[65] These findings were confirmed by the Fukuzumi group, who demonstrated that the character and WO mechanism of such iron-based WOCs were sensitive to the reaction conditions.[66] ChemSusChem 2014, 7, 2070 – 2080


CHEMSUSCHEM MINIREVIEWS WOC, the electrochemical WO mechanism of which is well characterized. Iron-based WOCs have only recently begun to be well characterized. Early successes in this field in finding catalysts that are either highly active[61] or highly stable[64] is extremely promising for their future development. Currently, the mechanism by which they catalyze WO is the subject of intense investigation. As the details of the mechanisms of these WOCs are clarified, it is expected that new iron-based WOCs which are both highly active and highly robust will be developed.

4. Cobalt

Figure 2. Structures of homogeneous iron WOCs discussed herein.

Cobalt has gained popularity in the development of WOCs, since Nocera and Kanan’s report of a self-repairing heterogeneous cobalt oxide with high WO activity.[72] Attempts to develop molecular WOCs with cobalt have since been split into attempts to mimic possible structures found in Nocera and Kanan’s heterogeneous system and those that focus on more traditional coordination chemistry. Both approaches have yielded highly active WOCs, although the high activity of the heterogeneous system has invited warranted criticism of the true nature of any reported homogeneous WOCs based on cobalt.

Recently, Lloret-Fillol et al. published a follow-up report on this family of WOCs, and demonstrated the formation of the FeIV (OH) CeIV adduct prior to the rate-determining O O bond formation through WNA.[67] Consistent with this mechanism, they found that the WO rate increased with increasing electrophilicity of the iron(V) oxo species formed. To further support 4.1. Cobalt polyoxometalate (POM) systems their proposed mechanism, they recently reported DFT calculaCobalt (POMs) were first investigated as WOCs by Hill and cotions performed in conjunction with the Luis group that show workers in 2010, and may be considered an attempt at harthat the iron(V) oxidation state was necessary to catalyze vesting the stability of metal oxides for use in homogeneous WO.[68, 69] catalysis.[73, 74] The POM [Co4(H2O)2(a-PW9O34)2]10 (Co4POM), reIn a recent report by Hołyn´ska and co-workers, dinuclear complexes derived from the initial mononuclear non-heme ported by Hill et al., contained a tetranuclear cobalt core and iron complexes were proposed to be the true active species was found to rapidly turn over WO with photogenerated for WO catalysis.[70] They found that a dinuclear m-oxo-bridged [Ru(bpy)3]3 + (Figure 3).[73] Following this report, the less active diiron tris(2-methylpyridyl)amine (TPA) complex gave higher [{Co4(m-OH)(H2O)3}(Si2W19O70)]11 (Co4Si2POM) was also reportturnover frequencies (TOFs) than those reported for the monoed,[75] and Car and co-workers reported [Co4(H2O)2(SiW9O34)2]12 [64] (Co4SiPOM).[76] Nevertheless, the initial report on Co4POM was meric [Fe(TPA)] complex in the initial report. Nevertheless, only the [Fe(TPA)] complex was investigated; it remains to be not without controversy, because the WO activity of this clarified whether these results extend to the more active WOCs system was somewhat lower than that of the heterogeneous in this family. system. This led to speculation that the true WOC was not Recently, [Fe(dpaq)] was reported to electrochemically drive Co4POM, but rather cobalt oxides formed in situ during WO. Indeed, when driven electrochemically, Co4POM converted into WO by the Meyer group.[71] This complex functions as a homogeneous WOC in aqueous solution, with a substantial rate enhancement upon addition of propylene carbonate to the solution. Interestingly, this WOC also catalyzes the oxidation of propylene carbonate, leading to a relatively low Faradaic efficiency of 45 %. The electrochemically determined kinetic parameters are consistent with O O bond formation through WNA; this corresponds to the rate-determining step of this WOC. Currently, this is the only iron-based Figure 3. Structures of homogeneous cobalt WOCs discussed herein.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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CHEMSUSCHEM MINIREVIEWS cobalt oxide on the electrode surface,[77] despite its hydrolytic stability in the absence of oxidizing agents.[78] However, characterization of the electrocatalytic behavior of Co4POM required concentrations much higher than those used in the initial report. This gave rise to a concern that the observed cobalt oxide formation was a result of the differences in conditions between the two studies. This possibility was explored further by Scandola et al.,[79] who found that photogenerated [Ru(bpy)3]3 + (Scheme 4) was not capable of oxidizing

Scheme 4. Photogeneration of [Ru(bpy)3]3 + in the presence of Na2S2O8.

Co4POM, and thus, Co4POM could not be the active catalyst observed in the initial report by Hill et al. However, these findings were obtained under much lower [Ru(bpy)3]2 + concentrations than those used the initial work (50 mm vs. 1.5 mm), leading to the possibility that Co4POM was being oxidized directly by the sulfate radicals generated during photolysis, thereby preventing the reaction of Co4POM with [Ru(bpy)3]3 + . The Galn-Mascars group attempted to address this ambiguity by using [Co9(H2O)6(OH)3(HPO4)2(PW9O34)3]16 (Co9POM).[80] In this study, free bpy ligand was added to a solution of the POM to sequester any free CoII ions generated during electrolysis as [Co(bpy)3]2 + . In this system, they found that addition of bpy did indeed prevent deposition of heterogeneous cobalt oxides. They also reported that hypochlorite could be used to drive Co9POM for a TON of over 800 (Table 1). We endeavored to clarify this issue by focusing on two cobalt POMs, [CoMo6O24H6]3 (CoPOM) and [Co2Mo10O38H4]6 (Co2POM),[81] which were based on polymolybdates rather than polytungstates, which were often used in the studies of Hill and co-workers.[73] Consequently, we found that no nanoparticles formed during WO with these WOCs by using the dynamic light scattering (DLS) technique. In addition, we also reported that simple free cobalt(II) species were not capable of catalyzing WO under these conditions; this demonstrated that heterogeneous cobalt oxide species were not responsible for the observed WO. This finding was subsequently confirmed to be the case for Co4POM by Stracke and Finke, who found that free CoII ions were indeed incapable of generating the electrocatalytic current they had observed previously.[82] In addition, they were unable to find evidence for cobalt oxide formation from Co4POM, although they were also unable to exclude its involvement in electrocatalysis under the conditions they adopted. Most recently, Geletii and co-workers indicated that the active WOC in this system was unmodified Co4POM through  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim selective extraction of the POM from the catalysis solution.[83] Their study also showed that the WO activity of Co4POM decreased upon subsequent runs, which suggested that the decomposition products of Co4POM were less catalytically active than the starting complex. Further confirmation of the homogeneity of Co4POM was provided by X-ray absorption nearedge structure (XANES) and EXAFS studies performed by Dau and co-workers, who showed that the WOC remained intact after WO catalysis.[84] Following this, the Hill group reported the more active catalyst Na10[Co4(H2O)2(VW9O34)2]·35 H2O (Co4VPOM).[85] The WO activity of this WOC was measured by using chemically generated [Ru(bpy)3]3 + , and had a significantly higher TOF than that of Co4POM under the conditions used (TOF > 1000 s 1). The TON of this WOC was substantially lower than that of Co4POM (Table 1); however, by using a variety of techniques, it was found that Co4VPOM was the active WOC. A series of Keggin-type POMs were tested for WO activity by Ding and co-workers.[86] Of the complexes tested, only [CoIIICoII(H2O)W11O39]7 (CoIIICoIIPOM) showed WO activity. The WO activity of this WOC is attributed to cooperation between the cobalt(II) and cobalt(III) centers. In the CoII,II oxidation state, this POM is rapidly hydrolyzed, which shows that the CoIII center is needed to prevent hydrolysis of the POM in aqueous solution. In addition, the buffering conditions adopted had a strong effect on the WO activity of CoIIICoIIPOM, with greatly reduced catalytic activity observed in weakly buffering solutions. Recently, Zhang et al. reported four cobalt phosphate POMs, [{Co4(OH)3(PO4)}4(MW9O34)4]n , in which M = Si, Ge, P, and As (n = 32 for M = Si and Ge; n = 28 for M = P and As).[87] These systems were confirmed to be homogeneous by using the same selective extraction method as that reported by Geletii et al.[83] The highest WO activity was found for M = Ge in this family of WOCs, although their activities were all comparable to those of previously reported cobalt-based POMs. This saga serves as an excellent example of the necessity of strictly matching reaction conditions between experiments, because the nature of the WOC may depend upon the methods used to study it. Under the conditions used in the initial report,[73] Co4POM serves as a homogeneous WOC. Nevertheless, modifications to the reaction conditions, in terms of relative species concentrations, the buffer/pH used, and other factors, can cause the formation of a heterogeneous WOC. When the WO characteristics of a previously established WOC are reexamined in further detail, considerable attention must therefore be paid to the reaction conditions employed. 4.2. Mononuclear cobalt systems In addition to the extensive work on cobalt-based POMs as WOCs, a number of groups have explored mononuclear cobalt-based WOCs. This approach affords more control over the structure and electronic properties of the WOCs, as well as the possibility of incorporating anchoring motifs for anchoring the WOC to a solid support. The first of these catalysts were reported independently by the Berlinguette[88] and Nocera[89] groups in early 2011 (Figure 3). ChemSusChem 2014, 7, 2070 – 2080



Berlinguette’s group designed a cobalt-based WOC with only one open coordination site: [Co(Py5)(OH2)]2 + (Py5 = 2,6-[bis(bis2-pyridyl)methoxymethane]pyridine).[88] This allowed them to conclude that the two PCET events observed electrochemically were attributable to depro- Scheme 5. Formation of singlet oxygen and its reaction with cobalt–porphyrin. tonation at the lone aqua site (Figure 3). The oxo ligand formed then served as the active species in O O bond formaFollowing these studies, Wang and Groves reported on election, as discussed below. In this study, a catalytic wave was obtrochemically driven WO by a family of cationic cobalt–porserved electrochemically at mildly basic pH. This wave is attribphyrins.[92] They were able to confirm that the cobalt–porphyruted to WO through nucleophilic attack of hydroxide on the ins were the true active catalyst in the reaction solution. ImCoIV oxo species, leading to generation of a cobalt(II) peroxyl, portantly, they found that the buffer solution used had a large which is then rapidly oxidized to a cobalt(IV) peroxyl species. effect on the onset potential for WO, with buffer ions with This species can then undergo further reaction to release O2 higher pKa values decreasing the required oxidation potential. and regenerate the starting CoII aqua species. In combination with kinetic studies, this demonstrated that Nocera et al. used hangman corroles to achieve single-site PCET during O O bond formation was the rate-determining WO by cobalt.[89] In these complexes, there are two vacant costep in WO by these WOCs. ordination sites at the cobalt center, one of which has a proxiAlthough care must be taken to avoid the contribution of mal base that is sterically hindered from coordinating to the metal oxide nanoparticles generated in situ during WO catalycobalt center. These complexes served as WOCs with one-elecsis, the high reaction rates observed for these cobalt-based tron oxidation of cobalt(III) to a formally cobalt(IV) species as WOCs suggested that they were a promising avenue for furthe rate-determining step when driven electrochemically. It ther investigation. Because the ambiguity surrounding the mowas also reported that the potential required to drive WO by lecular systems has been mostly resolved, the rapid developthese catalysts was reduced upon fluorination of the ligand; ment of this area is expected. As with all base-metal WOCs, imthis demonstrated the molecular nature of these WOCs and proved robustness is required before they can be used in practhe ability to tune the reactivity of WOCs by ligand modificatical devices. tion. Nevertheless, doubts about the ability of these mononuclear 5. Nickel systems to serve as true homogeneous WOCs carried over from the difficulties in conclusively determining the homogeDespite nickel oxide serving as one of the only two WOCs curneity of the cobalt-based POMs. Indeed, several cobalt comrently in commercial use, the development of homogeneous plexes synthesized to serve as WOCs were found by Nam and WOCs based on nickel has been relatively unexplored. The co-workers to rather serve as precursors for highly active only reported nickel-based WOC is a POM synthesized by Hill cobalt oxide nanoparticles.[90] In light of these findings, we set and co-workers: K10H2[Ni5(OH)6(OH2)3(Si2W18O66)] (Ni5POM).[93] Deposition of double salts formed from the Ni5POM anion with out to determine whether definitive proof of a homogeneous [Ru(bpy)3]2 + made it difficult to characterize the mechanism of mononuclear cobalt-based WOC could be obtained. We were WO by this WOC. However, Hill and co-workers were able to able to find water-soluble cobalt–porphyrin derivatives that exshow that the POM in solution was the true molecular catalyst hibited WO activity, but did not release free cobalt(II) into solufor WO. Based on the high activity of nickel oxide and Ni5POM tion or form cobalt oxides during catalysis. Consequently, the towards WO, research into nickel-based WOCs is expected to active WOC could be definitively assigned as the cobalt–poryield more highly active WOCs. phyrins, in agreement with previous work on cobalt POMs.[21, 81] Importantly, during photoinitiated WO with cobalt–porphyrins, degradation of the catalyst is greatly inhibited by filtering 6. Copper out light capable of exciting the Soret band of the porphyrin. This led us to investigate the possible causes for decomposiCopper complexes have rarely been explored as WOCs; this is tion of these WOCs. A survey of the literature allowed us to relikely to be due to the metal’s preference to undergo onealize that singlet oxygen generated from the reaction of O2 rather than two-electron redox events. To date, only two families of copper-based WOCs have been reported: the first by with the triplet excited state of either cobalt–porphyrin or the Mayer et al. and later independently by the Lin group,[94, 95] [Ru(bpy)3]2 + could rapidly attack the porphyrin ring to yield [91] ring-opening products (Scheme 5). Methods for inhibiting and the second by the Meyer group.[96] damage by singlet oxygen must therefore be established Mayer and co-workers first electrochemically examined the when developing WOCs for light-driven WO. WO activity of [Cu(bpy)(OH)2] (Figure 4) under alkaline conditions.[94] The complex was demonstrated to serve as a homoge 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 4. Structures of copper WOCs discussed herein.

neous WOC on the basis of EPR measurements and control studies, which showed no WO activity for copper oxide, even in the presence of bpy. Electrochemical studies on the system indicated that the WOC was mononuclear, although a detailed mechanistic study has not yet been completed. As with other base-metal WOCs, stability remains a challenge, with about 35 % of the catalyst decomposing after 30 turnovers. The WO activity of a modified version of [Cu(bpy)(OH)2] with hydroxyl groups on the 6 and 6’ positions of bpy, [Cu(bpyOH)(OH)2] (Figure 4), was recently reported by Lin and co-workers.[95] In this WOC, the ligand is both redox active and able to serve as an internal base for proton transfer. Because of these factors, this WOC can be driven at a potential about 200 mV lower than that of [Cu(bpy)(OH)2]. Using the squarewave voltammetric technique, they demonstrated that the ligand and copper(II/III) oxidation potentials were within about 20 mV of one another. This allows the catalyst to undergo twoelectron oxidation at closely spaced potentials, which allows the formation of a formally copper(IV) species capable of driving oxidation of water to peroxide at a relatively low potential. Nevertheless, the lower operating potential of this WOC also reduces the driving force for the reaction, which, in turn, reduces the WO rate of [Cu(bpyOH)(OH)2] relative to [Cu(bpy)(OH)2]. By separating the anode and cathode compartments during electrolysis, the Lin group was also able to greatly improve the apparent robustness of the WOC, and observed over 400 turnovers. This demonstrated that, without separating these compartments, the major degradation pathway for this WOC was reduction of [Cu(bpyOH)(OH)2] to CuI at the cathode with subsequent ligand loss. Recently, the Meyer group reported a second type of copper-based WOC: [Cu(TGG)(OH2)]2 (Figure 4). The WO activity of this WOC was characterized electrochemically. By varying the pH and concentration of the WOC in the reaction solution, the rate-determining step could be determined to be O O bond formation during WNA on a copper(III) oxyl formed by two-electron oxidation of the starting complex. Control studies with simple CuII salts, such as CuSO4, indicated that they could not be responsible for the observed WO activity due to poor solubility, although more detailed investigations would be required to fully understand the mechanism of WO by this WOC.

7. Summary and Outlook The field of water oxidation (WO) catalysis has been rapidly developing in the past decade. Although little was known about  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim base-metal coordination complexes capable of catalyzing WO prior to the turn of the century, attempts at modeling the natural photosystem led to the development of manganese-based water oxidation catalysts (WOCs) in the late 1990s. More recently, the drive to find carbon-neutral energy technologies has greatly accelerated the pace of research toward developing highly active and robust WOCs. Although great advancements have been made with WOCs based on precious metals, steady improvement has also been made with base-metal WOCs. In particular, cobalt-based WOCs have been the subject of substantial investigation, which has resulted in several families of efficient WOCs. Several factors must be considered during the study of base-metal WOCs. Of particular note is the importance of reaction conditions when studying WOCs, because even apparently small changes in reaction conditions have large effects on both the mechanism and stability of WOCs. This is especially evident in the case of CoPOMs, for which the homogeneity of the system was highly dependent on the concentration of the WOC, pH, and concentrations of other species in the reaction solution. The stability of WOCs based on base metals also remains a challenge. In almost all cases, TONs in the order of 100 are observed for base-metal WOCs (Table 1), and TONs greater than 1000 have been achieved for only a few systems. One factor contributing to this instability is the relative lability of first-row transition metals relative to those of their secondand third-row counterparts. Even complexes that are able to remain ligated during turnover must be tolerant of the highly oxidizing species produced during the reaction, including hydrogen peroxide and singlet oxygen. In nature, these issues are avoided by continually regenerating the oxygen-evolving center as it degrades. Artificial WOCs with such self-repair capabilities are promising candidates for the next generation of base-metal WOCs.

Acknowledgements This work was partly supported by a Grant-in-Aid for Scientific Research (B) (no. 24350029), a Grant-in-Aid for Scientific Research on Innovative Areas ‘Coordination Programming’ (no. 2107) (no. 24108732), and a Grant-in-Aid for Scientific Research on Innovative Areas ‘Artificial Photosynthesis’ (no. 2406) (no. 24107004) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. This was further supported by the International Institute for Carbon Neutral Energy Research (WPII2CNER), sponsored by the World Premier International Research Center Initiative (WPI), MEXT, Japan. Keywords: energy storage · homogeneous oxidation · transition metals · water chemistry [1] [2] [3] [4]



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Received: April 16, 2014 Published online on July 25, 2014

ChemSusChem 2014, 7, 2070 – 2080


Progress in base-metal water oxidation catalysis.

This minireview provides a brief overview of the progress that has been made in developing homogeneous water oxidation catalysts based on base metals ...
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