Transition metal complexes that catalyze oxygen formation from water: 1979–2010

Coordination Chemistry Reviews 256 (2012) 1115–1136

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Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

Transition metal complexes that catalyze oxygen formation from water: 1979–2010 Xien Liu ∗ , Fengying Wang Department of Materials Science and Engineering, the Pennsylvania State University, University Park, PA 16802, USA

Contents 1. 2.

3.

4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water-oxidation chemistry of manganese complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Mononuclear manganese complex catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Dinuclear manganese complex catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Tetranuclear manganese complex catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water-oxidation chemistry of ruthenium complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Mononuclear ruthenium complex catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Dinuclear ruthenium complex catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Multinuclear ruthenium complex catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water-oxidation chemistry of iridium, iron and cobalt complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

a r t i c l e

i n f o

Article history: Received 23 July 2011 Accepted 26 January 2012 Available online 14 February 2012 Keywords: Water oxidation Transition metal complexes Photosynthesis Energy conversion

1116 1116 1116 1116 1119 1120 1120 1127 1130 1131 1135 1135

a b s t r a c t The study of catalytic water oxidation continues to be one of the most active areas of research across many sub-disciplines of chemistry. From efforts toward developing artificial photosynthetic assemblies to the exploration of nanoscale materials to be used as a photoanode for splitting water, a detailed understanding of the mechanistic details of water oxidation in photosystem II (PSII) is paramount for the rational design of an artificial model. In addition, insight into the model’s mechanism of molecularlevel water-oxidation catalysis will provide us with a unique opportunity to elucidate the mechanistic pathways of water oxidation in the oxygen-evolving complex (OEC) of PSII. In this review, the proposed mechanisms, catalytic activities and reaction kinetics of catalytic water oxidation with transition metal complexes in homogeneous systems published from 1979 to 2010, with an emphasis on the last decade, are discussed. These metal complexes include mononuclear, dinuclear and multinuclear manganese, ruthenium, iridium, iron and cobalt complexes. Electrodeposited cobalt complexes are a type of heterogeneous water-oxidation catalyst; however, these complexes are discussed herein because they are topological analogs of the Mn cluster in the OEC. MV O species (M = Mn, Ru) as

Abbreviations: 3,6-tBu2 qui, 3,6-di(tert-butyl)-1,2-benzoquinone; bpm, 2,2 -bipyrimidine; bpp, 2,6-bis(pyridyl)pyrazolate; bpy, 2,2 -bipyridine; bpz, 2,2 -bipyrazine; btpyan, 1,8-bis{(2,2 :6 ,2 )terpyridyl}anthracene; CAN, cerium ammonium nitrate; Cp*, pentamethylcyclopentadienyl; DFT, density functional theory; DMPO 5, 5-dimethylpyrroline-1-oxide; EPR, electron paramagnetic resonance; ESI-MS, electrospray ionization mass spectrometry; ESR, electron spin resonance; EXAFS, X-ray absorption fine structure; GC–MS, gas chromatography–mass spectrometry; H2 pda, 1,10-phenanthroline-2,9-dicarboxylic acid; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital; ITO, indium-tin-oxide; KPi, potassium phosphate; LDI-TOF-MS, laser desorption–ionization time-of-flight mass spectrometry; mcbpen, Nmethyl-N -carboxymethyl-N,N -bis(2-pyridylmethyl)ethane-1,2-diamine; mCPBA, m-chloroperbenzoic acid; Mebimpy, 2,6-Bis(1-methylbenzimidazol-2-yl)pyridine; MIMS, membrane inlet mass spectrometry; NaPi, sodium phosphate; NIR, near-infrared; OEC, oxygen-evolving complex of photosystem II; PCET, proton-coupled electron transfer; pic, 4-picoline; PSII, photosystem II; P680, primary electron donor of photosystem II; salpd, propane-l,3-diylbis(salicylideneiminate); SEM, scanning electron microscope; SIRMS, stable isotope ratio mass spectrometry; SHE, standard hydrogen electrode; TBHP, tert-butylhydrogenperoxide; TEM, transmission electron microscopy; terpy, 2,2 :6,2 terpyridine; TN, turnover number; tpy, 2,2 :6 ,2 -terpyridine; trpy, 2,2 :6 ,2 -terpyridine; TOF, turnover frequency; UV/vis, ultraviolet/visible; XANES, X-ray absorption near-edge structure; XAS, X-ray absorption spectroscopy; XRD, X-ray diffraction. ∗ Correspondence address: Department of Materials Science and Engineering, the Pennsylvania State University, University Park, PA, USA, 16802. Tel.: +1 8144413976. E-mail address: [email protected] (X. Liu). 0010-8545/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ccr.2012.01.015

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the key active species in homogeneous catalytic evolution of O2 are common feature in many catalysts, in which formation of the O O bond can be achieved either by intramolecular elimination of dioxygen from two MV O groups or by nucleophilic attack of OH− species on MV O groups. Another common feature appears from tetranuclear ruthenium complex 71, manganese complex 10 and cobalt complex, all of these cubane structural complexes have self-repair properties similar to OEC–protein complex in nature. © 2012 Elsevier B.V. All rights reserved.

1. Introduction A major challenge facing humanity is the development of a renewable source of energy to replace fossil fuels. Solar energy is considered a decentralized and inexhaustible natural resource; 1 h of sunlight produces more energy than the energy consumed on the planet in one year [1–4]. In natural photosynthesis, solar energy is converted into chemical energy by utilizing water as a raw material. The water is catalytically oxidized by the CaMn4 Ox cluster in the oxygen-evolving complex (OEC) of PSII, which is embedded in the thylakoid membranes of green plants, cyanobacteria and algae. With inspiration from nature, constructing an artificial photosynthetic device has been a goal of utmost importance for scientists working on solving the world’s energy problem [5–28]. However, although a number of advances have been made in the studies of the OEC of PSII, many details of the structure, bioenergetics and catalytic mechanism of the CaMn4 Ox complex remain poorly understood [29–35]. Identifying catalysts to carry out this reaction at high rates for sustained periods has been problematic because removing four electrons and protons from two water molecules with the concomitant formation of an O–O bond (Eq. (1)) is a highly energetically unfavorable reaction (H◦ = 572 kJ/mol). Furthermore, the chemical intermediates formed during this reaction are so reactive that autoxidation of the catalysts often leads to selfdestruction. In contrast, the OEC can be resynthesized every half hour with little or no production of oxidized intermediates, such as hydroxyl radicals, hydrogen peroxide, or superoxide; thus, the functional stability of the OEC is maintained despite its structural instability [36]. This review is mainly focused on the molecularlevel homogenous catalytic water oxidation and divided into four main sections. Section 2 is devoted to the various types of manganese complex catalysts and several significant works are brought up to date. Section 3 covers more than 60 Ru complex catalysts and describes the reaction mechanism of evolved O2 , stability and activity of these water-oxidation catalysts. Iridium, iron and electrodeposited cobalt complex catalysts are discussed in Section 4. The very exciting and productive work being done on the use of transition metal complexes anchored to semiconductor surfaces in catalytic water oxidation will not be covered. 2H2 O → O2 + 4H+ + 4e− E ◦ = 1.23 V vs. SHE (pH 0);

(1) 0.82V vs. SHE (pH 7).

2. Water-oxidation chemistry of manganese complexes Manganese is within the active site of the OEC in PSII [37,38], in which proton-coupled electron transfer (PCET) is sequentially driven by P680+ as an oxidant (approximately 1.25 V vs. SHE) and a redox active tyrosine as a mediator that leads to accumulation of multiple oxidizing equivalents during the catalytic cycle and the O2 is evolved in a light-driven reaction. PCET reactions play an essential role in a broad range of energy conversion processes, including photosynthesis and respiration, which are described in terms of non-adiabatic transitions between the reactant and product electron–proton vibronic states, classified concerted and sequential PCET. The former refers to the transfer of an electron

and a proton in a single step without a stable intermediate and later involves stable intermediate. Although many mechanistic details remain unknown, functional OEC models have provided some mechanistic insights through experimental evidence. Wateroxidizing manganese systems have been studied extensively as models of the OEC. However, none of these manganese complexes is a true water-oxidation catalyst that can oxidize water under a light-driven process. In addition, all manganese compounds investigated for their catalytic properties require the use of oxygen transfer and/or two-electron oxidizing agents as oxidants to evolve O2 catalytically in a homogeneous solution. Herein, manganese water-oxidation catalysts 1–10 (Chart 1) will be discussed. 2.1. Mononuclear manganese complex catalysts Limburg, Brudvig, and Crabtree investigated the reaction of compound 1 with oxone in a buffered aqueous solution over the pH range of 3–6 using 0.023 M oxone [39], and found that a green Mn(III/IV) dimer (determined by the observation of a 16-line EPR signal) can be formed and can generate O2 with a byproduct MnO4 − . Experiments investigating the O2 evolution were carried out in 0.1 M acetate-H2 SO4 buffer (pH 4.3, [Mn] = 125 ␮M) with different ligands. Utilizing dipicolinate (dpa) as the ligand (1), the rate of O2 evolution was 3.7 ␮mol/h, and the turnover number (TN) was less than 1. Considerable improvement was made by replacing dpa with terpy; the rate of O2 evolution dramatically increased to 61 ␮mol/h, and the TN improved to 50. In fact, mononuclear compound 1 and Mn–terpy complexes are precursors which formed dinuclear Mn complexes under catalytic conditions, and then the dinuclear Mn complexes catalyzed water oxidation. Recently, Åkermark et al. studied the mechanism of water oxidation by a mononuclear 5,10,15-tris(4-nitrophenyl)MnIII corrole (2) using an oxygen isotope label and an online gas analysis of the mass spectrometry data [40]. They provided experimental evidence for a possible mechanism involving the nucleophilic addition of hydroxide on MV O species (Fig. 1), which was proposed previously based on density functional theory calculations [41]. The hydroperoxy complex can be produced by the attack of hydroxide on the active species MV O. Spectroscopic evidence exists for all of the species in this mechanism except for the MnIII hydroperoxy complex. The evolved O2 had the expected statistical isotope distribution when 18 O-labeled water was used, indicating that one of the two oxygen atoms of the O2 was derived from tert-butylhydrogenperoxide (TBHP) and the other from water. However, TN and turnover frequency (TOF) were not clearly reported here. 2.2. Dinuclear manganese complex catalysts Watkinson, Whiting, and McAuliffe reported a dinuclear MnIII compound (3) that evolves O2 in the presence of p-benzoquinone in water under conditions of visible light irradiation and room temperature [42], although the photoactivity of water oxidation is much lower than that of the complex [{Mn(salpd)(H2 O)}2 ][ClO4 ]2 reported previously [43]. This compound is the first example of a dinuclear manganese complex that evolves O2 . Brudvig and Limburg et al. reported compound 4, [H2 O(terpy)Mn(O)2 Mn(terpy)OH2 ](NO3 )3 , which is the first

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Chart 1. Structures of manganese water-oxidation catalysts.

functional model with di-␮-oxo dimeric Mn units that can catalyze homogeneous O2 evolution, similar to those observed in the OEC [44]. They emphasized the idea that the terminal MnV O is a key species for O O bond formation involving an attack by OH− to produce a peroxy intermediate. After releasing O2 , the MnV MnV species is reduced to MnIII MnIII dimer (Fig. 2), which is oxidized by the oxidant (NaOCl) to recover the original MnIV MnIV dimer, as verified by electron paramagnetic resonance (EPR) and ultraviolet/visible (UV/vis) spectroscopies.

To study the origin of the O atoms in the evolved O2 molecule, they used 18 O labeling combined with mass spectrometry to follow the evolution of O2 . The 18 O18 O/18 O16 O ratios observed from the spectra indicated that the 18 O content in the evolved O2 was 75%, which agrees with the 18 O content in water and indicates that water was oxidized under these conditions. However, a key issue in this 18 O labeling method is that the reaction rate of the primary oxidant with the catalyst must be faster than that of its exchange with free water. To solve this problem, Raman spectroscopy was

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NO2

N

=

MnIII

O2N

N NO2

Mn N

N

O tBuOOH MnIII

Mn V

O2 -e O MnIV

O-

nBu4NOH -H+ -e-

O

OH

MnIII

Fig. 1. Possible mechanism of oxygen evolution of 2. Source: The graphic was adapted from reference [40].

utilized to measure the exchange kinetics of both hypochlorite and oxone with water [45]. The results of these Raman experiments demonstrate that the O-atom exchange of hypochlorite with water was indeed rapid (reaching equilibrium after approximately 30 s), whereas the exchange of oxone with water was much slower (no exchange was detected even after an hour). Clearly, oxone performed much better as an oxidant in the 18 O-labeling experiments. Hence, they proposed the mechanism for water oxidation depicted in Fig. 3 and the three pathways for O O formation depicted in Scheme 1. The complex, including MnV O, is a key intermediate in

Fig. 2. A simplified proposal for the reaction mechanism of the formation of O2 from 4 with NaClO. Source: Reprinted with permission from [44].

Fig. 3. Proposed mechanism for the reaction between 4 and the oxygen atom transfer reagents XO. Source: Reprinted with permission from [45]. © 2001 American Chemical Society.

the O O bond formation that involves the attack of the MnV O by solvent or oxone, depending on the concentration of the oxidant (oxone), thereby producing unlabeled, single-labeled, and doublelabeled oxygen. Using EPR, UV–vis, electrospray ionization mass spectrometry (ESI-MS), X-ray absorption spectroscopy (XAS), and gas-phase stable isotope ratio mass spectrometry (SIR-MS) [46], the species involved in the catalytic cycles were identified, including the major steady-state product (MnIV MnIV dimer), a small amount of a MnIII MnIV dimer, and a minor amount of a species giving rise to a g = 4 EPR signal, such as [MnIV (terpy)L3 ]+ (L H2 O, OH, or OAc) or a MnIV trimer. Based on the above studies, the reaction

Scheme 1. Possible pathways for the formation of labeled and unlabeled O2 from the reaction between 4 and KHS16O5 in H2 18 O. Source: Reprinted with permission from [45]. © 2001 American Chemical Society.

X. Liu, F. Wang / Coordination Chemistry Reviews 256 (2012) 1115–1136

Fig. 4. Structure of Mn2 dimeric porphyrin complex 5 and a reaction pathway for O2 formation. Source: Reprinted with permission from [48]. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA.

procedure from MnIII MnIV to MnIV MnIV was elucidated via stopped-flow UV–vis absorption spectroscopy [14], and the MnIV MnIV dimer was a major steady-state species in the presence of excess HSO5 − ; however, this species is catalytically inactive. The biphasic nature of the time course of the absorbance change in the reaction of 4 with HSO5 − indicates that both MnIII and MnIV can act as the binding site for HSO5 − . Ligand exchange and intramolecular electron transfer between the Mn sites of complex 4 in the presence HSO5 − was slow (i.e., millisecond time scale). A MnIII 2 porphyrin dimer (5) was reported by Naruta et al., which is the first example involving a MnV O species during the formation of the O O bond (Fig. 4). [47,48], the MnV 2 porphyrin dimer was generated by the oxidation of the MnIII 2 porphyrin dimer with m-chloroperbenzoic acid (mCPBA) as an oxidant under basic conditions. Each Mn center is a six-coordinate HO-MnV O species, verified by comparing the results of its resonance Raman spectrum in the presence of the 18 O-labeled and deuterated Bu4 NOH in H2 O to the calculated values. Each compound shown in Fig. 4 is reasonably assigned on the basis of its electron spin resonance (ESR) and UV/vis spectra. Oxygen can be evolved in stoichiometric quantities upon the addition of an excess of triflic acid to the MnV O species. They think the O O bond was formed either by the attack of water on the active species H2 O-MnV O or by a coupling reaction between the oxo groups of each MnV O unit. The TN was approximately 1, this result is contrary to the early work by Naruta et al. [47]. The complex should not be referred as a catalyst due to its stoichiometric quantities of evolution of O2 . McKenzie et al. reported a four-electron water-oxidation reaction catalyzed by 6, [Mn2 (mcbpen)2 (H2 O)2 ](ClO4 )2 , with TBHP or cerium ammonium nitrate (CAN) in aqueous solutions [49]. The TN reached approximately 20 when TBHP was used as an oxidant. Using 18 O labeling experiments combined with a Clark electrode and a membrane inlet mass spectrometry (MIMS) technique, they demonstrated that one oxygen atom in the evolved O2 was from water, whereas the other oxygen atom was from the oxidants. According to the data from the ESI-MS and UV–vis spectroscopy of the intermediates, these authors proposed a mechanism for the reaction pathway (Fig. 5). In this mechanism, the binuclear MnII MnII compound was oxidized with TBHP and cleaved to form a mononuclear MnIII compound in which the oxygen atom of the terminal hydroxide moiety was derived from water as verified by ESI-MS studies of the reaction in D2 O and H2 18 O. The MnIII complex was then converted into a MnIII MnIII complex by dimerization.

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In the next step, the TBHP served as a one-oxygen-atom donor in the conversion from the MnIII MnIII dimer to a di-␮-oxo bridged MnIV MnIV species. The resonance structure ␮-peroxo-MnIII MnIII completes the oxygen release and restores the original state. The evolution of oxygen is less efficient when using ceric ammonium nitrate as an oxidant. In this mechanism, one of O atoms in the evolved O2 comes from the nitrate counter ion of the oxidant, which is a highly unusual conclusion that is disfavored by the recent studies of Kurz et al. [51]. In Kurz et al. experiment, no oxygen was detected when complex 6 was used the water-oxidation catalyst with CeIV as an oxidant. Furthermore, TBHP is a controversial reagent as an oxidant in an evolving O2 reaction catalyzed transition metal complexes, because it has a tendency toward radical chemistry and can produce O2 by itself [52]. Beckmann et al. studied the oxygen formation catalyzed by two dinuclear manganese compounds, 7 and 8, with oxone (HSO5 − ) as the oxidant in an aqueous solution. They carried out a systematic comparison of these compounds with two other dinuclear manganese compounds, 4 and 6, and the tetranuclear manganese complex 9 to investigate the abilities of these complexes to act as water-oxidation catalysts under various reaction conditions [50,51]. A key observation from these studies is that the use of 18 O resulted in labeling percentages that agreed well with the 2 theoretically expected values if it is assumed that both O atoms originate from water. These observations were confirmed by 18 O isotope-labeling experiments combined with MIMS and EPR spectroscopy. Moreover, these observations have never been observed for a homogeneous manganese-mediated reaction. Complexes 7 and 8 underwent rapid decomposition under the strongly oxidizing and very acidic reaction conditions used. Based on the detected EPR signals and well-documented reports, these complexes likely produced a few byproducts (Mn2 III,IV (␮-O)2 , monomeric MnII or MnIV , CO2 Mn2 O3 ) by comproportion and decomposition (Fig. 6). By comparing the results of 18 O-labeling studies of compounds 4, 7, and 9, a strong proof of principle was obtained: in the case of complex 8, the evolved O2 originates from two molecules of water instead of from HSO5 − . When PbIV (AcO)4 is used as a two-electron oxidant, complex 8 can successfully produce O2 , whereas complexes 4 and 6 cannot. This result indicates that the initial phase of the oxygenforming reaction mediated by 8 requires a two-electron acceptor as the oxidant. 2.3. Tetranuclear manganese complex catalysts The first tetramanganese-oxo core complex 10 [L6 Mn4 O4 , L (p-Ph)2 PO2 − ] and the more soluble complexes 10’, 10 [L (pMePh)2 PO2 − or (p-MeOPh)2 PO2 − ] were developed by Dismukes et al. [53–55], the compound 10 self-assembles spontaneously from MnII and permanganate salts in high yield in non-aqueous solvents and the complex 10+ can be obtained by oxidation of 10 using electrochemical or chemical methods. These complexes can efficiently mimic the function of the active site of the OEC to release molecular O2 in the gas phase. They compared the O2 -evolving ability of these complexes with those of other manganese-oxo core complexes and found that the cubane core topology is uniquely suited for O2 evolution [56–58]. The photofragments of 10 and the origin of the oxygen atoms were studied using a laser desorption–ionization time-offlight mass spectrometer (LDI-TOF-MS) in conjunction with an 18 O-isotopomer experiment and using a quadrupole mass spectrometer interfaced to a Nd:YAG laser, respectively. The results of these experiments indicated that the compound 10 spontaneously release O2 and produce a Ph2 PO2 2− ligand, and thus form the open “butterfly” L5 Mn4 O2 2+ under UV illumination. The mechanism goes through a so-called “butterfly” intermediate, the O O bond was formed across two corners of the cubane, and two of the manganese ions pivot outward (Fig. 7). The mechanism

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O Py

N

Mn

N

18 18

H

O=O

O

II

H 18

Py O O O

H 2+ N

II

Mn

N

Py H

O

tBuOH

Py

tBuOOH

2H218O O

N

Py OO MnIII 18O MnIII O Py

Py

O

2+

Py

N

H2O

N

Py

N

O

MnIII

N

N

+

18

OH

Py

O

H218O 2+

O N N

Py O Py

MnIV Py

O 18

O

-O

Py

N

N

MnIV N

2+

O

tBuOH

O

tBuOOH N

Py

O MnIII 18O Py

Py

N

MnIII O Py

N

O

Fig. 5. A speculative mechanism for water oxidation by tBuOOH catalyzed by 6. Source: The graphic was adapted from reference [49].

involving the “butterfly” intermediate has recently been the subject of a theoretical study by Kuznetsov, Hill and Musaev et al. [60], the results from their calculations indicated that the formation of “butterfly” intermediate makes the Mn4 O4 -core more  ˚ compact and shortens the O1 O1 distance from 2.74 A˚ to 2.45 A,  and thus increases feasibility of O1 O1 bond formation. The “butterfly” intermediate continuously undergoes 2 two-electron oxidation/reduction process and its Mn4 -core transforms from    [Mn1 (III)-Mn1 III-Mn2 (IV)-Mn2 (IV)] to [Mn1 (II)-Mn1 (II)-Mn2 (III)   2 1 1 1 1 Mn (III)]. The O O single bond and O O double bond are formed in each two-electron oxidation/reduction process, respectively. Finally, the “butterfly” intermediate is transformed into an open “butterfly” complex (Fig. 7). The complexes 10 and 10+ also can undergo reduction with H atom donors like phenothiazine by a proton-coupled electron-transfer (4e− /4H+ and 5e− /4H+ ) reaction to form the “pinned-butterfly” compound and two water molecules in solution, which were confirmed by electrospray mass spectrometry and Fourier transform infrared (FTIR) spectroscopy [55,61,62]. Brimblecombe, Dismukes, Swiegers and Spiccia et al. found that TN of 10+ was more than 1000 and its maximum of TOF was approximately 0.075 s−1 at an overpotential of 0.38 V (vs. Ag/AgCl) under illumination when suspended in a proton-conducting membrane (Nafion) coated onto a conducting electrode. A photoelectrochemical system consisted with TiO2 /Ru(bipy)2 bipy(COO− )2 /Nafion/10+ and a Pt cathode can catalyze water oxidation using sunlight as the only source of energy [28,59,63,64]. The average peak photocurrent density was approximately 31 ␮A cm−2 and the TOF was about 0.013 s−1 . They suggest a self-repair mechanism involving photolytic disruption of a phosphinate ligand and evolution of O2 successively and then reassembling the cubane structure. Hocking, Spiccia et al. found that the above Mn cubane-type complex embedded in Nafion decomposed into Mn(II) compounds during the electrochemical investigation and catalytic cycling to form a

disordered Mn(III/IV) oxide phase [65]. These results were consistent with the data from in situ Mn K-edge XAS and transmission electron microscopy (TEM) experiments. This tetranuclear Mn cluster is a precursor of water-oxidation catalysts, and cycling between the Mn(II) compounds and the disordered Mn(III/IV) oxide phase plays a key role for in the water-oxidation catalysis. Both Spiccia and Dismukes’ group emphasized that the catalyst is self-healing during catalytic cycling similar to OEC–protein complex in nature. Cubane structural ruthenium complex 71, cobalt complex (shown in Fig. 21b) also display this kind of self-repair properties that will be discussed later.

3. Water-oxidation chemistry of ruthenium complexes Although ruthenium is not within the active site of the OEC of PSII, several explorations of ruthenium complexes as effective catalysts for water oxidation have been conducted. Significant progress has been reported in the development of molecular catalysts for water oxidation based on the mononuclear, dinuclear, and multinuclear ruthenium complexes in the last five years. In particular, mononuclear Ru complexes (Chart 2) have rapidly grown in importance in the catalog of water-oxidation catalysts. 3.1. Mononuclear ruthenium complex catalysts Three mononuclear Ru-OH2 complexes 11–13 were reported by Thummel et al. [66,67] in which the Ru atom was fixed in a slight torsion plane of 4-tert-butyl-2,6-di([1 ,8 ]-naphthyrid-2 -yl) pyridine and coordinated by two axial 4-substituted pyridine ligands. A single-crystal X-ray analysis of these complexes indicated that the water molecule coordinated to the central Ru metal was stabilized by hydrogen bonding between this water and one of the outer

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Fig. 6. Reaction pathways suggested for the reaction of compound 8 with HSO5 − or PbIV . Source: Reprinted with permission from [51]. © 2008 Royal Society of Chemistry.

N atoms of the naphthyridine moiety. The catalytic water-oxidation activities of the series of mononuclear Ru compounds were measured in aqueous solution (pH 1) using CeIV as the oxidant, and the catalyst with 4-methyl-pyridine as axial ligands exhibited a maximum TN of 260. Its first-order rate constant is 0.169 ␮mol/min. Subsequently, they prepared a series of mononuclear Ru complexes (14–24) and compared the catalytic water-oxidation activities of these complexes [68]. The results indicated that complex 20 exhibited the highest TN (1170) with a rate of 0.403 ␮mol/min, and complex 21 had the lowest TN (95) with a rate of 0.021 ␮mol/min. the kinetic studies of complex 23 indicated that the catalyst was not decomposed into ruthenium dioxide, this result was obtained from a comparison of kinetic plots of 23 and RuO2 .The rates of O2 evolution (catalyzed by complex 14) were observed as a function of the catalyst concentration, which indicated that the reaction is first order. This observation was supported using the sterically hindered complex 24. They tentatively proposed a unimolecular reaction mechanism (Fig. 8) with support from DFT calculations. According to this mechanism, after the catalyst had two electrons removed by CeIV and was attacked by H2 O, a seven-coordinate RuIV (H2 O) species was formed. This species lost two protons and two electrons, thereby providing a resonance-stabilized RuVI O species. Moreover, the RuVI OOH species was produced by an attack of water on the RuVI O species. The proton was then released, accompanied by the evolution of O2 . Sun et al. reported a mononuclear catalytic water-oxidation complex, 25 (RuII (pic)2 L, where L 2,2 -bipyridine-6,6 dicarboxylic acid anion) [69], that exhibited a high catalytic activity for water oxidation. A single-crystal X-ray analysis revealed that

25 possesses a distorted octahedral structure. A seven-coordinate intermediate with a hydroxide ligand coordinated to the Ru ion was detected by high-resolution mass spectrometry (HR-MS) measurements during the catalytic water oxidation by 25. The RuIV dimer complex with a bridging ligand [HOHOH] together with two water molecules in the form of a hydrogen-bonding network was observed by X-ray crystallography. The turnover number for the 25 catalytic water oxidation in the presence of CeIV as an oxidant (pH 1, [25] = 2.0 × 10−6 M) was estimated to be approximately 120, and the second-order rate constant was 7.83 × 105 M−1 s−1 . The results of DFT calculations suggested that a bimolecular pathway was involved during the catalytic O2 evolution by 25 [70]. The mononuclear ruthenium complexes 26–28, RuII (pda)L2 , were sequentially reported [71] in which the 2,2 -bipyridine-6,6 -dicarboxylate ligand (flexible) was replaced with a 1,10-phenanthroline-2,9dicarboxylate ligand (rigid). Unlike complex 25, complexes 26–28 exhibited small structural changes, which made a significant difference in their mechanisms of CeIV -driven catalytic water oxidation. Based on the results of experiments and calculations, 26–28 utilize a mononuclear reaction mechanism instead of a bimolecular mechanism, and their catalytic stabilities are stronger than that of complex 25. The TNs of 26, 27 and 28 are 336, 310 and 190, respectively, after 6 h of catalytic water oxidation; additionally, initial turnover frequencies (0.092, 0.102 and 0.040 s−1 , respectively) were obtained. A study of visible light-driven water oxidation using 25, Ru(bpy)3 2+ , and the sacrificial electron acceptors [Co(NH3 )5 Cl]Cl2 or Na2 S2 O8 demonstrated that the TOFmax (maximum turnover frequency) was over 0.15 s−1 and 0.35 s−1 for the systems of [Co(NH3 )5 Cl]Cl2 and Na2 S2 O8 , respectively [72].

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+H atom

Mn

Mn

-2H2O

O Mn O Mn

10-3H

+H atom 10-2H

"Pinned butterfly" 1 Ligand

+H atom Mn

Mn

+

O Mn O Mn

H O Mn Mn O O Mn O Mn

"Open "butterfly" O2

O O Mn Mn O O Mn

Mn

10-H

+H atom

hν +

Mn1

hν

O2

O1' Mn1' O1 2' 2' Mn O Mn2

1 Ligand

H+

-e-

+

10

"butterfly"

P

O Mn O O Mn O Mn

Mn

10+

(L = -O2PPh2)

O O

(L' = -O2P(p-MePh)2)

= P O O

P O O

(L'' = -O2P(pMeOPh)2)

Fig. 7. Reaction pathways and a possible photocatalytic cycle for 10. Observed reduction reactions of 10 and 10+ in solution and the gas-phase photodissociation to yield O2 . Reverse arrows show the proposed reoxidation steps that regenerate the cubanes. Source: The graphic was adapted from reference [59].

Complexes 29 and 30 [RuL(pic)3 and RuL(bpy)(pic), respectively, where H2 L 2,6-pyridinedicarboxylic acid], with a negatively charged tridentate ligand, were also synthesized for studies of the chemical and photochemical catalytic water oxidation [73]. Complex 29 displayed a higher TOF (0.23 s−1 ) than the other ruthenium-based water-oxidation catalysts reported (with CeIV as the oxidant, pH 1.0). Complexes 31–34 were also active for the light- or CeIV -driven catalytic water oxidation [74]. Meyer et al. reported a family of mononuclear Ru complexes 35–46 [25,75–79] containing tpy, Mebimpy, and other ligands in this series. This family includes complexes such as [Ru(tpy)(bpm)(OH2 )]2+ , [Ru(tpy)(bpz)(OH2 )]2+ , [Ru(Mebimpy)(bpy)(OH2 )]2+ , [Ru(Mebimpy)(bpm)(OH2 )]2+ , and [Ru(Mebimpy)-(bpz)(OH2 )]2+ . The detailed water-oxidation mechanism of one of these complexes ([Ru(tpy)(bpm)(OH2 )]2+ ), which is one of the most well-defined pathways determined for a homogeneous water-oxidation catalyst to date, was studied. The results of their study indicated that these complexes are promising, robust single-site water-oxidation catalysts.

The detailed mechanisms of single-site water-oxidation catalysts were elucidated via stopped-flow experiments in conjunction with spectrophotometric monitoring and DFT calculations of intermediate structures (Fig. 9) [75,76]. The kinetic data are listed in Table 1. The key O O bond-forming step was a nucleophilic Table 1 Rate constants at 298 K in 0.1 M HNO3 (k1 –k4 ) and 1.0 M HNO3 (k5 ) for [Ru(tpy)(bpm)(OH2 )]2+ in the presence of CeIV . Rate constant

Value

Reaction

k1

2.4 × 102 M−1 S−1

RuII –OH2 2+

k2

5.0 M−1 S−1

RuIV = OH2 2+ −→RuV = O3+

k0–0

9.6 × 10−3 S−1

RuV O3+ + H2 O−→RuIII –OOH2+

k4 k5

−4

7.4 × 10 −1

10 M

−1

S

−1

S

Reprinted with permission from [76].

−→

−e−

RuIV = OH2 2+

−H+

RuIII

k3

−2e− ,−2H+

Ru

IV

Ru

IV

+ − 2+ −e ,−H

−→

OOH

2+

RuIV (OO)

− 2+ +H2 O −O2

−→

(OO)

2+ −e

(OO)

−

V

Ru

II 3+

−→ Ru (OO)

OH2 2+

X. Liu, F. Wang / Coordination Chemistry Reviews 256 (2012) 1115–1136

1123

Chart 2. Structures of mononuclear ruthenium catalysts.

attack of water on the [RuV O]3+ moiety with a rate constant of 9.6 × 10−3 s−1 . This [RuV O]3+ species was formed by continuous oxidation from [RuII -OH2 ]2+ to [RuIV O]2+ accompanied by the loss of two electrons and two protons to avoid charge buildup. The [RuIV O]2+ was then oxidized using CeIV as an oxidant.

[RuV O]3+ + H2 O → RuIII -O2 H + H+

(3)

The existence of the RuIII -O2 H intermediate (Eq. 3) was demonstrated by an experiment involving the redox titration of (NH4 )2 Fe(SO4 )2 combined with spectrophotometric monitoring. However, this intermediate was unstable, similar to RuIII -OH2+ , due to a disproportionation reaction. Kinetics studies demonstrated that RuIII -O2 H undergoes a rapid oxidation accompanied by the loss of one electron and proton (RuIII -O2 H → [RuIV (OO)]2+ ), followed by a competition between the first-order decomposition

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X. Liu, F. Wang / Coordination Chemistry Reviews 256 (2012) 1115–1136

Chart 2. ( Continued ).

of [RuIV (OO)]2+ and its further oxidation to [RuV (OO)]3+ with rate constants of 7.4 × 10−4 s−1 and 10 M−1 s−1 , respectively. The ratelimiting reaction for the catalytic water oxidation depends on the acid concentration under catalytic conditions. The former reaction is rate-limiting in 0.1 M HNO3 ; however, in 1.0 M HNO3 , the latter reaction determines the rate of water oxidation. David Milstein et al. reported the mononuclear Ru(II) pincer complex 47, which possesses a distorted octahedral coordination

geometry at the ruthenium and can evolve H2 and O2 in consecutive thermal- and light-driven steps [80]. A mononuclear aromatic Ru(II) hydrido–hydroxo complex (Fig. 10) was generated by a reaction of a de-aromatized RuII pincer complex with H2 O at room temperature, and its structure was elucidated by 1 H NMR, 17 O NMR, 31 P NMR, and X-ray diffraction analyses. This species can then react with H2 O at 100 ◦ C to form a cisdihydroxo complex accompanied with the evolution of H2 . The

X. Liu, F. Wang / Coordination Chemistry Reviews 256 (2012) 1115–1136

1125

Fig. 8. Proposed water-oxidation mechanism of 12. Source: Reprinted with permission from [68]. © 2008 American Chemical Society.

dihydroxo complex can be prepared independently by a reaction of the above hydrido–hydroxo complex with N2 O with a 60% yield. Moreover, 31 P NMR, 13 C NMR, IR, mass-spectrum and elemental analyses were utilized to characterize the structure of this dihydroxo complex. Finally, O2 can be released by photolysis of the cis-dihydroxo complex in the 320–420 nm range to generate the initial RuII hydrido–hydroxo complex. The O2 and H2 were detected via gas chromatography–mass spectrometry (GC–MS) of samples obtained from a reaction of a sample of the gases with (PEt3 )3 IrCl to form (PEt3 )3 Ir(O2 )Cl and (PEt3 )3 Ir(H)2 Cl, which is a highly sensitive method that is rarely used by other research groups detecting O2 and H2 . Hence, to verify the oxygen sources, an 18 O-labeled dihydroxo complex (18 O18 O) was prepared, and the irradiation of the compound was performed in H2 16 O. The analysis of the GC–MS data indicated that the major dioxygen product was 18 O 18 O, indicating that the exchange between the hydroxide of the dihydroxo complex and H2 O did not occur and that the oxygen in the generated O2 is from the dihydroxo complex rather than from the solvent. However, the question of how the O O bond formed remains unanswered. A mixed-labeled dihydroxo complex (18 O16 O) was generated by the reaction of the 18 O-labeled RuII hydrido–hydroxo complex with N2 16 O, and upon photolysis, the observed 16 O–16 O/16 O–18 O/18 O–18 O ratio was 3.8/16.2/1. In

contrast, a crossover experiment involving the photolysis of equimolar amounts of the 18 O16 O and 18 O18 O dihydroxo complexes produced a mixture of dioxygen products with a 16 O–16 O/16 O–18 O/18 O–18 O ratio of 13.1/1/15.6. Combining the above two experiments, an unambiguous conclusion was obtained that the O–O bond formation process in this system was intramolecular rather than intermolecular. Finally, they concluded that hydrogen peroxide was formed from the dihydroxo complex, which produced O2 and H2 O itself through a disproportionation reaction. These authors ruled out the possibility of a hydroxyl radical as an oxygen source by conducting experiments to detect the hydroxyl radical using the spin trap 5,5-dimethyl-pyrroline1-oxide (DMPO) and irradiating the dihydroxo complex in the presence of the enzyme catalase. Eisenberg, Hammarström, and Styring individually commented on this new catalyst [81,82]. The mechanisms of water-splitting and oxygen oxygen bond formation were also studied computationally by Hall et al. [83]. Berlinguette et al. reported a series of mononuclear polypyridyl Ru water-oxidation catalysts (48–58) to explore how the electron density of the metal affects the catalyst activity and stability [84,85]. They concluded that electron-withdrawing groups on the bpy ligands increased the TN and decreased the catalytic activity. Conversely, the presence of electron-donating groups decreased

Fig. 9. Proposed single-site water-oxidation mechanism of 36. Source: Reprinted with permission from [76]. © 2010 American Chemical Society.

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X. Liu, F. Wang / Coordination Chemistry Reviews 256 (2012) 1115–1136

Fig. 10. Proposed mechanism for the formation of H2 and O2 catalyzed by 47. Source: Reprinted with permission from [80]. © 2009 American Association for the Advancement of Science.

the catalyst stability and enhanced the catalytic rates. To evaluate the mechanistic details of the CeIV -driven oxidation of water mediated by the above catalysts, complexes 35, 57 and 58 were chosen as samples for studies of the catalytic activity, reaction kinetics, and the source of O-atoms, respectively. Complexes 57 and 58 contain more electron-donating (–OMe) and electron-drawing (COOH) substituents on the bpy ligands compared with 35, which has been reported by Masaoka and Sakai [86]. From the view of the reaction mechanism, an “acid–base” mechanism was largely followed [76,87]. However, a discrepancy occurred at the point of the [RuIV O]2+ species (Fig. 11), and a disproportionation pathway for 58 (kd = 1.2 M−1 s−1 ) was initiated to form [RuV O]3+ and [RuIII OH]2+ . For complex 57, the formation of [RuV O]3+ (k3 = 3.7 M−1 s−1 ) was much faster than that of [RuIII OOH]2+ (ko o = 3 × 10−5 s−1 ) when 1 eq of CAN was added to [RuIV O]2+ . Moreover, under the same conditions, complexes 35 and 58 exhibited slow reactivity. The rates of the first two PCET steps, k1 and k2 , were determined via stopped-flow spectroscopic techniques, and complex 58 was observed to have the fastest rate (k1 > 106 M−1 s−1 for 58, k1 = 4.4 × 104 M−1 s−1 , 1.7 × 105 M−1 s−1 for 35 and 57, respectively, and k2 = 6.6 × 103 M−1 s−1 , 4.6 × 106 M−1 s−1 , and 3.3 × 105 M−1 s−1 for 35, 57 and 58, respectively). An oxygentransfer pathway (kOAT ) was also investigated, wherein the higher oxidation state [RuV O]3+ species abstracted an oxygen atom from CAN to form [RuIV OO]2+ . Yagi et al. reported a series of mono-ruthenium complexes (59–62) for studying catalytic water oxidation [88]. Complex 60 was observed to have a higher turnover frequency (kO2 = 0.11 s−1 ) and TN of 690. For the mononuclear ruthenium compounds 35–46 reported by Meyer’s group, 48–58 reported by Berlinguette’s group and 59–62 reported by Yagi’s group, RuV O species play a key role for homogeneous catalytic formation of O O bond from water.

Fig. 11. Summary of the (possible) competing reaction pathways for 35, 57 and 58 describing relevant (proton-coupled) electron-transfer steps, O O bond formation, exclusion of dioxygen, and substrate binding (L) coordinating ligand; e.g., Cl− , MeCN; (R = H, 35; OMe, 57; CO2 H, 58). Rate-determining steps (RDSs) for 35 and 58 (RDS), and 57 (RDS ) at pH 1 are indicated. Source: Reprinted with permission from [85]. © 2010 American Chemical Society.

X. Liu, F. Wang / Coordination Chemistry Reviews 256 (2012) 1115–1136

Collin and Sauvage et al. synthesized mononuclear RuII complexes of sterically hindering diimine chelates [89], which were used to catalyze water oxidation by preventing the formation of inactive trans-isomer complexes. However, no O2 evolution was observed in the presence of excess CeIV . Kaneko et al. reported two mononuclear ruthenium-ammine complexes, Ru(NH3 )5 Cl2+ and Ru(NH3 )5 (H2 O)3+ , and studied the water-oxidation activities of the two complexes in the presence of excess CeIV [90]. Their TNs can approximately reach 12 and 15 in 90 min, respectively. Interestingly, the change in the pH of the reaction medium did not significantly affect the O2 production. The studies of the water-oxidation mechanism were conducted in detail and suggested that the Ru O O Ru bonded intermediate was formed by the reaction of two mononuclear RuV complexes and then released one molecule of oxygen. Taqui-Khan et al. reported four mononuclear RuIII complexes, which are represented by [RuIII (H-dmg)2 XY]n (dmg = dimethylglyoximato; axial ligand: X = Y = Cl− or ClO4 − (n = −1); X = Cl− , Y = imidazole or 2-methylimidazole (n = 0)) [91]. Electrochemical studies demonstrated that these compounds can undergo a four-electron redox change from RuIII to RuVII , involving steps with one electron (RuIV /RuIII ), two electrons (RuIV /RuVI ) and one electron (RuVI /RuVII ). The chemical and electrochemical oxidation of water was studied in 1 M HClO4 − with an excess of CAN. The compound containing 2-methylimidazole had the highest TNs, which were 7.6 and 21 in the chemical and electrochemical water oxidation, respectively.

3.2. Dinuclear ruthenium complex catalysts Dinuclear ruthenium complexes used for catalytic water oxidation (Chart 3) are among the most widely studied catalysts in recent decades. The first example of a well-defined molecular catalyst is 63, the -oxo-bridged cis,cis-[(bpy)2 (H2 O)RuORu(H2 O)(bpy)2 ]4+ complex reported by Meyer et al. in 1982 and referred to as the “blue dimer” [92]. The turnover frequency and turnover number of 63 were 0.0042 s−1 and 13.2, respectively. To clarify the water-oxidation mechanisms of this catalyst, the blue dimer and its structurally related derivatives have been studied in detail over the past three decades [93–110]. Meyer et al. emphasized that PCET could be the key to catalytic water oxidation by the blue dimer [107,111] (Chart 3) which allows many oxidative equivalents to accumulate sequentially at one site without the buildup of highly charged species. The loss of catalytic activity can be attributed primarily to anation induced by O2 evolution. The 18 O-labeled and global kinetic analyses of the blue dimer were conducted in a strongly acidic solution (pH ∼ 0–1) in the presence of CeIV [100,101]. As expected, the results suggested that the mechanism of water oxidation by the blue dimer is highly complex (Fig. 12). This mechanism involved not only multiple PCET reactions, higher oxidation states of the Ru-O-Ru core, and cross-electron transfer between non-adjacent oxidation states but also different intermediate acid–base equilibrium reactions and some anion exchanges between the surrounding solution and the coordinated water molecules. However, the catalytically active species (RuV ORuV ) in solution was not observed by optical or EPR techniques. Meyer et al. studied a black suspension in frozen solutions at 77 K and tentatively attributed the peaks that appeared at 816 cm−1 for (RuV O) and at approximately 357 cm−1 for sym (Ru–O–Ru) in the resonance Raman spectra to a RuV ORuV species. The results of the kinetic analysis suggested that RuIV ORuIV was an unstable oxidation state and that the rate of its disproportionation (k-3 ≥ 105 ) into RuIV ORuIII and RuV ORuIV was much faster than the formation of RuIV ORuIV (k3 ∼ 7.3 × 103 ). The oxidation of RuIV ORuIV in

1127

Fig. 12. Proposed mechanism for oxidative activation and water oxidation by the blue dimer at 23 ± 2 ◦ C. Source: Reprinted with permission from [107]. © 2008 American Chemical Society.

the presence of CeIV is also much faster (k4 ≥ 105 ) (Scheme 2). The 18 O-labeled experiment suggested the presence of 13% 18 O18 O, 64% 16 O18 O, and 23% 16 O16 O if no exchanges in the oxygen atoms located at the ␮-oxo-bridge, solvent, and Ru O occurred with O2 during the oxidative reaction. A conclusion about the sources of the oxygen atoms can be deduced from the different isotope proportions in the evolved O2 . Three different cases exist: (1) both O atoms are from the solvent to result in 23% 16 O16 O; (2) one O atom comes from Ru O, and the other comes from the solvent to result in 64% 16 O18 O; and (3) both O atoms are derived from Ru O to result in 13% 18 O18 O. Hence, different water-oxidation mechanisms correspond to these different cases. For the 64% 16 O18 O case, a proposed mechanism (Fig. 12) for the formation of a RuV ORuV species and the oxidative activation in 0.1 M HNO3 were derived from the results of kinetic studies. The pathway leading to oxygen–oxygen bond formation is thought to involve the nucleophilic attack of a solvent water molecule on a formal RuV O group, accompanied by a PCET step, to form a peroxidic intermediate in which the RuIII ORuIII is recovered concomitant with the evolution of O2 . For the 13% 18 O18 O

(a)

RuIIIORuIII

CeIV

(b)

RuIVORuIII

CeIV

k1 (2 × 104)

RuIVORuIII

CeIII

RuIVORuIV

CeIII

k2 (15)

k3 (7.3 × 10 ) 3

(c)

RuIVORuIII

RuVORuIV

2 RuIVORuIV

k-3 (≥ 10 ) 5

(d)

RuIVORuIV

CeIV

(e)

RuVORuIV

CeIV

(f)

2H2O RuVORuV (first order)

k4 (≥ 105) k4 (≥ 102)

(f') 2RuVORuV 2H2O (second order) (g)

RuIIIORuIII

(h)

2RuVORuIV

k6 (≥ 1S-1)

RuVORuIV

CeIII

RuVORuV

CeIII

RuIIIORuIII

O2

k6' (>2 × 104)

RuVORuIV k8 (22) k-8(<22)

RuIVORuIII

k7 (≥ 106)

RuIVORuIV

RuVORuIV

RuIVORuIII

O2

RuIVORuIV

RuVORuV

Scheme 2. The stepwise oxidation and rate constants of complex 63 catalytic water oxidation when CeIV is used as an oxidant. Reprinted with permission from [101]. © 2000 American Chemical Society.

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X. Liu, F. Wang / Coordination Chemistry Reviews 256 (2012) 1115–1136

Chart 3. Structures of dinuclear ruthenium water-oxidation catalysts.

case, a water-oxidation mechanism involving an intramolecular coupling or a bimolecular Ru-O· · ·O-Ru interaction was proposed. Hurst et al. also studied the mechanism of water oxidation by the blue dimer via 18 O-isotope-labeling, resonance Raman spectroscopy, cryogenic electron paramagnetic resonance, and “real-time” mass spectrometry [102–106,108–110]. They found that the 18 O16 O/16 O16 O ratio is approximately 0.40/0.60 in their 18 O labeling. Traces of 18 O18 O indicate that an intramolecular coupling or a bimolecular Ru–O· · ·O–Ru interaction will not occur. Finally, for the 60% 16 O16 O case, a previous study suggested that this pathway involved the nucleophilic attack of H2 O on an electrophilic RuV O moiety (Fig. 13, Left). Meyer et al. suggested a similar possible mechanism involving a bridging ozonide dianion (O3 2− ) by analogy with the formation of hydrogen trioxide (H2 O3 ) from the combination of perhydroxyl (HO2 · ) and hydroxyl (OH· ) radicals. A subsequent attack by the solvent on the central O atom could then yield O2 in which both O atoms are obtained from solvent. Based on the EPR signal, the NIR absorbance, and a comparison of a similar reaction of Ru (bpy)3 3+ and OH- , Hurst et al. proposed a completely different bipyridine ligand-based mechanism in which the ligand (bipyridine) of the catalyst is attacked by the OH− of the solvent to form an intermediate (L2 RuV (O)ORuIV (OH) (LOH)L4+ ) that could generate endoperoxides and further form unstable dioxetanes to evolve O2 (Fig. 13, Right). Density functional theory calculations were also performed to further understand the possible reaction mechanisms involved in the formation of oxygen [112]. Although water oxidation by the blue dimer is not yet understood completely and there are differing interpretations of the mechanism, a general consensus exists that the catalytically active species is the RuV ORuV species formed by the four-electron oxidation of RuIII ORuIII , which undergoes first-order decay with the concomitant release of O2 .

In 2004, Llobet et al. reported a new water-oxidation catalyst, 64 ({[RuII (trpy)(H2 O)]2 (-bpp)}3+ , which differed from the blue dimer in its structure. The oxo bridge was replaced by a rigid pyrazolate ligand, and the two metal centers were much closer. This new catalyst was called the “bpp dimer” [113]. The pH-dependence of E1/2 for the oxidative couples of the blue and bpp dimers were studied over a broad pH range by Meyer et al. and Llobet et al., respectively [93,113]. The species with the lowest oxidation state is RuII RuII in the bpp dimer at pH 1 in a CF3 COOH solution instead of RuIII ORuIII , which was the case for the blue dimer. The highest oxidation states involved in the O2 evolution are the RuIV RuIV and RuV ORuV in blue and bpp dimer catalysts, respectively. The composition of protons in the two catalysts differed with the varying pH values and oxidation states. The mechanism of water oxidation was studied using complex 64 [26,114–118]. The results indicate that the dimer was oxidized with CeIV in 0.1 M triflic acid and reached the highest oxidation state of RuIV RuIV quickly. Two pathways, called the “intramolecular O O bond formation” (Fig. 14, Right) and “H2 O nucleophilic attack” (Fig. 14, Left) pathways, were observed to result in O2 evolution, as deduced from a thorough kinetic analysis combined with 18 O-labeling experiments. Thummel et al. reported a series of water-oxidation catalysts (65–67) and their derivatives [66,119], which differ structurally from the above catalysts. Two RuII ions were coordinated by bistridentate rigid bridging ligands and four axial pyridine ligands or its 4-substituted derivatives. The rates of oxygen production and the turnover number (TN) were studied using gas chromatography (GC) analysis combined with an optical oxygen sensor in the presence of excess CeIV as a sacrificial oxidant at pH 1. The results indicated that the systems having 4-methoxypyridine as an axial ligand appeared to be more active than other related species. The turnover number and the rate of oxygen production for the most active catalysts were 689 and 0.924 ␮mol/min, respectively.

X. Liu, F. Wang / Coordination Chemistry Reviews 256 (2012) 1115–1136

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Fig. 13. Pathway for O2 formation from two solvent molecules (Left) based on the formation of a bridging ozonide ligand or (Right) based on the covalent hydration of a bipyridine ligand. Source: Reprinted with permission from [108]. © 2008 American Chemical Society.

However, the details of the mechanism of these catalytic water oxidations were not described. In 2000, Tanaka et al. reported the dinuclear complex 68, [Ru2 II (OH)2 (3,6-tBu2 qui)2 (btpyan)](SbF2 )2 [120], which could effectively catalyze the oxidation of water via a modification of the indium-tin-oxide (ITO) electrode in water. They studied the acid–base equilibrium and redox properties of the catalyst and the analogous species [Ru2 II (OH)2 (bpy)2 (btpyan)](SbF2 )2 and compared their catalytic activities [121,122]. They proposed an electrochemical water-oxidation mechanism of [Ru2 II (OH)2 (3,6tBu2 qui)2 (btpyan)](SbF2 )2 in which the two quinone ligands played a key role because these ligands could promote the formation of [RuII (O)(SQ)RuII (O)(SQ)]0 and [RuII (O)(Q)RuII (O)(Q)]2+ by transforming a quinone and semiquinone into each other accompanied with the loss of two protons from the hydroxide of [RuII (OH)(Q)RuII (OH)(Q)]2+ and the oxidation of two electrons on

the semiquinones of [RuII (O)(SQ)RuII (O)(SQ)]0 . The metal-centered oxidation resulted in the [RuIII (O)(Q)RuIII (O)(Q)]2+ species, which was viewed as the activated site of O O formation. O2 was generated, and H2 O was added to the RuIII core simultaneously. The resulting [RuIII (OH2 )(Q)RuIII (OH2 )(Q)]2+ species was converted to the original [Ru2 II (OH)2 (3,6-tBu2 qui)2 (btpyan)](SbF2 )2 after the loss of two protons. The electrochemical activation of the catalyst resulted in a TN of 33,500 after 40 h of operation. Sun et al. reported two dinuclear ruthenium complexes, 69 and 70, with four axial 4-methylpyridine ligands [123,124] that are similar in structure to the complexes reported by Thummel et al. However, the neutral ligands were replaced with negatively charged dicarboxylate ligands to lower the oxidation potentials of the catalysts. In complex 69 (trans), the two Ru ions occupied positions on opposite sides of the central pyridazine ring. No ␮Cl bridge was present in the molecule, which resulted in a TN of

Fig. 14. Potential water-oxidation pathways for complex 64. Left: water nucleophilic attack. Right: interaction of two M O units. Trpy ligands are not shown. Source: Reprinted with permission from [118]. © 2009 American Chemical Society.

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approximately 1700 and a rate constant of 2.6 × 10−4 s−1 (pH 1) in the presence of a high concentration CeIV (330 mM). Conversely, 70 has a symmetric cis structure with a ␮-Cl bridge and can be considered the most stable and most active ruthenium-based catalyst for water oxidation reported to date. Its TN is 3540 under the conditions described above. Moreover, its TN can reach more than 10,000 in the presence of CeIV (5 mM, pH 1) with a high TOF value of 1.2 s−1 . The visible light-driven water oxidation of 69 was also studied in a system involving Ru(bpy)3 2+ as a photosensitizer and [Co(NH3 )5 Cl]Cl2 or Na2 S2 O8 as an electron acceptor. A TN of 1270 was obtained under these conditions [125]. In the 1980s, Grätzel et al. reported a series of dinuclear oxoruthenium complexes [95–97] that have a structure similar to that of the blue dimer except that 2,2 -bipyridine was replaced with 4,4 dicarboxy-2,2 -bipyridine and 5,5 -dicarboxy-2,2 -bipyridine. The electron-withdrawing nature of the carboxylic acid groups had a dramatic effect on the redox potential of ruthenium and caused the water oxidation to be thermodynamically more feasible. In 1992, Elliot et al. reported a series of Ru-oxo dimers with saturated alkyl bridges between bipyridines [98]. In their study, the flexible alkyl bridges were designed to increase the stability of the molecules. The water-oxidation capabilities of these complexes were similar to those of the blue dimer and its derivatives. In 1995, Wong et al. reported two oxo-bridged Ru complexes [99] that differed slightly in structure from the blue dimer. They replaced 2,2 -bipyridine with 4,4 -dichloro- and 5,5 -dichloro-2,2 -bipyridine to facilitate the solvent nucleophilic attack on the terminal oxygen atoms. The two dimers in the catalytic oxidation of water were more stable than the blue dimer in the electrolysis experiment; however, their catalytic activities toward the oxidation of water to molecular dioxygen were very similar to those of the blue dimer.

3.3. Multinuclear ruthenium complex catalysts A trinuclear Ru complex [(NH3 )5 Ru-O-Ru(NH3 )4 -ORu(NH3 )5 ]Cl6 (Ru-red) was developed as a homogeneous water-oxidation catalyst by Yagi, Kaneko et al. [126,127], its TN and TOF were 75 and 0.051 s−1 at lower catalyst’s concentration. However, Ru-red decomposed to a mononuclear Ru species at a higher catalyst’s concentration during the catalytic procedure and produced N2 by oxidation of the ammine ligands. Most of transition metal complexes with organic ligands used the water-oxidation catalysts were often deactivated due to their decomposition at a higher catalyst concentration. In this review several catalysts only display a higher TOFs at an extremely low concentration (approximately 10−6 –10−7 M). Currently, homogeneous and heterogeneous water-oxidation catalysts have complementary strengths and weaknesses. In some heterogeneous water-oxidation catalysts, the metal oxide/hydroxide catalysts are usually both more robust. In contrast, most of the homogeneous water-oxidation catalysts are readily studied, optimized, and formulated, but they have unstable organic ligands. Polyoxometalates (POMs) including the tetraruthenium(IV)-oxo core are considered very distinctive water-oxidation catalysts because they display the structure of a metal oxide/hydroxide of the heterogeneous catalysts without organic ligands. Additionally, they have the advantages of the aforementioned homogeneous water-oxidation catalysts. Bonchio et al. reported a polyoxometalate (POM) embedding of the tetraruthenium(IV)-oxo core in complex 71, Cs10 [Ru4 (␮O)4 (␮-OH)2 (H2 O)4 (-SiW10 O36 )2 ] [128], which was produced by the reaction of K4 Ru2 OCl10 with K8 -SiW10 O36 ·12H2 O followed by the addition of excess CsCl (Scheme 3). POM metalation is fostered by the in situ generation of the tetranuclear [Ru4 O6 (H2 O)n]4+ , In situ formation also implies a self-repair mechanism. An XRD

Scheme 3. Metalation [Ru4 O6 (H2 O)n ]4+ .

of

SiW10

by

complementary

Lego

Assembly

of

Reprinted with permission from [128]. © 2008 American Chemical Society.

analysis indicated that two -SiW10 units were connected by a central core of [Ru4 (␮-O)4 (␮-OH)2 (H2 O)4 ]6+ , thereby forming a skewed dimeric structure. Four ruthenium and six oxygen atoms were located at the apices of a tetrahedron and an octahedron. In the central core, the Ru-O-Ru bond angle was 131.2(9)◦ and ˚ This type the average O O distance was approximately 2.89(3) A. of POM structure is stable in aqueous solution, as confirmed by ESI-MS, IR, and UV–vis spectroscopies. The experiment in water was performed using the lithium salt complex Li10 [Ru4 (␮-O)4 (␮OH)2 (H2 O)4 (-SiW10 O36 )2 ] as a catalyst and (NH4 )2 Ce(NO3 )6 as an oxidant in 10 ml H2 O (pH 0.6) at 20 ◦ C. Under this condition, the TN reached 90 in 2 h, and the maximum turnover frequency was 0.13 s−1 , corresponding to an overall 90% yield based on the added CeIV oxidant. The kinetics of O2 evolution catalyzed by the compound was studied over a concentration range from 0.05 to 1.45 ␮mol with CeIV (10.9 mmol) in 10 ml of water at 20 ◦ C. The initial rate of O2 evolution from the catalyst is linearly dependent with a pseudo-first-order kinetic constant of 9.92 × 10−3 s−1 . Under these conditions, the maximum TN is 500 in 60 h. pH titrations analyzed via UV–vis spectroscopy and an acid–base titration of the catalyst showed that the Ru4 O4 (OH)2 (H2 O)4 core complex undergoes a reversible monoprotonation equilibrium with a pKa of 3.62 in aqueous solution. In addition, the acid–base equilibrium is not concentration dependent, thereby ruling out POM dissociation or aggregation phenomena. Bonchio et al. also proposed a mechanism including the nucleophilic attack of water on the highvalent RuV O species to result in O O bond formation during the catalytic water oxidation, which was based on results from cyclic voltammetry (CV), UV–vis, resonance Raman, electron paramagnetic resonance (EPR), and DFT calculations [129]. Electrochemical studies indicated that the Ru4 core experienced a stepwise oxidation from S0 to S4 (S0 to S4 is an analogy to the OEC of PSII) and that the oxidation state changed from Ru4 {IV, IV, IV, IV} to Ru4 {V, IV, IV, IV} to Ru4 {V, V, IV, IV} to Ru4 {V, V, V, IV} to Ru4 {V, V, V, V} (Scheme 4). Electrocatalytic O2 evolution was observed at E = 1.15 V. In the IR spectra (10−2 M in H2 O at pH 0.6, 250–600 cm−1 , ex = 488 nm), the 456 cm−1 feature arises from a vibration within the Ru4 core. New components with frequencies greater than 500 cm−1 were produced together with PCET (4e− /4H+ ) after the addition of CeIV . The above catalytic pathway vividly mimics water oxidation in nature. Light-induced water-oxidation catalyzed by 71 was also investigated in homogeneous solution and on a sensitized nanocrystalline TiO2 surface with [Ru(bpy)3 ]2+ or its derivative as a photosensitizer and Na2 S2 O8 or TiO2 as an electron acceptor

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and 10%) experiment demonstrated that the oxygen atoms in the evolved O2 were from H2 O. 2S2 O8 2− + 2H2 O + 2h → 4SO4 2− + O2 + 4H+ 4Ru(bpy)3

Scheme 4. Stepwise transformation of the POM-embedded Ru4 core in 71 along the S0 –S4 oxidation states during oxygen-evolving catalysis. The structure of the POM ligand is omitted for clarity. Reprinted with permission from [129]. © 2009 American Chemical Society.

[130]. In both cases, the holes of photoinduced [Ru(bpy)3 ]3+ were removed by 71, and a clear acceleration of [Ru(bpy)3 ]2+ recovery (ns timescale) was observed compared with the electron-hole recombination between [Ru(bpy)3 ]3+ and Na2 S2 O8 or TiO2 according to the results of time-resolved spectroscopy. This feature is favorable for water-splitting photochemical devices. When a tetranuclear dendrimer is used as a photosensitizer system, 71 can reach a photon-to-oxygen top-record quantum yield, ˚ = 0.30, at an excitation wavelength of 550 nm [131]. Hill et al. simultaneously reported the same [Ru4 (␮-O)4 (␮OH)2 (H2 O)4 ]6+ core complex 72, Rb8 K2 [{Ru4 O4 (OH)2 (H2 O)4 }(␥SiW10 O36 )2 ]·25H2 O, an oxidative and hydrolytically stable complex [132]. A slight difference was evident in the synthetic procedure compared with that reported by Bonchio because RuCl3 ·H2 O was used as a starting material instead of K4 Ru2 OCl10 . The Raman spectra of the complex (0.15 mM) in water with e = 1064 nm exhibited an intense band at 487 cm−1 . This vibration is assigned to a symmetric Ru-O-Ru mode. The experimental data from magnetic susceptibility and EPR experiments, the rest potentials from the cyclic voltammetry experiments, the elemental analysis, and the bond valence sums are consistent with a RuIV core. Notably, in the study of pH titrations using UV–vis spectra and the acid–base titration, two pKa values in the pH range 3.5–4.5 were observed in this catalyst instead of the one pKa observed by Bonchio. A thermogravimetric analysis indicated that the POM structure of the catalyst was stable even to 450 ◦ C with loss of water. The results of the cyclic voltammograms showed that this catalyst had two RuV/IV couples with their oxidation peaks at approximately 940 and 1050 mV and four RuIV/III couples with reduction peaks at approximately 530, 370, −170 and −350 mV (pH 1). The current of 950–1050 mV was several-fold higher (0.6 mM, pH 7), which is consistent with electrocatalytic evolution of O2 . This unusually low potential indicates that the complex has potential as a catalyst for homogeneous water oxidation in an aqueous solution. The investigation of water oxidation catalyzed by this compound according to the Eqs. (4) and (5) was conducted under ambient conditions at pH 7 (phosphate buffer). The kinetics for this catalytic reaction were monitored spectrophotometrically using the accumulation of Ru(bpy)3 2+ , which obeys a non-exponential rate law. The evolution of O2 was typically completed in a considerably shorter reaction time (30–40 s). The reaction of 12 ␮mol of Ru(bpy)3 3+ with 0.1 ␮mol of the catalyst in a quartz UV–vis cell generated 10.7 ␮mol Ru(bpy)3 2+ and 18 TN of O2 , which are yields of approximately 90% and 66%, respectively, based on Ru(bpy)3 3+ . The possibility that RuO2 catalytically oxidized water was precluded by conducting a comparative experiment with RuCl3 under identical conditions. The 18 O-labeling (5%

2+

+ 2S2 O8

2−

+ 2h → 4Ru(bpy)3

3+

(4) + 4SO4

2−

(5)

Photocatalytic water oxidation was investigated by Hill et al. using the POM structure complex as a catalyst (1.25–10 ␮M), Ru(bpy)3 3+ generated from Ru(bpy)3 2+ (0.5–1.0 mM) as an oxidant, and S2 O8 2− (2.5–10 mM) as a sacrificial electron acceptor in a neutral environment (20–50 mM sodium phosphate, initial pH 7.2 (fig. 28) [133]. Under visible-light illumination (420–520 nm), 2 eq of the MLCT excited state Ru(bpy)3 2+ * were formed from the absorption of two photons by 4 eq of the ground state Ru(bpy)3 2+ . Moreover, the electron located on the ligand of the excited state species was removed by 2 eq of the electron acceptor S2 O8 2− . The net result of the reaction is the formation of 2 eq of Ru(bpy)3 3+ and 2 eq of SO4 −• . SO4 −• is also a strong oxidant and can oxidize the unexcited two equiv of Ru(bpy)3 2+ into two equiv of Ru(bpy)3 3+ . Finally, four equiv of Ru(bpy)3 3+ were produced after the above photochemical and chemical oxidations, sequentially oxidizing Rb8 K2 [{Ru4 O4 (OH)2 (H2 O)4 }(␥SiW10 O36 )2 ]·25H2 O, which in turn oxidized H2 O to O2 and regenerated Ru(bpy)3 2+ . Under the above photocatalytic system, an initial turnover frequency (TOF) of approximately 0.08 s−1 and a higher TN of 350 were achieved at lower catalyst and higher persulfate concentrations. However, a further increase in catalyst concentration resulted in a significant decrease in the reaction rate and precipitation of an adduct. The quantum efficiencies for generating Ru(bpy)3 3+ (∼44%) and its reaction with the catalyst to form O2 (∼60%) were major limiting factors in this system. They also prepared the complex H2 Ce2.5 K(NH4 )0.5 [{RuV RuIV 3 O6 (OH2 )4 }(SiW10 O36 )2 ], which is an electron oxidation species of 72, and the water-soluble diruthenium complex Cs6 [{Ru2 O2 (OH2 )2 }(SiW10 O36 )]·25H2 O [134]. Notably, both 71 and 72 have two protonated Ru-O-Ru oxygens; however, no [Ru4 O6 ] core oxygens in H2 Ce2.5 K(NH4 )0.5 [{RuV RuIV 3 O6 (OH2 )4 }(-SiW10 O36 )2 ] were protonated. They suggested a mechanism for the water oxidation catalyzed by 72 with [(Ru(bpy)3 )]3+ as an oxidant (Eq. 6-10) in which 72 (0)-Ru4 {IV, IV, IV, IV} was rapidly oxidized in a stepwise manner to 72 (+4)-Ru4 {V, V, V, V}. Eq. (10) is a rate-limiting step. A hyperbolic dependence of the O2 yield on the catalyst concentration indicated that the oxidation of bpy ligand occurred simultaneously. These authors also reported the phosphorus-centered species Cs9 [(␥-PW10 O36 )2 RuIV 4 O5 (OH)(OH2 )4 ], which can catalytically oxidize water in a visible light-driven process, and its TN and TOF can reach 120 and 0.13 s−1 (pH 5.8) [135] (Figs. 15–17). 4. Water-oxidation chemistry of iridium, iron and cobalt complexes Bernhard et al. reported a class of cyclometalated iridium(III) aquo complexes, 73–77 (Chart 4) [136–138], which are economically synthesized, robust, water-soluble, and capable of oxidization, making them highly tunable water-oxidation catalysts. These complexes can be synthesized by cleaving their precursor cyclometalated iridium chloride-bridged dimers with 9:1 mixtures of ethanol/water. Pressure transducers were used for the dynamic monitoring of the headspace pressure of the 40 ml reaction vials in which 1.7 mmol of a CeIV solution were injected into different amounts of catalyst with a total solution volume of 10 ml under an argon flow. Two different characterizations were observed in the shape of the kinetic traces. The first involved a constant linear evolution of oxygen from the systems, which is non-first-order dependent on the catalyst concentration. However, a parabolic curve of the kinetic traces started forming after the initial half hour,

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IrIII-Cl

2+ 2H

HO RuQ

QRu OH

+

H2O 2H +

QRu

H2O RuQ

O

O

2H2O

+

H+, eIrIV-OH

IrIII-OOH

+

2+

4+ O

IrIII-OH 2

RuQ

-2e-

O

Cl-

H2O

O2

QRu

O2

0

4+ QRu OH2

H+, 2e-

QRu

RuQ

O

O

RuQ

H+ IrV=O

-2eFig. 15. Proposed mechanism for water oxidation catalyzed by 68, (SbF6 )2 modified on an ITO electrode in water. Source: the graphic was adapted from reference [121].

Fig. 16. Structure of the polyanion of 72, highlighting the central {Ru4 (␮-O)4 (␮OH)2 (H2 O)4 }6+ core (ball-and-stick representation, Ru blue, ␮-O red, O(H2 ) orange; hydrogen atoms omitted for clarity) and the slightly distorted {Ru4 } tetrahedron (transparent blue). The polytungstate fragments are shown as gray polyhedra, and Si is in yellow. Source: Reprinted with permission from [132]. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA.

which was maintained until the CeIV concentration dwindled. A yield of less than 4% of the theoretical maximum of 430 ␮mol of O2 was obtained. The understanding of this characterization is limited. The catalyzed reactions are slow, more than 100 million times

+ H+ , e -

H2O Fig. 18. Reaction mechanism postulated for iridium-catalyzed water oxidation. Source: the graphic was adapted from reference [140].

slower than those in the OEC. Meyer also commented on this type of water-oxidation catalyst [139]. Brudvig and Crabtree et al. reported families of Cp* iridium complexes, 78–89 (Chart 4) [140,141], which are highly active and robust water-oxidation catalysts. This study employed stronger donating ligand sets to improve the catalytic activity compared with the report of Bernhard et al. The 18 O-labeling experiments confirmed that water was the source of O atoms in the evolved O2 . Complexes 88 and 89 exhibited higher activities with turnover frequencies of 0.33 s−1 (pH 0.89). A mechanism was proposed based on the electrochemistry data, the DFT calculations and the kinetic reaction in which IrV O was formed from IrIII -OH2 using the oxidant (CeIV ). The two-electron oxidation can be detected in the cyclic voltammetry measurement of the catalyst. The O O bond was then formed by the attack of H2 O on IrV O. The peroxo intermediate, IrIII -OOH, released O2 , which was accompanied by the binding of water and the transfer of one proton and two electrons (Fig. 18). Macchioni et al. reported the two water-soluble mononuclear IrIII complexes 90 and 91 (Chart 4) [142], which showed higher activity. Complexes 90 and 91 had TOF = 0.14 s−1 and 0.26 s−1 , respectively (Chart 5). Ellis et al. reported complex 92 and a series of its derivatives containing the iron-centered tetraamido macrocyclic ligands [143]. These derivatives efficiently catalyze water oxidation. A study of O2 evolution over time shows that if R1 = H, R2 = CH3 , the complex

Fig. 17. Light-induced catalytic water oxidation catalyzed by tetraruthenium polyoxometalate (72) using [Ru(bpy)3 ]2+ as a photosensitizer and persulfate as a sacrificial electron acceptor. Source: Reprinted with permission from [133]. © 2009 American Chemical Society.

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Chart 4. Structures of iridium water-oxidation catalysts.

R1

O

N

R1

R3

O

N

R1 = Cl R2 = F R3 = H2 O

Fe N

N

O

O R2

R2

92 Chart 5. Structures of iron water-oxidation catalysts.

does not have any catalytic activity. However, the introduction of electron-withdrawing substituents to the ligand can apparently increase the rates of evolution of O2 . The catalyst with R1 = Cl and R2 = F exhibited the highest turnover frequencies (TOF > 1.3 s−1 , TN > 16), and its initial rate was first-order. The mechanism of the generation of O2 from this series of catalysts remains unclear. Hill et al. reported the carbon-free, earth-abundant durable water-oxidation catalyst Na10 -93{[Co4 (H2 O) 2 (PW9 O34 )2 ] Na10 }(Fig. 19). The reaction center is composed of four cobalt atoms surrounded by polyoxotungstate (PW9 O34 −9 ) anions. The catalyst can be easily prepared from mixtures of Na2 WO4 ·2H2 O, Na2 HPO4 ·7H2 O and Co(NO3 )2 ·6H2 O in water. The four cobalt atoms are aligned in a near-linear array between the two POM ligands, which differs from the organization of the four ruthenium atoms in 71 and 72 (a cuboidal arrangement of four Ru cores) [144]. The turnover frequency (TOF) was greater than 5 s−1 (pH 8). Using a 10 ml reaction solution with the oxidant [Ru(bpy)3 ]3+

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Fig. 19. X-ray structure of Na10 -93 in combined polyhedral ([PW9 O34 ] ligands) and ball-and-stick (Co4 O16 core) notation. Co atoms are purple, O/OH2 (terminal) are red, PO4 are orange tetrahedral, and WO6 are gray octahedra. Hydrogen atoms, water molecules, and sodium cations are omitted for clarity.

Fig. 20. SEM image of the electrodeposited catalyst on electrode surface ([KPi] = 0.1 M, pH 7.0 and [Co2+ ] = 0.5 mM).

Reprinted with permission from [144]. © 2010 American Association for the Advancement of Science.

Source: Reprinted with permission from [146]. © 2008 American Association for the Advancement of Science.

(2.4 × 10−3 M), the catalyst (1.2 × 10−7 M) and a higher buffer capacity solution (pH 8.0, 3.0 × 10−2 M NaPi and 3.0 × 10−2 M sodium borate buffer), the TN could reached approximately 1000 in less than 3 min. The stability of the catalyst was demonstrated by the following experiments. No change was found in the absorption of the ultraviolet and visible spectra or the 31 P nuclear magnetic resonance (NMR) spectra under the catalytic reaction conditions, even with a pH range of 3.5–9. The catalyst is stable for at least 1 d. The water-oxidation activity exhibited a relatively minor change after the excess 2,2 -bipyridine was added to the catalyst, which did not indicate any formation of cobalt hydroxide/oxide in the catalyst. After the water-oxidation catalytic reaction, the 31 P NMR displayed the only species consistent with the original phosphate buffer solution. The precipitate was obtained by the addition of [Ru(bpy)3 ]2+ as a counter ion, presumably forming Na8 Ru(bpy)3 -[Co4 (H2 O)2 (PW9 O34 )2 ], which can reproduce the water-oxidation activity of Na10 [Co4 (H2 O)2 (PW9 O34 )2 ] exactly, and exhibits the same characteristic IR peaks (1037, 939, 882, and 767 cm−1 ) as its precursor. Upon a comparison of the current of a freshly prepared solution of [Ru(bpy)3 ]2+ and the catalyst to that of a solution that had already undergone catalytic chemical reduction of an equivalent amount of [Ru(bpy)3 ]3+ by the catalyst, no evident catalyst deactivation was observed. All of the above experiments demonstrated that Na10 -93 is a robust water-oxidation catalyst. It is also capable of self-assembly like 71 and 72 [145].

Nocera et al. reported an oxygen-evolving catalyst that was formed in situ through electrodeposition on an inert ITO electrode from an aqueous solution of KH2 PO4 and K2 HPO4 (pH 7) containing Co(OH2 )6 (NO3 )2 with concentrations of 0.1 M (KPi) and 0.5 mM (Co2+ ). This catalyst is easy to prepare, is an earth-abundant wateroxidation catalyst, and can generate O2 under conditions of low overpotential, neutral pH, 1 atm of pressure, and room temperature [16,146]. Its current density was more than 1 mA/cm2 after 7–8 h when electrolysis was carried out at 1.29 V in a KPi electrolyte (pH 7, [Co2+ ] = 0.5 mM). The results of SEM and X-ray powder diffraction of electrodeposited materials on the ITO substrate showed that individual micrometer-sized particles were formed on top of the film that displayed broad amorphous features (Fig. 20). The composition of the electrodeposited material was determined using energy-dispersive X-ray analysis, elemental analysis, and X-ray photoelectron spectroscopy. The spectra indicated a Co:P ratio of roughly approximately 2:1 in the electrodeposited catalyst. Water as the source of O2 was confirmed in an 18 O2 labeling experiment in a helium-saturated electrolyte containing 14.6% H2 18 O using a mass spectrometer in which electrolysis was carried out at 1.29 V vs. NHE. The ratio of isotopes detected (16 O–16 O/16 O–18 O/18 O–18 O = 73.4/24.5/2.1) was consistent with the statistical ratio (72.9%, 24.9%, and 2.1% relative abundances), and 95 mmol (3.0 mg) of O2 can be produced using the catalyst (∼0.2 mg). The stability of the KPi electrolyte was confirmed by the

Fig. 21. (a) Edge-sharing molecular cobaltate cluster (MCC) model for surface Co-Pi. Bridging oxo/hydroxo ligands are shown in red, and non-bridging oxygen ligands (including water, hydroxide, and phosphate) complete the octahedral coordination geometry of each peripheral Co ion (blue) and are shown in light red. (b) Proposed structural motif deduced from XAS data relating to the bulk of the Co-Pi film (cobalt in blue, oxygen in red). Source: Reprinted with permission from refs. [147,148]. © 2009 American Chemical Society.

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Fig. 22. Proposed pathway for OER by Co-Pi. A PCET equilibrium proceeded by a turnover-limiting O O bond forming step is consistent with current dependencies on proton and electron equivalencies. Curved lines denote phosphate or OHx terminal or bridging ligands. Source: Reprinted with permission from [150]. © 2010 American Chemical Society.

31 P nuclear magnetic resonance spectra of the electrolysis solutions. They also demonstrated that proton-accepting electrolytes (Pi, MePi, and Bi) were capable of maintaining the pH under catalytic conditions so that catalytic water oxidation can be performed with a low overpotential and allow O2 production to occur in salt water without the oxidation of Cl− [147]. Phosphate was a crucial component in the Co-Pi catalyst because it aided in catalyst self-repair, thereby maintaining the stability of the catalyst system [148]. The self-repair was carried out by the dynamic equilibrium between Co2+ -HPO4 −2 in solution and Co3+ -HPO4 −2 on the electrode. The structure and valence of the Co-Pi water-oxidation catalyst were studied by Dau et al. and Nocera et al. using X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS) spectroscopy, and Fourier transform (FT) EXAFS [149,150]. Nocera et al. proposed an edge-sharing CoO6 octahedral architecture with Co oxo/hydroxo clusters as a structural characterization of Co-Pi (Fig. 21a), which differed from the corner-sharing Co-oxo cubane reported previously by Dau et al. (Fig. 21b). A Co valence greater than 3 for the Co-Pi samples was supported using the XANES spectra and the electrochemical data when water oxidation was carried out (1.25 V vs. NHE). Previous studies using EPR have provided increasing evidence of a CoIV species forming from CoII during water oxidation [151] Although the mechanism of O O bond formation remains to be determined, based on the studies of electrochemistry, a reaction pathway that uses a model of a molecular cobaltate cluster (Fig. 22), in which a turnover-limiting chemical step involving the evolution of O2 is produced by a PCET equilibrium, has been proposed [151].

5. Conclusions This review discusses the advances of molecular-level wateroxidation catalysis applied in homogeneous system over the past 30 years with a particular focus on the more recent decades. This review demonstrates that many well-characterized model complexes have been developed. Detailed insights into the mechanisms of these complexes were explored. These studies could promote the production of a practical device for splitting water. However, much work needs to be performed for an industrial application that converts light to chemical energy to become practical. References

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