Developing good molecular catalysts for water oxidation remains one of the most challenging aspects of artificial photosynthesis. The oxidation process requires a number of difficult steps including the binding of at least two waters in close proximity, the removal of 4 electrons from these two waters, the formation of an O-O bond, the release of the product O2, and the regeneration of the catalyst starting state. Several comprehensive reviews on molecular water oxidation catalysts have ap-peared221-223 including one that focuses on the bioinorganic chemistry of the OEC in PSII.224 Heterogeneous, polyoxometallate and colloidal catalysts for oxygen evolution continue to be explored by a number of groups21,225-227 and are not covered here. Complexes which mimic the structure and spectroscopy of the OEC in PSII have been recently reviewed228,229 and this section will focus on some of the newer or most notable complexes that show functional similarities.
A collection of functional molecular oxygen-evolving catalysts is shown in Fig. 12. Of these, the oldest and best studied is the ruthenium oxo-bridged dimer, 31230-233 which was first reported to oxidize water in the presence of strong oxidants by Meyers and co-workers in 1982.234 It is known to proceed via a [(bpy)2RuV(O)(|-O)RuV(O)(bpy)2]4+ intermediate235 and although the formation of a peroxo bridge between adjacent oxo groups is appealing, recent isotopic labeling studies suggest the O2 is at least partially derived from solvent H2O.236 Yamada et. al propose oxo attack on water H-bonded to the catalyst to give a hydroperoxo intermediate prior to O2 evolution.236 The catalytic activity is attenuated by an anation side reaction which nonetheless is reversible by substitution of the anion with water.235 Overall the num-
Fig. 12. A collection of functional oxygen-evolving catalysts.
Fig. 12. A collection of functional oxygen-evolving catalysts.
ber of catalytic turnovers is limited (< 25) due to eventual decomposition of the complex. Catalyst 28 consists of two cofacial Mn(III) tetraarylporphyrins and recently it has been shown that a MnV(O)-MnV(O) intermediate is formed prior to oxygen evolution.237,238 This result is particularly interesting in that a terminal MnV(O) group is implicated in many O2 evolving mechanisms for PS II. 239-243 The Mnm(^-O)2MnIV dimer 29 was the first bis ^-oxo Mn dimer shown to evolve O2 catalytically under homogeneous conditions.244,245 The disposition of the two open coordination sites makes it difficult to see how the two putative MnV(O) groups could form an intramo lecular peroxo intermediate and thus suggests that water plays an active role in the O2 forming step. This is supported by isotopic labeling studies, however these results are not conclusive in that water may exchange with the oxo species bound to the Mn ions. Turnover numbers for this catalysts are modest (~30) with Oxone (KHSO5) as the terminal oxidant presumably as the complex is seen to decompose with the formation of permanganate.245
The MnnMnn dimer 30 has an unusual structure with two 7 coordinate Mn ions forming a bis |-oxygen dimer via an n1carboxylato oxygen ligand.246 Upon mixing 30 with BuOOH or Ce(IV), O2 is evolved with a catalytic mechanism invoking dissociation of the dimer to form monomeric MnIII(OH) complexes, reassociation to a Mnm(|-O)Mnm dimer, further oxidation to the MnIV(|-O)2MnIV dimer and formation of MnIII(| -OO)MnIII dimer which then loses O2 to reform 30.247 Unfortunately, the data is not reported in a way that turnover numbers are easily compared with other catalysts but it appears that at least 10 turnovers are possible. Interestingly, isotopic labeling studies show the O2 evolved consistently incorporates one oxygen atom from the solvent water and one from the ButOOH. Curiously, the same labeling studies show that when (NH4)2[Ce(NO3)6] is used as the oxidant, some of the nitrate oxygen atoms are incorporated into the dioxygen product. 247 The mechanism by which this occurs is unclear but it is notable in that many groups use this oxidant because it is 'oxygen-atom' free. It is worth mentioning that O2 evolution has been observed from a model complex containing a Mn4O46+ cubane core, however this result is only seen in the gas phase and the relevancy to condensed phase systems is unclear.248
The remaining water oxidation catalysts 31-34 are RunRun dimers with complexes 33 and 34 sharing similar bridging ligands. While these catalysts are less relevant as models of the natural photosystem, they are the most active and robust molecular water oxidation catalysts. Dimer 32 was reported by Tanaka and coworkers in 2000 and utilizes an unusual bridging ligand based on two terpyridine ligands which are held in a cofacial manner by an anthracene spacer.249,250 Coordination of an ortho-quinone to each Ru(II) center leaves one accessible substrate site per metal ion. The ortho-quinone ligand appears to have an integral role in the catalytic cycle as replacement of this ligand with bpy results in markedly diminished O2 evolving activity. In fact, one postulated reaction mechanism involves a base-catalyzed internal disproportionation of the quinone and hydroxo ligands in 32 resulting in the formation of a peroxo bridged dimer as indicated in reaction 21:
RuIIQ(OH)-RuIIQ(OH) ^ RuIISQ-(OO)-RuIISQ + 2 H+ (21)
Controlled potential electrolysis of 32 in10% water in CF3CH2OH gave 21 turnovers (O2 per 32) whereas the activity jumps to 33,500 turnovers for the catalyst immobilized on an ITO electrode in aqueous solution. Unfortunately, no attempts to use chemical oxidants in a completely homogeneous system for 32 are reported.
Complex 33 consists of two RuII ions are bridged by a pyrazol-based chelating li-gand and an acetate group.251 Terpy ligands are use to tie-up three coordination sites on each Ru(II) ion leaving the complex coordinatively saturated but with an acid labile acetate group. Displacement of the acetate group with water gives the active catalyst which is though to proceed through a RuIV(O)-RuIV(O) species as the highest oxidation state intermediate. At pH 1, the highest oxidation potential revealed by CV was at 1.05 V(Ep,a vs. SSCE). If this irreversible process is due to formation of the RuIV(O)-RuIV(O) intermediate then the overpotential for O2 evolution is nearly nil. Using CeIV as the oxidant, catalytic turnover numbers on the order of 19 after 48 h are observed.
Zong and Thummel recently reported on the water oxidation ability of complex 34 and some closely related derivatives.252 Complex 34 shows turnover numbers as high as 3200 in aqueous solution (pH 1, CF3SO3H) using CeIV as the oxidant. Interestingly, they also show that a closely related RuII monomer can also catalyze O2 evolution albeit with only 580 turnovers compared to the dimer. The CV of 34 shows an oxidation at 1.66 V vs SCE in acetonitrile which is more typical of the uppermost redox process in these water oxidation catalysts. The authors do not speculate on the mechanism at this time. Not included in Fig. 12 is the dinuclear complex [(NH3)3Ru(| -Cl)3Ru(NH3)3]2+ which has also been reported to catalyze water oxidation using CeIV as the oxidant.253 Immobilizing the catalysts in a Nafion membrane was shown to significantly enhance the catalyst lifetime.
Of the water oxidation catalysts mentioned, all use either a powerful chemical oxidant (e.g., CeIV, OCl-, Oxone, ButOOH) or an electrode to drive the reaction. Efforts to couple the oxidation to a photoprocess have not yielded an active photoca-talyst but nonetheless are beginning to yield some promising results as shown previously in Fig. 9 for the Ru-Mn2 dyad.254-256
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