Water oxidation is a key reaction in natural photosynthesis and in many schemes for artificial photosynthesis. Although metal complexes capable of oxidizing water based on Ru, Mn, and Ir are known, a significant question is whether or not dimeric or higher order structures are required for water oxidation. We report here single-site catalytic water oxidation by the monomeric complexes [Ru(tpy)(bpm)(OH2)]2+ and [Ru(tpy)(bpz)(OH2)]2+ (tpy is 2,2′:6′,2′′-terpyridine; bpm is 2,2′-bipyrimidine; bpz is 2,2′-bipyrazine) by a well-defined mechanism involving RuVO.
Mastering the production of solar fuels by artificial photosynthesis would be a considerable feat, either by water splitting into hydrogen and oxygen or reduction of CO(2) to methanol or hydrocarbons: 2H(2)O + 4hnu --> O(2) + 2H(2); 2H(2)O + CO(2) + 8hnu --> 2O(2) + CH(4). It is notable that water oxidation to dioxygen is a key half-reaction in both. In principle, these solar fuel reactions can be coupled to light absorption in molecular assemblies, nanostructured arrays, or photoelectrochemical cells (PECs) by a modular approach. The modular approach uses light absorption, electron transfer in excited states, directed long range electron transfer and proton transfer, both driven by free energy gradients, combined with proton coupled electron transfer (PCET) and single electron activation of multielectron catalysis. Until recently, a lack of molecular catalysts, especially for water oxidation, has limited progress in this area. Analysis of water oxidation mechanism for the "blue" Ru dimer cis,cis-[(bpy)(2)(H(2)O)Ru(III)ORu(III)(OH(2))(bpy)(2)](4+) (bpy is 2,2'-bipyridine) has opened a new, general approach to single site catalysts both in solution and on electrode surfaces. As a catalyst, the blue dimer is limited by competitive side reactions involving anation, but we have shown that its rate of water oxidation can be greatly enhanced by electron transfer mediators such as Ru(bpy)(2)(bpz)(2+) (bpz is 2,2'-bipyrazine) in solution or Ru(4,4'-((HO)(2)P(O)CH(2))(2)bpy)(2)(bpy)(2+) on ITO (ITO/Sn) or FTO (SnO(2)/F) electrodes. In this Account, we describe a general reactivity toward water oxidation in a class of molecules whose properties can be "tuned" systematically by synthetic variations based on mechanistic insight. These molecules catalyze water oxidation driven either electrochemically or by Ce(IV). The first two were in the series Ru(tpy)(bpm)(OH(2))(2+) and Ru(tpy)(bpz)(OH(2))(2+) (bpm is 2,2'- bipyrimidine; tpy is 2,2':6',2''-terpyridine), which undergo hundreds of turnovers without decomposition with Ce(IV) as oxidant. Detailed mechanistic studies and DFT calculations have revealed a stepwise mechanism: initial 2e(-)/2H(+) oxidation, to Ru(IV)=O(2+), 1e(-) oxidation to Ru(V)=(3+), nucleophilic H(2)O attack to give Ru(III)-OOH(2+), further oxidation to Ru(IV)(O(2))(2+), and, finally, oxygen loss, which is in competition with further oxidation of Ru(IV)(O(2))(2+) to Ru(V)(O(2))(3+), which loses O(2) rapidly. An extended family of 10-15 catalysts based on Mebimpy (Mebimpy is 2,6-bis(1-methylbenzimidazol-2-yl)pyridine), tpy, and heterocyclic carbene ligands all appear to share a common mechanism. The osmium complex Os(tpy)(bpy)(OH(2))(2+) also functions as a water oxidation catalyst. Mechanistic experiments have revealed additional pathways for water oxidation one involving Cl(-) catalysis and another, rate enhancement of O-O bond formation by concerted atom-proton transfer (APT). Surface-bound [(4,4'-((HO)(2)P(O)CH(2))(2)bpy)(2)Ru(II)(bpm)Ru(II)(Mebimpy)(OH(2))](4+) and its tpy analog are impressive electroca...
The mechanism of Ce(IV) water oxidation catalyzed by [Ru(tpy)(bpm)(OH(2))](2+) (tpy = 2,2':6',2''-terpyridine; bpm = 2,2'-bipyrimidine) and related single-site catalysts has been determined by a combination of mixing and stopped-flow experiments with spectrophotometric monitoring. The mechanism features O---O coupling by water attack on Ru(V)=O(3+) and three peroxidic intermediates that have been characterized by a combination of spectroscopy and DFT calculations.
The blue dimer, cis, cis-[(bpy)2(H2O)Ru(III)ORu(III)(H2O)(bpy)2](4+), is the first designed, well-defined molecule known to function as a catalyst for water oxidation. It meets the stoichiometric requirements for water oxidation, 2H2O --> -4e(-), -4H(+) O-O, by utilizing proton-coupled electron-transfer (PCET) reactions in which both electrons and protons are transferred. This avoids charge buildup, allowing for the accumulation of multiple oxidative equivalents at the Ru-O-Ru core. PCET and pathways involving coupled electron-proton transfer (EPT) are also used to avoid high-energy intermediates. Application of density functional theory calculations to molecular and electronic structure supports the proposal of strong electronic coupling across the micro-oxo bridge. The results of this analysis provide explanations for important details of the descriptive chemistry. Stepwise e(-)/H(+) loss leads to the higher oxidation states [(bpy)2(O)Ru(V)ORu(IV)(O)(bpy)2] (3+) (Ru(V)ORu(IV)) and [(bpy)2(O)Ru(V)ORu(V)(O)(bpy)2](4+) (Ru(V)ORu(V)). Both oxidize water, Ru(V)ORu(IV) stoichiometrically and Ru(V)ORu(V) catalytically. In strongly acidic solutions (HNO3, HClO4, and HSO3CF3) with excess Ce(IV), the catalytic mechanism involves O---O coupling following oxidation to Ru(V)ORu(V), which does not build up as a detectable intermediate. Direct evidence has been found for intervention of a peroxidic intermediate. Oxidation of water by Ru(V)ORu(IV) is far slower. It plays a role late in the catalytic cycle when Ce(IV) is depleted and is one origin of anated intermediates such as [(bpy)2(HO)Ru(IV)ORu(IV)(NO3)(bpy)2](4+), which are deleterious in tying up active components in the catalytic cycle. These intermediates slowly return to [(bpy)2(H2O)Ru(IV)ORu(III)(OH2)(bpy)2](5+) with anion release followed by water oxidation. The results of a recent analysis of water oxidation in the oxygen-evolving complex (OEC) of photosystem II reveal similarities in the mechanism with the blue dimer and significant differences. The OEC resides in the thylakoid membrane in the chloroplasts of green plants, and careful attention is paid in the structure to PCET, EPT, and long-range proton transfer by sequential local proton transfers. The active site for water oxidation is a CaMn 4 cluster, which includes an appended Mn site, Mn(4), where O---O coupling is thought to occur. Photochemical electron transfer results in oxidation of tyrosine Y Z to Y Z (.), which is approximately 7 A from Mn(4). It subsequently oxidizes the OEC through the stepwise stages of the Kok cycle. O---O coupling appears to occur through an initial peroxidic intermediate formed by redox nucleophilic attack of coordinated OH(-) in Ca-OH(-) on Mn (IV)=O.
As the terminal step in photosystem II, and a potential half-reaction for artificial photosynthesis, water oxidation (2H 2 O → O 2 þ 4e − þ 4H þ ) is key, but it imposes a significant mechanistic challenge with requirements for both 4e − ∕4H þ loss and O-O bond formation. Significant progress in water oxidation catalysis has been achieved recently by use of single-site Ru metal complex catalysts such as ½RuðMebimpyÞðbpyÞðOH 2 Þ 2þ [Mebimpy ¼ 2,6-bisð1-methylbenzimidazol-2-ylÞpyridine; bpy ¼ 2,2 0 -bipyridine]. When oxidized from Ru II -OH 2 2þ to Ru V ¼ O 3þ , these complexes undergo O-O bond formation by O-atom attack on a H 2 O molecule, which is often the rate-limiting step. Microscopic details of O-O bond formation have been explored by quantum mechanical/molecular mechanical (QM/MM) simulations the results of which provide detailed insight into mechanism and a strategy for enhancing catalytic rates. It utilizes added bases as proton acceptors and concerted atom-proton transfer (APT) with O-atom transfer to the O atom of a water molecule in concert with proton transfer to the base (B). Base catalyzed APT reactivity in water oxidation is observed both in solution and on the surfaces of oxide electrodes derivatized by attached phosphonated metal complex catalysts. These results have important implications for catalytic, electrocatalytic, and photoelectrocatalytic water oxidation.water split | O-O coupling | base effect | isotope effect I n natural photosynthesis, and in many schemes for artificial photosynthesis, water oxidation (is a key reaction with requirements for both 4e − ∕4H þ loss and O -O bond formation. Significant progress in water oxidation catalysis has been achieved recently by using single-site molecular catalysts (1-8). This includes elucidation of a mechanism in Ce (IV) catalyzed water oxidation by ½RuðtpyÞðbpmÞðOH 2 Þ 2þ and ½RuðtpyÞðbpzÞðOH 2 Þ 2þ (tpy ¼ 2; 2 0 ∶6 0 ; 2 00 -terpyridine; bpm ¼ 2; 2 0 -bipyrimidine; bpz ¼ 2; 2 0 -bipyrazine), Scheme 1 (1). Although undergoing multiple turnovers unchanged, these catalysts are relatively slow with O-O bond formation, often the rate-limiting step. In Scheme 1, O-atom transfer from ½Ru V ðtpyÞðbpmÞðOÞ 3þ to H 2 O is rate limiting and occurs with kð0.1 M HNO 3 ; 25°CÞ ¼ 8.9 × 10 −3 s −1 (5). To put rate into perspective, in a practical solar energy conversion scheme, a turnover rate on the millisecond or submillisecond time scale is required to match or exceed the rate of solar insolation. Achieving rates of this magnitude poses a considerable challenge (9).A useful strategy for achieving faster rates in metal complex catalysts exists based on an interplay between mechanism and systematic synthetic modifications. Ligand variations can be used to modify redox potentials, increase driving force, and decrease barriers (6). Mechanistic insight can uncover previously undescribed reaction pathways. We report here the use of a previously unidentified pathway to achieve greatly enhanced rates of electrocatalytic water oxidation for the catalyst, ½RuðMebim...
A series of monomeric ruthenium polypyridyl complexes have been synthesized and characterized, and their performance as water oxidation catalysts has been evaluated. The diversity of ligand environments and how they influence rates and reaction thermodynamics create a platform for catalyst design with controllable reactivity based on ligand variations.
Electrocatalytic water oxidation occurs through the use of the phosphonate-derivatized single-site catalyst [Ru(Mebimpy)(4,4'-((HO)(2)OPCH(2))(2)bpy)(OH(2))](2+) [Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine; bpy = 2,2'-bipyridine] at pH 1 and 5 on fluorine-doped SnO(2) or Sn(IV)-doped In(2)O(3) electrodes or on nanocrystalline TiO(2). The surface-bound catalyst appears to retain the water oxidation mechanism found for [Ru(tpy)(bpm)(OH(2))](2+) and [Ru(tpy)(bpz)(OH(2))](2+) (tpy = 2,2':6',2''-terpyridine; bpm = 2,2'-bipyrimidine; bpz = 2,2'-bipyrazine) in solution and acts as a surface electrocatalyst for sustained water oxidation.
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