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 RuVO.

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 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.

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.

TitleTowards a comprehensive understanding of visible-light photogeneration of hydrogen from water using cobalt(ii) polypyridyl catalysts Permalink https://escholarship.org/uc/item/9tw104fd Journal Energy and Environmental Science, 7(4) Homogeneous aqueous solutions of photocatalytic ensembles, consisting of [Ru(bpy) 3 ] 2+ as a photosensitizer, ascorbic acid/ascorbate as the electron source, and 10 distinct Co 2+ -based molecular catalysts, were evaluated for visible-light induced hydrogen evolution using high-throughput screening. The combined results demonstrate that Co 2+ complexes bearing tetradentate ligands yield more active photocatalytic compositions than their congeners with pentadentate ligands while operating with high catalyst stability. Additionally, molecular Co 2+ catalysts with cis open coordination sites appear to be significantly more active for hydrogen evolution than those with trans open sites. As evidenced by mass spectrometric analysis of the reactor headspace and associated deuteration experiments, the H 2 gas generated in all instances was derived from aqueous protons. One of the most promising cis-disposed Co 2+ species, [Co(bpyPY2Me)(CH 3 CN)(CF 3 SO 3 )](CF 3 SO 3 ) (1), engages in highly efficient hydrogen evolving photocatalysis, achieving a turnover number of 4200 (H 2 /Co) and a turnover frequency of 3200 (H 2 /Co per h) at pH 4 under simulated sunlight (AM 1.5G, 100 mW cm À2 ) at room temperature. At equimolar concentrations of photosensitizer and 1, the total hydrogen produced appears to be exclusively limited by the photostability of [Ru(bpy) 3 ] 2+ , which was observed to decompose into an Ru(bpy) 2 -ascorbate adduct, as evidenced by HPLC and ESI-MS experiments.Lowering the operating temperature from 27 to 5 C significantly attenuates bpy dissociation from the sensitizer, resulting in a net $two-fold increase in hydrogen production from this composition. The primary electron transfer steps of this photocatalytic ensemble were investigated by nanosecond transient absorption spectroscopy. Photoexcited [Ru(bpy) 3 ] 2+ undergoes reductive quenching by ascorbic acid/ascorbate (k q ¼ 2.6 Â 10 7 M À1 s À1 ), releasing [Ru(bpy) 3 ] + from the encounter solvent cage with an efficiency of 55 AE 5%. In the presence of catalyst 1, [Ru(bpy) 3 ] + generated in the initial flash-quench experiment transfers an electron (k et ¼ 2 Â 10 9 M À1 s À1 ) at an efficiency of 85 AE 10% to the catalyst, which is believed to enter the hydrogen evolution cycle subsequently. Using a combinatorial approach, all ten Co 2+ catalysts were evaluated for their potential to operate under neutral pH 7.0 conditions. Catalyst 7, [Co(PY4MeH 2 )(CH 3 CN)(CF 3 SO 3 )](CF 3 SO 3 ), was revealed to be most promising, as its performance metrics were only marginally affected by pH and turnover numbers greater than 1000 were easily obtained in photocatalytic hydrogen generation. These comprehensive findings provide guidelines for the development of molecular compositions capable of evolving hydrogen from purely aqueous ...

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.

Nanosecond laser flash photolysis has been used to investigate injection and back electron transfer from the complex [(Ru(bpy)(2)(4,4'-(PO(3)H(2))(2)bpy)](2+) surface-bound to TiO(2) (TiO(2)-Ru(II)). The measurements were conducted under conditions appropriate for water oxidation catalysis by known single-site water oxidation catalysts. Systematic variations in average lifetimes for back electron transfer, <τ(bet)>, were observed with changes in pH, surface coverage, incident excitation intensity, and applied bias. The results were qualitatively consistent with a model involving rate-limiting thermal activation of injected electrons from trap sites to the conduction band or shallow trap sites followed by site-to-site hopping and interfacial electron transfer, TiO(2)(e(-))-Ru(3+) → TiO(2)-Ru(2+). The appearance of pH-dependent decreases in the efficiency of formation of TiO(2)-Ru(3+) and in incident-photon-to-current efficiencies with the added reductive scavenger hydroquinone point to pH-dependent back electron transfer processes on both the sub-nanosecond and millisecond-microsecond time scales, which could be significant in limiting long-term storage of multiple redox equivalents.

We recently described single-site catalysts for water oxidation that operate by a well-defined mechanism involving stepwise three-electron oxidation to high-oxidation-