Making Oxygen with Ruthenium Complexes

Abstract

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...

Redox and electrochemical water splitting catalytic properties of hydrated metal oxide modified electrodes

“…This is highlighted in the work of Suntivich 303 and Subbaraman 304 outlined above and concurrently through the dramatic rise in interest in the area of homogeneous water oxidation catalysis. Notable works include those of Meyer, [305][306][307][308] Thummel, [309][310][311] Llobet, [312][313][314][315][316] Sun, [316][317][318][319] and Crabtree [320][321][322] to name but a few. One of the advantages of these systems is the ease with which mechanistic studies can be performed.…”

Electrocatalytic Metal–Organic Frameworks for Energy Applications

“…Further characterization of MAF-X27-OH reveals no change in the PXRD pattern, SEM images, LSV curves, and CV curves following OER testing, which suggestst hat MAF-X27-OH is stable in alkalinem edia under harsh oxidative conditions. Ap rominentm olecular water oxidationc atalyst, [Ru(tpy)(dcbpy)(OH) 2 ](ClO 4 ) 2 (tpy = 2,2':6',2"-terpyridine, dcbpy = 2,2'-bipyridine-5,5'-dicarboxylic acid), [13] was incorporated by postsynthetic ligand exchange into thin films of UiO-67 (UiO = University of Oslo), aM OF made from Zr 6 (O) 4 (OH) 4 clusters connected with biphenyl-4,4'-dicarboxylic acid linkers (Figure 3). [105] UiO67-[RuOH 2 ]-modifiedF TO was evaluated for the OERi np H8.4 solutions,a nd current enhancements were ChemSusChem 2017, 10,4374 -4392 www.chemsuschem.org 2017 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim observeda fter 1.1 Vv ersus Ag/AgCl.…”

Water oxidation surface mechanisms replicated by a totally inorganic tetraruthenium–oxo molecular complex

“…The drawback lies in the intrinsic weakness of the biological components, requiring multiple repair strategies of both the inorganic core and the enzyme proteins. Similar stability issues due to oxidative damage during water oxidation affect most of the synthetic homogeneous molecular catalysts (5,6).…”