Making Oxygen with Ruthenium Complexes

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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 CO2 to methanol or hydrocarbons: 2H2O + 4hν → O2 + 2H2; 2H2O + CO2 + 8hν → 2O2 + CH4. 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(H2O)RuIIIORuIII(OH2)(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)2P(O)CH2)2bpy)2(bpy)2+ on ITO (ITO/Sn) or FTO (SnO2/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)(OH2)2+ and Ru(tpy)(bpz)(OH2)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 RuIV═O2+, 1e− oxidation to RuV═O3+, nucleophilic H2O attack to give RuIII−OOH2+, further oxidation to RuIV(O2)2+, and, finally, oxygen loss, which is in competition with further oxidation of RuIV(O2)2+ to RuV(O2)3+, which loses O2 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)(OH2)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)2P(O)CH2)2bpy)2RuII(bpm)RuII(Mebimpy)(OH2)]4+ and its tpy analog are impressive electrocatalysts for water oxidation, undergoing thousands of turnovers without loss of catalytic activity. These catalysts were designed for use in dye-sensitized solar cell configurations on TiO2 to provide oxidative equivalents by molecular excitation and excited-state electron injection. Transient absorption measurements on TiO2−[(4,4′((HO)2P(O)CH2)2bpy)2RuII(bpm)RuII(Mebimpy)(OH2)]4+, (TiO2−RuII−RuIIOH2) and its tpy analog have provided direct insight into the interfacial and intramolecular electron transfer events that occur following excitation. With added hydroquinone in a PEC configuration, APCE (absorbed-photon-to-current-efficiency) values of 4−5% are obtained for dehydrogenation of hydroquinone, H2Q + 2hν → Q + H2. In more recent experiments, we are using the same PEC configuration to investigate water splitting.