Abstract
Water oxidation is critically important for the development of energy solutions based on the concept of artificial photosynthesis. In order to gain deeper insight into the mechanism of water oxidation, the catalytic cycle for the first designed water oxidation catalyst, cis,cis-[(bpy)2(H2O)RuIIIORuIII(OH2)(bpy)2]4+ (bpy is 2,2-bipyridine) known as the blue dimer (BD), is monitored in D2O by combined application of stopped flow UV-Vis, electron paramagnetic resonance (EPR) and resonance Raman spectroscopy on freeze quenched samples. The results of these studies show that the rate of formation of BD[4,5] by Ce(IV) oxidation of BD[3,4] (numbers in square bracket denote oxidation states of the ruthenium (Ru) centers) in 0.1 M HNO3, as well as further oxidation of BD[4,5] are slower in D2O by 2.1–2.5. Ce(IV) oxidation of BD[4,5] and reaction with H2O result in formation of an intermediate, BD[3,4]′, which builds up in reaction mixtures on the minute time scale. Combined results under the conditions of these experiments at pH 1 indicate that oxidation of BD[3,4]′ is a rate limiting step in water oxidation with the BD catalyst.
Highlights
Photosynthetic water oxidation is a fundamental process in the biosphere, which results in the sunlight driven formation of O2 from water
Isotope effects on the kinetics of intermediate formation in the catalytic cycle for water oxidation by the blue dimer have been analyzed by a combination of UV-Vis stopped flow kinetics, oxygen evolution, electron paramagnetic resonance (EPR) spectroscopy and resonance Raman measurements
Stopped flow kinetics coupled with parallel EPR and Raman measurements were used to delineate the steps in the overall catalytic cycle
Summary
Photosynthetic water oxidation is a fundamental process in the biosphere, which results in the sunlight driven formation of O2 from water. About 30 years ago, Meyer and coworkers reported the first ruthenium-based catalyst for water oxidation, known as the “blue dimer” (BD) This catalyst may be considered as an artificial analog of the oxygen-evolving complex (OEC) in the Photosystem II (PS II) as they both undergo oxidative activation by proton coupled electron transfer (PCET) to reach higher oxidation states where water oxidation occurs [5,6,7,8]. 2.1–2.5 was determined experimentally by a combined UV-Vis stopped flow and EPR analysis This value is consistent with observations made earlier for a single site water oxidation catalyst where it was concluded that the mechanism of O–O bond formation was Atom Proton Transfer (APT). This comparison reveals that the rate limiting step or steps in the overall water oxidation catalytic cycle is not the O–O bond forming step and is consistent with rate limiting oxidation of BD[3,4]′ by Ce(IV)
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