Water oxidation (WO) is one of the most important reactions for life in Earth. During photosynthesis, CO2 is reduced into sugars using the electrons that come from WO at the oxygen evolving complex of photosystem II, generating oxygen as by product. Mimicking this natural process at higher efficiencies by using sustainable robust catalysts holds the potential to revolutionise solar fuel and chemicals synthesis in a circular economy. Therefore, understanding the intricacies of WO is essential not only to improve the reaction efficiency but also to develop better catalysts via rational design. Unfortunately, the WO mechanism is complex. Moreover, the reaction intermediates are highly unstable, thus making their isolation and characterisation extremely challenging.
To shed light in the WO mechanism, molecular complexes have been used as models to characterise the intermediates and mechanisms that govern the WO reaction. 1-3 Despite extensive efforts to elucidate the WO mechanism, it is still not fully understood, prompting an ongoing debate with several proposals concerning the formation of the O-O bond. In general, two formal mechanisms have been considered to form the O-O bond: i) the coupling between two radical M–O· (or M–oxyl) species, and ii) the water nucleophilic attack over an electrophilic metal oxo species (Fig. 1) 1, 4-8. In both cases, the formation of the O–O bond is usually the step that determines the rate of the reaction, and therefore, facilitating the O–O bond formation should translate into better efficiencies. However, despite indirect proofs for both mechanisms, the evidence for direct formation of the O–O bond from a M=O moiety is circumstantial 1, 7-11. In part, the challenge is the characterization the intermediates after the O–O bond forming step; especially when this is the rate-determining step (RDS) of the reaction. In general, catalytic intermediates after the RDS are extremely challenging to capture and characterise.12 The formation of putative metal-peroxo complexes during catalysis has been a topic of debate over the last decades and remains so 13-22. Despite some proposed spectroscopic evidence for the formation of Ru-O2 species, compelling evidence for the formation of M–peroxo species formed under catalytic conditions after the O–O bond formation has not been reported.10, 16, 23-25
After several years developing well defined Fe and Ru catalysts for WO and studying how the high valent intermediates proposed in the WO catalytic cycle can be formed and stabilised to promote the O-O bond, scientists at ICIQ’s Lloret-Fillol group, have isolated and fully characterised an elusive intermediate generated after the oxygen-oxygen bond formation event – the reaction’s rate-determining step. By modifying the conditions in their catalytic system, Carla Casadevall, a former FPU PhD student of the group, has crystallised a Ru(IV) side-on peroxo complex that serves as a ‘missing link’ for the species that form after the rate-determining O-O bond forming step. The Ru(IV) side-on peroxo complex (h2–1IV–OO) is generated from the isolated Ru(IV) oxo complex (1IV=O) in the presence of an excess of oxidant. The oxidation (IV) and spin state (singlet) of h2–1IV–OO were determined by a combination of experimental and theoretical studies. 18O and 2H labelling studies evidence the direct evolution of O2 through the nucleophilic attack of a H2O molecule on the highly electrophilic metal-oxo species via the formation of h2–1IV–OO. For the first time we have been able to directly track the isotopic speciation of the O-O moiety before and after the O-O bond formation. This has been possible because we have isolated one species before (1IV=O) and one after (h2-1IV-OO) the RDS. This study shows direct evidence for the water nucleophilic attack as a viable mechanism for O–O bond formation, as previously proposed based on indirect evidence.
This work has direct implications in our capacity to look at the oxygen-oxygen bond formation step and the afterwards reaction intermediates and proves again that well-defined molecular complexes offer access to fundamental aspects of the WO reaction, otherwise very challenging, which will be useful for further efficient catalyst design. This study will help to better understand the mechanism of the O-O bond formation, since it shows direct evidence for a single-site mechanism to form the oxygen-oxygen bond, one of the mechanisms postulated for photosystem II.
More details of this work could be found here: “Isolation of a Ru(IV) side-on peroxo intermediate in the water oxidation reaction” in Nature Chemistry: https://www.nature.com/articles/s41557-021-00702-5
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