Today, as we struggle to meet the world’s energy needs, the (electro)chemistry of oxygen species, such as oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), is at the center of chemist’s attention: the interconversion between oxygen and water plays an important role in renewable energy technologies, such as fuel cells, metal-air batteries, water electrolyzers, and water splitting photoelectrochemical cells. The electrochemical OER/ORR often occur with sluggish kinetics and at sizeable overpotentials, so finding efficient catalysts for these reactions is of paramount importance for those applications. The mechanistic studies on well-defined molecular systems have contributed significantly to our understanding of OER/ORR catalysis and have accelerated the discovery of improved catalytic materials. Our group is interested in identifying metal-free molecular motifs capable of catalyzing OER/ORR and identifying the mechanism of catalysis, which could have significant impact in design of new catalytic materials.<br/>Three covalently-linked dimers (DPE, XAN, BP) composed of xanthenol units connected by appropriate linkers (diphenyl ether, xanthenol, biphenyl) were prepared and characterized, with the aim to investigate the feasibility of O–O bond formation from metal-free alcohols. The conformational studies were performed using X-ray and NMR spectroscopy, supported by DFT calculations. These studies showed that the BP dimer adopts an inward-facing orientation of the alcohol OH-groups, which is a favorable conformation for an O–O bond forming step. Furthermore, the alkoxy radicals of model dimers were generated using the Suárez reaction and their O–O coupling to form peroxides was investigated. The peroxide formation was observed only from the biphenyl-linked dimer BP, as expected from the results of the conformational analysis. This work has shown, for the first time, that the O–O bond can be formed by the oxidation of two organic alcohols, showing a proof-of-principle chemical reactivity that can be utilized for future metal-free (graphene-based) OER catalysts.

Metal-free covalently-linked xanthenol dimers as model catalysts for OER: The O–O bond formation and their oxidative behavior

255th National Meeting (2018)

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