Activation of small molecules by copper and cobalt complexes via proton-coupled multi-electron transfer reactions and the application in catalysis

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Many redox reactions in nature and chemistry proceed in the transfer of both, electrons and protons, and in fact, this concept is utilised in the water oxidation of the photosystem II (PS II). At the oxygen evolving centre in the PS II four consecutive single electron transfer steps lead to the oxidation of water. Three out of the four electron transfer steps are most likely coupled to a proton transfer. The (almost) simultaneous transfer of protons and electrons avoids the formation of high energy intermediates, which could be formed in consecutive transfer steps. The mimicking of this reaction is the key step for an energy change from fossil to sustainable fuels. The synthesis of C1-fuels or H2 requires electrons in order to reduce the precursors CO2 or protons. Water is a cheap and widely available source of electrons and the only side product in the water oxidation reaction would be the natural gas oxygen. Among others, the oxidation of water can be performed by transition metal complexes.The project targets the activation of small molecules, in particular the oxidation of water and the reductive cleavage of laughing gas (N2O) by cobalt and copper complexes. Complexes with the earth abundant metal ions copper and cobalt are able to accumulate two or more redox equivalents and thus, to facilitate multistep redox reactions. The complexes feature a very robust ligand scaffold. Furthermore, they may stabilise in-situ formed intermediates and avoid the formation of high energy intermediates in the reaction cascade by coupling electron donation/acceptance with proton donation/acceptance.Novel cobalt and copper complexes will be synthesised and subsequently investigated with respect to their ability to facilitate the proton coupled electron transfer. The concept will be then used to activate small molecules. Stable mono- and dinuclear compounds, which can accumulate multiple redox equivalents, will be exploited for (electro)chemical water splitting catalysis. In addition, copper complexes with a central RS−-unit will be synthesised and utilised for functional studies of N2O-decomposition as model compounds for the N2O-reductase. N2O is the second to last product of the denitrification and naturally decomposed by the N2O-reductase in N2 and water by transferring two electrons and two protons to N2O.