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High-Valent Metal-Oxo Coordination Complexes for Homogenous Water Oxidation

Subject Area Inorganic Molecular Chemistry - Synthesis and Characterisation
Term from 2017 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 390531541
 
Final Report Year 2025

Final Report Abstract

Metal-(hydro)peroxo and -superoxo intermediates play a crucial role in metal-aided oxidation of water and organic substrates, as well as in activating O2, H2O2 and O2–. Their reactivity is dictated by structural and electronic properties, protonation, and/or O–O bond cleavage. The latter, coupled with protonation, leads to highly reactive MIV/MV-oxo species, which drive oxidation reactions, H2O2 and even O2– dismutation. Alternatively, protonation of M-peroxo intermediates results in H2O2 release as a product of either the 2e– reduction of O2 or the 2e– oxidation of H2O, as well as one of the products of O2– dismutation. This study explores the formation, structure, and reactivity of these M-oxygen species, including MIV/MV-oxo intermediates, using Fe, Mn, Ru, Co, Cu, Ni, and Zn in mono-, di- and tetranuclear complexes with acyclic and macrocyclic ligands, both with and without redox-active or positively charged moieties. Key findings include: • Formation and Reactivity of MIV/MV-Oxo Species: i) a ligand with O-donor atoms and bulky groups that facilitates a trigonal-bipyramidal high-spin Fe complex enables formation of highly reactive FeIV-oxo; ii) adjusting the N4 tetradentate ligand rotational flexibility and pH value enhance RuIV/RuV-oxo-mediated water oxidation; iii) protonation of a biomimetic FeIII2-(μ-O)(μ1,2-peroxo) yields a reactive “closed” core FeIV2(μ-O)2 capable of methanol oxidation; iv) a tetranuclear {MnII4L4}4+ cluster, resembling the OEC Mn4O4 core, facilitates electrocatalytic water oxidation involving [MnIIMnIII2MnIV] or [MnII2MnIV2]/[MnII2MnIIIMnIV] and O2 reduction via [MnII4MnIII-peroxo]. • Interplay of Redox-Active Ligands and Metal Centers: i) an acyclic ligand with two quinolates supports the formation of MnIV-oxo as an SOD intermediate; ii) with macrocyclic ligands, H2O2 is dismutated via MnIV-oxo and FeIV-oxo without redox cycling of quinolates; Mn complex exhibits both catalase and SOD activity, while the Fe analog only has catalase activity; iii) ligand-centered catalysis enables H2O2 and O2– dismutation, requiring ZnII or NiII centers to modulate ligand redox potentials; iv) the high stability of the Ni SOD catalyst arises from balanced inner- and outer-sphere electron transfer, preventing ligand overoxidation. • Protonation and M-(Su)Peroxo Reactivity: i) positively charged ligand moieties inhibit FeIII-peroxo protonation, blocking H2O2 release, whereas CuII analogs remain catalytically active due to kinetic lability; ii) CoIII-superoxo species are unreactive due to high stability and inertness; iii) The first side-on Mn2II,III-η2-peroxo was characterized, transforming into a Mn2II,III-μ1,2-peroxo, which protonates to a μ1,1-hydroperoxo species, enhancing SOD activity via rapid H2O2 release. These findings offer insights into M-oxygen species structure and reactivity, guiding the development of more efficient catalysts for oxygen species transformations in water-splitting and fuel cell technologies.

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