High-valent metal oxo species stabilized by polyoxovanadates: synthesis, properties and catalytic activity

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High-valent metal oxo species are considered key intermediates in various industrial and biological catalytic processes. This project investigates the formation, reactivity and possible stabilization of high-valent metal oxo and metal-hydroxo species in novel molecular vanadium oxide clusters. Metal-functionalized vanadium oxides [M(L)V12O32X]n- (= {MV12}, M = transition metal, L = terminal ligand, X = halide-template) with a pre-formed metal binding site are used as suitable models for the generation and possible stabilization of high-valent metal species as their oxidative-catalytic activity has been shown previously. In particular, this project is focused on redox-active 3d-transition metals with multiple accessible redox-transitions (FeII, MnII, CoII) as potentially relevant models for industrial catalysts (e.g. Haber-Bosch process) as well as enzymes (e.g. cytochrome P450). In addition, selected 4d/5d metals (RuII, IrII) are employed as they can facilitate the isolation and structural characterization of high-valent oxo complexes. The binding mode of metal centres in {MV12} is fundamentally different to typical metal oxo ligands (often: nitrogen donors with trigonal coordination geometry), therefore new intermediates with a previously un-explored coordination behaviour and new reactivity can be expected. In the first step, the formation of high-valent species by reaction of existing {MV12} complexes with reactive ligands such as peroxides and azides as well as the activation of molecular oxygen by reduced {MV12} species is investigated. High-valent intermediates are characterized using experimental and theoretical methods to understand their electronic structure, binding modes and reactivity. Whenever possible, structural characterization using single-crystal X-ray diffraction will provide detailed insight into the binding situation. The reactivity of the high-valent species as well as their possible use as catalysts will be explored using synthetically important substrate oxidations, e.g. epoxidations, C-H-activations and hydroxylations. In situ stability studies will provide insight into possible cluster rearrangement or colloid formation. The project therefore provides access to a completely new compound class whose reactivity can be rationalized based on the intermediates formed and where reactivity can be tuned by chemical modification of the metal centres present.