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

高价金属氧化物被认为是各种工业和生物催化过程中的关键中间体。本项目研究新型分子氧化钒团簇中高价金属氧和金属羟基的形成、反应性和可能的稳定性。具有预先形成的金属结合位点的金属功能化钒氧化物[M(L)V12O32X]n- （= {MV12}, M =过渡金属，L =末端配体，X =卤化物模板）被用作生成和可能稳定高价金属的合适模型，因为它们的氧化催化活性已经被证明。特别是，该项目专注于具有多种可达氧化还原过渡（FeII, MnII, CoII）的氧化还原活性3d过渡金属，作为工业催化剂（例如Haber-Bosch工艺）以及酶（例如细胞色素P450）的潜在相关模型。此外，选择了4d/5d金属（RuII, IrII），因为它们可以促进高价氧配合物的分离和结构表征。{MV12}中金属中心的结合模式与典型的金属氧配体（通常是具有三角配位几何的氮供体）根本不同，因此可以期待具有以前未探索的配位行为和新的反应性的新中间体。第一步，研究了现有{MV12}配合物与活性配体（如过氧化物和叠氮化物）反应形成高价物质以及还原{MV12}物质对分子氧的活化。利用实验和理论方法对高价中间体进行表征，了解它们的电子结构、结合模式和反应性。只要有可能，使用单晶x射线衍射进行结构表征将提供对结合情况的详细了解。高价物质的反应活性及其作为催化剂的可能用途将通过合成上重要的底物氧化，如环氧化，碳氢活化和羟基化来探索。原位稳定性研究将为可能的团簇重排或胶体形成提供洞见。因此，该项目提供了一种全新的化合物类别，其反应性可以根据形成的中间体进行合理调整，并且可以通过对现有金属中心进行化学改性来调整反应性。

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.