Direct determination of spin-spin interactions in transition metal-radical complexes relevant in catalysis and molecular magnetism by high-frequency electron-paramagnetic resonance

金属-自由基络合物在生物催化中起着决定性的作用，作为单分子磁体变得越来越重要。因此，非无辜配体大大扩展了过渡金属离子(TMI)催化剂的化学，也可以作为底物瞬时结合到金属上，最突出的是小分子，如O2/O2·-/O22-。在含有自由基配体的分子磁体中，强交换耦合被证明能够有效地抑制弛豫路径，例如磁化或拉曼型过程的量子隧道。了解这类络合物的自旋态能量，从而了解金属自旋与配体自由基(S)之间的潜在交换相互作用，对于表征它们的电子结构，将结构性质分别与反应性或松弛行为联系起来是必不可少的。尽管它们针对不同的功能性质进行了优化，但金属自由基催化剂和SMM具有相同的自旋耦合机理。因此，同样有效的方法也可以用于它们的研究。本项目的目的是通过EPR光谱，特别是频域傅立叶变换(FD-FT)THz-EPR，研究与(生物)催化相关的Fe和Co基金属自由基络合物的自旋结构，重点研究氧的活化过程和单分子的磁性。这一方法采用了最新的研究成果，证明这是一种有效、准确和灵敏的方法来评估具有多个顺磁中心的高自旋TMI化合物中的电子交换作用。在生物催化领域，人们主要关注与氧气活化有关的化合物，如裂环双加氧酶催化循环中金属自由基中间体的Fe和Co模型以及CoII-超氧化合物的催化循环。在研究含有自由基配体的分子磁体之前，首先对单核CoII-自由基模型化合物进行了系统的研究，这为后续分析自由基桥联的双核Co和Fe SMM奠定了基础。磁态的EPR谱有助于开发基于哈密顿的强健的自旋系统描述，从而允许从模拟中精确地量化相互作用参数。这些结果分别用于建立金属-自由基络合物的自旋性质、结构性质和反应性或松弛行为之间的磁结构关联。

Metal-radical complexes play a decisive role in (bio-)catalysis and become increasingly important as single-molecule magnets (SMMs). So do non-innocent ligands substantially extend the chemistry of transition metal ion (TMI) catalysts, and can also occur as substrates transiently binding to the metal, most prominently small molecules such as O2/O2•-/O22-. In radical ligand-containing molecular magnets, strong exchange couplings have been shown to be able to efficiently suppress relaxation pathways, such as quantum tunneling of the magnetization or Raman-type processes. Knowledge of the spin state energies of such complexes and thus the underlying exchange interactions between the spins of the metal and the ligand radical(s) is essential for characterization of their electronic structure, to relate structural properties to reactivity or relaxation behaviour, respectively. Despite their optimization towards different functional properties, metal-radical based catalysts and SMMs share the same spin coupling mechanisms. Thus, the same efficient methods can be applied for their investigation.The aim of the project is to study the spin structures in Fe- and Co-based metal-radical complexes with relevance to (bio-)catalysis, with a focus on oxygen activation processes, and single-molecule magnetism by EPR spectroscopy, particularly frequency-domain Fourier-transform (FD-FT) THz-EPR. This approach employs recent achievements demonstrating this to be a powerful, accurate and sensitive method to evaluate electron exchange interactions in high-spin TMI compounds with multiple paramagnetic centers in paramount detail. In doing so, it makes use of the fact that the presence of substantial anisotropy in spin systems, e.g. zero-field splitting or exchange anisotropy, grants formally forbidden transitions between spin-coupled states of different total electron spin St sufficient probability for their detection.In the field of (bio-)catalysis, the focus is mainly on compounds relevant for dioxygen activation, such as Fe and Co models for metal-radical intermediates in the catalytic cycles of ring-cleaving dioxygenase enzymes and CoII-superoxo complexes. The investigation of radical ligand-containing molecular magnets is preceded by systematic studies of mononuclear CoII-radical model complexes, which lay the groundwork for the subsequent analysis of radical-bridged dinuclear Co and Fe SMMs. The EPR spectra of magnetic states serve to develop robust, Hamiltonian-based descriptions of the spin systems that allow precise quantification of interaction parameters from simulations. The results are used to establish magneto-structural correlations between spin properties, structural properties and reactivity or relaxation behaviour, respectively, in metal-radical complexes.