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

• Applicant: Dr. Thomas Lohmiller
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 469127238

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.

• DFG Programme: Research Grants