Nature utilizes extraordinarily reactive high-valent iron oxo species (Fe(IV)=O or Fe(V)=O) in combination with redox-active ligands and second-sphere environment to perform extremely efficient oxygenation reactions. Inspired by this biological approach, in this project we will synthesize a series of mono- or dimeric Fe(II)/Fe(III) complexes containing phenol or quinol redox non-innocent units. Taking advantage of our mechanistic understanding, drawn from our previous research, together with a strong expertise in the application of rather unconventional cryo- and high-pressure-techniques, we will employ the synthesized iron complexes to clarify still-open questions or dichotomies existing in the field. Special attention will be paid to the mechanisms of substrate oxidation by intermediate-spin (IS) and high-spin (HS) Fe(IV)=O species and to the factors affecting the generation and reactivity of Fe(V)=O species in comparison to those of IS and HS Fe(IV)=O intermediates. The iron complexes synthesized for this purpose also represent suitable systems to investigate possible participation of redox non-innocent quinol moieties and/or potential involvement of a (L•)Fe(IV)=O/(L- )Fe(V)=O valence tautomerism in the reactivity of Fe(IV)=O. In addition, they will allow us to explore effects of introducing oxygen donor atoms that provide a weak ligand field around the Fe center, on the spin state and reactivity of the resulting high-valent intermediates. With the aim of mechanistic elucidations and modulation of reaction rates, we will introduce, for the first time in this particular research area, pressure effect studies and combine them with trapping and detecting reaction intermediates by cryo-MS investigations. It should be highlighted that the accomplishment of the objectives intended in this project is only achievable within the collaborative work with other research groups involved in this Research Unit that will share with us their strong expertise in various spectroscopic and/or theoretical methods. Vice versa, our knowledge related to mechanistic investigations and application of rather rare methodologies will be offered to our research collaborators within this Research Unit. A synergic interaction between syntheses and application of a variety of physicochemical methodologies anticipated in the present project will not only advance our understanding of what happens at the active iron center, but can directly provide valuable feedback for the redesign of new generations of functionalized iron complexes displaying more desirable catalytic features.