How Does Tuning of the Primary Coordination Sphere Around Ferryl-Oxo Intermediates Affect Structure and Reactivity? Insights into Axial and Equatorial Ligand Field Effects.

Iron-containing oxygenases play key roles in nature as part of the biosynthesis of natural products as well as the biodegradation of xenobiotics. The biosynthesis of natural products often takes place with high stereo-, chemo-, and regioselectivity, which makes these enzymes of interest to biotechnological applications. Interestingly, iron-containing enzymes appear in biology in a variety of coordination environments, including octahedral and trigonal bipyramidal geometries. To understand the structure, spectroscopic properties, and reactivity of these short-lived enzymatic intermediates, biomimetic models have been created. These models lack the secondary coordination sphere environment of proteins but explain the structural, functional, and spectroscopic properties of the metal coordination and the mechanistic features of the reaction. In this perspective, an overview is given on the first-coordination sphere environment of iron-(IV)-oxo intermediates, which are highly reactive oxidants linked to enzymatic reaction mechanisms. In particular, the effect of the axial and equatorial perturbations on the structure, spin-state ordering, and reactivity of these iron-(IV)-oxo complexes is discussed. Although systems with a porphyrinoid ligand environment show a considerable axial ligand effect due to mixing of the axial ligand orbitals with those on the porphyrin scaffold, by contrast, in nonheme iron systems, the axial ligand contribution is much less dominant due to weaker interactions of the axial ligand with equatorial ligands. However, several recent examples have shown that nonheme iron-(IV)-oxo systems are influenced by equatorial perturbations. Overall, we show that biomimetic porphyrinoid and nonheme iron-(IV)-oxo oxidants are versatile catalysts that can be engineered for stereoselective and regiospecific reaction processes.