Exploring Ligand Field Effects on Fe(IV)-oxo Reactivity Channels for Versatility as a Potential Catalyst for Strong C-H Bond Activation

The non-heme iron(IV)-oxo enzyme intermediates can perform a variety of oxidation reactions in various mononuclear and dinuclear enzymatic systems. These species have generated a large amount of interest due to their potential to catalyze C-H bonds, specifically the sought-after conversion of methane to methanol. Various biomimetic model complexes with S = 2 spin have been synthesized allowing a much more detailed analysis of these high spin systems. Advanced spectroscopic analysis indicates the existence of three pathways for the oxyl radical to form. For the S = 2 complexes, one reactivity pathway occurs through the σ [sigma] channel arising from an electron excitation from the σ [sigma] to σ* [sigma star] orbital, and two degenerate pathways occur through the π [pi] channel arising from an electron excitation going from the π [pi] to the π* [pi star] channel. In molecular complexes that have hydrogen atoms in close proximity to the iron(IV)-oxo site, the π [pi] channel can cause a self-oxidation of ligand C-H bonds that will eventually lead to the self-decay of the catalyst. This distinction means in some cases it would be beneficial to prefer one channel over the other depending on if the p [pi] is contributing to a self-decay pathway. Using density functional theory and multireference wave function theory, we are able to gain an understanding of the characteristics of these reaction channels and how different ligand fields affect them. We benchmarked a Fe(IV)-oxo system to find the most appropriate level of theory to create the model systems. Model systems will allow the understanding of the reactive oxyl intermediate, which is the species responsible for the oxidation reaction. Finding trends of reactivity and understanding the development of the oxyl character will allow the best optimization of these systems and aid in the design of novel catalysts. By considering the reactivity of these systems under the influence of varying ligand field strengths, we can gain a better understanding of how these iron-oxo species can be tuned in order to obtain a desired level of reactivity. We are offering new insight with the use of multireference methods at the CASSCF/CASPT2 level in order to perform an examination of an array of strong and weak field ligand systems.