[Fe ^IV=O(TBC)(CH ₃CN)] ²⁺: Comparative reactivity of iron(IV)-oxo species with constrained equatorial cyclam ligation

[Fe ^IV=O(TBC)(CH ₃CN)] ²⁺ (TBC = 1,4,8,11-tetrabenzyl-1,4,8,11-tetraazacyclotetradecane) is characterized, and its reactivity differences relative to [Fe ^IV=O(TMC)(CH ₃CN)] ²⁺ (TMC = 1,4,8,11-tetramethyl-1,4,8,11- tetraazacyclotetradecane) are evaluated in hydrogen atom (H-atom) abstraction and oxo-transfer reactions. Structural differences are defined using X-ray absorption spectroscopy and correlated to reactivities using density functional theory. The S = 1 ground states are highly similar and result in large activation barriers (∼25 kcal/mol) due to steric interactions between the cyclam chelate and the substrate (e.g., ethylbenzene) associated with the equatorial π-attack required by this spin state. Conversely, H-atom abstraction reactivity on an S = 2 surface allows for a σ-attack with an axial substrate approach. This results in decreased steric interactions with the cyclam and a lower barrier (∼9 kcal/mol). For [Fe ^IV=O(TBC) (CH ₃CN)] ²⁺, the S = 2 excited state in the reactant is lower in energy and therefore more accessible at the transition state due to a weaker ligand field associated with the steric interactions of the benzyl substituents with the trans-axial ligand. This study is further extended to the oxo-transfer reaction, which is a two-electron process requiring both σ- and π-electron transfer and thus a nonlinear transition state. In oxo-transfer, the S = 2 has a lower barrier due to sequential vs concerted (S = 1) two electron transfer which gives a high-spin ferric intermediate at the transition state. The [Fe ^IV=O(TBC)(CH ₃CN)] ²⁺ complex is more distorted at the transition state, with the iron farther out of the equatorial plane due to the steric interaction of the benzyl groups with the trans-axial ligand. This allows for better orbital overlap with the substrate, a lower barrier, and an increased rate of oxo-transfer.