(Fe-IV=O(TBC)(CH3CN)](2+): Comparative Reactivity of Iron(IV)-Oxo Species with Constrained Equatorial Cyclam Ligation

Abstract

[Fe-IV=O(TBC)(CH3CN)](2+) (TBC = 1,4,8,11-tetrabenzyl-1,4,8,11-tetraazacyclotetradecane) is characterized, and its reactivity differences relative to [Fe-IV=O(TMC)(CH3CN)](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 (similar to 25 kcal/mol) due to steric interactions between the cyclam chelate and the substrate (e.g., ethylbenzene) associated with the equatorial pi-attack required by this spin state. Conversely, H-atom abstraction reactivity on an S = 2 surface allows for a a-attack with an axial substrate approach. This results in decreased steric interactions with the cyclam and a lower barrier (similar to 9 kcal/mol). For [Fe-IV=O(TBC)(CH3CN)](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 sigma-