[FeIV═O(TBC)(CH3CN)]2+: Comparative Reactivity of Iron(IV)-Oxo Species with Constrained Equatorial Cyclam Ligation
Year: 2012
Type: article
Abstract: [Fe(IV)═O(TBC)(CH(3)CN)](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(3)CN)](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(3)CN)](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(3)CN)](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.
DOI: 10.1021/ja3046298
Institutions: Stanford University, Ewha Womans University, Centre National de la Recherche Scientifique, Université Joseph Fourier, Commissariat à l'Énergie Atomique et aux Énergies Alternatives, CEA Grenoble, Laboratoire de Chimie et Biologie des Métaux, University of Hyogo, Hyogo University, SLAC National Accelerator Laboratory, Stanford Synchrotron Radiation Lightsource
Language: English
Subfield: Inorganic Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
Awards: G113944070, G2300450775, G4200735206, G5154212342, Spectroscopic Characterization of Oxygen Intermediates in Non-heme and Heme Iron Enzymes, G7566245846, UV resonance Raman investigation on detection of higher order structural changes of heme proteins and elucidation of functional regulation mechanism
OpenAlex ID: W2028047546