EPR spectroscopy elucidates the electronic structure of [FeV(O)(TAML)] complexes

Abstract

Multifrequency, multitechnique pulse EPR spectroscopy was employed to unravel the spin Hamiltonian parameters of ¹⁷O in the [Fe^VO] moiety with two different tetraamido macrocyclic ligands (TAMLs), [Fe^V(O)(TAML-1)]⁻ (1, H₄(TAML-1) = 3,4,8,9-tetrahydro-3,3,6,6,9-hexamethyl-1H-1,4,8,11-benzotetraazocyclotridecane-2,5,7,10-(6H,11H)-tetrone) and [Fe^V(O)(TAML-2)]⁻ (2, H₄(TAML-2) = H₄[(Me₂CNCOCMe₂NCO)₂CMe₂]), to investigate the electronic structure of Fe^V-oxo species. Although rigorous computational studies on high-valent iron-oxo species have been reported recently, experimental evidence to explicate the electronic structure of Fe^V-oxo species is sparse. In particular, a complete hyperfine tensor of ¹⁷O can hardly be detected. Herein, we successfully probed the hyperfine tensor of ¹⁷O of the Fe^V-oxo moiety using ENDOR spectroscopy. Hence, the EPR spectroscopic results reported here provide a conclusive experimental basis for elucidating the electronic structure of the Fe^V-oxo complex. Moreover, the reactivity of the two different complexes is very distinct, and our results may provide insight into how their electronic structure contributes to their reactivity.