EPR spectroscopy elucidates the electronic structure of [Fe^V(O)(TAML)] complexes

Multifrequency, multitechnique pulse EPR spectroscopy was employed to unravel the spin Hamiltonian parameters of 17O in the [FeVO] moiety with two different tetraamido macrocyclic ligands (TAMLs), [FeV(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 [FeV(O)(TAML-2)]- (2, H4(TAML-2) = H4[(Me2CNCOCMe2NCO)2CMe2]), to investigate the electronic structure of FeV-oxo species. Although rigorous computational studies on high-valent iron-oxo species have been reported recently, experimental evidence to explicate the electronic structure of FeV-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 FeV-oxo moiety using ENDOR spectroscopy. Hence, the EPR spectroscopic results reported here provide a conclusive experimental basis for elucidating the electronic structure of the FeV-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.