One-electron oxidation of an oxoiron(IV) complex to form an [O=Fe ^V=NR]⁺ center

Oxoiron(V) species are postulated to be involved in the mechanisms of the arene cis-dihydroxylating Rieske dioxygenases and of bioinspired nonheme iron catalysts for alkane hydroxylation, olefin cis-dihydroxylation, and water oxidation. In an effort to obtain a synthetic oxoiron(V) complex, we report herein the oneelectron oxidation of the S = 1 complex [Fe^IV(O)(TMC) (NCCH₃)]²⁺(1, where TMC is tetramethylcyclam) by treatment with tert -butyl hydroperoxide and strong base in acetonitrile to generate a metastable S = 1/2 complex 2 at -44 °C, which has been characterized by UV-visible, resonance Raman, Mössbauer, and EPR methods. The defining spectroscopic characteristic of 2 is the unusual x/y anisotropy observed for the ⁵⁷Fe and ¹⁷O A tensors associated with the high-valent Fe=O unit and for the ¹⁴N A tensor of a ligand derived from acetonitrile. As shown by detailed density functional theory (DFT) calculations, the unusual x/y anisotropy observed can only arise from an iron center with substantially different spin populations in the d_xz and d _yz orbitals, which cannot correspond to an Fe^IV=O unit but is fully consistent with an S = 1/2 Fe^V center, like that found for [Fe^V(O)(TAML)]^- (where TAML is tetraamido macrocyclic ligand), the only well-characterized oxoiron(V) complex reported. Mass spectral analysis shows that the generation of 2 entails the addition of an oxygen atom to 1 and the loss of one positive charge. Taken together, the spectroscopic data and DFTcalculations support the formulation of 2 as an iron(V) complex having axial oxo and acetylimido ligands, namely [Fe^V(O)(TMC)(NC(O)CH ₃)]⁺.