One-electron oxidation of an oxoiron(IV) complex to form an [O═Feⱽ═NR]⁺ center

• Published in: Proceedings of the National Academy of Sciences - PNAS 2012-07, Vol.109 (30), p.11933-11938
• Main Authors: Van Heuvelen, Katherine M, Fiedler, Adam T, Shan, Xiaopeng, De Hont, Raymond F, Meier, Katlyn K, Bominaar, Emile L, Münck, Eckard, Que, Lawrence
• Format: Article
• Language: English
• Subjects: acetonitrile; Alkanes; Anisotropy; catalysts; Chemistry; Electronic structure; Electrons; hydroxylation; Iron; Iron catalysts; Iron Isotopes - chemistry; Ligands; Macromolecular Substances - chemistry; Mass Spectrometry; Mathematical models; Molecular orbitals; Molecular Structure; nitrogen; Nitrogen Isotopes - chemistry; olefin; Oxidation; Oxidation-Reduction; oxygen; Oxygen Isotopes - chemistry; Physical Sciences; Raman scattering; spectral analysis; Spectroscopy; stable isotopes; Tensors
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1206457109

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 one-electron oxidation of the S = 1 complex [Fe ᴵⱽ(O)(TMC)(NCCH ₃)] ²⁺ (1 , where TMC is tetramethylcyclam) by treatment with tert -butyl hydroperoxide and strong base in acetonitrile to generate a metastable [Formula] 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 [Formula] 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 DFT calculations 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)] ⁺.