Octahedral iron(iv)–tosylimido complexes exhibiting single electron-oxidation reactivity

Abstract

High valent iron species are very reactive molecules involved in oxidation reactions of relevance to biology and chemical synthesis. Herein we describe iron(IV)–tosylimido complexes [Fe^IV(NTs)(MePy₂tacn)](OTf)₂ (1^(IV)NTs) and [Fe^IV(NTs)(Me₂(CHPy₂)tacn)](OTf)₂ (2^(IV)NTs), (MePy₂tacn = N-methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane, and Me₂(CHPy₂)tacn = 1-(di(2-pyridyl)methyl)-4,7-dimethyl-1,4,7-triazacyclononane, Ts = Tosyl). 1^(IV)NTs and 2^(IV)NTs are rare examples of octahedral iron(IV)–imido complexes and are isoelectronic analogues of the recently described iron(IV)–oxo complexes [Fe^IV(O)(L)]²⁺ (L = MePy₂tacn and Me₂(CHPy₂)tacn, respectively). 1^(IV)NTs and 2^(IV)NTs are metastable and have been spectroscopically characterized by HR-MS, UV-vis, ¹H-NMR, resonance Raman, Mössbauer, and X-ray absorption (XAS) spectroscopy as well as by DFT computational methods. Ferric complexes [Fe^III(HNTs)(L)]²⁺, 1^(III)–NHTs (L = MePy₂tacn) and 2^(III)–NHTs (L = Me₂(CHPy₂)tacn) have been isolated after the decay of 1^(IV)NTs and 2^(IV)NTs in solution, spectroscopically characterized, and the molecular structure of [Fe^III(HNTs)(MePy₂tacn)](SbF₆)₂ determined by single crystal X-ray diffraction. Reaction of 1^(IV)NTs and 2^(IV)NTs with different p-substituted thioanisoles results in the transfer of the tosylimido moiety to the sulphur atom producing sulfilimine products. In these reactions, 1^(IV)NTs and 2^(IV)NTs behave as single electron oxidants and Hammett analyses of reaction rates evidence that tosylimido transfer is more sensitive than oxo transfer to charge effects. In addition, reaction of 1^(IV)NTs and 2^(IV)NTs with hydrocarbons containing weak C–H bonds results in the formation of 1^(III)–NHTs and 2^(III)–NHTs respectively, along with the oxidized substrate. Kinetic analyses indicate that reactions proceed via a mechanistically unusual HAT reaction, where an association complex precedes hydrogen abstraction.