Octahedral iron(<scp>iv</scp>)–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 (<b>1^(IV)[double bond, length as m-dash]NTs</b>) and [Fe^IV(NTs)(Me₂(CHPy₂)tacn)](OTf)₂ (<b>2^(IV)[double bond, length as m-dash]NTs</b>), (MePy₂tacn = <i>N</i>-methyl-<i>N</i>,<i>N</i>-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). <b>1^(IV)[double bond, length as m-dash]NTs</b> and <b>2^(IV)[double bond, length as m-dash]NTs</b> 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). <b>1^(IV)[double bond, length as m-dash]NTs</b> and <b>2^(IV)[double bond, length as m-dash]NTs</b> 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⁺, <b>1^(III)-NHTs</b> (L = MePy₂tacn) and <b>2^(III)-NHTs</b> (L = Me₂(CHPy₂)tacn) have been isolated after the decay of <b>1^(IV)[double bond, length as m-dash]NTs</b> and <b>2^(IV)[double bond, length as m-dash]NTs</b> in solution, spectroscopically characterized, and the molecular structure of [Fe^III(HNTs)(MePy₂tacn)](SbF₆)₂ determined by single crystal X-ray diffraction. Reaction of <b>1^(IV)[double bond, length as m-dash]NTs</b> and <b>2^(IV)[double bond, length as m-dash]NTs</b> with different <i>p</i>-substituted thioanisoles results in the transfer of the tosylimido moiety to the sulphur atom producing sulfilimine products. In these reactions, <b>1^(IV)[double bond, length as m-dash]NTs</b> and <b>2^(IV)[double bond, length as m-dash]NTs</b> 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 <b>1^(IV)[double bond, length as m-dash]NTs</b> and <b>2^(IV)[double bond, length as m-dash]NTs</b> with hydrocarbons containing weak C-H bonds results in the formation of <b>1^(III)-NHTs</b> and <b>2^(III)-NHTs</b> respectively, along with the oxidized substrate. Kinetic analyses indicate that reactions proceed <i>via</i> a mechanistically unusual HAT reaction, where an association complex precedes hydrogen abstraction.