Octahedral iron(IV)–tosylimido complexesexhibiting single electron-oxidation reactivity

International audienceHigh 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)(MePy(2)tacn)](OTf)(2) (1((IV))& xe001;NTs) and [Fe-IV(NTs)(Me-2(CHPy2)tacn)](OTf)(2) (2((IV))& xe001;NTs), (MePy(2)tacn = N-methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane, and Me-2(CHPy2)tacn = 1-(di(2-pyridyl)methyl)-4,7-dimethyl-1,4,7-triazacyclononane, Ts = Tosyl). 1((IV))& xe001;NTs and 2((IV))& xe001;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 = MePy(2)tacn and Me-2(CHPy2)tacn, respectively). 1((IV))& xe001;NTs and 2((IV))& xe001;NTs are metastable and have been spectroscopically characterized by HR-MS, UV-vis, H-1-NMR, resonance Raman, Mossbauer, and X-ray absorption (XAS) spectroscopy as well as by DFT computational methods. Ferric complexes [Fe-III(HNTs)(L)](2+), 1((III))-NHTs (L = MePy(2)tacn) and 2((III))-NHTs (L = Me-2(CHPy2)tacn) have been isolated after the decay of 1((IV))& xe001;NTs and 2((IV))& xe001;NTs in solution, spectroscopically characterized, and the molecular structure of [Fe-III(HNTs)(MePy(2)tacn)](SbF6)(2) determined by single crystal X-ray diffraction. Reaction of 1((IV))& xe001;NTs and 2((IV))& xe001;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))& xe001;NTs and 2((IV))& xe001;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))& xe001;NTs and 2((IV))& xe001;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

Triggering the generation of an iron(IV)-oxo compound and its reactivity toward sulfides by Ru(II) photocatalysis.

International audienceThe preparation of [Fe(IV)(O)(MePy2tacn)](2+) (2, MePy2tacn = N-methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane) by reaction of [Fe(II)(MePy2tacn)(solvent)](2+) (1) and PhIO in CH3CN and its full characterization are described. This compound can also be prepared photochemically from its iron(II) precursor by irradiation at 447 nm in the presence of catalytic amounts of [Ru(II)(bpy)3](2+) as photosensitizer and a sacrificial electron acceptor (Na2S2O8). Remarkably, the rate of the reaction of the photochemically prepared compound 2 toward sulfides increases 150-fold under irradiation, and 2 is partially regenerated after the sulfide has been consumed; hence, the process can be repeated several times. The origin of this rate enhancement has been established by studying the reaction of chemically generated compound 2 with sulfides under different conditions, which demonstrated that both light and [Ru(II)(bpy)3](2+) are necessary for the observed increase in the reaction rate. A combination of nanosecond time-resolved absorption spectroscopy with laser pulse excitation and other mechanistic studies has led to the conclusion that an electron transfer mechanism is the most plausible explanation for the observed rate enhancement. According to this mechanism, the in-situ-generated [Ru(III)(bpy)3](3+) oxidizes the sulfide to form the corresponding radical cation, which is eventually oxidized by 2 to the corresponding sulfoxide