Octahedral Iron (IV)-Tosylimido Complexes Exhibiting Single Electron-Oxidation Reactivity

The reaction of [FeII(CH3CN)(L)](OTf)2, where L stands for pentadentate N-based aminopyridine ligands derived from triazacyclononane (1(II)-OTf, L = MePy2tacn = N-methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane, and 2(II)-OTf, L = Me2(CHPy2)tacn = 1-(di(2- pyridyl)methyl)-4,7-dimethyl-1,4,7-triazacyclononane) with PhINTs, (Ts = Tosyl) produces high-valent complexes [FeIV(NTs)(MePy2tacn)](OTf)2 (1(IV)=NTs) and [FeIV(NTs)(Me2(CHPy2)tacn)](OTf)2 (2(IV)=NTs). 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 [FeIV(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 [FeIII(HNTs)(L)]2+, 1(III)–NHTs (L = MePy2tacn) and 2(III)–NHTs (L = Me2(CHPy2)tacn) have been isolated after they decay in solution, spectroscopically characterized, and the molecular structure of [FeIII(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. Mechanistic studies reveal that the reactions entail a single e- transfer (SET) rate determining step. 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 reveal that reactions proceed via a two-step process; the first is a bimolecular reversible binding of the substrate to the iron(IV) species. The second step is irreversible and bears the characteristics of a hydrogen atom transfer (HAT) process. The data thus puts forward an unusual mechanistic frame for a HAT reaction, where an association complex precedes hydrogen abstraction.