Elucidating decay pathways of bispidine-iron(IV)-tosylimido complexes: insights gained from decay products

This study investigates the degradation pathways of three iron(IV)-tosylimido complexes coordinated by tetra- and pentadentate bispidine ligands, a class of rigid nitrogen-donor frameworks based on the 3,7-diazabicyclo[3.3.1]nonane scaffold. Using a combination of spectroscopic (UV-vis-NIR, EPR, Mössbauer) and spectrometric techniques (HR ESI-MS/MS) it was revealed that two of the complexes, derived from a tetradentate bispidine ligand and a pentadentate bispidine ligand, follow a common degradation route. This involves initial reduction to a high-spin iron(III) species, followed by ligand demethylation at a nitrogen donor. The demethylation step is confirmed by the elimination of a CH₂NTs (tosylaminomethyl) fragment, as detected by tandem mass spectrometry. This transformation is analogous to formaldehyde loss observed in the degradation of related bispidine-iron(IV)-oxido complexes. In contrast, an isomeric iron(IV)-tosylimido complex bearing a structurally similar pentadentate bispidine ligand follows a distinct pathway, forming a low-spin iron(III)-amido complex of the type [(L)Fe-NHTs]2+. Mössbauer parameters support this assignment and align with previously reported low-spin iron-amido species. We attribute the difference in reactivity to the orientation of the tosylimido group within the coordination sphere: axial in the isomeric pentadentate ligand, but equatorial in the other two. This spatial change influences both the electronic structure and the accessibility of the tosylimido moiety. Although direct observation of the transient iron(IV)-tosylimido intermediate remains elusive in the tetradentate system due to its high reactivity, analysis of its degradation products provides indirect but compelling evidence for its formation. All complexes mediate NTs transfer to styrene, supporting the presence of tosylimido intermediates and offering a platform for rational catalyst design through ligand and counter ion tuning.