Generation, Spectroscopic, and Chemical Characterization of an Octahedral Iron(V)-Nitrido Species with a Neutral Ligand Platform

Iron complex [Fe<sup>III</sup>(N<sub>3</sub>)­(MePy<sub>2</sub>tacn)]­(PF<sub>6</sub>)<sub>2</sub> (<b>1</b>), containing a neutral triazacyclononane-based pentadentate ligand, and a terminally bound azide ligand has been prepared and spectroscopically and structurally characterized. Structural details, magnetic susceptibility data, and Mössbauer spectra demonstrate that <b>1</b> has a low-spin (<i>S</i> = 1/2) ferric center. X-ray diffraction analysis of <b>1</b> reveals remarkably short Fe–N (1.859 Å) and long FeN–N<sub>2</sub> (1.246 Å) distances, while the FT-IR spectra show an unusually low N–N stretching frequency (2019 cm<sup>–1</sup>), suggesting that the FeN–N<sub>2</sub> bond is particularly weak. Photolysis of <b>1</b> at 470 or 530 nm caused N<sub>2</sub> elimination and generation of a nitrido species that on the basis of Mössbauer, magnetic susceptibility, EPR, and X-ray absorption in conjunction with density functional theory computational analyses is formulated as [Fe<sup>V</sup>(N)­(MePy<sub>2</sub>tacn)]<sup>2+</sup> (<b>2</b>). Results indicate that <b>2</b> is a low-spin (<i>S</i> = 1/2) iron­(V) species, which exhibits a short Fe–N distance (1.64 Å), as deduced from extended X-ray absorption fine structure analysis. Compound <b>2</b> is only stable at cryogenic (liquid N<sub>2</sub>) temperatures, and frozen solutions as well as solid samples decompose rapidly upon warming, producing N<sub>2</sub>. However, the high-valent compound could be generated in the gas phase, and its reactivity against olefins, sulfides, and substrates with weak C–H bonds studied. Compound <b>2</b> proved to be a powerful two-electron oxidant that can add the nitrido ligand to olefin and sulfide sites as well as oxidize cyclohexadiene substrates to benzene in a formal H<sub>2</sub>-transfer process. In summary, compound <b>2</b> constitutes the first case of an octahedral Fe<sup>V</sup>(N) species prepared within a neutral ligand framework and adds to the few examples of Fe<sup>V</sup> species that could be spectroscopically and chemically characterized.