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

Iron complex [Fe-III(N-3)(MePy(2)tacn)](PF6)(2) (1), 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 Mossbauer spectra demonstrate that 1 has a low-spin (S = 1/2) ferric center. X-ray diffraction analysis of 1 reveals remarkably short Fe-N (1.859 angstrom) and long FeN-N-2 (1.246 angstrom) distances, while the FT-IR spectra show an unusually low N-N stretching frequency (2019 cm(-1)), suggesting that the FeN-N-2 bond is particularly weak. Photolysis of 1 at 470 or 530 nm caused N-2 elimination and generation of a nitrido species that on the basis of Mossbauer, magnetic susceptibility, EPR, and X-ray absorption in conjunction with density functional theory computational analyses is formulated as [Fe-V(N)(MePy(2)tacn)](2+) (2). Results indicate that 2 is a low spin (S = 1/2) iron(V) species, which exhibits a short Fe-N distance (1.64 angstrom), as deduced from extended X-ray absorption fine structure analysis. Compound 2 is only stable at cryogenic (liquid N-2) temperatures, and frozen solutions as well as solid samples decompose rapidly upon warming, producing N-2. 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 2 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-2-transfer process. In summary, compound 2 constitutes the first case of an octahedral Fe-V(N) species prepared within a neutral ligand framework and adds to the few examples of Fe-V species that could be spectroscopically and chemically characterized.