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

Abstract

Iron complex [Fe^III(N₃)(MePy₂tacn)](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 Mössbauer 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 Å) and long FeN-N₂ (1.246 Å) distances, while the FT-IR spectra show an unusually low N-N stretching frequency (2019 cm^-1), suggesting that the FeN-N₂ bond is particularly weak. Photolysis of 1 at 470 or 530 nm caused N₂ 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^V(N)(MePy₂tacn)]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 Å), as deduced from extended X-ray absorption fine structure analysis. Compound 2 is only stable at cryogenic (liquid N₂) temperatures, and frozen solutions as well as solid samples decompose rapidly upon warming, producing N₂. 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 H₂-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.