Unusual Dinitrogen Binding and Electron Storage in Dinuclear Iron Complexes

A rare example of a dinuclear iron core with a non-linearly bridged dinitrogen ligand is reported in this work. One-electron reduction of [(pyrr₂py)Fe(OEt₂)] (1) (pyrr₂py^2- = 2,6-bis((3,5-di-tert-butyl)pyrrol-2-yl)pyridine) with KC₈ yields the complex [K]₂[(pyrr₂py)Fe]₂(μ₂-η¹:η¹-N₂) (2), where the unusual cis-divacant octahedral coordination geometry about each iron and the η₅-cation-π coordination of two potassium ions with four pyrrolyl units of the ligand cause distortion of the bridging end-on μ-N₂ about the FeN₂Fe core. Attempts to generate a Et₂O-free version of 1 resulted instead in a dinuclear helical dimer, [(pyrr₂py)Fe]₂ (3), via bridging of the pyridine moieties of the ligand. Reduction of 3 by two electrons under N₂ does not break up the dimer, nor does it result in formation of 2 but instead formation of the ate-complex [K(OEt₂)]₂[(pyrr₂py)Fe]₂ (4). Reduction of 1 by two electrons and in the presence of crown-ether forms the tetraanionic N₂ complex [K₂][K(18-crown-6)]₂(pyrr₂py)Fe]₂(μ₂-η¹:η¹-N₂) (5), also having a distorted FeN₂Fe moiety akin to 2. Complex 2 is thermally unstable and loses N₂, disproportionating to Fe nanoparticles among other products. A combination of single-crystal X-ray diffraction studies, solution and solid-state magnetic studies, and ⁵⁷Fe Mössbauer spectroscopy has been applied to characterize complexes 2-5, whereas DFT studies have been used to help explain the bonding and electronic structure in these unique diiron-N₂ complexes 2 and 5.