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 worIc One- electron reduction of [((tBu)pyrr(2)py)Fe(OEt2)] ((tBu)pyrr(2)py(2-) = 2,6- bis((3,5-di-tert-butyl)pyrrol-2-yl)pyridine) with KC8 yields the complex [K](2)[(tBu)pyrr(2)py)Fe](2)(mu(2)-eta(1):eta(1)-N-2) (2), where the unusual cis-divacant octahedral coordination geometry about each iron and the eta(2)-cation-pi coordination of two potassium ions with four pyrrolyl units of the ligand cause distortion of the bridging end-on mu-N-2 about the FeN2Fe core. Attempts to generate a Et2O-free version of 1 resulted instead in a dinuclear helical dimer, [((tBu)pyrr(2)py, Fe](2) (3), via bridging of the pyridine moieties of the ligand. Reduction of 3 by two electrons under N-2 does not break up the dimer, nor does it result in formation of 2 but instead formation of the ate-complex [K(OEt2)](2). [((tBu)Pyrr(2)py)Fe](2) (4). Reduction of 1 by two electrons and in the presence of crown-ether forms the tetraanionic N-2 complex [K-2] [K(18-crown-6)](2) ((tBu)pyrr(2)py)Fe]2(mu(2)-eta(1):eta(1)-N-2) (5), also having a distorted FeN2Fe moiety akin to 2. Complex 2 is thermally unstable and loses N-2, disproportionating to Fe nanoparticles among other products. A combination of single-crystal X-ray diffraction studies, solution and solid-state magnetic studies, and Fe-57 Mossbauer 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-N-2 complexes 2 and 5.