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 [(tBupyrr2py)Fe(OEt2)] (1) (tBupyrr2py2- = 2,6-bis((3,5-di-tert-butyl)pyrrol-2-yl)pyridine) with KC8 yields the complex [K]2[(tBupyrr2py)Fe]2(mu2-eta1:eta1-N2) (2) where the unusual cis-divacant octahedral coordination geometry about each iron and the eta5-cation-pi coordination of two potassium ions with four pyrrolyl units of the ligand cause distortion of the bridging end-on mu-N2 about the FeN2Fe core. Attempts to generate an Et2O free version of 1 resulted instead in a dinuclear helical dimer [(tBupyrr2py)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[(tBupyrr2py)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(tBupyrr2py)Fe]2(mu2-eta1:eta1-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 Mossbauer spectroscopy have 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.