Oxidation and Reduction of Bis(imino)pyridine Iron Dinitrogen Complexes: Evidence for Formation of a Chelate Trianion

Oxidation and reduction of the bis(imino)pyridine iron dinitrogen compound, (iPrPDI)FeN2 (iPrPDI = 2,6-(2,6-iPr2–C6H3–NCMe)2C5H3N) has been examined to determine whether the redox events are metal or ligand based. Treatment of (iPrPDI)FeN2 with [Cp2Fe][BArF 4] (BArF 4 = B(3,5-(CF3)2-C6H3)4) in diethyl ether solution resulted in N2 loss and isolation of [(iPrPDI)Fe(OEt2)][BArF 4]. The electronic structure of the compound was studied by SQUID magnetometry, X-ray diffraction, EPR and zero-field 57Fe Mössbauer spectroscopy. These data, supported by computational studies, established that the overall quartet ground state arises from a high spin iron(II) center (S Fe = 2) antiferromagnetically coupled to a bis(imino)pyridine radical anion (S PDI = 1/2). Thus, the oxidation event is principally ligand based. The one electron reduction product, [Na(15-crown-5)][(iPrPDI)FeN2], was isolated following addition of sodium naphthalenide to (iPrPDI)FeN2 in THF followed by treatment with the crown ether. Magnetic, spectroscopic, and computational studies established a doublet ground state with a principally iron-centered SOMO arising from an intermediate spin iron center and a rare example of trianionic bis(imino)pyridine chelate. Reduction of the iron dinitrogen complex where the imine methyl groups have been replaced by phenyl substituents, (iPrBPDI)Fe(N2)2 resulted in isolation of both the mono- and dianionic iron dinitrogen compounds, [(iPrBPDI)FeN2]− and [(iPrBPDI)FeN2]2‑, highlighting the ability of this class of chelate to serve as an effective electron reservoir to support neutral ligand complexes over four redox states.