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)₂C ₅H₃N) has been examined to determine whether the redox events are metal or ligand based. Treatment of (^iPrPDI)FeN ₂ with [Cp₂Fe][BAr^F₄] (BAr ^F₄ = B(3,5-(CF₃)₂-C ₆H₃)₄) in diethyl ether solution resulted in N₂ loss and isolation of [(^iPrPDI)Fe(OEt ₂)][BAr^F₄]. 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 Reduction of the iron dinitrogen complex where the imine methyl groups have been replaced by phenyl substituents, (^iPrBPDI)Fe(N₂)₂ resulted in isolation of both the mono-and dianionic iron dinitrogen compounds, [( ^iPrBPDI)FeN₂]^- and [(^iPrBPDI)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.