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]­[BAr^F₄] (BAr^F₄ = B­(3,5-(CF₃)₂-C₆H₃)₄) in diethyl ether solution resulted in N₂ loss and isolation of [(^iPrPDI)­Fe­(OEt₂)]­[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 (<i>S</i>_Fe = 2) antiferromagnetically coupled to a bis­(imino)­pyridine radical anion (<i>S</i>_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 Reduction of the iron dinitrogen complex where the imine methyl groups have been replaced by phenyl substituents, (^iPrBPDI)­Fe­(N₂)₂ resulted in isolation of both the mono- and dianionic iron dinitrogen compounds, [(^iPrBPDI)­FeN₂]⁻ and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