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]­[BArF4] (BArF4 = B­(3,5-(CF3)2-C6H3)4) in diethyl ether solution resulted in N2 loss and isolation of [(iPrPDI)­Fe­(OEt2)]­[BArF4]. 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 (SFe = 2) antiferromagnetically coupled to a bis­(imino)­pyridine
radical anion (SPDI = 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methyl groups have been replaced by phenyl substituents, (iPrBPDI)­Fe­(N2)2 resulted in isolation of both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.