Multiple, Disparate Redox Pathways Exhibited by a Tris(pyrrolido)ethane Iron Complex

Iron­(III)
complexes of the tris­(pyrrolide)­ethane trianion have
been synthesized by reaction of one- and two-electron oxidants with
[(tpe)­Fe­(THF)]­[Li­(THF)4] (tpe = tris­(5-mesitylpyrrolyl)­ethane).
X-ray crystallography, 57Fe Mössbauer, 1H NMR and EPR spectroscopy, SQUID magnetometry, and density functional
theory calculations were employed to rigorously establish the iron
3+ oxidation state. All oxidants employed are proposed to operate
via an inner-sphere electron transfer mechanism. Dialkyl peroxides
and dibenzyldisulfide served to oxidize iron by one electron, and
group transfer of an aryl nitrene unit to the Fe2+ starting
material resulted in formation of Fe3+ amido species following
H-atom abstraction by a presumed nitrenoid intermediate. Single electron
transfer to and from diphenyldiazoalkane was also observed to yield
a diphenyldiazomethanyl radical anion antiferromagnetically coupled
to the S = 5/2 Fe3+. Isolation of Fe3+ complexes of tpe, in comparison with previous results wherein
the tpe ligand was the redox active moiety, presents an unusual juxtaposition
of two noncommunicating redox reservoirs, each accessible via different
reaction pathways (namely, inner- and outer-sphere electron transfer).