Generation of a High-Valent Iron Imido Corrolazine Complex and NR Group Transfer Reactivity

The generation of a new high-valent
iron terminal imido complex prepared with a corrolazine macrocycle
is reported. The reaction of [FeIII(TBP8Cz)]
(TBP8Cz = octakis­(4-tert-butylphenyl)­corrolazinato)
with the commercially available chloramine-T (Na+TsNCl–) leads to oxidative N-tosyl transfer to afford [FeIV(TBP8Cz+•)­(NTs)] in dichloromethane/acetonitrile
at room temperature. This complex was characterized by UV–vis,
Mössbauer (δ = −0.05 mm s–1,
ΔEQ = 2.94 mm s–1), and EPR (X-band (15 K), g = 2.10, 2.00) spectroscopies,
and together with reactivity patterns and DFT calculations has been
established as an iron­(IV) species antiferromagnetically coupled with
a Cz-π-cation-radical (Stotal = 1/2 ground state). Reactivity studies with triphenylphosphine
as substrate show that [FeIV(TBP8Cz+•)­(NTs)] is an efficient NTs transfer agent, affording the phospharane
product Ph3PNTs under both stoichiometric and catalytic
conditions. Kinetic analysis of this reaction supports a bimolecular
NTs transfer mechanism with rate constant of 70(15) M–1 s–1. These data indicate that [FeIV(TBP8Cz+•)­(NTs)] reacts about 100 times
faster than analogous Mn terminal arylimido corrole analogues. It
was found that two products crystallize from the same reaction mixture
of FeIII(TBP8Cz) + chloramine-T + PPh3, [FeIV(TBP8Cz)­(NPPh3)] and [FeIII(TBP8Cz)­(OPPh3)], which were definitively
characterized by X-ray crystallography. The sequential production
of Ph3PNTs, Ph3PNH, and Ph3PO was observed by 31P NMR spectroscopy
and led to a proposed mechanism that accounts for all of the observed
products. The latter FeIII complex was then rationally
synthesized and structurally characterized from FeIII(TBP8Cz) and OPPh3, providing an important benchmark
compound for spectroscopic studies. A combination of Mössbauer
and EPR spectroscopies led to the characterization of both intermediate
spin (S = 3/2) and low spin
(S = 1/2) FeIII corrolazines,
as well as a formally FeIV corrolazine which may also be
described by its valence tautomer FeIII(Cz+•).