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

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posted on 2013-04-15, 00:00 authored by Pannee Leeladee, Guy N. L. Jameson, Maxime A. Siegler, Devesh Kumar, Sam P. de Visser, David P. Goldberg
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+•).

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