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 [Fe-III(TBP(8)Cz)] (TBP(8)Cz = octakis(4-tert-butylphenyl)corrolazinato) with the commercially available chloramine-T (Na+TsNCl-) leads to oxidative N-tosyl transfer to afford [Fe-IV(TBP(8)Cz(+center dot))(NTs)] in dichloromethane/acetonitrile at room temperature. This complex was characterized by UV-vis, Mossbauer (delta = -0.05 mm s(-1), Delta E-Q = 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-pi-cation-radical (S-total = 1/2 ground state). Reactivity studies with triphenylphosphine as substrate show that [Fe-IV(TBP(8)Cz(+center dot))(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 [Fe-IV(TBP(8)Cz(+center dot))(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 Fe-III(TBP(8)Cz) + chloramine-T + PPh3, [Fe-IV(TBP(8)Cz)(NPPh3)] and [Fe-III(TBP(8)Cz)(OPPh3)], which were definitively characterized by X-ray crystallography. The sequential production of Ph3P=NTs, Ph3P=NH, and Ph3P=O was observed by P-31 NMR spectroscopy and led to a proposed mechanism that accounts for all of the observed products. The latter Fe-III complex was then rationally synthesized and structurally characterized from Fe-III(TBP(8)Cz) and OPPh3, providing an important benchmark compound for spectroscopic studies. A combination of Mossbauer and EPR spectroscopies led to the characterization of both intermediate spin (S = 3/2) and low spin (S = 1/2) Fe-III corrolazines, as well as a formally Fe-IV corrolazine which may also be described by its valence tautomer Fe-III(Cz(+center dot)).