Catalytic Reactivity of a Meso-N-Substituted Corrole and Evidence for a High-Valent Iron-Oxo Species

It is shown that an iron(III) meso-N-substituted corrole (TBP(8)Cz)Fe-III (1) (TBP(8)CZ = octakis(4-tert-butylphenyl)corrolazinato), is a potent catalyst for the oxidation of alkenes in the presence of pentaflouroiodosylbenzene (C6F5IO) as oxidant. In the case of cyclohexene, complex 1 performs on a par with one of the best porphyrin catalysts ((TPPF20)FeCl), exhibiting rapid turnover and a high selectivity for epoxide (CzFe(III)/C6F5IO/cyclohexene (1:100:1000) in CH2Cl2/CH3OH (3:1 v:v) gives 33 turnovers of epoxide.in <2 min). Reaction rates for 1 are greatly enhanced compared to other Fe or Mn corroles under similar catalytic conditions, consistent with an increase in the electrophilicity of a high-valent iron-oxo intermediate induced by meso-N substitution. Reaction of dark-green 1 (lambda(max) = 440, 611, 747 nm) under single-turnover-ike conditions at -78 degrees C leads to the formation of a new dark-brown species (2) (lambda(max) = 396, 732, 843 nm). The Fe-III complex 1 is restored upon the addition of 2 equiv of ferrocene to 2, or by the addition of 1 equiv of PPh3, which concomitantly yields OPPh3. In addition, complex 2 reacts with excess cyclohexene at -42 degrees C to give 1. Complex 2 was also characterized by EPR spectroscopy, and all of the data are consistent with 2 being an antiferromagnetically coupled iron(IV)-oxo pi-cation-radical complex. Rapid-mixing stopped-flow UV-vis studies show that the low-temperature complex 2 is generated as a short-lived intermediate at room temperature.