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 (C(6)F(5)IO) as oxidant. In the case of cyclohexene, complex 1 performs on a par with one of the best porphyrin catalysts ((TPPF(20))FeCl), exhibiting rapid turnover and a high selectivity for epoxide (CzFe(III)/C(6)F(5)IO/cyclohexene (1:100:1000) in CH(2)Cl(2)/CH(3)OH (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-like 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 PPh(3), which concomitantly yields OPPh(3). 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.
McGown
Amanda J
AJ
Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore Maryland 21218, USA.
Kerber
William D
WD
Fujii
Hiroshi
H
Goldberg
David P
DP
D000475:Alkenes / Q000737:chemistry; D002384:Catalysis; D003510:Cyclohexanes / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D011166:Porphyrins / Q000737:chemistry*
D000475:Alkenes; D003510:Cyclohexanes; D009942:Organometallic Compounds; D011166:Porphyrins; C431863:corrole; D007501:Iron