Addition of O(2)(g) at low-temperature to a mononuclear, nonheme iron(ii) complex comprising a tetraazamacrocyclic N(4) donor and an arylthiolate S donor leads to the generation of a deep red complex assigned as a low-spin Fe(III)?OOH complex, formed via metal- and ligand-assisted oxidation. Full Article

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. Full Article

Iron peroxide species have been identified as important intermediates in a number of nonheme iron as well as heme-containing enzymes, yet there are only a few examples of such species either synthetic or biological that have been well characterized. We describe the synthesis and structural characterization of a new series of five-coordinate (N4S(thiolate))Fe(II) complexes that react with tert-butyl hydroperoxide ((t)BuOOH) or cumenyl hydroperoxide (CmOOH) to give metastable alkylperoxo-iron(III) species (N4S(thiolate)Fe(III)-OOR) at low temperature. These complexes were designed specifically to mimic the nonheme iron active site of superoxide reductase, which contains a five-coordinate iron(II) center bound by one Cys and four His residues in the active form of the protein. The structures of the Fe(II) complexes are analyzed by X-ray crystallography, and their electrochemical properties are assessed by cyclic voltammetry. For the Fe(III)-OOR species, low-temperature UV-vis spectra reveal intense peaks between 500-550 nm that are typical of peroxide to iron(III) ligand-to-metal charge-transfer (LMCT) transitions, and EPR spectroscopy shows that these alkylperoxo species are all low-spin iron(III) complexes. Identification of the vibrational modes of the Fe(III)-OOR unit comes from resonance Raman (RR) spectroscopy, which shows nu(Fe-O) modes between 600-635 cm(-1) and nu(O-O) bands near 800 cm(-1). These Fe-O stretching frequencies are significantly lower than those found in other low-spin Fe(III)-OOR complexes. Trends in the data conclusively show that this weakening of the Fe-O bond arises from a trans influence of the thiolate donor, and density functional theory (DFT) calculations support these findings. These results suggest a role for the cysteine ligand in SOR, and are discussed in light of the recent assessments of the function of the cysteine ligand in this enzyme. Full Article

The activation of dioxygen and its analogues, such as hydrogen peroxide, by metalloporphyrins leads to the generation of high-valent metal-oxo species. This process is critically important to heme-catalyzed reactions, such as for cytochrome P450, and synthetic porphyrin-catalyzed oxidations. We have synthesized a new ring-contracted porphyrinoid system called a corrolazine that is designed to stabilize high oxidation states, including high-valent metal-oxo species. The corrolazine ligand stabilizes manganese(V) terminal oxo and terminal imido complexes for isolation, both of which are only transiently observed with normal porphyrin macrocycles. Examination of both oxygen atom transfer and hydrogen atom abstraction reactions for the Mn(V)-oxo complex has led to a number of mechanistic insights regarding these transformations. The activation of H 2O 2 to give the Mn(V)-oxo complex exhibits some dramatic and unexpected axial ligand effects that call into question the normal role of axial ligands in O-O bond cleavage pathways. Full Article

Full Article