Artificial metalloenzymes for asymmetric catalysis were created by in vivo incorporation of unnatural metal binding amino acids into LmrR.

The reaction of [(PyNMe)Fe(CFSO)], 1, with excess peracetic acid at -40 °C generates a highly reactive intermediate, 2b(PAA), that has the fastest rate to date for oxidizing cyclohexane by a nonheme iron species. It exhibits an intense 490 nm chromophore associated with an S = 1/2 EPR signal having g-values at 2.07, 2.01, and 1.94. This species was shown to be in a fast equilibrium with a second S = 1/2 species, 2a(PAA), assigned to a low-spin acylperoxoiron(III) center. Unfortunately, contaminants accompanying the 2(PAA) samples prevented determination of the iron oxidation state by Mössbauer spectroscopy. Use of MeO-PyNMe (an electron-enriched version of PyNMe) and cyclohexyl peroxycarboxylic acid as oxidant affords intermediate 3b(CPCA) with a Mössbauer isomer shift δ = -0.08 mm/s that indicates an iron(V) oxidation state. Analysis of the Mössbauer and EPR spectra, combined with DFT studies, demonstrates that the electronic ground state of 3b(CPCA) is best described as a quantum mechanical mixture of [(MeO-PyNMe)Fe(O)(OC(O)R)] (∼75%) with some Fe(O)(OC(O)R) and Fe(OOC(O)R) character. DFT studies of 3b(CPCA) reveal that the unbound oxygen of the carboxylate ligand, O2, is only 2.04 Å away from the oxo group, O1, corresponding to a Wiberg bond order for the O1-O2 bond of 0.35. This unusual geometry facilitates reversible O1-O2 bond formation and cleavage and accounts for the high reactivity of the intermediate when compared to the rates of hydrogen atom transfer and oxygen atom transfer reactions of Fe(OC(O)R) ferric acyl peroxides and Fe(O) complexes. The interaction of O2 with O1 leads to a significant downshift of the Fe-O1 Raman frequency (815 cm) relative to the 903 cm value predicted for the hypothetical [(MeO-PyNMe)Fe(O)(NCMe)] complex.

Oxoiron(IV) units are often implicated as intermediates in the catalytic cycles of non-heme iron oxygenases and oxidases. The most reactive synthetic analogues of these intermediates are supported by tetradentate tripodal ligands with N-methylbenzimidazole or quinoline donors, but their instability precludes structural characterization. Herein we report crystal structures of two [Fe (O)(L)] complexes supported by pentadentate ligands incorporating these heterocycles, which show longer average Fe-N distances than the complex with only pyridine donors. These longer distances correlate linearly with log k ' values for O- and H-atom transfer rates, suggesting that weakening the ligand field increases the electrophilicity of the Fe=O center. The sterically bulkier quinoline donors are also found to tilt the Fe=O unit away from a linear N-Fe=O arrangement by 10°.

Herein, we report the formation of a highly reactive nickel-oxygen species that has been trapped following reaction of a Ni(II) precursor bearing a macrocyclic bis(amidate) ligand with meta-chloroperbenzoic acid (HmCPBA). This compound is only detectable at temperatures below 250 K and is much more reactive toward organic substrates (i.e., C-H bonds, C=C bonds, and sulfides) than previously reported well-defined nickel-oxygen species. Remarkably, this species is formed by heterolytic O-O bond cleavage of a Ni-HmCPBA precursor, which is concluded from experimental and computational data. On the basis of spectroscopy and DFT calculations, this reactive species is proposed to be a Ni(III) -oxyl compound.

The oxidation of the C–H and C=C bonds of hydrocarbons with H2O2 catalyzed by non-heme iron complexes with pentadentate ligands is widely accepted as involving a reactive FeIV=O species such as [(N4Py)FeIV=O]2+ formed by homolytic cleavage of the O–O bond of an FeIII–OOH intermediate (where N4Py is 1,1-bis(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine). We show here that at low H2O2 concentrations the FeIV=O species formed is detectable in methanol. Furthermore, we show that the decomposition of H2O2 to water and O2 is an important competing pathway that limits efficiency in the terminal oxidant and indeed dominates reactivity except where only sub-/near-stoichiometric amounts of H2O2 are present. Although independently prepared [(N4Py)FeIV=O]2+ oxidizes stoichiometric H2O2 rapidly, the rate of formation of FeIV=O from the FeIII–OOH intermediate is too low to account for the rate of H2O2 decomposition observed under catalytic conditions. Indeed, with excess H2O2, disproportionation to O2 and H2O is due to reaction with the FeIII–OOH intermediate and thereby prevents formation of the FeIV=O species. These data rationalize that the activity of these catalysts with respect to hydrocarbon/alkene oxidation is maximized by maintaining sub-/near-stoichiometric steady-state concentrations of H2O2, which ensure that the rate of the H2O2 oxidation by the FeIII–OOH intermediate is less than the rate of the O–O bond homolysis and the subsequent reaction of the FeIV=O species with a substrate.