O2 bubbling into a THF solution of FeII(BDPP) (1) at −80 °C generates a reversible bright yellow adduct 2. Characterization by resonance Raman and Mössbauer spectroscopy provides complementary ...insights into the nature of 2. The former shows a resonance-enhanced vibration at 1125 cm–1, which can be assigned to the ν(O–O) of a bound superoxide, while the latter reveals the presence of a high-spin iron(III) center that is exchange-coupled to the superoxo ligand, like the FeIII–O2 – pair found for the O2 adduct of 4-nitrocatechol-bound homoprotocatechuate 2,3-dioxygenase. Lastly, 2 oxidizes dihydroanthracene to anthracene, supporting the notion that FeIII–O2 – species can carry out H atom abstraction from a C–H bond to initiate the 4-electron oxidation of substrates proposed for some nonheme iron enzymes.
Thiolate-ligated oxoiron(IV) centers are postulated to be the key oxidants in the catalytic cycles of oxygen-activating cytochrome P450 and related enzymes. Despite considerable synthetic efforts, ...chemists have not succeeded in preparing an appropriate model complex. Here we report the synthesis and spectroscopic characterization of Fesuperscript IV(O)(TMCS)⁺ where TMCS is a pentadentate ligand that provides a square pyramidal N₄(SR)subscript apical, where SR is thiolate, ligand environment about the iron center, which is similar to that of cytochrome P450. The rigidity of the ligand framework stabilizes the thiolate in an oxidizing environment. Reactivity studies suggest that thiolate coordination favors hydrogen-atom abstraction chemistry over oxygen-atom transfer pathways in the presence of reducing substrates.
An unprecedentedly reactive iron species (2) has been generated by reaction of excess peracetic acid with a mononuclear iron complex FeII(CF3SO3)2(PyNMe3) (1) at cryogenic temperatures, and ...characterized spectroscopically. Compound 2 is kinetically competent for breaking strong CH bonds of alkanes (BDE ≈ 100 kcal·mol–1) through a hydrogen-atom transfer mechanism, and the transformations proceed with stereoretention and regioselectively, responding to bond strength, as well as to steric and polar effects. Bimolecular reaction rates are at least an order of magnitude faster than those of the most reactive synthetic high-valent nonheme oxoiron species described to date. EPR studies in tandem with kinetic analysis show that the 490 nm chromophore of 2 is associated with two S = 1/2 species in rapid equilibrium. The minor component 2a (∼5% iron) has g-values at 2.20, 2.19, and 1.99 characteristic of a low-spin iron(III) center, and it is assigned as FeIII(OOAc)(PyNMe3)2+ , also by comparison with the EPR parameters of the structurally characterized hydroxamate analogue FeIII(tBuCON(H)O)(PyNMe3)2+ (4). The major component 2b (∼40% iron, g-values = 2.07, 2.01, 1.95) has unusual EPR parameters, and it is proposed to be FeV(O)(OAc)(PyNMe3)2+, where the OO bond in 2a has been broken. Consistent with this assignment, 2b undergoes exchange of its acetate ligand with CD3CO2D and very rapidly reacts with olefins to produce the corresponding cis-1,2-hydroxoacetate product. Therefore, this work constitutes the first example where a synthetic nonheme iron species responsible for stereospecific and site selective CH hydroxylation is spectroscopically trapped, and its catalytic reactivity against CH bonds can be directly interrogated by kinetic methods. The accumulated evidence indicates that 2 consists mainly of an extraordinarily reactive FeV(O)(OAc)(PyNMe3)2+ (2b) species capable of hydroxylating unactivated alkyl CH bonds with stereoretention in a rapid and site-selective manner, and that exists in fast equilibrium with its FeIII(OOAc)(PyNMe3)2+ precursor.
In biological systems, the cleavage of strong C-H bonds is often carried out by iron centres-such as that of methane monooxygenase in methane hydroxylation-through dioxygen activation mechanisms. ...High valent species with Fe(2)(micro-O)(2) diamond cores are thought to act as the oxidizing moieties, but the synthesis of complexes that cleave strong C-H bonds efficiently has remained a challenge. We report here the conversion of a synthetic complex with a valence-delocalized Fe(3.5)(micro-O)(2)Fe(3.5)(3+) diamond core (1) into a complex with a valence-localized HO-Fe(III)-O-Fe(IV)=O(2+) open core (4), which cleaves C-H bonds over a million-fold faster. This activity enhancement results from three factors: the formation of a terminal oxoiron(iv) moiety, the conversion of the low-spin (S = 1) Fe(IV)=O centre to a high-spin (S = 2) centre, and the concentration of the oxidizing capability to the active terminal oxoiron(iv) moiety. This suggests that similar isomerization strategies might be used by nonhaem diiron enzymes.
FeIV(O)(TMG3tren)2+ (1; TMG3tren = 1,1,1-tris{2-N 2-(1,1,3,3-tetramethylguanidino)ethyl}amine) is a unique example of an isolable synthetic S = 2 oxoiron(IV) complex, which serves as a model for the ...high-valent oxoiron(IV) intermediates observed in nonheme iron enzymes. Congruent with DFT calculations predicting a more reactive S = 2 oxoiron(IV) center, 1 has a lifetime significantly shorter than those of related S = 1 oxoiron(IV) complexes. The self-decay of 1 exhibits strictly first-order kinetic behavior and is unaffected by solvent deuteration, suggesting an intramolecular process. This hypothesis was supported by ESI-MS analysis of the iron products and a significant retardation of self-decay upon use of a perdeuteromethyl TMG3tren isotopomer, d 36 -1 (KIE = 24 at 25 °C). The greatly enhanced thermal stability of d 36 -1 allowed growth of diffraction quality crystals for which a high-resolution crystal structure was obtained. This structure showed an FeO unit (r = 1.661(2) Å) in the intended trigonal bipyramidal geometry enforced by the sterically bulky tetramethylguanidinyl donors of the tetradentate tripodal TMG3tren ligand. The close proximity of the methyl substituents to the oxoiron unit yielded three symmetrically oriented short C−D···O nonbonded contacts (2.38−2.49 Å), an arrangement that facilitated self-decay by rate-determining intramolecular hydrogen atom abstraction and subsequent formation of a ligand-hydroxylated iron(III) product. EPR and Mössbauer quantification of the various iron products, referenced against those obtained from reaction of 1 with 1,4-cyclohexadiene, allowed formulation of a detailed mechanism for the self-decay process. The solution of this first crystal structure of a high-spin (S = 2) oxoiron(IV) center represents a fundamental step on the path toward a full understanding of these pivotal biological intermediates.
High-spin oxoiron(IV) species are often implicated in the mechanisms of nonheme iron oxygenases, their C–H bond cleaving properties being attributed to the quintet spin state. However, the few ...available synthetic S = 2 FeIVO complexes supported by polydentate ligands do not cleave strong C–H bonds. Herein we report the characterization of a highly reactive S = 2 complex, FeIV(O)(TQA)(NCMe)2+ (2) (TQA = tris(2-quinolylmethyl)amine), which oxidizes both C–H and CC bonds at −40 °C. The oxidation of cyclohexane by 2 occurs at a rate comparable to that of the oxidation of taurine by the TauD- J enzyme intermediate after adjustment for the different temperatures of measurement. Moreover, compared with other S = 2 complexes characterized to date, the spectroscopic properties of 2 most closely resemble those of TauD- J . Together these features make 2 the best electronic and functional model for TauD- J to date.
Streptomyces venezuelae CmlI catalyzes the six-electron oxygenation of the arylamine precursor of chloramphenicol in a nonribosomal peptide synthetase (NRPS)-based pathway to yield the nitroaryl ...group of the antibiotic. Optical, EPR, and Mössbauer studies show that the enzyme contains a nonheme dinuclear iron cluster. Addition of O2 to the diferrous state of the cluster results in an exceptionally long-lived intermediate (t 1/2 = 3 h at 4 °C) that is assigned as a peroxodiferric species (CmlI-peroxo) based upon the observation of an 18O2-sensitive resonance Raman (rR) vibration. CmlI-peroxo is spectroscopically distinct from the well characterized and commonly observed cis-μ-1,2-peroxo (μ-η1:η1) intermediates of nonheme diiron enzymes. Specifically, it exhibits a blue-shifted broad absorption band around 500 nm and a rR spectrum with a ν(O–O) that is at least 60 cm–1 lower in energy. Mössbauer studies of the peroxo state reveal a diferric cluster having iron sites with small quadrupole splittings and distinct isomer shifts (0.54 and 0.62 mm/s). Taken together, the spectroscopic comparisons clearly indicate that CmlI-peroxo does not have a μ-η1:η1-peroxo ligand; we propose that a μ-η1:η2-peroxo ligand accounts for its distinct spectroscopic properties. CmlI-peroxo reacts with a range of arylamine substrates by an apparent second-order process, indicating that CmlI-peroxo is the reactive species of the catalytic cycle. Efficient production of chloramphenicol from the free arylamine precursor suggests that CmlI catalyzes the ultimate step in the biosynthetic pathway and that the precursor is not bound to the NRPS during this step.
Following the heme paradigm, it is often proposed that dioxygen activation by nonheme monoiron enzymes involves an iron(IV)=oxo intermediate that is responsible for the substrate oxidation step. Such ...a transient species has now been obtained from a synthetic complex with a nonheme macrocyclic ligand and characterized spectroscopically. Its high-resolution crystal structure reveals an iron-oxygen bond length of 1.646(3) angstroms, demonstrating that a terminal iron(IV)=oxo unit can exist in a nonporphyrin ligand environment and lending credence to proposed mechanisms of nonheme iron catalysis.
A series of complexes FeIV(O)(TMC)(X)+ (where X = OH−, CF3CO2 −, N3 −, NCS−, NCO−, and CN−) were obtained by treatment of the well-characterized nonheme oxoiron(IV) complex FeIV(O)(TMC)(NCMe)2+ (TMC ...= tetramethylcyclam) with the appropriate NR4X salts. Because of the topology of the TMC macrocycle, the FeIV(O)(TMC)(X)+ series represents an extensive collection of S = 1 oxoiron(IV) complexes that only differ with respect to the ligand trans to the oxo unit. Electronic absorption, Fe K-edge X-ray absorption, resonance Raman, and Mössbauer data collected for these complexes conclusively demonstrate that the characteristic spectroscopic features of the S = 1 FeIVO unit, namely, (i) the near-IR absorption properties, (ii) X-ray absorption pre-edge intensities, and (iii) quadrupole splitting parameters, are strongly dependent on the identity of the trans ligand. However, on the basis of extended X-ray absorption fine structure data, most FeIV(O)(TMC)(X)+ species have FeO bond lengths similar to that of FeIV(O)(TMC)(NCMe)2+ (1.66 ± 0.02 Å). The mechanisms by which the trans ligands perturb the FeIVO unit were probed using density functional theory (DFT) computations, yielding geometric and electronic structures in good agreement with our experimental data. These calculations revealed that the trans ligands modulate the energies of the FeO σ- and π-antibonding molecular orbitals, causing the observed spectroscopic changes. Time-dependent DFT methods were used to aid in the assignment of the intense near-UV absorption bands found for the oxoiron(IV) complexes with trans N3 −, NCS−, and NCO− ligands as X−-to-FeIVO charge-transfer transitions, thereby rationalizing the resonance enhancement of the ν(FeO) mode upon excitation of these chromophores.
Iron-sulfur proteins are found in all life forms. Most frequently, they contain Fe$_2$S$_2$, Fe$_3$S$_4$, and Fe$_4$S$_4$ clusters. These modular clusters undergo oxidation-reduction reactions, may ...be inserted or removed from proteins, can influence protein structure by preferential side chain ligation, and can be interconverted. In addition to their electron transfer function, iron-sulfur clusters act as catalytic centers and sensors of iron and oxygen. Their most common oxidation states are paramagnetic and present significant challenges for understanding the magnetic properties of mixed valence systems. Iron-sulfur clusters now rank with such biological prosthetic groups as hemes and flavins in pervasive occurrence and multiplicity of function.
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