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High spin oxoiron(IV) complexes have been proposed to be a key intermediate in numerous nonheme metalloenzymes. The successful detection of similar complexes has been reported for only two synthetic systems. A new synthetic high spin oxoiron(IV) complex is now reported that can be prepared from a well-characterized oxoiron(III) species. This new oxoiron(IV) complex can also be prepared from a hydroxoiron(III) species via a proton-coupled electron transfer process--a first in synthetic chemistry. The oxoiron(IV) complex has been characterized with a variety of spectroscopic methods: FTIR studies showed a feature associated with the Fe-O bond at nu(Fe(16)O) = 798 cm(-1) that shifted to 765 cm(-1) in the (18)O complex; Mossbauer experiments show a signal with an delta = 0.02 mm/s and |DeltaE(Q)| = 0.43 mm/s, electronic parameters consistent with an Fe(IV) center, and optical spectra had visible bands at lambda(max) = 440 (epsilon(M) = 3100), 550 (epsilon(M) = 1900), and 808 (epsilon(M) = 280) nm. In addition, the oxoiron(IV) complex gave the first observable EPR features in the parallel-mode EPR spectrum with g-values at 8.19 and 4.06. A simulation for an S = 2 species with D = 4.0(5) cm(-1), E/D = 0.03, sigma(E/D) = 0.014, and g(z) = 2.04 generates a fit that accurately predicted the intensity, line shape, and position of the observed signals. These results showed that EPR spectroscopy can be a useful method for determining the properties of high spin oxoiron(IV) complexes. The oxoiron(IV) complex was crystallized at -35 degrees C, and its structure was determined by X-ray diffraction methods. The complex has a trigonal bipyramidal coordination geometry with the Fe-O unit positioned within a hydrogen bonding cavity. The Fe(IV)-O unit bond length is 1.680(1) A, which is the longest distance yet reported for a monomeric oxoiron(IV) complex.

Neutral three‐coordinate iron alkylidenes of the form PN−Fe=CHR have been proposed as viable candidates for alkene metathesis. Indeed, during the final stages of preparing this current study, a separate report disclosed that dearomatized PN−Fe‐alkyl complexes are active precatalysts for ring‐opening metathesis polymerization (ROMP) of norbornene implicating PN−Fe=CHR species as possible intermediates. In yet another separate report, we prepared Zn analogues of PN−Fe‐alkyl complexes and herein provide an account for the synthesis, characterization, and reactivity of some new iron complexes with the same tBu substituted PN platform. [ABSTRACT FROM AUTHOR]

Herein, we report a new tripodal tris‐benzimidazole ligand (Tbim) that structurally mimics the 3‐His coordination environment of certain nonheme mononuclear iron oxygenases. The coordination chemistry of Tbim was explored with iron(II) revealing a diverse set of coordination modes. The aerobic oxidation of biomimetic model substrate diethyl‐2‐phenylmalonate was studied using the Tbim−Fe and Fe(OTf)2. [ABSTRACT FROM AUTHOR]

Herein, we report a bulky variant of a bis-benzimidazole carboxylate ligand. The five-position of the benzimidazole groups were substituted with tertiary butyl groups with the intention of preventing bis-ligation and oligomerization, as was observed in other studies using similar ligands. Unfortunately, even with the bulkier substituents, the new ligand still formed a bis-ligated structure. Although the new ligand did not afford the desired mono-ligated species under the conditions studied here, the preparation of the new ligand and coordination compounds provides an added design principle in the ongoing efforts of preparing structurally faithful 2-histidine-1-carboxylate model ligands in biomimetic chemistry. [ABSTRACT FROM AUTHOR]

We describe herein the hydrogen-atom transfer (HAT)/proton-coupled electron-transfer (PCET) reactivity for FeIV–oxo and FeIII–oxo complexes (1–4) that activate C–H, N–H, and O–H bonds in 9,10-dihydroanthracene (S1), dimethylformamide (S2), 1,2-diphenylhydrazine (S3), p-methoxyphenol (S4), and 1,4-cyclohexadiene (S5). In 1–3, the iron is pentacoordinated by tris[N′-tert-butylureaylato)-N-ethylene]aminato ([H3buea]3-) or its derivatives. These complexes are basic, in the order 3 1 > 2. Oxidant 4, [FeIVN4Py(O)]2+ (N4Py: N,N-bis(2-pyridylmethyl)bis(2-pyridyl)methylamine), is the least basic oxidant. The DFT results match experimental trends and exhibit a mechanistic spectrum ranging from concerted HAT and PCET reactions to concerted-asynchronous proton transfer (PT)/electron transfer (ET) mechanisms, all the way to PT. The singly occupied orbital along the O...H...X (X = C, N, O) moiety in the TS shows clearly that in the PCET cases, the electron is transferred separately from the proton. The Bell–Evans–Polanyi principle does not account for the observed reactivity pattern, as evidenced by the scatter in the plot of calculated barrier vs reactions driving forces. However, a plot of the deformation energy in the TS vs the respective barrier provides a clear signature of the HAT/PCET dichotomy. Thus, in all C–H bond activations, the barrier derives from the deformation energy required to create the TS, whereas in N–H/O–H bond activations, the deformation energy is much larger than the corresponding barrier, indicating the presence of a stabilizing interaction between the TS fragments. A valence bond model is used to link the observed results with the basicity/acidity of the reactants. [ABSTRACT FROM AUTHOR]