Iron species with terminal oxo ligands are implicated as key intermediates in several synthetic and biochemical catalytic cycles. However, there is a dearth of structural information regarding these types of complexes because their instability has precluded isolation under ambient conditions. The isolation and structural characterization of an iron(III) complex with a terminal oxo ligand, derived directly from dioxygen (O2), is reported. A stable structure resulted from placing the oxoiron unit within a synthetic cavity lined with hydrogen-bonding groups. The cavity creates a microenvironment around the iron center that aids in regulating O2 activation and stabilizing the oxoiron unit. These cavities share properties with the active sites of metalloproteins, where function is correlated strongly with site structure.

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 stabilizing interaction between the TS fragments. A valence bond model is used to link the observed results with the basicity/acidity of the reactants.

High spin oxoiron(IV) complexes have been proposed to be a key intermediate in numerous non-heme 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) complexes can also be prepared from a hydroxoiron(III) species via a proton-coupled electron transfer process—a first in synthetic chemistry. The oxoiron(IV) complexes has been characterized with a variety of spectroscopic methods: FTIR studies showed a feature associated with the Fe–O bond at ν(Fe16O) = 799 cm−1 that shifted to 772 cm−1 in the 18O complex; Mössbauer experiments show a signal with an δ = 0.02 mm/s and |ΔEQ | = 0.43 mm/s, electronic parameters consistent with a Fe(IV) center; and optical spectra had visible bands at λmax = 440 (εM = 3100), 550 (εM = 1900) and 808 (εM = 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, σE/D = 0.014, and gz = 2.04 generates a fit that accurately predicted the intensity, lineshape, and position of the observed signals. These results showed the EPR spectroscopy can be a useful method for determining the properties of high spin oxoiron(IV) complexes. The oxoiron(VI) complex was crystallized at −35°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 FeIV–O unit bond length is 1.680(1) Å, which is the longest distance yet reported for monomeric oxoiron(IV) complex.