Keywordsbioinorganic chemistry iron-oxo; nonheme iron complexes; high-valent compounds; enzyme models High-valent oxoferryl intermediates have been proposed as the active oxidants in the catalytic cycles of a wide range of mononuclear non-heme oxygen activating enzymes.[1] These high-valent species have now been spectroscopically characterized for four enzymes and were found in all instances to contain high-spin (S = 2) iron(IV) centers.[2] Contemporaneously, the first examples of the existing family of synthetic nonheme oxoiron(IV) complexes were characterized, [3][4][5] which are exclusively octahedral and in all but one case exhibit the S = 1, rather than S = 2, spin-state. Given that DFT suggests higher reactivity for an S = 2 oxoiron(IV) unit, [6,7] it is perhaps not surprising that there is a scarcity of such complexes. Indeed, the only example to date is [Fe IV (O)(H 2 O) 5 ] 2+ (1),

Oxoiron(IV) species have been found to act as the oxidants in the catalytic cycles of several mononuclear nonheme iron enzymes that activate dioxygen. To gain insight into the factors that govern the oxidative reactivity of such complexes, a series of five synthetic S = 1 [FeIV(O)(LN5)]2+ complexes has been characterized with respect to their spectroscopic and electrochemical properties as well as their relative abilities to carry out oxo transfer and hydrogen atom abstraction. The Fe=O units in these five complexes are supported by neutral pentadentate ligands having a combination of pyridine and tertiary amine donors but with different ligand frameworks. Characterization of the five complexes by X-ray absorption spectroscopy reveals Fe=O bonds of ca. 1.65 Å in length that give rise to the intense 1s→3d pre-edge features indicative of iron centers with substantial deviation from centrosymmetry. Resonance Raman studies show that the five complexes exhibit ν(Fe=O) modes at 825–841 cm−1. Spectropotentiometric experiments in acetonitrile with 0.1 M water reveal that the supporting pentadentate ligands modulate the E1/2(IV/III) redox potentials with values ranging from 0.83 to 1.23 V vs. Fc, providing the first electrochemical determination of the E1/2(IV/III) redox potentials for a series of oxoiron(IV) complexes. The 0.4-V difference in potential may arise from differences in the relative number of pyridine and tertiary amine donors on the LN5 ligand and in the orientations of the pyridine donors relative to the Fe=O bond that are enforced by the ligand architecture. The rates of oxo-atom transfer (OAT) to thioanisole correlate linearly with the increase in the redox potentials, reflecting the relative electrophilicities of the oxoiron(IV) units. However this linear relationship does not extend to the rates of hydrogen-atom transfer (HAT) from 1,3-cyclohexadiene (CHD), 9,10-dihydroanthracene (DHA), and benzyl alcohol, suggesting that the HAT reactions are not governed by thermodynamics alone. This study represents the first investigation to compare the electrochemical and oxidative properties of a series of S = 1 FeIV=O complexes with different ligand frameworks and sheds some light on the complexities of the reactivity of the oxoiron(IV) unit.

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The spectroscopic and chemical characterization of a new synthetic non-heme iron(IV)-oxo species [Fe IV (O)( Me,H Pytacn)(S)] 2+ (2, Me,H Pytacn = 1-(2′-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane, S = CH 3 CN or H 2 O) is described. 2 has been prepared by reaction of [Fe II (CF 3 SO 3 ) 2 ( Me,H Pytacn)] (1) with peracetic acid. Complex 2 bears a tetradentate N 4 ligand that leaves two cis-sites available for binding an oxo group and a second external ligand but, unlike related iron(IV)-oxo of tetradentate ligands, it is remarkably stable at room temperature (t 1/2 > 2h at 288 K). Its ability to exchange the oxygen atom of the oxo ligand with water has been analyzed in detail by means of kinetic studies, and a mechanism has been proposed on the basis of DFT calculations. Hydrogen-atom abstraction from C-H bonds and oxygen atom transfer to sulfides by 2 have also been studied. Despite its thermal stability, 2 proves to be a very powerful oxidant that is capable of breaking the strong C-H bond of cyclohexane (BDE = 99.3 kcal·mol −1 ).