C–H Bond Cleavage by Bioinspired Nonheme Oxoiron(IV) Complexes, Including Hydroxylation of <i>n</i>‑Butane

The development of efficient and selective hydrocarbon oxidation processes with low environmental impact remains a major challenge of the 21st century because of the strong and apolar nature of the C–H bond. Naturally occurring iron-containing metalloenzymes can, however, selectively functionalize strong C–H bonds on substrates under mild and environmentally benign conditions. The key oxidant in a number of these transformations is postulated to possess an <i>S</i> = 2 Fe^IVO unit in a nonheme ligand environment. This oxidant has been trapped and spectroscopically characterized and its reactivity toward C–H bonds demonstrated for several nonheme iron enzyme classes. In order to obtain insight into the structure–activity relationships of these reactive intermediates, over 60 synthetic nonheme Fe^IV(O) complexes have been prepared in various laboratories and their reactivities investigated. This Forum Article summarizes the current status of efforts in the characterization of the C–H bond cleavage reactivity of synthetic Fe^IV(O) complexes and provides a snapshot of the current understanding of factors that control this reactivity, such as the properties of the supporting ligands and the spin state of the iron center. In addition, new results on the oxidation of strong C–H bonds such as those of cyclohexane and <i>n</i>-butane by a putative <i>S</i> = 2 synthetic Fe^IV(O) species that is generated in situ using dioxygen at ambient conditions are presented

Spectroscopic and Theoretical Study of Spin-Dependent Electron Transfer in an Iron(III) Superoxo Complex

It was shown previously (<i>J. Am. Chem. Soc.</i> <b>2014</b>, <i>136</i>, 10846) that bubbling of O₂ into a solution of Fe^II(BDPP) (H₂BDPP = 2,6-bis­[[(<i>S</i>)-2-(diphenylhydroxymethyl)-1-pyrrolidinyl]­methyl]­pyridine) in tetrahydrofuran at −80 °C generates a high-spin (<i>S</i>_Fe = ⁵/₂) iron­(III) superoxo adduct, <b>1</b>. Mössbauer studies revealed that <b>1</b> is an exchange-coupled system, Ĥex=JŜFe·ŜR, where <i>S</i>_R = ¹/₂ is the spin of the superoxo radical, of which the spectra were not well enough resolved to determine whether the coupling was ferromagnetic (<i>S</i> = 3 ground state) or antiferromagnetic (<i>S</i> = 2). The glass-forming 2-methyltetrahydrofuran solvent yields highly resolved Mössbauer spectra from which the following data have been extracted: (i) the ground state of <b>1</b> has <i>S</i> = 3 (<i>J</i> < 0); (ii) |<i>J</i>| > 15 cm^–1; (iii) the zero-field-splitting parameters are <i>D</i> = −1.1 cm^–1 and <i>E</i>/<i>D</i> = 0.02; (iv) the major component of the electric-field-gradient tensor is tilted ≈7° relative to the easy axis of magnetization determined by the <i>M</i>_<i>S</i> = ±3 and ±2 doublets. The excited-state <i>M</i>_<i>S</i> = ±2 doublet yields a narrow parallel-mode electron paramagnetic resonance signal at <i>g</i> = 8.03, which was used to probe the magnetic hyperfine splitting of ¹⁷O-enriched O₂. A theoretical model that considers spin-dependent electron transfer for the cases where the doubly occupied π* orbital of the superoxo ligand is either “in” or “out” of the plane defined by the bent Fe–OO moiety correctly predicts that <b>1</b> has an <i>S</i> = 3 ground state, in contrast to the density functional theory calculations for <b>1</b>, which give a ground state with both the wrong spin and orbital configuration. This failure has been traced to a basis set superposition error in the interactions between the superoxo moiety and the adjacent five-membered rings of the BDPP ligand and signals a fundamental problem in the quantum chemistry of O₂ activation

Characterization of a Paramagnetic Mononuclear Nonheme Iron-Superoxo Complex

O₂ bubbling into a THF solution of Fe^II(BDPP) (<b>1</b>) at −80 °C generates a reversible bright yellow adduct <b>2</b>. Characterization by resonance Raman and Mössbauer spectroscopy provides complementary insights into the nature of <b>2</b>. 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 Fe^III–O₂^– pair found for the O₂ adduct of 4-nitro­catechol-bound homo­proto­catechuate 2,3-dioxy­genase. Lastly, <b>2</b> oxidizes dihydro­anthracene to anthracene, supporting the notion that Fe^III–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

The Two Faces of Tetramethylcyclam in Iron Chemistry: Distinct Fe–O–M Complexes Derived from [Fe^IV(O_{<i>anti</i>/<i>syn</i>})(TMC)]²⁺ Isomers

Tetramethylcyclam (TMC, 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) exhibits two faces in supporting an oxoiron­(IV) moiety, as exemplified by the prototypical [(TMC)­Fe^IV(O_<i>anti</i>)­(NCCH₃)]­(OTf)₂, where <i>anti</i> indicates that the O atom is located on the face opposite all four methyl groups, and the recently reported <i>syn</i> isomer [(TMC)­Fe^IV(O_<i>syn</i>)­(OTf)]­(OTf). The ability to access two isomers of [(TMC)­Fe^IV(O_{<i>anti</i>/<i>syn</i>})] raises the fundamental question of how ligand topology can affect the properties of the metal center. Previously, we have reported the formation of [(CH₃CN)­(TMC)­Fe^III–O_<i>anti</i>–Cr^III(OTf)₄(NCCH₃)] (<b>1</b>) by inner-sphere electron transfer between Cr­(OTf)₂ and [(TMC)­Fe^IV(O_<i>anti</i>)­(NCCH₃)]­(OTf)₂. Herein we demonstrate that a new species <b>2</b> is generated from the reaction between Cr­(OTf)₂ and [(TMC)­Fe^IV(O_<i>syn</i>)­(NCCH₃)]­(OTf)₂, which is formulated as [(TMC)­Fe^III–O_<i>syn</i>–Cr^III(OTf)₄(NCCH₃)] based on its characterization by UV–vis, resonance Raman, Mössbauer, and X-ray absorption spectroscopic methods, as well as electrospray mass spectrometry. Its pre-edge area (30 units) and Fe–O distance (1.77 Å) determined by X-ray absorption spectroscopy are distinctly different from those of <b>1</b> (11-unit pre-edge area and 1.81 Å Fe–O distance) but more closely resemble the values reported for [(TMC)­Fe^III–O_<i>syn</i>–Sc^III(OTf)₄(NCCH₃)] (<b>3</b>, 32-unit pre-edge area and 1.75 Å Fe–O distance). This comparison suggests that <b>2</b> has a square pyramidal iron center like <b>3</b>, rather than the six-coordinate center deduced for <b>1</b>. Density functional theory calculations further validate the structures for <b>1</b> and <b>2</b>. The influence of the distinct TMC topologies on the coordination geometries is further confirmed by the crystal structures of [(Cl)­(TMC)­Fe^III–O_<i>anti</i>–Fe^IIICl₃] (<b>4</b>_<b>Cl</b>) and [(TMC)­Fe^III–O_<i>syn</i>–Fe^IIICl₃]­(OTf) (<b>5</b>). Complexes <b>1</b>–<b>5</b> thus constitute a set of complexes that shed light on ligand topology effects on the coordination chemistry of the oxoiron moiety