Upside Down! Crystallographic and Spectroscopic Characterization of an [Fe^IV(O_syn)(TMC)]²⁺ Complex

Fe^II(TMC)­(OTf)₂ reacts with 2-^tBuSO₂–C₆H₄IO to afford an oxoiron­(IV) product, <b>2</b>, distinct from the previously reported [Fe^IV(O_anti)­(TMC)­(NCMe)]²⁺. In MeCN, <b>2</b> has a blue-shifted near-IR band, a higher ν­(FeO), a larger Mössbauer quadrupole splitting, and quite a distinct ¹H NMR spectrum. Structural analysis of crystals grown from CH₂Cl₂ reveals a complex with the formulation of [Fe^IV(O_syn)­(TMC)­(OTf)]­(OTf) and the shortest Fe^IVO bond [1.625(4) Å] found to date

Hypervalent Iodine Reagent Mediated Oxidative Heterocyclization of Aldoximes with Heterocyclic Alkenes

An efficient cycloaddition of heterocyclic alkenes with nitrile oxides generated in situ from the corresponding aldoximes using organohypervalent iodine­(III) reagent, [hydroxy­(tosyloxy)­iodo]­benzene (Koser’s reagent), has been developed. The oxidative cyclization of various aldoximes with 1-propene-1,3-sultone affords the respective isoxazoline-ring-fused heterobicyclic products in moderate to good yields. Furthermore, the reaction of aldoxime with a cyclic phospholene-oxide under similar conditions produces the corresponding heterobicyclic phospholene oxides in moderate yields. The structures of bicyclic phospholene oxide and two sultones were established by single-crystal X-ray crystallography

High-Energy-Resolution Fluorescence-Detected X‑ray Absorption of the Q Intermediate of Soluble Methane Monooxygenase

Kα high-energy-resolution fluorescence detected X-ray absorption spectroscopy (HERFD XAS) provides a powerful tool for overcoming the limitations of conventional XAS to identify the electronic structure and coordination environment of metalloprotein active sites. Herein, Fe Kα HERFD XAS is applied to the diiron active site of soluble methane monooxygenase (sMMO) and to a series of high-valent diiron model complexes, including diamond-core [Fe^IV₂(μ-O)₂(L)₂]­(ClO₄)₄] (<b>3</b>) and open-core [(OFe^IV–O–Fe^IV(OH)­(L)₂]­(ClO₄)₃ (<b>4</b>) models (where, L = tris­(3,5-dimethyl-4-methoxypyridyl-2-methyl)­amine) (TPA*)). Pronounced differences in the HERFD XAS pre-edge energies and intensities are observed for the open versus closed Fe₂O₂ cores in the model compounds. These differences are reproduced by time-dependent density functional theory (TDDFT) calculations and allow for the pre-edge energies and intensity to be directly correlated with the local active site geometric and electronic structure. A comparison of the model complex HERFD XAS data to that of MMOH_Q (the key intermediate in methane oxidation) is supportive of an open-core structure. Specifically, the large pre-edge area observed for MMOH_Q may be rationalized by invoking an open-core structure with a terminal Fe^IVO motif, though further modulations of the core structure due to the protein environment cannot be ruled out. The present study thus motivates the need for additional experimental and theoretical studies to unambiguously assess the active site conformation of MMOH_Q

Tuning Electron-Transfer Properties in 5,10,15,20-Tetra(1′-hexanoylferrocenyl)porphyrins as Prospective Systems for Quantum Cellular Automata and Platforms for Four-Bit Information Storage

Metal-free (<b>1</b>) and zinc (<b>2</b>) 5,10,15,20-tetra­(1′-hexanoylferrocenyl)­porphyrins were prepared using an acid-catalyzed tetramerization reaction between pyrrole and 1′-(1-hexanoyl)­ferrocencarboxaldehyde. New organometallic compounds were characterized by combination of ¹H, ¹³C, and variable-temperature NMR, UV–vis, magnetic circular dichroism, and high-resolution electrospray ionization mass spectrometry methods. The redox properties of <b>1</b> and <b>2</b> were probed by electrochemical (cyclic voltammetry and differential pulse voltammetry), spectroelectrochemical, and chemical oxidation approaches coupled with UV–vis–near-IR and Mössbauer spectroscopy. Electrochemical data recorded in the dichloromethane/TBA­[B­(C₆F₅)₄] system (TBA­[B­(C₆F₅)₄] is a weakly coordinating tetrabutylammonium tetrakis­(pentafluorophenyl)­borate electrolyte) are suggestive of “1e^– + 1e^– + 2e^–” oxidation sequence for four ferrocene groups in <b>1</b> and <b>2</b>, which followed by oxidation process centered at the porphyrin core. The separation between all ferrocene-centered oxidation electrochemical waves is very large (510–660 mV). The nature of mixed-valence [<b>1</b>]^<i>n</i>+ and [<b>2</b>]^<i>n</i>+ (<i>n</i> = 1 or 2) complexes was probed by the spectroelectrochemical and chemical oxidation methods. Analysis of the intervalence charge-transfer band in [<b>1</b>]⁺ and [<b>2</b>]⁺ is suggestive of the Class II (in Robin–Day classification) behavior of all mixed-valence species, which correlate well with Mössbauer data. Density functional theory–polarized continuum model (DFT-PCM) and time-dependent (TD) DFT-PCM methods were applied to correlate redox and optical properties of organometallic complexes <b>1</b> and <b>2</b> with their electronic structures

Modeling TauD‑<i><b>J</b></i>: A High-Spin Nonheme Oxoiron(IV) Complex with High Reactivity toward C–H Bonds

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 <i>S</i> = 2 Fe^IVO complexes supported by polydentate ligands do not cleave strong C–H bonds. Herein we report the characterization of a highly reactive <i>S</i> = 2 complex, [Fe^IV(O)­(TQA)­(NCMe)]²⁺ (<b>2</b>) (TQA = tris­(2-quinolylmethyl)­amine), which oxidizes both C–H and CC bonds at −40 °C. The oxidation of cyclohexane by <b>2</b> occurs at a rate comparable to that of the oxidation of taurine by the TauD-<i><b>J</b></i> enzyme intermediate after adjustment for the different temperatures of measurement. Moreover, compared with other <i>S</i> = 2 complexes characterized to date, the spectroscopic properties of <b>2</b> most closely resemble those of TauD-<i><b>J</b></i>. Together these features make <b>2</b> the best electronic <i>and</i> functional model for TauD-<i><b>J</b></i> to date

Modeling TauD‑<i><b>J</b></i>: A High-Spin Nonheme Oxoiron(IV) Complex with High Reactivity toward C–H Bonds

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 <i>S</i> = 2 Fe^IVO complexes supported by polydentate ligands do not cleave strong C–H bonds. Herein we report the characterization of a highly reactive <i>S</i> = 2 complex, [Fe^IV(O)­(TQA)­(NCMe)]²⁺ (<b>2</b>) (TQA = tris­(2-quinolylmethyl)­amine), which oxidizes both C–H and CC bonds at −40 °C. The oxidation of cyclohexane by <b>2</b> occurs at a rate comparable to that of the oxidation of taurine by the TauD-<i><b>J</b></i> enzyme intermediate after adjustment for the different temperatures of measurement. Moreover, compared with other <i>S</i> = 2 complexes characterized to date, the spectroscopic properties of <b>2</b> most closely resemble those of TauD-<i><b>J</b></i>. Together these features make <b>2</b> the best electronic <i>and</i> functional model for TauD-<i><b>J</b></i> to date

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