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SUMMARYMononuclear non-haem iron (NHFe) enzymes catalyse a wide variety of oxidative reactions including halogenation, hydroxylation, ring closure, desaturation, and aromatic ring cleavage. These are highly important for mammalian somatic processes such as phenylalanine metabolism, production of neurotransmitters, hypoxic response, and the biosynthesis of natural products.1–3 The key reactive intermediate in the catalytic cycles of these enzymes is an S = 2 FeIV=O species, which has been trapped for a number of NHFe enzymes4–8 including the halogenase SyrB2, the subject of this study. Computational studies to understand the reactivity of the enzymatic NHFe FeIV=O intermediate9–13 are limited in applicability due to the paucity of experimental knowledge regarding its geometric and electronic structures, which determine its reactivity. Synchrotron-based nuclear resonance vibrational spectroscopy (NRVS) is a sensitive and effective method that defines the dependence of the vibrational modes of Fe on the nature of the FeIV=O active site.14–16 Here we present the first NRVS structural characterisation of the reactive FeIV=O intermediate of a NHFe enzyme. This FeIV=O intermediate reacts via an initial H-atom abstraction step, with its subsquent halogenation (native) or hydroxylation (non-native) rebound reactivity being dependent on the substrate.17 A correlation of the experimental NRVS data to electronic structure calculations indicates that the substrate is able to direct the orientation of the FeIV=O intermediate, presenting specific frontier molecular orbitals (FMOs) which can activate the selective halogenation versus hydroxylation reactivity.

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The charge-ordering states with lattice distortions of a halogen-bridged binuclear-metal mixed-valence complex (called MMX chain), Pt2(dta)4I (dta = CH3CS2 -), have been investigated by transport, magnetic, and optical measurements. This complex is a binuclear unit-assembled conductor containing metal−metal bonds. It exhibits metallic conduction above room temperature, representing the first example of a metallic halogen-bridged one-dimensional transition-metal complex. Below 300 K it shows semiconducting behavior, which is considered to be of the Mott−Hubbard type due to electron correlation. The metal−semiconductor transition at 300 K (= T M - S) is derived from a valence transition of Pt from an averaged-valence state of 2.5+ to a trapped-valence state of 2+ and 3+. The charge-ordering modes are considered to be −IPt2+−Pt3+−IPt2+−Pt3+−IPt2+−Pt3+−IPt2+−Pt3+−I for the semiconducting phase below T M - S and −I−Pt2.5+−Pt2.5+−I−Pt2.5+−Pt2.5+−I−Pt2.5+−Pt2.5+−I−Pt2.5+−Pt2.5+−I− for the metallic phase above T M - S. 129I Mössbauer spectroscopic study is reported for a low-temperature insulating phase below 80 K. The low-temperature electronic structure is considered to be an alternate charge-ordering state with lattice distortions of IPt2+−Pt3+−I−Pt3+−Pt2+IPt2+−Pt3+−I−Pt3+−Pt2+I. The present binuclear platinum complex inherently possesses valence instability of the intermediate valence 2.5+. X-ray photoelectron spectroscopy and polarized reflection measurements are also reported.

Recently, a variety of thiolated gold alloy clusters with well-defined compositions have been synthesized, and the effect of doping on their properties and stability has been studied extensively. We examined the occupation site of the Pd dopant within Au 24 Pd 1 (SC 12 H 25 ) 18 by probing complementarily the local environments of Au and Pd elements using 197 Au Mossbauer and Pd K-edge EXAFS spectroscopy, respectively. The experimental results suggest that the doped single Pd atom is preferentially located at the center of Au 24 Pd 1 (SC 12 H 25 ) 18 to form the superatomic Pd@Au 12 core, which supports recent theoretical predictions. These spectroscopic measurements also clarified intracluster electron transfer from the Pd atom to the surrounding Au atoms.

57 Fe Mössbauer spectroscopy was applied to an iron-based layered superconductor LaFeAsO 0:89 F 0:11 with a transition temperature of 26 K and to its parent material LaFeAsO. Throughout the temperature range from 4.2 to 298 K, a singlet pattern with no magnetic splitting was observed in the Mössbauer spectrum of the F-doped superconductor. Furthermore, no additional internal magnetic field was observed for the spectrum measured at 4.2 K under a magnetic field of 7 T. On the other hand, magnetically split spectra were observed in the parent LaFeAsO below 140 K, and this temperature is slightly lower than that of a structural phase transition from tetragonal to orthorhombic phase, which accompanies the electrical resistivity anomaly at around 150 K. The magnetic moment is estimated to be $0:35 B /Fe from the internal magnetic field of 5.3 T at 4.2 K in the orthorhombic phase, and the spin disorder appears to remain in the magnetically ordered state even at 4.2 K. The lack of a magnetic transition in LaFeAsO 0:89 F 0:11 down to 4.2 K suggests that this system exhibits a paramagnetic state or that the magnetic moment is small. The present results show that F doping effectively suppresses the magnetic and structural transitions in the parent material, leading to the emergence of superconductivity in the F-doped system.

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Binuclear non-heme iron enzymes activate O2 for diverse chemistries that include oxygenation of organic substrates and hydrogen atom abstraction. This process often involves the formation of peroxo-bridged biferric intermediates, only some of which can perform electrophilic reactions. To elucidate the geometric and electronic structural requirements to activate peroxo reactivity, the active peroxo intermediate in 4-aminobenzoate N-oxygenase (AurF) has been characterized spectroscopically and computationally. A magnetic circular dichroism study of reduced AurF shows that its electronic and geometric structures are poised to react rapidly with O2. Nuclear resonance vibrational spectroscopic definition of the peroxo intermediate formed in this reaction shows that the active intermediate has a protonated peroxo bridge. Density functional theory computations on the structure established here show that the protonation activates peroxide for electrophilic/single-electron-transfer reactivity. This activation of peroxide by protonation is likely also relevant to the reactive peroxo intermediates in other binuclear non-heme iron enzymes.