Terminal Oxygen Research Articles

Iridium Oxide Coordinatively Unsaturated Active Sites Govern the Electrocatalytic Oxidation of Water

AbstractA special membrane electrode assembly to measure operando X‐ray absorption spectra and resonant photoemission spectra of mesoporous templated iridium oxide films is used. These films are calcined to different temperatures to mediate the catalyst activity. By combining operando resonant photoemission measurements of different films with ab initio simulations these are able to unambiguously distinguish µ2‐O (bridging oxygen) and µ1‐O (terminal oxygen) in the near‐surface regions of the catalysts. The intrinsic activity of iridium oxide scales with the formation of µ1‐O (terminal oxygen) is found. Importantly, it is shown that the peroxo species do not accumulate under reaction conditions. Rather, the formation of µ1‐O species, which are active in O−O bond formation during the OER, is the most oxidized oxygen species observed, which is consistent with an O−O rate‐limiting step. Thus, the oxygen species taking part in the electrochemical oxidation of water on iridium electrodes are more involved and complex than previously stated. This result highlights the importance of employing theory together with true and complementary operando measurements capable of probing different aspects of catalysts surfaces during operation.

Synthesis, structure and electrochemical properties of a new γ-octamolybdate-based 3D inorganic-organic hybrid containing scarce Mo-N bond

A new inorganic–organic hybrid based on γ-[Mo8O26]4–, Ni(bbi)2(bbi)0.5[γ-Mo8O26]0.5·H2O (1) (bbi = 1,1′-(1,4-butanediyl)bis(imidazole)), has been synthesized and structurally characterized by X-ray single-crystal diffraction. [Ni2(bbi)2(bbi)]4+ chains are linked into a 3D framework structure by γ-[Mo8O26]4–, in which γ-[Mo8O26]4– adopts μ4-coodination mode to chelate two Ni2+ through terminal oxygen atoms and two coordinate imidazole-N of bbi ligands via five-coordinated Mo atoms. The Hirshfeld surface analyses have been calculated. The electrochemical performances studies showed that the hybrid not only had bi-functional electrocatalytic activities toward the reduction of BrO3−/Fe3+/H2O2 and the oxidation of AA, but also had the amperometric sensor functions of BrO3−.

Electrocatalytic epoxidation of cyclooctene to epoxides driven by cobalt-containing polyoxometalate

The electrocatalytic epoxidation directly using water as oxygen source is considered as a sustainable strategy to achieve epoxide production under ambient conditions. However, it is challenging to develop non-noble metal based heterogeneous electrocatalysts for efficient epoxidation with a deep understanding of the structure–activity relationships. In this work, we report a Weakley-polyoxometalate Na10[Co4(H2O)2(PW9O34)2] (Co4P2W18) with high efficiency toward electrocatalytic epoxidation of cyclooctene via directly transfer oxygen atom from water. A significantly higher faradaic efficiency of cyclooctene oxide was achieved over Co4P2W18 than a Keggin type Co substituted polyoxometalate Na5[Co(H2O)PW11O39] (CoPW11) and some other typical electrocatalysts including RuO2, IrO2, and Co3O4 in our system. Experimental results and theoretical calculation indicate that the oxygen atom directly transfers from water for the epoxidation of cyclooctene via the cleavage of O−H of pre-adsorbed water as the rate-determined step. Further calculation demonstrates that the water and cyclooctene were adsorbed by Co site and their adjacent terminal oxygen atom during the epoxidation reaction. With the assistance of this terminal oxygen atom for adsorption and as an electron capture center, lower energy barriers for activation of water and cyclooctene are observed on Co4P2W18 than on CoPW11. Our study provides atomic insight into how structure affects catalytic activity, which benefits designing efficient electrocatalysts.

Thermal Reactions of NiAl3O6+ and Al4O6+ with Methane: Reactivity Enhancement by Doping.

Investigation of the reactivity of heteronuclear metal oxide clusters is an important way to uncover the molecular-level mechanisms of the doping effect. Herein, we performed a comparative study on the reactions of CH4 with NiAl3O6+ and Al4O6+ cluster cations at room temperature to understand the role of Ni during the activation and transformation of methane. Mass spectrometric experiments identify that both NiAl3O6+ and Al4O6+ could bring about hydrogen atom abstraction reaction to generate CH3• radical; however, only NiAl3O6+ has the potential to stabilize [CH3] moiety and then transform [CH3] to CH2O. Density functional theory calculations demonstrate that the terminal oxygen radicals (Ot-•) bound to Al act as the reactive sites for the two clusters to activate the first C-H bond. Although the Ni atom cannot directly participate in methane activation, it can manipulate the electronic environment of the surrounding bridging oxygen atoms (Ob) and enable such Ob to function as an electron reservoir to help Ot-• oxidize CH4 to [H-O-CH3]. The facile reduction of Ni3+ to Ni+ also facilitates the subsequent step of activating the second C-H bond by the bridging "lattice oxygen" (Ob2-), finally enabling the oxidation of methane into formaldehyde. The important role of the dopant Ni played in improving the product selectivity of CH2O for methane conversion discovered in this study allows us to have a possible molecule-level understanding of the excellent performance of the catalysts doping with nickel.

Crystal-to-crystal polymerisation of monosubstituted [PW11O39Cu(H2O)]5- Keggin-type anions.

The reaction between neutral bis(picolinate)copper(II) complexes and copper(II)-monosubstituted Keggin-type phosphotungstate anions formed in situ leads to the formation of the hybrid [C(NH2)3]10[{PW11O39Cu(H2O)}2{Cu(pic)2}]·10H2O compound (1, pic = picolinate) in the presence of structure-directing guanidinium cations. Single-crystal X-ray diffraction studies demonstrate that 1 contains dimeric {PW11O39Cu(H2O)}2{Cu(pic)2} molecular species constituted by two Keggin-type anions linked by one {Cu(pic)2} octahedral complex through axial coordination to their terminal oxygen atoms. The extensive hydrogen-bonding network established by guanidium cations and Keggin clusters plays a key role in retaining the crystallinity of the system throughout dehydration to allow a single-crystal-to-single-crystal (SCSC) transformation into the anhydrous [C(NH2)3]10[{PW11O39Cu}2{Cu(pic)2}] (2a) at 170 °C. Structural modifications involve the re-orientation, shifting in ca. 1.5 Å and condensation of all the {PW11O39Cu} units to result in {PW11O39Cu}n chains in an unprecedented solid-state polymerisation. This phase transition also implies the cleavage of Cu-O bonds induced by the rotation and translation of Keggin-type anions, in such a way that hybrid dimeric units in 1 are dismantled and {Cu(pic)2} complexes become square planar. The irreversibility of the phase transition has been confirmed by combined thermal and diffractometric analyses, which evidence that the anhydrous phase adsorbs only one water molecule per cluster to become the [C(NH2)3]10[{PW11O39Cu}2{Cu(pic)2}]·2H2O (2h) hydrated derivative without any significant alteration in its cell parameters, nor in its crystalline structure. Phase transformations have been monitored by electron paramagnetic resonance spectroscopy.

Synthesis, crystal structure and investigation of ion-exchange possibility for sodium tellurate NaTeO3(OH).

A new sodium tellurate has been hydrothermally synthesized and comprehensively analysed using spectroscopic and thermogravimetric techniques, resulting in the determination of its composition as NaTeO3(OH). The analysis of synchrotron X-ray and neutron diffraction data indicates that NaTeO3(OH) has a crystal structure similar to that of the previously reported tellurate, KTeO3(OH), with the space group P21/a (No. 14). NaTeO3(OH) consists of zigzag one-dimensional chains built by edge-sharing TeO6 octahedra, running parallel to the c-axis and connected to sodium and hydrogen atoms. The hydrogen atoms covalently bond to the terminal oxygen atoms on the one-dimensional chain and also form hydrogen bonds with other terminal oxygen atoms on nearby chains. The structure has been confirmed by optimization using the pseudopotential method and performing Bond Valence Sum (BVS) analysis. Although Li+ ions in LiTeO3(OH) can be exchanged reversibly with H+ ions, no ion exchange behaviour is observed in NaTeO3(OH). The difference is attributed to the size of the alkali ions and their crystal structure.

Insights of the peroxychloroformyl radical ClC(O)OO via microwave spectrum.

The exploration of Venus has received much attention in the past and will keep growing due to the starting of the NASA DAVINCI project. To explain the extremely low O2 : CO ratio observed in Venus' atmosphere, a chlorine-initiated CO oxidation catalytic cycle has been proposed. However, relevant studies on the key intermediates, such as the peroxychloroformyl radical (ClC(O)OO), are rare. In this study, the ClC(O)OO radical was observed using Fourier-Transform Microwave (FTMW) spectroscopy under the supersonic expansion condition. Two conformers, trans-ClC(O)OO and cis-ClC(O)OO, and their chlorine isotopologues were detected. The molecular constants including the fine and hyperfine constants, were determined. Based on the experimental and the ab initio calculations, the unpaired electron is mostly located at the terminal oxygen atom, supported by the small magnetic hyperfine constants of chlorine. In addition, the angles between the Cl-C bond and the a-axis of the trans-35ClC(O)OO and trans-37ClC(O)OO are similar, but these angles are different for cis-ClC(O)OO, making the quadrupole coupling tensors in the inertial axes disagree with the ratio of the quadrupole moments of 35Cl and 37Cl. Finally, we concluded that the ClC(O)OO radicals should behave similarly to other peroxyl radicals, as assumed in the current photochemical model of Venus.

Rational modulation of Fe single-atom electronic structure in a Fe-N2B4 configuration for preferential 1O2 generation in Fenton-like reactions

The important role of optimizing the coordination environment of single-atom catalysts (SACs) for selective production of singlet oxygen (1O2) in Fenton-like reactions is revealed. Herein, we introduce electron-depletion boron atoms to manipulate the coordination number and atom types of Fe site simultaneously and construct a six-coordination Fe-N2B4 catalyst for peroxymonosulfte (PMS) activation. Particularly, it achieves 98.68% 1O2 generation selectivity superior to unregulated Fe-N4 catalyst (64.57%), exhibiting an exceptional bisphenol A (BPA) degradation performance with a reaction rate constant of 0.249 min−1. Experimental and theoretical results unveil that the tailored electronic structure of Fe not only enhances the adsorption selectivity of terminal oxygen atoms in PMS and alters the reaction pathway preference, but also facilitates the electron donation from PMS and lowers the energy barrier for 1O2 generation. This work provides a universal strategy for rational and precise modulation of SACs for specific reactive species conversion in environment remediation.

Enhancing ozone catalytic decomposition through acid treatment of α-MnO2 for improved activity and humidity resistance

Post-etching method using dilute acid solutions is an effective technology to modulate the surface compositions of metal-oxide catalysts. Here the α-MnO2 catalyst treated with 0.1 mol/L nitric acid exhibits higher ozone decomposition activity at high relative humidity than the counterpart treated with acetic acid. Besides the increases in surface area and lattice dislocation, the improved activity can be due to relatively higher Mn valence on the surface and newly-formed Brønsted acid sites adjacent to oxygen vacancies. The remnant nitro species deposited on the catalyst by nitric acid treatment is ideal hydrophobic groups at ambient conditions. The decomposition route is also proposed based on the DRIFTS and DFT calculations: ozone is facile to adsorb on the oxygen vacancy, and the protonic H of Brønsted acid sites bonds to the terminal oxygen of ozone to accelerate its cleavage to O2, reducing the reaction energy barrier of O2 desorption.

The Escherichia coli MFS-type transporter genes yhjE, ydiM, and yfcJ are required to produce an active bo3 quinol oxidase.

Heme-copper oxygen reductases are membrane-bound oligomeric complexes that are integral to prokaryotic and eukaryotic aerobic respiratory chains. Biogenesis of these enzymes is complex and requires coordinated assembly of the subunits and their cofactors. Some of the components are involved in the acquisition and integration of different heme and copper (Cu) cofactors into these terminal oxygen reductases. As such, MFS-type transporters of the CalT family (e.g., CcoA) are required for Cu import and heme-CuB center biogenesis of the cbb3-type cytochrome c oxidases (cbb3-Cox). However, functionally homologous Cu transporters for similar heme-Cu containing bo3-type quinol oxidases (bo3-Qox) are unknown. Despite the occurrence of multiple MFS-type transporters, orthologs of CcoA are absent in bacteria like Escherichia coli that contain bo3-Qox. In this work, we identified a subset of uncharacterized MFS transporters, based on the presence of putative metal-binding residues, as likely candidates for the missing Cu transporter. Using a genetic approach, we tested whether these transporters are involved in the biogenesis of E. coli bo3-Qox. When respiratory growth is dependent on bo3-Qox, because of deletion of the bd-type Qox enzymes, three candidate genes, yhjE, ydiM, and yfcJ, were found to be critical for E. coli growth. Radioactive metal uptake assays showed that ΔydiM has a slower 64Cu uptake, whereas ΔyhjE accumulates reduced 55Fe in the cell, while no similar uptake defect is associated with ΔycfJ. Phylogenomic analyses suggest plausible roles for the YhjE, YdiM, and YfcJ transporters, and overall findings illustrate the diverse roles that the MFS-type transporters play in cellular metal homeostasis and production of active heme-Cu oxygen reductases.

Determining gas-phase chelation of zinc, cadmium, and copper cations with HisHis dipeptide using action spectroscopy and theoretical calculations

Using light generated by an infrared free electron laser, action spectroscopy was performed on doubly charged complexes of the metalated dipeptide histidyl-histidine (HisHis). Metal cations used were zinc, cadmium, and copper. Molecular dynamics and quantum-chemical calculations were used to screen a large number of conformers, whose theoretical infrared spectra were compared to the recorded action spectra of these metalated complexes. The zinc and cadmium spectra display dominant features associated with an iminol binding motif of the HisHis ligand, where the metal ion coordinates with both pros (π) nitrogens of the imidazole sidechains, the terminal carbonyl oxygen, and the backbone nitrogen for which the hydrogen ordinarily bound here has migrated to a carbonyl. The copper complex was difficult to assign to a single species, because a few predicted bands are absent from the experimental spectrum. The theoretical single point energies were also calculated for all structures examined, and DFT methods were found to describe the ion conformer populations in the case of the zinc and cadmium chelates better than the MP2 prediction.

Spectroscopy and Photochemistry of OAlNO and Implications for New Metal Chemistry in the Atmosphere.

A new aluminum-bearing species, OAlNO, which has the potential to impact the chemistry of the Earth's upper atmosphere, is characterized via high-level, ab initio, spectroscopic methods. Meteor-ablated aluminum atoms are quickly oxidized to aluminum oxide (AlO) in the mesosphere and lower thermosphere (MLT), where a steady-state layer of AlO then builds up. Concurrent formation of nitric oxide (NO) in the same region of the atmosphere will lead to the bimolecular formation of the OAlNO molecule. Molecular orbital analysis provides fundamental insights into the chemical bonding and energetic arrangement of the triplet (1 3A″) ground state and singlet (1 1A') excited-state species of OAlNO. Additionally, unpaired electrons on the terminal oxygen atom of triplet (1 3A″) OAlNO cause it to be reactive to atmospheric species, potentially impacting climate science and high-altitude chemistry. The triplet (1 3A″) ground-state species exhibits a large permanent dipole moment useful for rotational spectroscopic detection; however, similar rotational constants to the singlet (1 1A') excited-state species will hamper differentiation in a spectrum. Strong infrared intensities will assist in detection and discrimination of the different spin states and isomers. Repulsive electronic excited states of OAlNO will lead to photolysis of the Al-N bond and formation of various electronic states of AlO + NO through nonadiabatic pathways. Reaction through the OAlNO intermediate represents a means for the production of electronically excited AlO, leading to new chemistry in the atmosphere. Excitation to higher-lying electronic states will lead to fluorescence with a minor Stokes shift, useful for laboratory investigation. Such physical properties of this molecule will allow for new, unexplored chemical pathways in the MLT to be considered.

OH Roaming and Beyond in the Unimolecular Decay of the Methyl-Ethyl-Substituted Criegee Intermediate: Observations and Predictions.

Alkene ozonolysis generates short-lived Criegee intermediates that are a significant source of hydroxyl (OH) radicals. This study demonstrates that roaming of the separating OH radicals can yield alternate hydroxycarbonyl products, thereby reducing the OH yield. Specifically, hydroxybutanone has been detected as a stable product arising from roaming in the unimolecular decay of the methyl-ethyl-substituted Criegee intermediate (MECI) under thermal flow cell conditions. The dynamical features of this novel multistage dissociation plus a roaming unimolecular decay process have also been examined with ab initio kinetics calculations. Experimentally, hydroxybutanone isomers are distinguished from the isomeric MECI by their higher ionization threshold and distinctive photoionization spectra. Moreover, the exponential rise of the hydroxybutanone kinetic time profile matches that for the unimolecular decay of MECI. A weaker methyl vinyl ketone (MVK) photoionization signal is also attributed to OH roaming. Complementary multireference electronic structure calculations have been utilized to map the unimolecular decay pathways for MECI, starting with 1,4 H atom transfer from a methyl or methylene group to the terminal oxygen, followed by roaming of the separating OH and butanonyl radicals in the long-range region of the potential. Roaming via reorientation and the addition of OH to the vinyl group of butanonyl is shown to yield hydroxybutanone, and subsequent C-O elongation and H-transfer can lead to MVK. A comprehensive theoretical kinetic analysis has been conducted to evaluate rate constants and branching yields (ca. 10-11%) for thermal unimolecular decay of MECI to conventional and roaming products under laboratory and atmospheric conditions, consistent with the estimated experimental yield (ca. 7%).

Methyl-terminated graphite carbon nitride with regulatable local charge redistribution for ultra-high photocatalytic hydrogen production and antibiotic degradation

Intramolecular-tailored graphite carbon nitride (g-C3N4) has great potential to greatly optimize the photo-response performance and carrier separation ability, but exquisite molecular structure engineering is still challenging. Firstly, a series of oxygen and terminal methyl moiety co-modified g-C3N4 (CNNx) has been systematically prepared by using N-Hydroxysuccinimide (HOSu) as a novel copolymerized precursor and urea. The density functional theory (DFT) calculations demonstrated that the presence of oxygen can lower the binding energy for the C–C bond to make the terminal modification easier. The terminal methyl and Oxygen not only caused abundant alveolar defects to break the periodic symmetry but also acted as an electron-accepting platform to tune the local charge redistribution within g-C3N4 molecular. The synthesized CNNx (CNN25) achieved ultra-high photocatalytic activity and chemical stability under visible light toward antibiotic degradation (99% tetracycline, 92% doxycycline, 65% ofloxacin and 74% sulfathiazole degradation within 30 min) and hydrogen production (an apparent quantum efficiency of 2.10% at 400 nm). CNN25 also maintains good efficiency in surface water and groundwater. Moreover, the TC solution treated with CNN25 had hardly any harm to the growth of E. coli. We believe our findings will provide a facile and green strategy for the preparation of non-metallic modified g-C3N4.

Infrared Spectroscopic and Theoretical Investigations of Group 13 Oxyfluorides OMF2 and OMF (M=B, Al, Ga, In).

Group 13 oxyfluorides OMF2 were produced by the reactions of laser-ablated group 13 atoms M (M=B, Al, Ga and In) with OF2 and isolated in excess neon or argon matrices at 5 K. These molecules were characterized by matrix-isolation infrared spectroscopy and isotopic substitution experiments in conjunction with quantum-chemical calculations. The calculations indicate that the OMF2 molecules have a 2 B2 ground state with C2v symmetry. The computed molecular orbitals and spin densities show that the unpaired electron is mainly located at the terminal oxygen atom. Oxo monofluorides OMF were only observed in solid argon matrices and exhibit a linear structure in the singlet ground state. The M-O bonding in the OMF molecules can be rationalized as highly polar multiple bonds based on the calculated bond lengths and natural resonance theory (NRT) analyses. In particular, the molecular orbitals of OBF exhibit the character of a triple bond B-O resulting from two degenerate electron-sharing π bonds and an O→B dative σ bond formed by the oxygen 2p lone pair which donates electron density to the boron empty 2p orbital.

Photochromic composites with fast light response, high contrast, and waterproof properties

Preparation of reusable photochromic materials is an effective strategy to alleviate resource waste, energy crisis and environmental problems. However, some weak links (such as slow light response rate, poor cycling stability, hydrophilicity, etc.) may limit their application in specific fields. Here, an intelligent material combines the advantages of inorganic compounds and organic substances, of which rapid charge transfer enables reversible color change assembly among semiconductors, polyoxometalates (POMs) and polymers. Specifically, Keggin-structure POMs are spontaneously attached to nitrogen-containing polymer by hydrogen bonds between terminal oxygen atoms of POMs and protons of amide nitrogen. Additionally, nano-TiO2 which acts as an extra electro donor is doped in this system. When exposed to UV light, the accelerated photoelectron transmission within the system achieves rapid color switching for a few seconds and achieves a reflection value difference of 64.12% within 60 s, displaying fast light response rate and high contrast. In addition, it also has stable cycling characteristics, with only a slight decrease in contrast after 20 times optical printing. On the premise of ensuring readability, a simple spraying treatment forms an anchorage of polydimethylsiloxane/TiO2 layer to photochromic layer, providing this material with waterproof properties. Encouragingly, this system can be applied by air-jet electrospinning, leading to a forward leap of self-fading time for compressing to 1 h. Overall, a waterproof photochromic composite has been developed based on the pain points of this material required overcoming, exhibiting potential in the fields of rewritable materials, anti-counterfeiting materials, indicator materials, etc.

LDI mass spectrometry and quantum chemical calculations of four, five, and six atomic clusters of tantalum oxochloride anion-radicals

Quantum-chemical calculations of isomers of bulk closed “cage” structures of four, five and six atomic clusters of tantalum oxochloride anion-radicals, identified by LDI mass spectrometry in the gas phase, have been performed. The structure and relative thermodynamic stability of the isomers of clusters [Ta4OxCly]- (x = 6–9; y = 9, 7, 5,3), [Ta5OxCly]- (x = 8–12; y = 8, 6, 4, 2) and [Ta6OxCly]- (x = 12–15; y = 7, 5, 3, 1) of various configurations (tetrahedron, trigonal bipyrmide, tetragonal pyramid, trigonal prism, pentagonal pyramid and octahedron position of tantalum atoms) and different positions of oxygen and chlorine atoms have been studied. Some patterns of relative thermodynamic stability of isomers have been established. In the series of cluster radical anions of the same geometric configuration [TanOxCly]- (n = 4–6), the relative stability of geometric isomers depends on the ratio of the number of oxygen and chlorine atoms. For all anion-radicals [TanOxCly]- (n = 4–6) isomers having an anionic fragment with terminal oxygen and chlorine atoms (≡ТаOCl-) are energetically more favorable, than those with three of thes chlorine atoms (≡ TaCl3-). The average length of the Ta–Ta distances, in a series of cluster anions of tantalum oxochlorides [TanOxCly]- (n = 4–6) of the same geometric configuration, increases with a decrease in the number of oxygen atoms (an increase in the number of chlorine atoms). In the closed “cage” structures of the cluster anion radicals of tantalum oxochlorides, together with the oxygen, the chlorine can also act as a bridging atom, but the total binding energy of such isomer is higher.

Insight into the In-Situ Encapsulation-Reassembly Strategy To Fabricate PW12@NiCo-LDH Acid-Base Bifunctional Catalysts.

Acid-base bifunctional catalysts have attracted increasing attention due to the improved overall efficiency of synthetic reactions. Herein, we reported the successful fabrication of a PW12@NiCo-LDH acid-base bifunctional catalyst by using the in-situ encapsulation-reassembly strategy. The evolution process of morphology and structure was monitored carefully by various time-dependent characterizations. X-ray absorption fine structure (XAFS) and density functional theory (DFT) calculations demonstrated that the terminal oxygen of PW12 in PW12@NiCo-LDH preferred to assemble with the oxygen vacancies on NiCo-LDH. When applied for deacetalization-Knoevenagel condensation, the PW12@NiCo-LDH displayed >99% conversion of benzaldehyde dimethyl acetal (BDMA) and >99% yield of ethyl α-cyanocinnamate (ECC). Moreover, PW12@NiCo-LDH can be recycled at least 10 cycles without obvious structural change, which can be attributed to the confinement of PW12 into the NiCo-LDH nanocage. Such excellent catalytic activity of PW12@NiCo-LDH was benefited from the short mass transfer pathway between acid sites and base sites, which was caused by the stable assembly between PW12 and NiCo-LDH.

Second-sphere tuning of analogues for the ferric-hydroperoxoheme form of Mycobacterium tuberculosis MhuD

Mycobacterium tuberculosis MhuD catalyzes the oxygenation of heme to mycobilin; experimental data presented here elucidates the novel hydroxylation reaction catalyzed by this enzyme. Analogues for the critical ferric–hydroperoxoheme (MhuD–heme–OOH) intermediate of this enzyme were characterized using UV/Vis absorption (Abs), circular dichroism (CD), and magnetic CD (MCD) spectroscopies. In order to extract electronic transition energies from these spectroscopic data, a novel global fitting model was developed for analysis of UV/Vis Abs, CD, and MCD data. A variant of MhuD was prepared, N7S, which weakens the affinity of heme-bound enzyme for a hydroperoxo analogue, azide, without significantly altering the protein secondary structure. Global fitting of spectroscopic data acquired in this study revealed that the second-sphere N7S substitution perturbs the electronic structure of two analogues for MhuD–heme–OOH: azide-inhibited MhuD (MhuD–heme–N3) and cyanide-inhibited MhuD (MhuD–heme–CN). The ground state electronic structures of MhuD–heme–N3 and MhuD–heme–CN were assessed using variable-temperature, variable-field MCD. Altogether, these data strongly suggest that there is a hydrogen bond between the Asn7 side-chain and the terminal oxygen of the hydroperoxo ligand in MhuD–heme–OOH. As discussed herein, this finding supports a novel hydroxylation reaction mechanism where the Asn7 side-chain guides a transient hydroxyl radical derived from homolysis of the OO bond in MhuD–heme–OOH to the β- or δ-meso carbon of the porphyrin ligand yielding β- or δ-meso-hydroxyheme, respectively.

Synthesis and characterization a new polyoxomolybdate C34H114Fe2Mo12N18Na2O66 and study of its catalytic activity in the production of 1,2,3-triazoles

A new polyoxomolybdate, [(C6H12N4-CH3)2Na2(H2O)8](C6H12N4-CH3)2[(H0.5)N(CH2O)3FeMo6O18.5(OH)2.5]2·10H2O (1), was synthesized. The structure of the synthesized polyoxomolybdate was investigated by single-crystal X-ray diffraction analysis and several other identification techniques such as FTIR and EDX analysis. Each unit cell of 1 contains one cation [(C6H12N4-CH3)2Na2(H2O)8]4+ and two hexamethylenetetramine cations (C6H12N4-CH3)+, and two polyanions [(H0.5)N(CH2O)3FeMo6O18.5(OH)2.5]3– with ten water molecules in the crystal lattice. In the polyanion, molybdenum ions are bonded to two terminal oxygen atoms and two μ2- (Mo-Mo) and μ3- (Mo-Fe-Mo) bridging oxygen atoms. Iron in the center of the polyanion 1 is also surrounded by three deprotonated oxygens of trimethanolamine ligand and three μ3- (Mo-Fe-Mo) bridging oxygen atoms. This new polyoxomolybdate was used as an efficient catalyst in the azide–alkyne cycloaddition reaction to produce various 1,2,3-triazoles with high yields. Additional investigations reveal this catalytic system can be refreshed and utilized for up to six successive applications.