Oxygen Coordination Research Articles

Oxidation processes at the surface of BaTiO3 thin films under environmental conditions

Dissociation and adsorption of water on ferroelectric oxide surfaces playimportant role in the processes of screening and switching dynamics of ferroelectric polarization, as well as in catalytic processes which can be additionally coupled with light, temperature or vibration stimuli. In this work,we present XPS study of ferroelectric BaTiO3thin films and determine the entanglement between surface chemistry, polarization direction and stability, by observing changes upon time exposure to environmental conditions, heating in O2atmosphereandirradiation in vacuum. We devote special attention to Ba 3d spectral region and identify two different oxidation states of O atoms in the compounds of Ba. While this second specie was generally attributed to Ba in surface compounds where it has different oxygen coordination than in the bulk, based on the XPS results of oxygen annealed thin films,we demonstrate that this so far neglected component, corresponds to barium peroxide (BaO2)and identify it as important active specie for the study of screening mechanisms closely related with catalytic activity present in this ferroelectric material. Finally, we report on chemically assisted polarization switching in thin films induced byheating in vacuum or exposure to X-Ray radiation due to the formation of positive surface electric field created by oxygen orelectron vacancies, respectively.

Novel Mixed Ligand Complexes of Alkaline Earth Metals with Coumarilic Acid and Nicotinamide

Coordination compounds with mixed ligands were synthesized with 2A group (Mg2+, Ca2+, Ba2+, Sr2+) alkaline earth metal cations of coumarilic acid and nicotinamide ligands. Afterward, the structural properties of these new molecules were investigated by melting point, elemental analysis, infrared spectroscopy, thermal analysis (TGA / DTA) curves, powder X-ray diffraction (P-XRD) spectroscopy. It has been suggested that the complex structure with the Mg2+ metal center is different from the other three structures. In this structure, it was determined that four aqua and two nicotinamide ligands were located in the coordination sphere, and the coordination number was six, as expected. With two monoanionic coumarilic acids located outside the coordination sphere, complex charge equivalence was achieved. The other three molecules, Sr2+ and Ba2+, have iso-structural properties, and it is suggested that both structures contain a dinuclear metal center, and two aqua ligands are located in the bridging position between metal centers. Besides, the two coumarilate ligands involved in coordination are thought to coordinate with the primary metal cation through carbonyl and acidic oxygens while coordinating with the secondary metal cation through furan oxygen, providing the third bridge connection between metal centers. Metal cations with nine coordination numbers complete the coordination sphere with two terminal aqua and one nicotinamide ligands, each included in the structure. In the molecule with Ca2+ cation, which differs little from these metal cation structures, the difference according to these structures can be interpreted as the coordination of furan oxygen with the secondary metal center due to the octet coordination of the Ca2+ cation. From the thermal analysis curves, it was determined that only the Mg2+ cation complex contained hydrate. As a result of thermal decomposition, it was determined that relevant metal oxide residues remained in all structures, and this situation was defined by powder XRD.

Hydrothermal Synthesis, Crystal Structure, and Spectroscopic Properties of Pure and Eu3+-Doped NaY[SO4]2 ∙ H2O and Its Anhydrate NaY[SO4

The water-soluble colorless compound NaY[SO4]2 ∙ H2O was synthesized with wet methods in a Teflon autoclave by adding a mixture of Na2[SO4] and Y2[SO4]3 ∙ 8 H2O to a small amount of water and heating it up to 190 °C. By slow cooling, single crystals could be obtained and the trigonal crystal structure of NaY[SO4]2 ∙ H2O was refined based on X-ray diffraction data in space group P3221 (a = 682.24(5) pm, c = 1270.65(9) pm, Z = 3). After its thermal decomposition starting at 180 °C, the anhydrate NaY[SO4]2 can be obtained with a monoclinic crystal structure refined from powder X-ray diffraction data in space group P21/m (a = 467.697(5) pm, b = 686.380(6) pm, c = 956.597(9) pm, β = 96.8079(5), Z = 2). Both compounds display unique Y3+-cation sites with eightfold oxygen coordination (d(Y–Os = 220–277 pm)) from tetrahedral [SO4]2− anions (d(S–O = 141–151 pm)) and a ninth oxygen ligand from an H2O molecule (d(Y–Ow = 238 pm) in the hydrate case. In both compounds, the Na+ cations are atoms (d(Na–Os = 224–290 pm) from six independent [SO4]2− tetrahedra each. Thermogravimetry and temperature-dependent PXRD experiments were performed as well as IR and Raman spectroscopic studies. Eu3+-doped samples were investigated for their photoluminescence properties in both cases. The quantum yield of the red luminescence for the anhydrate NaY[SO4]2:Eu3+ was found to be almost 20 times higher than the one of the hydrate NaY[SO4]2 ∙ H2O:Eu3+. The anhydrate NaY[SO4]2:Eu3+ exhibits a decay time of about τ1/e = 2.3 µm almost independent of the temperature between 100 and 500 K, while the CIE1931 color coordinates at x = 0.65 and y = 0.35 are very temperature-consistent too. Due to these findings, the anhydrate is suitable as a red emitter in lighting for emissive displays.

Boosting Selective Nitrogen Reduction via Geometric Coordination Engineering on Single-Tungsten-Atom Catalysts.

Atomic interface regulation that can efficiently optimize the performance of single-atom catalysts (SACs) is a worthwhile research topic. The challenge lies in deeply understanding the structure-properties correlation based on control of the coordination chemistry of individual atoms. Herein, a new kind of W SACs with oxygen and nitrogen coordination (W-NO/NC) and a high metal loading over 10 wt% is facilely prepared by introducing an oxygen-bridged [WO4 ] tetrahedron. The local structure and coordination environment of the W SACs are confirmed by high-angle annular dark-field scanning transmission electron microscopy, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure. The catalyst shows excellent selectivity and activity for the electrochemical nitrogen reduction reaction (NRR). Density functional theory calculation reveals that unique electronic structures of the N and O dual-coordinated W sites optimize the binding energy of the NRR intermediate, resulting in facilitating the electrocatalytic NRR. This work opens an avenue toward exploring the correlation between coordination structure and properties.

Infrared phonon spectroscopy on the Cairo pentagonal antiferromagnet Bi2Fe4O9 : A study through the pressure-induced structural transition

Magnetic and crystallographic transitions in the Cairo pentagonal magnet ${\mathrm{Bi}}_{2}{\mathrm{Fe}}_{4}{\mathrm{O}}_{9}$ are investigated by means of infrared synchrotron-based spectroscopy as a function of temperature (20--300 K) and pressure (0--15.5 GPa). One of the phonon modes is shown to exhibit an anomalous softening as a function of temperature in the antiferromagnetic phase below 240 K, highlighting spin-lattice coupling. Moreover, under applied pressure at 40 K, an even larger softening is observed through the pressure-induced structural transition. Lattice dynamical calculations reveal that this mode is indeed very peculiar as it involves a minimal bending of the strongest superexchange path in the pentagonal planes, as well as a decrease in the distances between second-neighbor irons. The latter confirms the hypothesis made by Friedrich et al., [J. Phys.: Condens. Matter 24, 145401 (2012)] about an increase in the oxygen coordination of irons being at the origin of the pressure-induced structural transition. As a consequence, one expects a new magnetic superexchange path that may alter the magnetic structure under pressure.

Identification of a copper ion recognition peptide sequence in the subunit II of cytochrome c oxidase: a combined theoretical and experimental study

The role of the pentapeptide, NHSFM, derived from the surface exposed part of the metal ion binding loop of the subunit II of cytochrome c oxidase on the maturation of the binuclear purple CuA center of the enzyme has been investigated using several experimental and computational methods. The copper ion was found to form 1:1 complex of the pentapeptide with a binding constant ~ 104M-1 to 105M-1,where a 4 ligand coordination from the peptide in a type 2 copper center was revealed. The pH dependence of the metal-peptide was associated with a [Formula: see text] of ~ 10 suggesting deprotonation of the N-terminal amine. EXAFS studies as well as DFT calculations of the metal-peptide complexes revealed pH dependent changes in the metal-ligand bond distances. Spectroscopic properties of the metal peptides calculated from TDDFT studies agreed with the experimental results. Restrained molecular dynamics (RMD) simulations indicated coordination of a carbonyl oxygen from the asparagine (N) side chain and of water molecules apart from histidine (H), methionine (M)and terminal amine of asparagine (N) in a distorted square planar geometry of Cu-NHSFM. Analyses of the backbone distances as well as B-factors for the metal peptide suggested that the peptide backbone becomes more compact and rigid on binding of the metal ion. This indicated that binding of copper ion to this pentapeptide in the protein possibly cause movement of the protein backbone bringing other coordinating residues closer to the copper ion, and thus helping in sequential uptake of copper ions to the protein.

Activated CuNi@Ni Core@shell structures via oxygen and nitrogen dual coordination assembled on 3D CNTs-graphene hybrid for high-performance water splitting

Herein, a unique nanohybrid, in which CuNi@Ni core@shell nanoparticles were dual-coordinated with oxygen and nitrogen heteroatoms (CuNi@Ni(ON) NPs) and uniformly assembled on 3D porous carbon nanotubes-graphene (CNTs-Gr), was well-designed via an effective synthesis process. The CuNi@Ni(ON)/CNTs-Gr material with tunable electronic properties and conductivity displayed favorite adsorption energies towards reactants, thus manifesting its remarkable bifunctional activities for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) operating in 1.0 M KOH. To achieve current responses of 10 and 100 mA cm−2, small overpotential values of 42.1 and 410 mV were needed for HER and OER, respectively. An electrolyzer employing CuNi@Ni(ON)/CNTs-Gr electrodes delivered an exciting cell voltage of 1.51 V at 10 mA cm−2 and good stability over 25 h operation, surpassing performance of a commercial Pt/C//RuO2-based system. These achievements suggested the CuNi@Ni(ON)/CNTs-Gr is one of the most effective nonprecious catalysts for high-performance production of green hydrogen gas via water splitting.

Mechanism and Selectivity Control in Ni- and Pd-Catalyzed Cross-Couplings Involving Carbon-Oxygen Bond Activation.

Transition-metal-catalyzed C-O bond activation provides a useful strategy for utilizing alcohol- and phenol-derived electrophiles in cross-coupling reactions, which has become a research field of active and growing interest in organic chemistry. The synergy between computation and experiment elucidated the mechanistic model and controlling factors of selectivities in these transformations, leading to advances in innovative C-O bond activation and functionalization methods.Toward the rational design of C-O bond activation, our collaborations with the Jarvo group bridged the mechanistic models of C(sp2)-O and C(sp3)-O bond activations. We found that the nickel catalyst cleaves the benzylic and allylic C(sp3)-O bonds via two general mechanisms: the stereoinvertive SN2 back-side attack model and the stereoretentive chelation-assisted model. These two models control the stereochemistry in a wide array of stereospecific Ni-catalyzed cross-coupling reactions with benzylic or allylic alcohol derivatives. Because of the catalyst distortion, the ligands can differentiate the competing stereospecific C(sp3)-O bond activations. The PCy3 ligand interacts with nickel mainly through σ-donation, and the Ni(PCy3) catalyst can undergo facile bending of the substrate-nickel-ligand angle, which favors the stereoretentive benzylic C-O bond activation. The N-heterocyclic carbene SIMes ligand has additional d(metal)-p(ligand) back-donation with nickel, which leads to an extra energy penalty for the same angle bending. This results in the preference of stereoinvertive benzylic C-O bond activation under Ni/SIMes catalysis. In addition to ligand control, a Lewis acid can increase the selectivity for stereoinvertive C(sp3)-O activation by stabilizing the SN2 back-side attack transition state. The oxygen leaving group complexes with the MgI2 Lewis acid in the stereoinvertive activation, leading to the exclusive stereoinvertive Kumada coupling of benzylic ethers. We also identified that the competing C(sp3)-O bond activation models have noticeable differences in charge separation. This leads to the solvent polarity control of the stereospecificity in C(sp3)-O activations. Low-polarity solvents favor the neutral stereoretentive C-O bond activation, while high-polarity solvents favor the zwitterionic stereoinvertive cleavage.In sharp contrast to the nickel catalysts, the C(sp2)-O bond activation under palladium catalysis mainly proceeds via the classic three-membered ring oxidative addition mechanism instead of the chelation-assisted mechanism. This is due to the lower oxophilicity of palladium, which disfavors the oxygen coordination in the chelation-assisted-type activation. The three-membered ring activation model selectively cleaves the weak C-O bond, resulting in the exclusive chemoselectivity of acyl C-O bond activation in Pd-catalyzed cross-coupling reactions with aryl carboxylic acid derivatives. This explains the overall acylation in the Pd-catalyzed Suzuki-Miyaura coupling with aryl esters. In collaboration with the Szostak group, we revealed that the three-membered ring model applies in the Pd-catalyzed C-O bond activation of carboxylic acid anhydride, which stimulated the development of a series of Pd-catalyzed decarbonylative functionalizations of aryl carboxylic acids.

Dimensional Control of Octahedral Tilt in SrRuO3 via Infinite-Layered Oxides.

Manipulation of octahedral distortion at atomic scale is an effective means to tune the ground states of functional oxides. Previous work demonstrates that strain and film thickness are variable parameters to modify the octahedral parameters. However, selective control of bonding geometry by structural propagation from adjacent layers is rarely studied. Here we propose a new route to tune the ferromagnetism in SrRuO3 (SRO) ultrathin layers by oxygen coordination of adjacent SrCuO2 (SCO) layers. The infinite-layered CuO2 exhibits a structural transformation from "planar-type" to "chain-type" with reduced film thickness. Two orientations dramatically modify the polyhedral connectivity at the interface, thus altering the octahedral distortion of SRO. The local structural variation changes the spin state of Ru and orbital hybridization strength, leading to a significant change in the magnetoresistance and anomalous Hall resistivity. These findings could launch investigations into adaptive control of functionalities in quantum oxide heterostructures using oxygen coordination.

Improving the Catalytic Efficiency of NiFe-LDH/ATO by Air Plasma Treatment for Oxygen Evolution Reaction

Developing efficient catalysts toward the oxygen evolution reaction(OER) is important for water splitting and rechargeable metal-air batteries. Although NiFe oxides are considered as potentially applicable catalysts in the alkaline media, there are still a limited numbers of researches working on membrane electrode assembly(MEA) fed with pure water due to their poor electrical conductivity. In this work, antimony doped tin oxide(ATO) has been employed as conductive supports where NiFe layered double hydroxide uniformly dispersed[named NiFe-LDH(layered double hydroxide)/ATO]. The catalysts have been synthesized by a one-step co-precipitation method, and then NiFe-LDH/ATO-air plasma was obtained through mild air plasma treatment. According to XPS analysis, binding energies of Ni2p and Fe2p were shifted negatively. Moreover, a new signal of low oxygen coordination appeared on O1s spectrum after air plasma treatment. These XPS results indicated that oxygen vacancies(Ov) were generated after air plasma treatment. Electrochemical measurement indicated that the vacancy-rich NiFe-LDH/ATO-air plasma exhibited better performance than NiFe-LDH/ATO not only in 1 mol/L KOH solutions but also in an alkaline polymer electrolyte water electrolyzer(APEWE) fed with deionized water. This work provides a feasible way to design practical catalysts used in electrochemical energy conversion systems by choosing corrosion resistance supports and defect engineering.

A Patternable and In Situ Formed Polymeric Zinc Blanket for a Reversible Zinc Anode in a Skin-Mountable Microbattery.

Owing to their high safety and reversibility, aqueous microbatteries using zinc anodes and an acid electrolyte have emerged as promising candidates for wearable electronics. However, a critical limitation that prevents implementing zinc chemistry at the microscale lies in its spontaneous corrosion in an acidic electrolyte that causes a capacity loss of 40% after a ten-hour rest. Widespread anti-corrosion techniques, such as polymer coating, often retard the kinetics of zinc plating/stripping and lack spatial control at the microscale. Here, a polyimide coating that resolves this dilemma is reported. The coating prevents corrosion and hence reduces the capacity loss of a standby microbattery to 10%. The coordination of carbonyl oxygen in the polyimide with zinc ions builds up over cycling, creating a zinc blanket that minimizes the concentration gradient through the electrode/electrolyte interface and thus allows for fast kinetics and low plating/stripping overpotential. The polyimide's patternable feature energizes microbatteries in both aqueous and hydrogel electrolytes, delivering a supercapacitor-level rate performance and 400 stable cycles in the hydrogel electrolyte. Moreover, the microbattery is able to be attached to human skin and offers strong resistance to deformations, splashing, and external shock. The skin-mountable microbattery demonstrates an excellent combination of anti-corrosion, reversibility, and durability in wearables.

Zinc catalysed electrophilic C-H borylation of heteroarenes.

Cationic zinc Lewis acids catalyse the C–H borylation of heteroarenes using pinacol borane (HBPin) or catechol borane (HBCat). An electrophile derived from [IDippZnEt][B(C6F5)4] (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) combined with N,N-dimethyl-p-toluidine (DMT) proved the most active in terms of C–H borylation scope and yield. Using this combination weakly activated heteroarenes, such as thiophene, were amenable to catalytic C–H borylation using HBCat. Competition reactions show these IDipp–zinc cations are highly oxophilic but less hydridophilic (relative to B(C6F5)3), and that borylation proceeds via activation of the hydroborane (and not the heteroarene) by a zinc electrophile. Based on DFT calculations this activation is proposed to proceed by coordination of a hydroborane oxygen to the zinc centre to generate a boron electrophile that effects C–H borylation. Thus, Lewis acid binding to oxygen sites of hydroboranes represents an under-developed route to access reactive borenium-type electrophiles for C–H borylation.

Field-induced single-ion magnet based on a quasi-octahedral Co(II) complex with mixed sulfur-oxygen coordination environment.

Synthesis and characterization of structure and magnetic properties of the quasi-octahedral complex (pipH2)[Co(TDA)2] 2H2O (I), (pipH22+ = piperazine dication, TDA2- = thiodiacetic anion) are described. X-ray diffraction studies reveal the first coordination sphere of the Co(II) ion, consisting of two chelating tridentate TDA ligands with a mixed sulfur-oxygen strongly elongated octahedral coordination environment. SQUID magnetometry, frequency-domain Fourier-transform (FD-FT) THz-EPR spectroscopy, and high-level ab initio SA-CASSCF/NEVPT2 quantum chemical calculations reveal a strong "easy-plane" type magnetic anisotropy (D ≈ +54 cm-1) of complex I. The complex shows field-induced slow relaxation of magnetization at an applied DC field of 1000 Oe.

Oxygen Coordination on Fe-N-C to Boost Oxygen Reduction Catalysis.

The coordination environments of iron (Fe) in Fe-N-C catalysts determine their intrinsic activities toward oxygen reduction reactions (ORR). The precise atomic-level regulation of the local coordination environments is thus of critical importance yet quite challenging to achieve. Here, atomically dispersed Fe-N-C catalyst with O-Fe-N2C2 moieties is thoroughly studied for ORR catalysis. Advanced synchrotron X-ray characterizations, along with theoretical modeling, explicitly unraveled the penta-coordinated nature of the Fe center in the catalytic domain and the energetically optimized ORR pathways on the well-tailored O-Fe-N2C2 moieties. The combined structure identification from both experiments and theory provides an opportunity to understand the role of the coordination environments in directing the catalytic activity of single-atom or single-site catalysts; not only the center metal atom but also the whole coordinating atoms participate in the catalytic cycle.

Direct Regulation of Double Cation Defects at the A1A2 Site for a High-Performance Oxygen Evolution Reaction Perovskite Catalyst.

Perovskites are one of the efficient catalysts for the oxygen evolution reaction (OER), and they belong to the primary ABO3 in which the A site and B site are site-substituted, and oxygen vacancies are introduced. Further improvement of these complex perovskites is the next necessary topic for specific applications. Herein, two complex perovskites, La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF) and Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), are exploited as the examples to demonstrate the double cation defects-introduced method of A1 and A2 to supply superimposed enhancement of the activity and stability. This is based on the fact that the increased content of oxygen vacancies and coordination can balance the oxygen vacancy and B-site element oxidation state. The electrochemical measurements revealed that the optimized A-LSCF10 and A-BSCF10 both exhibit outstanding OER catalytic activity. A small Tafel slope (57 mV dec-1) and a low overpotential (228 mV at 10 mA cm-2) for A-LSCF10 (vs 93 mV dec-1 and 345 mV at 10 mA cm-2 for A-LSCF0), and a small Tafel slope (65 mV dec-1) and an overpotential (242 mV at 10 mA cm-2) for A-BSCF10 (vs 66 mV dec-1 and 308 mV at 10 mA cm-2 for A-BSCF0) are determined, as well as good stability for 24 h.

On the Role of Alkali-Metal-Like Superatom Al12 P in Reduction and Conversion of Carbon Dioxide.

Developing efficient catalysts for the conversion of CO2 into fuels and value-added chemicals is of great significance to relieve the growing energy crisis and global warming. With the assistance of DFT calculations, it was found that, different from Al12 X (X=Be, Al, and C), the alkali-metal-like superatom Al12 P prefers to combine with CO2 via a bidentate double oxygen coordination, yielding a stable Al12 P(η2 -O2 C) complex containing an activated radical anion of CO2 (i.e., CO2 .- ). Thereby, this compound could not only participate in the subsequent cycloaddition reaction with propylene oxide but also initiate the radical reaction with hydrogen gas to form high-value chemicals, revealing that Al12 P can play an important role in catalyzing these conversion reactions. Considering that Al12 P has been produced in laboratory and is capable of absorbing visible light to drive the activation and transformation of CO2 , it is anticipated that this work could guide the discovery of additional superatom catalysts for CO2 transformation and open up a new research field of superatom catalysis.

Structural and dynamic properties of soda-lime-silica in the liquid phase.

Soda-lime-silica is a glassy system of strong industrial interest. In order to characterize its liquid state properties, we performed molecular dynamics simulations employing an aspherical ion model that includes atomic polarization and deformation effects. They allowed us to study the structure and diffusion properties of the system at temperatures ranging from 1400 K to 3000 K. We show that Na+ and Ca2+ ions adopt a different structural organization within the silica network, with Ca2+ ions having a greater affinity for non-bridging oxygens than Na+. We further link this structural behavior to their different diffusivities, suggesting that escaping from the first oxygen coordination shell is the limiting step for the diffusion. Na+ diffuses faster than Ca2+ because it is bonded to a smaller number of non-bridging oxygens. The formed ionic bonds are also less strong in the case of Na+.

Structural, surface and optical properties of nanoalumina produced by various ways

Start The compared study of the properties of liquid-phase and electric explosion alumina nanoparticles was made. The X-ray diffraction analysis demonstrated that these nanoparticles remain in amorphous and semi-amorphous states. The complete structural information of Al2O3 nanoparticles was obtained as a result of the full-profile refinement of their model phase parameters. The parameters of the unit cells, spatial distribution of atoms, and occupancy of nodes were also determined.The data of IR spectra of surface OH groups and adsorbed CO revealed that the surface imperfection of an electric explosive sample is lower than that obtained using the liquid-phase method. Using photoluminescent spectroscopy, impurities of Cr3+, Mn4+ and Ti3+ in the octahedral oxygen coordination and Fe3+ in the tetrahedral oxygen coordination were detected. It was observed that the concentration of these impurities was significantly higher in the electric explosion sample. Using UV-Vis DR spectroscopy, it was found that the liquid-phase sample was amorphous, and the electric explosive sample was well crystallized and most probably consisted of 2D and 3D nanostructures.

Growth of Titanium Oxide Nanostructures on γ-Аl2О3 by Atomic Layer Deposition

This paper presents results on control over the surface composition, surface structure, and pore texture of core/shell materials, as exemplified by the growth of conformal titanium oxide nanocoatings on γ‑Аl2О3 by atomic layer deposition via sequential and alternating exposure of the alumina to TiCl4 and H2O vapor. The alumina surface and growing titanium oxide layer are shown to influence the characteristics of the forming two-phase material. Increasing the amount of titanium via an increase in the number of deposition cycles leads to a systematic decrease in specific surface area, pore volume, and pore size, which points to conformal pore filling in the starting matrix by a titanium oxide layer. The composition and structure of the titanium oxide coating are influenced by its thickness and the nature of the starting matrix. The coordination state of the titanium oxide in monolayer structures is characteristic of the titanium oxide polyhedra in aluminum titanate. As the distance from the top monolayer to the surface of the matrix (coating thickness) increases, an X-ray amorphous layer is formed in which the oxygen coordination environment of the titanium is similar to that in an anatase-like phase of titanium dioxide.

Copper(II) complexes of tripodal ligand scaffold (N3O) as functional models for phenoxazinone synthase

The copper(II) complexes [Cu(L)NO3] (1–9) of newer N3O ligands (L1-L9) have been synthesized and characterized. The molecular structure of 1, 4, and 7 exhibited nearly a perfect square pyramidal geometry (τ, 0.04–0.11). The Cu-OPhenolate bonds (~ 1.91 Å) are shorter than the Cu-N bonds (~ 2.06 Å) due to the stronger coordination of anionic phenolate oxygen. The Cu(II)/Cu(I) redox potentials of 1–9 appeared around −0.102 to −0.428 V versus Ag/Ag+ in water. The electronic spectra of the complexes showed the d-d transitions around 643–735 nm and axial EPR parameter (g||, 2.243–2.270; A||, 164–179 × 10−4 cm−1) that corresponds to square pyramidal geometry. The bonding parameters α2, 0.760–0.825; β2, 0.761–0.994; γ2, 0.504–0.856 and K||, 0.698–0.954 and K⊥, 0.383–0.820 calculated from EPR spectra and energies of d-d transitions. The complexes catalyzed the conversion of substrate 2-aminophenol into 2-aminophenoxazine-3-one using molecular oxygen in the water and exhibited the yields of 41–61%. The formation of the product is accomplished by the appearance of a new absorption band at 430 nm and the rates of formation were calculated as 6.98–15.65 × 10−3 s−1 in water. The reaction follows Michaelis-Menten enzymatic reaction kinetics with turnover numbers (kcat) 9.11 × 105 h−1 for 1 and 4.66 × 105 h−1 for 9 in water. The spectral, redox and kinetic studies were performed in water to mimic the enzymatic oxidation reaction conditions.