Unusual Oxidation Research Articles

Pd nanocrystals grown on MXene and reduced graphene oxide co-constructed three-dimensional nanoarchitectures for efficient formic acid oxidation reaction

Although direct formic acid fuel cell (DFAFC) is regarded as one of the most promising energy-conversion systems, its commercialization process is impeded by the high costs of electrode catalysts as well as the sluggish catalytic reaction kinetics. Herein, we present a convenient bottom-up method to the synthesis of nanosized Pd crystals grown on 3D porous hybrid nanoarchitectures constructed from MXene (Ti3C2Tx) and reduced graphene oxide nanosheets (Pd/MX-rGO) through a co-assembly process. The as-derived 3D Pd/MX-rGO nanoarchitecture is equipped with a number of attractive textural features, such as 3D cross-linked porous networks, large specific surface area, uniform Pd dispersion, optimized electronic structure, and good electron conductivity. As a result, unusual formic acid oxidation properties in terms of high catalytic activity, strong poison tolerance, and reliable long-term stability are achieved for the 3D Pd/MX-rGO catalyst, significantly superior to those for conventional Pd catalysts supported by carbon black, graphene, and Ti3C2Tx matrixes.

A comparison study of levofloxacin degradation by peroxymonosulfate and permanganate: Kinetics, products and effect of quinone group

Permanganate (Mn(VII)) as a selective oxidant has been widely used in water treatment process. Recently, peroxymonosulfate (PMS) was recognized as an emerging selective oxidant, which showed appreciable reactivity toward organic compounds containing electron-rich functional groups. In this study, the oxidation of a model fluoroquinolone antibiotic levofloxacin (LEV) by Mn(VII) and PMS was comparatively investigated. Degradation of LEV by PMS followed second-order kinetics and showed strong pH dependency with apparent second-order rate constants (kapp) of 0.15-26.52 M−1 s−1 at pH 5.0–10.0. Oxidation of LEV by Mn(VII) showed autocatalysis at pH 5.0–7.0, while no autocatalysis was observed at pH 8.0–10.0 (kapp = 2.23–4.16 M−1 s−1). Such unusual oxidation kinetics was attributed to the in-situ formed MnO2 from Mn(VII) consumption. The performance of PMS and Mn(VII) for the degradation of LEV was also examined in real waters. PMS primarily react with the aliphatic N4 amine on the piperazine ring of LEV, and Mn(VII) reacted with both the aliphatic N4 amine and aromatic N1 amine. Both PMS and Mn(VII) could efficiently eliminate the antibiotic activity of LEV. Benzoquinone showed activating effect on both PMS and Mn(VII) oxidation, but their activation mechanisms were totally different.

Unusual Copper Oxide Dispersion Achieved by Combining the Confinement Effect and Guest–Host Interaction Modulation

Due to the nontoxicity, low cost, and versatility, CuO-functionalized materials have received much attention. Their performances are intensely dependent on the dispersion state of CuO. Herein we re...

Performance analysis of a planar solid oxide fuel cell stack between 750 °C and 500 °C

The performance of a planar SOFC stack using anode-supported cells (ASC) specifically developed for low temperature operation is discussed. The performance in single cell tests is compared to the stack performance, and important issues leading to a loss of performance in the stack are discussed. Based on the analysis of the contribution of the cell components to the decreasing performance below 600 °C, it is demonstrated that the thin electrolyte contributes only a small fraction of the ohmic resistance in stack operation, and that further improvement of the electrolyte may yield insignificant improvements in stack performance. The anode has the largest contribution to cell impedance in both cell and stack tests. The implications of a cell optimized for low-temperature operation on the stack sealing and anode reduction procedures are briefly discussed, exemplified by the performance loss of the cathode during stack sealing. In addition, an unusual breakaway-type oxidation is found on a thin Crofer 22 APU foil in the cathode air compartment, which is likely related to the low operation temperature of the stack. These findings highlight the most pressing issues of cell and stack development for low temperature applications, and provide a guide toward stack operation at 500 °C.

Biological and Electrochemical Studies of Macrocyclic Complexes of Iron and Cobalt

In this work, we synthesized two tetraaza [N4] macrocyclic complexes of FeIII and CoII metal ions. The synthesized macrocyclic complexes were fully characterized by using various analytical techniques IR, UV-Vis, and MS. The spectral analysis indicated an octahedral geometry for both macrocyclic complexes. The electrochemical behavior was carried out using cyclic voltammetry on the Pt dish (0.031 cm2) electrode. The complexes were shown to have unusual oxidation states for the metal ions. The “heterogeneous electron transfer rate constant” (Ko) was also calculated using “Nicholson and Kochi’s method” and observed in the order: KoCoII > KoFeIII. The antimicrobial activities of two complexes were computed against E. coli, P. aeruginosa, B. cereus, S. aureus, whereas antifungal activities against C. Albicans and were contrasted with the standard drug “Gentamycin”.

Stabilization of unusual metal oxidation state +4 in the iron, cobalt, nickel, and copper complexes with trans-di[benzo]porphyrazine and two fluoride anions: a DFT quantum chemical analysis

Molecular structure calculations of heteroligand (6666)macrotetracyclic metal (MIV) chelates (M = Fe, Co, Ni, Cu) with trans-di[benzo]porphyrazine as the (N,N,N,N)-donor ligand and two F− anions were carried out at the OPBE/TZVP level of density functional theory. The key bond lengths, bond angles, and non-bond angles in the complexes were determined and corresponding standard enthalpies, entropies, and Gibbs energies of formation were calculated. All complexes have slightly distorted tetrahedral structures. The complex-forming agent MIV lies in the plane formed by the donor nitrogen atoms. The metal—nitrogen bonds are usually pair-wise equal. The six-membered metal chelate rings in the complexes are identical to one another from the standpoint of both the sum and set of the bond angles.

(Invited) A New Family of Lithium-Rich Layered Sulfides (Li[LitTi1-t]S2, 0 < t ≤ 0.33) Belonging to the LiTiS2 – Li2TiS3 System

Li-rich layered oxide (Li[LixM1-x]O2, (M = Mn, Ni, Co,...) positive electrode materials are considered as a key component for next generation lithium ion batteries, as they display outstanding specific energy originating from a combination of additional lithium de-intercalation and the unusual oxidation of oxygen anions.Recently we have extended these studies to the homologous titanium sulfides belonging to the LiTiS2 – Li2TiS3 system. Layered Li2TiS3 was obtained for the first time by direct reaction of Li2S on TiS2. Its structure is similar to the Li2MnO3 one, with a small amount of stacking faults. Overlithiated compositions (Li[LitTi1-t]S2, (0 < t ≤ 0.33)) were synthesized by direct synthesis from Li2TiS3 and LiTiS2 and structurally characterized. While LiTiS2 exhibits the O1 type sulfur packing, all compositions with ~0.08 < t ≤ 0.33 exhibit the O3 type sulfur packing like the homologous oxides.High resolution HAADF images recorded for the Li2TiS3 end-member reveal that lithium and titanium are fully ordered within the [Li1/3Ti2/3]S2 slabs of the typical honeycomb lattice usually observed for Li-rich layered oxides. Nevertheless, stacking faults are observed along the hexagonal C axis. In the Li[LitTi1-t]S2, (0.08 < t ≤ 0.33) solid solution the honeycomb ordering is observed only in some domains thank to the departure from the ideal ratio required for the ordering.The electrochemical studies shows that up to 1.1 Li per Li[LitTi1-t]S2 can be reversibly cycled with a negligible irreversibility. The high capacity (≈240 mAh/g) and the shape of the charge curves suggest that Ti3+ ions are oxidized in a first step and the S2- ions in a second step to form S2 2- ions. The very small irreversibility results from the easy oxidation of S2- ions, which does not induce any sulfur release from the particle surface and subsequent structural densification. The XPS study of the deintercalated material confirms the formation of S2 2- species.Contrary to oxides, there is a very small fading and voltage decay upon cycling. This behavior suggests that there is no significant structure modification as it is observed in lithium-rich oxides. In a particular composition range, they exhibit high electrochemical performance with a reversible capacity and an energy density exceeding respectively 265 mAh·g-1 and 600 Wh·kg-1, a very low irreversible capacity in the first cycle, fast activation and a limited voltage decay.Their operation potential within the electrochemical stability window of sulfide-based fast ionic conductors makes them promising cathode materials for all-solid-state lithium and Li-ion batteries.

Syntheses of Bis(N-heterocyclic carbene)s and their application in main-group chemistry

Since the first isolation of an N-heterocyclic carbene (NHC) in the early 1990s, these species have been identified as valuable tools for the stabilization of main-group elements in unusual oxidation states and allowed for the isolation of novel types of compounds. While bis(NHC)s, i.e., compounds in which two NHC units are framed within one molecule by using suitable linker groups, found numerous application in rare-earth and transition-metal chemistry, this does not hold true for main-group elements. Although examples are found throughout the s- and p-block, their potential still needs to be realized. This review summarizes the syntheses of the related bis(NHC)s and gives an overview of mono-, di- and polynuclear complexes incorporating main-group elements and bis(NHC)s.

Original Layered OP4-(Li,Na)xCoO2 Phase: Insights on Its Structure, Electronic Structure, and Dynamics from Solid State NMR.

The OP4-(Li/Na)xCoO2 phase is an unusual lamellar oxide with a 1:1 alternation between Li and Na interslab spaces. In order to probe the local structure, electronic structure, and dynamics, 7Li and 23Na magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was performed in complementarity to X-ray diffraction and electronic and magnetic properties measurements. 7Li MAS NMR showed that NMR shifts result from two contributions: the Fermi contact and the Knight shifts due to the presence of both localized and delocalized electrons, which is really unusual. 7Li MAS NMR clearly shows several Li environments, indicating that, moreover, Co ions with different local electronic structures are formed, probably due to the arrangement of the Na+ ions in the next cationic layer. 23Na MAS NMR showed that some Na+ ions are located in the Li layer, which was not previously considered in the structural model. The Rietveld refinement of the synchrotron XRD led to the OP4-[Li0.42Na0.05]Na0.32CoO2 formula for the material. In addition, 7Li and 23Na MAS NMR spectroscopies provide information about the cationic mobility in the material: Whereas no exchange is observed for 7Li up to 450 K, the 23Na spectrum already reveals a single average signal at room temperature due to a much larger ionic mobility.

A trinuclear nickel(II) cluster containing a ditopic redox active ligand: Structural and magnetic properties

The preparation of a trinuclear Ni2+ ionic complex containing the bridging ligand 2,6-bis-(8-quinolylamino)-4-(tert-butyl)phenol (H3LtBu) as a phenolate monoanion and perchlorate counteranions is described. The structure contains a rare nearly planar disc-like Ni3O3 core with large Ni-O-Ni angles, with each equivalent of the ligand bridging two Ni2+ ions binding facially to each ion. Variable temperature magnetic susceptibility data from 1 indicate moderate intramolecular antiferromagnetic interactions between the three S = 1 Ni2+ ions (JNi-Ni = −20.7 cm−1) and a diamagnetic (S = 0) ground state. Broken symmetry density functional theory calculations (DFT) at the UB3LYP/def2-TZVP level support the experimental results. The structure and properties of 1 stand in contrast to the other reported complex of H3LtBu, Cu4(LtBu)4, which features the ligand in an unusual dianion-aminyl radical oxidation state. Oxidation of 1 produced a black intractable material that could not be adequately characterized.

Palladium-Catalyzed Oxidation of β-C(sp3)-H Bonds of Primary Alkylamines through a Rare Four-Membered Palladacycle Intermediate.

Site-selective functionalizations of C-H bonds are often achieved with a directing group that leads to five- or six-membered metallacyclic intermediates. Analogous reactions that occur through four-membered metallacycles are rare. We report a challenging palladium-catalyzed oxidation of primary C-H bonds β to nitrogen in an imine of an aliphatic amine, a process that occurs through a four-membered palladacyclc intermediate. The success of the reaction relies on the identification, by H/D exchange, of a simple directing group (salicylaldehyde) capable of inducing the formation of this small ring. To gain insight into the steps of the catalytic cycle of this unusual oxidation reaction, a series of mechanistic experiments and density functional theory (DFT) calculations were conducted. The experimental studies showed that cleavage of the C-H bond is rate-limiting and formation of the strained four-membered palladacycle is thermodynamically uphill. DFT calculations corroborated these conclusions and suggested that the presence of an intramolecular hydrogen bond between the oxygen of the directing group and hydroxyl group of the ligating acetic acid is crucial for stabilization of the palladacyclic intermediate.

Sites of Electron Transfer Reactivity in Organometallic Compounds

Three essentially different sites are identified for redox activity in organometallic compounds: The metal, the organometallic (C‐bonded) ligand(s), and potentially noninnocent co‐ligands. Typical reactivity patterns resulting from either of these alternatives will be pointed out by means of representative examples, possibly involving unusual metal oxidation states, carbon‐centered radicals, or ligand redox systems.

Oligonuclear Actinoid Complexes with Schiff Bases as Ligands-Older Achievements and Recent Progress.

Even 155 years after their first synthesis, Schiff bases continue to surprise inorganic chemists. Schiff-base ligands have played a major role in the development of modern coordination chemistry because of their relevance to a number of interdisciplinary research fields. The chemistry, properties and applications of transition metal and lanthanoid complexes with Schiff-base ligands are now quite mature. On the contrary, the coordination chemistry of Schiff bases with actinoid (5f-metal) ions is an emerging area, and impressive research discoveries have appeared in the last 10 years or so. The chemistry of actinoid ions continues to attract the intense interest of many inorganic groups around the world. Important scientific challenges are the understanding the basic chemistry associated with handling and recycling of nuclear materials; investigating the redox properties of these elements and the formation of complexes with unusual metal oxidation states; discovering materials for the recovery of trans-{UVIO2}2+ from the oceans; elucidating and manipulating actinoid-element multiple bonds; discovering methods to carry out multi-electron reactions; and improving the 5f-metal ions’ potential for activation of small molecules. The study of 5f-metal complexes with Schiff-base ligands is a currently “hot” topic for a variety of reasons, including issues of synthetic inorganic chemistry, metalosupramolecular chemistry, homogeneous catalysis, separation strategies for nuclear fuel processing and nuclear waste management, bioinorganic and environmental chemistry, materials chemistry and theoretical chemistry. This almost-comprehensive review, covers aspects of synthetic chemistry, reactivity and the properties of dinuclear and oligonuclear actinoid complexes based on Schiff-base ligands. Our work focuses on the significant advances that have occurred since 2000, with special attention on recent developments. The review is divided into eight sections (chapters). After an introductory section describing the organization of the scientific information, Sections 2 and 3 deal with general information about Schiff bases and their coordination chemistry, and the chemistry of actinoids, respectively. Section 4 highlights the relevance of Schiff bases to actinoid chemistry. Sections 5–7 are the “main menu” of the scientific meal of this review. The discussion is arranged according the actinoid (only for Np, Th and U are Schiff-base complexes known). Sections 5 and 7 are further arranged into parts according to the oxidation states of Np and U, respectively, because the coordination chemistry of these metals is very much dependent on their oxidation state. In Section 8, some concluding comments are presented and a brief prognosis for the future is attempted.

Tautomerism and Self-Association in the Solution of New Pinene-Bipyridine and Pinene-Phenanthroline Derivatives.

Two novel pinene-type ligands have been synthesized and their tautomeric and self-associating behavior studied in solution and in the solid state. The first ligand, an acetylated derivative of 5,6-pinene-bipyridine, displays keto-enol tautomerism in solution. This tautomeric equilibrium was studied by NMR and UV-Vis spectroscopy in various solvents, and the results were compared with the ones obtained through DFT calculations. The second ligand was obtained by an unusual oxidation of the phenanthroline unit of a pinene-phenanthroline derivative. This compound exists as a single tautomer in solution and aggregates both in solution (as observed by NMR) and in the solid state through H-bonding as observed by X-ray structure determination and confirmed by DFT studies.

A fully diastereoselective oxidation of a mesoionic dipole with triplet molecular oxygen.

Oxidations with molecular oxygen are ubiquitous processes in biological systems where cofactor-dependent enzymes activate either oxygen or hydrogen peroxide to induce multichannel pathways. In stark contrast, such slow atmospheric oxidations are seldom harnessed in chemical synthesis and analysis. The present study unveils an unusual aerobic oxidation of a mesoionic dipole leading easily to a more functionalized skeleton. Although the synthetic scope has not been explored, two key considerations emerge from this transformation, as it proceeds with complete diastereoselection and could be successfully extrapolated to structurally related mesoionic chirons without racemization. How this oxidation actually occurs proved to be puzzling from the onset and only high-level computation reveals a cascade transformation, whose results reconcile theory and experiment. Hopefully, the mechanistic insights should help us to understand better the autoxidative reactions of organic molecules.

Tailoring of an unusual oxidation state in a lanthanum tantalum(IV) oxynitride via precursor microstructure design

Perovskite-type oxynitrides hold great potential for optical applications due to their excellent visible light absorption properties. However, only a limited number of such oxynitrides with modulated physical properties are available to date and therefore alternative fabrication strategies are needed to be developed. Here, we introduce such an alternative strategy involving a precursor microstructure controlled ammonolysis. This leads to the perovskite family member LaTa(IV)O2N containing unusual Ta4+ cations. The adjusted precursor microstructures as well as the ammonia concentration are the key parameters to precisely control the oxidation state and O:N ratio in LaTa(O,N)3. LaTa(IV)O2N has a bright red colour, an optical bandgap of 1.9 eV and a low (optically active) defect concentration. These unique characteristics make this material suitable for visible light-driven applications and the identified key parameters will set the terms for the targeted development of further promising perovskite family members.

Ground state in the novel dimer iridate Ba13Ir6O30 with Ir6+(5d3) ions

We have synthesized and studied a new iridate, Ba13Ir6O30, with unusual Ir oxidation states: 2/3 Ir6+(5d3) ions and 1/3 Ir5+(5d4) ions. Its crystal structure features dimers of face-sharing IrO6 octahedra, and IrO6 monomers, that are linked via long, zigzag Ir-O-Ba-O-Ir pathways. Nevertheless, Ba13Ir6O30 exhibits two transitions at TN1 = 4.7 K and TN2 = 1.6 K. This magnetic order is accompanied by a huge Sommerfeld coefficient 200 mJ/mole K below TN2, signaling a coexisting frustrated/disordered state persisting down to at least 0.05 K. This iridate hosts unusually large Jeff=3/2 degrees of freedom, which is enabled by strong spin-orbit interactions (SOI) in the monomers with Ir6+ ions and a joint effect of molecular orbitals and SOI in the dimers occupied by Ir5+ and Ir6+ ions. Features displayed by the magnetization and heat capacity suggest that the combination of covalency, SOI and large effective spins leads to highly frustrated ferrimagnetic ordering, possibly into a skyrmion crystal, a novelty of this new high-spin iridate.

Plutonium Oxidation States in Complex Molecular Solids

Oxidation state is a key chemical quantity that allows the understanding and prediction of the majority of chemical reactions; however, the main deficiency using the formal oxidation state comes from the materials containing multivalent metals. Among them, the most complicated element plutonium (Pu) is an outstanding instance. Here, we calculate the orbital occupation numbers under the frameworks of first-principles density functional theory (DFT) + U methods to quantitatively determine the Pu-oxidation state of the recently reported complex molecular solids: [K(crypt)]Pu[C5H3(SiMe3)2]3, Pu3(DPA)5(H2O)2, and Pu3(DPA)6H. The results show that the oxidation state of Pu is an extremely low Pu2+ in the former and a mixed Pu3+/Pu4+ in the latter two compounds, which is consistent with the experimental identifications. The steric effects and the environmentally sensitive localization → delocalization transition of Pu 5f electrons can rationally elucidate the formation of the unusual oxidation states. Such atomi...

Probing the electronic structure of the CoB16− drum complex: Unusual oxidation state of Co−1

Since the discovery of the first drum-like CoB16− complex, metal-doped drum-like boron nanotubular structures have been investigated with various metal dopants and different tubular size, forming a new class of novel nanostructures. The CoB16− cluster was found to be composed of a central Co atom coordinated by two fused B8 rings in a tubular structure, representing the potential embryo of metal-filled boron nanotubes and providing opportunities to design one-dimensional metal-boron nanostructures. Here we report improved photoelectron spectroscopy and a more in-depth electronic structure analysis of CoB16−, providing further insight into the chemical bonding and stability of the drum-like doped boron tubular structures. Most interestingly, we find that the central Co atom has an unusually low oxidation state of −1 and neutral CoB16 can be viewed as a charge transfer complex (Co−@B16+), suggesting both covalent and electrostatic interactions between the dopant and the boron drum.

Perovskites as Catalysts in Advanced Oxidation Processes for Wastewater Treatment

Advanced oxidation processes (AOPs), based on the formation of highly reactive radicals are able to degrade many organic contaminants present in effluent water. In the heterogeneous AOPS the presence of a solid which acts as catalyst in combination with other systems (O3, H2O2, light) is required. Among the different materials that can catalyse these processes, perovskites are found to be very promising, because they are highly stable and exhibit a high mobility of network oxygen with the possibility of forming vacancies and to stabilize unusual oxidation states of metals. In this review, we show the fundaments of different kinds of AOPs and the application of perovskite type oxides in them, classified attending to the oxidant used, ozone, H2O2 or peroxymonosulfate, alone or in combination with other systems. The photocatalytic oxidation, consisting in the activation of the perovskite by irradiation with ultraviolet or visible light is also revised.