Lower Oxidation States Research Articles

Exploring Equilibria between Aluminium(I) and Aluminium(III): The Formation of Dihydroalanes, Masked Dialumenes and Aluminium(I) Species

AbstractThe design of new reductive routes to low oxidation state aluminium (Al) compounds offers the opportunity to better understand redox processes at the metal centre and develop reactivity accordingly. Here, a monomeric AlI compound acts as a stoichiometric reducing agent towards a series of AlIII dihydrides, leading to the formation of new low oxidation state species including symmetric and asymmetric dihydrodialanes, and a masked dialumene. These compounds are formed by a series of equilibrium processes involving AlI, AlII and AlIII species and product formation can be manipulated by fine‐tuning the reaction conditions. The transient formation of monomeric AlI compounds is proposed: this is shown to be energetically viable by computational (DFT) investigations and reactivity studies show support for the formation of AlI species. Importantly, despite the potential for the equilibrium mixtures to lead to ill‐defined reactivity, controlled reactivity of these low oxidation state species is observed.

Exploring Equilibria between Aluminium(I) and Aluminium(III): The Formation of Dihydroalanes, Masked Dialumenes and Aluminium(I) Species.

The design of new reductive routes to low oxidation state aluminium (Al) compounds offers the opportunity to better understand redox processes at the metal centre and develop reactivity accordingly. Here, a monomeric AlI compound acts as a stoichiometric reducing agent towards a series of AlIII dihydrides, leading to the formation of new low oxidation state species including symmetric and asymmetric dihydrodialanes, and a masked dialumene. These compounds are formed by a series of equilibrium processes involving AlI, AlII and AlIII species and product formation can be manipulated by fine‐tuning the reaction conditions. The transient formation of monomeric AlI compounds is proposed: this is shown to be energetically viable by computational (DFT) investigations and reactivity studies show support for the formation of AlI species. Importantly, despite the potential for the equilibrium mixtures to lead to ill‐defined reactivity, controlled reactivity of these low oxidation state species is observed.

Co3O4 crystal plane regulation to efficiently activate peroxymonosulfate in water: The role of oxygen vacancies

Crystal plane effect has attracted remarkable attention in the process of peroxymonosulfate (PMS) activation in water. In this work, nanocube-Co3O4 (Co3O4-NC), nanoplate-Co3O4 (Co3O4-NP) and nanorod-like Co3O4 (Co3O4-NR) with (100), (111) and (110) plane predominant exposure is prepared by a facile hydrothermal method. Co3O4-NR with (110) plane exposed possesses more lattice defects (oxygen vacancies, Ov) and low oxidation state Co (Co2+), consequently, it exhibits a superior activity for PMS activation to efficiently remove bisphenol A (BPA) in water. Furthermore, it could be used in a widely water pH values ranging from 5.0 to 9.0 with an excellent PMS activited effects. During Co3O4-NR/PMS oxidation process, it is found that singlet oxygen (1O2) plays a dominant role in BPA degradation. However, Co3O4-NR treated by H2O2 shows a poor PMS activation performance, confirming Ov acting as the active site during such oxidation process. The important effect of dissolved oxygen is tested by Ar introduction into the reaction system and the Ov-O* metastable intermediate is proposed. In situ Raman proves the interaction between dissolved oxygen and Ov and then the intermediate activates PMS to degrade BPA. This work not only explores the effect of different crystal plane exposures on PMS activation in Co3O4/PMS system, but investigates the evolution of Ov during the PMS activation.

Structure and Chemical Bonding in Medium-Size Boron Clusters Doped with Praseodymium.

After reports of unusually low oxidation states of lanthanide elements in Ln-B clusters and their inverse sandwich geometrical topologies, the interest shifted from boride clusters doped with transition metal (TM) elements to the boride clusters doped with lanthanide atoms. In this work, the results obtained by a combined approach consisting of CALYPSO structure predictions and density functional theory (DFT) calculations for the neutral and anionic PrBn series, n = 7-16, are reported. A close agreement between our calculated vertical detachment energies and experimental data supports the accuracy of the results obtained. Contrary to the medium-size TM-doped medium boron clusters, which prefer three types of structural configurations, all lowest-energy states of the medium-size Pr-doped boron clusters have half-sandwich geometries. An interesting structural evolution pattern was found for both neutral and anionic PrBn clusters at n = 7, 10, 13, and 16, which includes quasi-planar B7 units half-sandwiching the Pr atom. Unusual oxidation numbers of +2 and +1 were found for the Pr atom in the PrB7- and PrB8- anions, respectively. Chemical bonding analysis for the neutral PrB7 and PrB13 clusters revealed that their high stability stems from interactions between Pr 5d and B 2p orbitals. A stable tubular-shaped PrB30 cluster is proposed as a promising building block for boron-based nanotubes.

Zeolite encapsulated Cu(II)-salen complexes for the catalytic degradation of dyes in a neutral condition

A series of copper complexes derived from salen ligands bis(salicylaldimine) (L1), bis(pyrrole-2-carboxaldimine) (L2) and bis(pyridine-2-carboxaldimine) (L3) have been synthesized and encapsulated into the supercage of zeolite. These complexes were well investigated by various spectrochemical and physicochemical techniques with DFT calculations. Both free and encapsulated complexes were applied as catalyst to the degradation of methylene blue (MB) using H2O2 as oxidant. The alternation of coordinated sites N2O2 with N4 causes the enhanced catalytic activity, which is attributed to the increased molecular planarity and lower oxidation states for the free complexes. Moreover, encapsulated complexes showed modified geometric, optical and catalytic properties due to electronic effects and steric constraints from the host zeolite supercage. The encapsulated complexes show high catalytic efficiency in organic dye degradation in a neutral condition compared to other encapsulated catalyst studied and show good resistance to radical scavenger. Encapsulated complexes CuL2-Y and CuL3-Y coordinated with N4 sites, exhibiting enhanced catalytic reactivity and recyclability compared to free complexes in solution, which is important for the design of heterogeneous catalyst. The encapsulated complexes with the highest energy electronic band are more reactive. The zeolite and electronic effects from different coordination sphere play an important role in the functionality of the modified geometries.

Reprogramming thermodynamic-limiting oxidation cycle in NiFe-based oxygen evolution electrocatalyst through Mo doping induced surface reconstruction

Engineering of robust nonprecious electrocatalysts toward anodic oxygen evolution reaction (OER) is of great significance for lowering the cost and energy consumption for renewable fuel production. Herein, we report NiFeMoOx nanosheets as high-performance OER electrocatalyst through promoting the thermodynamic-limiting oxidation cycle process in NiFe oxyhydroxide via high-valence Mo doping. The NiFeMoOx nanosheets are prepared by an elaborate in-situ solvothermal etching-depositing process with NiFe alloy framework as substrate and metal precursors. The resultant nanosheets exhibit outstanding alkaline OER activity, requires only 235/282/327 mV overpotentials to achieve current density of 10/100/300 mA cm−2, respectively, with a good long-term stability at 20 mA cm−2 for 72 h. Besides, the Tafel slope low to 28.1 mV dec−1 indicates a favorable OER kinetics. The superior catalytic activity of NiFeMoOx nanosheets should be attributed to the lower oxidation states of Ni and Fe induced by high-valence dopant, leading to easier surface reconstruction at low charge oxidation cycling during OER, thereby effectively reducing the overpotential. The synergy between the electronic effect among multimetallic sites and the unique morphology is expected to inspire the development of robust OER electrocatalyst for industrial application.

Multi-electron Transfer by U(ɪɪ) and Masked U(ɪɪ) Complexes

Complexes of uranium in low oxidation state have shown the ability of activating non-reactive small molecules such as N2. However, the multi-electron transfer required for such activation, remains limited in uranium chemistry. Here, we review our recent research on the use of different strategies to overcome this issue, which has led to the isolation of a diuranium(ɪɪɪ) bridging oxide complex that reacts as a U(ɪɪ) synthon able to effect one-electron transfer per uranium center to N-heterocycles and multi-electron transfer to diphenylacetylene and azobenzene. We also showed that a closely related molecular U(ɪɪ) complex effects the same reactions providing the first unambiguous example of a monouranium four-electron transfer.

Computational Prediction of Graphdiyne-Supported Three-Atom Single-Cluster Catalysts

Computational Prediction of Graphdiyne-Supported Three-Atom Single-Cluster Catalysts

Desymmetric Partial Reduction of Malonic Esters

Desymmetrization of easily available disubstituted malonic esters is a rewarding strategy to access structurally diverse quaternary stereocenters. Particularly, asymmetric reduction of malonic esters would generate a functional group with a lower oxidation state than the remaining ester, thus allowing for more chemoselective derivatization. Here, we report a new set of conditions for the zinc-catalyzed desymmetric hydrosilylation of malonic esters that afford aldehydes as the major product. Compared with alcohol-selective desymmetrization, the partial reduction uses a higher concentration of silanes and new pipecolinol-derived tetradentate ligands, proposedly to switch the pathway of zinc hemiacetal intermediates from elimination to silylation. As a result, high aldehyde-to-alcohol ratios and enantioselectivity of aldehydes are obtained from malonic esters with a large collection of substituents. Together with the abundant reactivity of aldehydes, the partial reduction has enabled an expeditious synthesis of bioactive compounds and natural metabolites containing a quaternary stereocenter.

High concentrations of dissolved organic nitrogen and N-nitrosodimethylamine precursors in effluent from biological nutrient removal process with low dissolved oxygen conditions

The presence of dissolved organic nitrogen (DON) in biological nutrient removal (BNR) effluent has led to increased concern about its adverse effects on wastewater discharge and reuse applications. Previous studies have demonstrated efficient biological inorganic nitrogen removal in BNR under low dissolved oxygen (DO) conditions; however, information on DON is scarce. This study investigated low-DO effects on DON and N-nitrosodimethylamine (NDMA) precursor concentrations in BNR effluents. Identical BNR reactors consisting of an external real-time DO intelligent control system were operated at three different DO concentrations (0.3, 1.0, and 4.0 mgO2/L). Surprisingly, significantly higher values of effluent DON (p<0.05, t-test) and NDMA precursors (p<0.01, t-test) were observed at lower DO levels. Ultrahigh-resolution mass spectrometry analysis showed that molecules produced by microbes at low-DO levels exhibited high proteins/amino sugars-like and low normal oxidation state of carbon characteristics, which possibly acted critical roles in NDMA formation. Furthermore, path analysis by partial least-squares path modeling suggested that NDMA formation potential had strong associations with microbe-DON network stability of microbe-DON co-occurrence interactions (r=0.979, p<0.01). These results highlight the necessity of reconsidering the feasibility of BNR systems operating at low-DO concentrations considering the adverse effects of DON on wastewater discharge and reuse applications.

Effect of selected dopants on conductivity and moisture stability of Li3PS4 sulfide solid electrolyte: a first-principles study

The first principle computational screening was performed to investigate the effect of selected dopants for Li3PS4 sulfide solid electrolyte on its ionic conductivity and stability toward moisture. The results suggest that substitution P5+ using isovalent cations whose electronegativity (EN) value is closer to the value of S has more significant effects on the ionic conductivity, whereby W5+ and Sb5+ can improve most. Similarly, aliovalent cation substitutions with compensating changes in the lithium-ion concentration, particularly those with a lower oxidation state and higher EN, such as Cu2+, effectively enhance the lithium-ion conductivity in this structure. For cation dopants, it is found that ionic conductivity improvement of Li3PS4 is the synergetic effect of EN and oxidation number of the dopant as well as the material's lattice parameter change. Oxides of the considered cation dopants can also improve the ionic conductivity of the material but have much lower lithium-ion conductivity than the cases of cation dopants. However, the metal oxide dopants, particularly those derived from soft Lewis' acid cations, show a marginal improvement in moisture stability of the Li3PS4 electrolyte. The effect of halides and metal halide dopants on the lithium-ion conductivity and moisture stability of Li3PS4 electrolyte are also studied. It is found that metal halides are more effective than any other dopants in improving the ionic conductivity of Li3PS4.

Oxidative Additions of C-F Bonds to the Silanide Anion [Si(C2 F5 )3

Compounds exhibiting main group elements in low oxidation states were found to mimic the reactivity of transition metal complexes. Like the latter, such main group species show a proclivity of changing their oxidation state as well as their coordination number by +2, therefore fulfilling the requirements for oxidative additions. Prominent examples of such main group compounds that undergo oxidative additions with organohalides R−X (R=alkyl, aryl, X=F, Cl, Br, I) are carbenes and their higher congeners. Aluminyl anions, which like carbenes and silylenes oxidatively add to strong σ‐bonds in R−X species, have been recently discovered. We present the first anion based upon a Group 14 element, namely the tris(pentafluoroethyl)silanide anion, [Si(C2F5)3]−, which is capable of oxidative additions towards C−F bonds. This enables the isolation of non‐chelated tetraorganofluorosilicate salts, which to the best of our knowledge had only been observed as reactive intermediates before.

Catalytic effect of highly dispersed ultrafine Ru nanoparticles on a TiO2-Ti3C2 support: Hydrolysis of sodium borohydride for H2 generation

To improve the utilization rate of the precious metal, ruthenium, TiO2 nanospheres containing oxygen vacancies are grown on the surface of layered Ti3C2, and superfine Ru nanoparticles are fixed on the surface of TiO2 nanospheres via hydrogen reduction of a Ru precursor. The presence of titanium in a low oxidation state is verified by X-ray photoelectron spectroscopy, thus confirming the existence of oxygen vacancies. Spherical aberration transmission electron microscopy studies indicate the dispersion of uniform Ru nanoparticles (average diameter: ~2 nm) on TiO2, and the existence of a part of Ru in the form of single atoms. The catalyst with 0.33 wt% Ru loading exhibits the best catalytic performance in the hydrolysis of NaBH4 (NaBH4 hydrolysis rate at 303 K is 60 L·min−1·gRu−1). Further, the catalyst performs well even after five cycles of use.

Oxidative Addition von C−F‐Bindungen an das Silanid‐Anion [Si(C2F5)3]−

AbstractVerbindungen von Hauptgruppenelementen in niedrigen Oxidationsstufen sind in der Lage, die Reaktivität von Übergangsmetallkomplexen nachzuahmen. Wie Letztere neigen niedervalente Hauptgruppenelementverbindungen zur Änderung ihrer Oxidationsstufe und Koordinationszahl um +2, wodurch sie die wesentlichen Voraussetzungen für oxidative Additionen erfüllen. Prominente Beispiele für Hauptgruppenelementverbindungen, die mit Organohalogeniden R−X (R=Alkyl, Aryl, X=F, Cl, Br, I) oxidative Additionen eingehen, sind Carbene und ihre höheren Analoga. Kürzlich wurde auch über Aluminyl‐Anionen berichtet, die wie Carbene und Silylene starke σ‐Bindungen von R−X‐Spezies oxidativ addieren können. Hier stellen wir nun mit dem Tris(pentafluorethyl)silanid‐Anion, [Si(C2F5)3]−, das erste Anion aus der 14. Gruppe vor, welches C−F‐Bindungen im Sinne einer oxidativen Addition zu spalten vermag. Dies ermöglichte die Isolierung von nicht‐chelatierten Tetraorganofluorosilicat‐Salzen, die unseres Wissens bisher nur als reaktive Zwischenstufen nachgewiesen wurden.

3d Metal Complexes in T-shaped Geometry as a Gateway to Metalloradical Reactivity.

ConspectusLow-valent, low-coordinate 3d metal complexes represent a class of extraordinarily reactive compounds that can act as reagents and catalysts for challenging bond-activation reactions. The pursuit of these electron-deficient metal complexes in low oxidation states demands ancillary ligands capable of providing not only energetic stabilization but also sufficiently high steric bulk at the metal center. From this perspective, pincer ligands are particularly advantageous, as their prearranged, meridional coordination mode scaffolds the active center while the substituents of the peripheral donor atoms provide effective steric shielding for the coordination sphere. In a T-shaped geometry, the transition metal complexes possess a precisely defined vacant coordination site, which, combined with the often observed high-spin electron configuration, exhibits unusually high selectivity of these compounds with respect to one-electron redox chemistry. In light of the intractable reaction pathways typically observed with related electronically unsaturated 3d transition metal complexes, the pincer coordination mode enables the isolation of low-valent compounds with more controlled and unique reactivity. We have thus investigated a series of T-shaped metal(I) complexes using three different types of pincer ligands, which may be regarded as "metalloradicals" due to their selectively exposed unpaired electrons.These compounds display remarkably high thermal stability and represent rarely observed "naked" monovalent metal species featuring both monomeric and dimeric structures. Extensive reactivity studies using various organic substrates highlight a strong tendency of these paramagnetic compounds to undergo one-electron oxidation, leading to the isolation of a plethora of metal(II) species with reduced organic ligands as unusual structural elements. The exploration of C2 symmetric T-shaped Ni(I) complexes as asymmetric catalysts also shows success in enantioselective hydrodehalogenation of geminal dihalogenides. In addition, this specific class of low-valent, low-coordinate complexes can be further diversified by introducing redox-active pincer ligands or building homobimetallic systems with two T-shaped units.This Account focuses on the discussion of selected examples of iron, cobalt, and nickel pincer complexes bearing a [P,N,P] or [N,N,N] donor set; however, their electronic structure and radical-type reactivity can be broadly extended to other pincer systems. The availability of various types of pincer ligands should allow fine-tuning of the reactivity of the T-shaped complexes. Given the unprecedented reactivity observed with these compounds, we expect the studies of T-shaped 3d metal complexes to be a fertile field for advancing base metal catalysis.

Solution XAS Analysis for Reactions of Phenoxide-Modified (Arylimido)vanadium(V) Dichloride and (Oxo)vanadium(V) Complexes with Al Alkyls: Effect of Al Cocatalyst in Ethylene (Co)polymerization

V K-edge XANES (XANES = X-ray Absorption Near Edge Structure) spectra of the reaction solution of V(NAr)Cl2(OAr) (1, Ar = 2,6-Me2C6H3) with halogenated Al alkyls (Me2AlCl, Et2AlCl, EtAlCl2, 50 equiv) in toluene showed low energy shifts (2.6–3.6 eV on the basis of inflection point in the photon energy) in the edge absorption accompanying slight shift to low photon energy in the pre-edge peak (λmax values); a similar spectrum was observed when the reaction of 1 with Me2AlCl was conducted in n-hexane. These results strongly suggest a formation of similar vanadium(III) species irrespective of kind of Al alkyls and solvent (toluene or n-hexane). Significant low-energy shifts in the edge absorption accompanied with diminishing the strong pre-edge absorption were also observed when VOCl3 or VO(OiPr)3 was treated with Me2AlCl (10 equiv) in toluene, clearly indicating a formation of low oxidation state vanadium species accompanied with certain structural changes (from tetrahedral to octahedral) in solution.

Enhanced Chemodynamic Therapy by Cu-Fe Peroxide Nanoparticles: Tumor Microenvironment-Mediated Synergistic Fenton Reaction.

An urgent need in chemodynamic therapy (CDT) is to achieve high Fenton catalytic efficiency at small doses of CDT agents. However, simple general promotion of the Fenton reaction increases the risk of damaging normal cells along with the cancer cells. Therefore, a tailored strategy to selectively enhance the Fenton reactivity in tumors, for example, by taking advantage of the characteristics of the tumor microenvironment (TME), is in high demand. Herein, a heterogeneous CDT system based on copper-iron peroxide nanoparticles (CFp NPs) is designed for TME-mediated synergistic therapy. CFp NPs degrade under the mildly acidic conditions of TME, self-supply H2O2, and the released Cu and Fe ions, with their larger portions at lower oxidation states, cooperatively facilitate hydroxyl radical production through a highly efficient catalytic loop to achieve an excellent tumor therapeutic efficacy. This is distinct from previous heterogeneous CDT systems in that the synergism is closely coupled with the Cu+-assisted conversion of Fe3+ to Fe2+ rather than their independent actions. As a result, almost complete ablation of tumors at a minimal treatment dose is demonstrated without the aid of any other therapeutic modality. Furthermore, CFp NPs generate O2 during the catalysis and exhibit a TME-responsive T1 magnetic resonance imaging contrast enhancement, which are useful for alleviating hypoxia and in vivo monitoring of tumors, respectively.

Direct Laser Interference Ink Printing Using Copper Metal-Organic Decomposition Ink for Nanofabrication.

In this study, we developed an effective and rapid process for nanoscale ink printing, direct laser interference ink printing (DLIIP), which involves the photothermal reaction of a copper-based metal–organic decomposition ink. A periodically lined copper pattern with a width of 500 nm was printed on a 240 μm-wide line at a fabrication speed of 17 mm/s under an ambient environment and without any pre- or post-processing steps. This pattern had a resistivity of 3.5 μΩ∙cm, and it was found to exhibit a low oxidation state that was twice as high as that of bulk copper. These results demonstrate the feasibility of DLIIP for nanoscale copper printing with fine electrical characteristics.

Influence of MnO2-Birnessite Microstructure on the Electrochemical Performance of Aqueous Zinc Ion Batteries

KxMnO2 materials with birnessite-type structure are synthetized by two different methods which make it possible to obtain manganese oxides with different degrees of crystallinity. The XPS results indicate that the sample obtained at high temperature (KMn8) exhibits a lower oxidation state for manganese ions as well as a denser morphology. Both characteristics could explain the lower capacity value obtained for this electrode. In contrast, the sample obtained at low temperature (KMn4) or by hydrothermal method presents a manganese oxidation state close to 4 and a more porous morphology. Indeed, in this case higher capacity values are obtained. At current density of 30 mA g−1, the KMn8, KMn4, and HKMn samples display a capacity retention of 88, 82, and 68%, respectively. The higher capacity loss obtained for the HKMn compound could be explained considering that the incorporation of Zn2+ in the structure gives rise to the stabilization of a ZnMn2O4 spinel-type phase. This compound is obtained in the discharge process but remains in the charge stage. Thus, when this spinel-type phase is obtained the capacity loss increases. Moreover, the stabilization of this phase is more favorable at low current rates where 100% of retention for all samples, before 50 cycles, was observed.

Synthesis of a meltable polyzirconosilane precursor for SiZrNC multinary ceramics

Polymeric precursors with refractory metal in the main chain while still possess meltable/soluble characteristics are highly desired for ultra-high temperature ceramics (UHTCs). Herein, we proposed a stepwise synthesis of polyzirconosilane (PZCS) with Zr-C-Si-N polymeric chain. The reaction mechanism was discussed based on the frontier molecular orbital theory and atom transfer radical polymerization, which was initiated and propagated by a low oxidation state active specie of Cp2Zr(II). The PZCS precursor has excellent soluble and meltable properties with a softening point of 80.8 ~ 89.2 oC, which could be spun into green fibers by melt spinning technique. The ceramization process of PZCS was studied, and the resulted SiZrNC multinary ceramics were composed of ZrC/SiC nanocrystals embedded by graphitized carbon phase. The excellent mouldable properties, oxygen-free compositions and high Zr content of PZCS make it an ideal precursor for the preparation of UHTCs matrixes and fibers.