Oxidation State Of Cobalt Research Articles

A Cobalt Phosphine Complex in Five Oxidation States.

Here we report electrochemical, spectroscopic, and crystallographic characterization of a redox series of cobalt complexes in five sequential oxidation states. A simple bidentate phosphine ligand, cis-1,2-bis(diphenylphosphino)ethylene (dppv), allows for isolation of the 3+, 2+, 1+, 0, and 1- oxidation states of cobalt─the only known example of transition-metal complexes with redox-innocent ligands in five oxidation states. Electrochemistry of [Co(dppv)2]2+ reveals three reversible reductions and one reversible oxidation. Complexes in each oxidation state are characterized using single-crystal X-ray diffraction. The coordination number and geometry of the complex changes as a function of the oxidation state: including acetonitrile ligands, the Co3+ complex is pseudo-octahedral, the Co2+ complex is square-pyramidal, the Co+ complex is pseudo-square-planar, and the Co0 and Co- complexes approach pseudo-tetrahedral, illustrating structures predicted by crystal-field theory of inorganic transition-metal complexes.

The Roles of Composition and Mesostructure of Cobalt-Based Spinel Catalysts in Oxygen Evolution Reactions.

By using the crystalline precursor decomposition approach and direct co‐precipitation the composition and mesostructure of cobalt‐based spinels can be controlled. A systematic substitution of cobalt with redox‐active iron and redox‐inactive magnesium and aluminum in a cobalt spinel with anisotropic particle morphology with a preferred 111 surface termination is presented, resulting in a substitution series including Co3O4, MgCo2O4, Co2FeO4, Co2AlO4 and CoFe2O4. The role of redox pairs in the spinels is investigated in chemical water oxidation by using ceric ammonium nitrate (CAN test), electrochemical oxygen evolution reaction (OER) and H2O2 decomposition. Studying the effect of dominant surface termination, isotropic Co3O4 and CoFe2O4 catalysts with more or less spherical particles are compared to their anisotropic analogues. For CAN‐test and OER, Co3+ plays the major role for high activity. In H2O2 decomposition, Co2+ reveals itself to be of major importance. Redox active cations in the structure enhance the catalytic activity in all reactions. A benefit of a predominant 111 surface termination depends on the cobalt oxidation state in the as‐prepared catalysts and the investigated reaction.

Structural, Optical, and Magnetic Properties of Cobalt-Doped ZnAl2O4 Nanosheets Prepared by Hydrothermal Synthesis

Nanomaterials with collective optical and magnetic properties are called smart or functional materials and have promising applications in many fields of science and technology. Undoped and Co-doped ZnAl2O4 were prepared using a co-precipitation-assisted hydrothermal method. A systematic investigation was carried out to understand the effects of the Co concentration on the crystalline phase, morphology, and optical and magnetic properties of Co-doped ZnAl2O4. X-ray diffraction confirmed the cubic spinel structure with the Fd3m space group, and there was no impurity phase. X-ray photoelectron spectroscopy of Co-doped ZnAl2O4 confirmed the existence of Zn, Al and O, and the Co in the optimized sample of ZAO-Co-3 confirmed the oxidation state of cobalt as Co2+. Transmission electron microscopy of pure and Co-doped ZnAl2O4 revealed micro-hexagons and nanosheets, respectively. The optical absorption results showed that the bandgap of ZnAl2O4 decreased with increasing Co concentration. The hysteresis loop of Co-doped ZnAl2O4 revealed clear ferromagnetic behavior at room temperature. The as-prepared materials are suitable for energy storage applications, such as in supercapacitors and fuel cells. This work aims to focus on the effect of cobalt ions in different concentrations on structural, optical and magnetic properties.

Correlative operando microscopy of oxygen evolution electrocatalysts

Transition metal (oxy)hydroxides are promising electrocatalysts for the oxygen evolution reaction1-3. The properties of these materials evolve dynamically and heterogeneously4 with applied voltage through ion insertion redox reactions, converting materials that are inactive under open circuit conditions into active electrocatalysts during operation5. The catalytic state is thus inherently far from equilibrium, which complicates its direct observation. Here, using a suite of correlative operando scanning probe and X-ray microscopy techniques, we establish a link between the oxygen evolution activity and the local operational chemical, physical and electronic nanoscale structure of single-crystalline β-Co(OH)2 platelet particles. At pre-catalytic voltages, the particles swell to form an α-CoO2H1.5·0.5H2O-like structure-produced through hydroxide intercalation-in which the oxidation state of cobalt is +2.5. Upon increasing the voltage to drive oxygen evolution, interlayer water and protons de-intercalate to form contracted β-CoOOH particles that contain Co3+ species. Although these transformations manifest heterogeneously through the bulk of the particles, the electrochemical current is primarily restricted to their edge facets. The observed Tafel behaviour is correlated with the local concentration of Co3+ at these reactive edge sites, demonstrating the link between bulk ion-insertion and surface catalytic activity.

Exploring the High-Temperature Electrical Performance of Ca3−xLaxCo4O9 Thermoelectric Ceramics for Moderate and Low Substitution Levels

Aliovalent substitutions in Ca3Co4O9 often result in complex effects on the electrical properties and the solubility, and impact of the substituting cation also depends largely on the preparation and processing method. It is also well-known that the monoclinic symmetry of this material’s composite crystal structure allows for a significant hole transfer from the rock salt-type Ca2CoO3 buffer layers to the hexagonal CoO2 ones, increasing the concentration of holes and breaking the electron–hole symmetry from the latter layers. This work explored the relevant effects of relatively low La-for-Ca substitutions, for samples prepared and processed through a conventional ceramic route, chosen for its simplicity. The obtained results show that the actual substitution level does not exceed 0.03 (x < 0.03) in Ca3−xLaxCo4O9 samples with x = 0.01, 0.03, 0.05 and 0.07 and that further introduction of lanthanum results in simultaneous Ca3Co4O9 phase decomposition and secondary Ca3Co2O6 and (La,Ca)CoO3 phase formation. The microstructural effects promoted by this phase evolution have a moderate influence on the electronic transport. The electrical measurements and determined average oxidation state of cobalt at room temperature suggest that the present La substitutions might only have a minor effect on the concentration of charge carriers and/or their mobility. The electrical resistivity values of the Ca3−xLaxCo4O9 samples with x = 0.01, 0.03 and 0.05 were found to be ~1.3 times (or 24%) lower (considering mean values) than those measured for the pristine Ca3Co4O9 samples, while the changes in Seebeck coefficient values were only moderate. The highest power factor value calculated for Ca2.99La0.01Co4O9 (~0.28 mW/K2m at 800 °C) is among the best found in the literature for similar materials. The obtained results suggest that low rare-earth substitutions in the rock salt-type layers can be a promising pathway in designing and improving these p-type thermoelectric oxides, provided by the strong interplay between the mobility of charge carriers and their concentration, capable of breaking the electron–hole symmetry from the conductive layers.

Development of non-stoichiometric hybrid Co3S4/Co0.85Se nanocomposites for an evaluation of synergistic effect on the OER performance

Water electrolysis powered by renewable energy sources to produce green hydrogen is a very promising and sustainable alternative to fossil fuels, as well as one of the best approaches for significantly reducing carbon footprints and controlling pollution. In this study, novel hybrid non-stoichiometric nanocomposite Co3S4/Co0.85Se electrocatalyst materials were synthesized using a simple and one-pot eco-friendly hydrothermal method. The as-prepared samples were characterized using X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM) for structural features. Crystalline phases of Co3S4, Co0.85Se, and mixed phases in the hybrid samples were confirmed through XRD. Uniformly distributed agglomerated nanoparticles (30-80 nm) of Co3S4, clearly distinguishable nanoaggregates (20-90 nm) of Co0.85Se, and nanoflakes covered with nanoparticles (30-150 nm) of Co3S4/Co0.85Se were confirmed by FESEM. The synergistic effects of the hybrid nanocomposite due to the combination of Co3S4 (containing two different oxidation states of cobalt) and non-stoichiometric Co0.85Se, was investigated towards the oxygen evolution reaction (OER). The electrocatalytic studies confirmed that the Co3S4/Co0.85Se electrocatalyst exhibited a low overpotential of 362 mV @ +30 mA.cm−2 towards the OER in an alkaline solution in comparison with Co3S4 and Co0.85Se electrocatalysts. Furthermore, a low overpotential of 1.59 V at a high current density and a high electrochemical active surface area (ECSA) of 210 cm2 was achieved due to the combined effects of chemical coupling between the Co3S4 and Co0.85Se. Chronoamperometric studies revealed excellent stability of the hybrid nanocomposite samples with negligible changes in the overpotential, even after 30 hours of testing. The synergetic effect of cobalt chalcogenides due to the nanoscale interaction was confirmed by the enhanced electrocatalytic activity.

Observation of a potential-dependent switch of water-oxidation mechanism on Co-oxide-based catalysts

Summary O–O bond formation is a key elementary step of the water-oxidation reaction. However, it is still unclear how the mechanism of O–O coupling depends on the applied electrode potential. Herein, using water-in-salt electrolytes, we systematically altered the water activity, which enabled us to probe the O–O bond-forming mechanism on heterogeneous Co-based catalysts as a function of applied potential. We discovered that the water-oxidation mechanism is sensitive to the applied potential: At relatively low driving force, the reaction proceeds through an intramolecular oxygen coupling mechanism, whereas the water nucleophilic attack mechanism prevails at high driving force. The observed mechanistic switch has major implications for the understanding and control of the water-oxidation reaction on heterogeneous catalysts.

Catalytic Hydrogenation of Alkenes and Alkynes by a Cobalt Pincer Complex: Evidence of Roles for Both Co(I) and Co(II)

The Co(I) complex, [Co(N2)(CyPNP)] (CyPNP = anion of 2,5-bis-(dicyclohexylphosphinomethyl)pyrrole), is active toward the catalytic hydrogenation of terminal alkenes and the semi-hydrogenation of internal alkynes under 2 bar of H2 (g) at room temperature. The products of alkyne semi-hydrogenation are a mixture of E- and Z-alkenes. By contrast, use of the related cobalt(I) precatalyst, [Co(PMe3)(CyPNP)], results in formation of exclusively Z-alkenes. A semi-stable Co(II) species, [CoH(CyPNP)], can also be generated by treatment of degassed solutions of [Co(N2)(CyPNP)] with H2. The CoII-hydride displays activity toward both alkene hydrogenation and isomerization, but its instability hampers implementation as a catalyst. Several species relevant to potential catalytic intermediates have been isolated and detected in solution. These compounds include alkene and alkyne adducts of Co(I) as well as a Co(III) dihydride species. Catalytic results with the compounds examined are most consistent with a process involving shuttling between Co(I) and Co(III) states. However, generation of small quantities of Co(II) during catalytic turnover appears to be responsible for the isomerization observed for alkyne semi-hydrogenation. The interplay of cobalt oxidation states within the same catalyst system is discussed in the context of mechanistic scenarios for catalytic hydrogenation.

Complexity of a Co3O4 System under Ambient-Pressure CO2 Methanation: Influence of Bulk and Surface Properties on the Catalytic Performance

Although using supported noble-metal catalysts for CO2 hydrogenation is an effective solution due to their excellent catalytic properties, metal oxide supports themselves can exhibit good activity being more economically feasible. This work focuses on investigating the complexity of the Co3O4 system during the CO2 methanation reaction, which is usually accompanied by the formation of unstable dispersions of cobalt oxide and metallic Co. Herein, we have tested different types of Co3O4: synthetically prepared mesoporous m-Co3O4 (BET surface area, 95 m2/g) and commercial c-Co3O4 (BET surface area, 15 m2/g; purchased from Merck) in the CO2 methanation reaction under different reduction temperatures (273–673 K). The reduction temperature was adjusted to 573 K for both the catalysts to reach the optimal Co/cobalt oxide ratio and consequently the best catalytic performance. m-Co3O4 is more active (CO2 conversion 95%) and stable at higher temperatures compared to c-Co3O4 (CO2 conversion 63%) due to its morphology-induced ∼66 times higher surface basicity. DRIFTS results showed differences in the detected surface species: formate was observed on m-Co3O4 and was proven to contribute to the total methane formation. It was revealed that in CO2 methanation reaction, both bulk and surface properties such as morphology, cobalt oxidation states, acid–base properties, and presence of defect sites directly affect the catalytic performance and reaction mechanism. Furthermore, 1% 5 nm Pt nanoparticles were loaded onto the Co3O4s to check the competitiveness of the catalysts. This study evidences on a cheap noble-metal-free catalyst for CO2 methanation consisting of m-Co3O4 with competitive activity and ∼100% CH4 selectivity.

Accessing three oxidation states of cobalt in M6L3 nanoprisms with cobalt-porphyrin walls.

Nanocages with porphyrin walls are common, but studies of such structures hosting redox-active metals are rare. Pt2+-linked M6L3 nanoprisms with cobalt-porphyrin walls were prepared and their redox properties were evaluated electrochemically and chemically, leading to the first time that cobalt-porphyrin nanocages have been characterized in CoI, CoII, and CoIII states.

Solvothermal preparation of nano cobalt sulfide from tris (cyclohexylpiperazinedithiocarbamato)cobalt(III) and characterization, single crystal X-ray crystal structure of the precursor

Solvothermal formation of CoS nano particles is reported with a cobalt(III) dithiocarbamate as a single source precursor. Morphology and composition of the nano product have been characterized by PXRD and EDX analysis. Four unsymmetrically substituted tris cobalt(III) compounds, [Co(chmdtc)3] (1), [Co(chedtc)3] (2), [Co(achdtc)3] (3), and [Co(chpdtc)3] (4) (where chmdtc = cyclohexylmethylcarbodithioate, chedtc = cyclohexylethylcarbodithioate, achdtc = allylcyclohexylcarbodithioate and chpdtc = cyclohexylpiperazinecarbodithioate) were synthesized and characterized. Compounds (1)– (4) show bands at 634, 642, 635, and 643 nm respectively, due to d-d transitions. One electron quasi reversible reductions due to Co(III)/Co(II) redox process are observed in CV. In the 13C NMR spectra of the complexes, the thioureide (N13CS2) carbons resonate between 203.8 and 205.7 ppm. The observed deshielding of the –CH2 protons in 1H NMR of the compounds is attributed to the shift of electron density on the sulfur (or the metal) through the thioureide π system. TGA supported the proposed formulas of the compounds. Single crystal X-ray structure of [Co(chpdtc)3] (4) shows that the cobalt is in a distorted octahedral environment. Bond Valence Sum (BVS) calculations confirmed the formal oxidation state of cobalt as +3.

The significance of the local structure of cobalt-based catalysts on the photoelectrochemical water oxidation activity of BiVO4

The local structures of the water oxidation catalysts play an important role in reaction kinetics and the performance of the photoanodes. In this study, we deposited cobalt-based catalysts on nanoporous BiVO4 with controlled thicknesses by atomic layer deposition (ALD). Despite the similar oxidation states of cobalt in all depositions, different water oxidation activities in neutral pH conditions were observed. A dramatic photocurrent raise, lowered kinetic overpotential, and smaller charge transfer resistance across the photoanode/electrolyte interface were achieved when a uniform ultrathin Co(OH)2 layer was formed on BiVO4. Photocurrent density for water oxidation showed a 95% enhancement at 0.6 V vs. RHE when the catalyst was in the form of Co(OH)2, while an 80% increase was obtained for CoO. Ideal coordination of Co(OH)2 on hydroxylated BiVO4 surface assists the charge transfer between the electrolyte and BiVO4 without increasing surface recombination. The results of this study emphasize the importance of controlling the local structure of the catalysts in the performance of the water splitting photoanodes.

Aqueous degradation of artificial sweeteners saccharin and neotame by metal organic framework material

The artificial sweeteners (ASs) saccharin (SAC) and neotame (NEO) are widely used across the globe and are considered as emerging contaminants in surface, ground, and drinking waters. To degrade SAC and NEO, the metal organic framework material Co-based bio-MOF-11 was prepared by hydrothermal reaction and used with peroxymonosulfate (PMS) activator. The effects of the initial concentration of SAC and NEO, bio-MOF-11-Co dosage, PMS concentration, initial pH, temperature, and competitive anions were determined. The results revealed that bio-MOF-11-Co effectively catalyzed the degradation of SAC and NEO and possessed good stability and recycling efficiency. The degradation reaction was effective from pH 3.6–9.8 and followed quasi-first-order kinetics with degradation rate constants of 0.001–0.013 min−1 for SAC and 0.03–0.52 min−1 for NEO. Increased temperature was conducive to the degradation of both artificial sweeteners. The presence of Cl− inhibited the degradation of SAC and NEO, while the presence of CO32− promoted their degradation. Electron paramagnetic resonance (EPR) and free radical quenching demonstrated that the primary free radicals were sulfate radicals (SO4−·) and hydroxyl radicals (HO). The change of cobalt oxidation state and electron transfer in bio-MOF-11-Co mainly induces the production of SO4−·. A plausible mechanism for degradation is SO4−· and HO attack on CS bonds, NS bonds, and benzene rings.

In situ detection of the product of the CoPi-catalyzed oxygen evolution reaction

The in situ detection of the product of the oxygen evolution reaction (OER) catalyzed by cobalt phosphate (CoPi) was achieved with interdigitated array (IDA) electrodes. For this purpose, CoPi and Pt were electrodeposited on the two working electrodes of an IDA, the generator and the collector. The CoPi-catalyzed OER could then be analyzed on the IDA generator and the reaction product, O2, could be detected immediately by the closely arrayed Pt IDA collector electrode. The CoPi-Pt IDA reveals a clear O2 evolution process corresponding to the different oxidation states of the catalyst for the OER and provides information regarding the existence of higher cobalt oxidation states. The turnover frequencies are also calculated using the IDA results. In addition, continuous O2 production and detection was achieved by holding the CoPi generator at a constant higher potential with the Pt collector at a reducing potential measuring O2 from the generator. The collection efficiency is above 90%, meaning that 90% of the O2 product is measured in situ by the IDA collector. Other electrocatalytic reactions of interest could be similarly investigated both qualitatively and quantitatively using IDA electrodes operating in the generator–collector mode.

A highly selective catalyst of Co/La4Ga2O9 for CO2 hydrogenation to ethanol

Abstract A new catalyst of Co/La4Ga2O9 for CO2 hydrogenation to produce ethanol was prepared by reducing LaCo0.5Ga0.5O3, which showed excellent selectivity to ethanol (≈35C-mol%) at mild reaction conditions (270 °C, 3.5 MPa, 3000 mL g−1 h−1). The catalysts were characterized by N2 adsorption/desorption, XRD, XAFS, CO and CO2-TPD, H2 chemisorption, XPS and TEM techniques. The interaction between Co nanoparticles (NPs) and La4Ga2O9 oxide resulted in Co0-Co2+ on the surface of Co NPs. It was proposed that La4Ga2O9 could catalyze reverse water gas shift reaction (r-WGS), which converted CO2 to CO. Then, the CO migrated to Co0-Co2+ on Co NPs, where it was hydrogenated to form ethanol like higher alcohols synthesis from syngas. The results suggest that by controlling the oxidation state of cobalt, and combined with a kind of active site for activating CO2 to form CO, a catalyst with excellent selectivity to ethanol could be obtained for CO2 hydrogenation, which means that the complex reaction may be proceed with high selectivity using only one active metal component.

The Effects of Sr Content on the Performance of Nd1–xSrxCoO3−δ Air-Electrode Materials for Intermediate Temperature Solid Oxide Fuel Cells under Operational Conditions

The potential of the perovskite system Nd1-xSrxCoO3-δ (x = 1/3 and 2/3) as cathode material for solid oxide fuel cells (SOFCs) has been investigated via detailed structural, electrical, and electrochemical characterization. The average structure of x = 1/3 is orthorhombic with a complex microstructure consisting of intergrown, adjacent, perpendicularly oriented domains. This orthorhombic symmetry remains throughout the temperature range 373-1073 K, as observed by neutron powder diffraction. A higher Sr content of x = 2/3 leads to stabilization of the cubic perovskite with a homogeneous microstructure and with a higher oxygen vacancy content and cobalt oxidation state than the orthorhombic phase at SOFC operation temperature. Both materials are p-type electronic conductors with high total conductivities of 690 and 1675 S·cm-1 at 473 K in air for x = 1/3 and 2/3, respectively. Under working conditions, both compounds exhibit similar electronic conductivities, since x = 2/3 loses more oxygen on heating than x = 1/3, associated with a greater loss of p-type charger carriers. However, composite cathodes prepared with Nd1/3Sr2/3CoO3-δ and Ce0.8Gd0.2O2-δ present lower ASR values (0.10 Ω·cm2 at 973 K in air) than composites prepared with Nd2/3Sr1/3CoO3-δ and Ce0.8Gd0.2O2-δ (0.34 Ω·cm2). The high activity for the oxygen electrochemical reaction at intermediate temperatures is likely attributable to a large disordered oxygen-vacancy concentration, resulting in a very promising SOFC cathode for real devices.

Nickel and Cobalt Oxidation State Evolution at Ni-Rich NMC Cathode Surfaces during Treatment

Continued improvement of Ni-rich cathode materials is necessary to achieve higher energy density and cycle life targets for lithium-ion batteries. Despite having a significant influence on these pa...

Thermal Stability of LixCoO2 Electrodes for All-Solid-State Secondary Batteries Operating at High-Temperature

Over the last decades, the development and the miniaturization of portable electronic devices have stimulated many researches in the field of micro power sources. In this context, all-solid-state microbatteries have been developed and are currently commercialized for room temperature applications [1]. The thermal stability of the LiPON ceramic electrolyte used in these microbatteries suggests that they could be used for powering autonomous sensors located in harsh environment, and particularly those exposed to high temperatures (typically 200 °C) [2]. In order to estimate the sustainability of standard microbatteries LiCoO2/LiPON/Li at high temperature, the thermal stability of the positive electrode material and its compatibility with the LiPON electrolyte have been studied.Lithium cobalt oxide is the most used material for positive electrode in microbatteries. Its thermal stability was investigated for the first time by Dahn et al. [3] who showed that LixCoO2 materials start to decompose above 200 °C. Ever since, only few studies have evaluated their intrinsic thermal stability, i.e. without electrolyte and in airtight conditions, but never over extended periods of time and at lower temperatures. Here, LixCoO2 compounds with various compositions (0.45<x<1) were prepared by chemical oxidation of a commercial LiCoO2 powder using a NO2BF4 [4]. ICP-OES and XPS were used to determine the Li/Co ratio and the oxidation state of cobalt respectively. XRD and Raman spectroscopy analyses, conducted in air-tight cells, were achieved to characterize the structural evolution of the pristine delithiated samples. Their thermal stability was firstly evaluated by DSC in sealed crucible. For all the delithiated phases, an exothermic peak linked to the decomposition of the structure accompanied by oxygen loss is observed above 200 °C [3,5]. Long term thermal stability under argon atmosphere was assessed for temperatures ranging from 100 °C to 200 °C, by XRD and Raman spectroscopy. LixCoO2 compounds start to evolve from 100 °C to form a HT-LiCoO2 phase and a spinel Co3O4-like one. In situ XRD temperature analyses were achieved under helium atmosphere to study the decomposition mechanism of the LixCoO2 compounds. The kinetics of the decomposition reaction was followed by in situ XRD isothermal measurements. Kinetics parameters and model function were determined by conventional [6] and Coats-Redfern methods [7]. Identification of decomposition products at 200 °C was carried out by combining XRD, TEM, Raman, XPS and 7Li NMR spectroscopies. All these results suggest that the spinel phase is partially lithiated in this temperature range (100-200°C), whereas HT-LiCoO2 and Co3O4 were clearly identified as decomposition products at higher temperature (600 °C) in agreement with prior studies [3]. The decomposition mechanism appears therefore more complex than the previously reported one. The presence of lithium vacancies destabilizes to the layered structure which collapses locally to form a spinel-like phase due to cobalt moving into the vacancies. Given the ionic radius of cobalt, its presence in the interslab space traps some lithium in the spinel-like phase. At the same time, the major part of remaining lithium tends to segregate in other parts of the crystals where it re-stabilizes the layered scaffold.Raman spectroscopy analyses carried out on LixCoO2 films prepared by magnetron sputtering and delithiated in coin cells, showed that thin films exhibit the same thermal behaviour as bulk compounds.In parallel, LiCoO2/LiPON interface was studied before and after annealing at 200 °C. Thin film bilayers of LiCoO2 (5 µm)/LiPON (3 µm) were prepared by magnetron sputtering. GD-OES and ToF-SIMS analyses were achieved post deposition and after high temperature annealing under argon atmosphere. For pristine bilayers, a 500 nm lithium-rich interphase is observed [8,9]. A marked growth of this interphase, accompanied by phosphorous diffusion into the latter is highlighted for annealed samples.

Atomic layer deposition of cobalt phosphate from cobaltocene, trimethylphosphate, and O2 plasma

Electrodeposited cobalt phosphate has been reported in the literature as a robust alternative to noble metal-based electrocatalysts for the O2 evolution reaction. In parallel, atomic layer deposition (ALD) has been acknowledged as a key technology for the preparation of thin films for energy applications. With the present work, the authors have addressed the preparation of cobalt phosphate thin films by a plasma-assisted ALD process. The process developed consists of cobaltocene (step A) and trimethyl phosphate (step C) exposures alternated by O2 plasma (steps B and D) in an ABCD fashion. The process shows a linear growth with a growth per cycle of 1.12 ± 0.05 Å at 300 °C and no nucleation delay. The ALD saturation behavior has been demonstrated for each dosing step, and the process shows minimal inhomogeneity on 100 mm diameter wafers in terms of film thickness (&amp;lt;1.0%) and refractive index (&amp;lt;0.5%). The chemical characterization of the layers deposited shows that the composition is close to the stoichiometric (Co3.1P2O8.3 for a deposition temperature of 200 °C) and the oxidation states of cobalt and phosphorus and agrees with those reported in the literature. The cobalt-to-phosphorous ratio has been found to correlate with the values of absorption coefficient (k) and refractive index (n) of the layers. Furthermore, the authors have gained insights into the surface reactions occurring during each ALD step by quadrupole mass spectrometry investigation. The results suggest that cobaltocene undergoes associative adsorption and the cyclopentadienyl ligands are removed during subsequent O2 plasma exposure. Moreover, the authors have indirectly identified cyclopentadienone as an intermediate of the oxidation and removal of cyclopentadienyl ligand. On the other hand, the trimethyl phosphate dosing is characterized by chemisorption via the elimination of methanol.

Recent Advances in Four‐Coordinated Planar Cobalt Catalysis in Organic Synthesis

AbstractIn the field of transition metal‐catalyzed organic transformations, four‐coordinated planar cobalt complexes represent an active area of research due to their low cost, abundance on earth, and unique catalytic activity based on the three oxidation states of cobalt. These complexes can behave as various reactive intermediates, such as Co(II) metalloradicals, Co(III) hydrides, Co(I) bases, and Co(I) nucleophiles, according to the ligand field theory. For instance, planar Co(II) species have a d7 electronic configuration and act as metalloradical catalysts, which react with diazo and azide compounds to form reactive intermediates such as Co(III) carbene‐radicals and Co(III) nitrene‐radicals, respectively. These intermediates enable efficient cyclopropanation and aziridination of alkenes and C(sp3)‐H bond functionalization. Furthermore, the hydrogen bonding interaction between amide groups of planar ligands and polar substituents of the carbon radical on the Co(III) carbene‐radicals not only creates robust chiral cavities but also contributes to the stabilization of the Co(III) carbene‐radical and the transition state, consequently resulting in a dramatic improvement in reaction efficiency. The Co(III)‐hydrides provide a useful method for the generation of carbon radicals through hydrocobaltation of alkenes followed by homolytic cleavage of the Co−C bond. The carbon radical formation can participate in various hydrofunctionalizations and alkene‐isomerization. The d8 planar Co(I) complexes act as a base because they have an electronically filled dz2 orbital as the highest occupied molecular orbital (HOMO). This basicity enables various aromatic C−H functionalizations without a stoichiometric amount of oxidant. In contrast, the same planar Co(I) also acts as a superior nucleophilic catalyst, which can react with carbon electrophiles such as epoxides and organic (pseudo)halides to produce the corresponding alkyl‐Co(III) species. The generated alkyl‐Co(III) can be converted into the carbon radical or undergo transalkylation reaction with other transition metal catalysts. The strategies provide a new method for organic transformation with carbon electrophiles. This minireview covers recent developments in the field of four‐coordinated planar cobalt‐catalyzed organic transformations, paying particular attention to the relationship between their catalytic activities and oxidation states. The reactions have been categorized into those involving planar Co(II) complexes, Co(III) hydrides, and Co(I) complexes, with representative examples and insightful mechanistic discussions.