High-valent Cobalt Research Articles

Ti/TiO2-CoWO4-Co3(PO4)2 composites: Plasma electrolytic synthesis, optoelectronic properties, and solar light-driven photocatalytic activity

Co-, W-, and P-containing oxide heterostructures on titanium substrates have been obtained by plasma electrolytic oxidation at current densities of 0.1 and 0.15 A/cm2. The heterostructures have been studied by the X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. Conglomerates of nanoparticles with an elemental composition corresponding to CoWO4, CoO, and Co3(PO4)2 have been found on the surface of the samples. Air-annealing of samples at 700 °C for 1 h leads to the transformation of nanoparticles into nanocrystals with CoWO4 and Co3(PO4)2 phases. The annealed samples and the initial samples obtained at 0.15 A/cm2 are also contain the crystalline phase of TiO2. All samples are characterized by a small bandgap: Eg values for the direct (n = 2) and indirect (n = 1/2) transitions are equal 2.77 and 2.07 eV, respectively, for both initial samples. All samples exhibit photocatalytic activity in indigo carmine (IC) degradation at pH = 2.9 under solar light illumination. The proposed mechanism of solar light-driven photocatalytic activity of Ti/TiO2-CoWO4-Co3(PO4)2 structures includes the formation of p-n heterojunction between TiO2 and CoWO4, capture of holes by Co3(PO4)2, and producing a high-valence cobalt ions Co3+/Co4+, which could further induce oxidation reactions with water molecules and IC.

Insights into Cobalta(III/IV/II)-Electrocatalysis: Oxidation-Induced Reductive Elimination for Twofold C-H Activation.

The merger of cobalt‐catalyzed C−H activation and electrosynthesis provides new avenues for resource‐economical molecular syntheses, unfortunately their reaction mechanisms remain poorly understood. Herein, we report the identification and full characterization of electrochemically generated high‐valent cobalt(III/IV) complexes as crucial intermediates in electrochemical cobalt‐catalyzed C−H oxygenations. Detailed mechanistic studies provided support for an oxidatively‐induced reductive elimination via highly‐reactive cobalt(IV) intermediates. These key insights set the stage for unprecedented cobaltaelectro two‐fold C−H/C−H activation.

Autologous Cobalt Phosphates with Modulated Coordination Sites for Electrocatalytic Water Oxidation

AbstractThe correlation between metal coordination and electrocatalytic water oxidation performance is elusive in many cobalt‐based materials. Herein, we designed an ideal Co phosphate‐based platform to explore the effect of coordination environment on oxygen evolution reaction (OER) activity. The cobalt geometry was modulated from octahedral to tetrahedral by simple removal of water ligands in Co3(PO4)2⋅8 H2O. Other features except the coordination structure in the two autologous materials remain similar. The two analogues display the same OER kinetics, but the anhydrous Co3(PO4)2 exhibits a greatly enhanced OER activity. On the basis of Raman and operando XAS results, the higher intrinsic activity of the Co tetrahedral sites is because they facilitate the formation of active high valent cobalt (hydr)oxide intermediates during OER. This work not only brings insights of OER on Co‐based electrocatalysts but also provides a reference system to study the effect of metal geometry on electrocatalysis.

α-Alkylation of Ketones with Secondary Alcohols Catalyzed by Well-Defined Cp*CoIII -Complexes.

Although α-alkylation of ketones with primary alcohols by transition-metal catalysis is well-known, the same process with secondary alcohols is arduous and complicated by self-condensation. Herein a well-defined, high-valence cobalt(III)-catalyst was applied for successful α-alkylation of ketones with secondary alcohols. A wide-variety of secondary alcohols, which include cyclic, acyclic, symmetrical, and unsymmetrical compounds, was employed as alkylating agents to produce β-alkyl aryl ketones.

Operando Observation of Cobalt Catalysts for Water Splitting By X-Ray and Infrared Absorption Spectroscopy

Recently, electrochemical hydrogen production by water splitting has been receiving attention with regard to achieving a sustainable society. For commercial applications, improving overall water splitting efficiency has been required for the development of highly active oxygen evolution electrocatalysts. In this situation, Nocera and colleagues reported that a cobalt oxide electrodeposited from a dilute Co2+ solution in a potassium phosphate buffered electrolyte (Co-KPi) is efficient electrocatalyst for oxygen evolution reaction (OER) [1]. The local structure of Co species in the Co-KPi catalyst was investigated by Co K-edge X-ray absorption fine structure (XAFS) spectroscopy and X-ray pair distribution function (PDF) analysis, and it was revealed that the Co-KPi is composed of nano-sized clusters with an edge-sharing CoO6 octahedral structure [2]. However, the detailed information about oxygen species in this electrocatalyst under working condition is not yet obtained enough. In this situation, we have developed many operando observation techniques for XAFS and infrared absorption spectroscopy using attenuated total reflection mode (ATR-IR) to observe the oxygen evolution electrocatalysts. For example, hard X-ray XAFS can observe the electronic states and local structures of metal species in the catalysts. On the other hand, tender and soft X-ray XAFS and ATR-IR can reveal the chemical states of light elements in the catalysts. Therefore, we investigated the reaction mechanism of Co-KPi and similar catalysts for water splitting using various kinds of operando XAS and ATR-IR spectroscopy (Figure 1). At first, we prepared for Co-KPi and some similar cobalt catalysts in a potassium acetate (Co-KOAc) or a potassium chloride (Co-KCl) aqueous solution by electrodeposition method. It was confirmed that these cobalt catalysts are composed of the edge-sharing CoO6 octahedral structure (CoOOH) by Co K-edge XAFS using hard X-ray. Next, operando O K-edge XAFS spectra for cobalt catalysts were taken using soft X-ray under electrochemical control. When the electrode potential was changed from 0.0 to 1.1 V, a new absorption peak assigned to oxygen species of CoO2 was observed around 529 eV, indicating that the part of CoOOH cluster was oxidized to CoO2 with the high-valent cobalt species (Co4+). The CoO2 peak was obviously observed for Co-KPi of highly active OER electrocatalyst, compared with Co-KOAc and Co-KCl. Finally, operando P K-edge XAFS and ATR-IR revealed the phosphate ion is bounded on the cobalt clusters to stabilize the CoO2 structure. Therefore, we revealed that Co-KPi can function as highly active OER electrocatalyst due to the presence of CoO2 strucutre stabilized by phosphate ion. [1] D. G. Nocera et al, J. Am. Chem. Soc. 131, 2615 (2009). [2] (a) D. G. Nocera et al, J. Am. Chem. Soc. 132, 13692 (2010). (b) J. Am. Chem. Soc. 135, 6403 (2013). Figure 1

Unique Reactivity of High-valent Cobalt Catalysis in C-H Functionalization and Development of Catalytic Asymmetric C-H Functionalization Reactions

Unique Reactivity of High-valent Cobalt Catalysis in C-H Functionalization and Development of Catalytic Asymmetric C-H Functionalization Reactions

Ketenimine Formation Catalyzed by a High-Valent Cobalt Carbene in Bulky Alkoxide Ligand Environment

High-valent cobalt carbene Co(OR)2(═CPh2) (OR = OCtBu2Ph) undergoes reaction with various isocyanides CNR′ (CNR′ = 2,6-dimethylphenyl isocyanide, 4-methoxyphenyl isocyanide, 2-chloro-6-methylphenyl isocyanide, adamantyl isocyanide) to yield the corresponding ketenimine. The reaction is accompanied by the formation of cobalt bis(alkoxide) bis(isocyanide) complexes Co(OR)2(CNR)2, which were independently synthesized and characterized. DFT calculations suggest the mechanism proceeds through isocyanide binding to Co, followed by intramolecular insertion into the Co–carbene bond to form the ketenimine. We have also conducted an investigation of the catalytic formation of ketenimines at room temperature using mixtures of diazoalkanes (diphenyldiazomethane, methyl diazo(phenyl)acetate, and ethyl diazoacetate) and isocyanides (2,6-dimethylphenyl isocyanide, 4-methoxyphenyl isocyanide, adamantyl isocyanide, cyclohexyl isocyanide, and benzyl isocyanide). While no catalytic reactivity was observed for diphenyldiazomethane, ester-substituted diazoalkanes (diazoesters) demonstrate catalytic turnover. Relatively high yields are observed for the reactions involving bulkier aliphatic substrates adamantyl and cyclohexyl isocyanides. Mechanistic studies suggest that the lack of catalytic reactivity involving diphenyldiazomethane results from the inability of Co(OR)2(CNR)2 to undergo carbene formation upon reaction with N2CPh2. In contrast, facile reaction is observed between Co(OR)2(CNR)2 and diazoesters, which enables the overall catalytic reactivity.

Weakly Coordinating, Ketone‐Directed (η5‐Pentamethylcyclopentadienyl)cobalt(III)‐ and (η5‐Pentamethylcyclopentadienyl)rhodium(III)‐Catalyzed C−H Amidation of Arenes: A Route to Acridone Alkaloids

The weakly coordinating, ketone-directed, regioselective monoamidation of aromatic ketones, chalcone, carbazole, and benzophenones was achieved by employing high-valent cobalt and rhodium catalysis to access numerous biologically important molecular building blocks. This amidation proceeded smoothly with a variety of ketones and several amidating partners. The application of the products in the synthesis of various heterocycles, including acridones, indoles, quinoline, quinolones, quinolinones, and quinazolines, was also explored. The total synthesis of acridone-based alkaloids, namely, toddaliopsin A, toddaliopsin D, and arborinine, and the formal synthesis of acronycine and noracronycin were also accomplished by applying this method. A mechanistic study revealed this amidation reaction follows a base-assisted intermolecular electrophilic substitution pathway.

Cp*Co(iii)-catalyzed N-alkylation of amines with secondary alcohols

We report here the first high-valent cobalt(iii)-catalyzed direct alkylation of aromatic amines with secondary alcohols.

Reactive Cobalt⁻Oxo Complexes of Tetrapyrrolic Macrocycles and N-based Ligand in Oxidative Transformation Reactions.

High-valent cobalt–oxo complexes are reactive transient intermediates in a number of oxidative transformation processes e.g., water oxidation and oxygen atom transfer reactions. Studies of cobalt–oxo complexes are very important for understanding the mechanism of the oxygen evolution center in natural photosynthesis, and helpful to replicate enzyme catalysis in artificial systems. This review summarizes the development of identification of high-valent cobalt–oxo species of tetrapyrrolic macrocycles and N-based ligands in oxidation of organic substrates, water oxidation reaction and in the preparation of cobalt–oxo complexes.

Hydroarylations by cobalt-catalyzed C-H activation.

As an earth-abundant first-row transition metal, cobalt catalysts offer a broad range of economical methods for organic transformations via C–H activation. One of the transformations is the addition of C–H to C–X multiple bonds to afford alkylation, alkenation, amidation, and cyclization products using low- or high-valent cobalt catalysts. This hydroarylation is an efficient approach to build new C–C bonds in a 100% atom-economical manner. In this review, the recent developments of Co-catalyzed hydroarylation reactions and their mechanistic studies are summarized.

Weakly Coordinating, Ketone-Directed Cp*Co(III)-Catalyzed C-H Allylation on Arenes and Indoles.

Weakly coordinating, ketone-directed, regioselective monoallylation of arenes and indoles is reported using a stable and cost-effective high-valent cobalt(III)-catalyst to access several important molecular building blocks. The allylation proceeds smoothly with a variety of substrates in the presence of various electron-rich and -deficient substituents. The method was applied to the formal synthesis of an ancisheynine alkaloid, a highly conjugated azatetracene, and isochroman. The mechanistic study reveals that the allylation reaction follows a base-assisted intermolecular electrophilic substitution pathway.

Electrocatalytic Water Oxidation by Single Site and Small Nuclearity Clusters of Cobalt

Cobalt oxides are an earth abundant material that exhibits high electrocatalytic activity for the oxygen evolution reaction (OER) across a wide pH range. Recent studies suggest that OER catalysis can proceed through an active site comprised of one or two cobalt atoms but that multiple adjacent cobalt centers are preferred to stabilize high valent cobalt oxo-intermediates by delocalization. Utilizing molecular precursors to prepare single, isolated cobalt atoms (SS-Co) and small clusters of Co3O4 we find that OER proceeds more efficiently on Co3O4. Using electrochemical impedance spectroscopy (EIS), these results were rationalized at an atomic level. The EIS results support a hypothesis that charge transfer related to the formation of reaction intermediates proceeds more easily on Co3O4 than on SS-Co, which is attributed to the difficulty in forming Co(IV) = O and unlikely nucleophilic attack by water to form Co(II)-OOH.

Cobalt Catalysts for Oxygen Evolution Studied By Operando Soft X-Ray Absorption Spectroscopy

Recently, hydrogen production system by electrochemical water splitting has received attractive attention to realize a sustainable society that does not depend on fossil fuels. For commercial applications, the improvement of overall water splitting efficiency has been required by the development of highly active oxygen evolution electrocatalyst. In this situation, Nocera and colleagues reported that a cobalt oxide electrodeposited from a dilute Co2+ solution in a potassium phosphate buffered electrolyte (Co-KPi) is efficient electrocatalyst for oxygen evolution reaction (OER) [1]. The local structure of Co species in the Co-KPi catalyst was investigated by Co K-edge X-ray absorption fine structure (XAFS) spectroscopy and X-ray pair distribution function (PDF) analysis, and it was revealed that the Co-KPi is composed of nano-sized clusters with an edge-sharing CoO6 octahedral structure [2]. However, the detailed information about oxygen species in this electrocatalyst under working condition is not yet obtained enough. Thus, we have investigated the reaction mechanism by measuring operando O K-edge XAFS spectra for Co-KPiand similar catalysts under electrochemical potential control. Electrochemical O K-edge XAFS measurements using soft X-rays were performed with transmission mode at BL3U in the UVSOR Synchrotron, according to the previous works [3]. A home-made electrochemical cell was used with Au/Cr/SiC thin film substrates as working electrodes, a Pt counter electrode, and a Ag/AgCl reference electrode. Co-KPi electrocatalyst was electrodeposited on the Au/Cr/SiC working electrode in potassium phosphate buffered electrolyte containing Co(NO3)2. The O K-edge XAFS spectra for Co-KPi catalyst were taken under electrode potential control in potassium phosphate buffered electrolyte (Figure 1). At 0.0 V, an absorption peak attributed to oxygen species of CoOOH was detected around 531 eV. When the electrode potential was changed from 0.0 to 1.1 V, a new absorption peak assigned to oxygen species of CoO2 was observed around 529 eV, indicating that the part of CoOOH cluster was oxidized to CoO2 with the high-valent cobalt species (Co4+). Next, to discuss the relationship between spectroscopic result and catalytic activity, the O K-edge XAFS spectra were taken for the low active cobalt oxide catalysts with CoOOH local structures electrodeposited in a potassium acetate (Co-KOAc) or a potassium chloride (Co-KCl) aqueous solution. Then, the intensities of CoO2 species for Co-KOAc and Co-KCl were much lower than that for Co-KPi. Therefore, we found that Co-KPi can function as highly active OER electrocatalysts due to the presence of CoO2species. [1] D. G. Nocera et al, J. Am. Chem. Soc. 131, 2615 (2009). [2] (a) D. G. Nocera et al, J. Am. Chem. Soc. 132, 13692 (2010). (b) J. Am. Chem. Soc. 135, 6403 (2013). [3] (a) M. Nagasaka et al, J. Phys. Chem. C 117 16343 (2013). (b) M. Yoshida et al, J. Phys. Chem. C 119, 19279 (2015). Figure 1

Bioinspired Cobalt-Citrate Metal-Organic Framework as an Efficient Electrocatalyst for Water Oxidation.

Efficient and cost-effective oxygen evolution reaction (OER) electrocatalysts are closely associated with many important energy conversion technologies. Herein, we first report an oxygen-evolving cobalt-citrate metal-organic framework (MOF, UTSA-16) for highly efficient electrocatalytic water oxidation. Benefiting from synergistic cooperation of intrinsic open porous structure, in situ formed high valent cobalt species, and existing Co4O4 cubane, the UTSA-16 exhibits excellent activity toward OER catalysis in alkaline medium. The UTSA-16 needs only 408 mV to offer a current density of 10 mA cm-2 for OER catalysis, which is superior to that of most MOF-based electrocatalysts and the standard Co3O4 counterpart. The present finding provides a better understanding of electroactive MOFs for water oxidation.

Recent Developments in Cobalt Catalyzed Carbon–Carbon and Carbon–Heteroatom Bond Formation via C–H Bond Functionalization

Cobalt catalysts have evolved to be seen as versatile eco-compatible and economical catalysts in organic synthesis in recent years. Cobalt-catalyzed reactions are undoubtedly a classic in synthetic chemistry for the formation of carbon–carbon and carbon–heteroatom bonds. Another important aspect in this field is catalyst variants, such as low-valent and high-valent cobalt catalysts. This review summarizes the recent progress and synthetic utility of low-valent and high-valent cobalt catalysts towards C–H functionalization processes achieving C–C, C–O, C–N and C–B bond formation. Mechanistic insight is also discussed, with the goal of serving as a stepping stone for further development in this field. In addition, Csp3–H bond functionalization reactions provide many opportunities for novel synthesis approaches. 1 Introduction 2 Carbon–Carbon Bond Formation 2.1 Csp2–Csp3 Bond Formation 2.2 Csp2–Csp2 Bond Formation 3 Carbon–Nitrogen Bond Formation 4 Csp3–H Bond Functionalization 5 Carbon–Oxygen Bond Formation 6 Carbon–Boron Bond Formation 7 Conclusion

Switch to Allylic Selectivity in Cobalt-Catalyzed Dehydrogenative Heck Reactions with Unbiased Aliphatic Olefins

A unique C–H allylation has been discovered with unbiased aliphatic olefins. An intimate M–L affiliation between a high-valent cobalt catalyst and amino-quinoline derived benzamides has been found to be crucial for this unprecedented selectivity. An exemplary set of aliphatic olefins, high yields coupled with excellent regio- and stereoselectivity, and wide functional group tolerances are noteworthy. In addition, a catalytically competent organometallic Co(III) species has been identified through X-ray crystallography. This study is expected to facilitate new synthetic designs toward unconventional allylic selectivity with aliphatic olefins.

ChemInform Abstract: High‐Valent‐Cobalt‐Catalyzed C—H Functionalization Based on Concerted Metalation—Deprotonation and Single‐Electron‐Transfer Mechanisms

AbstractReview: [61 refs.

Synthesis and Characterization of a Stable High-Valent Cobalt Carbene Complex.

The formally Co(IV) carbene Co(OR)2(═CPh2) is formed upon the reaction of diphenyldiazomethane with the cobalt bis(alkoxide) precursor Co(OR)2(THF)2. Structural, spectroscopic, and theoretical studies demonstrate that Co(OR)2(═CPh2) has significant high-valent Co(IV)═CPh2 character with non-negligible spin density on the carbene moiety.

Square-Planar Cobalt(III) Pincer Complex.

A series of square-planar cobalt(II) complexes with pincer ligands {N(CH2CH2PtBu2)2}(-) ({L1(tBu)}(-)), {N(CH2CH2PtBu2)(CHCHPtBu2)}(-) ({L2(tBu)}(-)), and {N(CHCHPtBu2)2}(-) ({L3(tBu)}(-)) was synthesized. Ligand dehydrogenation was accomplished with a new, high-yield protocol that employs the 2,4,6-tri-tert-butylphenoxy radical as hydrogen acceptor. [CoCl{Ln(tBu)}] (n = 1-3) were examined with respect to reduction, protonation, and oxidation, respectively. One-electron oxidations of [CoCl(L1(tBu))] and [CoCl(L2(tBu))] lead to ligand-centered radical reactivity, like amide disproportionation into cobalt(II) amine and imine complexes. In contrast, oxidation of [CoCl{L3(tBu)}] with Ag(+) enabled the isolation of thermally stable, square-planar cobalt(III) complex [CoCl{L3(tBu)}](+), which adopts an intermediate-spin (S = 1) ground state with large magnetic anisotropy. Hence, pincer dehydrogenation gives access to a new platform for high-valent cobalt in square-planar geometry.