Transition metal-catalyzed tandem isomerization and cationic polymerization of allyl ethers. II. Structural and mechanistic studies

In the presence of organosilanes, dicobalt octacarbonyl catalyzes the polymerization of alkyl allyl ethers to give high molecular weight polymers. This article reports the results of a detailed mechanistic study of this new polymerization reaction. The evidence obtained in this study supports a stepwise process involving first, the reaction of dicobalt octacarbonyl with an organosilane to form HCo(CO)₄ and R₃SiCo(CO)₄. In subsequent steps, HCo(CO)₄ isomerizes the allyl ether to a 1-propenyl ether and then this compound is polymerized by the formal transfer of a silyl cation from R₃SiCo(CO)₄. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1985–1997, 1997     

Activation of Molecular Oxygen by a Cobalt(II) Tetra-NHC Complex**

The first dicobalt(III) μ₂-peroxo N-heterocyclic carbene (NHC) complex is reported. It can be quantitatively generated from a cobalt(II) compound bearing a 16-membered macrocyclic tetra-NHC ligand via facile activation of dioxygen from air at ambient conditions. The reaction proceeds via an end-on superoxo intermediate as demonstrated by EPR studies and DFT. The peroxo moiety can be cleaved upon addition of acetic acid, yielding the corresponding Co^III acetate complex going along with H₂O₂ formation. In contrast, both Co^II and Co^III complexes are also studied as catalysts to utilize air for olefin and alkane oxidation reactions; however, not resulting in product formation. The observations are rationalized by DFT-calculations, suggesting a nucleophilic nature of the dicobalt(III) μ₂-peroxo complex. All isolated compounds are characterized by NMR, ESI-MS, elemental analysis, EPR and SC-XRD.     

Kinetics and mechanism of the catalytic reaction between alcohols and dimethyl carbonate

The mechanism of the reaction between alcohols and dimethyl carbonate, catalyzed by dicobalt octacarbonyl Co₂(CO)₈, is studied by means of mathematical modeling. Kinetic models for possible schemes of chemical transformations are constructed at different initial concentrations of the catalyst. Based on a comparative analysis of activation energies of possible stages of chemical transformations, possible reaction pathways are determined and an appropriate mechanism is selected.     

Synthesis and thermal decomposition of cobalt-containing oligosilanes

Oligo(phenylcobaltcarbonylsilane) was prepared from oligo(phenylsilane) and dicobalt octacarbonyl. The reaction proceeds with elimination of H₂ and CO and insertion of cobalt carbonyl fragments into the silicone backbone of oligosilane. Oligosilane containing cobalt carbonyl groups in side organic substituents was obtained from oligolmethyl(phenylethynyl)Isilane and CO₂(CO)₈. The reaction of 1,2-bis(phenylethyny1)tetramethyldisilane with Co₂(CO)₈ proceeds with the sequential attachment of cobalt carbonyl fragments to ethynyl groups to form disilane derivatives [²-CCPhCo₂(CO)₆] Thermal decomposition of cobalt-containing oligosilanes affords a mixture of paramagnets and ferromagnets.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya. No. 10, pp. 2561–2567, October, 1996.     

The oxidation of Co2(CO)8, Mn2(CO)10 and Fe(CO)5 by dibenzoyl peroxide. Kinetics and mechanism

Summary Dicobalt octacarbonyl in toluene solution can be quantitatively oxidized at room temperature with dibenzoyl peroxide to cobalt(II) benzoate and carbon monoxide. The rate of CO evolution is first order in dicobalt octacarbonyl, first order in dibenzoyl peroxide, and negative first order in CO. Similar behaviour leading to manganese(II) benzoate is observed with dimanganese decacarbonyl. Sixteen electron rather than seventeen electron intermediates are involved in these reactions. In contrast to the dinuclear carbonyls, Fe(CO)₅ is oxidized by dibenzoyl peroxide in an autocatalytic reaction.     

Kinetics and mechanism of the reaction of tetracobalt dodecacarbonyl with carbon monoxide under pressure

The kinetics of the reaction of tetracobalt dodecacarbonyl with carbon monoxide to form dicobalt octacarbonyl in n-hexane have been investigated over a wide range of temperature and CO pressure. The reaction is first order in [Co₄(CO)₁₂]; the order in [CO] changes between one (at low pressures and high temperatures) and two (at high pressures and low temperatures).Activation parameters have been estimated and a mechanism involving initial reversible breaking of one CoCo bond, followed by irreversible breaking of a second, is proposed. The first step involves concerted addition of CO while the second can proceed with or without such addition.     

水热合成Co3O4的制备参数调变及其催化分解N2O性能

以十六烷基三甲基溴化胺（CTAB）为模板剂,通过调变CTAB浓度水热合成了氧化钴前驱体,焙烧制得棒状形貌的Co₃O₄,在其表面浸渍K₂CO₃溶液制得K改性的Co₃O₄催化剂,用于N₂O分解。用X射线衍射（XRD）、N₂物理吸附（BET）、扫描电镜（SEM）、X射线光电子能谱（XPS）、H₂程序升温还原（H₂-TPR）和O₂程序升温脱附（O₂-TPD）等技术对催化剂进行了表征,考察了CTAB/钴及尿素/钴物质的量比等制备参数对Co₃O₄催化分解N₂O活性的影响。结果表明,CTAB浓度为0.05 mol/L、CTAB/钴离子物质的量比为1、尿素/钴离子物质的量比为4时,所制备的Co₃O₄催化剂具有较高的N₂O分解活性,而K改性可以进一步提升其催化性能。K改性的Co₃O₄在有氧有水气氛中400℃下进行N₂O分解反应,50 h后N₂O转化率仍保持在91%以上。     

Co/AlPO4-5催化剂中Co的状态及其作用

本文用IR,TEM,FABMS,ESCA,XRD以及还原度的测定等方法,描述了Co/AlPO_4-5上Co_3O_4在氢作用下变成Co°,CoO以及未还原的Co_3O_4三种状态的还原过程和钴的颗粒分布的变化,并表明在还原过程中,可能有钴的簇状物生成,导致B酸形成。     

Synthesis and structure of the first diamagnetic trinuclear cobalt carbonyl sulfur complexes,SCo3(CO)7(μ-R1ĆNR2)

Reaction of primary thioamides with dicobalt octacarbonyl affords in good yield the first diamagnetic monomeric trinuclear cobalt carbonyl sulfur complexes SCO₃(CO)₇(μ-R¹?NR²), which contain a bridging bidentate imino ligand. The structure of the product with R¹  Me, R²  C₆H₁₁ has been determined by X-ray diffraction, and shown to include a very short cobaltcobalt bond distance and a long cobaltsulfur bond distance when compared to other cobaltsulfur clusters.     

Preparation of phenyl substituted propargylic alcohol dicobalt hexacarbonyls and their reactions with active methylene compounds in the presence of acid

Eight new complexes with the formula [PhC_2C(OH)R~2R~2]Co_2(CO)_6 were prepared fromphenyl substituted propargylic alcohols and dicobalt octacarbonyl.The reactions of these propargylioalcohol complexes with active methylene compounds,2,4-pentanedione or ethyl acetoacetate,in thepresnce of an acid,HBF_4(40％)+P_2O_5(in excess)or BF_3·Et_2O,at room temperature in dichlorome-thane were investigated.From the 1-alkyl substituted tertiary propargylic alcohol complexes,threenew conjugated ene-yne complexes produced by intramolecular dehydration reaction were isolated inhigh yields(82—95％).On the other hand,four new alkylated complexes were obtained withsatisfactory yields(44—66％)from the secondary propargylic alcohol complexes.The influence ofother acids,phosphorus pentoxide and polyphosphoric acid,on both dehydration reaction andalkylated reaction was also studied.     

Cobalt Hydroxide Nanosheets and Their Thermal Decomposition to Cobalt Oxide Nanorings

We demonstrate herein that single‐crystalline β‐cobalt hydroxide (β‐Co(OH)₂) nanosheets can be successfully synthesized in large quantities by a facile hydrothermal synthetic method with aqueous cobalt nitrate as the cobalt source and triethylamine as both an alkaline and a complexing reagent. This synthetic method has good prospects for the future large‐scale production of single‐crystalline β‐Co(OH)₂ nanosheets owing to its high yield, low cost, and simple reaction apparatus. Single‐crystalline porous nanosheets and nanorings of cobalt oxide (Co₃O₄) were obtained by a thermal‐decomposition method with single‐crystalline β‐Co(OH)₂ nanosheets as the precursor. A probable mechanism of formation of β‐Co(OH)₂ nanosheets, porous Co₃O₄ nanosheets, and Co₃O₄ nanorings was proposed on the basis of the experimental results.     

Redox behaviour of the dicobalt cofacial porphyrin Co2FTF4 and related complexes adsorbed on graphite electrodes

Adsorbed on graphite electrodes, Co₂FTF4 in a potent catalyst for O₂ reduction by a four-electron mechanism. The two observable redox surface waves have been previously assigned to the two cobalt centres. Using differential pulse polarography (DPP), the behaviour of this dicobalt cofacial dimer was re-examined at different pH values in aqueous solutions and in the presence of potential axial ligands for cobalt. From these observations and from a comparison with other adsorbed porphyrins it can be concluded that (a) the porphyrins are probably adsorbed by strong interactions between graphite and the aromatic rings, and (b) the more negative surface wave is cobalt-based but the more positive one is instead a porphyrin ring oxidation. This implies that the catalyst is in the Co^IICo^IIIFTF4 state when catalytic oxygen reduction begins.     

Electronic Structure and Hydrogen Bonding in a Dicobalt(III) Werner Complex

The bonding of the O-O group in the dicobalt cation 1a [(NH₃)₆Co₂(μ-O₂)(μ-OH)(μ-NH₂)]³⁺ was studied by DFT methods (ADF program) and the bridging O₂ ligand was characterized as superoxide(O₂⁻). In this complex, three bridging ligands connect the two cobalt atoms, forcing a cis conformation of the Co-O-O-Co atoms. A comparison was made with [(NH₃)₁₀Co₂(μ-O₂)]⁵⁺, 2a, where a trans arrangement is observed. Superoxide binds more strongly to the dicobalt(III) fragment in 2a than in 1a, both as a result of weaker Pauli repulsion and stronger covalent interaction. It was found that in 1a the electronic structure with one unpaired electron, where cobalt is formally Co(III), d⁶, and O₂ carries one negative charge gives rise to the most stable structure, compared to possibilities with three and five unpaired electrons. The hydrogen bonds in the crystal were analyzed and the interactions between one water molecule or one nitrate ion studied in more detail.     

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.     

Catalytic Active Site for NO Decomposition Elucidated by Surface Science and Real Catalyst

Comprehensive studies combining surface science and real catalyst were performed to get further insight into catalytic active site and reaction mechanism for NO decomposition over supported palladium and cobalt oxide-based catalysts. On palladium single-crystal model catalysts, adsorption, dissociation and desorption behavior of NO was found to be closely related to the surface structures, the stepped surface palladium being active for dissociation of NO. In accordance with this result, the activity of powder Pd/Al₂O₃ catalysts for NO decomposition was directly related to the number of step sites exposed on the surface, suggesting that the step sites act as the catalytic active site for NO decomposition on Pd/Al₂O₃. NO decomposition over cobalt oxide was found to be significantly promoted by addition of alkali metals. Surface science study and catalyst characterization led to the same conclusion that the interface between the alkali metal and Co₃O₄ serves as the catalytic active site. From the results of in situ Fourier transform infrared (FT-IR) spectroscopy and isotopic transient kinetic analysis, a reaction mechanism was proposed in which the reaction is initiated by NO adsorption onto alkali metals to form NO₂⁻ species and then NO₂⁻ species react with the adsorbed NO species to form N₂ over the interface between the alkali metal and Co₃O₄.     

Core-Shell-Type All-Inorganic Heterometallic Nanoclusters: Record High-Nuclearity Cobalt Polyoxoniobates for Visible-Light-Driven Photocatalytic CO2 Reduction

Only rarely have polyoxometalates been found to form core–shell nanoclusters. Here, we succeeded in isolating a series of rare giant and all-inorganic core–shell cobalt polyoxoniobates (Co−PONbs) with diverse shapes, nuclearities and original topologies, including 50-nuclearity {Co₁₂Nb₃₈O₁₃₂}, 54-nuclearity {Co₂₀Nb₃₄O₁₂₈}, 62-nuclearity {Co₂₆Nb₃₆O₁₄₀} and 87-nuclearity {Co₃₃Nb₅₄O₁₈₈}. They are the largest Co−PONbs and also the polyoxometalates containing the greatest number of Co ions and the largest cobalt clusters known thus far. These molecular Co−PONbs have intriguing and atomically precise core–shell architectures comprising unique cobalt oxide cores and niobate oxide shells. In particular, the encapsulated cobalt oxide cores with different nuclearities have identical compositions, structures and mixed-valence Co³⁺/Co²⁺ states as the different sized Co−O moieties of the bulk cubic-spinel Co₃O₄, suggesting that they can serve as various molecular models of the cubic-spinel Co₃O₄. The successful construction of the series of the Co−PONbs reveals a feasible and versatile synthetic method for making rare core–shell heterometallic PONbs. Further, these new-type core–shell bimetal species are promising cluster molecular catalysts for visible-light-driven CO₂ reduction.     

富氧条件下Co/ZSM-5催化剂对C_3H_8选择还原NO_x的性能

采用多种物理化学手段研究了在模拟的轻型柴油车尾气中不同Co担载量及Cu掺杂的Co/ZSM-5催化剂的Co组分分散状态、可还原性、NO吸附脱附性质对C3H8选择性催化还原NOx性能的影响。结果表明,浸渍法制备的Co/ZSM-5催化剂上既有外表面上的Co3+和Co2+物种,也有孔内的Co2+离子。富氧条件下Co/ZSM-5催化剂上C3H8选择性催化还原NOx的活性主要与ZSM-5载体孔外表面分散的CoOx物种中的钴离子可还原能力和NO吸附脱附性能密切相关。Co/ZSM-5催化剂上适宜的Co担载量约为4.0wt%,低担载量时随Co担载量增加,表面CoOx物种中钴离子可还原能力增强,C3H8选择性催化还原NOx的低温转化活性增加;高担载量时,随Co担载量增加,单位Co离子的NO吸附量的减少以及催化剂表面活性中心数的减少,导致了Co/ZSM-5催化剂NOx的转化率和催化剂比速率(k)的下降。孔外表面Co3O4晶体的存在使催化剂表面产生较强的NO吸附,并在高温时有利于C3H8的氧化燃烧,使C3H8选择性催化还原NOx的活性降低。     

Preparation, characterization and photocatalytic activity of Co3O4/LiNbO3 composite

The Co₃O₄/LiNbO₃ composites were synthesized by impregnation of LiNbO₃ in an aqueous solution of cobalt nitrate and next by calcination at 400°C. The activity of produced samples has been investigated in the reaction of photocatalytic hydrogen generation. The crystallographic phases, optical and vibronic properties were studied using X-ray diffraction (XRD), diffuse reflectance (DR) UV-vis and resonance Raman spectroscopic techniques, respectively. The influence of cobalt content (range from 0.5 wt.% to 4 wt.%) on the photocatalytic activity of Co₃O₄/LiNbO₃ composites for photocatalytic hydrogen generation has been investigated. Co₃O₄/LiNbO₃ composites exhibited higher than LiNbO₃ photocatalytic activity for hydrogen generation. The highest H₂ evolution efficiency was observed for Co₃O₄/LiNbO₃ composite with 3 wt.% cobalt content. The amount of H₂ obtained in the presence of LiNbO₃ and Co₃O₄/LiNbO₃ (3 wt.% of cobalt content) was 1.38 µmol/min and 2.59 µmol min^?1, respectively.      

富氧条件下Co/ZSM-5催化剂对C3H8选择还原NOx的性能

采用多种物理化学手段研究了在模拟的轻型柴油车尾气中不同Co担载量及Cu掺杂的Co/ZSM-5催化剂的Co组分分散状态、可还原性、NO吸附脱附性质对C₃H₈选择性催化还原NO_x性能的影响。结果表明,浸渍法制备的Co/ZSM-5催化剂上既有外表面上的Co³⁺和Co²⁺物种,也有孔内的Co²⁺离子。富氧条件下Co/ZSM-5催化剂上C₃H₈选择性催化还原NO_x的活性主要与ZSM-5载体孔外表面分散的CoO_x物种中的钴离子可还原能力和NO吸附脱附性能密切相关。Co/ZSM-5催化剂上适宜的Co担载量约为4.0wt%,低担载量时随Co担载量增加,表面CoO_x物种中钴离子可还原能力增强,C₃H₈选择性催化还原NO_x的低温转化活性增加;高担载量时,随Co担载量增加,单位Co离子的NO吸附量的减少以及催化剂表面活性中心数的减少,导致了Co/ZSM-5催化剂NO_x的转化率和催化剂比速率（k）的下降。孔外表面Co₃O₄晶体的存在使催化剂表面产生较强的NO吸附,并在高温时有利于C₃H₈的氧化燃烧,使C₃H₈选择性催化还原NO_x的活性降低。     

Nano-Co3O4-catalyzed microwave-assisted one-pot synthesis of some seleno [2 , 3-b ] pyridine/quinoline derivatives

For the efficient synthesis of transition-metal cobalt oxide nanoparticles (Co₃O₄ NPs) without using any costly and toxic solvent or complicated equipment, the co-precipitation method was used in this work. Using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), UV–Vis spectrophotometry, Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD), the prepared Co₃O₄ NPs were characterized and identified. The influence of prepared Co₃O₄ NPs on the developmental synthesis of some selenopyridine/quinoline derivatives under different microwave irradiation powers and irradiation times was investigated via click (reaction) chemistry. The reusable Co₃O₄ nanoparticles have high catalytic activity under microwave irradiation for the synthesis of organoselenium compounds with higher yields (>?90%), milder reaction conditions and shorter time without significantly decreasing the reaction rates and yields.