Real Catalytic Species Research Articles

Chemical and structural origin of lattice oxygen oxidation in Co–Zn oxyhydroxide oxygen evolution electrocatalysts

The oxygen evolution reaction (OER) is a key process in electrochemical energy conversion devices. Understanding the origins of the lattice oxygen oxidation mechanism is crucial because OER catalysts operating via this mechanism could bypass certain limitations associated with those operating by the conventional adsorbate evolution mechanism. Transition metal oxyhydroxides are often considered to be the real catalytic species in a variety of OER catalysts and their low-dimensional layered structures readily allow direct formation of the O–O bond. Here, we incorporate catalytically inactive Zn2+ into CoOOH and suggest that the OER mechanism is dependent on the amount of Zn2+ in the catalyst. The inclusion of the Zn2+ ions gives rise to oxygen non-bonding states with different local configurations that depend on the quantity of Zn2+. We propose that the OER proceeds via the lattice oxygen oxidation mechanism pathway on the metal oxyhydroxides only if two neighbouring oxidized oxygens can hybridize their oxygen holes without sacrificing metal–oxygen hybridization significantly, finding that Zn0.2Co0.8OOH has the optimum activity. Oxygen evolution is one half of the overall water splitting reaction to produce hydrogen. Although this reaction is well studied, there remains debate over the particulars of the catalytic mechanism. Here, the authors investigate Co–Zn oxyhydroxide electrocatalysts, and suggest that the mechanism depends on the amount of Zn2+ they contain.

OSSO]‐Type Fe(III) Metallate as Single‐Component Catalyst for the CO2 Cycloaddition to Epoxides

AbstractA new [OSSO]‐Fe(III) metallate complex was prepared and characterized. We demonstrated that such metallate is the real catalytic active species for the cycloaddition of CO2 to the epoxides, formed from the in situ reaction of the related [OSSO]‐Fe(III) neutral complexes and tetrabutylammonium bromide. The metallate complex was used as a single component catalyst for the formation of cyclic organic carbonates from ten epoxides and CO2 at 1 bar pressure with good activity.magnified image

Hybrid 2D Dual‐Metal–Organic Frameworks for Enhanced Water Oxidation Catalysis

AbstractMetal–organic frameworks (MOFs) and MOF‐derived nanostructures are recently emerging as promising catalysts for electrocatalysis applications. Herein, 2D MOFs nanosheets decorated with Fe‐MOF nanoparticles are synthesized and evaluated as the catalysts for water oxidation catalysis in alkaline medium. A dramatic enhancement of the catalytic activity is demonstrated by introduction of electrochemically inert Fe‐MOF nanoparticles onto active 2D MOFs nanosheets. In the case of active Ni‐MOF nanosheets (Ni‐MOF@Fe‐MOF), the overpotential is 265 mV to reach a current density of 10 mA cm−2 in 1 m KOH, which is lowered by ≈100 mV after hybridization due to the 2D nanosheet morphology and the synergistic effect between Ni active centers and Fe species. Similar performance improvement is also successfully demonstrated in the active NiCo‐MOF nanosheets. More importantly, the real catalytic active species in the hybrid Ni‐MOF@Fe‐MOF catalyst are unraveled. It is found that, NiO nanograins (≈5 nm) are formed in situ during oxygen evolution reaction (OER) process and act as OER active centers as well as building blocks of the porous nanosheet catalysts. These findings provide new insights into understanding MOF‐based catalysts for water oxidation catalysis, and also shed light on designing highly efficient MOF‐derived nanostructures for electrocatalysis.

Nitrido complex of high-valent Ru(VI) -catalyzed reduction of imines and alkynes with hydrosilanes: A theoretical study of the reaction mechanism

The reaction mechanism for hydrosilylation of imines and alkynes catalyzed by a nitrido complex of high-valent Ru(VI) has been studied using density functional theory. Four different reaction routes for RuVIN complex catalyzed hydrosilylation have been probed in detail, spanning those involving N, RuVI, or RuIII as active center in activating SiH bonds. The activation of SiH bond at the nitrido ligand can be discounted because they would lead to large activation barrier for reduction of imines and alkynes. Our computational results support an ionic outer-sphere mechanism involving silane initially binding with ruthenium metal. This proposed ionic outer-sphere mechanism is characterized by a rate-determining SiH bond cleavage upon imines nitrogen or alkynes carbon atoms backside attacking the silane-ruthenium complex, featuring a SN2@Si transition state. In addition, we found organic substrate is crucial in controlling the catalytic cycle. With imines, the ionic outer-sphere cycle occurs at the high-valent RuVI metal center is preferable. The highest energy barrier must overcome is 21.9 kcal/mol. With alkynes, the low-valent RuIII complex formed by NN coupling of RuVIN complex is the real catalytic species. The rate-determining energy barrier is calculated to be 12.7 kcal/mol.

Transformation of a Cp*-iridium carbene catalyst in water oxidation using Oxone as primary oxidant

Fundamental understanding of the real catalytic species in water oxidation is a major knowledge gap that must be overcome in order to develop artificial water splitting systems and produce chemical fuels from solar energy. In an effort to analyze the transformation and catalytic resting state of [Cp*Ir(pmNHC)Cl]Cl [pmNHC=1-(2-pyrimidyl)-3-n-butylimidazol-2-ylidene, Cp*=pentamethylcyclopentadienyl] in water oxidation using potassium peroxymonosulfate (Oxone) as primary oxidant, we found the precatalyst evolved into blue solution when a small amount of Oxone was added while further oxidation yield purple solution. These species have been proven to be homogeneous and they remain active in water oxidation. 1H NMR profiles and MALDI-MS data suggested a rapid oxidation of the Cp* ligand to acetone, formic and acetic acid. 1H NMR, TEM-EDX as well as MALDI-MS data indicated a continuous degradation of pyrimidyl, butyl and NHC groups. MALDI-MS, XANES and 17O NMR spectroscopy data of the blue species are most consistent with an iridium(III) compound with two waters, one dioxygen and pmNHC group. The catalytic resting purple species probably consists of an iridium(III) complex with three waters, one chloride and one bicarbonate ligand. Besides, the presence of a small amount of iridium aqua ion oligomers was observed in the purple species. The formation of polymers and nanoparticles is likely to be inhibited by the acid environment.

Novel CoP Hollow Prisms as Bifunctional Electrocatalysts for Hydrogen Evolution Reaction in Acid media and Overall Water-splitting in Basic media

Cobalt phosphide (CoP) with hollow prisms nanostructure has been successfully synthesized via a facile two-step micro-assisted method. Firstly, uniform Co3(CH3COO)5(OH) cubes are prepared by microwave method. Then as-prepared Co3(CH3COO)5(OH) cubes as precursors have been conducted by phosphorization at low temperature. Novel CoP hollow prisms with 1μm length, 500nm width and 500nm height have been synthesized. The obtained CoP hollow prisms can be used as both an efficient electrocatalyst for HER in acid media (0.5M H2SO4 solution) and a remarkable bifunctional electrocatalyst for overall water-splitting in alkaline media (1.0M KOH solution) which might be attributed to their high surface area, porosity and good distribution. The OER mechanisms of CoP in alkaline solution have been investigated through XPS and SEM Mapping, which demonstrate that the amorphous oxygen-containing nanostructure on the surface of CoP may be the real catalytic active species. In addition, phosphidation of uniform cobalt salt hydrates precursor with specific structure at low temperature is a newfangled and effective strategy to synthesis bifunctional electrocatalyst CoP.

Dehydration of Aldoximes Using PhSe(O)OH as the Pre-Catalyst in Air.

PhSe(O)OH was found to be a good pre-catalyst for aldoxime dehydrations in open air. Compared with the previously reported (PhSe)(2)-H(2)O(2) system, it is more stable and milder, affording broader application scopes due to a higher functional group tolerance. The control experiments for mechanism study disclosed that air was the key factor for the reaction to maintain enough concentration of PhSeOH, which should be the real catalytic species.

The Real Role of N-Heterocyclic Carbene in Reductive Functionalization of CO2: An Alternative Understanding from Density Functional Theory Study.

The mechanisms of reductive functionalization of CO2 to formamide catalyzed by N-heterocyclic carbene (NHC) were comprehensively studied with DFT calculations. New activation mode with much lower energy barrier than those proposed before was discovered. In this reaction, NHC acts as neither a CO2 nor a silane activator, but as a precursor of the real catalyst, i.e., the in situ formed ionic liquid [NHCH](+)[Carbamate](-). In this loose contact ion pair, the negatively charged O atom of the carbamate anion becomes the new active site and is free to do nucleophilic attack. When amine is absent, CO2 will be converted into methanol. In this case, the NHC-CO2 adduct is the real catalytic species, the active site shifted from the carbene C atom to the negatively charged O atom. These new activation modes follow a pattern of "S(N)2@Si-Acceptor", in which the Si-H bond is activated via concerted backside S(N)2 nucleophilic attack by the negatively charged O atom, and the leaving hydride is directly accepted by a free CO2 molecule. The advantages of these new activation modes originate from the following points: (1) The ionic liquid [NHCH](+)[Carbamate](-) and NHC-CO2 adduct are thermodynamically more stable than NHC. (2) The active site of the NHC catalyst is extended outside a lot. Consequently, the large steric effect between the NHC arms and the substrates in transition state can be avoided to some extent. (3) The O atom has good silicon affinity. In addition, a free CO2 molecule, whose carbon atom is more electrophilic than those of the CO2 moieties in NHC-CO2 adduct and carbamate, acts as an efficient hydride acceptor.

A closer look at ε-caprolactone polymerization catalyzed by alkyl aluminum complexes: the effect of induction period on overall catalytic activity.

Previous studies on the ring-opening polymerization of ε-caprolactone using structurally related aluminum complexes as pre-catalysts showed inconsistent trends in the total conversion time. We propose that an induction period for Al complexes for conversion to real catalytic species, Al alkoxide, should be considered because the total conversion time consists of both an induction period and polymer propagation time. Herein, the polymerization rate of a series of Al complexes bearing ketimine ligands was investigated. The catalytic results indicated complexes with more steric hindrance with an electron-withdrawing group on the ligands, or the fact that less chelating ligands demonstrated greater propagation activity. The opposite trend for these structural effects was observed on the measurement of induction periods. These features on ligands of aluminum complexes are responsible for facilitating the conversion process to Al alkoxides. The overall catalytic performance should consider both the induction period and the propagation time.

Using the Type II Cycloisomerization Reaction of 1,6‐Enynes as a Mechanistic Probe to Identify the Real Catalytic Species of GaX3 and InX3

AbstractPreviously, we found that InCl2+ is the real catalytic species in InCl3‐catalyzed type II cycloisomerization of 1,6‐enynes for generating nonconjugated dienes. We further used this cycloisomerization reaction as a mechanistic probe to study whether GaCl3, GaBr3, InBr3, InI3 also have GaCl2+, GaBr2+, InBr2+, and InI2+ as the real catalytic species. Experimental and computational results from the mechanistic probe reaction here support that the real catalytic species are positively charged GaCl2+, GaBr2+, InBr2+, and InI2+.

Catalytic role of TiO(2) terminal oxygen atoms in liquid-phase photocatalytic reactions: oxidation of aromatic compounds in anhydrous acetonitrile.

On the basis of experiments carried out with controlled amounts of residual oxygen and water, or by using oxygen-isotope-labeled Ti(18) O2 as the photocatalyst, we demonstrate that (18) Os atoms behave as real catalytic species in the photo-oxidation of acetonitrile-dissolved aromatic compounds such as benzene, phenol, and benzaldehyde with TiO2 . The experimental evidence allows a terminal-oxygen indirect electron-transfer (TOIET) mechanism to be proposed, which is a new pathway that involves the trapping of free photogenerated valence-band holes at Os species and their incorporation into the reaction products, with simultaneous generation of oxygen vacancies at the TiO2 surface and their subsequent healing with oxygen atoms from either O2 or H2 O molecules that are dissolved in the liquid phase. According to the TOIET mechanism, the TiO2 surface is not considered to remain stable, but is continuously changing in the course of the photocatalytic reaction, challenging earlier interpretations of TiO2 photocatalytic phenomena.

Mechanisms of the InCl3-Catalyzed Type-I, II, and III Cycloisomerizations of 1,6-Enynes

InCl3-catalyzed cycloisomerizations of 1,6-enynes can give either type-I dienes and cyclohexenes (type-III dienes), or type-II dienes, depending on the substitutions in the substrates. Previously, we studied how the type-II diene products were generated and found that the real catalytic species for the cycloisomerizations is InCl2(+) (J. Org. Chem. 2012, 77, 8527-8540). In the present paper, we used density functional theory (DFT) calculations to reveal how the type-I and type-III dienes were generated. A unified model to explain how substituents affect the regiochemistry of type-I, II, and III cycloisomerizations has been provided. Experimental and computational investigation of the InCl3-catalyzed cycloisomerization of 1,6-enynes with both substituents at the alkyne and alkene parts has also been reported in the present study.

In-situ and self-distributed: A new understanding on catalyzed thermal decomposition process of ammonium perchlorate over Nd2O3

Catalyzed thermal decomposition process of ammonium perchlorate (AP) over neodymium oxide (Nd2O3) was investigated. Catalytic performances of nanometer-sized Nd2O3 and micrometer-sized Nd2O3 were evaluated by differential scanning calorimetry (DSC). In contrast to universal concepts, catalysts in different sizes have nearly similar catalytic activities. Based on structural and morphological variation of the catalysts during the reaction, combined with mass spectrum analyses and studies of unmixed style, a new understanding of this catalytic process was proposed. We believed that the newly formed chloride neodymium oxide (NdOCl) was the real catalytic species in the overall thermal decomposition of AP over Nd2O3. Meanwhile, it was the “self-distributed” procedure which occurred within the reaction that also worked for the improvement of overall catalytic activities. This work is of great value in understanding the roles of micrometer-sized catalysts used in heterogeneous reactions, especially the solid–solid reactions which could generate a large quantity of gaseous species.

Cobalt‐Based Particles Formed upon Electrocatalytic Hydrogen Production by a Cobalt Pyridine Oxime Complex

An open-coordination-sphere cobalt(III) oximato-based complex was designed as a putative catalyst for the hydrogen evolution reaction (HER). Electrochemical alteration in the presence of acid occurs, leading to the formation of cobalt-based particles that act as an efficient catalyst for HER at pH 7. The exact chemical nature of these particles is yet to be determined. This study thus raises interesting issues regarding the fate of molecular-based complexes designed for the HER, and points to the challenging task of identifying the real catalytic species. Moreover, understanding and rationalizing the alteration pathways can be seen as a new route to reach catalytic particulates.

DFT and Experimental Exploration of the Mechanism of InCl3-Catalyzed Type II Cycloisomerization of 1,6-Enynes: Identifying InCl2+ as the Catalytic Species and Answering Why Nonconjugated Dienes Are Generated

InCl(3) and other In(III) species have been widely applied as catalysts in many reactions. However, what are the real catalytic species of these reactions? Through DFT calculations and experimental investigation of the mechanism and regioselectivity of InCl(3)-catalyzed cycloisomerization reactions of 1,6-enynes (here all discussed 1,6-enynes are ene-internal-alkyne molecules), we propose that the catalytic species of this reaction is the in situ generated InCl(2)(+). Further electrospray ionization high-resolution mass spectroscopy (ESI-HRMS) supported the existence of InCl(2)(+) in acetonitrile solution. This finding of InCl(2)(+) as the catalytic species suggests that other reactions catalyzed by In(III) species could also have cationic In(III) species as the real catalysts. DFT calculations revealed that the catalytic cycle of the cycloisomerization of 1,6-enynes catalyzed by InCl(3) starts from InCl(2)(+) coordination to the alkyne of the substrate, generating a vinyl cation. Then nonclassical cyclopropanation of the vinyl cation to the alkene part of the substrate gives a homoallylic cation, which undergoes a novel homoallylic cation rearrangement involving a [1,3]-carbon shift to give the more stable homoallylic cation 15. Finally InCl(2)(+) cation coordination assisted nonconjugated [1,2]-hydride shifts deliver the final nonconjugated diene products. The preference of generating nonconjugated dienes instead of conjugated dienes in the cycloisomerization reaction is mainly due to two reasons: coordination of the InCl(2)(+) to the alkene part in [1,2]-H shift transition states disfavors the conjugated [1,2]-H shifts that generate cations adjacent to the positively charged alkene, and coordination of InCl(2)(+) to the nonconjugated diene product is stronger than coordination to the conjugated diene, making nonconjugated [1,2]-H shift transition states lower in energy than conjugated [1,2]-H shift transition states, on the basis of the Hammond postulate. DFT calculations predicted that the conjugated [1,2]-H shifts could become favored if the electron-donating methyl substituent in the alkyne moiety of the 1,6-enyne is replaced by a H atom. This prediction of producing a conjugated diene has been verified experimentally. Rationalization about why type II rather than type I products were obtained using InCl(3) as the catalyst in the cycloisomerization of 1,6-enynes has also been investigated computationally.

Study on the Catalytic Properties of Bicomponent Nanooxides in the Thermal Decomposition of Ammonium Perchlorate

Nanostructured catalysts Nd2O3, Cr2O3 and NdCrO3 were prepared by precipitation method. Catalytic effect of the catalysts on the process of thermal decomposition of ammonium perchlorate (AP) was studied by differential thermal analysis. The structure and morphology of the catalysts prior to and after the thermal decomposition of AP were characterized by X-ray powder diffractometer and scanning electron microscopy. Our experiments suggest the real catalytic species play an important role in the overall thermal decomposition of AP for this bicomponent nanooxides catalytic system.

Pyrido[1,2-a][1,2,4]triazol-3-ylidenes as a New Family of Stable Annulated N-Heterocyclic Carbenes: Synthesis, Reactivity, and Their Application in Coordination Chemistry and Organocatalysis

General synthetic avenues to the pyrido-annulated triazolium salts with different steric and electronic properties have been developed. This architecture can be readily altered with different N-alkyl or aryl substituents at the N2 position of the triazole ring and modifications to the pyridine backbone. Deprotonation of the triazolium salts 12 with NaH led to formation of stable carbenes 11 at room temperature as clearly demonstrated through ESI mass spectra and by observation of the characteristic (13)C NMR resonance for the carbene carbon at delta = 202-208 ppm. In sharp contrast, treatment of these triazolium salts with K2CO3 led to dimerization of free carbenes 11. The dimeric enetetramine (11b)2 could react with elemental sulfur to deliver the corresponding thiourea 16 in toluene at 80 degrees C in good yield. A silver complex with the pyrido[1,2-a][1,2,4]triazol-3-ylidene is described, and the molecular structure of complex 17 was established by X-ray crystallography. The triazolium salts 12 turned out to be powerful catalysts in catalytic benzoin condensations and transesterifications at 25 degrees C. The catalytic activity was largely dependent on the steric and electronic nature of the R(1) and R(2) substituents of the triazolium salt. We rationalized that this type of triazolium-catalyzed benzoin condensations should undergo the "traditional" Breslow mechanism rather than the pathway of the dimer (11)2 as the real catalytic species.

Intramolecular [4+2] Cycloaddition Reactions Catalyzed by Rhodium Catalysts Derived from a Rhodium Anion Precursor

Development of effective cyclization reactions for the synthesis of carbocycles and heterocycles has been the subject of extensive study because of their relevance to medicines and other functional materials. Among the numerous methods available, the transition metal-promoted cyclization reaction has become one of the most popular. Although considerable efforts are still being devoted to the development of more efficient and practical catalytic systems, identification of true catalysts or utilization of new catalyst precursors is also important since all the key catalytic properties are inherently dependent upon the real catalytic species. Moreover, these studies may lead to development of new catalytic systems. Recently, we have found generation of an anionic rhodium species, [Rh(COD)Cl2] −, from the reaction of dimeric [Rh(COD)Cl]2 with dppe. 4 Thus, reaction between [Rh(COD)Cl]2 and dppe (1 equiv per Rh atom) in THF gave a yellow precipitate, and subsequent heating of the yellow solid above 120 C led to isolation of the well-known orange solid, [Rh(dppe)Cl]2. By the way, the exact composition of the yellow precipitate, a synthetic intermediate for [Rh(dppe)Cl]2, had not been reported until we recently solved its molecular structure by an X-ray crystallographic analysis (Figure 1). Thus, it was revealed that the intermediate yellow solid before heating was an ionic complex of [Rh(dppe)2] cation and [Rh(COD)Cl2] anion. Synthesis of the ionic complex would be attributed to a disproportionation reaction of the starting rhodium dimer. According to the literatures, the first report of this anion with a cation other than the present [Rh(dppe)2] cation appeared in 1957, but its crystal structure was solved thirty years later. While we studied chemistry of [Rh(dppe)2][Rh(COD)Cl2] (1), we realized this anion could be used as a catalyst precursor in many catalytic reactions. Herein, we report that [4+2] cycloaddition reactions with catalysts from a [Rh(COD)Cl2] anion precursor. The anionic rhodium species was prepared not only by the reaction of [Rh(COD)Cl]2 with dppe but also by the reaction of [Rh(COD)Cl]2 with PPNCl [PPN = bis(triphenylphosphoranyliene)ammonium] to study catalytic activity of Rh anion precursor. In order to confirm usefulness of the [Rh(COD)Cl2] anion as a catalyst precursor, catalytic intramolecular [4+2] cycloaddition of a dienyne was investigated as shown in Table 1. Both the complex 1 and an initially in-situ generated species from a reaction of [Rh(COD)Cl]2 with dppe were inactive as catalyst precursors in the intramolecular [4+2] cycloaddition reaction (entries 1 and 2). However, addition

Understanding the Accelerating Effect of ε-Caprolactam on the Formation of Urethane Linkages

The accelerating role of e-caprolactam on the formation of urethane linkages is studied in the case of the reaction between toluene 2,4-diisocyanate and n-propanol in carbon tetrachloride at room temperature. FT-IR spectroscopy is exploited to follow the consumption of the isocyanate groups. The comparison between the rate of the reactions carried out in the presence and absence of a catalytic amount of e-caprolactam shows its accelerating effect. The acylurea-like derivative 1-methyl-2,4-[(2-oxoazepane-1-carbonyl)amino]benzene has been prepared and identified as the real catalytic species formed in situ as a result of the reaction between toluene 2,4-diisocyanate and e-caprolactam. A kinetic model is proposed to analyze the experimental data, and B3LYP/6-31+G* calculations are exploited to investigate the structure of 1-methyl-2,4-[(2-oxoazepane-1-carbonyl)amino]benzene and clarify the structural features leading the catalytic activity.

Effect of Solvent Polarity on the Initiation and the Propagation of Ethylene Polymerization with Constrained Geometry Catalyst/MAO Catalytic System: A Density Functional Study with the Conductor-Like Screening Model

The effect of solvent polarity on the initiation and propagation of ethylene polymerization with a constrained geometry catalyst (CGC) and MAO counterion was investigated by the density functional theory (DFT) with the conductor-like screening model (COSMO). Structures and energetics of reactant, π-complex, transition state, γ-agostic product, and β-agostic product during insertion of ethylene monomer into the catalytic active species of ionic pair or cationic catalyst are analyzed using the software DMol3. A comparative analysis on the energetics of the ethylene insertion in the gas state and in solvents with different dielectric constants (ε) shows that the solvent polarity affects the energetics of monomer insertion very differently depending upon the model for active catalytic species and/or polymerization stage. This different behavior provides a very useful method for identifying the real active catalytic species in experiment. The two important features of molecular structure determining the dependency of enegetics on the solvent polarity are the degree of exposure of the Ti atom on the solvent medium and the degree of separation of ionic pair during monomer insertion.