Identifying the Types and Characterization of the Active Sites on M−X−C Single-Atom Catalysts

Single atom catalysts (SACs) have attracted much attention in recent years. As an essential group in SACs, M−X−C (X=nonmetallic element) materials have been demonstrated to be efficient in many reactions. However, identifying the active sites on M−X−C, especially under working conditions, is still challenging, which is crucial for chemists to further understand the mechanism underlying the reaction and better design proper SACs for specific reactions. Herein, the types and characterization of M−X−C are comprehensively summarized and discussed in this review. In addition to the basic information above, the challenges and opportunities remaining in this field will be also proposed to present a perspective to the research on the next step.     

Insight into Iron Leaching from an Ascorbate-Oxidase-like Fe−N−C Nanozyme and Oxygen Reduction Selectivity**

Ascorbate (H₂A) is a well-known antioxidant to protect cellular components from free radical damage and has also emerged as a pro-oxidant in cancer therapies. However, such “contradictory” mechanisms underlying H₂A oxidation are not well understood. Herein, we report Fe leaching during catalytic H₂A oxidation using an Fe−N−C nanozyme as a ferritin mimic and its influence on the selectivity of the oxygen reduction reaction (ORR). Owing to the heterogeneity, the Fe-N_x sites in Fe−N−C primarily catalyzed H₂A oxidation and 4 e⁻ ORR via an iron-oxo intermediate. Nonetheless, trace O₂⋅⁻ produced by marginal N−C sites through 2 e⁻ ORR accumulated and attacked Fe-N_x sites, leading to the linear leakage of unstable Fe ions up to 420 ppb when the H₂A concentration increased to 2 mM. As a result, a substantial fraction (ca. 40 %) of the N−C sites on Fe−N−C were activated, and a new 2+2 e⁻ ORR path was finally enabled, along with Fenton-type H₂A oxidation. Consequently, after Fe ions diffused into the bulk solution, the ORR at the N−C sites stopped at H₂O₂ production, which was the origin of the pro-oxidant effect of H₂A.     

Generation of FeIV=O and its Contribution to Fenton-Like Reactions on a Single-Atom Iron−N−C Catalyst

Generating Fe^IV=O on single-atom catalysts by Fenton-like reaction has been established for water treatment; however, the Fe^IV=O generation pathway and oxidation behavior remain obscure. Employing an Fe−N−C catalyst with a typical Fe−N₄ moiety to activate peroxymonosulfate (PMS), we demonstrate that generating Fe^IV=O is mediated by an Fe−N−C−PMS* complex—a well-recognized nonradical species for induction of electron-transfer oxidation—and we determined that adjacent Fe sites with a specific Fe₁−Fe₁ distance are required. After the Fe atoms with an Fe₁-Fe₁ distance <4 Å are PMS-saturated, Fe−N−C−PMS* formed on Fe sites with an Fe₁-Fe₁ distance of 4–5 Å can coordinate with the adjacent Fe^II−N₄, forming an inter-complex with enhanced charge transfer to produce Fe^IV=O. Fe^IV=O enables the Fenton-like system to efficiently oxidize various pollutants in a substrate-specific, pH-tolerant, and sustainable manner, where its prominent contribution manifests for pollutants with higher one-electron oxidation potential.     

Gelatin-Derived 1D Carbon Nanofiber Architecture with Simultaneous Decoration of Single Fe−Nx Sites and Fe/Fe3C Nanoparticles for Efficient Oxygen Reduction

Exploring high-performance non-precious-metal electrocatalysts for the oxygen reduction reaction (ORR) is critical. Herein, a scalable and cost-effective strategy is reported for the construction of one-dimensional carbon nanofiber architectures with simultaneous decoration of single Fe−N_x sites and highly dispersed Fe/Fe₃C nanoparticles for efficient ORR, through the Fe^III-complex-assisted electrospinning of gelatin nanofibers with subsequent pre-oxidation and carbonization. Results show that the presence of a Fe^III complex enables the 1D gelatin nanofibers to be well retained during the pre-oxidation process. Owing to the distinct 1D nanofiber structure and the synergistic effect of Fe/Fe₃C and Fe−N_x sites, the resulting electrocatalyst is highly active for ORR with a half-wave potential of 0.885 V (outperforming commercial Pt/C) and a superior electrochemical stability in alkaline electrolytes. Similarly, it also shows a high power density (144.7 mW cm⁻²) and a superior stability in Zn-air batteries. This work opens a path for the design and synthesis of 1D carbon electrocatalyst for efficient ORR catalysis.     

Hydrothermal-Induced Formation of Well-Defined Hollow Carbons with Curvature-Activated N−C Sites for Zn–Air Batteries

Metal-free carbons have been regarded as one of the promising materials alternatives to precious-metal catalysts for oxygen reduction reaction (ORR) due to their high activity and stability. In this paper, well-defined N-doped hollow carbons (NHCs) are firstly synthesized by using an ammonia-based hydrothermal synthesis that is environmentally friendly and suitable for mass production in industry and a commercial black carbon as raw material. Moreover, the shell thickness of the NHCs can be easily tuned by this hydrothermal strategy. Zn–air battery test results reveal shell thickness-dependent activity and durability for ORR over the NHCs, which exceeds that obtained by commercial Pt/C (20 wt %). The enhanced battery performance can be attributed to the curvature-activated N–C moieties on the hollow carbon surface, which served as the main active sites for ORR as evidenced by DFT calculations. The proposed approach may open a way for designing curved hollow carbons with high graphitization degree and dopant nitrogen level for metal–air batteries or fuel cells.     

Atramacronoids A−C,three eudesmanolide sesquiterpene-phenol hybrids with an unprecedented C−C linkage from the rhizomes of Atractylodes macrocephala

Three eudesmanolide sesquiterpene-phenol hybrids, atramacronoids A?C (1?3), featuring an unusual 6/6/5/5/6 skeleton furnished by forming an unexpected C-8?C-16 linkage, were obtained from the rhizomes of Atractylodes macrocephala. Their structures and absolute configurations were elucidated by spectroscopic data analysis, chemical calculations, combined with X-ray diffractions. The plausible biosynthetic pathways for compounds 1?3 are proposed. Surprisingly, compound 1 exhibited cytotoxicity against SGC-7901 cells by inducing cells apoptosis, which might relate to the promotion of synthesis of neutrophil elastase.     

Re-Catalyzed Annulations of Weakly Coordinating N-Carbamoyl Indoles/Indolines with Alkynes via C−H/C−N Bond Cleavage

Described herein are rhenium-catalyzed [3+2] annulations of N-carbamoyl indoles with alkynes via C−H/C−N bond cleavage, which provide rapid access to fused-ring pyrroloindolone derivatives. For the first time, the weakly coordinating O-directing group was successfully employed in rhenium-catalyzed C−H activation reactions, enabled by the unique catalytic trio of Re₂(CO)₁₀, Me₂Zn and ZnCl₂. Mechanistic studies revealed that aminozinc species plays an important role in the reaction. Based on the mechanistic understanding, a more powerful catalytic trio of Re₂(CO)₁₀, [MeZnNPh₂]₂ and Zn(OTf)₂ was devised and applied successfully in the [4+2] annulations of indolines and alkynes affording pyrroloquinolinone derivatives.     

Hydrolysis of Methylphenyldiethoxysilane with Rare Earth Superacid Catalysts SO2− 4/TiO2/Ln3+

Abstract

An environmentally benign hydrolysis of methylphenyldiethoxysilane (MePhSi(OEt)₂) catalyzed by a rare earth superacid catalyst SO ^2? ₄ /TiO₂/Ln³⁺ has been investigated. The hydrolysis rates decrease in the order SO ^2? ₄ /TiO₂/Nd³⁺ > SO ^2? ₄ /TiO₂/Y³⁺ > SO ^2? ₄ /TiO₂/Sm³⁺ > SO ^2? ₄ /TiO₂. The hydrolysis of MePhSi(OEt)₂ is a first-order reaction with respect to the concentration of MePhSi(OEt)₂, and the hydrolysis rate constant increases with increasing temperature. The activation energy E _a and the pre-exponential factor for this hydrolysis catalyzed by SO ^2? ₄ /TiO₂/Nd³⁺ have been determined as 322.50 kJ mol^?1 and 7.12 × 10⁴¹ s^?1, respectively. The products of the hydrolysis are oligomers of polymethylphenylsiloxane. The mechanism of MePhSi(OEt)₂ hydrolysis is also discussed.     

Pivotal Role of the Basic Character of Organic and Salt Catalysts in C−N Bond Forming Reactions of Amines with CO2

Organocatalysts promote a range of C−N bond forming reactions of amines with CO₂. Herein, we review these reactions and attempt to identify the unifying features of the catalysts that allows them to promote a multitude of seemingly unrelated reactions. Analysis of the literature shows that these reactions predominantly proceed by carbamate salt formation in the form [BaseH][RR′NCOO]. The anion of the carbamate salt acts as a nucleophile in hydrosilane reductions of CO₂, internal cyclization reactions or after dehydration as an electrophile in the synthesis of urea derivatives. The reactions are enhanced by polar aprotic solvents and can be either promoted or hindered by H-bonding interactions. The predominant role of all types of organic and salt catalysts (including ionic liquids, ILs) is the stabilization of the carbamate salt, mostly by acting as a base. Catalytic enhancement depends on the combination of the amine, the base strength, the solvent, steric factors, ion pairing and H-bonding. A linear relationship between the base strength and the reaction yield has been demonstrated with IL catalysts in the synthesis of formamides and quinazoline-2,4-diones. The role of organocatalysts in the reactions indicates that all bases of sufficient strength should be able to catalyze the reactions. However, a physical limit to the extent of a purely base catalyzed reaction mechanism should exist, which needs to be identified, understood and overcome by synergistic or alternative methods.     

Novel Preparation of Polyethylene from Nano‐extrusion Polymerization Inside the Nanochannels of MCM‐41/MgCl2/TiCl4 Catalysts

The MCM‐41 and SiO₂ supported TiCl₄ and TiCl₄/MgCl₂ catalysts with different molar ratios of Mg/Ti were synthesized and used for ethylene polymerization under atmospheric pressure. The nanochannels of MCM‐41 serve as nanoscale polymerization reactor and the polyethylene nanofibers were extruded during the reaction. The nanofibers were observed in SEM micrographs of resulting polyethylene. The effect of MgCl₂ on catalytic activity and thermal properties of resulting polyethylene is investigated too. In the presence of MgCl₂, the catalytic activity increased and more crystalline polyethylene with higher melting points were formed. However, no fibers could be observed in the polyethylene prepared by SiO₂ supported catalysts.     

The Mechanism of Stereospecific C?H Oxidation by Fe(Pytacn) Complexes: Bioinspired Non‐Heme Iron Catalysts Containing cis‐Labile Exchangeable Sites

A detailed mechanistic study of the hydroxylation of alkane C? H bonds using H₂O₂ by a family of mononuclear non heme iron catalysts with the formula [Fe^II(CF₃SO₃)₂(L)] is described, in which L is a tetradentate ligand containing a triazacyclononane tripod and a pyridine ring bearing different substituents at the α and γ positions, which tune the electronic or steric properties of the corresponding iron complexes. Two inequivalent cis‐labile exchangeable sites, occupied by triflate ions, complete the octahedral iron coordination sphere. The C? H hydroxylation mediated by this family of complexes takes place with retention of configuration. Oxygen atoms from water are incorporated into hydroxylated products and the extent of this incorporation depends in a systematic manner on the nature of the catalyst, and the substrate. Mechanistic probes and isotopic analyses, in combination with detailed density functional theory (DFT) calculations, provide strong evidence that C? H hydroxylation is performed by highly electrophilic [Fe^V(O)(OH)L] species through a concerted asynchronous mechanism, involving homolytic breakage of the C? H bond, followed by rebound of the hydroxyl ligand. The [Fe^V(O)(OH)L] species can exist in two tautomeric forms, differing in the position of oxo and hydroxide ligands. Isotopic‐labeling analysis shows that the relative reactivities of the two tautomeric forms are sensitively affected by the α substituent of the pyridine, and this reactivity behavior is rationalized by computational methods.     

Metal-free remote oxidative benzylic C−H amination of 4-methylanilides with N-fluorobenzenesulfonimide

A metal-free remote oxidative benzylic C?H amination of 4-methylanilides with N-fluorobenzenesulfonimide was reported. The reaction was promoted by a hypervalent iodine reagent and can be handled under mild and neutral conditions, providing the highly regioselective benzylic C?H amination products even with multi-substituted 4-methylanilides. It provided a novel and facile method for the construction of C(sp³)–N bonds.     

Iridium(III) Catalysts with an Amide-Pendant Cyclopentadienyl Ligand: Double Aromatic Homologation Reactions of Benzamides by Fourfold C−H Activation

The synthesis, characterization, and catalytic performance of iridium(III) catalysts that bear an amide-pendant cyclopentadienyl ligand ([Cp^AIrI₂]₂) is reported. These [Cp^AIrI₂]₂ catalysts were obtained from the complexation of a Cp^A ligand precursor with [Ir(cod)OAc]₂ followed by oxidation. Double aromatic homologation reactions of benzamides with alkynes by fourfold C−H activation proceeded in good to high yield with these [Cp^AIrI₂]₂ catalysts, demonstrating their superior catalytic performance in this challenging transformation.     

[{Fe(tim)}2]: An Fe?Fe Dimer Containing an Unsupported Metal–Metal Bond and Redox‐Active N4 Macrocyclic Ligands

Mixed doubles : The dimeric complex [{Fe(tim)}₂] (see structure, tim=2,3,9,10‐tetramethyl‐1,4,8,11‐tetraazacyclotetradeca‐1,3,8,10‐tetraene) represents an unprecedented complex containing an unsupported Fe? Fe bond. The crystal structure confirms the presence of reduced tim units, thus indicating ligand redox activity. Spectroscopic and computational studies establish a triplet ground state for [{Fe(tim)}₂] and suggest a mixed‐valence compound with respect to both the Fe ions and the ligands.

     

Two Coordination Modes of Bidentate Aminopyrazine Ligands in Cubane-type Cluster Complex Re4Te4Cl8(C4N3H4)4 · 2DMF

Abstract  A new cubane-type cluster complex Re₄Te₄Cl₄(C₄H₄N₃)₄ · 2DMF has been synthesized by reaction of Re₄Te₄Cl₈(TeCl₂)₄ with 2-aminopyrazine C₄H₅N₃ in DMF. The crystal structure of compound has been solved by X-ray single crystal diffraction method. Crystal data for Re₄Te₄Cl₄(C₄N₃H₄)₄ · 2DMF: a = 22.8718(16) ?, b = 8.5936(7) ?, c = 20.5720(17) ?, β ^o = 106.493(2), V = 3877.1(5) ?³, R ₁ = 0.0466, R _w(F ²) = 0.1191. In the complex bidentate aminopyrazine ligands are coordinated in two different types, namely, two of four aminopyrazine ligands bind to a single rhenium atom, and each of two other ligands is coordinated as bridge between two rhenium atoms. Graphical Abstract  A new cubane-type cluster complex Re₄Te₄Cl₄(C₄H₄N₃)₄  · 2DMF with two coordination modes of bidentate aminopyrazine ligands has been synthesized and structurally characterized.      

Catalytic enantioselective synthesis of atropisomeric 2-aryl-4-quinolinone derivatives with an N–C chiral axis

In the presence of an (R)-MOP-Pd₂(dba)₃ catalyst, the reaction of ortho-tert-butylaniline with 2-bromophenyl arylethynyl ketone proceeded via a tandem amination (1,4-addition of aniline to an ynone and subsequent intramolecular Buchwald–Hartwig amination) to afford axially chiral N-(2-tert-butylphenyl)-2-aryl-4-quinolinone derivatives with moderate enantioselectivity (up to 72% ee).     

Photovoltaic Properties of Polymer Film Composites with a Tetranuclear Copper Complex Cu4L4(OCH3)4 (L− = N,N′-Dibenzyl-N″-Trichloroacetylphosphotriamide)

Theoretical and Experimental Chemistry - A study was carried out on the photovoltaic and photoconductive properties of film composites of a copolymer of styrene with nonyl methacrylate and...     

CeIV- and HClO4-Promoted Assembly of an Fe2IV(μ-O)2 Diamond Core from its Monomeric FeIV=O Precursor at Room Temperature

Diiron(IV)-oxo species are proposed to effect the cleavage of strong C−H bonds by nonheme diiron enzymes such as soluble methane monooxygenase (sMMO) and fatty acid desaturases. However, synthetic mimics of such diiron(IV) oxidants are rare. Herein we report the reaction of (TPA*)Fe^II ( 1 ) (TPA*=tris(3,5-dimethyl-4-methoxypyridyl-2-methyl)amine) in CH₃CN with 4 equiv CAN and 200 equiv HClO₄ at 20 °C to form a complex with an [Fe^IV₂(μ-O)₂]⁴⁺ core. CAN and HClO₄ play essential roles in this unprecedented transformation, in which the comproportionation of Fe^III-O-Ce^IV and Fe^IV=O/Ce⁴⁺ species is proposed to be involved in the assembly of the [Fe^IV₂(μ-O)₂]⁴⁺ core.     

Synthesis of LiFe(1−x)V x PO4/C composite cathode materials with high performance via an aqueous solution–evaporation method

The modification techniques of applying carbon coating on particle surface and doping vanadium at Fe site were applied to make the LiFePO₄ cathode materials achieve high rate performance in lithium ion batteries. To design and synthesize these LiFe_(1?x)V _x PO₄/C (x?=?0, 0.02, 0.05, or 0.08) composites, an aqueous solution–evaporation method was taken, in which every kind of raw material was distributed at a high degree of uniformity. The LiFe_0.95V_0.05PO₄/2.57 wt% C composite displayed the best electrochemical performances. At rates of 0.1, 0.5, 2, 5, and 10 C (1 C?=?170 mAg^?1), it delivered a discharge capacity of 157.8, 156.9, 149, 139.6, and 130.1 mAh g^?1, respectively. The composite exhibited perfect cycle stabilities as well, maintaining 100 % (0.5 C), 99.7 % (2 C), 98.9 % (5 C), and 96.6 % (10 C) of the first discharge capacity after 100 cycles at different rates, respectively.     

Synthesis and characterization of LiMn0.8Fe0.2PO4/rGO/C for lithium-ion batteries via in-situ coating of Mn0.8Fe0.2C2O4·2H2O precursor with graphene oxide

Journal of Solid State Electrochemistry - LiMn0.8Fe0.2PO4 is a potential candidate cathode material to balance the energy density, safety, and cost of power lithium ion batteries. However, the low...