Synergistic Effects between Atomically Dispersed Fe−N−C and C−S−C for the Oxygen Reduction Reaction in Acidic Media

Various advanced catalysts based on sulfur‐doped Fe/N/C materials have recently been designed for the oxygen reduction reaction (ORR); however, the enhanced activity is still controversial and usually attributed to differences in the surface area, improved conductivity, or uncertain synergistic effects. Herein, a sulfur‐doped Fe/N/C catalyst (denoted as Fe/SNC) was obtained by a template‐sacrificing method. The incorporated sulfur gives a thiophene‐like structure (C−S−C), reduces the electron localization around the Fe centers, improves the interaction with oxygenated species, and therefore facilitates the complete 4 e⁻ ORR in acidic solution. Owing to these synergistic effects, the Fe/SNC catalyst exhibits much better ORR activity than the sulfur‐free variant (Fe/NC) in 0.5 m H₂SO₄.     

Dimetallapoly‐yn‐diylidynes: LnM≡C−(C≡C)x−C≡MLn (x=0–4)

Synthetic routes to dimetallated C_x carbon wires in which two metals are separated by a linear carbon chain involving terminal metal–carbon triple bonds are described for the complexes [(Tp*)(CO)₂W≡C?(C≡C)_n?C≡W(CO)₂(Tp*)] (Tp*=hydrotris(dimethylpyrazolyl)borate) where n=1, 3 or 4, joining the previously known examples with n=0, 1 and 2 to complete the series as models for linear carbyne C_∞.     

Reagent‐Free C−H/N−H Cross‐Coupling: Regioselective Synthesis of N‐Heteroaromatics from Biaryl Aldehydes and NH3

An unprecedented synthesis of N‐heteroaromatics from biaryl aldehydes and NH₃ through reagent‐free C−H/N−H cross‐coupling has been developed. The electrosynthesis uses NH₃ as an inexpensive and atom‐economic nitrogen donor, requires no oxidizing agents, and allows efficient and regioselective access to a wide range of phenanthridines and structurally related polycyclic N‐heteroaromatic products.     

Unprecedented Reaction Pathway of Sterically Crowded Calcium Complexes: Sequential C−N Bond Cleavage Reactions Induced by C−H Bond Activations

Five bis(quinolylmethyl)‐(1H ‐indolylmethyl)amine (BQIA) compounds, that is, {(quinol‐8‐yl‐CH₂)₂NCH₂(3‐Br‐1H ‐indol‐2‐yl)} ( L¹H ) and {[(8‐R³‐quinol‐2‐yl)CH₂]₂NCH(R²)[3‐R¹‐1H ‐indol‐2‐yl]} ( L^2–5H ) ( L²H : R¹=Br, R²=H, R³=H; L³H : R¹=Br, R²=H, R³=i Pr; L⁴H : R¹=H, R²=CH₃, R³=i Pr; L⁵H : R¹=H, R²=n Bu, R³=i Pr) were synthesized and used to prepare calcium complexes. The reactions of L^1–5H with silylamido calcium precursors (Ca[N(SiMe₂R)₂]₂(THF)₂, R=Me or H) at room temperature gave heteroleptic products ( L^{1, 2} )CaN(SiMe₃)₂ ( 1 , 2 ), ( L^{3, 4} )CaN(SiHMe₂)₂ ( 3 a , 4 a ) and homoleptic complexes ( L^{3, 5} )₂Ca ( D3 , D5 ). NMR and X‐ray analyses proved that these calcium complexes were stabilized through Ca⋅⋅⋅C−Si, Ca⋅⋅⋅H−Si or Ca⋅⋅⋅H−C agostic interactions. Unexpectedly, calcium complexes (( L^3–5 )CaN(SiMe₃)₂) bearing more sterically encumbered ligands of the same type were extremely unstable and underwent C−N bond cleavage processes as a consequence of intramolecular C−H bond activation, leading to the exclusive formation of (E )‐1,2‐bis(8‐isopropylquinol‐2‐yl)ethane.     

Halogenid‐Ionen als Katalysator: Metallzentrierte C–C‐Bindungsknüpfung ausgehend von Acetontril

Halide Ions as Catalyst: Metalcentered C–C Bond Formation Proceeded from Acetonitril AlMe₃ reacts at 20 ?C in acetonitrile to the complex [Me₃Al(NCMe)] ( 1 ). By addition of cesium halides (X^– = F^–, Cl^–, Br^–) a trimerisation to the heterocycle [Me₂Al{HNC(Me)}₂C(CN)] ( 2 ) has been observed. The reaction might be carried out under catalytic conditions (1–2 mol% CsX). The gallium complex [Me₂Ga{HNC(Me)}₂ · C(CN)] ( 3 ), generated under similar reaction conditions, can be converted to the silylated compound [Me₂Ga{Me₃SiNC(Me)}₂C(CN)] ( 4 ) by successive treatment with two equivalents n‐butyllithium and Me₃SiCl. 3 reacts under hydrolysis conditions (1 M hydrochloric acid) to the iminium salt [{H₂NC(Me)}₂C(CN)]Cl ( 5 ). A mixture of H₂O, Ph₂PCl and 3 in THF/toluene leads in a unusual conversion to the diphospane derivative [Ph₂P–P(O)(Me₂GaCl)] ( 6 ). 1 , 2 , 4 , 5 and 6 have been characterized by NMR, IR and MS techniques. X‐ray structure analyses were performed with 1 , 2 , 4 and 6 · 0.5 toluene. According this 1 possesses an almost linear axis AlNCC [Al1–N1–C3: 179,5(2)?; N1–C3–C4: 179,7(4)?]. 2 is an AlN₂C₃ six‐membered heterocycle with two iminium fuctions. One N–H group is responsible for a intermolecular chain‐formation through hydrogen bridges to an adjacent nitrile group along the direction [010]. The basic structural motif of the heterocycle 3 has been maintained after silylation to 4 . In 6 · 0.5 toluene an unit Me₂GaCl, originated from 3 , is coordinated to the oxygen atom of the diphosphane oxide Ph₂P–P(O)Ph₂.     

The Hexacyanodiborane(6) Dianion [B2(CN)6]2−

Diborane(6) dianions with substituents that are bonded to boron via carbon are very reactive and therefore only a few examples are known. Diborane(6) derivatives are the simplest catenated boron compounds with an electron‐precise B–B σ‐bond that are of fundamental interest and of relevance for material applications. The homoleptic hexacyanodiborane(6) dianion [B₂(CN)₆]²⁻ that is chemically very robust is reported. The dianion is air‐stable and resistant against boiling water and anhydrous hydrogen fluoride. Its salts are thermally highly stable, for example, decomposition of (H₃O)₂[B₂(CN)₆] starts at 200 °C. The [B₂(CN)₆]²⁻ dianion is readily accessible starting from 1) B(CN)₃²⁻ and an oxidant, 2) [BF(CN)₃]⁻ and a reductant, or 3) by the reaction of B(CN)₃²⁻ with [BHal(CN)₃]⁻ (Hal=F, Br). The latter reaction was found to proceed via a triply negatively charged transition state according to an S_N2 mechanism.     

Site‐Selective C−S Bond Formation at C−Br over C−OTf and C−Cl Enabled by an Air‐Stable,Easily Recoverable,and Recyclable Palladium(I) Catalyst

This report widens the repertoire of emerging Pd^I catalysis to carbon–heteroatom, that is, C−S bond formation. While Pd⁰‐catalyzed protocols may suffer from the formation of poisonous sulfide‐bound off‐cycle intermediates and lack of selectivity, the mechanistically diverse Pd^I catalysis concept circumvents these challenges and allows for C−S bond formation (S–aryl and S–alkyl) of a wide range of aryl halides. Site‐selective thiolations of C−Br sites in the presence of C−Cl and C−OTf were achieved in a general and a priori predictable fashion. Computational, spectroscopic, X‐ray, and reactivity data support dinuclear Pd^I catalysis to be operative. Contrary to air‐sensitive Pd⁰, the active Pd^I species was easily recovered in the open atmosphere and subjected to multiple rounds of recycling.     

Vacancy‐Rich Ni(OH)2 Drives the Electrooxidation of Amino C−N Bonds to Nitrile C≡N Bonds

Electrochemical synthesis based on electrons as reagents provides a broad prospect for commodity chemical manufacturing. A direct one‐step route for the electrooxidation of amino C?N bonds to nitrile C≡N bonds offers an alternative pathway for nitrile production. However, this route has not been fully explored with respect to either the chemical bond reforming process or the performance optimization. Proposed here is a model of vacancy‐rich Ni(OH)₂ atomic layers for studying the performance relationship with respect to structure. Theoretical calculations show the vacancy‐induced local electropositive sites chemisorb the N atom with a lone pair of electrons and then attack the corresponding N(sp³)?H, thus accelerating amino C?N bond activation for dehydrogenation directly into the C≡N bond. Vacancy‐rich nanosheets exhibit up to 96.5 % propionitrile selectivity at a moderate potential of 1.38 V. These findings can lead to a new pathway for facilitating catalytic reactions in the chemicals industry.     

C−H Activation from Iron(II)‐Nitroxido Complexes

The reaction of nitroxyl radicals TEMPO (2,2′,6,6′‐tetramethylpiperidinyloxyl) and AZADO (2‐azaadamantane‐N‐oxyl) with an iron(I) synthon affords iron(II)‐nitroxido complexes (^ArL)Fe(κ¹‐TEMPO) and (^ArL)Fe(κ²‐N,O‐AZADO) (^ArL=1,9‐(2,4,6‐Ph₃C₆H₂)₂‐5‐mesityldipyrromethene). Both high‐spin iron(II)‐nitroxido species are stable in the absence of weak C−H bonds, but decay via N−O bond homolysis to ferrous or ferric iron hydroxides in the presence of 1,4‐cyclohexadiene. Whereas (^ArL)Fe(κ¹‐TEMPO) reacts to give a diferrous hydroxide [(^ArL)Fe]₂(μ‐OH)₂, the reaction of four‐coordinate (^ArL)Fe(κ²‐N,O‐AZADO) with hydrogen atom donors yields ferric hydroxide (^ArL)Fe(OH)(AZAD). Mechanistic experiments reveal saturation behavior in C−H substrate and are consistent with rate‐determining hydrogen atom transfer.     

An NHC–Silyl–NHC Pincer Ligand for the Oxidative Addition of C−H,N−H,and O−H Bonds to Cobalt(I) Complexes

N‐Heterocyclic carbene based pincer ligands bearing a central silyl donor, [CSiC]⁻, have been envisioned as a class of strongly σ‐donating ligands that can be used for synthesizing electron‐rich transition‐metal complexes for the activation of inert bonds. However, this type of pincer ligand and complexes thereof have remained elusive owing to their challenging synthesis. We herein describe the first synthesis of a CSiC pincer ligand scaffold through the coupling of a silyl–NHC chelate with a benzyl–NHC chelate induced by one‐electron oxidation in the coordination sphere of a cobalt complex. The monoanionic CSiC ligand stabilizes the Co^I dinitrogen complex [(CSiC)Co(N₂)] with an unusual coordination geometry and enables the challenging oxidative addition of E−H bonds (E=C, N, O) to Co^I to form Co^III complexes. The structure and reactivity of the cobalt(I) complex are ascribed to the unique electronic properties of the CSiC pincer ligand, which provides a strong trans effect and pronounced σ‐donation.     

An NHC–Silyl–NHC Pincer Ligand for the Oxidative Addition of C−H,N−H,and O−H Bonds to Cobalt(I) Complexes

N‐Heterocyclic carbene based pincer ligands bearing a central silyl donor, [CSiC]⁻, have been envisioned as a class of strongly σ‐donating ligands that can be used for synthesizing electron‐rich transition‐metal complexes for the activation of inert bonds. However, this type of pincer ligand and complexes thereof have remained elusive owing to their challenging synthesis. We herein describe the first synthesis of a CSiC pincer ligand scaffold through the coupling of a silyl–NHC chelate with a benzyl–NHC chelate induced by one‐electron oxidation in the coordination sphere of a cobalt complex. The monoanionic CSiC ligand stabilizes the Co^I dinitrogen complex [(CSiC)Co(N₂)] with an unusual coordination geometry and enables the challenging oxidative addition of E−H bonds (E=C, N, O) to Co^I to form Co^III complexes. The structure and reactivity of the cobalt(I) complex are ascribed to the unique electronic properties of the CSiC pincer ligand, which provides a strong trans effect and pronounced σ‐donation.     

C−H and C−N Activation at Redox‐Active Pyridine Complexes of Iron

Pyridine activation by inexpensive iron catalysts has great utility, but the steps through which iron species can break the strong (105–111 kcal mol⁻¹) C−H bonds of pyridine substrates are unknown. In this work, we report the rapid room‐temperature cleavage of C−H bonds in pyridine, 4‐tert‐butylpyridine, and 2‐phenylpyridine by an iron(I) species, to give well‐characterized iron(II) products. In addition, 4‐dimethylaminopyridine (DMAP) undergoes room‐temperature C−N bond cleavage, which forms a dimethylamidoiron(II) complex and a pyridyl‐bridged tetrairon(II) square. These facile bond‐cleaving reactions are proposed to occur through intermediates having a two‐electron reduced pyridine that bridges two iron centers. Thus, the redox non‐innocence of the pyridine can play a key role in enabling high regioselectivity for difficult reactions.     

Ligand‐Promoted meta‐C−H Functionalization of Benzylamines

Meta‐C−H functionalization of benzylamines has been developed using a Pd^II/transient mediator strategy. Using 2‐pyridone ligands and 2‐carbomethoxynorbornene (NBE‐CO₂Me) as the mediator, arylation, amination, and chlorination of benzylamines are realized. This protocol features a broad substrate scope and is compatible with heterocylic coupling partners. Moreover, the loading of the Pd can be lowered to 2.5 mol % by using the optimal ligand.     

Modular Functionalization of Arenes in a Triply Selective Sequence: Rapid C(sp2) and C(sp3) Coupling of C−Br,C−OTf,and C−Cl Bonds Enabled by a Single Palladium(I) Dimer

Full control over multiple competing coupling sites would enable straightforward access to densely functionalized compound libraries. Historically, the site selection in Pd⁰‐catalyzed functionalizations of poly(pseudo)halogenated arenes has been unpredictable, being dependent on the employed catalyst, the reaction conditions, and the substrate itself. Building on our previous report of C−Br‐selective functionalization in the presence of C−OTf and C−Cl bonds, we herein complete the sequence and demonstrate the first general arylations and alkylations of C−OTf bonds (in <10 min), followed by functionalization of the C−Cl site (in <25 min), at room temperature using the same air‐ and moisture‐stable Pd^I dimer. This allowed the realization of the first general and triply selective sequential C−C coupling (in 2D and 3D space) of C−Br followed by C−OTf and then C−Cl bonds.     

The [Ag{Se2C2(CN)2}(Se6)]3−, [Sb{Se2C2(CN)2}2]3−, and [As(Se)3(CH2CN)]2− Anions: Facile Formation of the Maleonitrilediselenolate (mns) Ligand, [Se2C2(CN)2]2−

An unusual route to the maleonitrilediselenolate (mns) ligand has been discovered with the isolation of compounds that contain this ligand bound to silver (structure shown on the right) or antimony. The formation of the [As(Se)₃(CH₂CN)]²⁻ anion along with possible pathways to the mns ligands is discussed.     

Copper‐Catalyzed Oxidative C−H Bond Functionalization of N‐Allylbenzamide for Regioselective C−N and C−O Bond Formation

Copper‐catalyzed oxidative couplings of N‐allylbenzamides for C?N and C?O bond formations have been developed through C?H bond functionalization. To demonstrate the utility of this approach, it was applied to the synthesis of β‐aminoimides and imides. To the best of our knowledge, these are the first examples in which different classes of N‐containing compounds have been directly prepared from the readily available N‐allylbenzamides using an inexpensive catalyst/oxidant/base (CuSO₄/TBHP/Cs₂CO₃) system.     

Diastereoselective C−H Bond Amination for Disubstituted Pyrrolidines

We report herein the improved diastereoselective synthesis of 2,5‐disubstituted pyrrolidines from aliphatic azides. Experimental and theoretical studies of the C−H amination reaction mediated by the iron dipyrrinato complex (^AdL)FeCl(OEt₂) provided a model for diastereoinduction and allowed for systematic variation of the catalyst to enhance selectivity. Among the iron alkoxide and aryloxide catalysts evaluated, the iron phenoxide complex exhibited superior performance towards the generation of syn 2,5‐disubstituted pyrrolidines with high diastereoselectivity.     

Catalytic Oxidative Trifluoromethoxylation of Allylic C−H Bonds Using a Palladium Catalyst

A catalytic intermolecular allylic C−H trifluoromethoxylation reaction of alkenes has been developed based on the use of a palladium catalyst, CsOCF₃ as the trifluoromethoxide source, and benzoquinone as the oxidant. This reaction provides an efficient route for directly accessing allylic trifluoromethoxy derivatives with excellent regioselectivities from terminal alkenes via an allylic C−H bond activation process.     

Stoichiometric and Catalytic C−C and C−H Bond Formation with B(C6F5)3 via Cationic Intermediates

This work showcases a new catalytic cyclization reaction using a highly Lewis acidic borane with concomitant C−H or C−C bond formation. The activation of alkyne‐containing substrates with B(C₆F₅)₃ enabled the first catalytic intramolecular cyclizations of carboxylic acid substrates using this Lewis acid. In addition, intramolecular cyclizations of esters enable C−C bond formation as catalytic B(C₆F₅)₃ can be used to effect formal 1,5‐alkyl migrations from the ester functional groups to unsaturated carbon–carbon frameworks. This metal‐free method was used for the catalytic formation of complex dihydropyrones and isocoumarins in very good yields under relatively mild conditions with excellent atom efficiency.