Mechanism of Nickel(II)‐Catalyzed Oxidative C(sp2)−H/C(sp3)−H Coupling of Benzamides and Toluene Derivatives

The Ni‐catalyzed C(sp²)?H/C(sp³)?H coupling of benzamides with toluene derivatives was recently successfully achieved with mild oxidant iC₃F₇I. Herein, we employ density functional theory (DFT) methods to resolve the mechanistic controversies. Two previously proposed mechanisms are excluded, and our proposed mechanism involving iodine‐atom transfer (IAT) between iC₃F₇I and the Ni^II intermediate was found to be more feasible. With this mechanism, the presence of a carbon radical is consistent with the experimental observation that (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl (TEMPO) completely quenches the reaction. Meanwhile, the hydrogen‐atom abstraction of toluene is irreversible and the activation of the C(sp²)?H bond of benzamides is reversible. Both of these conclusions are in good agreement with Chatani's deuterium‐labeling experiments.     

Cross-Coupling Reaction of Alkenyl Sulfoximines and Alkenyl Aminosulfoxonium Salts with Organozincs by Dual Nickel Catalysis and Lewis Acid Promotion

In this article, the cross-coupling reaction (CCR) of exocyclic, axially chiral, and acyclic alkenyl (N-methyl)sulfoximines with alkyl- and arylzincs is described_. The CCR generally requires dual Ni catalysis and MgBr₂ promotion, which is effective in diethyl ether but not in THF. NMR spectroscopy revealed a complexation of alkenyl sulfoximines by MgBr₂ in diethyl ether, which suggests an acceleration of the oxidative addition through nucleofugal activation. The CCR of alkenyl sulfoximines generally proceeds in the presence of Ni(dppp)Cl₂ as a precatalyst and MgBr₂ with alkyl- and arylzincs with a high degree of stereoretention at the C and the S atom. CCR of axially chiral alkenyl sulfoximines with Ni(PPh₃)₂Cl₂ as a precatalyst and ZnPh₂ does not require salt promotion and is stereoretentive. The reaction with Zn(CH₂SiMe₃)₂, however, demands salt promotion and is not stereoretentive. CCR of axially chiral α-methylated alkenyl sulfoximines afforded persubstituted axially chiral alkenes with high selectivity. Alkenyl (N-triflyl)sulfoximines engage in a stereoretentive CCR with Grignard reagents and Ni(PPh₃)₂Cl₂. Ni-Catalyzed and MgBr₂-promoted CCR of E-configured acyclic alkenyl sulfoximines and aminosulfoxonium salts with ZnPh₂ and Zn(CH₂SiMe₃)₂ is stereoretentive with Ni(dppp)Cl₂ and Ni(PPh₃)₂Cl₂. CCRs of acyclic alkenyl sulfoximines and alkenyl aminosulfoxonium salts, carrying a methyl group at the α position, take a different course and give alkenyl sulfinamides under stereoretention at the S and C atom. CCR of acyclic, exocyclic, and axially chiral alkenyl sulfoximines has been successfully applied to the stereoselective synthesis of homoallylic alcohols, exocyclic alkenes, and axially chiral alkenes, respectively.     

Nickel-Catalyzed Selective C(sp2)−F Bond Alkylation of Industrially Relevant Hydrofluoroolefin HFO-1234yf

The selective transition-metal catalyzed C−F bond functionalization of inexpensive industrial fluorochemicals represents one of the most attractive approaches to valuable fluorinated compounds. However, the selective C(sp²)−F bond carbofunctionalization of refrigerant hydrofluoroolefins (HFOs) remains challenging. Here, we report a nickel-catalyzed selective C(sp²)−F bond alkylation of HFO-1234yf with alkylzinc reagents. The resulting 2-trifluoromethylalkenes can serve as a versatile synthon for diversified transformations, including the anti-Markovnikov type hydroalkylation and the synthesis of bioactive molecule analogues. Mechanistic studies reveal that lithium salt is essential to promote the oxidative addition of Ni⁰(L_n) to the C−F bond; the less electron-rich N-based ligands, such as bipyridine and pyridine-oxazoline, feature comparable or even higher oxidative addition rates than the electron-rich phosphine ligands; the strong σ-donating phosphine ligands, such as PMe₃, are detrimental to transmetallation, but the less electron-rich and bulky N-based ligands, such as pyridine-oxazoline, facilitate transmetallation and reductive elimination to form the final product.     

Mechanistic Insights into the Ni‐Catalyzed Reductive Carboxylation of C−O Bonds in Aromatic Esters with CO2: Understanding Remarkable Ligand and Traceless‐Directing‐Group Effects

The mechanism of the Ni⁰‐catalyzed reductive carboxylation reaction of C(sp²)?O and C(sp³)?O bonds in aromatic esters with CO₂ to access valuable carboxylic acids was comprehensively studied by using DFT calculations. Computational results revealed that this transformation was composed of several key steps: C?O bond cleavage, reductive elimination, and/or CO₂ insertion. Of these steps, C?O bond cleavage was found to be rate‐determining, and it occurred through either oxidative addition to form a Ni^II intermediate, or a radical pathway that involved a bimetallic species to generate two Ni^I species through homolytic dissociation of the C?O bond. DFT calculations revealed that the oxidative addition step was preferred in the reductive carboxylation reactions of C(sp²)?O and C(sp³)?O bonds in substrates with extended π systems. In contrast, oxidative addition was highly disfavored when traceless directing groups were involved in the reductive coupling of substrates without extended π systems. In such cases, the presence of traceless directing groups allowed for docking of a second Ni⁰ catalyst, and the reactions proceed through a bimetallic radical pathway, rather than through concerted oxidative addition, to afford two Ni^I species both kinetically and thermodynamically. These theoretical mechanistic insights into the reductive carboxylation reactions of C?O bonds were also employed to investigate several experimentally observed phenomena, including ligand‐dependent reactivity and site‐selectivity.     

Nickel‐Catalyzed Methylation of Aryl Halides with Deuterated Methyl Iodide

A nickel‐catalyzed methylation of aryl halides with cheap and readily available CH₃I or CD₃I is described. The reaction is applicable to a wide range of substrates and allows installation of a CD₃ group under mild reaction conditions without deuterium scrambling to other carbon atoms. Initial mechanistic studies on the stoichiometric and catalytic reactions of the isolated [(dppp)Ni(C₆H₄‐4‐CO₂Et)Br] [dppp=1,3‐bis(diphenylphosphanyl)propane] suggest that a Ni⁰/Ni^II catalytic cycle is favored.     

trans‐Bis(thioacetato‐S)bis(triphenylphosphine‐P)nickel(II)

In the title compound, [Ni(C₂H₃OS)₂(C₁₈H₁₅P)₂], the Ni atom lies on an inversion centre and the tri­phenyl phosphine and thio­acetate ligands are bonded to the central Ni^II atom in a trans fashion, with Ni—S = 2.2020 (8) and Ni—P = 2.2528 (8) Å, and angle S—Ni—P = 92.47 (3)°.     

Ammonia Activation by a Nickel NCN‐Pincer Complex featuring a Non‐Innocent N‐Heterocyclic Carbene: Ammine and Amido Complexes in Equilibrium

A Ni⁰‐NCN pincer complex featuring a six‐membered N‐heterocyclic carbene (NHC) central platform and amidine pendant arms was synthesized by deprotonation of its Ni^II precursor. It retained chloride in the square‐planar coordination sphere of nickel and was expected to be highly susceptible to oxidative addition reactions. The Ni⁰ complex rapidly activated ammonia at room temperature, in a ligand‐assisted process where the carbene carbon atom played the unprecedented role of proton acceptor. For the first time, the coordinated (ammine) and activated (amido) species were observed together in solution, in a solvent‐dependent equilibrium. A structural analysis of the Ni complexes provided insight into the highly unusual, non‐innocent behavior of the NHC ligand.     

State of Supported Nickel Nanoparticles during Catalysis in Aqueous Media

The state of Ni supported on HZSM‐5 zeolite, silica, and sulfonated carbon was studied during aqueous‐phase catalysis of phenol hydrodeoxygenation using in situ extended X‐ray absorption fine structure spectroscopy. On sulfonated carbon and HZSM‐5 supports, NiO and Ni(OH)₂ were readily reduced to Ni⁰ under reaction conditions (≈35 bar H₂ in aqueous phenol solutions containing up to 0.5 wt. % phosphoric acid at 473 K). In contrast, Ni supported on SiO₂ was not stable in a fully reduced Ni⁰ state. Water enables the formation of Ni^II phyllosilicate, which is more stable, that is, difficult to reduce, than either α‐Ni(OH)₂ or NiO. Leaching of Ni from the supports was not observed over a broad range of reaction conditions. Ni⁰ particles on HZSM‐5 were stable even in presence of 15 wt. % acetic acid at 473 K and 35 bar H₂.     

镍催化芳烃卤化物还原性交叉偶联的反应机理(英文)

利用密度泛函理论(DFT)计算研究了镍催化的溴苯(R1)与对溴苯甲酸甲酯(R2)生成非对称联芳化合物的还原性交叉偶联反应的机理.结果表明,相对于一价镍的催化机理,零价镍作为活性催化剂更有利于反应的进行,零价镍催化剂首先与R1络合或者首先与R2络合的反应机理十分相似,其决速步在气相时的能垒分别是70.50或者49.66 kJ?mol-1.零价镍催化机理包括如下步骤:一次氧化加成;还原;二次氧化加成;还原消除以及催化剂再生.另外,计算结果还表明有机锌试剂在这个反应体系中很难生成.     

Preparation and crystal structure of the Isotypic Carbides Ln4Ni2C5 (Ln = Er,Tm, Yb and Lu)

The title compounds were prepared from the elemental components in a lithium flux. Their crystal structure was determined for the ytterbium compound from single-crystal X-ray data. It is orthorhombic, Pmm2, a = 352.88(6) pm, b = 1 143.0(3) pm, c = 366.16(6) pm, Z = 1, R = 0.020 for 1 261 structure factors and 29 variable parameters. The structure may be viewed as an intergrowth of slabs consisting of the CeNiC₂ and the ScC (NaCl type) structures. It thus contains C₂ pairs with a C? C distance of 138(1) pm and isolated carbon atoms. Together with the nickel atoms the C₂ pairs form one-dimensionally infinite building elements [Ni₂C₄]_n. The fifth carbon atom is octahedrally coordinated by ytterbium atoms. Accordingly the compound may be rationalized to a first approximation with the formula (Yb³⁺)₄[Ni₂C₄^8?]C^4?. Yb₄Ni₂C₅ shows Curie-Weiss behaviour with a magnetic moment of μ_exp = 4.44 μ_B per ytterbium atom in good agreement with the theoretical moment of μ_eff = 4.53 μ_B for Yb³⁺.     

Impact of Oxidation State on Reactivity and Selectivity Differences between Nickel(III) and Nickel(IV) Alkyl Complexes

Described is a systematic comparison of factors impacting the relative rates and selectivities of C(sp³)?C and C(sp³)?O bond‐forming reactions at high‐valent Ni as a function of oxidation state. Two Ni complexes are compared: a cationic octahedral Ni^IV complex ligated by tris(pyrazolyl)borate and a cationic octahedral Ni^III complex ligated by tris(pyrazolyl)methane. Key features of reactivity/selectivity are revealed: 1) C(sp³)?C(sp²) bond‐forming reductive elimination occurs from both centers, but the Ni^III complex reacts up to 300‐fold faster than the Ni^IV, depending on the reaction conditions. The relative reactivity is proposed to derive from ligand dissociation kinetics, which vary as a function of oxidation state and the presence/absence of visible light. 2) Upon the addition of acetate (AcO^?), the Ni^IV complex exclusively undergoes C(sp³)?OAc bond formation, while the Ni^III analogue forms the C(sp³)?C(sp²) coupled product selectively. This difference is rationalized based on the electrophilicity of the respective M?C(sp³) bonds, and thus their relative reactivity towards outer‐sphere S_N2‐type bond‐forming reactions.     

Comparative Study of Hypophosphite H2PO2^- Adsorption on Ni（111） and Ag（111） Surfaces by DFT

Surface structures and electronic properties of hypophosphite H2PO2^- on Ni（111） and Ag（111） surfaces were investigated by means of density functional theory at B3LYP/6-31 ＋ ＋G（d,p） level. The most stable structure was that in which the H2PO2^- adsorbs with its two P--O bonds faced to the substrate surface. The results of the Mulliken population analysis showed that because of the subtle difference of electron configuration, the adsorption energy was larger on the Ni surface than on the Ag surface, and the amounts of both donation and back donation were larger on the Ni（111） surface than on the Ag（111） surface. There were more negative Mulliken charge transfer from H2PO2^- to substrate clusters on Ni surface than on Ag surface and more positive Mulliken charges on P atom in Ni4H2PO2^- than in Ag4H2PO2^-, which means that P atom in Ni4H2PO2^- is easily attacked by a nucleophile such as OH . Thus, H2PO2^- is more easily oxidated on Ni（111） surface than on Ag（111） suface. These results indicated that the silver surface is inactive for the oxidation reaction of the hypophosphite anion.     

Pd(OAc)2-catalyzed domino reactions of 1,2-dihaloarenes and 2-haloaryl arenesulfonates with Grignard reagents: efficient synthesis of substituted fluorenes

Pd(OAc)₂-catalyzed domino reactions of 1,2-dihalobenzenes and 2-haloaryl arenesulfonates with hindered Grignard reagents to form substituted fluorenes, which are believed to occur through palladium associated aryne intermediates, are described. Such palladium associated aryne reaction pathway was found to be favored by omitting the use of phosphine and N-heterocyclic carbene ligands for palladium catalysts and with better leaving groups. Our study suggested that Pd(leaving group)X associated arynes should be formed first and the sp³ C-H activation preferentially occurred at benzylic C-(1°)H bonds. The work described here provides a high yield, one-step access to substituted fluorenes from readily available 1,2-dihalobenzenes and 2-haloaryl arenesulfonates and hindered Grignard reagents, and this substituted fluorene-making method may find applications in the preparation of substituted fluorene-containing molecules including polymers.     

Synthesis of Ni–Ru Alloy Nanoparticles and Their High Catalytic Activity in Dehydrogenation of Ammonia Borane

We report the synthesis and characterization of new Ni_xRu_1?x (x=0.56–0.74) alloy nanoparticles (NPs) and their catalytic activity for hydrogen release in the ammonia borane hydrolysis process. The alloy NPs were obtained by wet‐chemistry method using a rapid lithium triethylborohydride reduction of Ni²⁺ and Ru³⁺ precursors in oleylamine. The nature of each alloy sample was fully characterized by TEM, XRD, energy dispersive X‐ray spectroscopy (EDX), and X‐ray photoelectron spectroscopy (XPS). We found that the as‐prepared Ni–Ru alloy NPs exhibited exceptional catalytic activity for the ammonia borane hydrolysis reaction for hydrogen release. All Ni–Ru alloy NPs, and in particular the Ni_0.74Ru_0.26 sample, outperform the activity of similar size monometallic Ni and Ru NPs, and even of Ni@Ru core‐shell NPs. The hydrolysis activation energy for the Ni_0.74Ru_0.26 alloy catalyst was measured to be approximately 37 kJ mol^?1. This value is considerably lower than the values measured for monometallic Ni (≈70 kJ mol^?1) and Ru NPs (≈49 kJ mol^?1), and for Ni@Ru (≈44 kJ mol^?1), and is also lower than the values of most noble‐metal‐containing bimetallic NPs reported in the literature. Thus, a remarkable improvement of catalytic activity of Ru in the dehydrogenation of ammonia borane was obtained by alloying Ru with a Ni, which is a relatively cheap metal.     

Palladium- and Ruthenium-Catalyzed Intramolecular Carbene CAr−H Functionalization of γ-Amino-α-diazoesters for the Synthesis of Tetrahydroquinolines

A synthetic method to prepare tetrahydroquinoline-4-carboxylic acid esters has been developed through the transition-metal-catalyzed intramolecular aromatic C−H functionalization of α-diazoesters. Both [{Pd(IMes)(NQ)}₂] (IMes=1,3-dimesitylimidazol-2-ylidene, NQ=1,4-naphthoquinone) and the first-generation Grubbs catalyst proved effective for this purpose. The ruthenium catalyst was found to be the most versatile, although in a few cases the palladium complex afforded better yields or selectivities. According to DFT calculations, Pd⁰- and Ru^II-catalyzed sp²-C_Ar−H functionalization proceeds through different reaction mechanisms. Thus, the Pd⁰-catalyzed reaction involves a Pd-mediated 1,6-H migration from the sp²-C_Ar−H bond to the carbene carbon atom, followed by a reductive elimination process. In contrast, electrophilic addition of the ruthenacarbene intermediate to the aromatic ring and subsequent 1,2-proton migration are operative in the Grubbs catalyst promoted reaction.     

Chiral Recognition of Racemic Proline based on Chiral Macrocyclic Nickel(II) Complex: Synthesis and Crystal Structures

The reactions of dl‐proline with chiral macrocyclic Ni^II complex [Ni(SS‐L)](ClO₄)₂ in acetonitrile/water gave a six‐coordinate enantiomer formulated as [Ni(SS‐L)(l‐Pro)](ClO₄)₂ · H₂O ( 1 ). Another enantiomer of [Ni(RR‐L)(d‐Pro)](ClO₄)₂ · H₂O ( 2 ) was obtained when [Ni(RR‐L)](ClO₄)₂ was used (L = 5,5,7,12,12,14‐hexamethyl‐1,4,8,11‐tetraazacyclotetradecane, Pro = proline). Single‐crystal X‐ray diffraction analyses of complexes 1 and 2 revealed that the Ni^II atom has a distorted octahedral coordination arrangement, being coordinated by four nitrogen atoms of L in a folded configuration, plus two carboxylate oxygen atoms of proline in mutually cis positions. Complexes 1 and 2 are supramolecular stereoisomers, which are constructed with hydrogen bonding linking of [Ni(SS‐L)(l‐Pro)]²⁺ and [Ni(RR‐L)(d‐Pro)]²⁺ monomers to form one‐dimensional zigzag chains. The homochiral natures of complexes 1 and 2 were confirmed by solid CD spectroscopy.     

Ni Oxidation State and Ligand Saturation Impact on the Capability of Octaazamacrocyclic Complexes to Bind and Reduce CO2

Two 15-membered octaazamacrocyclic nickel(II) complexes are investigated by theoretical methods to shed light on their affinity forwards binding and reducing CO₂. In the first complex ¹[Ni^IIL]⁰, the octaazamacrocyclic ligand is grossly unsaturated (π-conjugated), while in the second ¹[Ni^IILH]²⁺ one, the macrocycle is saturated with hydrogens. One and two-electron reductions are described using Mulliken population analysis, quantum theory of atoms in molecules, localized orbitals, and domain averaged fermi holes, including the characterization of the Ni-C_CO2 bond and the oxidation state of the central Ni atom. It was found that in the [NiLH] complex, the central atom is reduced to Ni⁰ and/or Ni^I and is thus able to bind CO₂ via a single σ bond. In addition, the two-electron reduced ³[NiL]²⁻ species also shows an affinity forwards CO₂.     

Correlated Coordination and Redox Activity of a Hemilabile Noninnocent Ligand in Nickel Complexes

The compound [Ni(Q_M)₂], Q_M=4,6‐di‐tert‐butyl‐N‐(2‐methylthiomethylphenyl)‐o‐iminobenzoquinone, is a singlet diradical species with approximately planar configuration at the tetracoordinate metal atom and without any Ni?S bonding interaction. One‐electron oxidation results in additional twofold Ni?S coordination (d_Ni?S≈2.38 Å) to produce a complex cation of [Ni(Q_M)₂](PF₆) with hexacoordinate Ni^II and two distinctly different mer‐configurated tridentate ligands. The O,O′‐trans arrangement in the neutral precursor is changed to an O,O′‐cis configuration in the cation. The EPR signal of [Ni(Q_M)₂](PF₆) has a very large g anisotropy and the magnetic measurements indicate an S=3/2 state. The dication was structurally characterized as [Ni(Q_M)₂](ClO₄)₂ to exhibit a similar NiN₂O₂S₂ framework as the monocation. However, the two tridentate (O,N,S) ligands are now equivalent according to the formulation [Ni^II(Q_M⁰)₂]²⁺. Cyclic voltammetry reflects the qualitative structure change on the first, but not on the second oxidation of [Ni(Q_M)₂], and spectroelectrochemistry reveals a pronounced dependence of the 800–900 nm absorption on the solvent and counterion. Reduction of the neutral form occurs in an electrochemically reversible step to yield an anion with an intense near‐infrared absorption at 1345 nm (ε=10400 M ^?1 cm^?1) and a conventional g factor splitting for a largely metal‐based spin (S=1/2), suggesting a [(Q_M ^{. ?})Ni^II(Q_M^2?)]^? configuration with a tetracoordinate metal atom with antiferromagnetic Ni^II–(Q_M ^{. ?}) interactions and symmetry‐allowed ligand‐to‐ligand intervalence charge transfer (LLIVCT). Calculations are used to understand the Ni?S binding activity as induced by remote electron transfer at the iminobenzoquinone redox system.     

A comparative study of the packing of two polymorphs of the nickel(II) pincer complex [2,6‐bis(di‐tert‐butylphosphinoyl)‐4‐(3,5‐dinitrobenzoyloxy)phenyl‐κ3P,C1,P′]chloridonickel(II)

Pincer complexes can act as catalysts in organic transformations and have potential applications in materials, medicine and biology. They exhibit robust structures and high thermal stability attributed to the tridentate coordination of the pincer ligands and the strong σ metal–carbon bond. Nickel derivatives of these ligands have shown high catalytic activities in cross‐coupling reactions and other industrially relevant transformations. This work reports the crystal structures of two polymorphs of the title Ni^II POCOP pincer complex, [Ni(C₂₉H₄₁N₂O₈P₂)Cl] or [NiCl{C₆H₂‐4‐[OCOC₆H₄‐3,5‐(NO₂)₂]‐2,6‐(OP^tBu₂)₂}]. Both pincer structures exhibit the Ni^II atom in a distorted square‐planar coordination geometry with the POCOP pincer ligand coordinated in a typical tridentate manner via the two P atoms and one arene C atom via a C—Ni σ bond, giving rise to two five‐membered chelate rings. The coordination sphere of the Ni^II centre is completed by a chloride ligand. The asymmetric units of both polymorphs consist of one molecule of the pincer complex. In the first polymorph, the arene rings are nearly coplanar, with a dihedral angle between the mean planes of 27.9 (1)°, while in the second polymorph, this angle is 82.64 (1)°, which shows that the arene rings are almost perpendicular to one another. The supramolecular structure is directed by the presence of weak C—H…O=X (X = C or N) interactions, forming two‐ and three‐dimensional chain arrangements.     

Doppelkomplexsalze von Co(II), Ni(II) und Cu(II) mit Thioharnstoffliganden im Kationkomplex und Thiocyanatliganden im anionkomplex

Three new double complex compounds of the following type were obtained: [CoThio ₄][Co(SCN)₄], [NiThio ₄][Ni(SCN)₄] and (CuThio ₄) (CuCo(SCN)₄). The melting points of the compounds were determined, and the molecular weight of the first. The IR-spectra were studied and the metal-ligand bond interpreted. It was shown that the metal-thiourea bond in all compounds is formed via the sulphur atom. In the complex anion of the first and second compounds Co(II) and Ni(II) are coordinated with SCN^– through the nitrogen atom. In the third, more complicated compound, Cu(II) is coordinated to SCN^– through the sulphur atom, and Co(II) through the nitrogen atom, a bridging bond being formed.