Air-stable Nickel Research Articles

Nickel(II)/BINOL-catalyzed enantioselective C–H activation via desymmetrization and kinetic resolution

The field of nickel catalysis has witnessed remarkable growth in recent years. However, the use of nickel catalysts in enantioselective C–H activation remains a daunting challenge because of their variable oxidation states, intricate coordination chemistry, and unpredictable reactivity patterns. Herein, we report an enantioselective C–H activation reaction catalyzed by commercially available and air-stable nickel(II) catalyst. Readily available and simple (S)-BINOL is used as a chiral ligand. This operationally simple protocol enables the synthesis of planar chiral metallocenes in high yields with excellent enantioselectivity through desymmetrization and kinetic resolution. Air-stable planar chiral nickelacycle intermediates are first synthesized via enantioselective C–H nickelation and shown to be possible intermediates of the reaction. Deuterium-labeling studies, alongside the characterization and transformation of chiral nickel(II) species, suggest that C–H cleavage is the enantio-determining step. Moreover, the large-scale synthesis and diverse synthetic transformations underscore the practicality of this protocol.

A general and robust Ni-based nanocatalyst for selective hydrogenation reactions at low temperature and pressure.

Catalytic hydrogenations are important and widely applied processes for the reduction of organic compounds both in academic laboratories and in industry. To perform these reactions in sustainable and practical manner, the development and applicability of non-noble metal-based heterogeneous catalysts is crucial. Here, we report highly active and air-stable nickel nanoparticles supported on mesoporous silica (MCM-41) as a general and selective hydrogenation catalyst. This catalytic system allows for the hydrogenation of carbonyl compounds, nitroarenes, N-heterocycles, and unsaturated carbon─carbon bonds in good to excellent selectivity under very mild conditions (room temperature to 80°C, 2 to 10 bar H2). Furthermore, the optimal nickel/meso-silicon dioxide catalyst is reusable (4 cycles) without loss of its catalytic activity.

Nickel-Catalyzed Enantioselective Hydrothiocarbonylation of Cyclopropenes.

Hydrothiocarbonylation of olefins using carbon monoxide and thiols is a powerful method to synthesize thioesters from simple building blocks. Owing to the intrinsic challenges of catalyst poisoning, transition-metal-catalyzed asymmetric thiocarbonylation, particularly when utilizing earth abundant metals, remains rare in the literature. Herein, we report a nickel-catalyzed enantioselective hydrothiocarbonylation of cyclopropenes for the synthesis of a diverse collection of functionalized thioesters in good to excellent yields with high stereoselectivity. This new method employs an inexpensive, air-stable nickel(II) precursor, which provides enhanced catalyst fidelity against CO poisoning compared to nickel(0) catalysts.

High-Activity and High-Selectivity Air-Stable Nickel and Copper Complexes for Copolymerization of Epoxides with Anhydrides.

Ring-opening copolymerization (ROCOP) of epoxides with anhydrides presents a highly promising pathway for the synthesis of diverse polyester copolymers. This process involves the simultaneous opening of the epoxide ring and the anhydride ring, resulting in the formation of copolymer chains with alternating repeating units. Here, we synthesized a series of nickel and copper complexes coordinated by amine-bis(benzotriazole phenolate) ligands (C1NNBiBTP-H2). Different acid-base conditions can result in the synthesis of diverse coordination structures. The six-coordinated mononuclear Ni catalyst synthesized in this study exhibited competitive turnover frequencies (TOF = 450 h-1) compared to previous studies involving copolymerization of epoxides with anhydrides. This indicates the catalyst's high activity in facilitating the copolymerization reaction. The results indicate that the catalyst exhibited a high molecular weight control ability and high polymer selectivity for the ROCOP of epoxides and anhydrides. Thus, this air-stable catalyst has the potential for use in industries.

Nickel Phosphite-Catalyzed Tetradehydro-Diels-Alder Reactions of (E)-3-ene-1,8-diynes.

A nickel-catalyzed tetradehydro-Diels-Alder reaction of (E)-3-ene-1,8-diynes for the preparation of isoindolines, dihydroisobenzofurans, and tetrahydroisoquinolines has been developed. A series of air-stable nickel catalysts were used in this study, including the novel nickel(0)-phosphite catalysts, Ni[P(O-3,5-Me-Ph)3]4, Ni[P(O-1-naphthyl)3]4, and Ni[P(O-2-naphthyl)3]4. To help understand the type of intermediate in the initial cycloisomerization process, the trapping of nickellacycle intermediates with pinacolborane to yield vinyl boronates is also discussed.

Nickel Catalyzed Cross-Coupling of Aryl and Alkenyl Triflates with Alkyl Thiols

We report a nickel catalyzed C-S cross-coupling of aryl and alkenyl triflates with alkyl thiols. A variety of the corresponding thioethers were synthesized using an air-stable nickel precatalyst under mild reaction conditions with short reaction times. A broad substrate scope, including pharmaceutically relevant compounds, could be demonstrated.

Nickel-catalyzed cooperative B-H bond activation for hydroboration of N‑heteroarenes, ketones and imines

We report two air-stable nickel(II) half-sandwich complexes, Cp*Ni(1,2-Cy2PC6H4O) (1) and Cp*Ni(1,2-Ph2PC6H4NH) (2), for cooperative B-H bond activation and their applications in catalytic hydroboration of unsaturated organic compounds. Both 1 and 2 react with HBpin by adding the B-H bond across the Ni−X bond (X = O or N), giving rise to the 18-electron Ni(II)−H active species, [H1(Bpin)] and [H2(Bpin)]. Subtle tuning of the Ni−X pair and the supporting ancillary phosphine have a significant effect on the reactivity and catalytic performance of Cp*Ni(1,2-R2PC6H4X). Unlike [H2(Bpin)], the activation of HBpin in [H1(Bpin)] is reversible, which enables the Ni−O complex to be an effective cooperative catalyst in the hydroboration of N-heteroarenes, and as well as ketones and imines.

Nickel-Catalyzed Reductive Borylation of Enaminones via C(sp2)-N Bond Cleavage.

The cleavage and transformation of alkenyl C(sp2)-N bonds is a significant synthetic challenge. Herein we described an unprecedented nickel-catalyzed reductive borylation of enaminones to synthesize β-ketone boronic esters. Notably, B2pin2 played the dual role in this process, and water served as a hydrogen source, which was transferred to target products. The air-stable nickel catalyst was applied to the cleavage of alkenyl C(sp2)-N bonds, concomitant with the reductive process of the alkenyl boronic ester intermediates, on the basis of the mechanism study.

Nickel(II) phosphine-catalysed hydrodehalogenation of aryl halides under mild ambient conditions

A convenient and low-cost hydrodehalogenation process catalysed by air-stable nickel(II) complexes containing bidentate phosphine ligands has been developed under ambient conditions with sodium borohydride acting as the reducing agent and hydrogen atom source. High conversions of aryl halides, including polyfluorinated substrates, into their hydrogenated counterparts have been achieved in dimethylformamide (DMF) and to a moderate extent, in DMF/isopropanol (10:90% v/v) solvent mixture. A mechanism has been proposed to account for the experimental data.

Development of an Operationally Simple, Scalable, and HCN-Free Transfer Hydrocyanation Protocol Using an Air-Stable Nickel Precatalyst

Hydrocyanation reactions enable access to synthetically valuable nitriles from readily available alkene precursors. However, hydrocyanation reactions using hydrogen cyanide (HCN) or similarly toxic reagents on laboratory scale can be particularly challenging due to their hazardous nature. In addition, such processes typically require air- and temperature-sensitive Ni(0) precatalysts, further reducing the operational simplicity of this transformation. Herein, we report a HCN-free transfer hydrocyanation of alkenes and alkynes that employs commercially available aliphatic nitriles as sacrificial HCN donors in combination with a catalytic amount of air-stable and inexpensive NiCl2 as a precatalyst and a cocatalytic Lewis acid. The scalability and robustness of the catalytic process were demonstrated by the hydrocyanation of α-methylstyrene on a 100 mmol scale (11.4 g of product obtained) using 1 mol % of the Ni catalyst. In addition, the feasibility of the dehydrocyanation protocol using the air-stable Ni(II) precatalyst and norbornadiene as a sacrificial acceptor was showcased by the selective conversion of an aliphatic nitrile into the corresponding alkene.

Electrocatalytic H2 Generation from Water Relying on Cooperative Ligand Electron Transfer in "PN3 P" Pincer-Supported NiII Complexes.

Water is the most sustainable source for H2 production, and the efficient electrocatalytic production of H2 from mixed water/acetonitrile solutions by using two new air-stable nickel(II) pincer complexes, [Ni(κ3 -2,6-{Ph2 PNR}2 (NC5 H3 )Br2 ] (R=H I, Me II) is reported. Hydrogen generation from H2 O/CH3 CN solutions is initiated at -2 V against Fc+/0 , and bulk electrocatalysis studies showed that the catalyst functions with an excellent Faradaic efficiency and a turnover frequency of 160 s-1 . A DFT computational investigation of the reduction behavior of I and II revealed a correlation of H2 formation with charge donation from electrons originating in a reduced ligand-localized orbital. As a result, these catalysts are proposed to proceed by a novel mechanism involving electron/proton transfer between a Ni0I species bonded to an anionic PN3 P ligand ("L- /Ni0I ") and a NiI -hydride ("Ni-H"). Furthermore, these catalysts are able to reduce phenol and acetic acid, more active proton sources, at lower potentials that correlate with the substrate pKa .

Air-stable nickel(II) borohydrides as prohydrides: Reactions with halocarbons and aerial carbon dioxide

Non-noble metal hydrides are highly attractive reagents owing to their involvement in the reactions of environmental and industrial interest. However, they are often cumbersome to isolate as a shelf-stable reagent. Herein, we have described the synthesis of non-noble metal hydrides by simple manual grounding of Ni(II)-chelates and NaBH4 in open air at room temperature without using organic solvents. Two nickel(II) complexes of tetradentate N4 ligands, [(LNHCy)Ni(CH3CN)(H2O)](ClO4)2 and [(tren)Ni(CH3CN)2](BPh4)2, were synthesized and their solid-state structures were confirmed by single crystal X-ray diffraction studies. Both the nickel complexes were manually ground with sodium borohydride in open air to produce corresponding nickel(II)-borohydrides that were characterized using FT-IR spectroscopy and powder X-ray diffraction techniques. Further, the borohydrides (prohydrides) activate the C-X bond in benzyl bromide, CDCl3, trityl chloride and iodobenzene to yield toluene, CHDCl2, triphenylmethane and benzene; the borohydrides reacted with trityl cation and air in acetonitrile solution to yield triphenylmethane and carbonate.

Well-Defined, Versatile and Recyclable Half-Sandwich Nickelacarborane Catalyst for Selective Carbene-Transfer Reactions.

Catalytic carbene-transfer reactions constitute a class of highly useful transformations in organic synthesis. Although catalysts based on a range of transition-metals have been reported, the readily accessible nickel(II)-based complexes have been rarely used. Herein, an air-stable nickel(II)-carborane complex is reported as a well-defined, versatile and recyclable catalyst for selective carbene transfer reactions with low catalyst loading under mild conditions. This catalyst is effective for several types of reactions including diastereoselective cyclopropanation, epoxidation, selective X-H insertions (X = C, N, O, S, Si), particularly for the unprotected substrates. This represents a rare example of carborane ligands in base metal catalysis.

Nickel don't care about no air.

Air-stable nickel precatalysts enable one to rapidly screen ligands, organic substrates and conditions in an accessible and scalable fashion. The best method can be identified and translated to an industrial setting.

Nickel catalysed construction of benzazoles via hydrogen atom transfer reactions

An air-stable nickel catalyst that promotes two consecutive HAT-mediated dehydrogenations towards furnishing benzazoles.

Scope and Mechanistic Aspect of Nickel-Catalyzed Alkenylation of Benzothiazoles and Related Azoles with Styryl Bromides

Alkenylation of benzothiazoles and related azoles with alkenyl bromides is achieved employing the well-defined and air-stable nickel complexes, (bpy)NiBr2 and [Ni(bpy)3][NiBr4], as catalysts. Numerous electronically distinct alkenyl bromides efficiently coupled with substituted benzothiazoles, oxazoles, and benzimidazoles under the catalytic conditions to afford 2-alkenylated azoles. An extensive mechanistic study of the alkenylation of benzothiazole using (bpy)NiBr2 highlights a single-electron transfer process for the reaction involving the two-step one-electron oxidative addition of alkenyl bromide. The substrate benzothiazole plays a significant and diverse role in the reaction, and C–H bond cleavage is reversible in nature. Detailed kinetic analysis and control reactivity studies are indicative of a Ni(I)/Ni(III) pathway for the alkenylation comprising the rate-influencing reductive elimination step.

Dehydrogenative Coupling of Aldehydes with Alcohols Catalyzed by a Nickel Hydride Complex

A nickel hydride complex, {2,6-(iPr2PO)2C6H3}NiH, has been shown to catalyze the coupling of RCHO and R′OH to yield RCO2R′ and RCH2OH, where the aldehyde also acts as a hydrogen acceptor and the alcohol also serves as the solvent. Functional groups tolerated by this catalytic system include CF3, NO2, Cl, Br, NHCOMe, and NMe2, whereas phenol-containing compounds are not viable substrates or solvents. The dehydrogenative coupling reaction can alternatively be catalyzed by an air-stable nickel chloride complex, {2,6-(iPr2PO)2C6H3}NiCl, in conjunction with NaOMe. Acids in unpurified aldehydes react with the hydride to form nickel carboxylate complexes, which are catalytically inactive. Water, if present in a significant quantity, decreases the catalytic efficiency by forming {2,6-(iPr2PO)2C6H3}NiOH, which causes catalyst degradation. On the other hand, in the presence of a drying agent, {2,6-(iPr2PO)2C6H3}NiOH generated in situ from {2,6-(iPr2PO)2C6H3}NiCl and NaOH can be converted to an alkoxide species, bec...

Competitive interplay of deposition and etching processes in atomic layer growth of cobalt and nickel metal films

Abstract

Electrocatalytic proton reduction by an air-stable nickel(ii)-thiolato PNN pincer complex.

We report an air-stable nickel(ii)-thiolato PNN pincer complex [(PNN)NiII(SC6H4Me)]+[MeC6H4SO3]- (PNN = 2-((di-tert-butylphosphinomethyl-6-diethylaminomethyl)pyridine)) which is capable of reducing protons at an overpotential of 0.54 V at low acid concentrations. The proton reduction can be catalysed using weak or strong acids such as acetic acid and trifluoroacetic acid respectively. In contrast, the chloro and nitrate derivatives of the nickel pincer complex behave as poorer catalysts. A mechanism accounting for the role of the ligand in proton reduction is also briefly outlined.

Synthesis of nickel nanopowders in water/ethylene glycol solutions. The influence of the solution composition on the particles’ size

Air stable nickel nanopowders have been synthesized via the reduction of as-prepared Ni(OH) 2 by hydrazine in water/ethylene glycol solutions under heterogeneous nucleation conditions at70 °C. The resulting product is a powder of black color; yield of product in all cases exceeded 95%. The influence of the water/ethylene glycol ratio on the reaction proceeding and the size and size distribution of obtained nickel nanoparticles have been investigated. Using the method of electron microscopy it was found that the obtained nickel particles are mainly globular. It was established that the mean diameter and polydispersity of the particles practically do not change in the range of ethylene glycol concentrations of 33–89 % by volume and are equal to 140±50 nm and increase drammatically when the concentration of ethylene glycol is reduced by less than 10 % by volume and the obtained particles are agglomerates of irregular shape with an average diameter of 500 nm and high polydispersity. Using the powder diffraction method it was found that the obtained products do not contain nickel oxide or hydroxide. Using Scherrer equation the average crystallites sizes for some nickel samples have been calculated. It was found that the sizes of crystallites of obtained nickel nanopowders increase from 20 to 33 nm with a decreasein the ethylene glycol concentration in the reaction mixture. It was determined that the regulation of the composition of the solvent can be used to obtain nickel nanopowders with predicted sizes and polydispersity of particles. Based on the comparison of the values of the induction period times and the average diameters of the resulting nickel particles, it was found that ethylene glycol is not only a particles stabilizer, but also participates in the topochemical reaction of reduction of nickel hydroxide with hydrazine. Keywords: nickel nanopowders, heterogeneous nucleation, ethylene glycol, hydrazine.