Nickel Pincer Complexes Research Articles

Solvent dependent C−H Bond Strength in a Nickel Pincer Complex

AbstractMetal coordination can substantially weaken the bond strength of X−H bonds in coordinated ligands. Despite the importance for redox catalysis, this phenomenon is surprisingly scarcely examined for C−H bonds of organometallic compounds. We here report a nickel(I) pincer complex with a remote methylene group in the ligand backbone that exhibits an unusually solvent dependent C−H bond dissociation free energy (BDFEC−H). Structural and thermochemical characterization in MeCN and THF indicates that the BDFEC−H strongly depends on solvent binding to the metal within the NiII/NiI redox couple.

Suzuki–Miyaura Csp2–Csp2 Cross-Couplings Employing Nickel(II) Pincer Precatalysts: Mechanistic Investigations

This study investigates the mechanism of Suzuki-Miyaura Csp2-Csp2 cross-couplings facilitated by PCP-type nickel(II) pincer complexes. By employing a combination of standard experimental and theoretical methods, it is proposed that a nickel(I)/(III) cycle is involved in this reaction and single-electron pathways likely operate in this transformation.

Asymmetric Synthesis of P-Stereogenic Secondary Phosphine-Boranes by an Unsymmetric Bisphosphine Pincer-Nickel Complex

The first highly enantioselective catalytic synthesis of P-stereogenic secondary phosphine-boranes was realized by the asymmetric addition of primary phosphine to electron-deficient alkenes with a newly developed unsymmetric bisphosphine (PCP') pincer-nickel complex. Various P-stereogenic secondary phosphine-boranes were obtained in 57-92% yields with up to 99% ee and >20:1 dr. The follow-up alkylation upon P-C bond formation with alkyl halides provided a practical way to access P-chiral compounds with diverse functional groups.

Computational study on the mechanism of hydroboration of CO2 catalysed by POCOP pincer nickel thiolate complexes: concerted catalysis and hydride transfer by a shuttle.

Hydroboration of carbon dioxide (CO2) catalysed by bis(phosphinite) (POCOP) pincer nickel complexes is among the most efficient homogeneous processes for the reduction of CO2 to the methanol level. Although both POCOP pincer nickel hydride and thiolate complexes are effective catalysts, the latter is far more effective under the same conditions. The mechanism for nickel hydride complexes catalysed reactions is well-established. However, that for nickel thiolate complex catalysed reactions remains elusive. In this work, the mechanism for the reduction of CO2 catalysed by POCOP pincer nickel thiolate complexes was investigated using density functional theory. The calculated results indicated that the reaction occurs via a concerted catalytic process involving two active species and the hydride is transferred by a shuttle species. Specifically, the reaction proceeds through four cycles: formation of two active species (cycle I) followed by further reaction of these two species to form a hydride transfer shuttle which is responsible for hydride transfers CO2→HCOOBcat (cycle II), HCOOBcat→CH2O (cycle III) and CH2O→catBOCH3 (cycle IV). The calculated mechanism is in good agreement with the experimental observation that the reaction is exothermic with simultaneous HBcat degradation.

Unsymmetrical nickel (PCN) pincer complexes with a benzothiazole side-arm: Synthesis, characterization and electrochemical properties

The newly prepared unsymmetrical pincer ligand with a benzothiazole side-arm 2-(3-((di-tert-butylphosphino)methyl)phenoxy)benzo[d]thiazole [BzTz(H)PCN] has been reacted with anhydrous NiBr2 to get the corresponding NiII square planar bromo complex (BzTzPCN)NiBr (1) after HBr elimination and CH activation on the pincer central phenyl ring. Starting from 1, reaction with AgF in toluene or with AgBF4 in THF led to bromide abstraction and formation of the fluoro complex (BzTzPCN)NiF (2) and the ionic aquo species [(BzTzPCN)Ni(H2O)]BF4 (3), respectively. All species have been characterized in solution (multinuclear 1H, 13C{1H}, 31P{1H} and 11B NMR spectroscopy) and in the solid state (single-crystal X-ray diffraction analysis). Finally, comparative electrochemical measurements (CV and in situ EPR-spectroelectrochemistry) carried out on the halide complexes 1 and 2 revealed that the anodic oxidation process leads to the formation of stable NiIII species bearing a coordinated bromide ligand in case of 1 and a fluoride-free complex in case of 2.

Enhancing the Stability of Aromatic PCN Pincer Nickel Complexes by Incorporation of Pyridine as the Nitrogen Side Arm

New PCNPy pincer nickel complexes have been synthesized through a short synthetic route. Incorporating pyridine as the nitrogen side arm facilitated the C–H activation in the PCN ligand and allowed the cyclometallation with nickel to take place at room temperature. Pyridine also enhanced the stability of β‐hydrogen‐containing alkyl complexes. Also, the symmetric NCN nickel complex with pyridine side arms was successfully obtained giving a rare example of such type of complexes to be prepared through direct C–H activation. Furthermore, preliminary results showed that the (PCNPy)Ni–Br is active in Kumada coupling reactions particularly the coupling of aryl halides with aryl Grignard reagents.

Application of bis(phosphinite) pincer nickel complexes to the catalytic hydrosilylation of aldehydes

A series of bis(phosphinite) (POCOP) pincer ligated nickel complexes, [2,6-(tBu2PO)2C6H3]NiX (X = SH, 1; SCH2Ph, 2; SPh, 3; NCS, 4; N3, 5), were used to catalyse the hydrosilylation of aldehydes. It was found that both complexes 1 and 2 are active in catalysing the hydrosilylation of aldehydes with phenylsilane and complex 1 is comparatively more active. The expected alcohols were isolated in good to excellent yields after basic hydrolysis of the resultant hydrosilylation products. However, no reaction was observed when complex 3 or 4 or 5 was used as the catalyst. The results are consistent with complexes 1 and 2 serving as catalyst precursors, which generate the corresponding nickel hydride complex [2,6-(tBu2PO)2C6H3]NiH in situ, and the nickel hydride complex is the active species that catalyses this hydrosilylation process. The in situ generation of the nickel hydride species was supported by both experimental results and DFT calculation.

Synthesis and Characterization of 3,5-Bis(di-tert-butylphosphinito)pyridine Pincer Complexes

The synthesis of some “reverse pyridine” bis(phosphinite) pincer complexes of nickel and rhodium is reported. N-Functionalization of a POCOP ligand with a borane Lewis acid was found to permit cycl...

Mechanistic Investigations of Nickamine- catalyzed Hydrosilylation of Alkenes: Nickel Nanoparticles Are the Active Species.

Hydrosilylation is an important chemical process for the synthesis of organosilanes and for the production of silicone polymers. The wide variety of catalysts developed for this reaction generally follow a Chalk-Harrod, or a sigma-bond metathesis mechanism. Recently, our group developed a nickel pincer complex, Nickamine, for highly selective hydrosilylation of alkenes. Preliminary mechanistic studies had suggested a pathway that deviates from both Chalk-Harrod and sigma-bond metathesis cycles. Here we used in situ NMR to monitor the hydrosilylation reaction. The observed induction period indicated that the species previously believed to be the resting state is merely a precatalyst. Via a combination of Transmission Electron Microscopy, mercury poisoning test, and competition reactions we show that the true catalyst is not a molecular nickel species, but rather nickel nanoparticles.

POCOP-type cobalt and nickel pincer complexes bearing an appended phosphinite group

The reaction of 1,3,5-(iPr2PO)3C6H3 with Co2(CO)8 leads to the isolation of a POCOP-type mononuclear pincer complex {κP,κC,κP-2,4,6-(iPr2PO)3C6H2}Co(CO)2 (1) or a tetranuclear species {κP-{κP,κC,κP-2,4,6-(iPr2PO)3C6H2}Co(CO)2}2Co2(CO)6 (2), depending on the ligand to cobalt ratio employed. The latter compound can be an impurity during the synthesis of {2,6-(iPr2PO)2-4-Me2N-C6H2}Co(CO)2, when the ligand precursor 5-(dimethylamino)resorcinol is contaminated with phloroglucinol due to incomplete monoamination. Similarly, the reaction of 1,3,5-(iPr2PO)3C6H3 with NiCl2 in the presence of 4-dimethylaminopyridine provides {κP,κC,κP-2,4,6-(iPr2PO)3C6H2}NiCl (3) bearing an appended phosphinite group. Structures 1–3 have been studied by X-ray crystallography.

Solvent-Free N-Alkylation and Dehydrogenative Coupling Catalyzed by a Highly Active Pincer-Nickel Complex

The synthesis and characterization of a pincer-nickel complex of the type (iPr2NNN)NiCl2(CH3CN) is reported here. We have demonstrated the utility of this pincer-nickel complex (0.02 and 0.002 mol ...

Selective Synthesis of Secondary Alkylboronates: Markovnikov-Selective Hydroboration of Vinylarenes with Bis(pinacolato)diboron Catalyzed by a Nickel Pincer Complex

A nickel pincer complex bearing a β-aminoketonato-based O,N,P-tridentate ligand (1a) has been employed for the highly Markovnikov-selective hydroboration of vinylarenes using bis(pinacolato)diboron. This reaction proceeds smoothly under mild reaction conditions and affords the corresponding Markovnikov products in good to high yield.

Carboxylation of the Ni–Me Bond in an Electron-Rich Unsymmetrical PCN Pincer Nickel Complex

The synthesis of a new unsymmetrical PCN ligand bearing tert-butyl groups on the phosphorus atom and isopropyl groups on the nitrogen donor atom is presented. It reacts with the commercially availa...

Bioinspired Design and Computational Prediction of SCS Nickel Pincer Complexes for Hydrogenation of Carbon Dioxide

Inspired by the structures of the active site of lactate racemase and H2 activation mechanism of mono-iron hydrogenase, we proposed a series of sulphur–carbon–sulphur (SCS) nickel complexes and computationally predicted their potentials for catalytic hydrogenation of CO2. Density functional theory calculations reveal a metal–ligand cooperated mechanism with the participation of a sulfur atom in the SCS pincer ligand as a proton receiver for the heterolytic cleavage of H2. For all newly proposed complexes containing functional groups with different electron-donating and withdrawing abilities in the SCS ligand, the predicted free energy barriers for the hydrogenation of CO2 to formic acid are in a range of 22.2–25.5 kcal/mol in water. Such a small difference in energy barriers indicates limited contributions of those functional groups to the charge density of the metal center. We further explored the catalytic mechanism of the simplest model complex for hydrogenation of formic acid to formaldehyde and obtained a total free energy barrier of 34.6 kcal/mol for the hydrogenation of CO2 to methanol.

Suzuki-Miyaura coupling catalyzed by a Ni(II) PNP pincer complex: Scope and mechanistic insights

The nickel(II) pincer complex, [NiCl(PhPNP)] (PhPNP = anion of 2,5-bis(diphenylphosphinomethyl)pyrrole), has been employed as a precatalyst for the Suzuki-Miyaura cross-coupling reaction of aryl halides and boronic acids. Both electron-rich and electron-deficient aromatic bromides were found to undergo coupling with boronic acids in modest yield at elevated temperature in the presence of K3PO4·H2O. Preliminary mechanistic studies of the reaction identified a novel species formulated as the boronate complex, [Ni(OB{OH}{2-tolyl})(PhPNP)], which most likely represents a catalyst deactivation pathway. The productive catalytic cycle was found to be most consistent with a Ni(I)/Ni(III) process where the boronic acid serves as both reductant and nucleophile in the presence of base.

Functional Models of the Nickel Pincer Nucleotide Cofactor of Lactate Racemase

AbstractA novel nickel pincer cofactor was recently discovered in lactate racemase. Reported here are three synthetic nickel pincer complexes that are both structural and functional models of the pincer cofactor in lactate racemase. DFT computations suggest the ipso‐carbon atom of the pyridinium pincer ligands act as a hydride acceptor for lactate isomerization, whereas an organometallic pathway involving nickel‐mediated β‐hydride elimination is less favored.

Functional Models of the Nickel Pincer Nucleotide Cofactor of Lactate Racemase.

A novel nickel pincer cofactor was recently discovered in lactate racemase. Reported here are three synthetic nickel pincer complexes that are both structural and functional models of the pincer cofactor in lactate racemase. DFT computations suggest the ipso-carbon atom of the pyridinium pincer ligands act as a hydride acceptor for lactate isomerization, whereas an organometallic pathway involving nickel-mediated β-hydride elimination is less favored.

Computational Prediction of Ammonia-Borane Dehydrocoupling and Transfer Hydrogenation of Ketones and Imines Catalyzed by SCS Nickel Pincer Complexes.

Inspired by the catalytic mechanism and active site structure of lactate racemase, three scorpion-like SCS nickel pincer complexes were proposed as potential catalysts for transfer hydrogenation of ketones and imines with ammonia-borane (AB) as the hydrogen source. Density functional theory calculations reveal a stepwise hydride and proton transfer mechanism for the dehydrocoupling of AB and hydrogenation of N-methylacetonimine, and a concerted proton-coupled hydride transfer process for hydrogenation of acetone, acetophenone, and 3-methyl-2-butanone. Among all proposed Ni complexes, the one with symmetric NH2 group on both arms of the SCS pincer ligand has the lowest free energy barrier of 15.0 kcal/mol for dehydrogenation of AB, as well as total free energy barriers of 17.8, 18.2, 18.0, and 18.6 kcal/mol for hydrogenation of acetone, N-methylacetonimine, acetophenone, and 3-methyl-2-butanone, respectively.

Access to β-Ketonitriles through Nickel-Catalyzed Carbonylative Coupling of α-Bromonitriles with Alkylzinc Reagents.

Herein, we report a nickel-catalyzed carbonylative coupling of α-bromonitriles and alkylzinc reagents with near stoichiometric carbon monoxide to give β-ketonitriles in good yields. The reaction is catalyzed by a readily available and stable nickel(II) pincer complex. The developed protocol tolerates substrates bearing a variety of functional groups, which would be problematic or incompatible with previous synthetic methods. Additionally, we demonstrate the suitability of the method for carbon isotope labeling by the synthesis of 13 C-labeled β-ketonitriles and their transformation into isotopically labeled heterocycles.

Cross-Coupling Reaction of Allylic Ethers with Aryl Grignard Reagents Catalyzed by a Nickel Pincer Complex

A cross-coupling reaction of allylic aryl ethers with arylmagnesium reagents was investigated using β-aminoketonato- and β-diketiminato-based pincer-type nickel(II) complexes as catalysts. An β-aminoketonato nickel(II) complex bearing a diphenylphosphino group as a third donor effectively catalyzed the reaction to afford the target cross-coupled products, allylbenzene derivatives, in high yield. The regioselective reaction of a variety of substituted cinnamyl ethers proceeded to give the corresponding linear products. In contrast, α- and γ-alkyl substituted allylic ethers afforded a mixture of the linear and branched products. These results indicated that the coupling reaction proceeded via a π-allyl nickel intermediate.