Pincer Nickel Research Articles

Synthesis, characterization, photophysics, and a ligand rearrangement of CCC-NHC pincer nickel complexes: Colors, polymorphs, emission, and Raman spectra

The metallation/transmetallation strategy has been successfully applied to the preparation of CCC-NHC pincer nickel complexes. Manipulation of the coordination sphere lead to cationic CCC-NHC pincer nickel complexes. These were found to exhibit polymorphism in the solid state with significant differences in the intermolecular distances observed by X-ray crystallography. Three polymorphs were observed for the PF6− salt, and two were observed for the BF4− salt. When BPh4− was the counterion only one morphology was observed, and crystals grown of the triflate salt were too small for analysis. Details of the differences are documented for comparison. Additionally, during crystallization it was found that a rearrangement of the CCC-NHC ligand had occurred, involving coupling of an NHC from two different ligands, which were oxidized.

Electrocatalytic reduction of CO2 with CCC-NHC pincer nickel complexes

A CCC-NHC pincer Ni(ii)Cl complex was prepared according to the metallation/transmetallation methodology. It was fully characterized by electrochemical, NMR spectroscopic, theoretical, and X-ray crystallographic methods. The complex and its cation were evaluated for electrocatalytic reduction of CO2 under a variety of conditions and found to provide some of the fastest catalytic rates and highest substrate selectivities (CO2vs. H+) reported. Rates improved in the presence of water and, significantly, catalysis occurred at the first reduction potential, presumably at the Ni(i) state. Controlled potential electrolysis (CPE) was found to yield CO at 34% and formate at 47% Faradaic efficiency (FE).

Synthesis of Quinoline-Based NNN-Pincer Nickel(II) Complexes: A Robust and Improved Catalyst System for C–H Bond Alkylation of Azoles with Alkyl Halides

The quinoline-based pincer nickel(II) complexes κN,κN,κN-{R2N-C6H4-(μ-N)-C9H6N}NiX ((R2NNNQ)NiCl: R = Me, 2a; R = Et, 2b) were synthesized by the reaction of the ligand precursors (R2NNNQ)H (R = Me, 1a; R = Et, 1b) with (DME)NiCl2 in the presence of Et3N. Similarly, the pincer nickel(II) derivatives (R2NNNQ)NiX (R = Me, X = Br, 3a; R = Et, X = Br, 3b; R = Me, X = OAc, 4a) were obtained by treatment of the ligands (R2NNNQ)H with the nickel precursor (THF)2NiBr2 or Ni(OAc)2. All of these complexes were characterized by 1H and 13C NMR spectroscopy as well as by elemental analysis. Further, the molecular structures of 2a and 3a,b were elucidated by X-ray crystallography. Complex 2a is found to be an efficient catalyst for the direct C–H bond alkylation of substituted benzothiazoles and oxazoles with various unactivated alkyl halides containing β-hydrogens under mild reaction conditions. The catalyst 2a is very robust and was recycled and reused five times for the alkylation reaction without a decrease in its ...

Reactions of Phenylhydrosilanes with Pincer-Nickel Complexes: Evidence for New Si-O and Si-C Bond Formation Pathways.

This contribution presents evidence for new pathways manifested in the reactions of the phenylhydrosilanes PhnSiH4-n with the pincer complexes (POCsp(2)OP)Ni(OSiMe3), 1-OSiMe3, and (POCsp(3)OP)Ni(OSiMe3), 2-OSiMe3 (POCsp(2)OP = 2,6-(i-Pr2PO)2C6H3; POCsp(3)OP = (i-Pr2POCH2)2CH). Excess PhSiH3 or Ph2SiH2 reacted with 1-OSiMe3 to eliminate the disilyl ethers Ph(n)H(3-n)SiOSiMe3 (n = 1 or 2) and generate the nickel hydride species 1-H. Subsequent reaction of the latter with more substrate formed corresponding nickel silyl species 1-SiPhH2 or 1-SiPh2H and generated multiple Si-containing products, including disilanes and redistribution products. The reaction of 1-OSiMe3 with excess Ph2SiH2/Ph2SiD2 revealed a net KIE of ca. 1.3-1.4 at room temperature. Treating 1-OSiMe3 with excess Ph3SiH also gave 1-H and the corresponding disilyl ether Ph3SiOSiMe3, but this reaction also generated the new siloxide 1-OSiPh3 apparently via an unconventional σ-bond metathesis pathway in which the Ni center is not involved directly. The reaction of excess PhSiH3 and 2-OSiMe3 gave polysilanes of varying solubilities and molecular weights; NMR investigations showed that these polymers arise from Ni(0) species generated in situ from the reductive elimination of the highly reactive hydride intermediate, 2-H. The stoichiometric reactions of 2-OSiMe3 with Ph2SiH2 and Ph3SiH gave, respectively, siloxides 2-OSiPh2(OSiMe3) and 2-OSiPh3. Together, these results demonstrate the strong influence of pincer backbone and hydrosilane sterics on the different reactivities of 1-OSiMe3 and 2-OSiMe3 toward Ph(n)SiH(4-n) (dimerization, polymerization, and redistribution vs formation of new siloxides). The mechanisms of the reactions that lead to the observed Si-O, Si-C, and Si-Si bond formations are discussed in terms of classical and unconventional σ-bond metathesis pathways.

Homolytic H2 cleavage by a mercury-bridged Ni(I) pincer complex [{(PNP)Ni}2{μ-Hg}

Reduction of the pincer nickel(ii) complex [(PNP)NiBr] with sodium amalgam (Na/Hg) forms the mercury-bridged dimer [{(PNP)Ni}2{μ-Hg}], which homolytically cleaves dihydrogen to form [(PNP)NiH]. Reversible CO2 insertion into the Ni-H bond is observed for [(PNP)NiH], forming the monodentate κ(1)O-formate complex [(PNP)NiOC(O)H].

Highly efficient pincer nickel catalyzed cross-coupling of aryltrimethylammonium triflates with arylzinc reagents

N,N,P-Pincer nickel complexes [Ni(Cl){N(2-R2PC6H4)(2′-Me2NC6H4)}] highly effectively catalyze the cross-coupling of aryltrimethylammonium triflates with arylzinc reagents.

Mono- and Dinuclear Pincer Nickel Catalyzed Activation and Transformation of C–Cl, C–N, and C–O Bonds

Condensation of 2-NH2C6H4P(Et)Ph (2) with pyrrole-2-carboxaldehyde generated 2-(C4H4N-2′-CH═N)C6H4P(Et)Ph (3). Treatment of 3 with NaH and followed by (DME)NiX2 (X = Cl, Br) afforded mononuclear pincer nickel complexes [Ni{2-(C4H3N-2′-CH═N)C6H4P(Et)Ph}X] (4a, X = Cl; 4b, X = Br). Reaction of [2-NH2C6H4P(Ph)]2(CH2)n (5a, n = 3; 5b, n = 4) with pyrrole-2-carboxaldehyde or 5-tert-butyl-1H-pyrrole-2-carbaldehyde formed [2-(C4H4N-2′-CH═N)C6H4P(Ph)]2(CH2)n (6a, n = 3; 6b, n = 4) and [2-(5′-tBuC4H3N-2′-CH═N)C6H4P(Ph)]2(CH2)4 (6c). Respective treatment of 6a–c with NaH followed by (DME)NiX2 (X = Cl, Br) gave the dinuclear nickel complexes [Ni{2-(5′-RC4H2N-2′-CH═N)C6H4P(Ph)}X]2(CH2)n (7a, R = H, X = Cl, n = 3; 7b, R = H, X = Cl, n = 4; 7c, R = H, X = Br, n = 4; 7d, R = tBu, X = Cl, n = 4). Catalysis of the complexes for the activation and transformation of C–Cl, C–N, and C–O bonds was evaluated. Complex 7c exhibited excellent catalytic activity in the cross-coupling of aryl chlorides or aryltrimethylammonium iodides with arylzinc reagents as well as of aryl sulfamates with aryl Grignard reagents. The dinuclear nickel complexes 7b–d showed higher catalytic activity than the mononuclear complexes in each type of reaction.

ChemInform Abstract: Suzuki—Miyaura Cross‐Coupling of Bulky Anthracenyl Carboxylates by Using Pincer Nickel N‐Heterocyclic Carbene Complexes: An Efficient Protocol to Access Fluorescent Anthracene Derivatives.

AbstractThe spectroscopic properties of the compounds are investigated in CH2Cl2 and they show strong fluorescence.

ChemInform Abstract: Amido Pincer Nickel Catalyzed Kumada Cross‐Coupling of Aryl, Heteroaryl, and Vinyl Chlorides.

Amido pincer nickel complexes {Ni(Cl)[2-P(Ph2)C6H4NC(Ph)=NAr]} (Ar = 4-MeC6H4, 1; Ar = 4-ClC6H4, 2; Ar = 4-MeOC6H4, 3) were shown to efficiently catalyze the cross-coupling of activated, unactivated, and deactivated aryl chlorides, N-heteroaryl chlorides, 1,4-dichlorobenzene, and vinyl chlorides with aryl Grignard reagents.

CNN]-pincer nickel(II) complexes of N-heterocyclic carbene (NHC): synthesis and catalysis of the Kumada reaction of unactivated C-Cl bonds.

Three novel [CNN]-pincer nickel(ii) complexes with NHC-amine arms were synthesized in three steps. Complex was proven to be an efficient catalyst for the Kumada coupling of aryl chlorides or aryl dichlorides under mild conditions.

Amido Pincer Nickel Catalyzed Kumada Cross-Coupling of Aryl, Heteroaryl, and Vinyl Chlorides

Amido pincer nickel complexes {Ni(Cl)[2-P(Ph2)C6H4NC(Ph)=NAr]} (Ar = 4-MeC6H4, 1; Ar = 4-ClC6H4, 2; Ar = 4-MeOC6H4, 3) were shown to efficiently catalyze the cross-coupling of activated, unactivated, and deactivated aryl chlorides, N-heteroaryl chlorides, 1,4-dichlorobenzene, and vinyl chlorides with aryl Grignard reagents.

Cross-Coupling of ArX with ArMgBr Catalyzed by N-Heterocyclic Carbene-Based Nickel Complexes

N-Heterocyclic carbene-based pincer nickel complexes were synthesized and characterized. These complexes efficiently catalyze cross-coupling of aryl Grignard reagents with aryl chlorides or fluorides under mild conditions.

Suzuki–Miyaura cross-coupling of bulky anthracenyl carboxylates by using pincer nickel N-heterocyclic carbene complexes: an efficient protocol to access fluorescent anthracene derivatives

A series of fluorescent (hetero)-aryl substituted anthracene derivatives were readily accessible from the corresponding bulky anthracen-9-yl carboxylates via Suzuki-Miyaura cross-coupling reactions by using pincer nickel N-heterocyclic carbene complex even at the catalyst loading as low as 0.1 mol% in the presence of catalytic amounts of PCy3.

Cross coupling reactions of multiple C[sbnd]Cl bonds of polychlorinated solvents with Grignard reagent using a pincer nickel complex

The nickel(II) complex of a bulky pincer-type ligand, N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamido, was examined for sp3–sp3 coupling of Grignard reagents with polychlorinated solvents. The nickel(II) complex catalyzed CC coupling of polychlorinated alkyl halides, such as dichloromethane (CH2Cl2), chloroform (CHCl3), and carbon tetrachloride (CCl4), with various Grignard reagents. The effective activation of multiple CCl bonds proceeded under ambient reaction conditions and within a short time (20min). This catalyst displays the highest activity yet reported for this reaction type, with catalyst loading as low as 0.4mol% and turnover frequency (TOF) as high as 724h−1. The catalyst is capable of replacing all chlorine atoms with CC bond formations for all of the polychlorinated solvents under investigation. The catalytic process could prove to be an efficient method of remediation of toxic polychlorinated solvents while generating synthetically and commercially important chemicals.

ChemInform Abstract: Cross‐Coupling of Aryltrimethylammonium Iodides with Arylzinc Reagents Catalyzed by Amido Pincer Nickel Complexes.

The cross-coupling reaction of aryltrimethylammonium iodides with aryl- or heteroarylzinc chlorides catalyzed by amido pincer nickel complexes was performed. The reaction requires low catalyst loading and displays broad substrate scope.

Cross-Coupling of Aryltrimethylammonium Iodides with Arylzinc Reagents Catalyzed by Amido Pincer Nickel Complexes

The cross-coupling reaction of aryltrimethylammonium iodides with aryl- or heteroarylzinc chlorides catalyzed by amido pincer nickel complexes was performed. The reaction requires low catalyst loading and displays broad substrate scope.

Kumada Coupling of Aryl, Heteroaryl, and Vinyl Chlorides Catalyzed by Amido Pincer Nickel Complexes

A series of amido pincer complexes of nickel were examined for their catalysis in the Kumada cross-coupling reaction. The P,N,O-pincer nickel complexes tested are active catalysts for the cross-coupling of aryl, heteroaryl, and vinyl chlorides with aryl Grignard reagents. The reactions can proceed at room temperature and tolerate functional groups in aryl chlorides with the aid of LiCl and ZnCl(2) additives.

Trinuclear copper(I) complex of 1,3-bis(2-pyridinylmethyl)imidazolylidene as a carbene-transfer reagent for the preparation of catalytically active nickel(II) and palladium(II) complexes

Reactions of 1,3-bis(pyridin-2-ylmethyl)-1H-imidazol-3-ium hexafluorophosphate, ([HL1](PF 6), L1 = 1,3-bis(pyridin-2-ylmethyl)imidazolylidene) and 1,3-bis(pyridin-2-ylmethyl)-1H-benzimidazol-3-ium hexafluorophosphate ([HL2](PF 6), L2 = 1,3-bis(pyridin-2-ylmethyl)benzoimidazolylidene) with cuprous oxide in acetonitrile readily yielded trinuclear complexes [Cu 3(L1) 3(PF 6) 3] ( 1) and [Cu 3(L2) 3(PF 6) 3] ( 2). Treatment of 1 with Ni(PPh 3) 2Cl 2 and Pd(cod)Cl 2 gave [Ni(L1)Cl](PF 6) ( 3) and [Pd(L1)Cl](PF 6) ( 4), respectively, due to transmetalation. [Ni(L1) 2](PF 6) 2 ( 5) was obtained from the reaction of [Cu 3(L1) 3(PF 6) 3] and Raney nickel in acetonitrile. All these complexes have been fully characterized. Both 1 and 2 consist of a triangular Cu 3 core with each Cu–Cu bond capped by an imidazolylidene group. Each imidazolylidene acts as a bridging ligand in a μ 2 mode and is bonded equally to two Cu(I) ions. The pincer nickel and palladium complexes are square-planar and contain a tridentate NCN ligand. Complexes 3 and 4 are efficient catalyst precursors for Kumada–Corriu and Suzuki–Miyaura coupling reactions of aryl halides with organometallic reagents.

ChemInform Abstract: A Pyridine‐Bridged Bis‐benzimidazolylidene Pincer Nickel(II) Complex: Synthesis and Practical Catalytic Application Towards Suzuki—Miyaura Coupling with Less‐Activated Electrophiles.

AbstractThe title catalyst is found to be very efficient in promoting cross coupling of less reactive aryl halides and ‐sulfonates with phenylboronic acid.

Neutral and cationic chiral NCN pincer nickel(ii) complexes with 1,3-bis(2′-imidazolinyl)benzenes: synthesis and characterization

Chiral 1,3-bis(2'-imidazolinyl)benzenes 1a-e easily undergo direct nickelation at the C2 position of the central benzene ring via the C-H bond activation in the reaction with anhydrous NiCl(2) giving neutral NCN pincer nickel(II) complexes 2a-e in 40-87% yields. Treatment of the nickel pincers 2a or 2c with AgBF(4) in CH(3)CN-CH(2)Cl(2) afforded the cationic nickel pincers 3a or 3c in good yields. All the complexes were characterized by elemental analysis, (1)H, (13)C NMR, and IR spectra. Molecular structures of the neutral complexes 2a, 2b and 2c as well as the cationic complex 3c have been determined by X-ray single-crystal diffraction. The cationic nickel pincers 3 are found to be effective catalysts for the Michael addition of ethyl 2-cyanopropionate to methyl vinyl ketone in the presence of i-Pr(2)NEt base with a catalyst loading of 5 mol% even at -78 °C, producing the adduct in >99% yield after 24 h albeit with no ee.