Nickel Complexes Of General Formula Research Articles

Synthesis and Catalytic Activity in Suzuki Coupling of Nickel Complexes Bearing n-Butyl- and Triethoxysilylpropyl-Substituted NHC Ligands: Toward the Heterogenization of Molecular Catalysts

Cyclopentadienyl N-heterocyclic carbene (NHC) nickel complexes of general formula [Ni(R-NHC-nBu)XCp] [R-NHC-nBu = 1-butyl-3-methyl-, 1-isopropyl-3-butyl-, 1-phenyl-3-butyl-, 1-(2,4,6-trimethylphenyl)-3-butyl-, 1-(2,6-diisopropylphenyl)-3-butyl-imidazol-2-ylidene; X = Cl or I; Cp = η5-C5H5], which bear an n-butyl side-chain attached to one of the nitrogen atoms of the NHC ring, were synthesized as models for trialkoxysilylpropyl-substituted complexes. They were prepared by the direct reactions of nickelocene with the corresponding imidazolium salts (R-NHC-nBu·HX). The new complexes [Ni(Me-NHC-nBu)ClCp] (1a), [Ni(iPr-NHC-nBu)ClCp] (1b), [Ni(Ph-NHC-nBu)ICp] (1c), [Ni(Mes-NHC-nBu)ICp] (1d), and [Ni(iPr2Ph-NHC-nBu)ICp] (1e) were obtained in moderate to good yields and were fully characterized by standard spectroscopic techniques, and in the cases of 1a,b,d,e by single-crystal X-ray crystallography. The bulky electron-rich pentamethylcyclopentadienyl derivatives, [Ni(Mes-NHC-nBu)ICp*] (2d) and [Ni(iPr2Ph-NHC-nB...

Intramolecular Nitrile C–H Bond Activation in Nickel NHC Complexes: A Route to New Nickelacycles

Cyclopentadienyl N-heterocyclic carbene (NHC) nickel complexes of general formula [Ni(NHC)XCp] (NHC = 1-(propylnitrile)-3-methylimidazol-2-ylidene, 1-(2,4,6-trimethylphenyl)-3-(butylnitrile)imidazol-2-ylidene, 1-(2,4,6-trimethylphenyl)-3-(pentylnitrile)imidazol-2-ylidene, 1-(2,4,6-trimethylphenyl)-3-(hexylnitrile)imidazol-2-ylidene; X = Cl, Br, I; Cp = η5-C5H5), which bear an alkylnitrile side chain attached to one of the nitrogen atoms of the NHC ring, were prepared by the direct reaction of nickelocene with the corresponding imidazolium salts (NHC·HX). The new complexes [Ni{Me-NHC-(CH2)2CN}ICp] (1a), [Ni{Mes-NHC-(CH2)3CN}ClCp] (1b), [Ni{Mes-NHC-(CH2)4CN}ICp] (1c), and [Ni{Mes-NHC-(CH2)5CN}BrCp] (1d) were obtained in good yields and were fully characterized by standard spectroscopic techniques and elemental analyses and, in the cases of 1a,b, by single-crystal X-ray crystallography. Structural studies established their two-legged piano-stool geometry. The cationic derivatives [Ni{Mes-NHC-(CH2)nCN}(NCMe)Cp]+ (2b–d; n = 3–5) were prepared from the reaction of their neutral homologues 1b−d with KPF6 in acetonitrile at room temperature. Upon treatment of the neutral complexes 1 or of the cationic compounds 2 with KO-t-Bu, a C–H bond α to the nitrile group in each molecule underwent a base-promoted C–H activation and the new nickelacyclic complexes [Ni{Me-NHC-CH2CH(CN)}Cp] (3a) and [Ni{Mes-NHC-(CH2)nCH(CN)}Cp] (3b–d; n = 2–4) were obtained. All of these metallacycles contain Ni–C σ-bonds, and their synthesis generated a new asymmetric carbon center. The cyclic complexes 3 were fully characterized by 1H and 13C NMR spectroscopy, IR spectroscopy, and elemental analyses. The structures of complexes 3b and 3d, which contain six- and eight-membered metallacyclic rings, respectively, were determined by single-crystal X-ray diffraction studies. DFT studies, carried out to probe the mechanism of these cyclonickelation reactions, indicated that the mechanism of formation of these nickelacycles was similar to that observed for the formation of cyanomethyl ligands from coordinated acetonitrile. Nevertheless, the base deprotonation of an α-C–H ligand in a side arm, while thermodynamically comparable to that of a simultaneously coordinated acetonitrile ligand, is kinetically favored, and this leads to the formation of nickelacycles rather than cyanomethyl complexes in the case of the base-assisted activation of the cationic complexes 2.

Half-sandwich NHC-nickel(ii) complexes as pre-catalysts for the fast Suzuki coupling of aryl halides: a comparative study

Cationic half-sandwich nickel complexes of general formula [Ni(NHC)(NCMe)(eta(5)-C5R5)](PF6) [NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) a, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) b; R = H, Me] were prepared from the reaction of their neutral homologues [Ni(NHC)Cl(eta(5)-C5R5)] with 1 equiv. of KPF6 in acetonitrile at room temperature. The new cationic complexes [Ni(IPr)(NCMe)(eta(5)-C5Me5)](PF6) 3a, [Ni(IMes)(NCMe)(eta(5)-C5Me5)](PF6) 3b and [Ni(IMes)(NCMe)(eta(5)-C5H5)](PF6) 4b were obtained in high yield and were fully characterized by 1H and 13C NMR spectroscopy, IR spectroscopy, elemental analyses, and in the case of 3a by a single-crystal X-ray diffraction study. The neutral analogue of 3a, [Ni(IPr)Cl(eta(5)-C5Me5)] 1a was also structurally characterized. Their geometries were compared and no significant structural differences were observed. Nevertheless solution NMR spectroscopy established that the acetonitrile ligand of the cationic species is labile in solution. This results in the absence of any rotational significant barrier about the nickel-carbene carbon bond at ambient temperature in solution in the sterically congested cationic complexes 3a and 3b, in contrast to their neutral analogues 1a and [Ni(IMes)Cl(eta(5)-C5Me5)] 1b. The neutral and the cationic complexes catalyzed the cross-coupling of phenylboronic acid with aryl halides in the absence of co-catalysts or reductants. Surprisingly, the neutral or cationic nature of the complexes proved to have almost no influence on the reaction yields and rates. However, complexes bearing the bulky electron-rich pentamethylcyclopentadienyl ligand were much more active than those bearing the cyclopentadienyl ligand, and TOF of up to 190 h(-1), a high rate for nickel(II) complexes under similar conditions, were observed with these species.

Synthesis, Structure, and Solution Dynamics of Pentamethylcyclopentadienyl Nickel Complexes Bearing N-Heterocyclic Carbene Ligands

Neutral pentamethylcyclopentadienyl nickel complexes of general formula [Ni(NHC)X(η5-C5Me5)] [NHC = 1,3-dimethylimidazol-2-ylidene (Me-NHC), 1,3-bis(2,4,6,-trimethylphenyl)imidazol-2-ylidene (Mes-NHC), 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (iPr-NHC); X = Cl, I] were prepared from the reaction of pentamethylcyclopentadienyl acetylacetonate nickel(II) with 1 equiv of the corresponding imidazolium salt (NHC·HX). The new complexes [Ni(Me-NHC)I(η5-C5Me5)] 1a, [Ni(Mes-NHC)Cl(η5-C5Me5)] 2, and [Ni(iPr-NHC)Cl(η5-C5Me5)] 3 were obtained in moderate to good yields and were fully characterized by 13C and 1H NMR spectroscopy, and in the cases of 1a and 2 by single-crystal X-ray crystallography. The related cyclopentadienyl complex [Ni(Me-NHC)I(η5-C5H5)] 1b was also synthesized and structurally characterized; its geometry and spectroscopic data are comparable to those of complex 1a. The variable-temperature (VT) 1H NMR spectra of the sterically constrained complexes 2 and 3 are consistent with restricted ro...

From isothiocyanato– to silyl–nickel complexes

Nickel complexes of general formula [Ni(PR 3) 2(SiMe n Cl 3− n ) 2], where R=Bu, Ph; n=0–2, were synthesised from isothiocyanato–phosphine nickel complexes and fully characterised spectroscopically. They are the first chlorosilyl nickel complexes with at least one alkyl (methyl) group at silicon atom.