Towards Nickel-NHC Fluoro Complexes-Synthesis of Imidazolium Fluorides and Their Reactions with Nickelocene.

While hundreds of complexes of the general formula [Ni(η5-C5H5)(NHC)(X)] exist (NHC = a N-heterocyclic carbene, X = Cl, Br, I), none is yet known with X = F. We attempted to prepare such a species by reacting nickelocene with imidazolium fluorides. Three imidazolium fluorides (ImH)+ F- [Im = (N,N'-bis-(R)-imidazolium: 1a, IMe, R = Me; 1b, IMes, R = 2,4,6-trimethylphenyl; 1c, IPr, R = 2,6-diisopropylphenyl)] were prepared and characterized by spectroscopic methods. In addition, the salts 1b [(IMesH)+ F-] and 1c [(IPrH)+ F-] were subjected single-crystal X-ray diffraction experiments. The reactions of these imidazolium fluorides with nickelocene did not lead to [Ni(η5-C5H5)(NHC)(F)] species. Instead, the reaction of 1a [(IMeH)+ F-] and 1b [(IMesH)+ F-] with nickelocene led to the salt 2 [Ni(η5-C5H5)(IMe)2]+ F- and to the square planar complex 3atrans-[NiF2(IMes)2] respectively. Both complexes were characterized spectroscopically and by single crystal X-ray diffraction. All four X-ray diffraction studies reveal hydrogen bonding and hydrogen interactions with the F atom or anion, and in some cases with solvent molecules of crystallization, and these phenomena are all discussed. Complex 2, in particular, exhibited a wide range of interesting H-bonded interactions in the solid state. Complexes 2 and 3a were tested as catalysts for Suzuki-Miyaura coupling but were not promising: complex 2 was inactive, and while 3a did indeed catalyze the reaction, it gave widely diverging results owing to its instability in solution.

Synthesis of Imidazolium Cations Linked to Para-t-Butylcalix[4]arene Frameworks and Their Use as Synthons for Nickel-NHC Complexes Tethered to Calix[4]arenes.

A series of cationic p-tert-butylcalix[4]arenes, with side-arms that are functionalized with imidazolium groups, have been synthesized in good yields. The parent tetrahydroxy para-t-butyl-calix[4]arene was dialkylated at the phenolic hydrogen atoms using α,ω-dibromo-alkanes to yield bis(mono-brominated) alkoxy-chains of variable length. The brominated side-arms in these compounds were then further alkylated with substituted imidazoles (N-methylimidazole, N-(2,4,6-trimethyl-phenyl)imidazole, or N-(2,6-di-isopropylphenyl)imidazole) to yield a series of dicationic calixarenes with two imidazolium groups tethered, via different numbers of methylene spacers (n = 2-4), to the calixarene moiety. Related tetracationic compounds, which contain four imidazolium units linked to the calix[4]arene backbone, were also prepared. In all of these compounds, the NMR data show that the calixarenes adopted a cone configuration. All molecules were characterized by NMR spectroscopy and by MS studies. Single crystal X-ray diffraction studies were attempted on many mono-crystals of these cations, but significant disorder problems, partly caused by occluded solvent in the lattice, and lack of crystallinity resulting from partial solvent loss, precluded the good resolution of most X-ray structures. Eventually, good structural data were obtained from an unusually disordered single crystal of 5a, (1,3)-Cone-5,11,17,23-tetra-t-butyl-25,27-di-hydroxy-26,28-di-[2-(N-2,6-diisopropylphenyl-imidazolium)ethoxy]calix[4]arene dibromide and its presumed structure was confirmed. The structure revealed the presence of H-bonded interactions and some evidence of π-stacking. Some of these imidazolium salts were reacted with nickelocene to form the nickel N-heterocyclic carbene (NHC) complexes 7a-7d. A bis-carbene nickel complex 8 was also isolated and its structure was established by single crystal X-ray diffraction studies. The structure was disordered and not of high quality, but the structural data corroborated the spectroscopic data.

Hydroboration of Alkenes Catalysed by a Nickel N-Heterocyclic Carbene Complex: Reaction and Mechanistic Aspects.

The pentamethylcyclopentadienyl N-heterocyclic carbene nickel complex [Ni(η5 -C5 Me5 )Cl(IMes)] (IMes=1,3-dimesitylimidazol-2-ylidene) efficiently catalyses the anti-Markovnikov hydroboration of alkenes with catecholborane in the presence of a catalytic amount of potassium tert-butoxide, and joins the very exclusive club of nickel catalysts for this important transformation. Interestingly, the regioselectivity can be reversed in some cases by using pinacolborane instead of catecholborane. Mechanistic investigations involving control experiments, 1 H and 11 B NMR spectroscopy, cyclic voltammetry, piezometric measurements and DFT calculations suggest an initial reduction of the NiII precursor to a NiI active species with the concomitant release of H2 . The crucial role of the alkoxo-catecholato-borohydride species resulting from the reaction of potassium tert-butoxide with catecholborane in the formation of an intermediate nickel-hydride species that would then be reduced to the NiI active species, is highlighted.

(NHC-olefin)-nickel(0) nanoparticles as catalysts for the (Z)-selective semi-hydrogenation of alkynes and ynamides.

Nickel(0) nanoparticles coordinated to NHC ligands bearing N-coordinated cinnamyl moieties were readily prepared by reduction of a [NiCpBr(NHC-cinnamyl)] complex with methyl magnesium bromide. The combination of a strong σ-donor NHC ligand with a π-coordinating appended cinnamyl moiety likely prevents nickel(0) nanoparticle aggregation to larger inactive species, and allows the effective and (Z)-selective semi-hydrogenation of alkynes and ynamides.

C-H activation of acetonitrile at nickel: ligand flip and conversion of N-bound acetonitrile into a C-bound cyanomethyl ligand.

Nickel joins the fairly exclusive list of metals that can activate nitrile C-H bonds. We report the first example of the C-H activation of an acetonitrile ligand on a nickel center. The acetonitrile ligand formally loses a proton and undergoes a sharp flip to give a cyanomethyl ligand that is coordinated to the nickel atom. Structures of an initial N-bound acetonitrile-nickel complex and of a final cyanomethyl-nickel complex are both presented.

Synthesis, characterization, and catalytic application in aldehyde hydrosilylation of half-sandwich nickel complexes bearing (κ1-C)- and hemilabile (κ2-C,S)-thioether-functionalised NHC ligands.

Neutral nickel-N-heterocyclic carbene complexes, (κ1-C)-[NiCpBr{R-NHC-(CH2)2SR'}] [Cp = η5-C5H5; R-NHC-(CH2)2SR' = 1-mesityl-3-[2-(tert-butylthio)ethyl]- (1a), 1-mesityl-3-[2-(phenylthio)ethyl]- (1b), 1-benzyl-3-[2-(tert-butylthio)ethyl]- (1c), 1-benzyl-3-[2-(phenylthio)ethyl]-imidazol-2-ylidene (1d)], which bear a N-bound thioether side arm, were prepared by the reaction of nickelocene with the corresponding imidazolium bromides [R-NHC-(CH2)2SR'·HBr] (a-d), via conventional or microwave heating. The 1H NMR spectra of the benzyl-substituted species 1c and 1d showed signals for diastereotopic NCH2CH2S protons at room temperature. However, structural studies established the absence of coordination of the sulphur atom in the solid state, and solvent DFT calculations showed that bromide displacement by sulphur is an unfavourable process (ΔG = +13.5 kcal mol-1 for 1d), thereby suggesting that the observed disatereotopicity is more likely due to significant steric congestion rather than to a possible C,S-chelation in solution. Treatment of these complexes with KPF6 in tetrahydrofuran (THF) led to bromide abstraction to afford the cationic complexes [NiCp{R-NHC-(CH2)2SR'}](PF6) (2a-c). Alternatively, 2a-c could also be prepared by the direct reaction of nickelocene with the corresponding imidazolium hexafluorophosphate salts [R-NHC-(CH2)2SR'·HPF6]. Inversely to the neutral species, whereas X-ray crystallography established C,S-chelation in the solid state, the 1H NMR spectra (CDCl3, CD2Cl2, or thf-d8) at room temperature showed no diastereotopic NCH2CH2S protons, thus suggesting the possible displacement of the sulphur atom by the respective solvents and/or very fast sulphur inversion. DFT calculations established a low energy inversion process in all cases (+9 ≤ΔG‡≤ +13 kcal mol-1) as well as a favourable solvent coordination process (ΔG‡≈ +11 kcal mol-1; ΔG≈-7 kcal mol-1) with a solvent such as THF, thus suggesting that sulphur inversion and/or solvent coordination can both account for the absence of diastereotopy at room temperature, depending on the solvent. While all complexes catalysed the hydrosilylation of benzaldehyde in the absence of any additive, the cationic C,S-chelated complexes 2 proved more active than the sterically constrained neutral species 1. In particular, 2c proved to be the most active pre-catalyst and its catalytic charge could be lowered down to 2 mol% with PhSiH3 as the hydrogen source.

Displacement of η5-cyclopentadienyl ligands from half-sandwich C,C-(NHC-cyanoalkyl)-nickel(ii) metallacycles: further insight into the structure of the resulting Cp-free nickelacycles and a catalytic activity study.

Four cationic C,C-(NHC-cyanoalkyl)-nickel(ii) metallacyclic complexes, [Ni{Me-NHC-CH2CH(CN)}(NCMe)](PF6) (2a), [Ni{Mes-NHC-CH2CH(CN)}(NCMe)](PF6) (2b), [Ni{Mes-NHC-(CH2)2CH(CN)}(NCMe)](PF6) (2c) and [Ni{DiPP-NHC-(CH2)2CH(CN)}(NCMe)](PF6) (2d), were prepared by the removal of the Cp ligand under acidic conditions at 0 °C from the corresponding half-sandwich nickelacycles [NiCp{R-NHC-(CH2)nCH(CN)}] (1a-1d; Cp = η5-C5H5; n = 1 or 2; R-NHC-(CH2)nCH(CN) = 1-R-3-[(CH2)nCH(CN)]-imidazol-2-ylidene). Full characterization of 2a-d by 1H and 13C{1H} NMR spectroscopy in CD3CN and pyridine-d5, ATR-FTIR spectroscopy, mass spectrometry, and CHN microanalyses established the presence of only one acetonitrile ligand per nickel atom in the solid state. A DFT structural study conducted on the cations of the methyl-substituted 5-membered nickelacycle 2a and the mesityl-substituted 6-membered cycle 2c found a small energetic cost (ΔG = 7-12 kcal mol-1) for the loss of one acetonitrile ligand from the square-planar structures existing in solution, that should be easily amenable upon solvent evaporation (ΔG‡ = 14 kcal mol-1 in the case of 2c). Two structures with one acetonitrile ligand could be optimized in both cases: (i) a truly T-shaped 14-electron structure with an end-on acetonitrile ligand, and (ii) a masked T-shaped structure stabilized by the π-coordination of the dangling CN group of the metallated alkyl chain, the latter being favoured by 2.4 kcal mol-1 in the case of the flexible 6-membered ring 2c. A comparison of calculated vibrational frequencies with experimental FTIR spectra ruled out π-coordination of the dangling CN group as a ν(C[triple bond, length as m-dash]N) band at low frequency was absent. Complexes 2a-d thus probably exist as rare three-coordinate T-shaped 14-electron species in the solid state. Their catalytic activity was studied for the direct arylation of azoles, and 2c proved to be moderately active for the coupling of benzothiazole with aryl iodides. Mechanistic insights suggest that competing processes or a radical process catalysed by nickel particles could follow an initial reduction of 2c by the dimerization of a sacrificial amount of benzothiazole.

From acetone metalation to the catalytic α-arylation of acyclic ketones with NHC-nickel(II) complexes.

Air-stable N-heterocyclic carbene-nickel(ii) complexes at concentrations as low as 1 mol% exhibit high catalytic activity for the α-arylation of acyclic ketones and join a highly restricted list of nickel catalysts for this key reaction. Mechanistic investigations suggest a radical pathway.

One-step synthesis of a highly homogeneous SBA-NHC hybrid material: en route to single-site NHC-metal heterogeneous catalysts with high loadings.

The one-step synthesis of a mesoporous silica of SBA type, functionalized with a 1-(2,6-diisopropylphenyl)-3-propyl-imidazolium (iPr2Ar-NHC-propyl) cation located in the pore channels, is described. This material was obtained by the direct hydrolysis and co-condensation of tetraethylorthosilicate (TEOS) and 1-(2,6-diisopropylphenyl)-3-[3-(triethoxysilyl)propyl]-imidazolium iodide in the presence of Pluronic P123 as a non-ionic structure-directing agent and aqueous HCl (37%) as an acid catalyst. Small-angle X-ray diffraction measurements, scanning and transmission electron microscopies, as well as dinitrogen sorption analyses revealed that the synthesized material is highly mesoporous with a 2D hexagonal arrangement of the porous network. (13)C and (29)Si CP-MAS NMR spectroscopy confirmed that the material contains intact iPr2Ar-NHC-propyl cations, which are covalently anchored via silicon atoms fused into the silica matrix. Moreover, comparison of the latter data with those of an analogous post-synthetic grafted SBA-NHC material allowed us to establish that, as expected, (i) it is most probably more homogeneous and (ii) it shows a more robust anchoring of the organic units. Finally, elemental mapping by energy dispersive X-ray spectroscopy in the scanning electron microscope demonstrated a very homogeneous distribution of the imidazolium units within the one-pot material, moreover with a high content. This study thus demonstrates that a relatively bulky and hydrophilic imidazolium unit can be directly co-condensed with TEOS in the presence of a structure-directing agent to provide in a single step a highly ordered and homogeneous mesoporous hybrid SBA-NHC material, possessing a significant number of cationic NHC sites.

Facile displacement of η5-cyclopentadienyl ligands from half-sandwich alkyl,NHC-nickel complexes: an original route to robust cis-C,C-nickel square planar complexes.

The η(5)-cyclopentadienyl (Cp) ligands of 18-electron half-sandwich alkyl,NHC-nickel complexes are readily displaced under acidic conditions to afford a novel class of cis-C,C-nickel square planar complexes. Remarkably, the nickel-alkyl and nickel-carbene bonds are not ruptured in these unprecedented Cp acidolysis reactions.

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 of mono-, di- and tetra-alkyne functionalized calix[4]arenes: reactions of these multipodal ligands with dicobalt octacarbonyl to give complexes which contain up to eight cobalt atoms.

The functionalized mono-alkyne cone-monopropargyl p-tert-butylcalix[4]arene was synthesized by the reaction of p-tert-butylcalix[4]arene with K(2)CO(3) and 3-bromo-1-propyne. More prolonged reaction times led to the formation of the 1,3 cone bis(propargyl)calix[4]arene . The tetra-alkyne species cone-tetrapropargyl p-tert-butylcalix[4]arene and its conformational isomer, 1,3-alternate-tetrapropargylcalix[4]arene may both be prepared via related reaction sequences. The structures of the molecules and were both re-determined by single crystal X-ray diffraction studies. All four functionalized calixarenes react rapidly with dicobalt octacarbonyl to give [(calix[4]arene).{Co(2)(CO)(6)}(n)] species (n = 1, 2 or 4) in which the alkyne functionalities of the propargylcalix[4]arenes are ligated to one or more [Co(2)(CO)(6)] groups. Two products could be harvested from the reaction of [Co(2)(CO)(8)] with the bis-propargyl-calixarene , depending on the reaction conditions and relative molar quantities of the reactants: complex , [1,3-cone bis(propargyl)calix[4]arene.{Co(2)(CO)(6)}(2)], in which each alkyne group is bonded to a [Co(2)(CO)(6)] group, and complex , [1,3-cone bis(propargyl)calix[4]arene.{Co(2)(CO)(6)}], which contains a unligated alkyne and an alkyne group bonded to a Co(2)(CO)(6) unit. The structures of the tetracobalt and octacobalt complexes and were established by single crystal X-ray diffraction studies.

Unsaturated dinickel-molybdenum clusters with N-heterocyclic carbene ligands.

The first examples of mixed metal trinuclear clusters carrying N-heterocyclic carbene (NHC) ligands were isolated from reactions of the complexes [Ni(NHC)ClCp] [NHC = bis-(2,6-diisopropylphenyl)- or bis-(2,4,6-trimethylphenyl)-imidazol-2-ylidene] with [Mo(CO)(3)Cp](-); the unsaturated 46-electron clusters have triangular MoNi(2) cores and the reaction pathway activates usually inert Ni-Cp and Ni-NHC bonds.

Structure-reactivity relationships in SHOP-type complexes: tunable catalysts for the oligomerisation and polymerisation of ethylene.

For over 30 years complexes with the general formula [NiPh(P,O)L] (L = tertiary phosphine; P,O = chelating phosphanylenolato ligand) have been used as highly efficient oligomerisation catalysts suitable for the production of linear alpha-olefins. The same complexes, which are usually referred to as SHOP-type catalysts (SHOP = Shell Higher Olefin Process) can also be used as ethylene polymerisation catalysts, provided they are treated with a phosphine scavenger that selectively removes the tertiary phosphine ligand (L). This Perspective examines the impact of various parameters (influence of the substituents, backbone size, solvent, use of co-catalysts, etc.) on the catalytic outcome of the complexes. Overall, this review shows that the selectivity and activity of the catalyst may be tuned efficiently through directed modification of the P,O chelator.