Michael J. Chetcuti

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.

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.

Cyclopentadienyl N-heterocyclic carbene-nickel complexes as efficient pre-catalysts for the hydrosilylation of imines†

The in situ generated nickel hydride complex, [Ni(Mes2NHC)HCp], and its cationic analogue, [Ni(Mes2NHC)(NCMe)Cp](PF6), are efficient and chemoselective pre-catalysts for the hydrosilylation of both aldimines and ketimines under mild conditions.

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.

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.

LIGAND REACTIONS WITH UNSATURATED NICKEL – GROUP 6 COMPLEXES

The complexes [(η5–C5Me5)Ni–M(CO)3(η5–C5H5)] (Ni–M, M=Mo, W) can be considered to contain Ni=Mo or Ni=W double bonds and may be represented as [(η5–C5Me5)Ni=M(µ–CO)(CO)2(η5–C5H5)] (Ni=M, M=Mo, W). These species, and their closely related group 6 methylcyclopentadienyl congeners [(η5–C5Me5)Ni=M(µ–CO)(CO)2(η5–C5H4Me] (Ni=M, M=Mo, W), have a rich chemistry that stems from their unsaturation. This review relates their discovery, and summarizes the chemistry of these complexes with simple two-electron donor ligands, with alkynes, and with alkenes.

Bimetallic-induced tail-to-tail dimerization and C–H activation of methyl acrylate

An organometallic complex resulting from tail-to-tail dimerization and C-H activation of methyl acrylate (MA), [Mo(CO2Cp(eta 3-(MeO2C)CH[symbol: see text]CH[symbol: see text]CHCH2(CO2Me)] 2, has been fully characterized from the reaction of the heterobimetallic complex [Cp*Ni=Mo(mu-CO)(CO)2Cp] with MA and an exclusively eta 3-allyl bonding mode of the coupled ligand was established for the first time by X-ray diffraction; formation of 2 is accompanied by that of the mu 3-alkylidyne-capped cluster [NiMo2(mu 3-CCH2CO2Me)(CO)4Cp*Cp2] 3 which results from a double C-H activation of the CH2 group of MA; none of these reactions occur with the corresponding homodinuclear complexes.