Reaction of Phenyl Iso(thio)cyanate with N-Heterocyclic Carbene-Supported Nickel Complexes: Formation of Nickelacycles

Diphosphorus Release and Heterocumulene Oligomerisation by Nickel Complexes

The generation of diphosphorus molecules P2 under mild conditions in solution is a useful strategy to generate diphosphines via [4+2] cycloadditions. We recently described the release of P2 units from the nickel butterfly complex [{(IMes)Ni(CO)}2(μ2,η2:η2-P2)] (IMes=1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) upon addition of CO gas. Herein, we developed an alternative protocol for the same process using heterocumulenes. In addition to formation of P4 (the dimerisation product of P2), the reactions afford nickel complexes of novel pincer-type ligands. Aryl isothiocyanates undergo a trimerisation within the coordination sphere of nickel and afford square planar nickel complexes with S-C-S pincer-ligand frameworks. Carbon disulfide coordinates to the [(IMes)Ni]-fragment in an η2-fashion, affording a dinuclear complex. Similar products are formed when the N-heterocyclic carbene nickel(0) complex [(IMes)Ni(vtms)2] is used as a precursor (vtms=vinyltrimethylsilane).

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

Reversible homolytic activation of water via metal-ligand cooperativity in a T-shaped Ni(ii) complex

A T-shaped Ni(ii) complex [Tol,PhDHPy]Ni has been prepared and characterized. EPR spectra and DFT calculations of this complex suggest that the electronic structure is best described as a high-spin Ni(ii) center antiferromagnetically coupled with a ligand-based radical. This complex reacts with water at room temperature to generate the dimeric complex [Tol,PhDHPy]Ni(μ-OH)Ni[Tol,PhDHPyH] which has been thoroughly characterized by SXRD, NMR, IR and deuterium-labeling experiments. Addition of simple ligands such as phosphines or pyridine displaces water and demonstrates the reversibility of water activation in this system. The water activation step has been examined by kinetic studies and DFT calculations which suggest an unusual homolytic reaction via a bimetallic mechanism. The ΔH ‡, ΔS ‡ and KIE (k H/k D) of the reaction are 5.5 kcal mol-1, -23.8 cal mol-1 K-1, and 2.4(1), respectively. In addition to the reversibility of water addition, this system is capable of activating water towards net O-atom transfer to substrates such as aromatic C-H bonds and phosphines. This reactivity is facilitated by the ability of the dihydrazonopyrrole ligand to accept H-atoms and illustrates the utility of metal ligand cooperation in activating O-H bonds with high bond dissociation energies.

Reactivities of zero-valent group 10 complexes toward organic isocyanates: synthesis of metallacycles containing dimeric isocyanate units, isocyanate cyclotrimerization, and computational chemistry

Five-membered metallacycles involving dimeric isocyanate units converted into cyclic tetramers and isocyanurates, depending on the central metal or organic isocyanates, and their synthetic and theoretical studies were carried out.

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.

Complexes of Ni(i): a “rare” oxidation state of growing importance

The synthesis and diverse structures, reactivity (small molecule activation and catalysis) and magnetic properties of Ni(i) complexes are summarized.