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International audience; Fast (60 kHz) magic angle spinning solid-state NMR allows very sensitive proton detection in highly paramagnetic organometallic powders. We showcase this technique with the complete assignment of 1 H and 13 C resonances in a high-spin Fe(II) polymerisation catalyst with less than 2 mg of sample at natural abundance.

International audience; Brought to life more than half a century ago and successfully applied for high-value petrochemical intermediates production, nickel-catalyzed olefin oligomerization is still a very dynamic topic, with many fundamental questions to address and industrial challenges to overcome. The unique and versatile reactivity of nickel enables the oligomerization of ethylene, propylene and butenes into a wide range of oligomers that are highly sought-after in numerous fields to be controlled. Interestingly, both homogeneous and heterogeneous nickel catalysts have been scrutinized and employed to do this. This rare specificity encouraged us to interlink them in this review so as to open up opportunities for further catalyst development and innovation. An in-depth understanding of the reaction mechanisms in play is essential to being able to fine-tune the selectivity and achieve efficiency in the rational design of novel catalytic systems. This review thus provides a complete overview of the subject, compiling the main fundamental/industrial milestones and remaining challenges facing homogeneous/heterogeneous approaches as well as emerging catalytic concepts, with a focus on the last 10 years.

International audience; The reactivity of a phenoxy-imine-ether system (FI)TiCl3/MAO was studied toward selective ethylene trimerization. This system was shown to either trimerize or polymerize ethylene depending on the reaction temperature. Its selectivity switches from a significant production of the trimerization product, 1-hexene (85 wt %, 520-450 kg1-hexene gTi-1 h-1) between 30 and 40 °C, to a moderate polyethylene formation (70-80 wt %, 60-70 kgpolyethylene gTi-1 h-1) at higher reaction temperature (T > 60 °C). Polymerization was investigated based on an original "polymer-to-catalyst" strategy aiming at identifying the active species responsible for this side reaction. Using DSC, SEC and high temperature 13 C NMR analyses, polyethylenes were found to exhibit high molar masses (> 10 5 g mol-1) and a low 1-hexene content (< 1 mol %) at any temperature. Kinetic studies support that trimerization and polymerization species are generated from the catalyst precursor at 40 °C but a parallel process may occur at higher temperature. The increase dispersity to 4.6 at 80 °C suggests a change from single to multisite catalysis. The poor comonomer incorporation ability of the active species is reminiscent of a molecular Ziegler Natta or a bulky postmetallocene catalyst.

International audience; The reduction of [Ni(DME)Cl 2 ] with 2 equiv. of bis(trimethylsilyl)-1,4-tetramethyldihydropyrazine in presence of a ligand L and an excess of olefin cleanly leads to [Ni(L)(alkene) 2 ] complexes. When reduction is done in presence of 1,5-cyclooctadiene (COD), [Ni(COD) 2 ] is obtained. Such approach also allows access to the Ni I dimer [Ni(bis(dicyclohexylphosphino)propane)Cl] 2. Low valent, ligand stabilized (phosphines, N-heterocyclic carbenes = NHC) Ni(0) complexes are important species, being catalytically relevant in many transformations, such as cross coupling processes. 1 Among the known Ni(0) complexes, stable [Ni(L)(alkene) 2 ] or [Ni(L)(bis-alkene)] species (L=phosphines, NHC) are rare because of a lack of efficient, versatile synthetic route or availability of the appropriate precursors. 2 Indeed, [Ni(phosphine)(C 2 H 4) 2 ] have been reported by displacement of the poly-ene ligand from the extremely sensitive [Ni(CDT)] precursor (CDT = 1,5,9-cyclododecatriene), whereas other [Ni(L)(bis-alkene)] species have been prepared from air, light and heat sensitive [Ni(COD) 2 ]. 3-8 In turn, both Ni(0) complexes [Ni(CDT)] and [Ni(COD) 2 ] require the use of pyrophoric alkylaluminium derivatives to be synthesized. It is thus desirable to devise a rational, experimentally simpler, versatile method to access families of Ni(0) species. Recently, bis(trimethylsilyl)-1,4-tetramethyl-dihydropyrazine 1 has been shown to behave as a one or two-electron reducing agent for numerous transition metal complexes. 9,10,11 It was also used to reduce several late transition metal halide precursors to the corresponding metallic nanoparticles in the absence of stabilizing ligand. 12 In the case of Ni, nanoparticles active in carbon-carbon cross coupling process were obtained. We have hypothesized that if the in situ generated reduced Ni center could be efficiently trapped by ligands the aforementioned desirable Ni(0) complexes would be readily accessible. In the present work, we show that from the simple, commercially available, [Ni(DME)Cl 2 ] precursor (DME = dimethoxy-ethane), several Ni(0) complexes could be synthesized in one pot. We also report that with strongly donating bis-phosphine ligands, the reaction leads to Ni(I) complexes instead. Our work started with the synthesis of the emblematic and sensitive [Ni(COD) 2 ] complex. To date it is synthesized by reduction of Ni(acac) 2 with alkylaluminum compounds such as triethylaluminium or DIBAL-H in presence of COD and butadiene at low temperature, followed by low temperature crystallization and filtration to eliminate aluminium side products. 13 The room temperature reduction of [Ni(DME)Cl 2 ] with two equivalents of 1 was carried out in a first stage in THF with the presence of a large excess (20 equiv. of COD). Formation of black precipitate (likely Ni metal) pointed to a fast reduction compared to coordination by COD. The low temperature reduction (-78°C) was attempted to prevent Ni(0) precipitate formation, but the reaction does not proceed at this temperature within a day. The optimized reduction temperature was found to be at-20°C. In this case, we were able to synthesize [Ni(COD) 2 ] as crystalline material in 54% isolated yield, fully characterized by 1 H and 13 C NMR (Scheme 1). The moderate yield can be explained by the remaining formation of nickel black particles during the reduction, which can nevertheless be readily eliminated by filtration prior to crystallization of [Ni(COD)2] from cold toluene. Scheme 1 Synthesis of [Ni(COD) 2 ] by organic reduction of Ni(II) halide Beyond the synthesis of [Ni(COD) 2 ] itself, the reaction proved our hypothesis right, clearly indicating that monometallic Ni(0) can be

Sulfonamido-phosphorus and aminophosphine ligands self-assemble to readily form active and stable nickel catalysts that are highly selective for the dimerization of ethylene to 1-butene. The self-assembled allyl-nickel complexes are zwitterionic and are stabilized by hydrogen bond interactions between the two ligands. These organometallic cis-diphosphine complexes rearrange under an ethylene atmosphere to give trans-diphosphine catalysts, with one monoanionic P,O METAMORPhos ligand and an aminophosphine.

International audience; The new nickel(II) complexes containing NHC ligands N-substituted by a CH2CH2OR ether group (R = Me or Ph) [NiCl2{ImMes(C2OMe)}2] (6), [NiCl2{Imn-Bu(C2OMe)}2] (7), [NiBr2{ImDiPP(C2OMe)}2] (8), [NiBr2{ImMes(C2OMe)}2] (9), [NiBr2{Imn-Bu(C2OMe)}2] (10), NiBr2{ImMes(C2OPh)}2] (18), [NiI2{ImDiPP(C2OMe)}2] (21), [NiI2{ImMes(C2OMe)}2] (22) and [NiI2{Imn-Bu(C2OMe)}2] (23) were synthesized in good yields and fully characterized by NMR spectroscopy and X-ray diffraction analysis. The reaction conditions were optimized and further applied to thioether or non-functionalized NHC ligands, affording [NiBr2{ImDiPP(C2SPh)}2] (19) and [NiBr2{ImDiPP(n-Bu)}2] (20), respectively. Equilibriainvolving syn/anti isomers were unveiled for complexes [NiCl2{ImDiPP(C2OMe)}2] (5), 6−10 and 18−23. Reactions of 6 and 20 with an halide abstractor afforded the dicationic aquo complexes cis-[Ni{ImMes(C2OMe)}2(H2O)2][PF6]2 (27) and cis-[Ni{ImDiPP(n-Bu)}2(H2O)2][PF6]2 (28), in which a cis arrangement of the carbene ligands is evidenced, which contrasts with that in their precursors. These molecules represent rare examples of nickel aquo NHC complexes and of complexes with two cis monodentate NHC ligands. The new complexes reported in this work (15 crystal structures) displayed moderate activities as pre-catalysts forethylene oligomerization and favored dimerization.

International audience; Multinuclear aluminum cocatalysts have been obtained by the reaction of various phenols, alcohols or diois with trimethylaluminum and were used in situ or as isolated, welldefined species, for the activation of an iron(ll) or an iron(lll) precatalyst for the oligomerization of ethylene. The best cocatalyst candidate involves 2,2'-biphenol (10) in a 10/AIMe3 ratio of 2/3.

The new alcohol- and ether-functionalised-NHC silver(I) complexes bis(1-(2,6-diisopropylphenyl)-3-(2-hydroxyethyl)-1H-imidazol-2(3H)-ylidene)silver(I) chloride, [Ag{ImDiPP(C2OH)}2]Cl (4), bis(1-(2-hydroxyethyl)-3-mesityl-1H-imidazol-2(3H)-ylidene)silver(I) chloride, [Ag{ImMes(C2OH)}2]Cl (5), bis(1-(2-hydroxyethyl)-3-methyl-1H-imidazol-2(3H)-ylidene)silver(I) chloride, [Ag{ImMe(C2OH)}2]Cl (6), bis(1-(2,6-diisopropylphenyl)-3-(2-hydroxyethyl)-1H-imidazol-2(3H)-ylidene)silver(I) tetrafluoroborate, [Ag{ImDiPP(C2OH)}2]BF4 (9), and bis(1-(2,6-diisopropylphenyl)-3-(2-methoxyethyl)-1H-imidazol-2(3H)-ylidene)silver(I) chloride, [Ag{ImDiPP(C2OMe)}2]Cl (13), were synthesized and fully characterized by NMR spectroscopy and single crystal X-ray diffraction. For some complexes, an uncommon heteronuclear coupling (4)J((107/109)Ag-H) was unveiled. Their ability to transfer the NHC ligand to Ni(II) was assessed in the presence of different nickel(II) sources; the bis-NHC Ni(II) complex bis(1-(2,6-diisopropylphenyl)-3-(2-methoxyethyl)-1H-imidazol-2(3H)-ylidene)nickel(II) chloride, [NiCl2{ImDiPP(C2OMe)}2] (15), was obtained from 13 and shown by X-ray diffraction study to have a trans-arrangement of the two NHC ligands. However, in contrast to other Ag(I) NHC complexes the transmetallation reaction failed with the hydroxyl-functionalised silver complexes, possibly due to the acidity of the alcohol OH function, leading overall to reprotonation of the C(NHC) and isolation of the corresponding imidazolium salts.

International audience; Since the discovery that bis(imino)pyridine ligands are able to confer high activities in ethylene oligomerization and polymerization to their iron complexes, considerable attention has been focused on catalyst design for these reactions and this research constitutes an ever-growing area in molecular catalysis. The tuning of the ligand structures and properties, and thus of catalysts, generally represents the basis for subsequent work contributing to process development and industrialization. Significant effort is therefore devoted to generate structural diversity in order to access the required catalyst stability and selectivity. This feature article outlines nitrogen-containing ligands that have been developed for the iron-catalyzed oligomerization of ethylene since the seminal discovery of the properties of bis(imino)pyridine ligands.

International audience; The new molybdenum imido alkylidene complex Mo(N(2,6-iPr2C6H3))(CHCMe2Ph)(NC4H2Me2)(OB(Mes)2) (1;Mes = 2,4,6-MePh) containing both boroxide and pyrrolide ligands is reported. Its formation results from the reaction between bis(mesityl)borinic acid ((Mes)2BOH) and the bis-pyrrolide Schrock-type precursor Mo(N-2,6-iPr2C6H3)(CHCMe2Ph)(NC4H2Me2)2. The complex was fully characterized by 1H, 13C, 11B, and 95Mo NMR spectroscopy, X-ray diffraction, and elemental analysis. Complex 1 proved to be active for homometathesis reactions of 1- and 2-octene at 0.1 mol % loading. The synthesis of mixed pyrrolide boroxide imido molybdenum alkylidene complexes was extended to other borinic acids. The catalytic activity of these new complexes was evaluated in the homometathesis of linear olefins.