Simplified and versatile access to low valent Ni complexes by metal-free reduction of Ni II precursors

Abstract : 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
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Emile Moser, Erwann Jeanneau, Nicolas Mézailles, Hélène Olivier-Bourbigou, Pierre-Alain Breuil. Simplified and versatile access to low valent Ni complexes by metal-free reduction of Ni II precursors. Dalton Transactions, Royal Society of Chemistry, 2019, 48 (13), pp.4101-4104. ⟨10.1039/c9dt00668k⟩. ⟨hal-02122151⟩

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