Heteroleptic Ni(?) Complexes Containing A Sterically Demanding N-heterocyclic Carbene:Synthesis,Characterization And Catalysis

A series of novel heteroleptic Ni(?) complexes containing a sterically demanding N-heterocyclic carbene(NHC)have been synthesized and fully characterized.The catalytic behaviors of these Ni(?) complexes in the reductive cross-couplings of aryl chlorides and naphthyl methyl ethers with benzyl chlorides have also been studied.1.Heteroleptic Ni(?) complexes of the general formular Ni(NHC)(PPh3)X2(1-5)were easily prepared via the reaction of Ni(PPh3)2X2 with one equivalent of a sterically demanding NHC.Complexes 6-8 were synthesized by the reaction of complexes 1,3-5 and Ni(SIMes)[P(OEt)3]Br2 with one equivalent of tricyclohexylphosphine.All of Ni(?) complexes obtained are air-and moisture-stable solid powders and have been characterized by element analysis and NMR spectra.Complexes 1 and 8 have also been characterized by X-ray crystallography.2.Catalytic performances of complexes 1-3 have been studied in the reductive cross-coupling of aryl chlorides with benzyl chlorides,wherein the significant synergic effect between NHCs and triphenylphosphine ligands was observed.Moreover,it is found that an increasing in the steric hindrance of a NHC plays a significant role in catalysis,and complex 1 is the most active Ni(?)-based catalyst in this type of reaction.In addition,the preliminary investigation indicates that the introduction of the oxygen atom in the NHC ligand is expected to further increase the catalytic activity of this kind of Ni(?) complexes.3.The catalytic activity of Ni(?) complexes 6-8 in the reductive cross-coupling of naphthyl methyl ether with benzyl chlorides has been investigated.It is found that the introduction of a tricyclohexylphosphine ligand is of benefit to the carbon-oxygen bond activation,whereas a sterically demanding NHC is detrimental to this type of reaction.Complex 8 and Ni(I'Bu)(PCy3)Br2 can be used as efficient catalysts in the reductive cross-coupling of naphthyl methyl ethers with benzyl chlorides.