Chiral nickel(II) and palladium(II) catalysts bearing strong electron-withdrawing fluorine groups: synthesis, characterization and their application in catalytic polymerization for ethylene and styrene

A series of new chiral and achiral nickel(II) and palladium(II) complexes, {bis[N,N-(2,6-diethyl-4-naphthylphenyl)imino]-1,2-dimethylethane}dibromonickel 3a, {bis[N,N-(4-fluoro-2-methyl-6-sec-phenethylphenyl)imino]-1,2-dimethylethane}dibromonickel rac-(RS)-3b, {bis[N,N-(4-fluoro-6-sec-phenethylphenyl)imino]-1,2-dimethylethane}dibromonickel rac-(RR/SS)-3c and {bis[N,N-(4-fluoro-6-sec-phenethylphenyl)imino]-1,2-dimethylethane}dichloropalladium rac-(RR/SS)-3d were successfully synthesized and characterized. The molecular structures of representative ligand rac-(RS)-2b, nickel complex 3a, rac-(RR/SS)-3c and palladium complex rac-(RR/SS)-3d were determined by X-ray crystallography. The structures of complexes 3a and rac-(RR/SS)-3c have pseudo-tetrahedral geometry about the nickel center, showing C2 molecular symmetry. However, the structure of palladium complex rac-(RR/SS)-3d has pseudo-square planar geometry about the palladium center, showing C2 molecular symmetry. Complex 3e {bis[N,N-(2,6-dimethylphenyl)imino]-1,2-dimethylethane}dibromonickel was also synthesized for comparison. Nickel complex rac-(RS)-3b bearing strong electron-withdrawing fluorine group in the para-aryl position and a chiral sec-phenethyl group in the ortho-aryl position of the ligand (one methyl group in the ortho-aryl position) displays the highest catalytic activity for ethylene and styrene polymerization, and produced highly branched polyethylene and syndiotactic-rich polystyrene. However, palladium complex rac-(RR/SS)-3d shows low catalytic activity for ethylene and styrene polymerization due to the poor leaving group, Cl, attached to palladium and the unfavorable molecular structure. Copyright (c) 2014 John Wiley & Sons, Ltd.