Chiral {bis[N,N′-(2-ethyl-6-methylphenyl)imino]-1,2-dimethylethane}dibromonickel bearing sec-phenethyl groups: Synthesis, characterization and application in synthesis of nanosized dendritic polyethylene

Abstract

A series of {bis[N,N’N′-(2-ethyl-6-methylphenyl)imino]-1,2-dimethylethane}dibromonickels have been synthesized and characterized. The crystal structures of representative chiral ligand, bis[N,N’N′-(2-ethyl-6-methyl-4-s-phenethylphenyl)imino]-1,2-dimethylethane (S,R)–2a and its chiral complex, {bis[N,N’N′-(2-ethyl-6-methyl-4-sec-phenethylphenyl)imino]-1,2-dimethylethane}dibromonickel (S,R)–3a, have been confirmed by X-ray crystallographic analysis. The asymmetric unit of chiral (S,R)–2a contains one half-molecule which exhibits a crystallographically imposed center of symmetry. The single C-–C bond in 1,4-diazabutadiene fragment is trans-configured and situated on an inversion center. However, the trans-configured ligand can be transformed into cis-configured ligand in order to facilitate the formation of chiral complex (S,R)–3a. Complex (S,R)–3a has pseudo-tetrahedral geometry about the nickel center. Complex (S,R)–3a bearing one ethyl and one methyl in the ortho-aryl position and an electron-donating sec-phenethyl groups in the para-aryl position, activated by diethylaluminum chloride (DEAC), shows highly catalytic activity for the polymerization of ethylene [3.22×106 106g gPE/(mol Ni.h.bar)]. Interestingly, complex 3b bearing one ethyl and one methyl groups in the ortho-aryl position and a strong electron-withdrawing Cl substituent group in the para-aryl position produced the more dendritic polyethylenes (branching degree: 92, 114 and 139 branches/1000 C at 20, 40 and 60 60(°C, respectively) under low ethylene pressure. The dendritic polyethylene particle size can be controlled in the 1–20nm range, and could be expected to become the nano-targeted drug carrier after modified with water-soluble oligo(ethylene glycol) (OEG).