Structural analysis of imino- and amino-pyridine ligands for Ni(II):Precatalysts for the polymerization of ethylene

Abstract Imino- and amino-pyridine ligands are versatile scaffolds to probe how ligand sterics influence polymerization outcomes for nickel(II) ethylene polymerization catalysts. Six imino- and one amino-pyridine nickel(II) complexes with varied imino/amino-carbon substitution (R 1  = methyl, ethyl, isopropyl or tert -butyl) were synthesized ( 1 - 7 ). Five of these complexes were analyzed crystallographically: dimeric {[2-[(2,6-Me 2 -C 6 H 3 )N=C(R 1 )]C 5 H 4 N]NiBr 2 } 2 (R 1  = Et 2 ; R 1  =  t -Bu 4 ), monomeric [2-[(2,6-Me 2 -C 6 H 3 )N=C(R 1 )]C 5 H 4 N]NiBr 2 NCCH 3 (R 1  = Me 5 ; R 1  =  t -Bu 6 ), and monomeric [2-[(2,6-Me 2 -C 6 H 3 )NHCH( t -Bu)]C 5 H 4 N]NiBr 2 ( 7 ). Complexes 1 - 7 are competent ethylene polymerization precatalysts at 0 ° C, and tert -butyl substituted imino-pyridine precatalysts 4 and 6 and amino-pyridine precatalyst 7 display the best activity, highest polyethylene molecular weight, and narrowest dispersity values of series. 13 C-NMR analyses of polyethylene samples reveal a lightly branched morphology with a modest amount of methyl branching; polymer T m values are consistent with this assessment. Ongoing work includes further exploration of precatalysts 4 , 6 , and 7 in attempt to maximize polyethylene molecular weight and achieve living polymerization.