Ethylene polymerization catalyzed by substituted pyrazole nickel complexes

(Pyrazole)nickel dibromide complexes, (3,5-Me2pz)2NiBr2 (1), (3-Mepz)4NiBr2 (2), (pz)4NiBr2 (3) and (3,5-tBu2pz)2NiBr2 (4), were prepared by the reaction of the appropriate pyrazole with (DME)NiBr2. Solid-state structures of these complexes show a direct relation between the steric bulk of the pyrazole ligand and structure, with more bulky ligands forming four-coordinate complexes (1 and 4) whereas the less bulky ligands formed six-coordinate complexes (2 and 3). Activation of selected complexes (1 and 3) with methylaluminoxane (MAO) produced species that catalyzed the polymerization of ethylene to form high density polyethylene.