Synthesis and evaluation of substituted pyrazoles palladium(II) complexes as ethylene polymerization catalysts

The substituted pyrazole palladium complexes, (3,5-tBu2pz)2PdCl2 (1) (3,5-Me2pz)2PdCl2 (2), (3-Mepz)2PdCl2 (3) and (pz)2PdCl2 (4) (pzH=pyrazole), can be prepared from the reaction of (COD)PdCl2 with the appropriate pyrazole. The chloromethyl derivative, (3,5-tBu2pz)2PdCl(Me) (5), was prepared from (COD)PdClMe and tBu2pzH. X-ray crystal structure determination of 1 and 5 established their structures in the solid state to be the trans-isomer. After activation of 1–4 and 5 with methylaluminoxane (MAO) the resulting palladium complexes were used as catalysts in ethylene polymerization, yielding linear high-density polyethylene (HDPE). The highest activity was observed for (3,5-tBu2pz)PdClMe.