The Effects of Activators on Zirconium Phosphinimide Ethylene Polymerization Catalysts

Synthetic routes to the species CpZr(NPt-Bu3)2Cl, 7, Cp2Zr(NPt-Bu3)Cl, 8, CpZr(NPt-Bu3)2Me, 9, Cp2Zr(NPt-Bu3)Me, 10, and CpZr(NPt-Bu3)2Bn, 11, were developed in a manner similar
to that previously reported for zirconium phosphinimide complexes. Rather than employing
metathesis routes, transamination was considered to synthesize bis-phosphinimide zirconium
complexes. At ambient temperature, Zr(NPt-Bu3)3(NMe2), 15, was isolated in less than 5%
yield, but could be obtained cleanly via reaction of Zr(NPt-Bu3)3Cl, 14, with LiNMe2. However,
thermolysis of Zr(NEt2)4 with HNPt-Bu3 afforded Zr(NPt-Bu3)2(NEt2)2, 12, which was
subsequently converted to Zr(NPt-Bu3)2Cl2, 13, upon reaction with trimethylsilyl chloride.
Cationic products were generated from the reaction of Lewis acids in the presence of a donor
to provide the salts [CpZr(NPt-Bu3)Me(THF)][MeB(C6F5)3], 16, [Cp*Zr(NPt-Bu3)((i-PrN)2CMe)][MeB(C6F5)3], 17, and [CpZr(NPt-Bu3)((i-PrN)2CMe)][MeB(C6F5)3], 18. Similarly, reaction of [HNMe2Ph][B(C6F5)4] with 4 generated the salt [CpZr(NPt-Bu3)Me(NMe2Ph)][B(C6F5)4], 19, while reaction of 11 with B(C6F5)3 gave the base-free product [CpZr(NPt-Bu3)2][BnB(C6F5)3], 20. Structural considerations and preliminary MO calculations support
the reactivity studies that augur well for olefin polymerization activity. Experimentally,
previously reported screening using MAO as a solvent scrubber/activator with 1−4 showed
only moderate polymerization activities. However, use of 20 equiv of Al(i-Bu)3 as scavenger
and 2 equiv of B(C6F5)3 as cocatalyst resulted in a significant increase in activity relative to
that observed upon activation with MAO. Use of [Ph3C][B(C6F5)4] as the cocatalyst led to
even higher ethylene polymerization activities.