The Effects of Activators on Zirconium Phosphinimide Ethylene Polymerization Catalysts

Synthetic routes to the species CpZr(NP t- Bu 3 ) 2 Cl, 7, Cp 2 Zr(NP t- Bu 3 )Cl, 8, CpZr(NP t -Bu 3 ) 2 Me, 9, Cp 2 Zr(NP t- Bu 3 )Me, 10, and CpZr(NP t -Bu 3 ) 2 Bn, 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(NP t -Bu 3 ) 3 (NMe 2 ), 15, was isolated in less than 5% yield, but could be obtained cleanly via reaction of Zr(NP t -Bu 3 ) 3 Cl, 14, with LiNMe 2 . However, thermolysis of Zr(NEt 2 ) 4 with HNP t -Bu 3 afforded Zr(NP t -Bu 3 ) 2 (NEt 2 ) 2, 12, which was subsequently converted to Zr(NP t -Bu 3 ) 2 Cl 2, 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(NP t -Bu 3 )Me(THF)][MeB(C 6 F 5 ) 3 ], 16, [Cp*Zr(NP t- Bu 3 )(( i -PrN) 2 CMe)][MeB(C 6 F 5 ) 3 ], 17, and [CpZr(NP t- Bu 3 )(( i -PrN) 2 CMe)][MeB(C 6 F 5 ) 3 ], 18 . Similarly, reaction of [HNMe 2 Ph][B(C 6 F 5 ) 4 ] with 4 generated the salt [CpZr(NP t -Bu 3 )Me(NMe 2 Ph)][B(C 6 F 5 ) 4 ], 19, while reaction of 11 with B(C 6 F 5 ) 3 gave the base-free product [CpZr(NP t -Bu 3 ) 2 ][BnB(C 6 F 5 ) 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(C 6 F 5 ) 3 as cocatalyst resulted in a significant increase in activity relative to that observed upon activation with MAO. Use of [Ph 3 C][B(C 6 F 5 ) 4 ] as the cocatalyst led to even higher ethylene polymerization activities.