Evaluation of Zirconium and Hafnium Complexes that Contain the Electron-Withdrawing Diamido/Donor Ligand [(2,6-Cl2C6H3NCH2CH2)2NCH3)]2- for the Polymerization of 1-Hexene

The triamine (2,6-Cl 2 C 6 H 3 NHCH 2 CH 2 ) 2 NMe (H 2 [Ar Cl N 2 NMe]) can be prepared through the Pd-catalyzed coupling between 2-bromo-1,3-dichlorobenzene and (H 2 NCH 2 CH 2 ) 2 NH to give (2,6-Cl 2 C 6 H 3 NHCH 2 CH 2 ) 2 NH, followed by methylation of the central nitrogen with MeI. Zirconium and hafnium complexes that were prepared include [Ar Cl N 2 NMe]M(NMe 2 ) 2 (M = Hf or Zr), [Ar Cl N 2 NMe]MCl 2, [Ar Cl N 2 NMe]MMe 2, [Ar Cl N 2 NMe]HfCl(i-Bu), [Ar Cl N 2 NMe]HfMe(i-Bu), and [Ar Cl N 2 NMe]Hf(i-Bu) 2 . An X-ray crystal structure shows [Ar Cl N 2 NMe]Hf(i-Bu) 2 to be a square pyramid in which one of the isobutyl groups occupies the apical position. Activation of dimethyl species with {Ph 3 C}{B(C 6 F 5 ) 4 }, B(C 6 F 5 ) 3, or {PhNMe 2 H}{B(C 6 F 5 ) 4 } yielded {[Ar Cl N 2 NMe]MMe} + cations paired with {B(C 6 F 5 ) 4 } - or {MeB(C 6 F 5 ) 3 } - anions; these species are active for the polymerization of 1-hexene. Activation of [Ar Cl N 2 NMe]Hf(i-Bu) 2 with {Ph 3 C}{B(C 6 F 5 ) 4 } or B(C 6 F 5 ) 3 produced {[Ar Cl N 2 NMe]Hf(i-Bu)}{B(C 6 F 5 ) 4 } and {[Ar Cl N 2 NMe]Hf(i-Bu)}{HB(C 6 F 5 ) 3 }, which are also active for the polymerization of 1-hexene. Isobutene was found to insert slowly into the M−R bond of these initiators in a 1,2 manner and was oligomerized at high concentrations of isobutene. Activation of [Ar Cl N 2 NMe]HfMe(i-Bu) with B(C 6 F 5 ) 3 resulted in selective abstraction of a methyl group to generate 95% {[Ar Cl N 2 NMe]Hf(i-Bu)}{MeB(C 6 F 5 ) 3 }. The rate of consumption of 1-hexene followed a first-order dependence on 1-hexene (and hafnium or zirconium), and zirconium was faster than hafnium. Although poly[1-hexene] generated by the zirconium catalyst was characteristic of a living system, significant β-hydride elimination was observed in polymerizations by hafnium catalysts. The hafnium product of β-hydride elimination was observed to reinitiate the polymerization of 1-hexene. We conclude that by replacing the mesityl substituents in the analogous [(MesitylN 2 NMe] 2 - ligand with 2,6-Cl 2 C 6 H 3 groups, decomposition is attenuated significantly, although living behavior is observed for zirconium catalysts only.