Cationic Zirconium Complexes that Contain Mesityl-Substituted Diamido/Donor Ligands. Decomposition via CH Activation and Its Influence on 1-Hexene Polymerization

The dialkyl complexes [MesNMe]ZrMeNp (2b) and [MesNMe]ZrNp₂ ([MesNMe]^2- = [(MesNCH₂CH₂)₂NMe]^2-; Np = CH₂CMe₃) were prepared and shown to have distorted trigonal bipyramidal structures in which the two amido groups occupy equatorial positions. The neopentyl group in 2b was found in the axial position. Compound 2b was found to convert to another species (2a) in a first-order manner with k = 1.68 × 10^-3 min^-1 at 20 °C and to reach an equilibrium with K_eq = [2a]/[2b] = 0.43 in C₆D₆. Activation of [MesNMe]ZrMeNp with [Ph₃C][B(C₆F₅)₄] led to formation of unobservable {[MesNMe]ZrNp}[B(C₆F₅)₄], which decomposed by CH activation of an ortho mesityl methyl group to give an inactive dimer of {[activ-MesNMe]Zr}[B(C₆F₅)₄], or in the presence of dimethylaniline by β methyl elimination to give {[MesNMe]ZrMe(PhNMe₂)}[B(C₆F₅)₄]. {[MesNMe]ZrMe}[B(C₆F₅)₄] also decomposes to give the dimer of {[activ-MesNMe]Zr}[B(C₆F₅)₄] with k_d = 6.0 × 10^-5 s^-1 at 20 °C. A dimethyl complex was prepared that contained a second “internal amine” donor, i.e., {[(MesNCH₂CH₂)₂NCH₂CH₂NMesMe]ZrMe₂. Upon activation of the dimethyl species with [Ph₃C][B(C₆F₅)₄], one of the (now six) ortho methyl groups was CH activated; the methyl cation was not observed. 1-Hexene was polymerized too rapidly by {[MesNMe]ZrMe}[B(C₆F₅)₄] to follow readily by NMR methods, while polymerization by {[MesNMe]Zr(PhNMe₂)Me}[B(C₆F₅)₄] and {{[MesNMe]ZrMe}₂(μ-Me)}[B(C₆F₅)₄] was found to be retarded by dimethylaniline and [MesNMe]ZrMe₂, respectively. The marked curvature of the log plot of the consumption of 1-hexene by {[MesNMe]Zr(PhNMe₂)Me}[B(C₆F₅)₄] could be modeled by assuming that intermediates in the polymerization process decompose by CH activation of a mesityl methyl group and are thereby removed from the system as propagating species.