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

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journal contribution
posted on 2001-07-10, 00:00 authored by Yann Schrodi, Richard R. Schrock, Peter J. Bonitatebus
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 Keq = [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 kd = 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.