Propylene Polymerization with Cyclopentadienyltitanium(IV) Hydroxylaminato Complexes

Abstract

Complexes of the type Cp*TiX2(ONR′R′′) (Cp* = η5-C5Me5; X = Me, Cl; R = R′ = Et, TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl); X = R′ = Me, R′′ = tBu) were synthesized by several routes. Upon activation with [Ph3C]+[B(C6F5)4]− and AliBu3, these complexes generate highly active catalysts for propylene polymerization, although significant catalyst deactivation was observed. Activation with B(C6F5)3/AliBu3 or methylaluminoxane (MAO) resulted in reduced polymerization activity, the latter leading to increased catalyst lifetime. Model studies showed that the interaction of AlMe3 with Cp*Ti(Me)2(ONEt2) led to the formation of Cp*Ti(Me)2(η2-O(AlMe3)NEt2). The X-ray crystal structure confirmed that the hydroxylaminato ligand remained η2 bound to the titanium center with the AlMe3 bound to the complex through the oxygen atom of the hydroxylaminato ligand. Exchange reactions with organic ethers revealed the metalloether to be a comparable donor to PhOMe. Cp*Ti(Me)2(ONtBu(Me)) was revealed to be a weaker donor than Cp*Ti(Me)2(ONEt2); Cp*Ti(Me)2(TEMPO) did not bind to AlMe3. AliBu3 bound more weakly to Cp*Ti(Me)2(ONEt2) than AlMe3. Reaction of Cp*Ti(Me)2(ONEt2) and Cp*Ti(Me)2(ONtBu(Me)) with B(C6F5)3 resulted in clean formation of the zwitterionic contact ion pairs [Cp*Ti(Me)(η2-ONEt2)]+[MeB(C6F5)3]− and [Cp*Ti(Me)(η2-ONtBu(Me))]+[MeB(C6F5)3]−, respectively, whereas reaction of Cp*Ti(Me)2(η2-ONtBu(Me)) with [Ph3C]+[B(C6F5)4]− resulted in the clean formation of the solvent-separated ion pair [Cp*Ti(Me)(η2-ONtBu(Me))]+[B(C6F5)4]−. Reaction of Cp*Ti(Me)2(η1-TEMPO) with B(C6F5)3 results in the elimination of methane to result in the formation of the contact ion paired [Cp*Ti(η2-TEMPO)]+[MeB(C6F5)3]−, in which one of the TEMPO methyl groups has undergone C−H activation, resulting in a η2-bound TEMPO ligand, confirmed by 1H, gROESY, and 1H−15N HMBC NMR. Addition of AliBu3 or AlMe3 to the cationic [Cp*Ti(Me)(ONtBu(Me))]+[B(C6F5)4]− resulted in decomposition of the cations.