Sterically less-hindered half-titanocene(IV) Phenoxides: Ancillary-ligand effect on Mono-, Bis-, and Tris(2-Alkyl-/arylphenoxy) Titanium(IV) Chloride complexes

Abstract

A series of mono-, bis-, and tris(phenoxy)-titanium(IV) chlorides of the type [Cp*Ti(2-R-PhO)n,Cl3,-n, ] (n = 1 -3 ; Cp* = pentamethylcyclopentadienyl) was prepared, in which R = Me, iPr, tBu, and Ph. The formation of each mono-, bis-, and tris(2-alkyl-/arylphenoxy) series was authenticated by structural studies on representative examples of the phenyl series including [Cp*Ti(2-Ph-PhO)Cl2] (1PhC12), [Cp*Ti(2-Ph-PhO)2Cl] (2PhCI), and [Cp*Ti(2-Ph-PhO) 3] (3Ph). The metalcoordination geometry of each compound is best described as pseudotetrahedral with the Cp* ring and the 2-PhPhO and chloride ligands occupying three leg positions in a piano-stool geometry. The mean Ti-O distances, observed with an increasing number of 2Ph-PhO groups, are 1.784(3), 1.802(4), and 1.799(3) Å for 1PhCl2, 2PhCl, and 3Ph, respectively. All four alkyl/aryl series with Me, iPr, tBu, and Ph substituents were tested for ethylene homopolymerization after activation with Ph 3C+[B(C6F5) 4]- and modified methyaluminoxane (7% aluminum in isopar E; mMAO-7) at 140°C. The phenyl series showed much higher catalytic activity, which ranged from 43.2 and 65.4 kg (mmol of Ti-h)-1, than the Me, iPr, and tBu series (19.2 and 36.6kg (mmol of Ti-h)-1). Among the phenyl series, the bis(phenoxide) complex of 2PhCl showed the highest activity of 65.4 kg (mmol of Ti-h) -1. Therefore, the catalyst precursors of the phenyl series were examined by treating them with a variety of alkylating reagents, such as trimethylaluminum (TMA), triisobutylaluminum (TIBA), and methylaluminoxane (MAO). In all cases, 2PhCl produced the most catalytically active alkylated species, [Cp*Ti(2-Ph-PhO)MeCl]. This enhancement was further supported by DFT calculations based on the simplified model with TMA. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.