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-, his-, and tris(phenoxy)-titanium(IV) chlorides of the type [Cp*Ti(2-R-PhO)(n)Cl(3-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-/aryl-phenoxy) series was authenticated by structural studies on representative examples of the phenyl series including [Cp*Ti(2-Ph-PhO)Cl(2)] (1PhCl2), [Cp*Ti(2-Ph-PhO)(2)Cl] (2PhCl), and [Cp*Ti(2-Ph-PhO)(3)] (3 Ph). The metal-coordination geometry of each compound is best described as pseudotetrahedral with the Cp* ring and the 2-Ph-PhO and chloride ligands occupying three leg positions in a piano-stool geometry. The mean Ti-O distances, observed with an increasing number of 2-Ph-PhO groups, are 1.784(3), 1.802(4), and 1.799(3) angstrom for 1PhCl2, 2 PhCl, and 3Ph, respectively. All four alkyl/aryl series with Me, iPr, tBu, and Ph substituents were tested for ethylene homopolymerization after activation with Ph(3)C(+)[B(C(6)F(5))(4)](-) and modified methyaluminoxane (7% aluminum in iso-par 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, 2 PhCl 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.