Understanding the Roles of Triethylaluminum in Phosphinimide-Supported Titanium Catalyst Systems for Ethylene Polymerization

Treatment of [(Ind)­(tBu₃PN)­TiCl₂] (1; Ind = indenyl) with AlEt₃ in a range of solvents affords the emerald green heterobimetallic complex [(Ind)­(tBu₃PN)­Ti­(μ₂-Cl)₂AlEt₂] (2). Single-crystal X-ray diffraction studies, as well as nuclear magnetic and X-band electron paramagnetic resonance spectra, support formulation of 2 as a paramagnetic, formally [Ti^IIIAl^III] complex. Both 1 and 2 are active precatalysts for ethylene polymerization in the presence of AlEt₃ and a solid superacid; the product polyethylene materials are found to have similar molecular weight distributions in each case (3.29 with 1 and 3.05 with 2), implicating involvement of similar active catalysts in both. Electrochemical studies reveal that 1 can undergo one-electron reduction to Ti^III near −2.0 V versus ferrocenium/ferrocene in tetrahydrofuran electrolyte, whereas 2 undergoes one-electron oxidation at a significantly more positive potential (ca. −1.30 V). Spectroelectrochemical studies show that reduction of 1 in the presence of AlEt₃ leads to clean formation of 2. These findings support key roles for AlEt₃ in (1) catalyst alkylation, (2) chemical reduction of 1 by one electron, and (3) provision of the highly Lewis acidic [AlEt₂] fragment that stabilizes the Ti^III center in 2, as judged by the voltammetric data.