Titanium Ester Enolate Complex Supported by a Tetradentate Bis(phenolato) Ligand: Synthesis, Structure, and Activity in Methacrylate Polymerization

Reaction of the titanium dichloro complex containing a [OSSO]-type bis(phenolate) ligand (edtbp)TiCl2 (1) (edtbpH2 = (HOC6H2-tBu2-4,6)2(SCH2CH2S)) with 2 equiv of AlMe3 produces the methylchloro complex (edtbp)TiMeCl (2) in 77% yield, alternatively prepared from comproportionation of 1 and the dimethyl complex (edtbp)TiMe2 (3). The titanium ester enolate complex (edtbp)TiMe{O(iPrO)C=CMe2} (4) was synthesized by reaction of 2 with lithium isopropylisobutyrate, LiO(iPrO)C=CMe2, in a 1:1 molar ratio. This complex reacted with alcohols ROH (R = iPr, Ph, Ph3C) to give alkoxy complexes (edtbp)TiMe(OR) (6−8) and with acetone under C−C coupling to afford an aldolate complex (edtbp)TiMe(OCMe2CMe2CO2iPr) (9). Methyl abstraction from the neutral complexes 4, 7, and 9 using with B(C6F5)3 gave the corresponding cationic species [(edtbp)Ti(OR)(THF)n][MeB(C6F5)3]. All complexes have been characterized by multinuclear NMR spectroscopy and elemental analysis. Single-crystal X-ray diffraction studies were performed for complexes 2, 4, and 9. Cationic titanium enolate species initiate the polymerization of methyl methacrylate (MMA) at room temperature, producing syndiotactic-enriched poly(methyl methacrylates) (PMMAs). In the presence of the neutral enolate complex, MMA polymerization proceeds in a living fashion. The neutral titanium enolate complex 4 was found to be moderately active in n-butyl acrylate (n-BA) polymerization.