A novel phenolate "constrained geometry" catalyst system. Efficient synthesis, structural characterization, and α-olefin polymerization catalysis

This paper reports the convenient "one-pot" synthesis of a bifunctional ^-Me₄Cp^̂ phenolate^- ligand and the efficient "one-step" synthesis of the corresponding Ti^IV and Zr^IV complexes. The reaction of 2-bromo-4-methylphenol with 2 equiv of ⁿBuLi followed by addition of 2,3,4,5-tetramethyl-2-cyclopentenone produces the bifunctional mono-Cp phenol ligand precursor 2-(tetramethylcyclopentadienyl)-4-methylphenol ((TCP)H₂, 1). Reaction of 1 with Ti(CH₂-Ph)₄ at 60°C in toluene cleanly generates (TCP)Ti(CH₂Ph)₂ (2), while the corresponding reaction with Zr(CH₂Ph)₄ at higher temperatures affords the chelated C₂-symmetric zirconocene (TCP)₂Zr (3). In solution at room temperature, the two benzyl groups of 2 are magnetically equivalent, however, in the solid state, X-ray diffraction reveals that one benzyl group is coordinated in a normal η¹-fashion and the other in an η²-mode. The small Cp-(centroid)-Ti-O angle of 107.7(2)° in 2 indicates sterically open features in common with amido-based "constrained geometry" polymerization catalysts. Low-temperature NMR-scale reactions of 2 with B(C₆F₅)₃ and Ph₃C⁺B(C₆F₅)₄^- indicate the formation of the corresponding cationic complexes (TCP)TiCH₂Ph⁺ PhCH₂B(C₆F₅)₃^- (4) and (TCP)TiCH₂Ph⁺B(C₆F₅) ₄^- (5). Upon activation with Ph₃C⁺B(C₆F₅)₄ ^-, complex 2 is highly active for ethylene, propylene, and styrene polymerization.