Preparation and characterization of half-sandwich (pentamethylcyclopentadienyl)(triflato)titanium(IV) complexes: Solid-state structural motifs and catalysis studies

Half-sandwich (pentamethylcyclopentadienyl)(triflato)titanium(IV) complexes of the type [Ti(Cp*)-(TfO)₂X] (X=MeO (1), Me (2), 2,4,6-Me₃C₆H₂O (5)) or [Ti(Cp*)(o-OC₆H₄O)(TfO)] (7) were readily synthesized via methathesis of the corresponding chloride complexes with silver triflate (Cp* = (η⁵-1,2,3,4,5- pentamethylcyclopenta-2,4-dien-1-yl)). In addition, the complex 3 with X = OH was prepared by controlled hydrolysis of 2. The solid-state structures of these new complexes were determined by single-crystal X-ray-diffraction techniques. Three different structural motifs were identified; 1, 2, 3, and 7 are dimeric, while 5 is monomeric. The complexes were screened for their ability to stereospecifically polymerize styrene under homogeneous conditions. In the absence of activators, such as MAO (methylaluminoxane), 1 and 2 readily catalyzed the formation of atactic polystyrene; a strong dependence on the steric size of X was noted. In the presence of MAO, all of the complexes showed high activity and strong preference for the synthesis of syndiotactic polystyrene that was superior to that of [TiCl₃(Cp*)]/MAO.