Tris(pyrazolyl)borate Ti(IV) Complexes Containing Phenoxy Ligands: Effective Catalyst Precursors for Ethylene Polymerization That Proceeds via Cationic Ti(IV) Species

A series of phenoxy-substituted tris(pyrazolyl)borate titanium complexes, Tp′TiCl2(OAr) and Tp*TiMe2(OAr) [Tp = HB(pyrazolyl)3, Tp* = HB(3,5-Me2-pyrazolyl)3; Ar = Ph, 2,6-Me2C6H3, 2,6-iPr2C6H3], were prepared, and their structures were determined by X-ray crystallography. These complexes exhibited from moderate to remarkable catalytic activities for ethylene polymerization in the presence of MAO; TpTiCl2(OPh) exhibited the highest catalytic activity for ethylene polymerization. The cationic Ti(IV)−methyl complex, [Tp*TiMe(O-2,6-Me2C6H3)]+[MeB(C6F5)3]−·(THF)2, polymerizes ethylene to give polyethylene with a uniform molecular weight distribution, clearly indicating that the cationic Ti(IV) species plays an essential role as the active species.