Diverse Pathways of Activation and Deactivation of Half-Sandwich Aryloxide Titanium Polymerization Catalysts

A series of half-sandwich aryloxide titanium complexes, [CpTi(OAr)Me2] (Cp = C5H5; OAr = OC6H3Me2-2,6, OC6H3Et2-2,6, OC6H3iPr2-2,6, OC6H3tBu2-2,6, and OC6HPh4-2,3,5,6), have been synthesized. These compounds react with B(C6F5)3 to give thermally unstable complexes [CpTi(OAr)Me][MeB(C6F5)3]. Two different deactivation pathways have been identified within the series. The tetraphenylphenoxide cationic methyl compound decomposes cleanly at room temperature to give [CpTi(OC6HPh4-2,3,5,6)(C6F5){CH2B(C6F5)2}] and methane with a first-order rate constant of 7.6(2) × 10-4 s-1 at 25 °C. For relatively smaller aryloxide ligands, OAr = OC6H3iPr2-2,6, OC6H3tBu2-2,6, a Me/C6F5 exchange takes place, yielding CpTi(OAr)Me(C6F5) and MeB(C6F5)2. The cationic titanium complexes are shown to be active for the polymerization of 1-hexene. At −20 and 0 °C, first-order dependence on the concentration of 1-hexene is observed. The rate of polymerization decreases with increasing steric hindrance of aryloxides except for OAr = OC6HPh4-2,3,5,6.