Olefin Polymerization Reactivity of Group 4 Post‐Metallocene Catalysts Bearing a Four‐Membered C(sp3)‐Donor Chelate Ring

Abstract

AbstractThe synthesis, spectroscopic and structural characterization, and olefin polymerization behavior of a family of pyridine‐2‐phenolate‐6‐arylmethine [O,N,CH(Ar)] Group 4 catalysts bearing a four‐membered N,C(sp3)‐donor metallacycle are described. The racemic [O,N,CH(Ar)] complexes adopt C1 symmetry, as confirmed by NMR spectroscopy and X‐ray crystallography. Advanced NMR experiments have been conducted to probe for possible C−H⋅⋅⋅F−C interactions within fluorinated derivatives. Use of a bis‐pyridyl ligand (Ar=py) resulted in the formation of C2v‐symmetric [O,NCHN] complexes containing a six‐membered partially delocalized N,N‐donor chelate. All complexes have been evaluated as ethylene polymerization catalysts. Notably, the Ti derivatives in conjunction with [Ph3C][B(C6F5)4]/iBu3Al display excellent catalytic efficiencies (TOF over 4×105 h−1 [atm C2H4]−1) at 22 °C, and are considerably more active than previously reported Ti−[O,N,C(σ‐aryl)] relatives. DFT calculations have been performed to gain insights into catalytic behavior. These studies indicate that although the ethylene assimilation process (comprising initial ethylene insertion into the Ti−C(methine) bond of the four‐membered chelate) is accessible, there exists a kinetic preference for normal chain propagation for some catalysts. The DFT results for the Ti−[O,N,CH(Ar)] (Ar=Ph) catalyst are consistent with the narrow molecular weight distributions of the polymers produced (Mw/Mn down to 2.2), suggesting close to single‐site character.