Probing the effect of ortho-cycloalkyl ring size on activity and thermostability in cycloheptyl-fused N,N,N-iron ethylene polymerization catalysts

Abstract

The syntheses of six bis(imino)-5,6,7,8-tetrahydrocycloheptapyridine-iron(II) chloride complexes, [2-{(Ar)NCMe}-9-{N(Ar)}C₁₀H₁₀N]FeCl₂ (Ar = 2-(C₅H₉)-6-MeC₆H₃Fe1, 2-(C₆H₁₁)-6-MeC₆H₃Fe2, 2-(C₈H₁₅)-6-MeC₆H₃Fe3, 2-(C₅H₉)-4,6-Me₂C₆H₂Fe4, 2-(C₆H₁₁)-4,6-Me₂C₆H₂Fe5, 2-(C₈H₁₅)-4,6-Me₂C₆H₃Fe6), are reported in which the ring size of the ortho-cycloalkyl group has been varied as has the type of para-substituent. The molecular structures of Fe3 and Fe6 reveal square pyramidal geometries at iron while the ortho-cyclooctyl rings adopt boat-chair conformations. On treatment with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all six complexes showed optimal activities at 80 °C [up to 1.9 × 10⁷ g of PE per mol Fe per h for Fe5/MMAO] for ethylene polymerization forming linear polyethylene (T_m's > 126 °C). Notably, the catalytic activities showed a marked correlation with the ring size of the ortho-cycloalkyl substituent: cyclohexyl (Fe2 and Fe5) > cyclooctyl (Fe3 and Fe6) > cyclopentyl (Fe1 and Fe4) for either para-substituent, H or Me. Furthermore, this family of iron catalysts exhibited remarkable thermostability by remaining highly active even at temperatures as high as 100 °C (1.1 × 10⁷ g of PE per mol Fe per h); the wide variation in polymer molecular weights (M_w: 2.4–166 kg mol⁻¹), influenced through choice of precatalyst and co-catalyst as well as by temperature and pressure, further highlights the versatility of these catalysts.