Binuclear half-metallocene chromium(iii) complexes mediated ethylene polymerization with alkylaluminium as cocatalyst

Abstract

Binuclear half-metallocene chromium complexes {Cp*[3-(CHNR)-2-O-C₁₀H₅]CrCl}₂ [Cp* = C₅Me₅; R = ⁱPr (1), Ph (2), 2,6-ⁱPr₂C₆H₃ (3)] based on 1,1′-binaphthyl ligands, as well as their mononuclear analogues Cp*[3-(CHNR)-2′-R′-2-O-C₂₀H₁₁]CrCl [R = ⁱPr, R′ = ⁿBuO (4), R = Ph, R′ = ⁿBuO (5), R = 2,6-ⁱPr₂C₆H₃, R′ = ⁿBuO (6), R = ⁱPr, R′ = H (7)], were synthesized and characterized by mass spectrometry, elemental analysis, magnetic measurement, and UV-vis spectroscopy. The molecular structures of complexes 1, 3, 5 and 6 were further confirmed by single-crystal X-ray crystallographic analysis. When activated with a small amount of AlMe₃, these binuclear complexes exhibited higher activities in catalyzing ethylene polymerization in comparison with their mononuclear analogues, affording high molecular weight polymers with unimodal molecular weight distributions. The highest activity up to 2.87 × 10⁶ g PE (mol Cr)⁻¹ h⁻¹ was achieved in the catalyst system of complex 3 bearing a bulky 2,6-ⁱPr₂C₆H₃ group on the imine nitrogen atom in the presence of 25 equiv. AlMe₃ as activator at 20 °C. ¹³C NMR analysis indicates the resultant polymers are linear and no evidence on branch was found.