Thorium(iv) trialkyl complexes of non-carbocyclic ligands as highly active isoprene polymerisation catalysts

Abstract

A series of mono-anionic non-carbocyclic ligands, including bidentate benzamidinate [PhC(NDipp)₂] (Dipp = C₆H₃-2,6-ⁱPr₂), iminophosphinamide [Ph₂P(NDipp)₂] and phosphinoamide [Ph₂PNDipp], and tridentate hydrotris(3,5-dimethyl-1-pyrazolyl)borate (Tp^Me2) were used to stabilize the corresponding thorium(IV) trialkyl complexes [PhC(NDipp)₂]Th(CH₂SiMe₃)₃ (1), [Ph₂P(NDipp)₂]Th(CH₂SiMe₃)₃ (2), [Ph₂P(NDipp)]Th(p-CH₂-C₆H₄-Me)₃ (3) and (Tp^Me2)Th(CH₂SiMe₃)₃ (4), which were characterized by NMR spectroscopy and single-crystal X-ray analysis. Complexes 1–4 in combination with [Ph₃C][B(C₆F₅)₄] and AlⁱBu₃ form non-Cp-ligated actinide catalyst systems to show high activity and high cis-1,4-selectivity (89.9%) or trans-1,4-selectivity (91.4%) for the polymerization of isoprene. The reaction rate and selectivity of complexes 1 and 2 were controlled by the crowded space around the thorium centre, corroborated by the kinetics of the polymerization and the steric maps.