Unprecedentedly high activity and/or high regio-/ stereoselectivity of fluorenyl-based CGC allyl-type η³:η¹-tert-butyl(dimethylfluorenylsilyl)amido ligated rare earth metal monoalkyl complexes in olefin polymerization

A series of fluorenyl-based constrained-geometry-configuration (CGC) allyl-type rare earth metal monoalkyl complexes bearing the divalent anionic η³:η¹-tert-butyl(dimethylfluorenylsilyl)amido (η³:η¹-FluSiMe₂N^tBu) ligand (η³:η¹-FluSiMe₂N^tBu)Ln(CH₂SiMe₃)(THF)2 (1-3) have been synthesized via the alkane elimination reaction between the FluHSiMe2NHtBu ligand and rare earth metal tri(trimethylsilylmethyl) complexes Ln(CH₂SiMe₃)3(THF)n. Their structures are characterized by means of NMR spectrum, elemental analyses, and X-ray diffraction. These complexes 1-3 are isostructural and isomorphous, and each of them adopts a distorted-trigonal-bipyramidal configuration containing one η³:η¹-FluSiMe₂N^tBu ligand, one CH₂SiMe₃ ligand, and two THF molecules. Unlike traditional CGC allyl-type rare earth metal complexes showing no or low activity and regio-/stereoselectivity in styrene or MMA polymerization, these complexes 1-3 exhibit high catalytic activities and/or high regio-/stereoselectivities in the cis-1,4-polymerization of isoprene and myrcene or in the syndiotactic polymerization of styrene under the aid of different activators (borate or borane) and AlR3. The in situ 1H NMR spectra suggest that the exchanges of chelating ligands such as alkyl groups and divalent anionic η³:η¹-FluSiMe₂N^tBu ligands between rare earth metal centers and Al centers result in the formation of a heterobimetallic tetraalkylaluminate complex R₂Al(μ-R)₂Ln(R)(μ-R)₂AlR₂, which is activated by activators to form a divalent cationic species [Ln(μ-R)₂AlR₂]²⁺ as a catalytically active species in the coordination-insertion polymerization of olefins.