Half-sandwich scandium dibenzyl complexes bearing penta- or tetra-arylcyclopentadienyl ligands: synthesis, structure and syndiospecific styrene polymerization activity

Abstract

A series of half-sandwich scandium dibenzyl complexes has been synthesized via a one-pot reaction of potassium derivatives of pentaarylcyclopentadienes (Cp^(Ar*)5 or Cp^Ar5) (Ar* = 3,5-^tBu₂–C₆H₃; Ar = 3,5-ⁱPr₂–C₆H₃) or tetraarylcyclopentadienes (Cp^(Ar*)4H, Cp^Ar4H and Cp^Ph4H) with the cationic scandium dibenzyl complex [Sc(p-CH₂C₆H₄-Me)₂(THF)_x][BPh₄], generated in situ by the reaction of [Sc(p-CH₂C₆H₄-Me)₃(THF)₂] with [NEt₃H][BPh₄] in THF. [(Cp^Ar5)Sc(p-CH₂C₆H₄-Me)₂(THF)] (2) bearing a pentaarylcyclopentadienyl ligand displays two η¹-coordinated benzyl ligands without any obvious interaction of the ipso-carbon of the phenyl groups. In comparison, the two benzyl ligands in [(Cp^Ph4H)Sc(p-CH₂C₆H₄-Me)₂(THF)] (5) are supported by a tetraphenylcyclopentadienyl bond to the scandium center in two different patterns: one is η¹-coordination and the other is the η²-coordination mode. In combination with [Ph₃C][B(C₆F₅)₄] and AlⁱBu₃, these complexes showed highly active syndiospecific polymerization of styrene, up to 587 (kg(sPS) mol_(Sc)⁻¹ h⁻¹). The catalytic activity of these complexes decreased with the increasing steric hindrance of Cp ligands around the scandium centre in the order of Cp^Ph4H > Cp^Ar4H > Cp^(Ar*)4H > Cp^Ar5 > Cp^(Ar*)5H.