Synthesis of C2 and Cs symmetric zinc complexes supported by bis(phosphinimino)methyl ligands and their use in ring opening polymerisation catalysis

Abstract

Synthetic routes to zinc complexes supported by sterically demanding bis(phosphinimino)methyl ligands are reported. Two aryl-substituted ligand precursors have been utilised, [CH₂(Ph₂PNC₆H₂Me₃-2,4,6)₂], 1, and [CH₂(Ph₂PNPh)(Ph₂PNC₆H₂Me₃-2,4,6)], 2. The second of these is the first example of an asymmetric bis(phosphinimino)methane. These ligands are converted to three-coordinate RZnX complexes (R = 1; X = Me, N(SiMe₃)₂; R = 2; X = Me) with either C₂ or C_s symmetry in solution by reaction with either ZnMe₂ or Zn[N(SiMe₃)₂]₂ in toluene. All three derivatives have been shown to exist as three-coordinate monomers in the solid state by X-ray diffraction analysis. Protonolysis of these compounds with the bulky phenol 2,4-^tBu₂C₆H₃OH or triphenylmethanol resulted in the isolation of a series of three-coordinate aryloxy or alkoxyzinc derivatives. In contrast, reaction with less sterically demanding alcohols resulted in protonation of the bis(phosphinimino)methyl ligand. A similar result was obtained from reaction of RZnMe (R = 1) and triphenylsilanol and the product is the first example of a bis(triorganosiloxy)zinc compound to be structurally characterised. All the compounds have been examined for activity in ring opening polymerisation catalysis of rac-lactide. The aryloxy and triphenylmethoxy derivatives are active catalysts; however, no evidence of true ‘living’ behaviour or stereocontrol of diastereomer insertion has been observed.