Cationic organozinc complexes of a bis(phosphinimine) pincer ligand: synthesis, structural and polymerization studies

Abstract

Cationic organozinc complexes of a neutral bis(phosphinimine) pincer ligand (L) have been prepared and structurally characterized. This recently introduced ligand was constructed from a dibenzofuran (dbf) framework with symmetric attachment of phosphinimine groups at the 4 and 6 positions. Starting from protonated derivatives [LH][B(C₆F₅)₄] (1a), [LH][BPh₄] (1b), or [LH₂][BPh₄]₂ (1c), the complexes [LZnCH₃][B(C₆F₅)₄] (2a), [LZnCH₃][BPh₄] (2b), and [LZnOAc][BPh₄] (3), were prepared via protonolysis of an appropriate alkylzinc precursor. The complex [LZnPh][BPh₄] (4) is generated as a side-product in the synthesis of 2b. Solid-state structural studies have revealed the compounds to be charge separated cationic zinc species with 3-coordinate trigonal planar geometry. Preliminary studies have shown these complexes to be inactive for the polymerization of lactide. Upon modification of the initiating group to a methyl-(S)-lactate, however, complex [LZnOCH(Me)CO₂Me][B(C₆F₅)₄] (5) demonstrated significant polymerization activity at 60 °C. Additionally, NMR and mass spectrometry data confirmed a coordination-insertion mechanism was operative for this catalyst.