Synthesis of Sterically Demanding Bis(phosphinimine) Dibenzofuran Ligands and Subsequent Zinc Metalation

In light of previous success surrounding the use of bis(phosphinimine)dibenzofuran ligands for zinc-mediated lactide polymerization, a series of sterically demanding P=N pincer compounds have been prepared with important steric and electronic modifications at both P- and N-sites (L, 3a-d). These systems are highly crystalline and have been extensively characterized using multinuclear NMR spectroscopy, elemental analysis, and X-ray diffraction. The ligands can be transformed into their protonated analogues [HL][BAr4F] (4a-d, [BAr4F]=[B(m-(CF3)(2)-C6H3)(4)]) by reaction with Brookhart's acid, and subsequently coordinated to zinc via an alkane elimination reaction with [ZnEt2] at ambient temperature to afford the corresponding [LZnEt][BAr4F] cationic complexes 5a-d. In addition, an unusual chloridozinc species [LZnCl][BAr4F] (5c) has been isolated and structurally characterized, providing comparisons to previously established ligand sets with similar geometries.