Exploring the versatility of a bis(phosphinimine) pincer ligand: effect of sterics on structure and lactide polymerization activity of cationic zinc complexes

Abstract

Cationic zinc complexes of a neutral pincer framework 4,6-(ArNPPh₂)-dibenzofuran (L¹11: Ar = 2-ⁱPrPh; L²2: Ar = o-tolyl; L³3: Ar = Ph), have been prepared and characterized. Crystallographic and NMR studies of the methylzinc complexes [LZnCH₃⁺][BAr₄⁻] (4a–6a: Ar = m-(CF₃)₂–C₆H₃; 4b–6b: Ar = Ph) demonstrated that the steric demands of the ligand dramatically affect the solid-state geometry. The cationic zinc–lactate complexes [LZnOR⁺][B(m-(CF₃)₂–C₆H₃)₄⁻] (7: L = L²2; 8: L = L³3; R = CH(Me)CO₂Me) were also studied, and their efficacy as lactide polymerization catalysts was examined. Polymerization using 7 requires heating to 60 °C, while complex 8 displays high activity at ambient temperature. The difference in activity can be attributed to κ²versus κ³ binding modes of the ligand, providing important insight into structure–activity relationships for this system. Complex 8 gives modestly heteroenriched PLA (P_r = 0.70), which represents the best stereocontrol yet achieved by a cationic metal catalyst.