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

Cationic zinc complexes of a neutral pincer framework 4,6-(ArN=PPh2)-dibenzofuran (L-1 : Ar= 2-(PrPh)-Pr-i; L-2: Ar = o-tolyl; L-3: 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 = L-2; 8: L = L-3; R = CH(Me)CO2Me) were also studied, and their efficacy as lactide polymerization catalysts was examined. Polymerization using 7 requires heating to 60 degrees C, while complex 8 displays high activity at ambient temperature. The difference in activity can be attributed to kappa(2) versus kappa(3) 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.