Bis(imino)aryl NCN pincer aluminum and zinc complexes: Synthesis, characterization, and catalysis on L -lactide polymerization

Reactions of 2,6-(ArN=CH)₂C₆H₃Li with AlEt₂Cl afford a number of NCN pincer aluminum complexes (2,6-(ArN=CH)₂C₆H₃)AlEt₂ (Ar = Ph (1), 2,6-Me₂C₆H₃ (2), 2,6-Et₂C ₆H₃ (3), 2,6-ⁱPr₂C₆H ₃ (4)). Similar reactions of 2,6-(ArN=CH)₂C ₆H₃Li with ZnEtCl produce bisligated zinc complexes (2,6-(ArN=CH)₂C₆H₃)₂Zn (Ar = Ph (5), 2,6-Me₂C₆H₃ (6), 2,6-Et₂C ₆H₃ (7)) and monoligated NCN pincer zinc complex (2,6-(ArN=CH)₂C₆H₃)ZnEt (Ar = 2,6- ⁱPr₂C₆H₃ (8)). All complexes were characterized by ¹H and ¹³C NMR spectroscopy, and the molecular structures of complexes 3, 4, 6, 7, and 8 were determined by X-ray crystallography. The X-ray diffraction analysis reveals that both complexes 3 and 4 adopt a distorted trigonal-bipyramidal geometry around the aluminum central metal with three carbon atoms in the equator and the two imine nitrogen atoms in the apical positions. Complexes 6 and 7 adopt a distorted tetrahedral geometry around their zinc metal centers, while complex 8 adopts a square-planar geometry around its metal center. All these Al and Zn complexes are efficient initiators for l-lactide ring-opening polymerization in the presence of benzyl alcohol, and the polymerization reaction takes place in an immortal manner. The productivity of the Zn complexes is generally higher than that of the Al complexes under similar conditions.