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-Et2C6H3(3), 2,6-iPr2C6H3(4)). Similar reactions of 2,6-(ArNCH)2C6H3Li with ZnEtCl produce bisligated zinc complexes (2,6-(ArNCH)2C6H3)2Zn (Ar = Ph (5), 2,6-Me2C6H3(6), 2,6-Et2C6H3(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 8were determined by X-ray crystallography. The X-ray diffraction analysis reveals that both complexes 3and 4adopt 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 6and 7adopt a distorted tetrahedral geometry around their zinc metal centers, while complex 8adopts 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. [ABSTRACT FROM AUTHOR]