Well-Designed Unsymmetrical Salphen-Al Complexes: Synthesis, Characterization, and Ring-Opening Polymerization Catalysis

The unsymmetrical salphen ligand (3,5-Bu-t-1-OH- C6H2) CH=N-C6H4-N,=CH (3-Ph-1-OH-C6H3) (tBu-PuLH2,) was designed and synthesized to support aluminum complexes. Its symmetric analogues, (3,5-Bu-t-1-OH-C6H2)CH=N-C6H4-N=CH(3,5-Bu-t-1-OH-C6H2) ((LH2)-L-tBu) and (3-Ph-1-OH-C6H3)CH=N-C6H4-N=CH(3-Ph-1-OH-C6H3) (PhLH,), were also explored and compared. The methane elimination reactions between ligands and AlMe3 resulted in formation of ah tBu-PhLAlMe (1),(LAlMe)-L-tBu (2), and (LAlMe)-L-Ph (3) in high yields, which were characterized by elemental analysis, H-1 and C-13 NMR. The coordination geometries of unsymmetrical and symmetric ligands in 1 and 2 were studied by X-ray diffractions, which revealed a five coordinated distorted square-pyramidal geometry around Al centers. The aluminum methyl compounds 1-3 reacted with BnOH at 70 degrees C to give tBu-PhLAlOBn (4), (LAlOBn)-L-tBu (5), and (LAlOBn)-L-Ph (6), respectively, which existed as monometallic species in solution. as indicated by NMR studies. However, the aluminum isopropoxide prepared by the reaction of tBu-PhLH2, with 1 equiv of Al((OPr)-Pr-i)(3) contained three species, one monometallic (tBu-PhLAlOPr)-Pr-i (7) and two bridged dimers,mu-O-2-(cis-(tPu-PhLAlOPr)-Pr-i)(2) (8) and mu-O-2-(trans-(tBu-PhLAlOPr)-Pr-i)(2), (9). The catalytic performances of unsymmetrical 4 for the ring-opening polymerization of racemic lactide (rac-LA) were preliminarily studied and compared to those of symmetric 5 and 6.