Unbridged bidentate aluminum complexes supported by diaroylhydrazone ligands: Synthesis, structure and catalysis in polymerization of epsilon-caprolactone and lactides

A series of novel diaroylhydrazone aluminum complexes have been synthesized and well-defined structurally, and their catalytic performance in the polymerization of epsilon-caprolactone and lactides have also been evaluated. Complexes [(L1-4)(2)AlMe] (1-4) {[L-1 = (3,5-Bu-t(2)-2-OMe-C6H2)CH=NNCOC6H5], [L-2 = (3,5-Bu-t(2)-2-OMe-C6H2)CH=NNCO(C6H4-4-OCH3)], [L-3 = (3,5-Bu-t(2)-2-OMe-C6H2)CH=NNCO(C6H4-4-Br)] and [L-4 = (2-OMe-C6H4)CH=NNCO(C6H4-4-Bu-t)]} were prepared through treatment of AlMe3 with the corresponding proligands L1-4H in molar ratios of 1: 1 or 1: 2. Chemical structures of all the complexes were well-defined by elemental analysis, NMR spectra as well as single-crystal X-ray study. Complexes [(L1-4)(2)AlMe] (1-4) in this work represent the first examples of aluminum complexes of aroylhydrazone ligands with crystallographic characterization. Specifically, they are all in monomeric form with a penta-coordinated aluminum center, including two approximately co-planar five-membered metallacycles with aluminum. Introduced bulky tert-butyl substituents in aroylhydrazone ligands could affect the geometry around the central metal which is a distorted square-based pyramid in complexes 1-3 while being a trigonal bipyramidal in complex 4, thus affecting their catalytic behaviors. The complexes can successfully catalyze the ring-opening polymerization of epsilon-caprolactone and (L)-lactide under mild conditions without any activator. In addition, complexes 1-4 could also polymerize rac-lactide, affording atactic polylactides with high conversions and good controllability in relatively short reaction time.