Four- and five-coordinate aluminum complexes supported by N,O-bidentate β-pyrazylenolate ligands: synthesis, structure and application in ROP of ε-caprolactone and lactide

In this paper we report a series of Al(iii) complexes supported by N,O-bidentate beta-pyrazyl functionalized enolate ligands HL1-HL5 (L = (6-Me-2,5-C4H2N2)-CHC(R)-O-), (R = Bu-t, Ph, p-tolyl, p-OMePh, o-tolyl) and their exploitation for the ring-opening polymerization of epsilon-caprolactone (epsilon-CL) and rac-lactide (rac-LA). The structures of the form LAlMe2 (1a-5a) or L2AlMe (1b-4b) were isolated depending on the ligand or stoichiometry of complexation. The investigation of these complexes toward the ROP of both epsilon-CL and rac-LA under the same conditions showed that dimethyl-aluminum complexes LAlMe2 (1a-5a) gave a higher activity than monomethyl-aluminum complexes L2AlMe (1b-4b). Meanwhile, monomethyl-aluminum complexes 1b-4b promoted the ring-opening polymerization with a better control over molecular weights and polydispersities than 1a-5a. Moreover, all of the LAlMe2 and L2AlMe produced PLA with strong isotactic bias (P-m up to 0.77) and narrow distributions. Generally, a beta-pyrazyl enolate system exhibited significantly higher catalytic activity for the ROP of epsilon-CL and rac-LA than the analogous beta-quinolyl enolate systems reported recently. The results are discussed in terms of electronic and steric properties affected by the substituents on the ligands.