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

Abstract

In this paper we report a series of Al(III) complexes supported by N,O-bidentate β-pyrazyl functionalized enolate ligands HL¹–HL⁵ (L = (6-Me-2,5-C₄H₂N₂)–CHC(R)–O–), (R = ^tBu, Ph, p-tolyl, p-OMePh, o-tolyl) and their exploitation for the ring-opening polymerization of ε-caprolactone (ε-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 ε-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 β-pyrazyl enolate system exhibited significantly higher catalytic activity for the ROP of ε-CL and rac-LA than the analogous β-quinolyl enolate systems reported recently. The results are discussed in terms of electronic and steric properties affected by the substituents on the ligands.