Phenoxy-imine/-amide aluminum complexes with pendant or coordinated pyridine moieties: Solvent effects on structural type and catalytic capability for the ROP of cyclic esters

Abstract

Depending on the solvent employed, the dimeric aluminum phenoxyamide complexes [2-O,4-R4C6H3CHMeN(3′-R1,4′-R2,5′-R3C5HN)]2Al2Me2 (R1 = Me, R2 = R3 = R4 = H Al1; R2 = Me, R1 = R3 = R4 = H Al2; R3 = Me, R1 = R2 = R4 = H Al3; R1 = R2 = R3 = R4 = H Al4; R1 = Me, R4 = OMe, R2 = R3 = H Al5) or their monoaluminum phenoxyimine counterparts, [2-O-C6H4CH = N(3′-R1,4′-R2,5′-R3C5HN)]AlMe2 (R1 = Me, R2 = R3 = H Al6; R2 = Me, R1 = R3 = H Al7; R3 = Me, R1 = R2 = H Al8; R1 = R2 = R3 = H Al9), were obtainable by the treatment of the corresponding 2-pyridyl substituted salicylaldimine pro-ligand with AlMe3. Structural characterization of Al1 – Al4 highlights the Npy,N,O-chelation and bridging capacity of the dianionic pyridyl substituted phenoxyamide ligand. By contrast, the monoanionic phenoxyimine ligand in Al8 serves as an N,O-bidentate ligand with the Npy unit pendant. In the presence of benzyl alcohol (BnOH), all nine complexes exhibited high efficiency for the ring-opening polymerization (ROP) of ϵ-caprolactone (ϵ-CL), in which the activity displayed by dinuclear Al1 – Al5 in general exceeding that seen by mononuclear Al6 – Al9. Analysis of the polycaprolactone (PCL) generated using Al1/BnOH by 1H NMR spectroscopy and MALDI-TOF mass spectrometry showed the polymer to adopt mainly a linear structure with BnO groups constituting the end groups. By contrast, when Al1 was used in the absence of BnOH, the PCL was mainly cyclic in nature. For the ROP of L-LA or rac-LA good efficiency was again achieved albeit at a lower level than that seen for ϵ-CL. In common with that seen with ϵ-CL, the amount of BnOH employed proved crucial in determining both the linearity and end group composition of the polylactide (PLA).