Zinc, aluminum and group 3 metal complexes of sterically demanding naphthoxy-pyridine ligands: Synthesis, structure, and use in ROP of racemic lactide and β-butyrolactone

• New tridentate ortho-Ph3Si-substituted naphthol-pyridine ligands were synthesized.
• Zn, Al, Y and La complexes were prepared.
• Some of these complexes exhibit high activity in ROP of lactide and β-butyrolactone.
• Good molecular weight control and narrow distributions are achieved.

New potentially tridentate, bulky ortho-Ph3Si-substituted naphthol-pyridine proligands a–f were synthesized and introduced onto zinc, aluminum and group 3 metal centers (M = Sc, Y, La) using straightforward one-step alkane or amine elimination protocols. The solid-state structures of these mononuclear zinc (1a, 1c, 1d, 2c and 2e) and aluminum (3c) complexes were determined by single-crystal X-ray diffraction studies, while the solution structures were established using 1H, 13C{1H} and 29Si/29Si{1H} (when appropriate) NMR spectroscopy. For all complexes, only one species (isomer) of C1 symmetry was observed by NMR spectroscopy in the broad temperature range. Most of these complexes are effective initiators for the ring-opening polymerization (ROP) of racemic lactide (rac-LA) at 25–100 °C, affording poly(lactides)s (PLAs) generally with unimodal dispersities and molecular weights in good agreement with calculated values. An yttrium complex (4e) proved the most active (TOF = 1840 mol(LA) mol(Y)−1 h−1 at 25 °C) and yielded heterotactic-enriched PLAs (Pr up to 0.80) in toluene, while atactic PHBs were formed in the ROP of racemic β-butyrolactone (rac-BBL) under the same conditions.

Zinc, aluminum and group 3 metal complexes supported by tridentate naphthoxy-pyridine ligands were prepared and used as initiators/catalysts for the ring-opening polymerization of racemic-lactide and racemic-β-butyrolactone, affording heterotactic- enriched PLAs and atactic PHBs, respectively.Figure optionsDownload as PowerPoint slide