Phenoxy-Amidine Ligands: Toward Lactic Acid-Tolerant Catalysts for Lactide Ring-Opening Polymerization
Valentin Vaillant-Coindard, Benjamin Théron, Gaël Printz, Florian Chotard, Cédric Balan, Yoann Rousselin, Philippe Richard, Iogann Tolbatov, Paul Fleurat‐Lessard, Ewen Bodio, Raluca Malacea‐Kabbara, Jérôme Bayardon, Samuel Dagorne, Pierre Le Gendre
Abstract
The replacement of the imine functionality in the ubiquitous phenoxy-imine (FI) ligands by a more robust and donor N,N,N′-trisubstituted amidine function was examined and gave rise to the synthesis of five new phenoxy-amidine (FA) ligands (L1–L5). The solid-state structure of four proligands has been determined by X-ray diffraction analysis and showed that the amidine moiety is in a trans-configuration. The reaction of the phenol-amidine proligands with AlMe3 afforded mononuclear (L1–L5)AlMe2 (1a–5a). A similar alkane elimination route was used from ZnEt2 and led to dinuclear [(L1–L5)ZnEt]2 complexes (1b–5b) or to homoleptic (L2/L4)2Zn complexes (2b′, 4b′) depending on the metal/ligand ratio used. The structure of these complexes has been determined by NMR spectroscopy (1H, 13C, HMBC, HSQC, DOSY, and NOESY experiments) and X-ray diffraction study for seven of them. The crystal structure of the Al complexes showed FA ligands coordinated in a chelate fashion via the O atom of the aryloxy group and the imino-N atom, indicating that the amidine function has undergone trans–cis isomerization upon coordination. A similar chelating coordination mode was observed for the FA ligands with Zn metal ions. These complexes were used as initiators for the ring-opening polymerization of rac-lactide. FA–Zn complexes gave the best performance, affording polylactic acid with a narrow molecular weight distribution and heterotactic bias (Pr up to 0.75). Remarkably, some of these complexes were able to tolerate the presence of a large amount of lactic acid to the point of using it as a co-initiator during the polymerization reaction.