Alkali aminoether-phenolate complexes: synthesis, structural characterization and evidence for an activated monomer ROP mechanism

Abstract

Several monometallic {LOi}M complexes of lithium (M = Li; i = 1 (1), 2 (2), 3 (3)) or potassium (M = K, i = 3 (4)) and the heteroleptic bimetallic lithium complex {LO3}Li·LiN(SiMe2H)2 (5), all supported by monoanionic aminoether-phenolate {LOi}− (i = 1–3) ligands, have been synthesized and structurally characterized. A large range of coordination motifs is represented in the solid state, depending on the chelating ability of the ligand, the size of the metal and the number of metallic centres found in the complex. Pulse-gradient spin-echo NMR showed that 1–4 are monomeric in solution, irrespective of their (mono- or di)nuclearity in the solid-state. VT 7Li and DOSY NMR measurements conducted for 5 indicated that the two Li atoms in the complex do not exchange positions even at 80 °C. Upon addition of 1–10 equiv. of BnOH, the electron-rich and sterically congested {LO3}Li complex (3) promotes the controlled living and immortal ring-opening polymerisation of L-lactide. The combination of polymer end-group analyses and stoichiometric model reactions unambiguously provided evidence that ROP reactions catalyzed by these two-component {LOi}Li/BnOH catalyst systems operate according to an activated monomer mechanism, and not via the coordination–insertion scenario frequently assumed for similar alkali phenolate–alcohol systems.