Al and Zn phenoxy-amidine complexes for lactide ROP catalysis

Abstract

We report the synthesis of a new generation of phenoxy-amidine ligands based on an aryloxy moiety possessing an ortho-N-linked trisubstituted amidine. The reaction of the phenol-amidine proligands with aluminum and zinc alkyls gave mono- or bis-ligated complexes depending on the metal/ligand ratio used. The solid-state structure of four proligands and thirteen Zn and Al complexes has been determined by X-Ray diffraction analysis. The mono-ligated complexes present an aryloxy-bridged dimeric structure, which is retained in solution in the case of Zn complexes but not with aluminum according to DOSY NMR experiments. Bis(ligated) Al and Zn complexes exhibit fluxional behaviour in solution attributed to coordination–decoordination of the amidine moiety and the rotation around the amidine C–NR′₂ and C–Ar bonds. These complexes were tested for the ROP of rac-lactide in solution and under bulk conditions. In both cases, the most performant catalysts are Zn complexes featuring a phenoxy-amidine ligand with a pendant additional dimethylamino arm.