Gallium and Indium Complexes with Bulky Pyridine-Bis(phenolate) Ligands: Synthesis, Structure, and Activity in Ring-Opening Polymerization of Cyclic Esters

A series of gallium and indium complexes bearing pyridine-2,6-bis(phenolate) ligands with ortho-substituents of increasing steric bulk (tBu, SiMe3, SiPh3) was synthesized via SiMe4 elimination from M(CH2SiMe3)3 (M = Ga, In). The gallium complexes 2a–c were structurally characterized by X-ray diffraction, revealing monomeric four-coordinate distorted trigonal pyramidal geometries. DOSY NMR studies revealed that the indium complexes 3a (tBu) and 3b (SiMe3) are dimeric in solution, whereas the bulkier 3c (SiPh3) is monomeric. All complexes were tested as initiators for ring-opening polymerization (ROP) of ε-caprolactone, L-lactide, and rac-lactide in the presence of benzyl alcohol. The M-CH2SiMe3 bond is inert toward alcohols, pointing to an activated monomer mechanism. Indium complexes exhibited higher activity than their gallium analogues, and the activity strongly depends on the ligand steric bulk: the most hindered SiPh3 derivative 3c showed the highest activity, reaching full ε-CL conversion in 24 h at 80 °C, whereas the analogous gallium complex 2c was almost inactive.