Amido rare-earth(iii) and Ca(ii) complexes coordinated by tridentate amidinate ligands: synthesis, structure, and catalytic activity in the ring-opening polymerization of rac-lactide and ε-caprolactone

Abstract

Complexes [(2-MeOC₆H₄N)C(^tBu)N(2,6-Me₂C₆H₃)]₂LnN(SiMe₃)₂ (Ln = Y (2), Sm (3)) and [(2-MeOC₆H₄N)C(^tBu)N(2,6-Me₂C₆H₃)]CaN(SiMe₃)₂(THF) (4) were obtained by reacting Ln[N(SiMe₃)₂]₃ (Ln = Y, Sm) or Ca[N(SiMe₃)₂]₂ with [2-MeOC₆H₄N]C(^tBu)NH(2,6-Me₂C₆H₃) (1) in 1 : 2 or 1 : 1 molar ratios, respectively, and were isolated in 76 (2), 68 (3) and 63% (4) yields. In complexes 2 and 3, the amidinate ligands are tridentate due to the intramolecular coordination of the 2-MeO-group to the metal center. Complex 4 exhibits an unusual κ²-N,O:η³-arene coordination mode of the amidinate ligand. Complexes 2–4 initiate ring-opening polymerization (ROP) of rac-lactide (rac-LA) and ε-caprolactone (ε-CL) at room temperature and allow for conversion of 1000 equivalents of monomer into polymer within 180–230 min for rac-LA and 1–7 min for ε-CL. Complexes 2 and 3 enable the formation of atactic polymers (P_r = 0.52–0.58) featuring moderate polydispersities (PDI = 1.30–3.06 for polylactide; PDI = 1.48–2.84 for polycaprolactone). When 4 was applied, PDIs were somewhat narrower (1.28–2.43 for PLA; 1.24–2.48 for PCL). Application of the initiating systems 2–4/ⁱPrOH (1 : 1) leads to the formation of the polymers having close values of the experimental and calculated molecular weights however does not affect strongly the molecular weight distribution. In the presence of 5 equivalents of ⁱPrOH, complexes 2–4 initiate immortal polymerization for both rac-LA and ε-CL. The polymers feature a good match with the experimental and calculated M_n but have moderate polydispersities (PDI = 1.28–1.90 for polylactide; PDI = 1.24–1.96 for polycaprolactone).