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

• Published in: New journal of chemistry 2020-05, Vol.44 (19), p.7811-7822
• Main Authors: Rad'kova, Natalia Yu, Kovylina, Tatyana A, Shavyrin, Andrei S, Cherkasov, Anton V, Fukin, Georgy K, Lyssenko, Konstantin A, Trifonov, Alexander A
• Format: Article
• ISSN: 1144-0546; 1369-9261
• DOI: 10.1039/d0nj00042f

Complexes [(2-MeOC 6 H 4 N)C( t Bu)N(2,6-Me 2 C 6 H 3 )] 2 LnN(SiMe 3 ) 2 (Ln = Y ( 2 ), Sm ( 3 )) and [(2-MeOC 6 H 4 N)C( t Bu)N(2,6-Me 2 C 6 H 3 )]CaN(SiMe 3 ) 2 (THF) ( 4 ) were obtained by reacting Ln[N(SiMe 3 ) 2 ] 3 (Ln = Y, Sm) or Ca[N(SiMe 3 ) 2 ] 2 with [2-MeOC 6 H 4 N]C( t Bu)NH(2,6-Me 2 C 6 H 3 ) ( 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 κ 2 -N,O:η 3 -arene coordination mode of the amidinate ligand. Complexes 2-4 initiate ring-opening polymerization (ROP) of rac -lactide ( rac -LA) and -caprolactone ( -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 -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 / i PrOH (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 i PrOH, complexes 2-4 initiate immortal polymerization for both rac -LA and -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). Y( iii ), Sm( iii ) and Ca( ii ) monoamido complexes coordinated by tridentate amidinate ligand were synthesized and evaluated as initiators of ring-opening polymerization of rac -lactide and -caprolactone.