Zinc(II) complexes containing N′-aromatic group substituted N,N′,N-bis((1H-pyrazol-1-yl)methyl)amines: Synthesis, characterization, and polymerizations of methyl methacrylate and rac-lactide

A novel series of Zn(II) complexes [L_nZnCl₂] (L_n = L_A − L_F) based on N,N′,N-bis((1H-pyrazol-1-yl)methyl)amine bidentate ligands, N,N-bis((1H-pyrazol-1-yl)methyl)-3,5-dimethylaniline [L_A], N,N-bis((1H-pyrazol-1-yl)methyl)-2,6-dimethylaniline [L_B], N,N-bis((1H-pyrazol-1-yl)methyl)-2,6-diethylaniline [L_C], N,N-bis((1H-pyrazol-1-yl)methyl)-2,6-diisopropylaniline [L_D], N,N-bis((1H-pyrazol-1-yl)methyl)-4-bromoaniline [L_E] and N,N-bis((1H-pyrazol-1-yl)methyl)benzhydrylamine [L_F], has been synthesized and characterized. X-ray structures of these Zn(II) complexes showed a distorted tetrahedral geometry. No interaction exists between the N_amine and the Zn(II) center in the [L_nZnCl₂] (L_n = L_A − L_F) complexes, resulting in formation of an eight-membered chelate ring. [L_FZnCl₂] exhibited the highest catalytic activity (3.95 × 10⁴ g PMMA/mol·Zn·h) for the polymerization of methyl methacrylate (MMA) in the presence of modified methylaluminoxane (MMAO) at 60 °C and yielded high molecular weight (M_w) (11.0 × 10⁵ g/mol) of poly(methylmethacrylate) (PMMA). All the complexes resulted in syndiotactic enriched PMMA with high T_g (125–131 °C). The steric bulk of ligand architecture plays an influential role in controlling the catalytic activity and stereoregularity of the resultant PMMA. Further, alkyl derivatives [L_nZnMe₂] (L_n = L_A − L_F) of synthesized Zn(II) complexes, generated in situ, showed moderate to high activities toward ring opening polymerization (ROP) of rac-lactide (rac-LA) and yielded heterotactic polylactide (PLA) with P_r up to 0.95 at −50 °C. The activity and stereoselectivity toward ROP of rac-LA by these dimethyl Zn(II) complexes should be considered as a combined effect of steric hindrance and electronic density around the metal center.