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

<p>A novel series of Zn(II) complexes <b>[L</b>_<b>n</b><b>ZnCl</b>_<b>2</b><b>]</b> (L_n = L_A − L_F) based on <i>N</i>,<i>N</i>′,<i>N</i>-bis((1H-pyrazol-1-yl)methyl)amine bidentate ligands, <i>N,N</i>-bis((1H-pyrazol-1-yl)methyl)-3,5-dimethylaniline [<b>L</b>_<b>A</b>], <i>N</i>,<i>N</i>-bis((1H-pyrazol-1-yl)methyl)-2,6-dimethylaniline [<b>L</b>_<b>B</b>], <i>N</i>,<i>N</i>-bis((1H-pyrazol-1-yl)methyl)-2,6-diethylaniline [<b>L</b>_<b>C</b>], <i>N</i>,<i>N</i>-bis((1H-pyrazol-1-yl)methyl)-2,6-diisopropylaniline [<b>L</b>_<b>D</b>], <i>N</i>,<i>N</i>-bis((1H-pyrazol-1-yl)methyl)-4-bromoaniline [<b>L</b>_<b>E</b>] and <i>N</i>,<i>N</i>-bis((1H-pyrazol-1-yl)methyl)benzhydrylamine [<b>L</b>_<b>F</b>], 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 <b>[L</b>_<b>n</b><b>ZnCl</b>_<b>2</b><b>]</b> (L_n = L_A − L_F) complexes, resulting in formation of an eight-membered chelate ring. <b>[L</b>_<b>F</b><b>ZnCl</b>_<b>2</b><b>]</b> 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 (<i>M</i>_w) (11.0 × 10⁵ g/mol) of poly(methylmethacrylate) (PMMA). All the complexes resulted in syndiotactic enriched PMMA with high <i>T</i>_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 <b>[L</b>_<b>n</b><b>ZnMe</b>_<b>2</b><b>]</b> (L_n = L_A − L_F) of synthesized Zn(II) complexes, generated <i>in situ</i>, showed moderate to high activities toward ring opening polymerization (ROP) of <i>rac</i>-lactide (<i>rac</i>-LA) and yielded heterotactic polylactide (PLA) with <i>P</i>_r up to 0.95 at −50 °C. The activity and stereoselectivity toward ROP of <i>rac</i>-LA by these dimethyl Zn(II) complexes should be considered as a combined effect of steric hindrance and electronic density around the metal center.