Synthesis, structures, and catalytic efficiency in ring opening polymerization of rac -lactide with tridentate vs. bidentate cobalt(ii), zinc(ii), and cadmium(ii) complexes containing N -substituted N , N -bis((3,5-dimethyl-1 H -pyrazol-1-yl)methyl)amine ligands.

A series of Co(ii), Zn(ii), and Cd(ii) complexes supported by 1-(3,5-dimethyl-1 H -pyrazol-1-yl)- N -((3,5-dimethyl-1 H -pyrazol-1-yl)methyl)- N -(furan-2-ylmethyl)methanamine (L A ) and N , N -bis((3,5-dimethyl-1 H -pyrazol-1-yl)methyl)-4-isopropylaniline (L B ) were synthesized. The direct chelation of CoCl 2 ·6H 2 O, ZnCl 2 , and CdBr 2 ·4H 2 O by the ligands produced [L n MX 2 ] (L n = L A or L B ; M = Zn or Co, with X = Cl; M = Cd, with X = Br) complexes in high yields. Structural studies revealed that [L B CoCl 2 ] and [L B ZnCl 2 ] adopted distorted tetrahedral geometries, as L B coordinated the metal centers in a bidentate fashion, while L A coordinated the metal centers in a tridentate fashion through the nitrogen atoms of the pyrazole and amine moieties, so that [L A CoCl 2 ] and [L A ZnCl 2 ] exhibited trigonal bipyramidal geometries and [L A CdBr 2 ] a square pyramidal geometry. [L B CdBr 2 ] has two Cd-containing structures per unit cell, whereby one Cd center adopted a distorted tetrahedral geometry and the other exhibited square bipyramidal geometry. The in situ- generated alkyl derivatives of the synthesized complexes were assessed in the ring-opening polymerization of rac- lactide. Heterotactic polylactides (PLAs) were furnished with all complexes. The [L B ZnCl 2 ]/MeLi system produced PLA with a superior heterotactic bias ( P r up to 0.94) at -25 °C. PLAs with wide-ranging polydispersity indices (1.16-2.23) and low molecular weights were produced in all cases, irrespective of the specific M(II) center and ancillary ligand utilized.