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-1H-pyrazol-1-yl)methyl)amine ligands

A series of Co(ii), Zn(ii), and Cd(ii) complexes supported by 1-(3,5-dimethyl-1H-pyrazol-1-yl)-N-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-N-(furan-2-ylmethyl)methanamine (L(A)) and N,N-bis((3,5-dimethyl-1H-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.