Diverse coordination geometry of cobalt (II), zinc (II), and cadmium (II) complexes comprising N,N-bis(1H-pyrazol-1-yl)methyl)amines derivatives: Synthesis, structures, and ring opening polymerization of rac-lactide

A series of six new complexes of Co (II), Zn (II), and Cd (II) supported by bis(1H-pyrazol-1-yl)methyl)amine derivatives (N,N-bis((1H-pyrazol-1-yl)methyl)-1-phenylethanamine (L_A), N,N-bis((1H-pyrazol-1-yl)methyl)-1-(furan-2-yl)methanamine (L_B), and N,N-bis((1H-pyrazol-1-yl)methyl)-4-isopropylaniline (L_C) synthesized. The direct chelation of the ligands (L_n = L_A − L_C) with ZnCl₂, CoCl₂.6H₂O, and CdBr₂·4H₂O produced [L_nMX₂] (L_n = L_A − L_C; M = Zn, Co; X = Cl; M = Cd, X = Br) and [L_CCd(μ-Br)Br]₂ in high yields. Structural studies revealed that [L_nZnCl₂] (L_n = L_A − L_C), [L_ACdBr₂], and [L_CCoCl₂] adopted the distorted tetrahedral geometry, whereas [L_ACoCl₂] and [L_CCd(μ-Br)Br]₂ adopted a distorted square pyramidal geometry by coordinating with corresponding ligands in a bidentate fashion. Additionally, [L_BCdBr₂] possessed a distorted trigonal bipyramidal geometry that was obtained by the chelation with corresponding ancillary ligands in the tridentate coordination mode. The dimethyl derivatives of [L_nMMe₂] (L_n = L_A − L_C; M = Zn, Co; X = Cl; M = Cd, X = Br), which were generated in situ, were employed to catalyze the ring-opening polymerization (ROP) of rac-lactide (rac-LA). Hetero-enriched polylactides (PLAs) were furnished with all these complexes (Complex [L_BZnCl₂] produced PLA with superior heterotactic bias (P_r of up to 0.93) at −25°C). PLAs with wide-ranged polydispersity indices (PDI = 1.43–1.54) were produced in all the cases irrespective of the M (II) center and variations in ancillary ligands.