Synthesis and Characterization of Zinc(II), Cadmium(II), and Palladium(II) Complexes with the Thiophene-Derived Schiff Base Ligand

Zn(II), Pd(II), and Cd(II) complexes, [LTHMCl2] (M = Zn, Pd; X = Br, Cl) and [LTHCd(mu-X)X-]n (X = Cl, Br; n = n, 2), supported by the (E)-N-1,N-1-dimethyl-N-2-(thiophen-2-ylmethylene)ethane-1,2-diamine (LTH) ligand are synthesized and structurally characterized. Density functional theory (DFT) electronic structure calculations and variable-temperature NMR support the presence of two conformers and a dynamic interconversion process of the minor conformer to the major one in solution. It is found that the existence of two relevant complex conformers and their respective ratios in solution depend on the central metal ions and counter ions, either Cl- or Br-. Among the two relevant conformers, a single conformer is crystallized and X-ray diffraction analysis revealed a distorted tetrahedral geometry for Zn(II) complexes, and a distorted square planar and square pyramidal geometry for Pd(II) and Cd(II) complexes, respectively. It is shown that [LTHMCl2]/(LiOPr)-Pr-I (M = Zn, Pd) and [LTHCd(mu-Cl)Cl](n)/(LiOPr)-Pr-I can effectively catalyze the ring-opening polymerization (ROP) reaction of rac-lactide (rac-LA) with 94% conversion within 30 s with [LTHZnCl2]/(LiOPr)-Pr-I at 0 degrees C. Overall, hetero-enriched poly(lactic acid)s (PLAs) were provided by these catalytic systems with [LTHZnCl2]/(LiOPr)-Pr-I producing PLA with higher heterotactic bias (Pr up to 0.74 at 0 degrees C).