Zn (II) Pyridinyl Amine Complexes, Synthesis and Crystal Structure Studies: A Comparative Study of the Effect of Nuclearity and Benzoate Type On Ring-Opening Polymerization of Cyclic Esters
29 Pages Posted: 12 Oct 2021
A series of N-(pyridinylmethyl)aniline Zn(II) carboxylate complexes were synthesized and fully characterized by NMR, IR, mass spectroscopy, and elemental analysis. The reaction of N-(pyridin- 4-ylmethyl)aniline ligand (L1) with Zn(II) carboxylate promoted the formation of di-nuclear species with the formula [Zn(CH3COO)(RCOO)(L1)]2 and [Zn(RCOO)2(L1)]2 whereas the N-(pyridin-3-ylmethyl)aniline ligand (L2) exclusively afforded mononuclear complexes of the form Zn(L1)2(RCOO)2(where R = C6H5-, 4-Me-C6H4-, 4-F-C6H4-, and 4-Br-C6H4-). At elevated temperatures, the carboxylate complexes are active for the ring-opening polymerization (ROP) of ɛ-caprolactone (ɛ-CL) lactides (rac/L-LA) in bulk and toluene yielding polymers with low molecular weights between 2320 g mol-1 and 4980 g mol-1. The mononuclear complexes, which rearranged easily to their active species, were generally more active than their di-nuclear counterparts. The electronic properties of the carboxylate co-ligands significantly influenced the catalytic performance of all the complexes. The complexes [Zn(CH3COO)(4-F-C6H4COO)(L1)]2 and Zn[(4-Br-C6H4COO)(L2)]2, having electron-withdrawing substituents on the carboxylate coligand, gave the most promising results with a conversion of 99% after 24 h with apparent rate constants (κapp) of 18.13 x 10-2 h-1 and 19.11 x 10-2-1, respectively.
Keywords: Pyridynyl amines, Zinc, Ring-opening polymerization, ɛ-Caprolactone, Lactide
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