Zn(II) pyridinyl amine complexes, synthesis and crystal structure studies: A comparative study of the effect of nuclearity and benzoate type on the ring-opening polymerization of cyclic esters

Abstract

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 the N-(pyridin-4-ylmethyl)aniline ligand (L1) with Zn(II) acetate and benzoic acid derivatives promoted the formation of dinuclear 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 in 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 dinuclear 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 (3) and Zn(4-Br-C6H4COO)2(L2)2 (6) having electron-withdrawing substituent on the carboxylate co-ligands, gave the most promising results with a conversion of 99% after 24 h with apparent rate constants (kapp) of 18.13 × 10−2 h−1 and 19.11 × 10−2 h−1, respectively.