Well-Defined and Highly Effective Nickel Catalysts Coordinated on Tridentate SNO Schiff-Base Derivatives for Alternating Copolymerization of Epoxides and Anhydrides.

Abstract

A series of Ni complexes supported by SNO Schiff-base derivatives were synthesized in this study. Complex synthesis and characterization data are reported herein. Treatment of the pro-ligands [L1-H = 2-(((2-(methylthio)ethyl)imino)methyl)phenol, L2-H = 2,4-di-tert-butyl-6-(((2-(methylthio)ethyl)imino)methyl)phenol, L3-H = 2-(((2-(methylthio)ethyl)imino)methyl)-4,6-bis(2-phenylpropan-2-yl)phenol, L4-H = 4-bromo-2-(((2-(methylthio)ethyl)imino)methyl)phenol, and L5-H = 4-chloro-2-(((2-(methylthio)ethyl)imino)methyl)phenol] with Ni(OAc)2·4H2O in refluxing ethanol afforded six-coordinate mono-Ni(II) complexes [L2nNi] (n = 1-5). Noteworthy, a heptanuclear nickel(II) octacarboxylate species complex 6 and dinuclear nickel complex 6a resulted from treatment of L6-H [4-fluoro-2-(((2-(methylthio)ethyl)imino)methyl)phenol] with different metal precursors [Ni(OAc)2·4H2O for 6; NiBr2 for 6a] giving a quantitative yield. The reaction of nickel acetate tetrahydrate and L7-H to L9-H [L7-H = 2-methoxy-6-(((2-(methylthio)ethyl)imino)methyl)phenol, L8-H = 5-methoxy-2-(((2-(methylthio)ethyl)imino)methyl)phenol, and L9-H = 4-methoxy-2-(((2-(methylthio)ethyl)imino)methyl)phenol] produced the four-coordinate complexes [L2nNi] (n = 7-9). The highest performing catalyst was complex 3, which was highly efficient for the ring-opening copolymerization of phthalic anhydride (PA) and cyclohexene oxide (CHO) in the presence of a cocatalyst (4-dimethylaminopyridine). In addition, the same copolymerization conditions produced narrowly dispersed polyesters, with high selectivity and polymerization control. In addition to PA-CHO copolymerization, efficient diglycolic anhydride-PA and PA-propene oxide copolymerization was achieved under the same conditions. These catalysts are straightforward to produce and extend the scope of potential substrates.