Zinc(II), palladium(II) and cadmium(II) complexes containing 4-methoxy-N-(pyridin-2-ylmethylene)aniline derivatives: Synthesis, characterisation and methyl methacrylate polymerization

Abstract

A series of Zn(II), Pd(II) and Cd(II) complexes, [(L)nMX2]m (L = L-a – L-c; M = Zn, Pd, X = Cl; M = Cd, X = Br; n, m = 1 or 2), containing 4-methoxy-N-(pyridin-2-ylmethylene)aniline (L-a), 4-methoxy-N-(pyridin-2-ylmethyl)aniline (L-b) and 4-methoxy-N-methyl-N-(pyridin-2-ylmethyl)aniline (L-c) have been synthesized and characterized. The X-ray crystal structures of Pd(II) complexes [L1PdCl2] (L = L-b and L-c) revealed distorted square planar geometries obtained via coordinative interaction of the nitrogen atoms of pyridine and amine moieties and two chloro ligands. The geometry around Zn(II) centre in [(L-a)ZnCl2] and [(L-c)ZnCl2] can be best described as distorted tetrahedral, whereas [(L-b)2ZnCl2] and [(L-b)2CdBr2] achieved 6-coordinated octahedral geometries around Zn and Cd centres through 2-equivalent ligands, respectively. In addition, a dimeric [(L-c)Cd(μ–Br)Br]2 complex exhibited typical 5-coordinated trigonal bipyramidal geometry around Cd centre. The polymerization of methyl methacrylate (MMA) in the presence of modified methylaluminoxane (MMAO) was evaluated by all the synthesized complexes at 60 °C. Among these complexes, [(L-b)PdCl2] showed the highest catalytic activity (3.80 × 104 g PMMA/mol·Pd·h) yielding high molecular weight (9.12 × 105 g/mol) poly(methyl methacrylate) (PMMA). Syndio-enriched PMMA (characterized using 1H NMR spectroscopy) of ca. 0.68 was obtained with Tg in the range 120 – 128 °C. Unlike imine and amine moieties, the introduction of N-methyl moiety has an adverse effect on the catalytic activity, but the syndiotacticity remained unaffected.