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

A series of Zn (II), Pd (II) and Cd (II) complexes, [(L) _n MX ₂ ] _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 [L ₁ PdCl ₂ ] (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) center in [(L-a)ZnCl ₂ ] and [(L-c)ZnCl ₂ ] can be best described as distorted tetrahedral, whereas [(L-b) ₂ ZnCl ₂ ] and [(L-b) ₂ CdBr ₂ ] achieved 6-coordinated octahedral geometries around Zn and Cd centers 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 center. The polymerization of methyl methacrylate in the presence of modified methylaluminoxane was evaluated by all the synthesized complexes at 60°C. Among these complexes, [(L-b)PdCl ₂ ] showed the highest catalytic activity [3.80 × 10 ⁴ g poly (methyl methacrylate) (PMMA)/mol Pd hr ⁻¹ ], yielding high molecular weight (9.12 × 10 ⁵ g mol ⁻¹ ) PMMA. Syndio-enriched PMMA (characterized using ¹ H-NMR spectroscopy) of about 0.68 was obtained with T _g 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.