N,N′,X-bidentate versus N,N′,X-tridentate n-substituted 2-iminomethylpyridine- and 2-iminomethylquinolinecoordinated palladium(II) complexes

A new series of N,N′,X-tridentate complexes, [L_nPdCl]ClO₄ (L_n = L₁, L₂, L₃) and N,N′,X-bidentate complexes, [L_nPdCl₂] (L_n = L₄, L₅) containing N-functional group-substituted 2-iminomethylpyridine and 2-iminomethylquinoline ligands (L_n) were synthesized and characterized by X-ray crystallography. It showed that the coordination geometry around the palladium center of all Pd(II) complexes was slightly distorted square planar. The catalytic activity of Pd(II) complexes for methyl methacrylate polymerization was investigated. The reaction of [Pd(CH₃CN)₂Cl₂] with N-functional group-substituted 2-iminomethylpyridine and 2-iminomethylquinoline can produce N,N′,X-bidentate Pd(II) complexes and N,N′,X-tridentate Pd(II) complexes depending on the functional group substitution on the nitrogen of the imine moiety. For example, N,N-dimethyl-3-((pyridin-2-ylmethylene)amino)propan-1-amine (L₁), N,N-dimethyl-3- ((quinolin-2-ylmethylene)amino)propan-1-amine (L₂), and N-(3-(methylthio)propyl)-1-(pyridin-2-yl) methanimine (L₃) were coordinated to palladium as N,N′,X-tridentate mode in the presence of NaClO₄ to yield two fused ring metallacyclic [(NN’X)PdCl]ClO₄ complexes with high yield, i.e. [L_nPdCl]ClO₄ (L_n = L₁, L₂, L₃), respectively. However, N-methyl-N-(3-((pyridin-2-ylmethylene) amino)propyl)aniline (L₄) and N-(3-methoxypropyl)-1-(pyridin-2-yl)methanimine (L₅) yield (in the absence of NaClO₄) N,N’,X-bidentate metallacyclic [(NN’)PdCl₂] complexes with high yield, i.e. [L₄PdCl₂] and [L₅PdCl₂], respectively. The X-ray crystal structure of Pd(II) complexes revealed that the palladium in [L_nPdCl]ClO₄ (L_n = L₁, L₂, L₃) and [L_nPdCl₂] (L_n = L₄, L₅) formed a slightly distorted square planar geometry involving two nitrogens of iminomethylpyridine ligand, one X, and one or two chlorides. The N, N’,X-tridentate complex [L₂PdCl]ClO₄ and the N,N’,X-bidentate complex [L₅PdCl₂] showed high catalytic activity for polymerization of methyl methacrylate in the presence of co-catalyst, modified methylaluminoxane at 60 °C compared to the reference complex anhydrous [PdCl₂]. Specifically, the activities of [L₂PdCl]ClO₄ and [L₄PdCl₂] were 1.43 × 10⁵ and 1.08 × 10⁵ g PMMAM⁻¹ Pd h, respectively. The syndiotacticity of poly(methylmethacrylate) (PMMA), which was characterized using¹H NMR spectroscopy, was about 0.70 for the [L₂PdCl]ClO₄ and [L₄PdCl₂] complexes.