Abstract
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. [LnPdCl]ClO4 (Ln = 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 [LnPdCl]ClO4 (Ln = L1, L2, L3) and [LnPdCl2] (Ln = 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 PMMA M−1 Pd h, respectively. The syndiotacticity of poly(methylmethacrylate) (PMMA), which was characterized using 1H NMR spectroscopy, was about 0.70 for the [L2PdCl]ClO4 and [L4PdCl2] complexes.
Graphical Abstract
A new series of N,N′,X-tridentate complexes, [LnPdCl]ClO4 (Ln = L1, L2, L3) and N,N′,X-bidentate complexes, [LnPdCl2] (Ln = L4, L5) containing N-functional group-substituted 2-iminomethylpyridine and 2-iminomethylquinoline ligands (Ln) 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.