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

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.