Ha-Jin Lee

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

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.

Synthesis and Structural Characterisation of Palladium(II) Complexes with N,N′,N-Tridentate N′-Substituted N,N-Di(2-picolyl)amines and their Application to Methyl Methacrylate Polymerisation

The reaction of [Pd(CH3CN)2Cl2] with N′-substituted N,N-di(2-picolyl)amine-based ancillary ligands, for example N,N-di(2-picolyl)cyclohexylmethylamine (L1), N,N-di(2-picolyl)benzylamine (L2), N,N-di(2-picolyl)aniline (L3), and 1,4-bis[bis(2-pyridylmethyl)aminomethyl]benzene (L4), in the presence of NaClO4 in ethanol yields a new series of [(NN′N)PdCl]X (X = ClO4, Cl) complexes, i.e. mononuclear [LnPdCl]ClO4 (Ln = L1, L2, L3) and binuclear [L4Pd2Cl2]Cl2. X-Ray crystallographic analysis determined that the Pd atom in complexes [(NN′N)PdCl]X showed a slightly distorted square-planar geometry involving three nitrogen atoms and a chlorido ligand. Moreover, the unit cell included a ClO4– or Cl– anion as the counterion. The complex [L1PdCl]ClO4 showed the highest catalytic activity for the polymerisation of methyl methacrylate in the presence of modified methylaluminoxane at 60°C among the mononuclear PdII complexes. Specifically, the activity of binuclear [L4Pd2Cl2]Cl2 was 2-fold higher than the corresponding mononuclear [L2PdCl]ClO4 per active palladium metal centre.