Synthesis of N,N′,X-tridentate 2-iminomethylpyrrole-coordinated palladium(II) complexes via N─H bond activation of pyrrole moiety

The reaction of [Pd(CH₃CN)₂Cl₂] with N-functional group-substituted 2-iminomethylpyrrole-based ligands, namely N¹-((1H-pyrrol-2-yl)methylene)-N³,N³-dimethylpropane-1,3-diamine (L_A), N¹-((1H-pyrrol-2-yl)methylene)-N³-methyl-N³-phenylpropane-1,3-diamine (L_B), N-((1H-pyrrol-2-yl)methylene)-3-(methylthio)propan-1-amine (L_C) and N-((1H-pyrrol-2-yl)methylene)-3-methoxypropan-1-amine (L_D), resulted in [L_nPdCl] (L_n = L_A–L_D) complexes in high yield via N─H bond activation of pyrrole moiety without use of base. [L_nPdCl] existed as monomeric four-coordinated complexes with slightly distorted square planar geometries around the palladium metal center. The ligands show N,N′,X-tridentate binding mode to the palladium metal center to give two fused ring metallacycles. [L_BPdCl] gave the highest activity (3.29 × 10⁵ g PMMA (mol Pd)⁻¹ h⁻¹) for a methyl methacrylate (MMA) polymerization in the presence of modified methylaluminoxane at 60 °C compared to the other Pd(II) analogues, and resulted in PMMA with higher molecular weight (M_w = 7.16 × 10⁵ g mol⁻¹) and narrower polydispersity index. Syndiotactic-enriched PMMA resulted in all cases.