An Imidazole-Rich Pd(II)-Polymer Pre-catalyst for the Suzuki-Miyaura Coupling: Stability Influenced by Dissolved Oxygen and Reactants Concentration

Herein a novel Pd(II)-polymeric pre-catalyst was prepared by coordinating Pd(II) ions to a low cost imidazole/carboxylate-rich polymer. The material displays good activity in the Suzuki-Miyaura coupling towards a range of aryl bromides and iodides in iPrOH/H2O mixtures at 25 or 60 degrees C. Catalyst longevity and recyclability proved to be sensitive to the concentration of the cross-coupling partners. When the concentration of PhBr is high ([PhBr]=250 mmol L-1), inactive Pd(0) aggregates were detected. On the other hand, when the reaction was performed at 20-fold dilution ([PhBr]=12.5 mmol L-1) the material could be reused up to 12 times without significant loss of catalytic activity. In this condition, minimum amount of Pd(0) was detected by XPS analysis and no Pd(0) aggregates were observed by XRPD. Importantly, it was found that the presence of oxygen is fundamental for avoiding formation of inactive Pd(0) aggregates.