Title : Pd/DNA as highly active and recyclable catalyst of Suzuki-Miyaura coupling and aminocarbonylation. XPS investigation
Palladium is common known as best catalyst in the Suzuki–Miyaura and carbonylative coupling reactions due to its very high efficiency. Despite this, searching for new palladium catalysts is still a challenge. Not only complexes but also palladium nanoparticles (Pd NPs) should be taken into account, due to their unique physical properties differ from those of bulk metals and metal complexes. The successful application of Pd NPs requires their good stabilization against agglomeration.
DNA is a biopolymer with many unique features. It has been proved suitable as a template for metal and metal oxide nanoparticles, only scarcely employed in catalysis. DNA-supported Pd NPs were prepared out of cheap natural DNA at mild conditions in EtOH/H2O solution, without additional reducing agent. An effect of the palladium precursor and its dosage, on the various palladium nanoparticles sizes and morphology of Pd/DNA was observed (XPS, TEM, XRD, FTIR, SEM). The chemical shift in binding energy of XPS Pd 3d increased with the increase in the Pd(0) content indicated the interaction (immobilization) of Pd NPs with the DNA surface.
The Pd/DNA catalysts prepared from Pd(OAc)2 exhibited various activities; however, the best result was noted for Pd/DNA containing mainly Pd(II) and only a small amount of Pd(0). The calculated content of Pd(0) present in the catalysts prepared at RT (C1) and at 80° C (C2) was 20% and 45%, respectively. It was proved that the most catalytically active Pd(0) was formed under catalytic reaction conditions. For both, C1 and C2 catalysts, the proportion of metallic palladium increased to above 80%.
The catalyst was recovered by simple phase separation and then reused in seven consecutive cycles with a high activity. For the first time, Pd/DNA was applied with very good results in the carbonylative coupling of iodobenzene, leading to amides, benzoic acid, or benzophenone depending on the kind of nucleophile used. Aminocarbonylation of iodobenzene with n-hexylamine was performed with excellent selectivity using Mo(CO)6 as a CO source, while a mixture of products was formed with gaseous CO. The recovered Pd/DNA catalyst was used in the next four runs with high activity.