A Straightforward Access to Sustainable and Reusable Melanin-Supported Palladium Catalysts: Characterization and Application in Sonogashira Cross-Coupling Reactions

In the last decades, the impact of the emergent concept of sustainable development has driven the impressive evolution of catalytic methodologies, also stimulated by the breakthrough of new materials and nanotechnologies. Based on this evidence, we report herein the design of sustainable catalysts by addressing the unexploited potential of melanin, a class of pigments widely diffused in nature. By pursuing a straightforward and sustainable approach, we prepared a series of Pd@melanin hybrids inspired from mammalian eumelanin and fungal allomelanin. XPS analyses revealed that DHImel (DHI: 5,6-dihydroxyindole) can support palladium by promoting the reduction of Pd(II) to Pd(0) thanks to the catechol groups that can be converted into quinones. HR-TEM and XRD analyses revealed that palladium atoms are uniformly distributed, forming nanoaggregates in the face-centered cubic crystal structure with an average dimension of 8 nm. The catalytic activity of the hybrids was tested in the Sonogashira cross-coupling reaction. Overall, the collected data suggested that Pd@DHImel, with the highest product yields and lowest Pd loading, is the most efficient catalyst, even when working with deactivated aromatic iodides. Finally, stability tests indicated that the hybrid did not suffer leaching phenomena, leading to high yields for up to eight cycles.