Elucidate the Metal-Catalyzed Decarboxylative Coupling of Carboxylic Acids on Metal Surfaces

Metal-catalyzed thermodynamic difference in the stepwise decarboxylative coupling of carboxylic acids on metal surfaces is reported. Our study reveals that the decarboxylative coupling includes three steps: deprotonation, decarboxylation, and C-C coupling. Furthermore, it exhibits a strong underlying-metal-substrate dependency that shows no activity on Au(111), certain activity on Ag(111), and the highest activity on Cu(111). It is demonstrated that the central π-system of carboxylic acids plays a pivotal role, raising the barrier for deprotonation and lowering it for decarboxylation. Nevertheless, deprotonation remains the kinetically preferred first step on all surfaces. A combination of temperature-dependent X-ray photoelectron spectroscopy and scanning tunneling microscopy provides an intuitive map of the chemical states along temperature, enabling a clear determination of the deprotonation and decarboxylation temperature windows. Density functional theory calculations further support the proposed stepwise reaction mechanism and the pivotal dual role of the central π-system in carboxylic acids.