The Role of Kinetics versus Thermodynamics in Surface-Assisted Ullmann Coupling on Gold and Silver Surfaces

Abstract

Surface-assisted Ullmann coupling is the workhorse of on-surface synthesis. Despite its obvious relevance, many fundamental and mechanistic aspects remain elusive. To shed light on individual reaction steps and their progression with temperature, temperature-programmed X-ray photoelectron spectroscopy (TP-XPS) experiments are performed for a prototypical model system. The activation of the coupling by initial dehalogenation is tracked by monitoring Br 3d core levels, whereas the C 1s signature is used to follow the emergence of metastable organometallic intermediates and their conversion to the final covalent products upon heating in real time. The employed 1,3,5-tris­(4-bromophenyl)­benzene precursor is comparatively studied on Ag(111) versus Au(111), whereby intermolecular bonds and network topologies are additionally characterized by scanning tunneling microscopy (STM). Besides the well-comprehended differences in activation temperatures for debromination, the thermal progression shows marked differences between the two surfaces. Debromination proceeds rapidly on Ag(111), but is relatively gradual on Au(111). While on Ag(111) debromination is well explained by first-order reaction kinetics, thermodynamics prevail on Au(111), underpinned by a close agreement between experimentally deduced and density functional theory (DFT) calculated reaction enthalpies. Thermodynamically controlled debromination on Au(111) over a large temperature range implies an unexpectedly long lifetime of surface-stabilized radicals prior to covalent coupling, as corroborated by TP-XPS of C 1s core levels. These insights are anticipated to play an important role regarding our ability to rationally synthesize atomically precise low-dimensional covalent nanostructures on surfaces.

Initiating Ullmann-like coupling of Br2Py by a semimetal surface

“…In addition, the linear chain is uniformly organized with the peanut shape, while no metal atom can be recognized inside the chain, indicating that OM intermediates might have already been transformed into the covalently coupled nanostructures. Nevertheless, these chains are still terminated with Br atoms with the typical bright protrusion ends as generally reported in literature 43 . Notably, dissociated Br atoms from parent molecules also desorb from surface during thermal annealing, as discovered in Fig.…”

Large-extended 2D supramolecular network of dipoles with parallel arrangement on a Si(111)–B surface

“…We report the formation of ordered and extended 2D supramolecular network of 1-(4"-cyanobiphenyl)-4-(4"-methoxybiphenyl)-2,5-bis(propyloxy)benzene (LDipCC) molecule on a Si(111)-B surface. Scanning Tunnelling Microscopy (STM), Density Functional Theory (DFT) calculations, which is a powerful tool to elucidate molecular adsorption on surfaces, 25,26,27 and STM simulations were used to demonstrate that the LDipCC molecule is self-assembled into a highly homogeneous parallel arrangement on the Si(111)-B surface. In addition, Si(111)-B is a weakly reactive surface, anti-parallel arrangement has been mostly observed on Cu(111), but not on less reactive surfaces such as Au(111) and HOPG.…”

Reversible Dehalogenation in On‐Surface Aryl–Aryl Coupling

“…The assessment of reaction pathways and products is usually performed employing scanning tunneling microscopy (STM) in combination with x ‐ray photoelectron spectroscopy (XPS), but neither of these techniques provide detailed time and temperature resolution of a dynamically evolving process. In this respect, temperature‐programmed XPS (TP‐XPS) is a unique tool to discern fundamental mechanistic aspects of thermally activated surface reactions that has also been applied to study dehalogenative aryl–aryl coupling reactions . Such investigations have shown that the chemical transformations reflected in the halogen XPS signal are identical for many substrates, starting with the halogen's dissociation from the molecular precursor, accompanied by its chemisorption to the substrate, and its eventual desorption from the surface.…”