Reversible Dehalogenation in On‐Surface Aryl–Aryl Coupling

Abstract

In the emerging field of on‐surface synthesis, dehalogenative aryl–aryl coupling is unarguably the most prominent tool for the fabrication of covalently bonded carbon‐based nanomaterials. Despite its importance, the reaction kinetics are still poorly understood. Here we present a comprehensive temperature‐programmed x‐ray photoelectron spectroscopy investigation of reaction kinetics and energetics in the prototypical on‐surface dehalogenative polymerization of 4,4′′‐dibromo‐p‐terphenyl into poly(para‐phenylene) on two coinage metal surfaces, Cu(111) and Au(111). We find clear evidence for reversible dehalogenation on Au(111), which is inhibited on Cu(111) owing to the formation of organometallic intermediates. The incorporation of reversible dehalogenation in the reaction rate equations leads to excellent agreement with experimental data and allows extracting the relevant energy barriers. Our findings deepen the mechanistic understanding and call for its reassessment for surface‐confined aryl–aryl coupling on the most frequently used metal substrates.

Asymmetric Elimination Reaction on Chiral Metal Surfaces

“…Interestingly, the onset temperatures for the debromination process on PdGa{111} correspond well to those reported for Cu(111). [20,44] This observation supports the presumption that these surfaces have a comparable catalytic activity due to the similar average energy of d-band electrons (d-band center). [27,45,46] An intriguing difference is that on PdGa{111} surfaces the BMA debromination occurs over a wider temperature range than on Cu(111).…”

“…While TPD is an established method to determine enantiospecific reaction kinetics, [ 5,15 ] applications of TP‐XPS have been limited to symmetric reactions so far. [ 16–20 ]…”

“…[27,45,46] An intriguing difference is that on PdGa{111} surfaces the BMA debromination occurs over a wider temperature range than on Cu(111). [20,44] The temperature range for debromination is particularly wide on Au(111), which has been attributed to reversible debromination processes. [19,20] Therefore we simulate the debromination reaction on the PdGa{111} surfaces with a rate equation model including reversibility and enantiospecific energy barriers.…”

Oxygen-promoted synthesis of armchair graphene nanoribbons on Cu(111)

“…On-surface synthesis has received great attention as a method to create atomically-precise one-dimensional (1D) and two-dimensional (2D) polymers with intriguing properties [1][2][3][4][5][6][7][8]. In particular, graphene nanoribbons (GNRs), a category of quasi-1D nanomaterials derived from graphene, have been widely studied due to their tuneable electronic properties and potential applications in semiconductor de-vices, such as field-effect transistors and spintronics [9][10][11][12][13].…”

“…So far, few studies have focused on GNR synthesis on Cu(111) due to the stronger surface interaction, despite the lower temperature for dehalogenation [23,24,26,27]. It has been shown that chiral GNRs can be synthesized on Cu(111) [24] using the same precursor that yields non-chiral 7-AGNR on Au(111) [1] and that dehalogenation can be reversible on Au(111) but not Cu(111) [7], which implies that the reaction pathway and products achieved could be controlled through the choice of substrate.…”