A Single‐Molecule‐Level Mechanistic Study of Pd‐Catalyzed and Cu‐Catalyzed Homocoupling of Aryl Bromide on an Au(111) Surface

Abstract

On-surface Pd- and Cu-catalyzed C-C coupling reactions between phenyl bromide functionalized porphyrin derivatives on an Au(111) surface have been investigated under ultra-high vacuum conditions by using scanning tunneling microscopy and kinetic Monte Carlo simulations. We monitored the isothermal reaction kinetics by allowing the reaction to proceed at different temperatures. We discovered that the reactions catalyzed by Pd or Cu can be described as a two-phase process that involves an initial activation followed by C-C bond formation. However, the distinctive reaction kinetics and the C-C bond-formation yield associated with the two catalysts account for the different reaction mechanisms: the initial activation phase is the rate-limiting step for the Cu-catalyzed reaction at all temperatures tested, whereas the later phase of C-C formation is the rate-limiting step for the Pd-catalyzed reaction at high temperature. Analysis of rate constants of the Pd-catalyzed reactions allowed us to determine its activation energy as (0.41±0.03) eV.

On‐Surface Reactions

“…Similarly, the synthesis of poly(3,4‐ethylenedioxythiophene) by polymerization of 2,5‐diiodo‐3,4‐ethylenedioxythiophene on Cu(110) has been shown 16. Further examples for Ullmann‐like on‐surface syntheses have been shown on Cu,17–20 Ag,20–26 Au,19, 26–34 CaCO 3 ,35–37 NaCl@Au,38 h‐BN@Ni,39 sapphire,40 and graphene@Ni 39…”

“…Cu,11, 14–20, 42 Ag,20–26, 42 Au,13, 19, 26–34, 41, 81 CaCO 3 ,35–37 HOPG,42 NaCl@Au,38 h‐BN@Ni39 graphene@Ni39…”

Cross‐Coupling of Aryl‐Bromide and Porphyrin‐Bromide on an Au(111) Surface

“…Homocoupling of molecules of 1 on an Au(111) surface by Ullmann reaction occurs at 180 °C, forming polymeric chains consisting of alternating porphyrin and biphenyl moieties 17. 40, 41 Figure 1 a shows a 9‐member chain and the inset shows an unreacted monomer. The scanning tunneling microscopy (STM) topograph of the molecules in the chain retains the features of the unreacted monomer, displaying a square‐shape morphology with a depression trough in the middle, as marked by the dashed lines in Figure 1 a.…”

Steering On‐Surface Reactions by a Self‐Assembly Approach

“…[1][2][3][4] A number of homogeneous reactions including Ullmann coupling, [5][6][7][8] Glaser coupling, [9][10][11] imine formation, [12,13] Bergman cyclization [14] and the like have been explored at metal surfaces.H owever,t heir mechanisms are largely elusive except for well-studied Ullmann [15][16][17][18] and Glaser [10,11,[19][20][21] coupling reactions.D ifferent from the intrinsic reactivity of homogeneous synthesis in wet chemistry,on-surface synthesis can be seriously influenced by various surface effects,such as catalytically active adatoms, [22] reconstruction and step edges, [23] poison adatoms, [15] surface mobility, [16] space confinement, [24] and lattice guiding [10] as well.Ty pically,s ubstrate-induced confinement effect could result in the formation of ordered low dimensional surface nanostructures. [1][2][3][4] A number of homogeneous reactions including Ullmann coupling, [5][6][7][8] Glaser coupling, [9][10][11] imine formation, [12,13] Bergman cyclization [14] and the like have been ex...…”