Combined Photoemission and Scanning Tunneling Microscopy Study of the Surface-Assisted Ullmann Coupling Reaction

Abstract

The adsorption and reaction of 4,4″-dibromopara-terphenyl (DBTP) and 1,3,5-tris(4-bromophenyl)benzene (TBB) on Cu(111) surface were studied with X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and density functional theory (DFT) calculations. In addition, complementary scanning tunneling microscopy (STM) data are presented. At submonolayer coverage, scission of C−Br bonds occurs between 170 and 240 K. The estimated activation energy for this process is considerably lower than the C−Br bond energy, indicating that bond scission is assisted by Cu atoms of the substrate. The remaining molecular backbones undergo linkage by C−Cu−C bonds to form organometallic oligomers. Annealing of these oligomers leads to the formation of C−C bonded covalent two-dimensional networks. Above monolayer coverage, complete C− Br cleavage requires higher temperature, confirming the role of the Cu surface in the reaction. The results provide insight into the C−Br bond scission as the initial step of the surface-assisted Ullmann reaction.

Surface chemistry of porphyrins and phthalocyanines

“…Instead, C-C bonds were formed between the β-positions and C atoms of the phenyl rings after annealing to 573 K, indicating that also partial dehydrogenation of the phenyl rings occurred [489]. Note that C-C bond formation on the basis of surface-assisted dehalogenation of aromatic bromo-or iodocompounds (also referred to as Ullmann coupling) has also been studied with numerous non-porphyrinic compounds [485][486][487][488].…”

“…Even larger rhombic and Kagome pores with diameters of up to 9.2 nm where obtained by using an extended bis-terpyridyl linker, which was made by on-surface Ullmann reaction [487] using a brominated mono-terpyridyl linker as the starting compound [501]. Coordinative bonds formed by pyridyl groups were also used to synthesize cyclic and other supramolecules based on porphyrins with extended pyridyl linkers.…”

“…55. SAT complexes were first observed at room temperature by XPS for Zn-2HTPP on Ag(1 1 1) [12] and Cu-2HTBrPP on Cu(1 1 1) [715], although in the latter work the results may be influenced by chemisorbed Br atoms resulting from the C-Br bond scission, which occurs already below 240 K on Cu(1 1 1) [487]. A Cu-2HTPyP SAT complex and its transition to CuTPyP were observed in STM images taken after the deposition of Cu onto a 2HTPyP submonolayer on Au(1 1 1) [498].…”

“…The different reactivity of the two molecules -NiDBrDPP reacted faster -was attributed to the different bonding distances to the surface: For NiDBrDPP, it was proposed that the two Br atoms at the porphyrin core bind directly to the surface, whereas the four periphenyl rings in NiTBrPP were believed to result in a larger distance. However, it has been shown that aryl-Br bonds undergo dissociation on Cu(1 1 1) already below 240 K [487], which means that arguments based on intact C-Br bonds are probably not valid. In addition, the resulting chemisorbed Br atoms may play a role in the metal exchange.…”

Recent Progress in the Fabrication of Low Dimensional Nanostructures via Surface-Assisted Transforming and Coupling

“…Actually on Au surface, not obtaining organometallic intermediates is a common phenomenon, and the generally accepted opinion is that the SSR are mobile, diffuse around on the surface of substrate, and form covalent bonds upon encounter. On the contrary, the organometallic intermediates with C−M−C bonds are constantly obtained on Cu [16,[72][73][74] and Ag surfaces [75,76]. In a word, according to the explored experiments of Ullmann reaction, different experimental results can be briefly concluded in Table 1.…”