Kinetically Controlled Synthesis of Four‐ and Six‐Member Cyclic Products via Sequential Aryl‐Aryl Coupling on a Au(111) Surface

Abstract

We synthesize four-and six-member cyclic products via sequential multi-step aryl-aryl coupling reactions of 2,3,6,7,10,11-hexabromotriphenylene molecules on a Au(111) surface. The final products as well as the organo-gold intermediate structures are identified using scanning tunneling microscopy and density-functional theory simulation. By adjusting reaction temperature and post-annealing temperature, we enhance/suppress the yields of the four-member and sixmember cyclic products. We propose an underlying mechanism which is associated with different reaction kinetics of the firstorder and second-order reactions. This work exemplifies intricate kinetically-controlled on-surface synthesis when multiple reactions of different reaction order are involved.Over the past decade, on-surface reactions have attracted tremendous attention and dozens of well-known "textbook" reactions have been shown to proceed on surface. [1] Among these reactions, ring formation is a particular interesting type. Several ring-formation reactions have been realized on metal surfaces, such as the Diels À Alders reaction, [2] the 1,3-dipolar cycloaddition (Huisgen reaction), [3] [2 + 2 + 2] cyclotrimerization of alkyne groups, [4] and [2 + 2] cyclodimerization reaction. [5] Apart from these intermolecular couplings, intramolecular reactions of Bergman [6] and C1-C5 enediyne cyclization [7] also form cyclic structures. The cycloaddition reactions involving alkenes and alkynes were also reported. [8] Besides cycloaddition reactions, six-member and four-member cyclic structures were synthesized via the surface-mediated Ullmann-type reactions, [9] which evidenced that the formation of six-and four-member rings depends on crystal orientation [9b,c] or catalytic behavior [9a,c] of the metal substrates.Here we report on multi-step aryl-aryl coupling reactions of 2,3,6,7,10,11-hexabromotriphenylene (HBTP) molecules taking place on a Au(111) surface. The reaction pathway can be alternated by controlling the reaction temperature or postannealing temperature. As illustrated in Scheme 1, deposition of HBTP at room-temperature substrate followed by post annealing yields organo-gold compounds (C1) which comprises of one aryl-aryl bond by reaction I. Next, C1 is transformed into a four-member cyclic product consisting of two aryl-aryl bonds (C2) by reaction II, or a six-member cyclic product consisting of three aryl-aryl bonds (C3) by reaction III. Deposition of HBTP onto a hot Au(111) surface significantly enhances the yield of C2 but almost completely suppresses C3. We systematically varied the annealing temperature and surface temperature to investigate the underlying reaction mechanism.Deposition of HBTP molecules on a Au(111) surface followed by post-annealing at 100°C for 10 minutes generates regularly-packed molecular islands consisting of unreacted HBTP molecules as shown in Figure S1. After annealing the sample at 100°C for 12 hours, the molecular islands, highlighted in the blue colored area in Figure 1a, starts to dissolve from the bo...

Pseudo-atomic orbital behavior in graphene nanoribbons with four-membered rings

“…Nanoribbons with four-membered rings, however, have so far remained elusive due to challenges in their chemical synthesis (38). Although [2 + 2] cycloaddition has been shown to generate four-membered rings via on-surface synthesis, it has so far only been used to couple smaller acenes and triphenylenes (31,(39)(40)(41)(42). The CBD units in these systems exhibit relatively little antiaromaticity due to their fusion pattern (i.e., because the CBD bonds have more single bond character in the dominant resonance structures that maximize the number of Clar sextets), resulting in a large bandgap (31,36,43).…”

Dissociative Electron Attachment to 2,3,6,7,10,11-Hexabromotriphenylene

“…8,9 Recent studies of the aryl−aryl coupling of HBTP on the Au(111) surface by means of scanning transmission electron microscopy (STEM) and density functional theory (DFT) simulations revealed four possible reaction pathways depending on the reaction temperature and post-annealing temperature. 18 Obviously, these reactions are largely controlled by the electron-transfer processes and, therefore, by the electron affinity of HBTP and stability of its anion-radical. Surprisingly, no data have been reported so far regarding the electron affinity of HBTP.…”

“…Transition-metal-catalyzed cross-coupling reactions − enable highly efficient and selective functionalization of polyhalogenated polyaromatic hydrocarbons at their peripheral sites. Among polyhalogenated polyaromatic hydrocarbons, 2,3,6,7,10,11-hexabromotriphenylene (HBTP) is one of the most common and readily available building blocks for the synthesis of star-shaped molecules, discotic liquid crystals, ,,, and two-dimensional π-conjugated COFs. , Recent studies of the aryl–aryl coupling of HBTP on the Au(111) surface by means of scanning transmission electron microscopy (STEM) and density functional theory (DFT) simulations revealed four possible reaction pathways depending on the reaction temperature and post-annealing temperature . Obviously, these reactions are largely controlled by the electron-transfer processes and, therefore, by the electron affinity of HBTP and stability of its anion-radical.…”

Highly Selective On-Surface [2 + 2] Cycloaddition Induced by Hierarchical Metal–Organic Hybrids

“…Phenylene, known as a novel π-system in which benzenoid and cyclobutadienoid rings are aligned in an alternating manner, has received extensive attention owing to its unique structural, optical, and electronic properties. − In particular, incorporating the phenylene moiety into azaacenes − can form extended and dramatically stable π-conjugated frameworks, which offer considerable potential for n-type semiconductors in organic electronic devices. − The on-surface dehalogenative [2 + 2] cycloaddition provides a powerful methodology enabling the atomically precise synthesis of phenylene-containing scaffolds under ultrahigh-vacuum (UHV) conditions. − Catalyzed by noble-metal surfaces, precursor molecules of ortho- brominated aromatics are capable of generating the four-membered carbonic rings (i.e., the biphenylene moiety). To date, two types of mechanisms have been observed (Scheme a): (1) direct cycloaddition of biradicals (arynes) upon complete debromination − and (2) the metal–organic hybrids (MOHs) intermediate pathway. , The former mechanism proceeds by [2 + 2] and [2 + 2 + 2] cycloadditions simultaneously whether in solution , or on surfaces. − , Some fruitful efforts to improve the selectivity have been made from the perspective of raising the energy barriers of side reactions, achieved by restraining the orientation of monomers through the templating of the Au(100) substrate or utilizing the sterically hindered precursor (Scheme b), while the latter mechanism exists solely in on-surface synthesis.…”