& Surface Chemistry Cross-Coupling of Aryl-Bromide and Porphyrin-Bromide on an Au(111) Surface Guowen Kuang, [a] Qiushi Zhang, [a] Deng Yuan Li, [b] Xue Song Shang, [b] Tao Lin, [a] Pei Nian Liu,* [b] and Nian Lin* [a] Abstract: Cross-coupling is of great importance in organic synthesis. Here it is demonstrated that cross-coupling of aryl-bromide and porphyrin-bromide takes place on a Au(111) surface in vacuo. The products are oligomers consisting of porphyrin moieties linked by p-phenylene at porphyrin’s meso-positions. The ratio of the cross-coupled versus homocoupled bonds can be regulated by the reac- tant concentrations. Kinetic Monte Carlo simulations were applied to determine the activation barrier. It is expected that this reaction can be employed in other aryl-bromide precursors for designing alternating co-polymers incorpo- rating porphyrin and other functional moieties. Cross-coupling is a powerful tool available to synthetic chem- ists in their quest to create new carbon Àcarbon bonds, while at the same time introducing new carbon- and heteroatom- based functional groups. [1, 2] Cross-coupling reactions allow chemists to design and manipulate delicate and complex mol- ecules. [3–8] These reactions have been proved useful for the synthesis of many important products, such as drugs, materi- als, and optical devices. [9–11] Recently, it has been demonstrated that coupling reactions can take place on solid surfaces, known as on-surface synthesis. [12–16] In an on-surface reaction, covalent bonds form between molecular precursors that adsorb on a surface; during this process, reactants, intermedi- ates, catalysts and products are confined to a two-dimensional space defined by the surface. Various on-surface reactions, in- cluding Ullmann coupling, [17–22] Glaser coupling, [23–25] alkane polymerization, [26] boronic acid condensation, [27, 28] decarboxyla- tive polymerization, [29] imine coupling, [30, 31] acylation reac- tion, [32, 33] dimerization of N-heterocyclic carbenes, [34] azide– alkyne cycloadditions, [35, 36] and Bergman cyclizations [37] have been demonstrated. To a certain extent, on-surface reactions share fundamental similarities with their counterpart reactions in solution. Nevertheless, on-surface reactions can also follow a significantly different path or mechanism. The special condi- tions of surface chemistry can be used to synthetic advantage, as on-surface reactions under ultrahigh vacuum (UHV) can be conducted under a much wider range of temperatures than re- actions in solution, and two-dimensional confined geometry can invoke reactions not accessible in three dimensions. On-surface synthesis has been used to generate diverse organic systems, including macromolecules, [18] polymeric chains, [17, 19, 26, 38] two-dimensional organic networks, [20, 22, 24, 27, 28] and graphene ribbons. [21] To date, carbon Àcarbon bond formation by homocoupling reactions has been demonstrated in on-surface synthesis. Cross-coupling reactions, however, have rarely been demon- strated on-surface, [39] despite their significance in organic syn- thesis. Here we report on cross-coupling of aryl-bromide (1) and porphyrin-bromide (2 ; Scheme 1) on Au(111) surface. This reaction, to our knowledge, has not been reported in organic synthesis. As illustrated in Scheme 1, 1 undergoes homocou- pling by Ullmann reaction, but steric hindrance inhibits homo- coupling of 2. Mixing of 1 and 2 generates cross-coupled prod- ucts of p-phenylene-linked porphyrin oligomers. Through ana- lysing the yields of homocoupled and cross-coupled bonds, we quantitatively evaluated the activation barriers of the two coupling reactions, aided by kinetic Monte Carlo simulations. The yield of the cross-coupled bonds could be boosted with excess amount of 2 in the reactant mixture. We found that at a ratio of [2]/[1] = 13 ([2] and [1] stand for molecular dosage of 2 and 1, respectively, in a unit surface area), 95% of all formed bonds are cross-coupled ones and the products are oligomers with alternating (12) n morphology. Homocoupling of molecules of 1 on an Au(111) surface by Ullmann reaction occurs at 180 8C, forming polymeric chains consisting of alternating porphyrin and biphenyl moiet- ies. [17, 40, 41] Figure 1 a shows a 9-member chain and the inset shows an unreacted monomer. The scanning tunneling micros- copy (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. The molecules thus exhibit twofold symmetry which reflects a nonplanar porphyrin core. This feature corroborates the previous studies reporting the porphyrin core is distorted to a saddle-shape conformation when adsorbed on a metal surface. [42] Adjacent molecules in the chains are 1.73 nm apart; this is in good agreement with [a] G. Kuang, Q. Zhang, Dr. T. Lin, Prof. Dr. N. Lin Department of Physics The Hong Kong University of Science and Technology Clear Water Bay (Hong Kong) E-mail : phnlin@ust.hk [b] D. Y. Li, X. S. Shang, Prof. Dr. D. N. Liu Shanghai Key Laboratory of Functional Materials Chemistry and Institute of Fine Chemicals, East China University of Science and Technology, Meilong Road 130, Shanghai(P. R. China) E-mail : liupn@ecust.edu.cn Supporting information for this article is available on the WWW under http ://dx.doi.org/10.1002/chem.201501095. Chem. Eur. J. 2015, 21, 8028 – 8032 # 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 8028 Communication DOI: 10.1002/chem.201501095