Article pubs.acs.org/JPCC
Combined Photoemission and Scanning Tunneling Microscopy Study of the Surface-Assisted Ullmann Coupling Reaction Min Chen,† Jie Xiao,‡ Hans-Peter Steinrück,‡ Shiyong Wang,∥ Weihua Wang,∥ Nian Lin,∥ Wolfgang Hieringer,§ and J. Michael Gottfried*,† †
Fachbereich Chemie, Physikalische Chemie, Philipps-Universität Marburg, Hans-Meerwein-Strasse, 35032 Marburg, Germany Lehrstuhl für Physikalische Chemie II, and §Lehrstuhl für Theoretische Chemie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen, Germany ∥ Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China ‡
S Supporting Information *
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.
1. INTRODUCTION The surface-confined synthesis of molecular or polymeric covalent nanostructures based on aromatic building blocks has attracted considerable attention as an approach for the controlled nanostructuring of surfaces.1−4 As compared to solution-based organic synthesis, the range of reactions suitable for organic synthesis on surfaces in a vacuum is still very limited, especially with respect to C−C bond formation. One of the few examples is the Ullmann reaction, which was first described in 1901 as a method for the synthesis of substituted biphenyls from substituted bromobenzenes with metallic Cu.5 As an inherently heterogeneous reaction, the Ullmann reaction can be used for C−C coupling on Cu surfaces in an ultrahigh vacuum (UHV).2,6−16 Iodoarenes,11,17 fluoro-, and chlorobenzene18 undergo similar reactions on surfaces of Cu and other metals such as Ag, Au, Pd17−21 as was studied with highresolution electron energy loss spectroscopy (HREELS),8,9 scanning tunneling microscopy (STM),6,7,13,14,19,20 H/D atom titration experiments,8,9 temperature-programmed reaction (TPR),8,9 and near-edge X-ray absorption fine structure spectroscopy (NEXAFS).22 As a general finding, haloarenes on metal surfaces can undergo scission of the carbon−halogen bond, followed by C−C bond formation.15 On surfaces with low barriers for the C−H scission, side reactions such as dehydrogenation can occur.23,24 Model studies have mainly © 2014 American Chemical Society
been performed with small aromatic halides such as iodobenzene and para-diiodobenzene on Cu(111),8−10,25,26 Cu(110),11 Pd(111),23,24 Pd(110),24 and Au(111).17 In the case of iodobenzene on Cu or Ag surfaces, phenyl groups resulting from C−I bond scission form biphenyl around 370 K.8,9,22,27 Large aromatic halides and their reaction to C−C bonded polymers have rarely been investigated with the focus on fundamental questions.6,7,13,19−21 Most of these studies are based on STM, which provides atomic-level insight into the organometallic or covalent polymer structures2,6,7,11,13,19 but is of limited use when information about reactions and the chemical nature of intermediates is required. As an example, organometallic intermediates formed by reaction of 4,4″dibromo-para-terphenyl on Cu(111) and their further reaction to poly(para-phenylene) were observed by STM. The intermediate consists of formal biradical terphenyl units, which are connected through C−Cu−C bridges.6 Similar organometallic intermediates with C−Ag−C bonds were observed for 4,4″-dibromo-para-terphenyl on Ag(111)19 and 9,10-dibromoanthracene on Ag(111).28 Received: December 11, 2013 Revised: February 21, 2014 Published: March 4, 2014 6820
dx.doi.org/10.1021/jp4121468 | J. Phys. Chem. C 2014, 118, 6820−6830
The Journal of Physical Chemistry C
Article
Figure 1. Molecular structures of 4,4″-dibromo-para-terphenyl (DBTP, left) and 1,3,5-tris(4-bromophenyl)benzene (TBB, right).
similar reactivity, especially with respect to C−Br bond cleavage, indicate that the reaction center is rather localized and not significantly affected by the molecular backbone. The conclusions drawn from this study may therefore also apply to other bromoarenes.
The mechanism of the C−C coupling has also been studied with density functional theory (DFT).2,19,29,30 Adsorbed phenyl fragments on Cu(111) were predicted to couple via an intermediate C−Cu−C bonded state, in which the bridging Cu atom is embedded in a (111) terrace and is only slightly lifted from its equilibrium position.29 This arrangement requires substantial tilt of the phenyl rings (predicted 36°), which is not possible with large haloarenes, especially if they carry halide atoms at opposite ends. In this case, it has been shown by experiment and theory that organometallic intermediates with carbon−metal−carbon bridge bonds are formed, in which the bridging metal atom is an adatom that sits on top of the terrace atoms.2,6 Specifically, periodic dispersioncorrected DFT calculations for poly(meta-terphenyl-Cu), formed by room-temperature deposition of 4,4″-dibromometa-terphenyl (DMTP) onto Cu(111), showed zigzag chains with a buckled, nonplanar geometry, in which the bridging Cu atoms are closest to and the central phenyl rings are furthest away from the surface plane, in agreement with STM results.2 These structures with linkage by carbon−metal−carbon bonds represent the organometallic counterparts of surface-supported one- and two-dimensional coordination polymers, in which electronegative atoms such as N or O form coordinative bonds to metal adatoms. Such structures have been widely studied in the past decade,31,32 and there are also examples that include Cu atoms.33−35 While structural aspects have previously found considerable attention, less information is available about the chemical transformations during the processes of carbon−halogen bond scission, formation of an organometallic intermediate, and linkage by carbon−carbon bonds. In particular, in-depth studies with photoelectron spectroscopy are rare.2 The only relevant XPS and NEXAFS study of a related system does not provide a detailed study of the T
