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Trimethyltin Mediated Covalent Gold-Carbon Bond Formation Arunabh Batra, Gregor Kladnik, Narjes Gorjizadeh, Jeffrey Meisner, Michael Steigerwald, Colin Nuckolls, Su Ying Quek, Dean Cvetko, Alberto Morgante, and Latha Venkataraman J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja5061406 • Publication Date (Web): 14 Aug 2014 Downloaded from http://pubs.acs.org on August 25, 2014
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Trimethyltin Mediated Covalent Gold-Carbon Bond Formation Arunabh Batra,† Gregor Kladnik,‡,¶ Narjes Gorjizadeh,§,k Jeffrey Meisner,⊥ Michael Steigerwald,⊥ Colin Nuckolls,⊥ Su Ying Quek,∗,§,k Dean Cvetko,¶ Alberto Morgante,∗,¶,# and Latha Venkataraman∗,† Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY, Department of Physics, University of Ljubljana, Ljubljana, Slovenia, CNR-IOM Laboratorio Nazionale TASC, Basovizza SS-14, km 163.5, I-34012 Trieste, Italy, Department of Physics, National University of Singapore, Singapore, Institute of High Performance Computing, Agency for Science, Technology and Research, Singapore, Department of Chemistry, Columbia University, New York, NY, and Department of Physics,University of Trieste, Trieste, Italy Received July 17, 2014; E-mail:
[email protected];
[email protected];
[email protected] Abstract: We study the formation of covalent gold-carbon bonds in benzyltrimethylstannane (C10 H16 Sn) deposited on gold in ultra high vacuum conditions. Through X-ray Photoemission Spectroscopy and X-ray absorption measurements, we find that the molecule fragments at the Sn-Benzyl bond when exposed to gold surfaces at temperatures as low as −110 C. We show that the resulting benzyl species is stabilized by the presence of Au(111), but only forms covalent Au-C bonds on more reactive Au surfaces like Au(110). In addition, we present spectroscopic proof for the existence of an electronic ‘gateway’ state localized on the Au-C bond that is responsible for its unique electronic properties. Finally, we use density functional theory based nudged elastic band calculations to elucidate the crucial role played by the undercoordinated Au surface in the formation of Au-C bonds.
A well-defined, robust, metal-organic contact is a fundamental ingredient in organic electronics systems. Such contacts not only offer mechanical anchoring of organic components, but also define and enhance the electronic characteristics of devices. A popular method of achieving such contacts is the gold-thiol bond; 1 the covalent nature of the gold-sulfur bond provides superior mechanical stability and high electronic transparency. 2 Additionally, these bonds can form well-packed, uniform self-assembled monolayers 3,4 making experimental studies on thin films convenient. However, the non-specific nature of the gold-sulfur bond means that well-defined, 5,6 uniform molecule-metal geometries, and consequently electronic properties, 7 are difficult to obtain. 8 Additionally, problems with thermal stability and degradation due to oxidation 9 have led researchers to explore alternatives such as a variety of donor-acceptor bonds 10 and recently, covalent gold-carbon bonds. 11 Direct gold-carbon bonds are often formed electrochemically, by the reduction of aryldiazonium salts in solution. 12,13 Covalent sigma-coupled gold-carbon bonds have also been formed using terminal alkynes, 14 organomercury salts, 15 trimethylsilyl linkers, 16 and trimethyltin leaving groups. 11 Recently, Chen and coworkers 17 have demonstrated the use of benzyltrimethyltin molecules to form gold-carbon bonds that can couple into the π system of a conjugated molecular backbone. † Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY ‡ Department of Physics, University of Ljubljana, Ljubljana, Slovenia ¶ CNR-IOM Laboratorio Nazionale TASC, Basovizza SS-14, km 163.5, I34012 Trieste, Italy § Department of Physics, National University of Singapore, Singapore k Institute of High Performance Computing, Agency for Science, Technology and Research, Singapore ⊥ Department of Chemistry, Columbia University, New York, NY # Department of Physics,University of Trieste, Trieste, Italy
In single-molecule experiments, they have shown near-resonant molecular conductance, nearly 100 times that of molecules with conventional linker groups. 17 The electronic properties of these πcoupled Au-C bonds have also been shown to facilitate desirable thermoelectric characteristics, 18 and have been integrated into tunable molecular diode designs. 19 Theoretical calculations suggest that a hybridized gold-molecule ‘gateway’ state, 17,19 localized on the Au-C bond, may be responsible for many of the unique electronic properties of these π-coupled Au-C bonded systems, though there is no experimental evidence for the existence of such a state. Here, we study the formation of Au-C bonds using benzyltrimethylstannane (C10 H16 Sn) molecules on a variety of gold surfaces in ultra high vacuum (UHV) conditions. In contrast with studies conducted on similar systems in organic solvent solutions in ambient conditions, 11,17 this UHV study allows us to isolate the role played by gold in such reactions. Through X-Ray Photoemission Spectroscopy (XPS) measurements, we show that, even at temperatures as low as −110 C in the presence of a gold substrate, C10 H16 Sn cleaves at the Sn-C bond to form trimethyltin (C3 H9 Sn) and benzyl (C7 H7 ) species. The resulting trimethyltin fragments readily form Sn-Au bonds on all surfaces, while the fate of the benzyl species is determined by the reactivity of the gold surface. On Au(111), XPS core level shifts point to the formation of a surfacestabilized benzyl radical. In contrast, XPS on Au(110) shows no such shift; instead, Near-Edge X-Ray Absorption Fine Structure Spectroscopy (NEXAFS) on this surface shows the formation of an Au-C bond with a well-hybridized electronic ‘gap’state. Finally, through Density Functional Theory (DFT) based implementations of Nudged Elastic Band (NEB) calculations, 20 we determine a reaction pathway for covalent Au-C bond formation, and understand the essential role played by undercoordinated gold surfaces in facilitating these reactions. We characterize the cleavage of benzyltrimethylstannane (Figure 1a) using XPS measurements of monolayer films of the molecule deposited on Au(111), Au(110), and sputtered Au surfaces. These measurements are carried out at the ALOISA/HASPES beamline (Elettra Synchrotron, Trieste). 21 The Au substrates are first cleaned by repeated cycles of Ar sputtering and annealing to 800K. XPS measurements were conducted with incident photon energy of 650 eV. Measurements of cleaned Au are made to ensure no contamination on the sample. The base pressure for the measurement and sample preparation chamber is maintained at 10−10 mbar. Benzyltrimethylstannane is synthesized using previously reported methods 17 and is deposited on this substrate from a quartz Knudsen-type cell in line-of-sight with the sample preparation chamber. For monolayer and multilayer depositions, the Au substrate is cooled to −110 C and the Knudsen cell is heated to 50 C. Deposition of monolayers is controlled by monitoring XPS core level positions for C1s, along with C1s to Au 4f inten-
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tion of charge density difference (Figure 4c) of the transition state, image 2, shows Au-C1 bonding in the transition state by hybridization of dz2 orbital of Au adatom with pz orbital of C1. The fact that C1 is the active site in the reaction is consistent with our observation that binding between the intact molecule and the Au adatom takes place between Au and C1 (instead of Au and Sn). This is in contrast to previous reports of Au-Sn bond formation in solutionphase chemistry carried out in ambient conditions. 32 Finally, an analysis of the projected density of states on atom C1 (Figure 4d) shows a significant density around EFermi , which is consistent with NEXAFS results showing a new hybridized state close to the Fermi edge. The above NEB calculation readily explains the facile cleavage of Sn-C1 bonds and formation of Au-C1 bonds on Au(110), a surface characterized by highly undercoordinated ridge sites. On the Au(111) surface, experiments do not evidence the formation of the Au-C bond despite demonstrating clearly that the molecule cleaves on this surface even at −110 C. To understand the difference between these surfaces, we first note that on the Au(110) surface, the benzene ring tilts towards the Au surface as we go from Image 2 to 6 in Figure 4a. This tilt also allows the C1-Au bond to achieve the distance and angle necessary for its sp3 configuration. In contrast, on an Au(111) surface without the adatom, the π-system of the benzene ring lies flat on Au to maximize the energy gained from π-Au interactions, and cannot tilt to enable the formation of an sp3 C1Au bond. This restricts the orientation of the C1 carbon relative to the Au surface, and prevents the formation of the covalent bond on Au(111). In summary, we have shown that Sn-C bonds in benzyltrimethylstannane are cleaved by Au surfaces. We show through XPS and NEXAFS studies that Au-C bonds are only formed on undercoordinated Au surfaces. Importantly, we find direct evidence for a new electronic state near the Fermi energy, which is similar to the gateway states hypothesized in previous literature. 11 Finally, we use DFT-NEB to gather a detailed understanding of the reaction pathway for the formation of Au-C bonds on gold surfaces. These results together provide a general understanding of this reaction that can be used to engineer new molecule-metal interfaces and molecular devices incorporating direct gold-carbon bonds. ASSOCIATED CONTENT Supporting Information. Experimental and theoretical methods, additional details and supporting data can be found free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author. *
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[email protected].
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Notes. The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported in part by NSF Career Award (CHE-0744185) and the Packard Foundation. AB supported by NSF GRFP (Grant No. DGE-07-07425). Part of this work was carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory, supported by the DOE Office of Basic Energy Sciences (Contract no. DE-AC02-98CH10886). D.C. acknowledge support by Slovenian Ministry of Science (No. J2-4287). Support from MIUR (PRIN 20105ZZTSE) and MAE (US14GR12) is acknowledged. SYQ thanks A*STAR for funding via the IHPC Independent Investigatorship and Singapore NRF for the NRF Fellowship
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