20077
2006, 110, 20077-20080 Published on Web 09/15/2006
Assembly of Linear Clusters of Iodobenzene Dimers on Cu(110) Daniel B. Dougherty, Junseok Lee, and John T. Yates, Jr.* Department of Chemistry, Surface Science Center, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260 ReceiVed: July 18, 2006; In Final Form: August 31, 2006
Low-temperature STM observations of the low-coverage chemisorption behavior of iodobenzene on Cu(110) are presented at two annealing temperatures. When a random distribution of isolated iodobenzene molecules deposited at T < 100 K is annealed to approximately 170 K, the undissociated molecules assemble into chainlike clusters composed of identical subunits. An STM tip-induced dissociation reaction is used to determine that the individual units in the chains are composed of pairs of iodobenzene molecules. A model is proposed in which iodine atoms on each member of the pair are directed toward one another. The driving force for the formation of such clusters is suggested to be the dispersion interactions between the polarizable iodobenzene molecules.
Catalytic processes must be understood at a mechanistic level in order to control and improve industrially important reactions.1,2 Often, catalytic activity on surfaces can be traced to defect sites such as step edges that reduce activation barriers for key reaction steps.3-6 For processes involving complex molecules, however, stereochemistry may also play a role. The efficiency of a catalyst may partially reside in its ability to assemble molecules into a configuration favorable for a desired reaction. Here, we describe STM observations that suggest a possible role of such stereochemistry in the surface-catalyzed Ullmann reaction for the production of biphenyl (C12H10) from iodobenzene (C6H5I) on a copper surface. The Ullmann reaction can be written as7
2C6H5I + 2Cu f C12H10 + 2CuI
(1)
Studies relevant to the Ullmann reaction mechanism have been fruitfully carried out on single-crystal surfaces even though, in practice, it is performed in solution using polycrystalline Cu powder. A significant recent example was the demonstration that the products could be formed by STM tip-induced manipulation of the reactants on Cu(111) at cryogenic temperatures.8 Surface chemistry issues related to the thermal mechanism of the spontaneous Ullmann reaction on a Cu(111) surface were studied by Xi and Bent using temperature-programmed reaction spectroscopy (TPRS).9 Their results suggested that, at low iodobenzene coverage, the formation of biphenyl occurs by the cleavage of the C-I bond in iodobenzene followed by the diffusion and combination of the resulting phenyl surface fragments. This letter presents STM evidence showing that, on a Cu(110) surface, significant thermal clustering of iodobenzene molecules occurs prior to C-I bond cleavage. Experiments were performed in a UHV system with a preparation chamber (base pressure ∼1 × 10-9 mbar) isolated by a gate valve from a STM observation chamber (base pressure
