Ninety years ago, Ullmann and co-workers discovered that copper powder catalyzes the coupling of aromatic halides to produce biaryls, yet the role of the copper surface in this reaction remains controversial today. Recently, it has been shown that Ullmann coupling of submonolayer coverages of iodobenzene to form biphenyl occurs with 100% selectivity on a Cu(111) single crystal surface under ultrahigh-vacuum conditions (Xi, M.; Bent, B. E. Surf. Sci. 1992, 278,19). We report here on studies in which surface analysis techniques have been applied to determine the mechanism of this reaction. Two mechanisms are found. At low surface coverages (<1 monolayer), iodobenzene dissociates at approximately 175 K on the Cu(111) surface to form adsorbed phenyl groups, which subsequently couple at 300-400 K to form biphenyl. At high surface coverage (1-2 monolayers), biphenyl is also formed by a second pathway at temperatures as low as 210 K. Isotope labeling and surface vibrational spectroscopy are used to show that this low-temperature pathway involves a reaction between adsorbed phenyl groups and molecular iodobenzene. A key aspect of the high coverages required for this low-temperature coupling pathway is that the molecular iodobenzene assumes an orientation in which its pi-ring is tilted significantly away from the surface plane. It is suggested that such a tilted configuration allows phenyl radicals formed by C-I bond scission to react with adjacent phenyl groups as opposed to bonding directly to the copper surface. Potential implications of these results for the solution-phase Ullmann coupling reaction are discussed.
