Formation of Organometallic Intermediate States in On-Surface Ullmann Couplings

Possible origins of the formation of organometallic intermediates in on-surface Ullmann couplings have been investigated by surface tunneling microscopy (STM) and density functional theory (DFT) calculations. We consider the case of iodobenzenes on the coinage metals copper, silver, and gold. We found experimental evidence for the formation of surface vacancies and the presence of metal adatoms in these coupling reactions, which are taken as a hint for the reactive extraction of surface atoms by adsorbates. In a second step, we demonstrate by ab initio molecular dynamics calculations for aryl-iodides on copper that metal atoms can be pulled out of the surface to form metal-organic species. By contrast, a thermally activated provision of a metal atom from the surface to form an adatom is energetically unfavorable. Finally, we investigate the mechanism and energetics of the reactive extraction of surface metal atoms by means of (climbing-image) nudged elastic band density-functional theory calculations for iodobenzene on copper, silver, and gold, and analyze our results in the light of the experimental findings.