Abstract
Ullmann coupling has been one of the most important organic reactions for the formation of an aryl-aryl bond, which is of great significance in medicinal chemistry, natural product synthesis, and optoelectronic material fabrication. However, the associated reaction mechanism has not been determined with certainty and has mostly relied on theoretical calculations, since the identification of reaction intermediates lacked experimental evidence. Herein, we report the visualization of an unprecedented C-Cu-Br-Cu-C bonded intermediate state of Ullmann coupling by means of on-surface synthesis. These intermediates tend to form nanorings on a Cu(111) surface, as thermodynamically stable structures. Advanced techniques, including scanning tunneling microscopy, non-contact atomic force microscopy, and synchrotron radiation photoemission spectroscopy, together with density functional theory calculations, were used to scrutinize the structural assignments and intermediate transition process at the sub-molecular level. The C-Cu-Br-Cu-C structure is confirmed to be the precursor state of the conventional C-Cu-C intermediate during an on-surface Ullmann reaction, since their coexistence and transformation were observed experimentally. Our findings offer insights into revisiting and understanding the reaction mechanism of Ullmann coupling.