  Within the collection of surface-supported reactions currently accessible for
the production of extended molecular nanostructures under ultra-high vacuum,
Ullmann coupling has been the most successful in the controlled formation of
covalent single C-C bonds. Particularly advanced control of this synthetic tool
has been obtained by means of hierarchical reactivity, commonly achieved by the
use of different halogen atoms that consequently display distinct activation
temperatures. Here we report on the site-selective reactivity of certain
carbon-halogen bonds. We use precursor molecules halogenated with bromine atoms
at two non-equivalent carbon atoms and found that the Ullmann coupling occurs
on Au(111) with a remarkable predilection for one of the positions.
Experimental evidence is provided by means of scanning tunneling microscopy and
a rationalized understanding of the observed preference is obtained from
density functional theory calculations.
