Ranking the Stability of Transition-Metal Complexes by On-Surface Atom Exchange

Surface-adsorbed macrocycles exhibit a number of interesting physical and chemical properties; many of them are determined by their transition-metal centers. The hierarchical exchange of the central metal atom in such surface-adsorbed complexes is demonstrated, specifically in the porphyrin-like macrocycle pyrphyrin adsorbed on Cu(111). Using scanning tunneling microscopy and X-ray photoelectron spectroscopy, we show that Cu as central metal atom is easily exchanged with Ni or Fe atoms supplied in trace amounts to the surface. Atom exchange of Ni centers with Fe atoms also occurs, with moderate yield. These results allow ranking the stability of the surface-adsorbed Cu, Ni, and Fe complexes. The fact that the atom exchange occurs at 423 K shows that surface-adsorbed macrocycles can be surprisingly easily transformed.