Single-molecule imaging of dinitrogen molecule adsorption on individual iron phthalocyanine

Nitrogen fixation is a vital process for both nature and industry. Whereas the nitrogenase can reduce nitrogen in ambient environment in nature, the industrialized Haber-Bosch process is a high temperature and high-pressure process. Since the discovery of the first dinitrogen complex in 1965, many dinitrogen complexes are prepared in a homogeneous solution to mimic the nitrogenase enzyme in nature. However, studies of the heterogeneous process on surface are rarely addressed. Moreover, molecular scale characterization for such dinitrogen complex is lacking. Here, we present a simple model system to investigate, at the single-molecule level, the binding of dinitrogen on a surface confined iron phthalocyanine (FePc) monolayer through the combination of in-situ low-temperature scanning tunneling microscopy (LT-STM) and X-ray photoelectron spectroscopy (XPS) measurements. The iron center in FePc molecule deposited on Au(111) and highly oriented pyrolytic graphite (HOPG) surface can adsorb dinitrogen molecule at room temperature and low pressure. A comparative study reveals that the adsorption behaviors of FePc on these two different substrates are identical. Chemical bond is formed between the dinitrogen and the Fe atom in the FePc molecule, which greatly modifies the electronic structure of FePc. The bonding is reversible and can be manipulated by applying bias using a STM tip or by thermal annealing.