On-Surface Synthesis and Characterization of an Iron Corrole

Abstract

To explore the possibility of tuning the oxidation state of a coordinated metal ion in an adsorbed tetrapyrrole complex by a minor modification of the ligand structure, multilayers and monolayers of 3H-hexaethyl-dimethyl-corrole (3H-HEDMC) on Ag(111) were in situ metalated under ultrahigh vacuum (UHV) conditions and subsequently analyzed with spectroscopic and microscopic methods. The metalation reaction leads to the formation of Fe(III)-hexaethyl-dimethyl-corrole (Fe-HEDMC). The results were compared with those for Fe-octaethyl-porphyrin (Fe-OEP), in which the Fe metal center has a formal Fe(+II) character. The analysis of the nearly unperturbed iron-corrole in the multilayer regime with X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy shows that the electronic structure of the corrole-coordinated iron is consistent with an Fe(+III) state in Fe-HEDMC. In the monolayers, the Fe centers in Fe-HEDMC and Fe-OEP interact similarly with the Ag(111) surface, as indicated by contributions to the Fe 2p signals at a typical Fe(0) position. Nevertheless, a closer investigation of the Fe-HEDMC and Fe-OEP monolayers with NEXAFS, UV photoelectron spectroscopy (UPS), and scanning tunneling microscopy (STM) reveal significant differences between electronic states of the Fe centers, depending on the type of ligand. Apparently, the interfacial interactions between the coordinated metal centers and the surface modify the electronic state of Fe but do not fully equalize the differences induced by the corrole and porphyrin ligand structures. Thus, the experiments show that it is not only possible to perform a direct in situ metalation of corroles with iron atoms under UHV conditions but also that the differences in the electronic structures between Fe-corroles and Fe-porphyrins persist even in the presence of interfacial interactions with Ag(111). Especially, the differences in the Fe-related density of states around the Fermi energy are expected to result in different chemical reactivities and potential catalytic activities of the two supported Fe complexes.