Bottom-Up Fabrication of a Metal-Supported Oxo–Metal Porphyrin

Abstract

In situ preparation of oxotitanium tetraphenylporphyrin (TiO-TPP) on Ag(111) under ultrahigh vacuum conditions was achieved in a multistep procedure starting from adsorbed free-base tetraphenylporphyrin (2H-TPP). The final product as well as the intermediate titanium tetraphenylporphyrin (Ti-TPP) was characterized by a suite of surface-sensitive spectroscopic tools combined with scanning tunneling microscopy and density functional theory (DFT), and compared against the parent 2H-TPP species. Facile oxidation of Ti-TPP with molecular oxygen was observed at 300 K, with X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) from the Ti 2p core levels supporting a change in the oxidation state from Ti2+ to Ti4+. N K-edge and Ti L-edge NEXAFS suggest that the tetrapyrrole macrocycle conformation is modified upon binding to oxygen, in agreement with DFT calculations that predict a marked change of the local environment of the Ti centers upon oxygen attachment. O K-edge NEXAFS and O 1s energy-scanned photoelectron diffraction from the resulting TiO-TPP monolayer provide strong evidence for the presence of a titanium–oxygen double bond, with the latter technique yielding a bond length of 1.56 ± 0.02 Å. The majority of adsorbed TiO-TPP species have the oxo group pointing away from the surface rather than toward it, and thus the oxygen atom can potentially interact with external species. Both the highly reactive, intermediate Ti-TPP species and the final product TiO-TPP are of great interest for catalytic applications.