On-Surface Chemistry of Pb(II) Tetraphenylporphyrin on Au(111): Reversible Metalation, Thermal Degradation, and Formation of a Covalent Organic Framework

Abstract

Heavy-metal compounds play important roles in modern functional materials, enabling advancements in optoelectronics, catalysis, and beyond. Nanostructured surfaces incorporating heavy main group elements have shown significant promise for novel applications but often suffer from insufficient thermal stability. To address these challenges resulting from thermally induced reactions, we study the on-surface chemistry of Pb(II) 5,10,15,20-tetraphenylporphyrin, Pb(TPP), on the Au(111) surface as a model system. Pb(TPP) is formed either by vapor deposition of Pb atoms onto a monolayer of free-base tetraphenylporphyrin, H2(TPP), or by direct deposition of pre-synthesized Pb(TPP). Using XPS and STM, we compare these monolayers, finding that in-situ metalation yields exclusively Pb↑ configurations. Thermal degradation of Pb(TPP) on Au(111) differs mechanistically from previously reported transmetalation on Cu(111). Below 550 K, the Pb atom in Pb(TPP) is replaced by hydrogen through demetalation, resulting in a reduction of the released Pb atom and formation of H2(TPP). This process is enabled by cyclodehydrogenation of the TPP ligand, which releases the necessary hydrogen. Above 550 K, free-base porphyrins undergo remetalation with Au atoms from the substrate, while simultaneous C-H bond activation induces intermolecular coupling, resulting in the formation of a two-dimensional covalent organic frameworks (COFs) of Au porphyrins. These porphyrin-based COFs with embedded metal centers exemplify a promising platform for surface-supported functional materials with MN4 centers, paving the way for innovative applications in heterogeneous (electro)catalysis and related fields.