Metalation of Porphyrins by Lanthanide Atoms at Interfaces: Direct Observation and Stimulation of Cerium Coordination to 2H-TPP/Ag(111)

Although on-surface metalation protocols of tetrapyrroles with 3d metals are well established, reports on the formation of lanthanide tetrapyrrole complexes are scarce. Here, we address the synthesis of lanthanide-tetrapyrrole units in detail, refining earlier findings. Specifically, the formation of cerium tetraphenylporphyrin (Ce-TPP) was induced on Ag(111) either by thermal annealing or by a manipulation procedure using a scanning tunneling microscope (STM) tip. While the self-assembled TPP arrays are not altered upon Ce metalation, our STM observations show distinct modifications of submolecular features reflecting a multistep reaction pathway. The metalation proceeds from an initial configuration with a 2H-TPP molecule sitting atop a Ce atom via an intermediate state, where the macrocycle is partially deprotonated, to metalated Ce-TPP. The hitherto elusive 1H species - hypothesized in several 3d metalation studies - is visualized directly. Our study provides novel insights into the on-surface synthesis of tetrapyrroles and lanthanide-based nanosystems.