Single cobalt atoms and two oxidation states: on-surface synthesis of a 2D metal-organic framework via transmetallation.

Characterizing the chemical and physical properties of metal-organic frameworks (MOFs) is crucial for harnessing their unique functionalities in the design of next-generation catalysts and spintronic materials. Here, we report the formation of a homometallic MOF via co-deposition of Mn-tetrapyridylporphyrin (MnTPyP) molecules and Co atoms on an Au(100) surface. Low-energy electron diffraction and scanning tunneling microscopy measurements show that the predominant unit cell of the self-assembled MnTPyP film is commensurate with Au(100) and exhibits square symmetry, reflecting the substrate symmetry. As confirmed by scanning tunneling microscopy, Co deposition results in the formation of a long-range-ordered phase in which, as demonstrated by X-ray absorption spectroscopy, individual Co atoms coordinate the pyridyl groups of four neighboring molecules, indicating the formation of a metal-organic framework. This is accompanied by a loss of dichroism at the Mn L-edge in the X-ray absorption spectra, suggesting that the Mn atoms in the macrocycle are substituted with Co, a phenomenon known as "transmetallation". Further X-ray photoemission measurements, supported by density functional theory calculations, reveal the presence of two non-equivalent Co species with different oxidation states: +2 for the Co in the macrocycle and +1 for the pyridyl-coordinated Co. Together, these findings highlight the critical role of the Au(100) support in promoting the transmetallation process, enabling the MOF to stabilize two non-equivalent Co centers, which can be exploited for catalytic or spintronic applications.