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On-surface synthesis of low-dimensional metal-organic architectures provides a versatile platform to create ordered assemblies with tuneable structural and electronic properties [1,2]. One vector for this flexibility lies in the choice of the active functional groups bonding with the metal atoms. Among them, carboxyl (-COOH) groups are particularly interesting because they can coordinate in both a mono- and bi-dentate manner as well as host a strong nucleophilic site. Uphoff et al. have shown a possible route for activating these carboxylate groups through deprotonation, by employing a hot-deposition step in the formation of a TPA-based MOF using Ho atoms on Ag(100) [3]. It remains unclear whether this requirement of pre-activation is intrinsic to TPA or if the incorporation of a different metal atom can directly promote ligand deprotonation without prior substrate-induced activation. We address this question using scanning tunnelling microscopy, X-ray photoelectron and valence band spectroscopy, complemented by density functional theory calculations.
We investigate metal-organic coordination by examining nickel atoms interacting with a preassembled hydrogen-bonded terephthalic acid (TPA) monolayer on Ag(100), for which we explicitly do not follow the hot-deposition step outlined by Uphoff et al. [3] and find the majority of ligands still in the protonated carboxyl state. Upon Ni incorporation, X-ray photoelectron spectroscopy unambiguously reveals Ni(I) centers stabilized by a single-electron charge transfer process, as well as a quenching of the -COOH peak, clearly demonstrating the direct deprotonation of the previously inactive TPA ligands. The resulting formation of an extended metal-organic framework is further confirmed by valence band spectroscopy showing coordination-induced electronic reorganization between Ni and TPA via emergent hybrid states [4]. Furthermore, we find that the +1 oxidation state of Ni, together with the network structure, fundamentally limits the Ni-induced deprotonation to 50%. Our results demonstrate that charge-transfer-driven deprotonation is the central mechanism for linear metal-organic chain formation, deepening the understanding of structural motifs and electronic properties in low-dimensional systems.
[1] S. Mearini et al., Adv. Science 11, 38 (2024)
[2] S. Mearini et al., Small 21, 12 (2025)
[3] M. Uphoff et al., ACS Nano 12, 11 (2018)
[4] D. Baranowski et al. ACS Nano 18, 30 (2024)