Coordination and Structural Control of Trinuclear Ruthenium Clusters Using Organic Ligands at HOPG Surface
Authors: Inoue Aoi , Yoshimoto Soichiro , Abe Masaaki 
Published: December 2024

Controlling the self-assembly and nanoarchitectures of metal complexes at the molecular scale is essential for the development of new functional materials and devices. Since two-dimensional (2D) sheets consisting of metal complexes can be designed and controlled with various bonding modes, geometric structures, and chemical functions, they have been studied using various approaches by selecting metal ions and organic ligands. Trinuclear metal clusters contain three metal ions in one complex and are expected to be building blocks for the formation of 2D sheets of metal complexes by selecting their shape and axial ligands.
In this study, a trinuclear cluster of ruthenium ions, Ru3O(EtCOO)6(CO)(THF)2:1, was used as the core to form nanostructures on highly oriented pyrolytic graphite (HOPG) substrates via axial ligand exchange with 1,4-Di (4-pyridyl) benzene:2 and 4,4’-di(4-pyridyl) biphenyl: 3. The adlayer structures and electrochemical properties of the nanoarchitectures were investigated by atomic force microscopy (AFM) and cyclic voltammetry (CV).
In the experimental procedure, powders of 1, 2, and 3 were dissolved in methanol (MeOH) and tetrahydrofuran (THF), respectively, and a 0.78 µM solution was prepared. A predetermined amount of this was cast on HOPG, dried to form a thin film, and the thin film was observed in air using AFM. Next, equal amounts of 1 and 2 and 1 and 3 solutions were mixed in each solvent, and after heating a water bath at 60°C for 1 h, thin films were prepared in the same manner and observed by AFM. Ultraviolet–visible (UV-vis) absorption spectra and electrochemical measurements by CV were performed before and after heating the mixed solutions.
First, in the AFM image of the thin film prepared by casting the MeOH mixture of 1 and 2, two stripe-like domains with different heights were observed, which matched the heights of the structures obtained in the AFM images of the single-domain formation of 1 and 2. In addition, no new absorption peaks were observed in the UV-vis absorption spectrum after mixing the solution; it was inferred that 1and 2 did not exchange ligands. However, when the mixed solution was heated in a water bath for 1 h, a few striped domains were observed, and a new ring-like structure was formed. The CV results also supported the change in the AFM images and absorption spectra after heating the solution. Before heating the mixed solution, a small redox peak was observed around 0.8 V, whereas after the warm bath, a large redox peak was observed around 0.6 V. A dramatic change in the magnitude of the current density was observed, indicating a significant improvement in conductivity. These results suggest that heating the mixed solution replaced THF ligand 1 with 2, forming a cyclic structure consisting of 1 and 2. When the same experiment was carried out in THF solution, a different structure was observed from that of the methanol solution, and a one-dimensional (1D) molecular wire structure was formed after heating the water bath. The absorption spectra and CV results also supported that ligand exchange occurred in the 1D wire structure, revealing the solvent dependence of the nanoarchitecture. When 3 was used as the axial ligand, larger structures were formed compared to 2, and one-dimensional wire structures of several micrometers were observed in the THF solution after the warm bath. Larger current densities and peak separations are also observed in the voltammograms.
Thus, the length dependence of the organic ligand for the nanoarchitecture formation on the HOPG surface was demonstrated.

Figure 1