In Situ Scanning Tunneling Microscopy Observation of Metal-Cluster Redox Interconversion and CO Dissociation Reactions at a Solution/Au(111) Interface

We present here an in situ scanning tunneling microscopy (STM) study of potential-induced reactions of oxo-centered acetato-bridged triruthenium clusters on Au(111) under electrochemical conditions, in which (i) reversible interconversion between two different redox states of the cluster and (ii) spontaneous dissociation of CO from the cluster were probed and visualized. It is known that the triruthenium complex [Ru-3(mu O3-)(mu-CH3COO)(6)(CO)(4-methylpyridine){(NC5H4)-CH2NHC(O)(CH2)(10)S}] (1), which is in the mixed-valent {(Ru-II-CO)(RuRuIII)-Ru-III} state, on Au(111) is one-electron oxidized to cationic complex 1(+) which corresponds to a {(Ru-III-CO)(RuRuIII)-Ru-III} oxidation state. The redox reaction is reversible on a cyclic voltammetry timescale. Complex 1(+) undergoes irreversible dissociation of CO in an aqueous phase on a longer timescale (ca. minutes) to fonn an aqua ligand-coordinated complex, 2(+), which is expressed as ((Ru-III-OH2)(RuRuIII)-Ru-III). The reversible redox reaction (1 reversible arrow 1(+)) and the irreversible ligand substitution reaction (1(+) -> 2(+)) were independently examined with in situ STM in a monolayer level by visualizing molecular-sized spots showing a different extent of brightness in STM images. We show here that the molecular-sized spots corresponding to 1, 1(+), and 2(+) are resolved by their brightness, which strongly depends on both the oxidation states and the ligand nature of the clusters. By employing multiple fast scans at an applied potential of +0.80 V vs. Ag/AgCl, we obtained STM images that follow the irreversible 1(+) -> 2(+) reaction on the surface, from which a rate constant of the CO release was calculated to be 1.9 (+/- 0.2) x 10(-2)s(-1) (25 degrees C; in contact with 0.1 mol dm(-3) aqueous HClO4 solution). The difference in brightness of the molecular spots is rationalized in terms of orbital-mediated tunneling by considering the difference in electronic states of the d pi-p pi system in the mu 3-O trirutheniurn cluster.