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

Abstract

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 (® 3 -O)(®-CH 3 COO) 6 ) 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 + ¼ 2 + reaction on the surface, from which a rate constant of the CO release was calculated to be 1.9 («0.2) © 10 ¹2 s ¹1 (25°C; in contact with 0.1 mol dm ¹3 aqueous HClO 4 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 d³p³ system in the ® 3 -O triruthenium cluster.Direct probe and analysis of atomic and molecular processes that occur at solidsolution interfaces have become increasingly essential for nanometer-scale science and technology. One crucial experimental challenge is to develop practical methods that allow direct observation of chemical reactions at a molecular or monolayer level. Scanning tunneling microscopy (STM) is a powerful technique for structural characterization of atoms, molecules, and supramolecular assemblies on surfaces and the enormous utility has been particularly manifested in probing atomic-scale processes in ultrahigh-vacuum. 15 While the in situ technique of STM has recently seen rapid progress for structural identification of molecules on well-defined electrode surfaces, 614 direct probe of chemical reactions which are triggered by applied potentials still remains a significant challenge.In this paper, we describe a successful probe and analysis of potential-induced chemical reactions of self-assembled monolayers (SAMs) of an oxo-centered acetato-bridged trinuclear ruthenium cluster 15,16 [{Ru 3 (® 3 -O)(®-CH 3 COO) 6 -(mpy)(CO)} 2 (®-C 10 py)] (mpy = 4-methylpyridine, C 10 py = {(NC 5 H 4 )CH 2 NHC(O)(CH 2 ) 10 S} 2 ) (Figure 1a) 17 at a solution/Au(111) interface with the in situ STM technique. Interfacial reactions examined here are illustrated in Figure 1b + is the one-electron-oxidized form of 1, while 2 + represents a cat-

Electroactive chain-like compounds constructed from trimetallic clusters and 4,4′-bipyridine spacers: one-pot synthesis, characterization and surface binding

“…15,42-45 In the negative potential region, compound 2a is reduced at -0.66 V to [2a] 2-(a dianionic form of 2a) in which both redox moieties are in the {Ru II -CO}Ru 2 II,III state. 15,[42][43][44][45] The negligible extent of electronic communication is in contrast in a previous report on 4,4¢-bpy-bridged triruthenium dimer system [{Ru 3 O(CH 3 CO 2 ) 6 (CO)L} 2 (m-4,4¢-bpy)] (L = 4-(dimethylamino)pyridine, pyridine and 4-cyanopyridine) in 0.1 M n-Bu 4 NPF 6 -CH 2 Cl 2 , 28 in which the reductive process splits into two steps whose peak-to-peak separation (DE p ) depends on the nature of the terminal ligand L. In our system, 4,4¢-bpy as terminal ligands seems to reduce the extent of mutual communication.…”

“…This is readily ascribed to [2a]/[2a] 2+ ({Ru II -CO}Ru 2 III,III /{Ru III -CO}Ru 2 III,III ) on the basis of comparison with solution electrochemical data (Table 2) and reference data reported earlier. 13,15,38,[40][41][42][43][44][45] Integration of the redox wave yielded the surface coverage of pmla to be 1.9 ¥ 10 -11 mol cm -2 , suggesting that the pmla SAMs is not fully covered with 2a. Scheme 3 illustrates two possible molecular orientations of 2a on the surface.…”