Oxidation States and CO Ligand Exchange Kinetics in a Self‐Assembled Monolayer of a Triruthenium Cluster Studied by In Situ Infrared Spectroscopy

Abstract

Oxidation states and CO ligand exchange kinetics in a self-assembled monolayer (SAM) of an oxo-centered triruthenium cluster [Ru(3)(mu3-O)(mu-CH3COO)6(CO)(L1)(L2)] (L1 = [(NC5H4)CH2NHC(O)(CH2)10S-]2, L2 = 4-methylpyridine) have been extensively investigated on the surface of a gold electrode in aqueous and nonaqueous solutions. The SAM exhibits three consecutive one-electron transfers and four oxidation states, which have been characterized by electrochemistry, in situ infrared spectroscopy, and in situ sum frequency generation (SFG) vibrational spectroscopy measurements. The original electron-localized state of the Ru cluster center was changed to electron delocalization states by oxidation or reduction of the central Ru ions. These changes are revealed by the IR absorptions of the CO ligand and the bridging acetate ligands of the triruthenium cluster in the SAM. The IR absorptions of the two kinds of ligands are strongly dependent on the oxidation state of the Ru cluster center. One-electron oxidation of the central Ru ion in the SAM triggers a CO ligand liberation process. Solvent molecules may then occupy the CO site to result in a CO-free SAM. One-electron reduction of this CO-free SAM in a CO-saturated solution leads to re-coordination of the CO ligand into the SAM. Both processes can be precisely controlled by tuning the electrode potential. The kinetics of the CO exchange cycle in the SAM, including liberation and coordination, has been investigated by in situ IR and SFG measurements for the first time. The CO exchange cycle is significantly dependent on the temperature. The reaction rate greatly decreases with decreasing solution temperature, which is an important factor in the CO ligand exchange process. The activation energies of both CO liberation and coordination have been evaluated from the reaction rate constants obtained at various temperatures.

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.…”

Interference effects in the sum frequency generation spectra of thin organic films. II: Applications to different thin-film systems

“…I, in situ SFG observations have been carried out for electrochemical systems. [39][40][41][42][43][44] In this case, in order to reduce the IR absorption by the aqueous electrolyte solution, a thin-layer geometry has usually been employed in which the electrode is usually placed on an optical window ͑usually, CaF 2 or BaF 2 is used͒ to form a thin solution layer estimated to be several microns thick. Since it is not possible to control the thickness of the thin layer exactly, it is necessary to consider the possibility of interference in the thin layer of electrolyte solution contained between the electrode and the optical window.…”

“…In fact, this prediction concurs with the reported results even though the thickness of the thin solution layer cannot be accurately controlled, and the spectral line shapes obtained from different groups for the same sample were similar to each other. [39][40][41][42][43][44] On the other hand, the intensity of the SFG signal for different film thicknesses exhibits some thickness dependent features. For example, Fig.…”

Interfacial Structure of Soft Matter Probed by SFG Spectroscopy

“…In the present Personal Account, our recent SFG studies on the molecular structures at interfaces of functional soft matter such as polymers, self‐assembled monolayers (SAMs) and Langmuir–Blodgett (LB) ultrathin layers and lipid bilayers in various environments will be briefly reviewed. A general model dealing with the optical interference effect in thin layers of soft matter, which can dramatically affect the SFG spectral shape and intensity, will be introduced first.…”