/pdf/the-localized-to-delocalized-transition-in-mixed-valence-2a82oxt4jv.pdf

The localized-to-delocalized transition in mixed-valence chemistry.

In the Robin and Day classification, mixed-valence systems are characterized as Class I, II or III depending on the strength of the electronic interaction between the oxidized and reduced sites, ranging from essentially zero (Class I), to moderate (Class II), to very strong electronic coupling (Class III). The properties of Class I systems are essentially those of the separate sites. Class II systems possess new optical and electronic properties in addition to those of the separate sites. However, the interaction between the sites is sufficiently weak that Class II systems are valence trapped or charge localized and can the be described by a double-well potential. In Class III systems the interaction of the donor and acceptor sites is so great that two separate minima are no longer discernible and the energy surface features a single minimum. The electron is delocalized and the system has its own unique properties.
 The Robin and Day classification has enjoyed considerable
success and most of the redox systems studied to date are readily assigned to Class II. However the situation becomes much more complicated when the system shows borderline Class II/III behavior. Such “almost delocalized” mixed-valence systems are difficult to characterize. In this article spectral band shapes and intensities are calculated utilizing increasingly complex models including two to four states. Free-energy surfaces are constructed for harmonic diabatic surfaces and characterized as a function of increasing electronic coupling to simulate the Class II to III transition. The properties of the charge-transfer absorption bands predicted for borderline mixed-valence systems are compared with experimental data. The treatment is restricted to symmetrical (ΔG0 = 0) systems.

https://www.rsc.org/suppdata/cs/b0/b008034i/b008034i.pdf

Optical transitions of symmetrical mixed-valence systems in the Class II–III transition regime

The mixed-valence character of a set of six bistriarylamine derivatives with varying π-electron spacers has been investigated. The distances of triarylamine redox centers vary from 0.5 nm (N,N,N‘,N...

The Class II/III Transition in Triarylamine Redox Systems

Five Re(bipy)(CO)(3)Cl complexes were prepared and studied where bipy was 4,4'-dicarboxyl-2,2'-bipyridine (1), 2,2'-bipyridine (2), 4,4'-dimethyl-2,2'-bipyridine (3), 4,4'-di-tert-butyl-2,2'-bipyridine (4), and 4,4'-dimethoxy-2,2'-bipyridine (5). From this group, a significantly improved catalyst, Re(bipy-tBu)(CO)(3)Cl (4), for the reduction of carbon dioxide to carbon monoxide was found. The complex shows two one-electron reductions under argon, one reversible at -1445 mV (vs SCE), and one irreversible at -1830 mV. Under CO(2) the second irreversible wave displays a large catalytic enhancement in current. Diffusion coefficients were determined using the Levich-Koutecky method (1.1 × 10(-5) cm(2)/s for the neutral complex and 8.1 × 10(-6) cm(2)/s for the singly reduced species), and a second order rate constant for the electrochemical reduction with CO(2) of 1000 M(-1) s(-1) was measured. Bulk electrolysis studies were performed to measure Faradaic efficiencies for the primary gaseous products, η(CO) = 99 ± 2% in acetonitrile.

Re(bipy-tBu)(CO)3Cl−improved Catalytic Activity for Reduction of Carbon Dioxide: IR-Spectroelectrochemical and Mechanistic Studies

Personalized energy (PE) is a transformative idea that provides a new modality for the planet’s energy future. By providing solar energy to the individual, an energy supply becomes secure and available to people of both legacy and nonlegacy worlds and minimally contributes to an increase in the anthropogenic level of carbon dioxide. Because PE will be possible only if solar energy is available 24 h a day, 7 days a week, the key enabler for solar PE is an inexpensive storage mechanism. HY (Y = halide or OH−) splitting is a fuel-forming reaction of sufficient energy density for large-scale solar storage, but the reaction relies on chemical transformations that are not understood at the most basic science level. Critical among these are multielectron transfers that are proton-coupled and involve the activation of bonds in energy-poor substrates. The chemistry of these three italicized areas is developed, and from this platform, discovery paths leading to new hydrohalic acid- and water-splitting catalysts are...

/pdf/chemistry-of-personalized-solar-energy-2pmvswxtcu.pdf

Chemistry of Personalized Solar Energy

Single-electron reductions of linked triruthenium clusters of the general type Ru3-pyrazine-Ru3 produced mixed valence systems showing spectroscopic characteristics of rapid intramolecular electron transfer. Reflectance infrared spectroelectrochemistry was used to characterize the vibrational spectra of mixed valence systems that contained one carbon monoxide ligand on each Ru3 cluster. Infrared spectra in the CO stretching region showed two resolved, partially coalesced, and coalesced ν(CO) bands for clusters with rate constants for intramolecular electron transfer k e increasing from = 1 × 109 s– 1 up to 5 × 1011 and 9 × 1011s– 1, respectively. These data provide a strong correlation between rates of intramolecular electron transfer and infrared spectral bandshape.

http://science.sciencemag.org/content/sci/277/5326/660.full.pdf

Effects of Rapid Intramolecular Electron Transfer on Vibrational Spectra

Intramolecular electron transfers within the mixed valence states of the ligand bridged hexaruthenium clusters Ru3(μ3-O)(μ-CH3CO2)6(CO)(L)(μ-L‘)Ru3(μ3-O)(μ-CH3CO2)6(CO)(L) (L‘ = 1,4-pyrazine; L = 4-dimethylaminopyridine (1), pyridine (2), 4-cyanopyridine (3), or L‘ = 4,4‘-bipyridine; L = 4-dimethylaminopyridine (4), pyridine (5), 4-cyanopyridine (6)) were examined. Two discrete and reversible single electron reductions are evident by cyclic voltammetry in the redox chemistry of 1−5, and the intercluster charge-transfer complexes are well-defined. The splitting of the reduction waves, ΔE, is related to the electronic coupling HAB between the triruthenium clusters, and varies from 80 mV for 5 to 440 mV for 1. In the case of 6, the splitting of the reduction waves, ΔE, is <50 mV and the intercluster charge-transfer complex is not defined. The mixed valence states of 1−3 also exhibit intervalence charge transfer (ICT) bands in the region 12 100 (1) to 10 800 cm-1 (3) which provide spectroscopic estimates of H...

Electron Transfer on the Infrared Vibrational Time Scale in the Mixed Valence State of 1,4-Pyrazine- and 4,4‘-Bipyridine-Bridged Ruthenium Cluster Complexes

The synthetic method for the stepwise Ru3 unit elongation of pyrazine-bridged linear oligomer of trinuclear ruthenium complex with the Ru3(μ3-O)(μ-CH3COO)6 core has been developed, using a newly designed [Ru3(μ3-O)(μ-CH3COO)6(py)(CO)(H2O)] (py = pyridine) with labile and photoactive sites.

Stepwise Elongation of a Trinuclear Ruthenium Unit in Pyrazine-Bridged Linear Oligomers with Use of [Ru3(μ3-O)(μ-CH3COO)6(pyridine)(CO)(H2O)]

Pyrazine-bridged trimeric and tetrameric oligomers of triruthenium clusters, [{Ru3O(CH3CO2)6(CO)(py)}-(μ-pz)-{Ru3O(CH3CO2)6(CO)}-(μ-pz)-{Ru3O(CH3CO2)6(py)2}]+ (1) and [{Ru3O(CH3CO2)6(CO)(py)}-(μ-pz...

Haruko Nagino

Papers

Effects of Rapid Intramolecular Electron Transfer on Vibrational Spectra

Electron Transfer on the Infrared Vibrational Time Scale in the Mixed Valence State of 1,4-Pyrazine- and 4,4‘-Bipyridine-Bridged Ruthenium Cluster Complexes

Stepwise Elongation of a Trinuclear Ruthenium Unit in Pyrazine-Bridged Linear Oligomers with Use of [Ru3(μ3-O)(μ-CH3COO)6(pyridine)(CO)(H2O)]

14 Step-15 Electron Reversible Redox Behavior of Tetrameric Oligomer of Oxo-Bridged Triruthenium Cluster