Formation Of Mixed-valence Species Research Articles

Non-Covalent and Covalent Binding of New Mixed-Valence Cage-like Polyoxidovanadate Clusters to Lysozyme.

The high-resolution X-ray structures of the model protein lysozyme in the presence of the potential drug [VIVO(acetylacetonato)2] from crystals grown in 1.1 M NaCl, 0.1 M sodium acetate at pH 4.0 reveal the binding to the protein of different and unexpected mixed-valence cage-like polyoxidovanadates (POVs): [V15O36(OH2)]5-, which non-covalently interacts with the lysozyme surface, [V15O33(OH2)]+ and [V20O51(OH2)]n- (this latter based on an unusual {V18O43} cage) which covalently bind the protein. EPR spectroscopy confirms the partial oxidation of VIV to VV and the formation of mixed-valence species. The results indicate that the interaction with proteins can stabilize the structure of unexpected - both for dimension and architecture - POVs, not observed in aqueous solution.

Polyoxothiometalate-Derivatized Silicon Photocathodes for Sunlight-Driven Hydrogen Evolution Reaction.

Silicon photocathodes coated with drop-casted {Mo3S4}-based polyoxothiometalate assemblies are demonstrated to be effective for sunlight-driven hydrogen evolution reaction (HER) in acid conditions. These photocathodes are catalytically more efficient than that coated with the parent thiomolybdate incorporating an organic ligand, as supported by a higher onset potential and a lower overvoltage at 10 mA cm–2. At pH 7.3, the trend is inversed and the beneficial effect of the polyoxometalate for the HER is not observed. Moreover, the polyoxothiometalate-modified photocathode is found to be also more stable under acid conditions and can be operated at the light-limited catalytic current for more than 40 h. Furthermore, X-ray photoelectron spectroscopy and atomic force microscopy measurements indicate that the cathodic polarization of both photocathodes leads to the release of a large amount of the deposited material into the electrolyte solution concomitantly with the formation of mixed valence species {Mo(IV)3–xMo(III)xO4–nSn}(4–x)+ resulting from the replacement of S2– sulfido ligands in the cluster by oxo O2– groups; these combined effects are shown to be beneficial for the photoelectrocatalysis.

Electrochemical and structural investigation of Mg-doped Li3V(2-2x/3)Mgx(PO4)3

X-ray absorption spectroscopy was used to characterize the local structural environment of the vanadium ions in Mg doped Li3V(2-2x/3)Mgx(PO4)3; x = 0.15, 0.30, 0.45, (LVP) in a functioning battery. For all three compositions the x-ray absorption near-edge structure spectra show evidence for only two spectroscopically distinguishable species when the potential is cycled between 3 and 4.5 V. These are attributed to octahedral V3+ and V4+ in LVP. If the potential is increased above 4.5 V a third species that is consistent with V5+ in a distorted, possibly tetrahedral environment is formed. For x = 0.30 and x = 0.45 this species disappears when the potential is lowered. In contrast, for x = 0.15 a fourth species is formed on reduction. This new species is consistent with V4+ in a distorted tetrahedral environment and may explain the loss of capacity in the 15% sample during cycling. Although the potential shows discrete plateaus, consistent with formation of mixed-valence species, these are not seen in the x-ray absorption. Rather, we see apparent deviations from Faraday's law that are consistent with a lag in oxidation of the bulk vanadium.

Studies of Electronic Coupling and Mixed Valency in Metal−Metal Quadruply Bonded Complexes Linked by Dicarboxylate and Closely Related Ligands

Complexes of the form [(tBuCO2)3M2]2(mu-O2C-X-CO2), where M is Mo or W and X is a pi conjugated organic group, are ideally suited for studies of electronic coupling between the two redox centers via M2 delta-bridge pi conjugation. The complexes have intense metal-to-bridge charge-transfer transitions in the visible or near-IR region of the spectrum and exhibit thermo-, solvato- and electrochromic behavior. Chemical oxidation results in the formation of mixed-valence species that are particularly well-suited for the study of the class II/III border. The extent of electronic coupling is determined by a variety of spectroscopic techniques and, in particular, by EPR and electronic absorption spectroscopy. The latter provides a direct measure of the electronic coupling parameter Hab in pairs (Mo and W) of otherwise identical complexes. Similarly, the substitution within the bridge of the CO2 group by COS or RNCO allows an evaluation of the mechanism of the electronic coupling in closely related complexes. Electronic structure calculations employing density functional theory complement frontier molecular orbital theory in the interpretation of the physicochemical properties of these complexes.

Binuclear ruthenium and osmium mixed-valence complexes containing fused and flexible polypyridyl bridging ligands

{Os(bpy) 2} 2+ and {Ru(CN) 4} 2− mononuclear and binuclear complexes with ligands 2,3-di-(2-pyridyl)quinoxaline (dpq) and dipyrido[2,3- a:3′,2′- c]phenazine (ppb) have been prepared. For the binuclear complexes a splitting in oxidation potentials is observed consistent with the formation of mixed-valence species with comproportionation constants ( K com) ranging from 2.5 × 10 4 to 1.8 × 10 6. The electronic absorption spectra of the mixed-valence species reveal IVCT transitions in the near infrared region. The absorption maximum for the IVCT band ranges from 5800 to 9980 cm −1 and the extinction coefficients from 80 to 6300 M −1 cm −1. In general the {Os(bpy) 2} 2+ complexes show larger K com values and more intense IVCT bands than the corresponding {Ru(CN) 4} 2− complexes.