Trapped-valence State Research Articles

Mixed-valence μ3-oxo-centered triruthenium cluster [Ru3(II,III,III)(μ3-O)(μ-CH3CO2)6(H2O)3]·2H2O: Synthesis, structural characterization, valence-state delocalization and catalytic behavior

The oxo-centered, trinuclear, mixed valence [Ru3(II,III,III)O(CH3CO2)6(H2O)3]·2H2O (2) acetate complex has been prepared with high yield through reduction of [Ru3(III,III,III)O(CH3CO2)6(CH3OH)3]·CH3CO2 precursor compound in presence of muccic acid under hydrothermal conditions. The crystalline trinuclear oxo-cluster has been obtained as crystalline powder and characterized by single-crystal and powder X-ray diffraction, elemental analysis, SEM, TGA, IR spectroscopy. Complex 2 composes of μ3-oxocentered trinuclear ruthenium array and exhibits the oxidation state delocalization between three Ru atoms at 293K. Accurate single-crystal analysis along with valence bond calculations reveal trapped-valence state delocalization at room temperature, whereas three-site relaxation occurs at 100K leading to Ru(II) and Ru2(III) formal states. Moreover, the mixed valence of RuIIRu2III unit in compound 2 has been confirmed by XANES spectroscopy. The catalytic behavior of oxo-centered triruthenium complex 2 has been examined in hydration of nitriles and isomerization of allylic alcohols reactions both realized in aqueous media.

Halogen effect on mixed-valence state of biosmocenium(II, IV) salts

The mixed-valence biosmocenium(II, IV)–Cl adduct [(C5H5)OsII(μ2-η5:η5-C10H8) (C5H5)OsIVCl]Y(1a:YPF6, 1b:YBF4) and biosmocenium-Br adduct [(C5H5)OsII(μ2-η5:η5-C10H8) (C5H5)OsIVBr]PF6(2) were prepared. The structures of 1b and 2 were determined by single crystal X-ray analysis, and it was found that Os-X bond was fixed on cis position to the other half of osmocenyl moiety. Although 1b and 2 had the trapped-valence state in solid state, valence-detrapping depending on temperature was observed in solution state by using variable-temperature 1H NMR spectroscopy. It was found that intramolecular electron exchange reaction OsII–OIV ⇄ OsIV–OsII occurred accompanied by halogen migration. The activation energy of the reaction was determined. The energy decreased in the order A > 2 > 1a depending on a kind of halogen ligand bonded to Os center, and the tendency can be explained by HSAB theory.

A GSO–HDFT study of noncollinear spin structures of [2Fe–2S] cluster

The electronic and spin structures of a [2Fe–2S] model complex: Fe 2 S 2 ( SH ) 4 3 - ( 1), were investigated by hybrid density functional theory (HDFT) calculations. Generalized spin orbital (GSO) approach has been examined to investigate the delocalization nature at the reduced form. The calculated results showed that the delocalization is largely dependent on the geometrical [2Fe–2S] core structure. At the symmetrical geometry, the GSO solution was a mixed-valence state: Fe 2.5+Fe 2.5+, with a noncollinear spin state. The energy difference between the GSO solution and an unrestricted state was extremely small (4 cm −1). On the other hand at the asymmetrical geometry, the GSO solution converged into a trapped valence state: Fe 3+Fe 2+ with a collinear spin state, which was almost the same as an unrestricted result. Natural orbital analysis was performed to elucidate the magnetic orbital interactions at the GSO and unrestricted solutions. These results indicated that the delocalization of the odd electron is largely suppressed for the strong antiferromagnetic interactions between two irons. In conclusion, the GSO descriptions were not drastically improved the ground state energies of the [2Fe–2S] cluster, however, the GSO approach has opened to investigate a variety of electronic structures in the mixed valence states, which could not be investigated by a broken symmetry (BS) approach.

PH-dependent transition between delocalized and trapped valence states of a CuA center and its possible role in proton-coupled electron transfer.

A pH-dependent transition between delocalized and trapped mixed valence states of an engineered CuA center in azurin has been investigated by UV-visible absorption and electron paramagnetic resonance spectroscopic techniques. At pH 7.0, the CuA azurin displays a typical delocalized mixed valence dinuclear [Cu(1.5)....Cu(1.5)] spectra with optical absorptions at 485, 530, and 760 nm, and with a seven-line EPR hyperfine. Upon lowering of the pH from 7.0 to 4.0, the absorption at 760 nm shifted to lower energy toward 810 nm, and a four-line EPR hyperfine, typical of a trapped valence, was observed. The pH-dependent transition is reversible because increasing the pH restores all delocalized spectral features. Lowering the pH resulted in not only a trapped valence state, but also a dramatically increased reduction potential of the Cu center (from 160 mV to 340 mV). Mutation of the titratable residues around the metal-binding site ruled out Glu-114 and identified the C-terminal histidine ligand (His-120) as a site of protonation, because the His120Ala mutation abolished the above pH-dependent transition. The corresponding histidine in cytochrome c oxidases is along a major electron transfer pathway from CuA center to heme a. Because the protonation of this histidine can result in an increased reduction potential that will prevent electron flow from the CuA to heme a, the CuA and the histidine may play an important role in regulating proton-coupled electron transfer.

Mixed-valence state of optically active 1',1'"-bis(2-phenylbutyl)-1,1''-biferrocenium pentaiodide: effects of cation symmetry and intermolecular interaction on trapped-/detrapped-valence states.

The effects of cation symmetry and packing on the mixed-valence state of binuclear ferrocene derivatives are discussed separately by using chiral isomers and racemic modification of 1',1' "-bis(2-phenylbutyl)-1,1' '-biferrocenium pentaiodide. The pentaiodide anion has a polymeric structure and is composed of triiodide anion and iodine molecule units. The (R,S) isomer having an inversion center shows a detrapped-valence state even at 78 K. On the other hand, the (R,R) and the (S,S) isomers having no inversion center show a trapped-valence state at room temperature. The racemic modification, however, consisting of the (R,R) and the (S,S) isomers shows a perfect detrapped-valence state at room temperature. This finding shows that the packing effect overcomes the effect of cation asymmetry.

Structures and Mixed-valence State in Ni(mnt) 2 Salts of Biferrocenes

Ni(mnt) 2 salts of 1′, 1′′′-diisopropyl-1,1′′-biferrocene ( 1 ) and 1′,l′′′-dibenzyl-1 ,1′′-biferrocene ( 2 ) have been prepared. Both complexes show segregated-stack crystal structure, consisting of a mixed-valent biferrocenium monocation and a Ni(mnt) 2 monoanion. The D-A ratio is 1:1 in both compounds. The trapped-valence state is demonstrated for 2 at room temperature.

The role of a terminal benzene ring on the mixed-valence state of a series of 1′,1‴-bis( n-phenylalkyl)-1,1″-biferrocenium triiodides

The relation between the mixed-valence state of the central Fe atoms and the crystal structure is discussed in a series of 1′,1‴-bis( n-phenylalkyl)-1,1″-biferrocenium triiodides ( n=2, 3, 4, 6). The mixed-valence states depend strongly on the methylene substituent in these salts. The 2-phenylethyl and 4-phenylbutyl derivatives show valence detrapping with increasing temperature, although their quadrupole splitting values are considerably different from each other at lower temperatures. On the other hand, the 6-phenylhexyl derivative shows the temperature-independent trapped-valence state. The initiation temperature of valence detrapping in 4-phenylbutyl derivative is lowered by changing the recrystallizing solvent from hexane to dichloromethane, accompanied by a change in the powder X-ray diffraction pattern. The temperature dependences of the quadrupole splitting values in the salts under investigation can be explained using constant quadrupole splitting values both for typical ferrocene- and ferrocenium-like doublets and only by changing the energy difference between the Fe II–Fe III and Fe III–Fe II states in the solid state. Both the cation and anion in the 2-phenylethyl derivative are located on the center of symmetry, which is in good agreement with the 57Fe Mössbauer spectrum results at room temperature. By comparison of the present results with those of the corresponding dialkyl derivatives, the important role of the terminal benzene ring in the mixed-valence state is clarified. The role is based on intermolecular interaction via the benzene ring.

Charge Ordering with Lattice Distortions in a Conductive MMX-Chain Complex, Pt2(dta)4I (dta = CH3CS2-)

The charge-ordering states with lattice distortions of a halogen-bridged binuclear-metal mixed-valence complex (called MMX chain), Pt2(dta)4I (dta = CH3CS2-), have been investigated by transport, magnetic, and optical measurements. This complex is a binuclear unit-assembled conductor containing metal−metal bonds. It exhibits metallic conduction above room temperature, representing the first example of a metallic halogen-bridged one-dimensional transition-metal complex. Below 300 K it shows semiconducting behavior, which is considered to be of the Mott−Hubbard type due to electron correlation. The metal−semiconductor transition at 300 K (= TM-S) is derived from a valence transition of Pt from an averaged-valence state of 2.5+ to a trapped-valence state of 2+ and 3+. The charge-ordering modes are considered to be −IPt2+−Pt3+−IPt2+−Pt3+−IPt2+−Pt3+−IPt2+−Pt3+−I for the semiconducting phase below TM-S and −I−Pt2.5+−Pt2.5+−I−Pt2.5+−Pt2.5+−I−Pt2.5+−Pt2.5+−I−Pt2.5+−Pt2.5+−I− for the metallic phase above TM-S. 129I Mössbauer spectroscopic study is reported for a low-temperature insulating phase below 80 K. The low-temperature electronic structure is considered to be an alternate charge-ordering state with lattice distortions of IPt2+−Pt3+−I−Pt3+−Pt2+IPt2+−Pt3+−I−Pt3+−Pt2+I. The present binuclear platinum complex inherently possesses valence instability of the intermediate valence 2.5+. X-ray photoelectron spectroscopy and polarized reflection measurements are also reported.

Division of the Effects of Cation Symmetry and Its Packing on the Mixed-Valence State of 1‘,1‘ ‘‘-Bis(2-methylbutyl)-1,1‘ ‘-biferrocenium Triiodide Having Asymmetric Carbon in the Substituent

The effects of the cation symmetry and packing on the mixed-valence state of binuclear ferrocene derivatives are discussed separately by paying attention on the differences between the racemic and (S,S) samples and between the salts from hexane and from dichloromethane in 1‘,1‘ ‘‘-bis(2-methylbutyl)-1,1‘ ‘-biferrocenium triiodide having asymmetric carbon. An (R,S) isomer can sit on the center of symmetry, while (S,S) and (R,R) isomers cannot. Such a difference plays an important role on the mixed-valence state in the samples from dichloromethane but not in the samples from hexane. Both the racemic and (S,S) salts in the disordered state showed the same temperature-dependent mixed-valence state. The disordered state of racemic and (S,S) salts from hexane changed to the ordered state by recrystallization from dichloromethane. The ordered state for (S,S) salts showed a typical temperature-independent trapped-valence state. The packing effect in the present salts appears different from that of the derivatives...

Intermediate mixed-valence state between the valence-trapped and valence-detrapped states in 1′,1‴-bis(3-phenylpropyl)-1,1″-biferrocenium triiodide

57Fe Mössbauer spectroscopy and X-ray crystallography reveal that 1′,1‴-bis(3-phenylpropyl)-1,1″-biferrocenium triiodide shows an intermediate mixed-valence state between the typical temperature-independent trapped-valence state and typical `fusion type' valence detrapping.

Mixed-valence states of iron long-chain carboxylate complexes

Mixed-valence oxo-centered trinuclear iron carboxylate complexes with long alkyl-chain fatty acid anions, myristate [Fe 3O(O 2CC 13H 27) 6(py) 3], palmitate [Fe 3O(O 2CC 15H 31) 6(py) 3], stearate [Fe 3O(O 2CC 17H 35) 6(py) 3], and pyridine-solvated stearate [Fe 3O(O 2CC 17H 35) 6(py) 3]py, were studied by means of 57Fe Mössbauer spectroscopy. Each nonsolvated complex showed a temperature-dependent valence transformation between being valence-detrapped at room temperature (r.t.) and valence-trapped at low temperatures, while the solvated stearate complex showed a trapped-valence state at any temperature below r.t. The complexes showed no thermal anomaly in their DSC thermograms in the temperature range of 100–298 K, although the temperature dependence of the Mössbauer parameters and the powder X-ray diffraction patterns suggested a phase transition in the solvated stearate complex at ∼190 K.

129I Mössbauer Spectroscopic Study in a MMX-Chain System, M 2(dta) 4I (M = Ni, Pt, dta = CH 3CS 2−)

The title Pt compound is a highly-conductive mixed-valence MMX-chain system. It exhibits a metal-semiconductor transition at 300 K, which is attributed to a valence transition of Pt from an average-valence state of +2.5 to a trapped-valence state of +2 and +3. From 129I Mössbauer spectroscopic study, the 1-D structural mode was strongly suggested to be a -Pt II-Pt III-I-Pt III-Pt II-I-Pt II-Pt III-I-Pt III-Pt II-- charge ordering state at 16 K. The charge-ordering mixed-valence state is discussed, as well as the valence state of the Ni compound, which is considered to be an averaged-valence Mott insulator.

The Valence Transition in a Conductive MMX-Chain System, Pt 2(dta) 4I (dta = CH 3CS 2−)

The title compound is a highly-conductive mixed-valence MMX chain. It exhibits a metal-semiconductor transition at 300 K, which is associated with a valence transition of Pt from an averaged-valence state of +2.5 to a trapped-valence state of +2 and +3, from the infra-red study of v(C=S) of the dta ligands and the Raman study of v(Pt-Pt) and v(Pt-I) in Pt 2(dta) 4I.

57Fe Mössbauer spectra and X-ray structural analyses of iodide salts for 1′,1‴-bis(α- and β-naphthylmethyl)-1,1″-biferrocenes

The relation between the mixed-valence state and crystal structure of iodide salts for 1′,1‴-bis(α- and β-naphthylmethyl)-1,1″-biferrocenes has been studied by means of 57Fe Mössbauer spectroscopy and X-ray structural analysis. Time change of 57Fe Mössbauer spectroscopy was first observed for only the α-naphthylmethyl derivative; the trapped-valence state (metastable phase) of the Fe atom in the salt gradually changes into the detrapped-valence state (stable phase) on standing at room temperature for one month. There are two independent monocations in the β-naphthylmethyl derivative. Both salts (α-naphthylmethyl derivative in stable phase and β-naphthylmethyl derivative) gave the detrapped-valence state of 57Fe Mössbauer spectra, implying the equivalence of the oxidation state of the two Fe atoms. X-ray diffraction studies of both salts support the above results, i.e. both monocations sit on the center of symmetry and the FeC(Cp) distances (2.064(2) Å for the α-naphthylmethyl derivative and 2.061(9) and 2.056(7) Å for the β-naphthylmethyl derivative) are close to the mean values of ferrocene and ferrocenium ion.

Phase Transition in 1′, 1′″-Dihexylbiferrocenium Triiodide Accompanied by a Change In Mixed-Valence State

Abstract Two different crystals of 1′, 1′″-dihexylbiferrocenium triiodide were obtained from the different solution (hexane or dichloromethane). The salt prepared from hexane has a longer interlayer distance and gives trapped-valence state of Fe atoms, while the salt recrystallized from dichloromethane has a shorter interlayer distance and gives typical fusion-type valence detrapping of the mixed- valence state on the 57Fe Mössbauer spectroscopy. By annealed at 323 K the latter changes into the former salt. The X-ray diffraction study of the latter supports the results of the 57Fe Mössbauer spectroscopy.

Mixed-Valence States of [Fe3O(CH2XCO2)6(H2O)3]·nH2O (X = H, Cl, and Br) Characterized by X-ray Crystallography and 57Fe-Mössbauer Spectroscopy

Mixed-valence states of μ-oxo-centered trinuclear iron carboxylate complexes, [Fe3O(CH2BrCO2)6(H2O)3] (1), [Fe3O(CH3CO2)6(H2O)3]·2H2O (2), and [Fe3O(CH2ClCO2)6(H2O)3]·3H2O (3), were characterized by X-ray crystallography and 57Fe-Mössbauer spectroscopy. Compound 1 has a trapped-valence-type structure, i.e., one Fe(II)-type atom and two Fe(III)-type atoms. The central Fe3O atoms construct nearly an isosceles triangle array with Fe−O bond distances of 1.86(1), 1.83(1), and 2.00(1) Å at 293 K. In the case of compound 2, the X-ray structure shows good equivalence in geometry around the three iron atoms. Results of X-ray crystallography of 1 and 2 are consistent with the results of Mössbauer spectroscopy. The Mössbauer spectra of 1 show a trapped-valence state below 296 K. The Mössbauer spectra of compound 2 are simulated using a three-site relaxation model where an increased rate of intramolecular electron transfer is assumed among the three iron atoms with an increase in temperature. Black crystals of 3 were converted to red powders of [Fe3O(CH2ClCO2)6(H2O)3] (4) due to loss of all three crystal water molecules when kept in air at ambient temperature. Mössbauer spectra of the dehydrated compound 4 show a temperature-independent trapped-valence state. On the other hand, there are three quadrupole doublets in the Mössbauer spectrum of the hydrated compound 3 measured at 86 K, one of which was ascribed to an FeII state and the remaining two to an FeIII (FeIIIA and FeIIIB) state. It is found that two of the three doublets (FeII and FeIIIA) converge without appreciable line broadening with an increase in temperature, while the other doublet (FeIIIB) remains unchanged, indicating valence delocalization over the two iron atoms. The X-ray structure of compound 3 also indicates that valence delocalization takes place only between two iron atoms. At 112 K the central Fe3O atoms array with the Fe−O bond distances of 2.003(2) (Fe1), 1.853(2) (Fe2), and 1.849(2) Å (Fe3). A temperature increase to 293 K changes the bond distances to 1.957(3) (Fe1) and to 1.895(3) Å (Fe2), while the bond distance of Fe3−O is almost unchanged (1.846(3) Å). There are intra- and intermolecular hydrogen bonds formed by the crystal water and coordinated water molecules in compound 3. The results suggest that hydrogen-bonding interaction affects the local environment of iron atoms and induces valence delocalization between the two iron atoms.

Crystal structure and mixed-valence state in 1′,1‴-dibutylbiferrocenium triiodide

In 1′, 1‴-dibutylbiferrocenium triiodide crystals A show temperature-independent trapped-valence state, while crystals B show a valence-delocalization of the mixed-valence state. The mixed-valence states of both crystals are discussed in relation to their crystal structures.

Mixed-valence States of 57Fe Atoms Produced in 57Co-labelled [CoFe2O(CH2ICO2)6(H2O)3] and [CoFe2O(CH2BrCO2)6(H2O)3

Abstract Mixed-valence states of 57Fe atoms produced by EC-decay in 57Co-labelled [CoFe2O(CH2ICO2)6(H2O)3] and [CoFe2O(CH2BrCO2)6(H2O)3] were studied by means of emission Mössbauer spectroscopy. It was found that [CoFe2O(CH2ICO2)6(H2O)3] and [CoFe2O(CH2BrCO2)6(H2O)3] show a temperature-dependent trapped-to-averaged valence state and a temperature-independent trapped-valence state, respectively.

Enzymes and other iron proteins

The phenomenology of a new group of binuclear iron proteins is discussed. The common feature of these proteins is a pair of exchange coupled high spin irons in the diferric, diferrous or ferric-ferrous, trapped valence state. Of particular interest are the cases of intermediate coupling, where the zero-field splittings are comparable to the exchange and possibly the Zeeman interaction. Examples of the Mossbauer analysis of such systems in terms of a spin Hamiltonian are discussed.

Mixed-valence states in TCNQ salts of monoalkylbiferrocenes and their mixed crystals with dialkyl derivative

TCNQ salts of 1′-monoalkylbiferrocenes showed a temperature-independent trapped-valence state as expected for the unsymmetrical structure of cations. On the other hand, mixed crystals of 1′-monobutyl- and 1′.1‴-dibutylbiferrocenium+(TCNQ) 3 − showed a temperature dependence of the mixed-valence states, showing that the lattice affects the mixed-valence states.