Mixed-valence Nature Research Articles

Orbital Analysis Captures the Existence of a Mixed-Valent CuIII -O-CuII Active-Site and its Role in Water-Assisted Aliphatic Hydroxylation.

The Cu-O-Cu core has been proposed as a potential site for methane oxidation in particulate methane monooxygenase. In this work, we used density functional theory (DFT) to design a mixed-valent CuIII -O-CuII species from an experimentally known peroxo-dicopper complex supported by N-donor ligands containing phenolic groups. We found that the transfer of two-protons and two-electrons from phenolic groups to peroxo-dicopper core takes place, which results to the formation of a bis-μ-hydroxo-dicopper core. The bis-μ-hydroxo-dicopper core converts to a mixed-valent CuIII -O-CuII core with the removal of a water molecule. The orbital and spin density analyses unravel the mixed-valent nature of CuIII -O-CuII . We further investigated the reactivity of this mixed-valent core for aliphatic C-H hydroxylation. Our study unveiled that mixed-valent CuIII -O-CuII core follows a hydrogen atom transfer mechanism for C-H activation. An in-situ generated water molecule plays an important role in C-H hydroxylation by acting as a proton transfer bridge between carbon and oxygen. Furthermore, to assess the relevance of a mixed-valent CuIII -O-CuII core, we investigated aliphatic C-H activation by a symmetrical CuII -O-CuII core. DFT results show that the mixed-valent CuIII -O-CuII core is more reactive toward the C-H bond than the symmetrical CuII -O-CuII core.

Exploration of noncovalent interactions in the solid state structures of carboxylate bridged trinuclear mixed valence cobalt complexes using computational tools based on the topological analysis of the electron density

Three linear mixed valence trinuclear cobalt(III)-cobalt(II)-cobalt(III) complexes, [CoII{(μ-L1)(μ-OOCCH3)CoIII(OOCCH3)}2]⋅4H2O (1), [CoII{(μ-L2)(μ-OOCCH3)CoIII(OOCCH3)}2]⋅4H2O (2) and [CoII{(μ-L1)(μ-OOCPh)CoIII(OOCPh)}2] (3), have been synthesized using two N2O2 donor 'reduced Schiff base' ligands, H2L1 (2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(4-chlorophenol) and H2L2 (2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(4-bromophenol), and acetate or bezoate as anionic co-ligands. The complexes have been characterized by spectroscopic measurements and their solid state structures have been determined by single crystal X-ray diffraction analysis. DFT calculations have been performed to study the mixed valence nature of the complexes. Moreover, the structure directing role of several noncovalent interactions in the solid state of the complexes has been investigated. The interactions have been characterized by a combination of two computational tools based on the topological analysis of the electron density, which are the quantum theory of atoms-in-molecules (QTAIM) and the noncovalent interaction plot, since they are useful to reveal noncovalent interactions in the solid state.

Spectral analysis of radical metalloenzyme using EPR in Hydrolases

One of the categories of hydrolase is acid phosphatase which is isolated from sweet potato (spAP). It contains metal ions such as Zn, Mn, Fe etc. with molecular weight 110kDa. An aqueous epr spectrum in buffer (pH 5.6) exhibits class II mixed valence nature of Fe+3 together with Fe+2, Mn+2, Zn+2 and Cu+2 metallobiosites. It indicates reduced form of enzyme with gMn(II)= 2.0084 and grad= 2.0041 signals with radical signal. The localized electron interaction with 55Mn+2 nucleus is evidenced by six-hyperfine splitted spin allowed transitions flanked by ten satellite lines. Allowed and forbidden doublets are computed with A (55Mn) coupling constant~96 G and Zfs parameter D=0.029 cm-1. The spin integration of Tyrradical signal is shown in lyophilized sample spectrum (1.029 x 1015 spins /gm). It also shows DNA cleavage activity with pUC 19 plasmid DNA.

A synergistic catalyst based on a multivalence monodisperse-porous microspheres with oxygen vacancies for benzyl alcohol oxidation

A heterogeneous catalyst for selective oxidation of benzyl alcohol (BzOH) was synthesized by immobilization of Pd nanoparticles (Pd NPs, 3 nm) on monodisperse-porous manganese oxide (MnOx, 4 μm) microspheres obtained by “staged shape templating protocol”. MnOx microspheres containing Mn5O8 (97 %) and MnO2 (78 %) phases were evaluated as supports. The catalysts carrying immobilized Pd NPs were also synthesized, using monodisperse-porous SiO2 and TiO2 microspheres with similar size and porous properties (i.e. Pd@SiO2 and Pd@TiO2). Pd NP decorated-Mn5O8 (Pd@Mn5O8) microspheres provided the highest catalytic activity with BzOH conversion of 92.2 % and benzaldehyde (BzCHO) selectivity of 96.3 %. Higher catalytic activity of Pd@Mn5O8 was explained by mixed valence nature of Mn5O8 microspheres assisting the formation of abundant oxygen vacancies. Mn (II), Mn(III) and Mn(IV) oxidation states were shown by deconvoluted x-ray photoelectron spectra. A synergistic catalytic activity was reported for the first time, by combination of Pd NPs with Mn5O8 microspheres. Pd@Mn5O8 microspheres could be reused up to 5 times without remarkable loss in the activity and selectivity towards BzCHO. A methodology for comparison of catalytic activity and synergistic interaction between active site and support was also exemplified by choosing a nanoparticle type and a monodisperse-porous microsphere type.

Tuning Fe-Se Tetrahedral Frameworks by a Combination of [Fe(en)3]2+ Cations and Cl- Anions.

A one-dimensional (1D) chain compound (en = ethylenediamine), featuring tetrahedral FeSe2 chains separated by [Fe(en)3]2+ cations and Cl- anions, has been synthesized by a low temperature solvothermal method using simple starting materials. The degree of distortion in the Fe-Se backbone is similar to previously reported compounds with isolated 1D FeSe2 chains. 57Fe Mössbauer spectroscopy reveals the mixed-valent nature of [Fe(en)3]3(FeSe2)4Cl2 with Fe3+ centers in the [FeSe2]- chains and Fe2+ centers in the [Fe(en)3]2+ complexes. SQUID magnetometry indicates that is paramagnetic with a reduced average effective magnetic moment, μeff = 9.51 μB per formula unit, and a negative Weiss constant, θ = -10.9(4) K, indicating antiferromagnetic (AFM) nearest neighbor interactions within the [FeSe2]- chains. Weak antiferromagnetic coupling between chains, combined with rather strong intrachain AFM coupling, leads to spin-glass behavior at low temperatures, as indicated by a frequency shift of the peak observed at 3 K in AC magnetic measurements. A combination of [Fe(en)3]2+ and Cl- ions is also capable of stabilizing mixed-valent 2D Fe-Se puckered layers in the crystal structure of [Fe(en)3]4(Fe14Se21)Cl2, where Fe14Se21 layers have a unique topology with large open pores. Property measurements of [Fe(en)3]4(Fe14Se21)Cl2 could not be performed due to the inability to either grow large crystals or synthesize this material in single-phase form.

Short-range magnetic interactions and spin-glass behavior in the quasi-two-dimensional nickelate Pr4Ni3O8

The nickelate Pr4Ni3O8 features quasi-two-dimensional layers consisting of three stacked square-planar NiO2 planes, in a similar way to the well-known cuprate superconductors. The mixed-valent nature of Ni and its metallic properties makes it a candidate for potentially unconventional superconductivity. We have synthesized Pr4Ni3O8 by topotactic reduction of Pr4Ni3O10 in 10 percent hydrogen gas, and report on measurements of powder-neutron diffraction, magnetization and muon-spin rotation (uSR). We find that Pr4Ni3O8 shows complicated spin-glass behavior with a distinct magnetic memory effect in the temperature range from 2 to 300 K and a freezing temperature T_s ~ 68 K. Moreover, the analysis of uSR spectra indicates two magnetic processes characterized by remarkably different relaxation rates: a slowly-relaxing signal, resulting from paramagnetic fluctuations of Pr/Ni ions, and a fast-relaxing signal, whose relaxation rate increases substantially below ~ 70 K which can be ascribed to the presence of short-range correlated regions. We conclude that the complex spin-freezing process in Pr4Ni3O8 is governed by these multiple magnetic interactions. It is possible that the complex magnetism in Pr4Ni3O8 is detrimental to the occurrence of superconductivity.

Synthesis and Characterization of Cu(II) and Mixed-Valence Cu(I)Cu(II) Clusters Supported by Pyridylamide Ligands.

A group of copper complexes supported by polydentate pyridylamide ligands H2bpda and H2ppda were synthesized and characterized. The two Cu(II) dimers [CuII2(Hbpda)2(ClO4)2] (1) and [CuII2(ppda)2(DMF)2] (2) were constructed by using neutral ligands to react with Cu(II) salts. Although the dimers showed similar structural features, the second-sphere interactions affect the structures differently. With the application of Et3N, the tetranuclear cluster (HNEt3)[CuII4(bpda)2(μ3-OH)2(ClO4)(DMF)3](ClO4)2 (3) and hexanuclear cluster (HNEt3)2[CuII6(ppda)6(H2O)2(CH3OH)2](ClO4)2 (4) were prepared under similar reaction conditions. The symmetrical and unsymmetrical arrangement of the ligand donors in ligands H2bpda and H2ppda led to the dramatic conformation difference of the two Cu(II) complexes. As part of our effort to explore mixed-valence copper chemistry, the triple-decker pentanuclear cluster [CuII3CuI2(bpda)3(μ3-O)] (5) was prepared. XPS examination demonstrated the localized mixed-valence properties of complex 5. Magnetic studies of the clusters with EPR evidence showed either weak ferromagnetic or antiferromagnetic interactions among copper centers. Due to the trigonal-planar conformation of the trinuclear Cu(II) motif with the μ3-O center, complex 5 exhibits geometric spin frustration and engages in antisymmetric exchange interactions. DFT calculations were also performed to better interpret spectroscopic evidence and understand the electronic structures, especially the mixed-valence nature of complex 5.

Highly Stable Organic Bisradicals Protected by Mechanical Bonds.

Two new highly charged [2]catenanes-namely, mHe[2]C·6PF6 and mHo[2]C·6PF6-were synthesized by exploiting radical host-guest templation between derivatives containing BIPY•+ radical cations and the meta analogue of cyclobis(paraquat-p-phenylene). In contrast to related [2]catenanes that have been isolated as air-stable monoradicals, both mHe[2]C·6PF6 and mHo[2]C·6PF6 exist as air-stable singlet bisradicals, as evidenced by both X-ray crystallography in the solid state and EPR spectroscopy in solution. Electrochemical studies indicate that the first two reduction peaks of these two [2]catenanes are shifted significantly to more positive potentials, a feature which is responsible for their extraordinary stability in air. The mixed-valence nature of the mono- and bisradical states endows them with unique NIR absorption properties, e.g., NIR absorption bands for the mono- and bisradical states observed at ∼1800 and ∼1450 nm, respectively. These [2]catenanes are potentially useful in applications that include NIR photothermal conversion, UV-vis-NIR multiple-state electrochromic materials, and multiple-state memory devices. Our findings highlight the principle of "mechanical-bond-induced stabilization" as an efficient strategy for designing persistent organic radicals.

Synthesis and characterization of nanorod-structured vanadium oxides

Herein, we reported a facile method to synthesize the nanostructured vanadium oxides by electrochemical deposition, followed by thermal treatment. Polymer surfactant was selected as a template agent to control the nanostructures. Nanorod-, nanoparticle- and platelet-structured vanadium oxides with different sizes were obtained by simply changing the surfactant concentrations. The crystallinity of various nanostructured vanadium oxide was confirmed by X-ray diffraction. X-ray photoelectron spectroscopic analysis reveals a mixed valence nature of vanadium specie in which the amount of V(V) and V(IV) species is not significantly affected by varying the surfactant concentrations. Cyclic-voltammetry curves show that the smallest potential separation between the cathodic and anodic peaks for the nanorod-structured vanadium oxide. The nanorod-structured vanadium oxide exhibits excellent Li-ion intercalation properties, much better than that of the nanoparticle- and platelet-structured vanadium oxides. Thus, the excellent performance with low-cost make the nanorod-structured vanadium oxide promising potential as a cathode material for the Li-ion battery applications.

The Complexity of the CaF2:Yb System: A Huge, Reversible, X-ray-Induced Valence Reduction

Lanthanides are multivalent chemical systems whose luminescence activates numerous optical devices. Their multivalent nature sets a complex scenario that challenges widespread single-valent-ion models. Here we study the complexity inherent in the 2+/3+ mixed-valence nature of Yb in CaF2. We report X-ray absorption spectroscopy of CaF2:Yb that shows a maximal, huge Yb3+ X-ray-induced valence reduction at 200 K for low Yb concentration (0.01%) samples, which reverts to the initial state upon warming to 300 K. Although reduction doubles the number of Yb2+ ions, the anomalous emission that is normally observed at low temperatures completely disappears and never recovers. Surprisingly, however, after annealing at 900 K, some anomalous emission is again observed below 150 K. Under X-ray exposure electrons are ejected from the F– ions, including charge compensating interstitials, Fi–. Removing an electron from Fi– leaves a neutral atom which appears metastable. Ab initio calculations of the Fi–-to-Yb3+ electron ...

Systematic Band Gap Tuning of BaSnO3 via Chemical Substitutions: The Role of Clustering in Mixed-Valence Perovskites

By combining high-throughput experiments and first-principles calculations based on the DFT-ACBN0 approach, we have investigated the energy band gap of Sr-, Pb-, and Bi-substituted BaSnO3 over wide concentration ranges. We show that the band gap energy can be tuned from 3 to 4 eV by chemical substitution. Our work indicates the importance of considering the mixed-valence nature and clustering effects upon substitution of BaSnO3 with Pb and Bi. Starting from the band gap of ∼3.4 eV for pure BaSnO3, we find that Pb substitution changes the gap in a nonmonotonic fashion, reducing it by as much as 0.3 eV. Bi substitution provides a monotonic reduction but introduces electronic states into the energy gap due to Bi clustering. Our findings provide new insight into the ubiquitous phenomena of chemical substitutions in perovskite semiconductors with mixed-valence cations that underpin their physical properties.

Enhanced generation of H2O2 and radicals on Co9S8/partly-graphitized carbon cathode for degradation of bio-refractory organic wastewater

In electrochemical advanced oxidation processes (EAOPs), the hydrogen peroxide (H2O2) could be in-situ generated from the two-electron (2e−) oxygen reduction reaction (ORR), which was then converted to OH radicals in the role of cathode catalyst. To improve the efficiency of H2O2 generation, cobalt sulfide/partly-graphitized carbon (Co9S8/PGC) composites as EAOPs cathodes were synthesized via a simple simultaneous carbonization method. The structural and electrochemical properties of Co9S8/PGC were investigated. Results demonstrated that Co9S8 with coexistence of Co2+/Co3+ was tightly embedded into the PGC skeleton. Abundant oxygen-containing groups on the PGC surface could improve the wettability and hydrophilicity of the Co9S8/PGC (800°C), contributing to the formation of more catalytic active sites in the porous structure. Synergistic effect between Co9S8 and PGC could improve the electron transfer efficiency and O2 mass transfer by increasing the contact interface area between electrode and electrolyte, inducing the high selectivity for 2e−ORR. The mixed-valence nature of Co9S8 (Co2+/Co3+) exhibited high catalytic activity for promoting the OH generation from H2O2. Degradation rates of phenol and TOC removal rate of ceftazidime reached approximately 89.1 and 84.3% at 120 and 480min, respectively, indicating that Co9S8/PGC cathode had promise for further application.

Polaronic charge transfer and large dielectric constant inCa3CoRhO6

We report the dc electrical conductivity and ac impedance study of the spin chain compound CaCoRhO. The sample shows a variable range hopping-assisted dc conduction below 250 K, while a nearest neighbor small polaron hopping is observed at higher temperatures. A reasonably large frequency-independent ac dielectric permittivity (3200) is observed around room temperature. The activation energy associated with the relaxation peaks in loss tangent below 200 K is found to be close to the variable range hopping energy of the dc resistivity data which signifies the active role of the charge carrier hopping toward the ac dielectric data. The characteristic time constant associated with the relaxation (7.2 × 10−11 s) and the value of activation energy (116 meV) suggest a polaronic charge transfer in the system. The modulus analysis suggests that the relaxation in permittivity is due to the intrinsic grain effect and it is likely to be associated with charge transfer between transition metal ions in a system where their mixed valence nature is already reported. The sample also shows dielectric anomaly close to the low-temperature magnetic transition, although we failed to observe any ground state spontaneous electric polarization.

(Invited) Selective Synthesis of Photoelectrochemical Active Co8S15 Cluster inside the Bowl-Shaped Templating Pentaorgano[60]Fullerene Ligand

Metal chalcogenide clusters are structural motifs utilized, for example, in enzymatic active sites, catalysts, and optical/optoelectronic materials. Among non-biomimetic cluster complexes, those consisting of cobalt and sulfur are particularly attractive due to their potential as catalysts, electrocatalysts, semiconducting materials, and so forth. Moreover, intriguing properties have been reported for cobalt sulfide nanoparticles, giving rise to interest in cobalt–sulfur cluster complexes. However, synthetic routes to cobalt–sulfur cluster complexes are quite limited and their structural diversity is very low. Thus, development of facile cobalt–sulfur sources and terminal ligands is strongly desired. In this presentation, we discuss the selective synthesis of the well-defined Co8S15 cluster complex starting from a cobalt trisulfide complex. The precursor cobalt trisulfide complex served as a cobalt–sulfur source as well as a powerful sterically templating terminal ligand. A corner-sharing double-cubane-like cobalt–sulfur cluster was selectively synthesized from an easily prepared cobalt trisulfide complex of pentaaryl[60]fullerene. The obtained cluster contained two Co4S5 cubane-like units coordinated by thiolate and disulfide ligands. The bowl-shaped steric template as well as the metastable cobalt trisulfide moiety played important roles in the exclusive cluster formation. Crystallographic, electrochemical, and magnetic measurements elucidated a mixed-valence nature of the cluster with two high-spin cobalt(II) centers. The present study will provide new opportunities for development of artificial functional metal–sulfur cluster complexes for various applications. Figure 1

Eu3Ir2In15: A Mixed-Valent and Vacancy-Filled Variant of the Sc5Co4Si10 Structure Type with Anomalous Magnetic Properties.

A new compound, Eu3Ir2In15, has been synthesized using indium as an active metal flux. The compound crystallizes in the tetragonal P4/mbm space group with lattice parameters a = 14.8580(4) Å, b = 14.8580(4) Å, and c = 4.3901(2) Å. It was further characterized by SEM-EDX studies. The effective magnetic moment (μeff) of this compound is 7.35 μB/Eu ion with a paramagnetic Curie temperature (θp) of -28 K, suggesting antiferromagnetic interaction. The mixed-valent nature of Eu observed in magnetic measurements was confirmed by XANES measurements. The compound undergoes demagnetization at a low magnetic field (10 Oe), which is quite unusual for Eu-based intermetallic compounds. Temperature-dependent resistivity studies reveal that the compound is metallic in nature. A comparative study was made between Eu3Ir2In15 and hypothetical vacancy-variant Eu5Ir4In10, which also crystallizes in the same crystal structure. However, our computational studies along with control experiments suggest that the latter is thermodynamically less feasible compared to the former, and hence we propose that it is highly unlikely that an RE5T4X10 would exist with X as a group 13 element.

Rhenium Complex with Noninnocent Dioxolene Ligand: Combined Experimental and ab Initio Study of [(3,5-di-tert-Bu2C6H2O2)ReCl3(OPPh3)

Reaction of [ReOCl3(PPh3)2] with 3,5-di-tert-butyl-1,2-benzoquinone (3,5-DTBQ) in hot toluene produces a new complex [(3,5-di-tert-Bu2C6H2O2)Re(OPPh3)Cl3] (1), which was isolated and characterized by elemental analysis, IR, UV-vis spectroscopy, and cyclic voltammetry. In order to clarify the charge state of rhenium and the coordinated dioxolene ligand, X-ray experiments at 150 and 290 K were carried out. The C-O, C-C, and Re-O bond distances at both 150 and 290 K fall between those for semiquinolate (3,5-DTBSQ) and catecholate (3,5-DTBCat) forms; an empirical "metrical oxidation state" of the dioxolene ligand was estimated to be -1.5. High-level ab initio calculations (SOC-CASSCF/NEVPT2) revealed a mixed valence nature of the triplet ground state of complex 1 corresponding to a superposition of the Re(IV)-SQ and Re(V)-cat forms. In agreement with the high-level ab initio and DFT calculations, the temperature dependence of the magnetic susceptibility (5-300 K) is well described in the assumption of the triplet ground state, with the anomalously large zero-field splitting (ZFS) arising from the spin-orbit coupling. According to the ab initio calculations, all absorption bands in the visible region of the electronic absorptions spectrum are assigned to the LMCT bands, with significant contribution of the intraligand transition in the most intense band at 555 nm.

Transport Properties of the Au-Al-Yb Quasicrystal and Approximant under Hydrostatic Pressure

The mixed-valence Au Al Yb quasicrystal exhibits unusual quantum criticality. Its crystalline approximant behaves like a conventional heavy fermion metal. Due to these novel features, they have attracted much attention recently. To examine the sample dependence of this new type of materials, we have prepared several samples and measured the electrical resistivity both at ambient and high pressures as well as the magnetic susceptibility at ambient pressure. The residual resistivity ratio of the quasicrystals is almost sample independent while the power law exponent n of the electrical resistivity varies from sample to sample. The e ective magnetic moment peff of the quasicrystals is also sample dependent, ranging from about 3.4 to 3.9 μB/Yb, and these values are all smaller than the free Yb ion value, con rming the mixed-valence nature. Although the magnitude of n and peff is sample-dependent, the principal feature of the mixed-valence is con rmed in all the samples studied thus far. A combination of these results indicates correlation between n and peff , suggesting that the 4f electrons may contribute to a scattering mechanism of the conduction electrons. External pressure increases the magnitude of the electrical resistivity and decreases the index n of both the quasicrystal and the approximant. Up to the highest pressure measured in the present study no experimental evidence for a phase transition was found.

Crystal growth, structural, electrical, and magnetic properties of mixed-valent compounds YbOs2Al10 and LuOs2Al10.

Single crystals of YbOs2Al10 and LuOs2Al10 were grown for the first time using an aluminum self-flux method. The compounds crystallized into a cagelike structure in space group Cmcm, similar to the prototype compound YbFe2Al10. YbOs2Al10 exhibited a mixed-valent nature, as determined by magnetic susceptibility measurements over a wide temperature range from 2 to 900 K, in which the inter-configuration-fluctuation model revealed a broad peak around 400 K. In contrast, LuOs2Al10 displayed Pauli-like paramagnetic behavior over the same temperature range. Both compounds were metallic in nature between 2 and 300 K. The electronic specific heat coefficient of 21.3(2) mJ mol(-1) K(-2) for YbOs2Al10 was determined to be larger than that for LuOs2Al10 [8.9(1) mJ mol(-1) K(-2)], reflecting the mixed-valent nature of the former. First-principles calculations predicted the presence of a mixed-valent state in YbOs2Al10, in agreement with the experimental observations. The novel compound YbOs2Al10 elucidates the evolution of the mixed-valent nature of the Yb-based ternary transition metal aluminides from the 3d to 5d elements.

Synthesis and Theoretical Investigations of the Solid Solution CeRu1–xNixAl (x = 0.1–0.95) Showing Cerium Valence Fluctuations

Members of the solid solution series of CeRu(1-x)Ni(x)Al can be obtained directly by arc melting of the elements. The presented compounds with 0.1 ≤ x ≤ 0.85 crystallize in the orthorhombic space group Pnma (No. 62) in the LaNiAl structure type, while for 0.9 ≤ x ≤ 1, the hexagonal ZrNiAl-type structure is found. The orthorhombic members exhibit an anomaly in the trend of the lattice parameters as well as an interesting behavior of the magnetic susceptibility, suggesting that the cerium cations exhibit no local moment. Besides the mixed-valent nature of the cerium cations, valence fluctuations along with a change in the cerium oxidation state depending on the nickel content have been found. The oxidation state has been determined from the magnetic data and additionally by XANES. Density functional theory calculations have identified the shortest Ce-Ru interaction as decisive for the stability of the orthorhombic solid solution.

Charge transfer phase transition with reversed thermal hysteresis loop in the mixed-valence Fe9[W(CN)8]6·xMeOH cluster

A bimetallic pentadecanuclear cyanido-bridged {Fe9[W(CN)8]6 (MeOH)24}·xMeOH cluster of an Fe(II/III)-W(IV/V) mixed valence nature, reveals a reversible single-crystal-to-single-crystal transformation, concomitant with metal-to-metal charge transfer between Fe and W ions. The dominance of (HS)Fe(II)-NC-W(V) units at a high temperature, and (HS)Fe(III)-NC-W(IV) units at a low temperature, leads to an unprecedented reversed thermal hysteresis loop in magnetic measurements.