Heptanuclear high-spin complex compounds based on S-donors

The precursors [Fe(III)(^SYL)Cl] (^SYLH₂) = N,N′-bis(1-hydroxy-Y-2-benzyliden)-1,6-diamino-3-thiohexane, (Y = H, 3EtO, 5Me) are high-spin (S = 5/2) complexes. The precursors are combined with [Fe(II)(CN)₆]⁴⁻ and [Co(III)(CN)₆]³⁻ to yield star-shaped heptanuclear clusters, [Fe(II)(CN–Fe(III)^SYL)₆]Cl₂ and [Co(III)(CN–Fe(III)^SYL)₆]Cl₃. The star-shaped compounds are high-spin (HS) systems at room temperature. On cooling to 20 K some of the iron(III) centers perform some HS–HS transition.     

The electrochemical properties of thermophilic cytochrome P450 CYP119A2 at extremely high temperatures in poly(ethylene oxide)

The electrochemical measurements were carried out by using thermophilic cytochrome P450 CYP119A2 (P450st) modified with poly(ethylene oxide) (PEO) in PEO₂₀₀ as an electrochemical solvent. The PEO modified P450st gave clear reduction–oxidation peaks by cyclic voltammetry in oxygen-free PEO₂₀₀ up to 120 °C. The midpoint potential measured for the P450st was −120 mV vs. [Fe(CN)₆]⁴⁻/[Fe(CN)₆]³⁻ at 120 °C. The peak separation, ΔE, was 16 mV at 100 mV/s. The estimated electron transfer rate of PEO-P450st at 120 °C was 35.1 s⁻¹. The faster electron transfer reaction was achieved at higher temperatures. The electrochemical reduction of dioxygen was observed at 115 °C with the PEO-modified P450st system.     

Lithium Ferrocyanide Catholyte for High-Energy and Low-cost Aqueous Redox Flow Batteries**

Aqueous redox flow batteries (ARFBs) are a promising technology for grid-scale energy storage, however, their commercial success relies on redox-active materials (RAM) with high electron storage capacity and cost competitiveness. Herein, a redox-active material lithium ferrocyanide (Li₄[Fe(CN)₆]) is designed. Li⁺ ions not only greatly boost the solubility of [Fe(CN)₆]⁴⁻ to 2.32 M at room temperature due to weak intermolecular interactions, but also improves the electrochemical performance of [Fe(CN)₆]^4−/3−. By coupling with Zn, ZIRFBs were built, and the capacity of the batteries was as high as 61.64 Ah L⁻¹ (pH-neutral) and 56.28 Ah L⁻¹ (alkaline) at a [Fe(CN)₆]⁴⁻ concentration of 2.30 M and 2.10 M. These represent unprecedentedly high [Fe(CN)₆]⁴⁻ concentrations and battery energy densities reported to date. Moreover, benefiting from the low cost of Li₄[Fe(CN)₆], the overall chemical cost of alkaline ZIRFB is as low as $11 per kWh, which is one-twentieth that of the state-of-the-art VFB ($211.54 per kWh). This work breaks through the limitations of traditional electrolyte composition optimization and will strongly promote the development of economical [Fe(CN)₆]^4−/3−-based RFBs in the future.     

Nanocomposite of a chromium Prussian blue with TiO2. Redox reactions and the synthesis of Prussian blue molecule-based magnets

The reaction of [NEt₄]₃[Cr(CN)₆] with titanium(III) p-toluenesulfonate at a pH of 2 affords a gray solid whose metal content and spectroscopic and magnetic properties are fully consistent with it being a Prussian blue material of stoichiometry “Ti^III[Cr^III(CN)₆] · H₂O”. The carbon, nitrogen, and hydrogen content, however, are not consistent with this stoichiometry, and further investigation showed that the gray material has a powder X-ray diffraction profile, infrared spectrum, and magnetic properties very similar to those of the “all-chromium” Prussian blue Cr^II[Cr^III(CN)₆]_0.67 · 6H₂O. All data, including the C, H, and N weight percentages, are consistent with the conclusion that the material isolated is a nanocomposite of Cr^II[Cr^III(CN)₆]_0.67 · xH₂O and TiO₂ in the ratio of 1–1.6. These results suggest that Ti^III reduces some of the [Cr^III(CN)₆]³⁻ ions to generate Ti^IV and Cr^II; the former hydrolyzes to amorphous TiO₂ · 2H₂O, the latter loses its bound CN ligands and reacts with unreacted [Cr^III(CN)₆]³⁻ ions to generate the crystalline all-chromium PB species. The electrochemical potentials suggest that the [Cr^III(CN)₆]³⁻ ion should not be reduced by Ti^III; evidently, this unfavorable reaction is driven by the insolubility of the reaction products. The results constitute a cautionary tale in two respects: first, that the characterization of Prussian blue materials must be conducted with care and, second, that the insolubility of Prussian blue analogues can sometimes drive reactions that in solution are thermodynamically unfavorable.     

Unprecedented multistability in dodecanuclear complex compound observed by Mössbauer spectroscopy

The precursors [Fe(III)(^5XL)Cl] (^5XLH₂ = N,N′-bis(1-hydroxy-2-benzyliden)-1,6-diamino-3-X-hexane, X = N,S) are high-spin (S = 5/2) complexes. This precursors are combined with the bridging unit [(NC)₅Fe(II)-CN-Co(III)(CN)₅]⁶⁻ to yield star-shaped dodecanuclear clusters, [(^5XLFe(III)-NC)₅Fe(II)-CN-Co(III)(CN-Fe(III)^5XL)₅]Cl₄. The star-shaped compounds are high-spin systems at room temperature. On cooling to 20 K some of the iron(III) centers in the N-star switch to the low-spin state as proven by Mössbauer spectroscopy, i.e. multiple electronic transitions, while the S-star remains in the high-spin state.     

Mediated bioelectrocatalytic O2 reduction to water at highly positive electrode potentials near neutral pH

Combinations of bilirubin oxidase and metal complexes: [W(CN)₈]^3−/4−, [Os(CN)₆]^3−/4− and [Mo(CN)₈]^3−/4− (the formal potentials, E^0′(M), being 0.320, 0.448, and 0.584 V vs. Ag|AgCl, respectively, at pH 7.0), allowed bioelectrocatalytic reduction of O₂ to water at their formal potentials near neutral pH. The O₂ reduction current appeared even at the standard potential of the O₂/H₂O redox couple, E^0′(O₂/H₂O), when [Mo(CN)₈]^3−/4− was used at pH 7.4, though the magnitude was small. The magnitude of the bioelectrocatalytic current systematically decreased with the decrease in the potential difference between E^0′(O₂/H₂O) and E^0′(M). A limiting current as large as 17 mA/cm² of a projected electrode surface area was obtained at 0.25 V (−0.37 V vs. E^0′(O₂/H₂O)) for the O₂ reduction at pH 7.0 with a carbon felt electrode modified with electrostatically entrapped bilirubin oxidase and [W(CN)₈]^3−/4− at the electrode rotation rate of 4000 rpm.     

Electrochemical detection of asparaginase antibodies using bifunctionalized carbon interfaces

The early detection of anti-asparaginase biomarker can facilitate timely modification of asparaginase chemotherapy, thereby avoiding serious complications. Herein we describe the preparation of a novel electrochemical biosensing interface for rapid detection of anti-asparaginase in the picomolar range (1–10 000 pM). Coimmobilization of ferrocene and asparaginase on a carbon interface (via diazonium grafting) facilitates transduction through attenuation of the surface-bound ferrocene redox couple. The limit of detection of 0.8 pM for this point-of-care applicable method compares favourably to that of traditional faradaic assaying (2.0 pM) where transduction occurs by the target blocking the diffusion of the solution redox probe [Fe(CN)₆]^3−/4−.     

Direct electrochemical response of Myoglobin using a room temperature ionic liquid, 1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate,as supporting electrolyte

Direct electrochemical response of Myoglobin (Myb) at the basal plane graphite (BPG) electrode was observed when a room temperature ionic liquid (RTIL), 1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate ([HEMIm][BF₄]), was used as the supporting electrolyte. In a 0.17 M [HEMIm]BF₄ aqueous solution, a couple of well-defined redox peaks of Myb could be obtained, whose anodic and cathodic peak potentials were at −0.158 and −0.224 V (vs. Ag/AgCl), respectively. Both anodic and cathodic peak currents increased linearly with the potential scan rate. Compared with the supporting electrolyte of phosphate buffer, [HEMIm][BF₄] played an obvious promotion for the direct electron transfer between Myb and the BPG electrode. Further investigation suggested that Myb was adsorbed tightly on the surface of the BPG electrode in the presence of [HEMIm][BF₄] to form a stable, approximate monolayer Myb film. Myb adsorbed on the BPG electrode surface showed a remarkable electrocatalytic activity for the reduction of oxygen in a [HEMIm][BF₄] aqueous solution. Based on these, a third-generation biosensor could be constructed to directly detect the concentration of oxygen in aqueous solution with a limit of detection of 2.3 × 10⁻⁸ M.     

Synthesis,structure and magnetic properties of 2-D and 3-D [cation]{M[Au(CN)2]3} (M = Ni, Co) coordination polymers

In order to study the templating effect of the cation and the resulting impact on the magnetic properties, reactions of M(II) salts with [cation][Au(CN)₂] were conducted, yielding a series of coordination polymers of the form [cation]{M[Au(CN)₂]₃} (cation = ⁿBu₄N⁺, PPN⁺ (bis(triphenylphosphoranylidene)ammonium); M = Ni(II) and Co(II)). The structures of ⁿBu₄N{M[Au(CN)₂]₃} and PPN{M[Au(CN)₂]₃} (M = Ni and Co) contain two distinct 3-D anionic frameworks of {M[Au(CN)₂]₃}⁻, hence the framework was sensitive to the cation, but not to the identity of the metal center. In ⁿBu₄N{M[Au(CN)₂]₃}, the metal centers are connected by [Au(CN)₂] units to form six 2-D (4, 4) rectangular grids that are fused through the M centers to yield a complex three-dimensional framework which accommodates the ⁿBu₄N⁺ cations. In PPN{M[Au(CN)₂]₃}, the framework adopts a simpler non-interpenetrated Prussian-blue-type pseudo-cubic array, with the PPN⁺ cations occupying each cavity; no reduction in dimensionality occurs despite the large cation size. In the presence of water, {Co(H₂O)₂[Au(CN)₂]₂} · ⁿBu₄N[Au(CN)₂] was obtained, a 2-D layered polymer that contains neutral sheets of {Co(H₂O)₂[Au(CN)₂]₂} which are separated by ⁿBu₄N[Au(CN)₂] layers; aurophilic interactions of 3.4250(13) Å and hydrogen-bonding connect the layers. The magnetic properties of all compounds were investigated by SQUID magnetometry. The Ni(II) polymers have similar magnetic behaviour, which are dominated by zero-field splitting with very weak antiferromagnetic interactions at low temperature (D ∼ 2–3 cm⁻¹, zJ < 1 cm⁻¹). The magnetic behaviour of all of the Co(II) polymers were found to be very similar, and dominated by single-ion effects (i.e. a large first-order orbital contribution). No significant magnetic coupling is observed in any of these coordination polymers, suggesting that the [Au(CN)₂] bridging unit behaves as a poor mediator of magnetic exchange in these high-dimensionality systems.     

Second-order non-linear optical response in LB films for the metal complexes

The complexes with long alkyl chains {[Fe(C₁₆-trz)₃](ClO₄)₂}_n (1), [Fe(C₁₅-BPT)₂(NCS)₂] (2), [Fe(C₁₆-salen)Cl] (3), [Fe(C₁₆-salmmen)Cl] (4), K[Fe(C₁₆-salen)(CN)₂] (5), K[Fe(C₁₆-salmmen)(CN)₂] (6), Na[Fe(C₁₆-salmmen)(CN)₂] (7), [Mn(C₁₆-salen)Cl] (8), [Ni(C₁₆-salen)] (9), [Cu(C₁₆-salen)] (10) were synthesized (C₁₆-trz = 4-hexadecyl-1,2,4-triazole, C₁₅-BPT = N-(3,5-di-2-pyridinyl-4H-1,2,4-triazol-4-yl)-hexadecanecarboxamide, C₁₆-salen = N,N-bis[4-(hexadecyloxy)salicylidene]ethylenediamine, C₁₆-salmmen = N,N′-bis[4-(hexadecyloxy)salicylidene]-1,2-diaminopropane). Langmuir–Blodgett (LB) films of compounds 1–10 were prepared (Scheme 1). The transfers of the molecules from onto the gas–water surface to glass substrate were confirmed by UV–Vis spectra. The second harmonic generation (SHG) were estimated for the LB films formed by the metal complexes. The SHG was observed for the complexes with the long alkyl chains in LB film. The order of the intensity for the SHG related with the number of unpaired d electrons or the d electron configurations.     

Self-assembly of bilayer lipid membrane at multiwalled carbon nanotubes towards the development of photo-switched functional device

Bilayer lipid membrane (BLM) was self-assembled on a uniquely fabricated hydrophilic surface, containing N atoms from the carbon source of ethylene amine, of the multi-walled carbon nanotubes (MWNTs) to form the BLM/MWNTs nanocomposites. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and photoelectric experiments were taken to study the properties of the BLM/MWNTs nanocomposites. The thickness of the BLM, which was calculated from the CV data obtained at BLM/MWNTs electrode, turned out to be 4.38 nm, suggesting that the lipid self-assembled at the nanotubes surface was consistent with a bilayer structure. C₆₀-incorporated BLM could also be self-assembled at the nanotubes surface (C₆₀-BLM/MWNTs). The formation of BLM on the MWNTs surface blocked the diffusion of [Fe(CN)₆]^3/4− redox ions across BLM to the MWNTs electrode as no redox current was observed by CV measurement, whereas the incorporation of the electron mediator, C_60, resumed a pair of redox peaks at C₆₀-BLMs/MWNTs electrode. Moreover, the incorporation of C₆₀ led to a four order of magnitude reduction of the resistance of C₆₀-BLM/MWNTs (369.3 Ω) than that of BLM/MWNTs (3.238 × 10⁶ Ω). MWNTs electrode exhibited an intrinsic cathodic photocurrent (166 μA cm⁻²) while BLM/MWNTs electrode blocked photocurrent response of the MWNTs. Interestingly, C₆₀-BLM/MWNTs electrode resumed partial photoelectric properties (photo current: 65 μA cm⁻²) due to the electron mediation effect of C₆₀ incorporated into the lipid membrane. As a result, the novel self-assembled BLM/MWNTs nanocomposites provided a simple yet useful model to study the C₆₀-mediated photoelectric properties of the BLM/MWNTs which may be applicable to develop new biosensors and molecular devices.     

EDL structure and peculiarities of ferricyanide cyclic voltammetry for silver deposits on gold

The evolution of electrochemical characteristics of a gold electrode upon the deposition of one and more atomic silver layers was studied by means of cyclic voltammetry and the method of potential temperature jump induced by the laser irradiation. Characteristics of the electric double layer on Ag monolayer are determined to be close to those of a massive silver electrode. Meanwhile, the electron-transfer parameters for the model redox system Fe(CN)₆^{3 −/4 −} correspond to a gold electrode. The silver beyond the first atomic layer in multilayer deposits was shown to transform into Ag hexacyanoferrate (II) due to the spontaneous chemical reaction with K₃Fe(CN)₆ from the solution. For the Fe(CN)₆^{3 −/4 −} redox system, the difference between oxidation and reduction peak potentials on a cyclic voltammogram increases with the growth of the silver layers number. This effect results from the corresponding increase in the ohmic resistance of the silver hexacyanoferrate (II) film and is not attributed to the changes in the electron-transfer kinetics.     

Cyclic voltammetric studies of carbohydrate–protein interactions on gold surface

A simple electrochemical method for the determination of association constants between carbohydrates and carbohydrate-binding proteins using cyclic voltammetry (CV) is described. The binding of concanavalin A (Con A) and cholera toxin (CT) to their specific α-mannose and β-galactose derivatives self-assembled on gold electrodes is electrochemically monitored with a redox probe of K₃Fe(CN)₆/K₄Fe(CN)₆. Upon binding of the proteins to the carbohydrate-modified electrodes, the redox current in CV decreases. The binding-induced change in electrochemical signal is thus used to construct Langmuir adsorption isotherm for the carbohydrate–protein interactions and to obtain the association constants. The association constants of carbohydrate–protein interactions determined by CV ((5.8 ± 1.2) × 10⁷ M^− 1 for mannose–Con A, (2.6 ± 0.5) × 10⁸ M^− 1 for galactose-CT) were in good agreement with those measured with electrochemical impedance spectroscopy and quartz crystal microbalance.     

Stereo-electrochemistry by a self-assembled monolayer of sulfur-bridged calixthiophene on gold

Sulfur-bridged calixthiophene formed a self-assembled mono-molecular layer on polycrystalline gold, and it regulated an electrochemical electron transfer by the host–guest interaction between the cavity and reactants. 1,7,13,19,25-Tetrathia[1.₅](2,5)thiophenophane (thiacalix[5]thiophene) perfectly passivated the gold electrode for relatively large reversible metal complexes: [Fe(CN)₆]^4−/3− and [IrCl₆]^3−/2−. However, for mono-atomic ions, such as silver and some of the halogen ions, the electrode behaved reversibly. For copper reduction, a large activation overpotential was observed to induce an initial copper reduction in the cavity.     

A unique 2-D hollow sheet structure and magnetic behavior of a cyanide- and triamine-bridged MnIICrIII ferrimagnet

A 2-D cyanide- and triamine-bridged Mn^IICr^III ferrimagnet, [Mn₃(dien)₂(H₂O)₂][Cr(CN)₆]₂ · 4H₂O (1), has been prepared by the combination of Mn²⁺, diethylenetriamine (dien) co-ligand and [Cr(CN)₆]³⁻. This compound forms a unique 2-D hollow sheet structure constructed by 1-D ribbon networks on the basis of triamine (dien)-bridged trinuclear Mn^II units. Compound 1 readily looses all lattice water molecules and irreversibly changes to a dehydrated form, [Mn₃(dien)₂(H₂O)₂][Cr(CN)₆]₂ (1a), in the air. Cryomagnetic studies of 1 and 1a reveal an antiferromagnetic interaction between Cr^III and Mn^II ions, and an unusual long-range ferrimagnetic ordering below 30 K (1) and 40 K (1a) with multiple magnetic phase changes below T_C. MCD spectra of 1a show a strong Faraday ellipticity associated with the LMCT band of the Cr–CN below 300 nm. Faraday ellipticity is remarkably enhanced below T_C in line with the characteristics long-range ferrimagnetic ordering.     

Prospects for a widely applicable reference potential scale in ionic liquids based on ideal reversible reduction of the cobaltocenium cation

Oxidation of ferrocene, Fe(cp)₂ or reduction of the cobaltocenium cation, [Co(cp)₂]⁺ represent reversible processes that are widely used to provide a voltammetric potential reference scale. However, the [Fe(cp)₂]^0/+ process has been reported to exhibit complexities which may restrict its usefulness for this purpose in ionic liquids. In this study, the reduction of [Co(cp)₂]⁺ in the ionic liquids, [bmpyr][Ntf₂], [emim][Ntf₂], and [bmim][PF₆] (bmpyr = 1-butyl-1-methylpyrrolidinium, emim = 1-ethyl-3-methylimidazolium, bmim = 1-butyl-3-methylimidazolium, Ntf₂ = bis(trifluoromethanesulfonyl)amide) is reported at macro, micro, and rotating disk electrodes. Reversible behaviour, after allowance for ohmic drop, and linear current–concentration relationships are attained over wide concentration ranges for all electrode configurations. Results support the use of the [Co(cp)₂]^+/0 process for reference potential purposes, with non-idealities of the kind reported for ferrocene not being detected.     

Spin crossover FeII complexes as templates for bimetallic oxalate-based 3D magnets

We present the synthesis and structural characterization of the salt [Fe(bpp)₂][MnCr(ox)₃]₂ · bpp · CH₃OH. It crystallizes in the monoclinic space group. This material contains an anionic [MnCr(ox)₃]⁻ 3D 10-gon ferromagnetic network, that orders below 3.0 K. The channels created by this architecture are filled by the spin crossover cations [Fe(bpp)₂]²⁺ (bpp = 2,6(bispyrazol-3-yl)pyridine), free ligand and solvent molecules. No spin transition has been observed at ambient pressure.     

Kinetic spectrophotometric method for trace determination of thiocyanate based on its inhibitory effect

A kinetic spectrophotometric method for the determination of thiocyanate, based on its inhibitory effect on silver(I) catalyzed substitution of cyanide ion, by phenylhydrazine in hexacyanoferrate(II) is described. Thiocyanate ions form strong complexes with silver(I) catalyst which is used as the basis for its determination at trace level. The progress of reaction was monitored, spectrophotometrically, at 488 nm (λ_max of [Fe(CN)₅PhNHNH₂]^3?, complex) under the optimum reaction conditions at: 2.5 × 10^?3 M [Fe(CN)₆]^4?, 1.0 × 10^?3 M [PhNHNH₂], 8.0 × 10^?7 M [Ag⁺], pH 2.8 ± 0.02, ionic strength (μ) 0.02 M (KNO₃) and temperature 30 ± 0.1 °C. A linear relationship obtained between absorbance (measured at 488 nm at different times) and inhibitor concentration, under specified conditions, has been used for the determination of [thiocyanate] in the range of 0.8–8.0 × 10^?8 M with a detection limit of 2 × 10^?9 M. The standard deviation and percentage error have been calculated and reported with each datum. A most plausible mechanistic scheme has been proposed for the reaction. The values of equilibrium constants for complex formation between catalyst–inhibitor (K_CI), catalyst–substrate (K_s) and Michaelis–Menten constant (K_m) have been computed from the kinetic data. The influence of possible interference by major cations and anions on the determination of thiocyanate and their limits has been investigated.     

Novel inorganic ionic compounds based on Re6 chalcocyanide cluster complexes: synthesis and crystal structures of [CuNH3(trien)]2[Re6S8(CN)6] · 7H2O, [CuNH3(trien)]2[Re6Se8(CN)6] and [CuNH3(trien)]2[Re6Te8(CN)6] · H2O

Three new octahedral rhenium chalcocyanide cluster compounds [CuNH₃(trien)]₂[Re₆S₈(CN)₆] · 7H₂O (1), [CuNH₃(trien)]₂[Re₆Se₈(CN)₆] (2) and [CuNH₃(trien)]₂[Re₆Te₈(CN)₆] · H₂O (3) exhibiting ionic structures have been obtained by the diffusion of an ammonia solution of KCs₃[Re₆S₈(CN)₆] (for 1), K₄[Re₆Se₈(CN)₆] · 3.5H₂O (for 2) or Cs₄[Re₆Te₈(CN)₆] · 2H₂O (for 3) into a glycerol solution of CuCl₂ · 2H₂O in the presence of trien (trien=triethylenetetramine). The compounds have been characterized by single-crystal X-ray diffraction. All three compounds contain a cationic complex [CuNH₃(trien)]²⁺ which was not described previously.     

Synthesis,physico-chemical and spectral investigations of novel homo-bimetallic mixed-ligand complexes: 57Fe Mössbauer parameters of [Fe2(imda)2(H2O)3Cl]

The newly prepared homo-bimetallic complexes [M₂(imda)₂(H₂O)₄], [M₂(imda)₂(Bipy)₂] (M = Co, Ni or Cu) and [Fe₂(imda)₂(H₂O)₃Cl] (H₂imda = iminodiacetic acid and Bipy = 2,2′-bipyridine) have been studied employing IR, FAB-mass, ¹H and ¹³C NMR, EPR and ligand field spectra, which indicated a high-spin state of metal ion with hexa-coordinate environment. ⁵⁷Fe Mössbauer data of the homo-bimetallic complex [Fe₂(imda)₂(H₂O)₃Cl] confirm a high-spin configuration with Fe (±3/2 → 1/2) nuclear transitions and the presence of Kramer's double degeneracy. At RT, the spin–spin interactions of the neighbouring nuclei (Fe³⁺–Fe³⁺ = S_5/2–S_5/2) are anti-ferromagnetically coupled. However, at LNT, the complex acquires a mixed-valent [Fe^III–Fe^II] composition corroborated from the X-band EPR data. CV studies indicated the presence of quasi-reversible redox Cu^II/I, Cu^II/III, Fe^III/II, Fe^III/I and Fe^II/I couples.